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Habitability at the surface of a planet depends on having an atmosphere long enough for life to develop. The loss of atmosphere to space is an important component in assessing planetary surface habitability. Current models of atmospheric escape from exoplanets are not well constrained by observations. Atmospheric escape observations from the terrestrial planets are available in public data archives. We recast oxygen escape rates from Earth derived from an instrument on Dynamics Explorer‐1 as function of solar wind and compare them to similar data from Mars. Analysis demonstrates that oxygen escape rates from Mars are not as sensitive to variations in solar power components as those from Earth. Available data from Venus can confirm or refute the assertion that oxygen escape from magnetized planets is more sensitive than that from unmagnetized planets. Plain Language Summary: Habitability of a planet depends on having an atmosphere long enough for life to develop. NASA and ESA data archives contain information about atmospheric escape from the terrestrial planets. For these planets oxygen ions dominate atmospheric escape. The data archives are just beginning to be analyzed and presented in a form that allows comparison with, and validation of, models of the interaction of stellar winds with exoplanets. We derive oxygen escape rates from Earth as a function of solar power components from a recasting of Dynamics Explorer‐1 data and compare them to similar data from Mars. Our analysis demonstrates that oxygen escape rates from Mars are not as sensitive to variations in the solar power components as those from Earth. These data and similar data from Venus will prove to be important constrains on models of stelar wind/atmosphere interactions and atmospheric escape from exoplanets. Key Points: We recast oxygen escape rates from Earth derived from an instrument on Dynamics Explorer‐1 as a function of solar energy inputsWe compare escape rates for a magnetized planet (Earth) and an unmagnetized planet (Mars) as a function of solar energy inputsOxygen escape rates from Mars are not as sensitive to variations in the solar power components as those from Earth [ABSTRACT FROM AUTHOR]

Mars once had a dense atmosphere enabling liquid water existing on its surface, however, much of that atmosphere has since escaped to space. We examine how incoming solar and solar wind energy fluxes drive escape of atomic and molecular oxygen ions (O+ and O2+ ${\mathrm{O}}_{2}^{+}$) at Mars. We use MAVEN data to evaluate ion escape from 1 February 2016 through 25 May 2022. We find that Martian O+ and O2+ ${\mathrm{O}}_{2}^{+}$ both have increased escape flux with increased solar wind kinetic energy flux and this relationship is generally logarithmic. Increased solar wind electromagnetic energy flux also corresponds to increased O+ and O2+ ${\mathrm{O}}_{2}^{+}$ escape flux, however, increased solar wind electromagnetic energy flux seems to first dampen ion escape until a threshold level is reached, at which point ion escape increases with increasing electromagnetic energy flux. Increased solar irradiance (both total and ionizing) does not obviously increase escape of O+ and O2+ ${\mathrm{O}}_{2}^{+}$. Our results suggest that the solar wind electromagnetic energy flux should be considered along with the kinetic energy flux as an important driver of ion escape, and that other parameters should be considered when evaluating solar irradiance's impact on O+ and O2+ ${\mathrm{O}}_{2}^{+}$ escape. Plain Language Summary: Mars was once like Earth with a dense atmosphere enabling liquid water to exist on its surface. However, in the billions of years since then, Mars has lost much of its atmosphere to space. We study how energy inputs from the Sun and from the solar wind can drive escape of the ionized constituents of water from Mars' atmosphere. Ion escape is one of several processes of atmospheric loss, and it is a particularly effective process for removing species heavier than hydrogen and helium from terrestrial atmospheres. We find that previously unconsidered energy fluxes may play an important role in driving ion escape. Key Points: Increased solar wind electromagnetic energy flux increases escape of O+ and O2+ ${\mathrm{O}}_{2}^{+}$O+ and O2+ ${\mathrm{O}}_{2}^{+}$ have increased escape rates with increased solar wind kinetic energyUnclear dependence on increased solar irradiance for O+ and O2+ ${\mathrm{O}}_{2}^{+}$ escape [ABSTRACT FROM AUTHOR]

We investigate how the Martian dayside ionospheric structure is modified by crustal magnetic field (CMF) strength and upstream solar wind pressure by analyzing electron density data from the Langmuir Probe and Waves instrument onboard the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft. We find that the electron density above the exobase is anticorrelated with the ratio of solar wind's normal dynamic pressure (PSW⊥ ${P}_{\text{SW}\perp }$) to CMF magnetic pressure (PCMF ${P}_{\text{CMF}}$). We also analyze the electron density behavior across different magnetic topologies as a function of PSW⊥/PCMF ${P}_{\text{SW}\perp }/{P}_{\text{CMF}}$. The extremely low electron density in the draped topology relates to ionopause‐like structures. The lower electron density in the closed and open topology under higher PSW⊥/PCMF ${P}_{\text{SW}\perp }/{P}_{\text{CMF}}$ may be attributed to a downward force, potentially the J × B force in the case of closed topology. This study highlights the complex interplay between solar wind and CMF in influencing the Martian dayside upper ionosphere. Plain Language Summary: Mars is unique in the solar system because it lacks a global dipole field like Earth and instead has crustal magnetic fields (CMF, i.e., pockets of magnetic fields unevenly distributed on its surface). Such a magnetic scenario yields a very special picture of the interaction between solar wind (a stream of charged particles from the Sun) and the Martian upper atmosphere. For decades, people have found that the structure of the Martian ionosphere (an ionized layer in its upper atmosphere) can be heavily influenced by solar wind dynamic pressure (ram pressure of the stream of charged particles) and CMF strength, but the physics behind this is unclear. Our results indicate that the competition between the solar wind dynamic pressure and CMF strength can induce electromagnetic force, which affects the electron density in the Martian ionosphere. This study sheds light on the detailed physics of the interaction between solar wind and CMF and its implication for the behaviors of the Martian ionosphere. Key Points: The electron density in the Martian dayside upper ionosphere is anticorrelated with pressure ratio of solar wind to crustal magnetic fieldThe electron density in closed, open, and draped topology behaves differently as a function of this ratioThe J × B force may play an important role in the effect of crustal magnetic field and solar wind conditions on the Martian upper ionosphere [ABSTRACT FROM AUTHOR]

A unique event known as the disappearing solar wind (DSW), characterized by an extremely low-density solar wind stream, occurred at Mars on 2022 December 26–27. As this stream flowed past Mars, several properties of the Mars–solar wind interaction changed in response to the density drop. We utilize in situ plasma measurements from the Mars Atmosphere and Volatile EvolutioN spacecraft to examine heavy ion escape during this event. We find that escaping ions with energies above and below 30 eV responded differently to the reduction in solar wind density. High-energy ions experienced a decrease in flux, whereas low-energy ions experienced an increase, regardless of whether they were escaping through the plume or tailward channel. Furthermore, we observe a net reduction in the flux of plume escaping ions during the DSW period, primarily due to a considerable reduction in the high-energy component, which typically dominates plume escape. In contrast, the overall flux of tailward escaping ions increased during this period. These variations in heavy ion escape are mainly attributed to substantial reductions in solar wind dynamic pressure and momentum density. This paper provides new insights into the dynamics of heavy ion escape under an exceptional state of the Mars–solar wind interaction. [ABSTRACT FROM AUTHOR]

We analyzed time‐of‐flight (TOF) data from the Arase satellite to investigate temporal variations of the molecular ion group (O2+, NO+, and N2+) at 19.2 keV/q in the inner magnetosphere for 6 years from the solar declining to rising phase. The molecular ions counts were estimated by subtracting the background contamination of oxygen counts. While the number of clear molecular ion events was small, the estimated counts exhibited good correlation with the solar wind dynamic pressure and SYM‐H index. Long‐term variations of the molecular ions differed from those of counts of the O+ and N+ group. Additionally, we discuss the importance of the solar wind dynamic pressure in causing variations of molecular ions in the inner magnetosphere. Plain Language Summary: Molecular ions (O2+, NO+, and N2+) have been observed in the magnetosphere. Molecular ions in the ionosphere are required to obtain energy through short‐lived reactions with electrons to escape from low altitudes to the magnetosphere. The escaping mechanisms of molecular ions are not fully understood. This study analyzed 6 years of data from the Arase satellite to investigate long‐term variations of molecular ion group in the inner magnetosphere. We discuss the importance of the solar wind dynamic pressure in causing variations of molecular ions in the inner magnetosphere. Key Points: Continuous molecular ion group observations in the inner magnetosphere for 6 yearsCounts of molecular ion group increase following enhancements in the solar wind dynamic pressureThe ratio of molecular ion group to O+ and N+ was lower in the rising phase of the solar cycle than that during the solar minimum [ABSTRACT FROM AUTHOR]

Previous simulations have suggested that O+ outflow plays a role in driving the sawtooth oscillations. This study investigates the role of O+ by identifying the differences in ionospheric outflow between sawtooth and non‐sawtooth storms using 11 years of FAST/Time of flight Energy Angle Mass Spectrograph (TEAMS) ion composition data from 1996 through 2007 during storms driven by coronal mass ejections. We find that the storm's initial phase shows larger O+ outflow during non‐sawtooth storms, and the main and recovery phases revealed differences in the location of ionospheric outflow. On the pre‐midnight sector, a larger O+ outflow was observed during the main phase of sawtooth storms, while non‐sawtooth storms exhibited stronger O+ outflow during the recovery phase. On the dayside, the peak outflow shifts significantly toward dawn during sawtooth storms. This strong dawnside sector outflow during sawtooth storms warrants consideration. Plain Language Summary: A sawtooth event is a convection mode in Earth's magnetosphere, which transports solar wind plasma and energy into the inner magnetosphere and ionosphere. Despite three decades since their discovery, the mechanism behind sawtooth oscillations remains uncertain. One theory suggests that O+ outflow induces sawtooth oscillations through an internal feedback mechanism. In line with this theory, some simulations have generated sawtooth oscillations under steady geomagnetic conditions. Furthermore, previous observations indicate that some, but not all, geomagnetic storms exhibit sawtooth oscillations. This study utilizes data from the FAST/TEAMS instrument (1996–2007) and compares O+ outflow variations during geomagnetic storms with and without sawtooth oscillations. Findings indicate that during the storms' initial phase, sawtooth storms produce less O+ outflow than non‐sawtooth storms. Additionally, non‐sawtooth storms exhibit higher O+ outflow in the dayside during the main phase and in the pre‐midnight sector during the recovery phase, challenging the key role of O+ outflow in driving the feedback mechanism. However, observing large O+ outflow in the dawnside sector of sawtooth events suggests more investigation is needed. Key Points: The intensity and location of O+ outflow during storms are different in storms with and without sawtooth oscillationsThe peak dayside outflow is significantly shifted toward dawn during storms with sawtooth observationsThe nightside picture is mixed; the pre‐midnight O+ outflow is higher in the main phase but lower in the recovery phase of sawtooth storms [ABSTRACT FROM AUTHOR]

The long‐standing "energy crisis" at the giant planets refers to the anomalous heating of planetary thermospheres compared to the available energy from solar irradiance. The coupling between planetary magnetospheres and their upper atmospheres is thought to address these crises, though the sources and pathways of energy transport have not been fully explored at each system. In particular, the total available energy from the upstream solar wind at each planet has not been comprehensively quantified. Here we apply recently developed models of energy conversion by magnetic reconnection and the Kelvin‐Helmholtz instability to each of the Giant Planets, providing estimates of the average external energy inputs for each system between 1985 and 2020. We find that external energy associated with solar‐wind‐magnetospheric coupling significantly exceeds that from solar extreme ultraviolet photons. While internal energy sources are known to dominate at Jupiter and Saturn, external sources may be significant at Uranus and Neptune. Plain Language Summary: The upper atmospheres of Jupiter, Saturn, Uranus, and Neptune are all hotter than would be predicted by their exposure to sunlight alone. The space plasma environment around each planet provides an additional reservoir of energy that can be used to provide this heating. Here we quantify the average energy input to each planetary system due to variation of the photons and plasmas emitted from the Sun. We find that the interaction between a planetary magnetic field and the solar interplanetary magnetic field provides a strong source of external energy at each of the planets, on average significantly more than that of light from the Sun. Key Points: Solar wind‐magnetospheric‐coupling provides higher energy input than solar extreme ultraviolet at the giant planetsSolar activity drives solar‐wind‐magnetospheric coupling much more strongly than planetary seasonThe solar wind may drive the energy input to the upper atmospheres at Uranus and Neptune [ABSTRACT FROM AUTHOR]

Interplanetary (IP) shocks are perturbations observed in the solar wind. IP shocks correlate well with solar activity, being more numerous during times of high sunspot numbers. Earth‐bound IP shocks cause many space weather effects that are promptly observed in geospace and on the ground. Such effects can pose considerable threats to human assets in space and on the ground, including satellites in the upper atmosphere and power infrastructure. Thus, it is of great interest to the space weather community to (a) keep an accurate catalog of shocks observed near Earth, and (b) be able to forecast shock occurrence as a function of the solar cycle (SC). In this work, we use a supervised machine learning regression model to predict the number of shocks expected in SC25 using three previously published sunspot predictions for the same cycle. We predict shock counts to be around 275 ± 10, which is ∼47% higher than the shock occurrence in SC24 (187 ± 8), but still smaller than the shock occurrence in SC23 (343 ± 12). With the perspective of having more IP shocks on the horizon for SC25, we briefly discuss many opportunities in space weather research for the remainder years of SC25. The next decade or so will bring unprecedented opportunities for research and forecasting effects in the solar wind, magnetosphere, ionosphere, and on the ground. As a result, we predict SC25 will offer excellent opportunities for shock occurrences and data availability for conducting space weather research and forecasting. Plain Language Summary: Solar activity is quite correlated with sunspot numbers. Alternating periods between solar minima and minima, termed solar cycle, usually occur every ∼11 years. As a result, researchers often attempt to predict sunspot occurrences for the following solar cycle. Solar perturbations occur more frequently during periods of high solar activity, and Earth‐bound perturbations can disturb the Earth's magnetic field in geospace and on the ground, affecting satellites and power infrastructure. In this work, we use an artificial intelligence supervised model to predict the number of shock occurrences in the ongoing solar cycle (beginning December 2019) by training the model with observations of sunspots and solar perturbations in the previous two solar cycles (August 1996–December 2019). Then, sunspot number predictions for the ongoing solar cycle are applied to the model, and predictions for the solar perturbations are obtained. We find that the number of predicted solar perturbations is ∼50% higher than their occurrence number in the previous solar cycle (December 2008–December 2019). Finally, we discuss how this relatively higher number of predicted solar perturbations can impact space weather research given the unprecedented number of data sets available in geospace and on the ground in the upcoming years. Key Points: SSN and shock count data in SC23‐24 are used with SSN predictions for SC25 in a supervised regression model to estimate shock counts in SC25We predict SC25 (275) will have ∼48% more shocks in comparison to SC24 (187), but it will have fewer shocks in comparison to SC23 (343)SC25 will offer unprecedented opportunities for space weather research given the availability of many data sets from space to the ground [ABSTRACT FROM AUTHOR]

The dayside particle precipitation during the passage of a solar wind rotational discontinuity (RD) event on 10 April 2015 is examined and reviewed. The RD leads to complex structures at the magnetopause, boundary layer, mantle, and cusp even though the geomagnetic activity level remains low. Particle precipitation data from DMSP F17 reveal the formation of an unusual boundary layer where the low energy (cold) ions exhibit energy-latitude dispersion that is usually associated with mantle while the high energy (hot) ions look like typical magnetospheric ions. DMSP F17 and F19 observe a double cusp that is a signature of magnetic reconnection occurring at both high- and low-latitudes due to the dominant IMF By. A global MHD simulation of the event supports the existence of the simultaneous reconnections at high- and low-latitude magnetopause that are consistent with the anti-parallel and component merging models, respectively. Finally, Cluster C2, located at high-latitude and high-altitude in the southern hemisphere, observes velocity fluctuations and reversals with peak-to-peak amplitudes >800 km s-1 as it crosses the magnetopause. Guided by the MHD simulation, the Cluster observation can be interpreted as the spacecraft crossing reconnection outflows while moving from one side of the X-line to the other. [ABSTRACT FROM AUTHOR]

Flux ropes are ubiquitous at Earth's magnetopause and play important roles in energy transport between the solar wind and Earth's magnetosphere. In this study, structure and coalescence of the magnetopause flux ropes formed by multiple X line reconnection in cases with different southward interplanetary magnetic field (IMF) clock angles are investigated by using three-dimensional global hybrid simulations. As the IMF clock angle decreases from 180°, the axial direction of the flux ropes becomes tilted relative to the equatorial plane, the length of the flux ropes gradually increases, and core field within flux ropes is formed by the increase in the guide field. The flux ropes are formed mostly near the subsolar point and then move poleward toward cusps. The flux ropes can eventually enter the cusps, during which their helical structure collapses, their core field weakens gradually, and their axial length decreases. When the IMF clock angle is large (i.e., the IMF is predominantly southward), the flux ropes can coalesce and form new ones with larger diameter. The coalescence between flux ropes can occur both near the subsolar point when they are newly formed and away from the subsolar point (e.g., in the southern hemisphere) when they move toward cusps. However, when the IMF clock angle is small (≤135°), we do not find coalescence between flux ropes. [ABSTRACT FROM AUTHOR]

Using the Arase and Van Allen Probes satellite observations, we investigate the nonlinear electromagnetic ion cyclotron (EMIC) rising‐tone (RT) emissions with an increase of the solar wind dynamic pressure in the dayside magnetosphere. We find that EMIC RT emissions are accompanied by the extended dayside uniform zone (DUZ) over |MLAT| < 25° due to the dayside magnetospheric compression by an increase in Pdyn. Using the observed plasma and magnetic field data, we modeled the threshold amplitude for the nonlinear EMIC waves and compared it with the observation. The small gradient of the ambient magnetic field strongly contributes to the reduction in the threshold amplitude of nonlinear wave growth compared to other parameters. When the threshold amplitude falls to comparable level of pre‐existing EMIC waves, EMIC RT emissions are immediately triggered, suggesting direct evidence that the DUZ is the preferred condition to cause the nonlinear EMIC RT emission in the dayside magnetosphere. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves play an important role in controlling the dynamics of charged particles in the inner magnetosphere. Especially, nonlinear EMIC rising‐tone (RT) emissions can cause the rapid loss of relativistic electrons and ring current ions. Here, we present direct evidence demonstrating that the distortion of the dayside magnetic field causes nonlinear EMIC RT emission in response to the intensification of the solar wind dynamic pressure. Remarkably, these nonlinear EMIC waves are generated through a reduction in the threshold wave amplitudes by the distortion of the magnetic fields, even in the absence of any significant change in the pre‐existing EMIC wave amplitude. The present result provides new insights into a triggering process of nonlinear plasma waves in the magnetosphere. Key Points: Electromagnetic ion cyclotron (EMIC) waves with rising‐tone (RT) elements were observed in the dayside magnetosphere during an increase in the solar wind dynamic pressureIncreasing solar wind dynamic pressure extends the dayside uniform zone of the magnetic field to higher magnetic latitudesThe uniform zone leads to the reduction of the nonlinear threshold wave amplitude, which triggers nonlinear EMIC RT emissions [ABSTRACT FROM AUTHOR]

We analyzed the contribution of electromagnetic ion cyclotron (EMIC) wave driven electron loss to a flux dropout event in September 2017. The evolution of electron phase space density (PSD) through the dropout showed the formation of a radially peaked PSD profile as electrons were lost at high L*, resembling distributions created by magnetopause shadowing. By comparing 2D Fokker Planck simulations of pitch angle diffusion to the observed change in PSD, we found that the μ and K of electron loss aligned with maximum scattering rates at dropout onset. We conclude that, during this dropout event, EMIC waves produced substantial electron loss. Because pitch angle diffusion occurred on closed drift paths near the last closed drift shell, no radial PSD minimum was observed. Therefore, the radial PSD gradients resembled solely magnetopause shadowing loss, even though the local pitch angle scattering produced electron losses of several orders of magnitude of the PSD. Plain Language Summary: Extremely energetic charged particles become trapped by Earth's geomagnetic field, forming the Van Allen radiation belts. The total amount of radiation trapped within these belts varies depending on the solar wind conditions, which can disturb the geomagnetic field to produce geomagnetic storms. At the beginning of a geomagnetic storm, there is a relative calm in the radiation belt, produced by the rapid drainage of electrons from the geomagnetic field. It is not fully understood if these electrons are primarily lost into the solar wind, or if they are lost into Earth's atmosphere. In this study, we analyze the remaining trapped electrons to reconstruct the mechanisms of electron escape at the beginning of a geomagnetic storm in September 2017. While previous work found that electrons were primarily lost into the solar wind, we found that loss into the atmosphere also played an important role. Furthermore, we showed that drainage of electrons into the atmosphere can be mistaken for loss into the solar wind if the energy and trajectory of lost electrons are not carefully considered. Key Points: Characterizing electron loss through peaks and minima in radial phase space density can misrepresent simultaneous loss mechanismsAnalysis of electron loss across all adiabatic invariants μ, K, and L*, is necessary to correctly identify loss mechanismsObservational analysis of phase space density data alone cannot be used to quantify individual contributions of simultaneous loss processes [ABSTRACT FROM AUTHOR]

In situ produced photoelectrons and precipitating solar wind electrons are two distinct hot electron populations in the dayside Martian upper atmosphere. While each population has been known for decades, its relative contribution to the measured hot electron flux has not been adequately characterized up to now. In this study, we implement a two‐stream kinetic model to compute the hot electron flux for a open magnetic field configuration. By comparing model results to realistic data acquired by the Mars Atmosphere and Volatile Evolution mission, we show that the electron fluxes predicted by the pure photoelectron model are enormously underestimated, especially in the direction toward Mars, but the measurements can be adequately reproduced once solar wind electron precipitation is taken into account. Such a precipitation plays a crucial role above 180 km, not only for hot electrons at all energies that move toward Mars but also for electrons above 1,000 eV that move away from Mars due to the backscattering of precipitating electrons. In contrast, the hot electron population is predicted to be mostly composed of photoelectrons below 180 km. The significance of precipitating solar wind electrons is also evaluated, revealing an appreciable impact on the structure of the dayside Martian upper atmosphere at high altitudes, where electron impact ionization is enhanced by a factor of 7 and cold electron heating enhanced by a factor of 4. Plain Language Summary: This study examines two hot electron populations in the dayside Martian upper atmosphere: locally produced photoelectrons and precipitating solar wind electrons. We intend to figure out how much each population contributes to the total hot electron flux. For this purpose, we construct a kinetic model and compare the model results to the realistic data collected by the Mars Atmosphere and Volatile Evolution mission. We find that the measurements are seriously underestimated by the model with pure photoelectrons, but adequately reproduced when both electron populations are included. In particular, solar wind electrons account for all hot electrons moving toward Mars and part of them moving away. Meanwhile, solar wind electrons likely exert an appreciable impact on the structure of the dayside Martian upper atmosphere, enhancing both ionization and cold electron heating. Key Points: A two‐stream kinetic model is employed to provide a preliminary distinction between photoelectrons and solar wind electrons at MarsPhotoelectrons are found to dominate the hot electron population in all directions below 180 km in the dayside Martian ionospherePrecipitating solar wind electrons are vital above 180 km, for all hot electrons moving toward Mars and part of them moving away from Mars [ABSTRACT FROM AUTHOR]

We report the properties of low frequency (LF) electromagnetic fluctuations (≤11 Hz) in relation to input electron and ion outflow response observed by Cluster in the mid‐altitude cusp during high solar wind dynamic pressure PSW and active magnetospheric conditions (Kp = 4+). The multipoint observations reveal the dynamic interplay between the spatial‐temporal properties of the wave and electron inputs and the ion outflow response enabling an assessment of causal connections. The LF waves are identified as ingoing traveling Alfvén waves that become dispersive at the ion gyroradius scale (i.e., kinetic Alfvén waves KAWs). The KAWs are collocated with ingoing field‐aligned electrons and ion outflow at keV energies and below. The KAWs are associated with earthward directed Poynting fluxes and energy densities with peak amplitudes occurring at multiple energy enhancements ("step‐ups") exhibited in the electrons and ions, attributed to IMF BZ and/or PSW variations. Indicative of a causal connection, KAW Poynting fluxes and energy densities are strongly correlated with precipitating electron energy fluxes and outflowing O+ energies and energy fluxes. These yield mid‐altitude cusp empirical relationships that can be incorporated into or constrain cusp transport models. These results demonstrate the important role played by KAWs in enhancing electron precipitation into the cusp ionosphere and subsequent O+ energization upward along the magnetic field into the mid‐altitude cusp and beyond. The results also suggest the importance of PSW and/or IMF BZ variations in driving/controlling the reconnection source Alfvén wave and electron inputs into the cusp that significantly impact the ion outflow process. Key Points: Impulsive traveling kinetic Alfven waves are observed in the mid‐altitude cusp associated with solar wind pressure and IMF BZ variationsAlfven wave energy densities and Poynting fluxes in the mid‐altitude cusp are strongly correlated with ingoing soft electron energy fluxesSpace‐time variations in cusp O+ outflow energies and energy fluxes are correlated with kinetic Alfven waves and ingoing soft electrons [ABSTRACT FROM AUTHOR]

MESSENGER observations of large numbers of flux transfer events (FTEs) during dayside crossings of Mercury's magnetopause have shown that the highly dynamic Hermean magnetosphere is strongly driven by frequent and intense magnetic reconnection. Since FTEs are products of reconnection, study of them can reveal information about whether reconnection sites favor points of maximum shear on the magnetopause. Here, we analyze 201 FTEs formed under relatively stable upstream solar wind conditions as observed by MESSENGER during inbound magnetopause crossings. By modeling paths of these FTEs along the magnetopause, we determine the conditions and locations of the reconnection sites at which these FTEs were likely formed. The majority of these FTE formation paths were found to intersect with high‐magnetic shear regions, defined as shear angles above 135°. Seven FTEs were found where the maximum shear angle possible between the reconnecting magnetic field lines was less than 80° and three of these had shear angles less than 70°, supporting the idea that very low‐shear reconnection could be occurring on Mercury's dayside magnetopause under this global‐scale picture of magnetic reconnection. Additionally, for the FTEs formed under these low‐shear reconnection conditions, tracing a dominant X‐line connecting points of maximum shear along the magnetopause that passes through a region of very low‐shear may be difficult to justify, implying reconnection could be occurring anywhere along Mercury's magnetopause and may not be confined to points of maximum shear. Plain Language Summary: Like at Earth, a magnetic field is generated in Mercury's iron core and this field forms a structure, known as a magnetosphere, around the planet that protects it from the stream of charged particles ejected from the Sun. Mercury's magnetosphere is driven by magnetic reconnection processes, where magnetic field lines from the Sun and Mercury's own field "break" and reconfigure. During this reconnection process, helical magnetic field structures termed magnetic flux ropes (FRs) can form. At Mercury, these FRs are of particular interest as they occur in large numbers suggesting reconnection occurs more frequently in the Mercury system than at Earth. We here analyze and model 201 of these FRs, using data from the MESSENGER mission, to investigate under what conditions the reconnection that formed these FRs occurred. We find for the majority of the FRs the maximum magnetic shear angle between the reconnecting field lines is larger than 135°, suggesting most form under higher shear conditions, as expected. However, we find a small number of cases where the shear angles between the reconnecting field lines is much lower than expected, supporting the idea that magnetic reconnection processes occur much more easily at Mercury. Key Points: Studying Flux Transfer Events (FTEs) can give information about the properties of magnetic reconnection on Mercury's dayside magnetopauseMost reconnection sites of modeled FTEs are found to pass through points of high‐magnetic shear on the magnetopauseFind evidence of low‐shear reconnection at shear angles <70°, suggesting reconnection may not be confined to points of maximal shear [ABSTRACT FROM AUTHOR]

Reconnection at Earth's magnetopause drives magnetospheric convection and provides mass and energy input into the magnetosphere/ionosphere system thereby driving the coupling between solar wind and terrestrial magnetosphere. Despite its importance, the factors governing the location of dayside magnetopause reconnection are not well understood. Though a few models can predict X‐line locations reasonably well, the underlying physics is still unresolved. In this study we present results from a comparative analysis of 274 magnetic reconnection events as observed by the Magnetospheric Multiscale (MMS) mission to determine what quantities affect the accuracy of such models and are most strongly associated with the occurrence of dayside magnetopause reconnection. We also attempt to determine under what upstream solar wind conditions each global X‐line model becomes least reliable. Plain Language Summary: Magnetic reconnection is an energy transferring process that happens at the Earth's magnetopause, the boundary between Earth's local plasma environment and the Sun's interplanetary environment. Reconnection helps move energy and particles into the region of space surrounding the earth called the magnetosphere. Dayside reconnection is important because it is a defining element to the amount of energy deposited into our magnetosphere. Despite its importance, scientists still do not completely understand how it works and where it happens. This study looks at many observations of magnetic reconnection to try and understand its precise location and figure out under which conditions it is more likely to occur. This paper compares in situ data with different models for predicting reconnection locations. Key Points: Compared four different reconnection line modelsOn average, the Maximum Bisection Field Model gives the best prediction of reconnection line locationThe Maximum Exhaust Velocity model gives the worst prediction of the reconnection line location [ABSTRACT FROM AUTHOR]

Using Magnetospheric Multiscale mission (MMS) orbits in the Earth's magnetotail from 2017 to 2020, plasma conditions and the 3D spatial structure of inner‐magnetotail plasma environments (with a focus on the plasma sheet (PS)) are studied with different approaches. Threshold conditions for distinguishing the PS, PS boundary layers, and lobes are derived from the statistical properties of background plasma parameters. Our results support previous studies that employed similar methods using Cluster data. However, stronger currents are observed in both the lobes and PS, likely due to the smaller spacecraft separation (≲70 km) that can resolve thin electron‐scale currents. Threshold conditions are used together with magnetic field and electric field measurements to image the spatial structure of the PS. Results are in good agreement with a global neutral sheet model based on solar wind conditions and magnetospheric configurations. Furthermore, the Earth's dipole tilts toward the Sun around June solstice, which warps the magnetotail as much as ∼2–4 RE in Z geocentric solar magnetospheric. This warping effect is relaxed toward September equinox. Consequently, as MMS travels through the magnetotail from dawn to dusk during this period, there is an apparent dawn‐dusk asymmetry in plasma conditions between June and September. Kink‐like flapping waves and interplanetary magnetic field twisting are other mesoscale processes attributed with a few RE of flaring near the flanks. These findings reveal important insights into the mesoscale structure and dynamics of the magnetotail. Plain Language Summary: Data from 4 years of observations by NASA's MMS mission are used to statistically identify distinctive regions within the Earth's magnetospheric tail. This study reveals insights into the spatial structure of this "magnetotail" and seasonal variations attributed with changes in the Earth's magnetic field configurations, particularly those of the orientation of the Earth's dipole. Our results agree with reported findings from ESA's Cluster mission. However, certain aspects unique to MMS lead to some improved measurements and features relating to MMS orbital design. The presented results are highly beneficial to future large statistical studies with MMS data. Key Points: Inner‐magnetotail environments are statistically identified with background plasma conditions and their global 3D structure is studiedWarping effects attributed to changes in the Earth's dipole tilt angle leads to an apparent dawn‐dusk asymmetry during the summer monthsWe utilize a large volume of Magnetospheric Multiscale mission data with partial plasma moments calculated from low‐energy plasma and energetic particle instruments [ABSTRACT FROM AUTHOR]

Foreshock transient (FT) events are frequently observed phenomena that are generated by discontinuities in the solar wind. These transient events are known to trigger global‐scale magnetic field perturbations (e.g., ULF waves). We report a series of FT events observed by the Magnetospheric Multiscale mission in the upstream bow shock region under quiet solar wind conditions. During the event, ground magnetometers observed significant Pc1 wave activity as well as magnetic impulse events in both hemispheres. Ground Pc1 wave observations show ∼8 min time delay (with some time differences) from each FT event which is observed at the bow shock. We also find that the ground Pc1 waves are observed earlier in the northern hemisphere compared to the southern hemisphere. The observation time difference between the hemispheres implies that the source region of the wave is the off‐equatorial region. Key Points: Magnetospheric Multiscale observed a series of foreshock transients near the Earth's bow shockPc1 waves and magnetic impulse events are observed by ground magnetometers in both hemispheres following the foreshock transientsThe difference in observation times between hemispheres implies that Pc1 waves are generated in the off‐equatorial region [ABSTRACT FROM AUTHOR]

Vlasiator is a high‐performance ion‐kinetic code that is now conducting 3D hybrid‐Vlasov simulations of the global magnetosphere. We use Vlasiator to investigate the impact of a pressure pulse with southward‐oriented magnetic field on the Earth's magnetosphere. The simulation driving parameters are comparable to conditions that have led to geomagnetic storms. Our pressure pulse simulation reproduces many physical effects, namely the expansion of the auroral oval, the development of field‐aligned currents, enhanced particle precipitation near the open/closed field line boundary, and compression of Earth's magnetopause. This demonstrates the effectiveness of the hybrid‐Vlasov approach for moderate driving conditions. Our investigation of the time‐dependent magnetopause compression motivates a generalization of the existing theory. Specifically, we find that accounting for the finite transition time of the solar wind dynamic pressure improves the model's description of the magnetopause oscillations. Plain Language Summary: We perform state‐of‐the‐art simulations of the interaction of near‐Earth space environment with the incoming material ejected from the Sun. The response of Earth's magnetic field to such ejecta is known as a "geomagnetic storm." Our simulation approach captures the motion of protons more accurately than commonly used fluid models, but at the cost of making the simulations more computationally intensive. To validate this novel approach, we compare the output of our simulation with established observational signatures of the initial storm phase. We find that our simulation, of relatively moderate storm conditions, reproduces known effects that are relevant to society. Such effects include particle precipitation into the atmosphere and reconfiguration of global magnetic field. Our simulation does diverge from established models on how the magnetopause, the boundary between Earth's magnetic environment and the solar wind, moves in response to a sudden increase of incoming pressure. This inspires a modification of the theory of magnetopause motion, which better accounts for the detailed time profile of the incoming solar wind. Key Points: Vlasiator's 3D hybrid‐kinetic model of the global magnetosphere produces expected behavior for a pressure pulse arriving at EarthThe finite transition time of the pressure pulse causes magnetopause oscillations to be weak and elongated relative to established modelsThe magnetopause oscillations are explained with a generalized model that accounts for the finite transition time of the pressure pulse [ABSTRACT FROM AUTHOR]

This study presents a re‐evaluation of the Kullen and Janhunen (2004, https://doi.org/10.5194/angeo-22-951-2004) global northward interplanetary magnetic field (IMF) simulation, using the Grand Unified Magnetosphere–Ionosphere Coupling Simulation version 4 (GUMICS‐4), a global MHD model. We investigate the dynamic coupling between northward IMF conditions and the Earth's magnetotail and compare the results to observation‐based mechanisms for the formation of transpolar arcs. The results of this study reveal that under northward IMF conditions (and northward IMF initialization), a large closed field line region forms in the magnetotail, with similarities to transpolar arc structures observed from spacecraft data. This interpretation is supported by the simultaneous increase of closed flux measured in the magnetotail. However, the reconnection configuration differs in several respects from previously theorized magnetotail structures that have been inferred from both observations and simulations results and associated with transpolar arcs. We observe that dawn–dusk lobe regions form as a result of high‐latitude reconnection during the initialization stages, which later come into contact as the change in the IMF By component causes the magnetotail to twist. We conclude that in the GUMICS simulation, transpolar arc‐like structures are formed as a result of reconnection in the magnetotail, rather than high‐latitude reconnection or due to the mapping of the plasma sheet through a twisted magnetotail as interpreted from previous analysis of GUMICS simulations. Plain Language Summary: When the magnetic field associated with the solar wind is directed "northward," the Earth's aurora can adopt a formation where a "bar" of emission crosses the otherwise dim region that lies poleward of the usual auroral emissions. These phenomena are called "theta auroras" (due to their resemblance to the Greek letter), or "transpolar arcs," and have been observed from spacecraft observing the auroral regions. As spacecraft cannot directly observe the global configuration of magnetic field lines within planetary magnetospheres, simulations can be useful to gain insight into the possible structures that occur during different solar wind conditions. We use a global simulation to model the interaction between the solar wind and the Earth's magnetosphere (the region of space around our planet) and see that a large‐scale field line structure can form during certain northward‐directed magnetic field conditions. When we trace these field lines to the ionospheric boundary, we find that they resemble the global structures that have been observed for auroral arcs that stretch across the polar cap (also named theta aurora or transpolar arcs). We also note some key differences in their formation process from observational results. Key Points: Signatures consistent with magnetotail reconnection are observed in a simulation driven by northward interplanetary magnetic field conditionsThe simulation results in the production of a closed magnetotail structure and polar cap arcsThe above structure occurs within a magnetotail that is highly twisted such that open lobe regions cross the equatorial plane [ABSTRACT FROM AUTHOR]

The polar wind, consisting of low‐energy ions and electrons, is an outflow along the open magnetic field lines from the polar cap ionosphere to the magnetosphere. Previous studies found that both solar radiation and solar wind electromagnetic energy are the two main energy sources for the polar wind. The polar rain, being field‐aligned precipitating electrons from the solar wind to the polar cap, may provide additional energies for the polar wind. This scenario is complicated as simulation studies show that polar rain changes the electric potential structures over the polar cap ionosphere. It is unclear how the polar rain affects the polar wind ion outflow. In this study, we show a positive correlation between the polar wind and the polar rain. Meanwhile, the polar wind is generally diminished in regions with strong Earth's magnetic field, suggesting the |B| modulates the penetration depth of the polar rain through the magnetic mirror force and thus the energy dissipation of the polar rain. Therefore, the polar rain can be an additional energy source for the polar wind although the polar rain has generally smaller energies and intensities than the particle precipitations in the auroral regions. Key Points: Both the density and the particle flux of the polar wind ion outflow are positively correlated with the energy flux of the polar rainDensity, velocity, and particle flux of the polar wind ion outflow are higher in regions with smaller Earth's magnetic strengthThe polar rain is an energy source for the polar wind from the nightside ionosphere during geomagnetic quiet times [ABSTRACT FROM AUTHOR]

Magnetosonic (MS) waves are dominant plasma waves causing severe Martian ionospheric erosion. They are generally considered to originate upstream of Martian bow shock with frequencies near the upstream proton gyrofrequency. However, whether MS waves can be locally excited lacks theoretical analysis. Here we present an event of MS waves with frequencies above and closely related to the local proton gyrofrequency in the Martian ionosphere. Concurrently, ring beam hot proton distributions are observed due to the penetration of magnetosheath protons. By employing the observed plasma and magnetic field data, the calculated linear growth rates for MS waves agree well with the observed wave power spectra, demonstrating that they can be locally excited by unstable ring beam hot protons at Mars. Our results could be of great help in understanding the excitation of MS waves in a heavy ion‐rich environment around unmagnetized planets. Plain Language Summary: Magnetosonic (MS) wave is one of the most important plasma waves contributing to the Martian atmospheric loss. They are generally considered to originate upstream of the Martian bow shock. Recent observations at Mars have shown that some MS waves with frequencies near the local proton gyrofrequency were accompanied by ring/shell‐like hot proton distributions. However, whether these waves can be locally excited by such protons lacks the support of the theoretical analysis. In this letter, on the basis of linear instability analysis, we show that MS waves with frequencies well above the local proton gyrofrequency can be locally excited by unstable ring beam hot protons in the Martian ionosphere. These results advance our knowledge of the MS wave excitation in a heavy ion‐rich environment around planets without an intrinsic magnetic field. Key Points: Magnetosonic waves above the local proton gyrofrequency are observed in the Martian ionosphereRing beam hot proton distribution associated with magnetosonic waves is formed by the penetration of solar wind protons simultaneouslyMagnetosonic waves are locally generated by the ring beam hot protons [ABSTRACT FROM AUTHOR]

Numerous questions remain about how solar wind directional discontinuities, for example, rotational discontinuities (RDs), affect energy and transport processes at the magnetopause. The impact on the dayside magnetosphere of a solar wind RD is studied. The study shows that the RD leads to complex structures at the magnetopause, boundary layer, mantle, and cusp even though the geomagnetic activity level remains low. At low altitudes, the Defense Meteorological Satellite Program spacecraft observe a double cusp that is a signature of magnetic reconnection occurring at both high and low latitudes due to the dominant IMF By, while Cluster C2, located at high‐latitude and high‐altitude in the southern hemisphere, observes velocity fluctuations and reversals with peak‐to‐peak amplitudes >800 km s−1 as it crosses the magnetopause. A global MHD simulation of the event shows that the C2 observations are consistent with the spacecraft crossing reconnection outflows while moving from one side of the X‐line to the other. Plain Language Summary: Many studies of the geoffectiveness of the solar wind structures tend to focus on large structures such as coronal mass ejections or corotating high‐speed streams, which can drive large geomagnetic activities such as storms or substorms. Fewer studies focus on the smaller scale structures such as solar wind directional discontinuity (DD). Numerous questions remain about how DDs affect energy conversion and transport processes at the magnetopause. We present an example that shows that a solar wind rotational discontinuity (RD), which is a class of DD, and its accompanying flow rotation can lead to complex structures at the magnetopause, boundary layer, mantle, and cusp even though the geomagnetic activity level remains low. At low altitudes, the Defense Meteorological Satellite Program spacecraft observe a double cusp that is a signature of magnetic reconnection occurring at both high and low latitudes due to the dominant IMF By, which is corroborated in a global MHD simulation. Cluster C2, located at high‐latitude and high‐altitude, observes velocity fluctuations and reversals with peak‐to‐peak amplitudes >800 km s−1 as it crosses the magnetopause. The MHD simulation of the event shows that the C2 observations are consistent with the spacecraft crossing reconnection outflows while moving from one side of the X‐line to the other. Key Points: A solar wind rotational discontinuity can lead to complex structures at the magnetopause, boundary layer, and cusp even during quiet timesDefense Meteorological Satellite Program observes double cusp signatures indicating that reconnection is occurring at both high and low latitudes due to dominant IMF ByThe observed large velocity reversals at the magnetopause are consistent with Cluster moving from one side of the X‐line to the other [ABSTRACT FROM AUTHOR]

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Mars Express and Mars Atmosphere and Volatile Evolution (MAVEN) observations have demonstrated the influence of Mars's spatially variable crustal magnetic fields upon the configuration of the plasma in the ionosphere. This influence furthermore leads to variations in ionospheric escape, conceivably in part through the modification of the plasma density and electron temperature in the upper ionosphere. In this study, we examine MAVEN Langmuir Probe and Waves data, finding a clear correspondence between the structure of the crustal fields and both the measured electron temperatures and densities, by first constructing an "average" profile from which departures can be quantified. Electron temperatures are shown to be lower in regions of strong crustal fields over a wide altitude range. We extend previous analyses to cover the nightside ionosphere, finding the same effects present to a lesser degree, in contrast to previous studies where the opposite relationship was found between densities and crustal fields. We further determine the altitude range over which this coupling between both plasma density (and temperature) and crustal fields is effective and use measurements made by MAVEN in the solar wind to explore the dependence of this crustal field control on the coupling to the solar wind and the interplanetary magnetic field (IMF). Based on this, there is some suggestion that variations in the solar wind dynamic pressure are associated with modulation of the effects of the crustal fields on plasma density, whereas the strength of the IMF modulates the crustal fields effects on both electron densities and temperatures. Plain Language Summary: Mars's atmosphere is exposed to ultra‐violet light from the Sun, forming a layer of plasma at high altitudes around the planet. This plasma layer, termed the ionosphere, is strongly affected in terms of its density and temperature, both by external factors like the solar wind and the interplanetary magnetic field (IMF), and internal factors, like Mars's crustal magnetic field. Here, we study how effective the crustal magnetic fields are at shaping the ionosphere in both density and temperature, by first determining the average structure of the ionosphere, and then examining departures from this average. We find that plasma densities are elevated and temperatures reduced in regions where the crustal field is stronger. We also find the same effect on the nightside. This result is in apparent opposition to previous related studies, now based on a much larger data set, which we suggest may be the result of poor seasonal sampling in earlier studies. We further investigate how this crustal field control varies with the solar wind and IMF, and see evidence for weak modulation according to both the dynamic pressure exerted by the solar wind, and the strength of the IMF. Key Points: We present a statistical analysis of Mars Atmosphere and Volatile Evolution Langmuir Probe and Wave densities and temperatures at MarsDensities are elevated and temperatures reduced in strong crustal field regions, on both the dayside and nightsideVariations in the driving solar wind conditions are able to affect these correlations to a modest degree [ABSTRACT FROM AUTHOR]

Using high‐resolution Solar Wind Electron Analyzer data from the Mars Atmosphere and Volatile EvolutioN spacecraft, we report two new types of electron pitch angle distributions (PADs) in the Martian plasma environment. The first type of electron PADs, showing pitch angle primally around 45° $45{}^{\circ}$, is termed funnel distribution, which was observed near the terminator and at about 1,200 km altitude. The second type of electron PADs, showing pitch angle primally around 135° $135{}^{\circ}$, is termed skirt distribution, which was detected on the nightside and at about 1,250 km altitude. The electrons of funnel and skirt distributions do not exhibit any photoelectron signatures and are shown to originate from the solar wind. Through the fitting analysis, we find that the electrons showing both funnel and skirt PADs are thermal electron populations and suprathermal electron populations. In addition, the possibilities that such two types of electron pitch angle distributions corresponding to magnetic field configurations are also discussed. Plain Language Summary: Electron pitch angle distribution (PAD) plays an important role in studying electron dynamics and magnetic field topologies. Various types of electron PADs in the Martian space environment have been widely reported and studied, including pancake, one‐sided loss cone, isotropy, double field‐aligned beam, butterfly, and so on. In this study, we report two new types of electron PADs, funnel and skirt distributions. The funnel distribution shows pitch angle primally around 45°, and the skirt distributions shows pitch angle primally around 135°. Additionally, we also analyze the sources and populations of these electrons showing funnel and skirt distributions, and discuss the magnetic field configurations that correlate to these two distributions. Such electron PADs may be important for understanding the Martian plasma environment, especially electron precipitation and ion escape. Key Points: In Martian space environment, we report two new types of electron pitch angle distributions, funnel and skirt distributionsThe funnel distribution shows pitch angles primally around 45° $45{}^{\circ}$, and the skirt distribution shows pitch angles primally around 135° $135{}^{\circ}$The electrons of funnel and skirt distributions are identified as thermal and suprathermal electron populations originating from solar wind [ABSTRACT FROM AUTHOR]

During a storm‐time interval around winter solstice, observations by the Magnetospheric Multi‐Scale (MMS) Mission show multiple distinct magnetopause boundary layers (BLs) in the vicinity of the southern cusp. The microphysics of the solar wind‐magnetosphere interaction during storm times are not well understood, because the observations are relatively lacking. This event enables the opportunity to probe the storm‐time magnetopause, and observations support that MMS was near a reconnection site equatorward of the southern cusp, suggesting active reconnection in close proximity to closed magnetic flux regions in the BL. The Grid Agnostic magnetohydrodynamics (MHD) for Extended Research Applications global MHD simulation shows evidence for transient secondary reconnection sites near the southern cusp, demonstrating mechanisms to form closed field line regions of the BL. Plain Language Summary: The physical processes by which the solar wind plasma penetrates Earth's magnetic shield are hotly debated. Furthermore, it is even less understood how the solar wind‐magnetosphere interaction plays out during geomagnetic storms, since observations are harder to obtain. Large geomagnetic storms sparked by strong solar activity occur only a few times yearly, and planning spacecraft observations during these times is extremely difficult. Fortunately, the Magnetospheric Multi‐Scale spacecraft constellation obtained storm‐time observations which provide the opportunity to reveal properties of the disturbed outer magnetosphere, in particular, at the interface of solar wind and geomagnetic fields. The event in this study shows magnetic reconnection occurring in an unusual location, which helps to understand the possible origin of the different observed boundary layers and how plasma is transported across them. Key Points: Around the minimum storm‐time SYM‐H, Magnetospheric Multi‐Scale encountered different magnetopause boundary layers (BLs) with multiple entries into the magnetosphereBz remains negative during one magnetopause crossing, indicating an encounter with the magnetosphere in the vicinity of the southern cuspObservations and global simulations support that secondary reconnection near the southern cusp produces closed BL magnetic flux [ABSTRACT FROM AUTHOR]

The response of the Mars ionosphere to changes in solar irradiance is an important aspect of how conditions on Mars are shaped by our dynamic Sun. Changes in the composition of ionospheric plasma with changes in solar irradiance will affect how the ionosphere mediates the interaction of the neutral atmosphere with the surrounding space environment. Here we use MAVEN ion and neutral density measurements acquired at low (Deep Dip 8) and high (Deep Dip 2) solar irradiance conditions to determine how ion and neutral densities change when the ionizing solar irradiance doubles. We find that the neutral composition does not change significantly when examined at fixed total neutral number density. Furthermore, we find that CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ and O+ densities increase by a factor of 2, but O2+ ${\mathrm{O}}_{2}^{+}$ densities increase by a factor of 1.5. The relative abundance of O2+ ${\mathrm{O}}_{2}^{+}$ decreases as solar irradiance increases. These results are explained by straightforward theoretical considerations of changes in ion production and loss rates. However, a photochemical model fails to reproduce these results with its default set of inputs. Consistent with previous modeling efforts, CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ densities are over‐predicted. Acceptable model–data agreement requires significant adjustments to important model inputs, such as reduction in irradiance by 15%, reduction in CO2 density by a factor of 2, and increase in O density by a factor of 2. These large adjustments are suggestive of the need for improvements to the state of Mars ionosphere models based on MAVEN inputs rather than issues with the underlying data. Key Points: The composition of Mars's ionosphere changes with changes in solar irradianceThese compositional changes vary with respect to altitude but are remarkably constant with respect to total neutral number densityPhotochemical modeling illuminates the photochemical pathways responsible for the observed changes [ABSTRACT FROM AUTHOR]

We present the first measurements of Mars discrete aurora in the extreme ultraviolet (<110 nm) and the first synoptic aurora images in the far ultraviolet (110–180 nm). Auroral emission is detected in >75% of nightside images, with patterns shifting visibly over 15–20 min. Aurora is observed most frequently in regions of open magnetic topology (where crustal magnetic fields are very weak and/or vertical), with the brightest aurora where crustal fields are strongest. We present the first disk‐averaged spectrum of discrete aurora, with several O, C, and CO features as expected for electron impact primarily on CO2. We categorize discrete auroral morphology into three types: crustal field aurora, non‐crustal field patchy aurora, and a new type we call "sinuous" aurora, an elongated serpentine structure that stretches thousands of kilometers into the nightside from near midnight in the northern hemisphere. These observations point to a highly dynamic environment in Mars' magnetotail. Plain Language Summary: In this study, we present near‐global images of localized aurora on Mars and the first measurements of these aurora at very short ultraviolet wavelengths (<110 nm). They are caused by energetic electrons from the solar wind smashing into Mars' upper atmosphere. We find auroras in >75% of images, with their patterns shifting visibly over 15–20 min. They are observed most frequently in regions where magnetic fields are very weak or both strong and vertical, with the brightest aurora where magnetic fields are strongest. We present the first disk‐averaged spectrum of these auroras, showing features expected for electrons striking CO2 (the most abundant gas in Mars' atmosphere). We categorize discrete auroral patterns into three types: those near strong vertical crustal magnetic field, patchy aurora near very weak crustal fields, and a new type we call "sinuous," an elongated serpentine structure that stretches thousands of kilometers into the nightside from near midnight in the northern hemisphere. These observations point to a highly dynamic environment in Mars' nightside space environment. Key Points: We present the first disk measurements of Mars discrete aurora in the EUV end FUV, with the oxygen feature at 130.4 nm being the brightestAuroras are detected in ∼75% of nightside images and are more likely where crustal fields are either very weak or both strong and verticalAn elongated, sinuous discrete aurora is discovered, extending far into the nightside. It may be related to the magnetotail current sheet [ABSTRACT FROM AUTHOR]

The cusp is a region in which the solar wind plasma can directly penetrate into the ionosphere. Most previous studies mainly focused on the low‐ and high‐latitude boundaries of the cusps, the identification of the dawnside and duskside boundaries of them appears to be questionable. The auroras are the ionospheric manifestations of solar‐terrestrial energy coupling. This paper presents a new approach to diagnose the duskside boundary of the cusp with the observation of a particular polar cap arc (PCA) located near the ∼1500 magnetic local time sector (15MLT‐PCA). Using measurements from Defense Meteorological Satellite Program satellite, we selected several 15MLT‐PCA examples where the satellite flew over different regions and analyze the particle characteristics. We identified the precipitating particles in dawnside and duskside regions next to 15MLT‐PCA are from the low latitude boundary layer/cusp and boundary plasma sheet/central plasma sheet respectively. The statistical result further confirms our findings. Therefore, we proposed that 15MLT‐PCA can be used as an identifiable optical signature of the duskside boundary of the cusp. This paper provides a valuable approach to identify the cusp boundaries, and the results are critical for understanding the generation mechanism of 15MLT‐PCA. Plain Language Summary: The polar cusps are important regions in which the particles originating from the solar wind can directly enter the upper atmosphere. Those precipitating particles can produce auroras in the ionosphere. The polar cusps have been studied in great detail for several decades and their properties have been characterized under different interplanetary magnetic field conditions. Until now, how to determine the cusp boundaries is still a topic of investigation. This study emphasizes one special auroral phenomenon called 1500 magnetic local time‐polar cap arc (15MLT‐PCA), and we check the particle distributions around 15MLT‐PCA using the observations provided by the Defence Meteorological Satellite Program satellite. We found that the 15MLT‐PCA occurs at the duskside boundary of the cusp, thus allowing the possibility that 15MLT‐PCA can be regarded as an optical signature of duskside boundary of the cusp. Key Points: The 1500 magnetic local time‐polar cap arc (15MLT‐PCA) is suggested to be an auroral indicator of the duskside boundary of the cuspAt the dawnside (duskside) auroral oval of 15MLT‐PCA, the precipitating particles appear as low latitude boundary layer (LLBL)/cusp (boundary plasma sheet/central plasma sheet [BPS/CPS]) characteristicsThe 15MLT‐PCA can be viewed as a separator between dayside LLBL/cusp and duskside CPS/BPS auroras [ABSTRACT FROM AUTHOR]

This paper presents updated methods for locating the Poleward and Equatorward Auroral Luminosity Boundaries (PALB and EALB) directly from IMAGE Far UltraViolet (FUV) images of the Northern Hemisphere auroral oval. Separate boundaries are determined from images measured at different FUV wavelengths. In addition, new methods for indirectly estimating the Open‐Closed magnetic field line Boundary (OCB) and the Equatorward Precipitation Boundary (EPB) locations are presented; these new boundaries are derived from a combination of the auroral luminosity boundary estimates with statistical latitudinal offsets derived from comparisons with low‐altitude spacecraft Particle Precipitation Boundaries (PPBs). Subsequently, we derive new circle model fits for all these boundary data sets, as well as new quality control criteria for these model fits. The suitability of circle fits for each of the data sets is discussed, and the OCB and PALB circle fits are validated against the Convection Reversal Boundary (CRB), as measured by low‐altitude in situ spacecraft. All the new boundary data sets, covering the epoch May 2000 to October 2002, are freely available online. Plain Language Summary: The ability to measure and model near‐Earth space, its response to driving forces in the interplanetary solar wind, and the impact of these forces on the Earth's upper atmosphere is the basis for the understanding and forecasting of space weather. Because there is a strong physical connection between the Earth's magnetosphere (the region of near‐Earth space dominated by the Earth's magnetic field) and ionosphere (the ionized region of the upper atmosphere) near the poles, observations of the ionosphere provide information about the magnetospheric state. Dynamic processes within the magnetosphere accelerate electrons and ions within the space environment giving them the energy to precipitate in the upper atmosphere. These high‐energy charged particles excite atmospheric atoms and molecules, creating aurora. The light from the aurora can be observed at ultraviolet wavelengths by space‐based imagers. In this study we identify the higher and lower latitude edges of the aurora and determine their relationship to important particle boundaries. Circles are then fitted to the spatial boundary variations to make sure they are defined across all longitudes. These data sets can be used to identify and track the spatial extent and dynamics of magnetospheric regions and boundaries. Key Points: Improved poleward and equatorward auroral luminosity boundaries are determined from IMAGE FUV measurementsNew Open‐Closed magnetic field line Boundary (OCB) and Equatorward Precipitation Boundary (EPB) data sets are derived from these observationsNew circle model fits are derived for all the boundary data sets [ABSTRACT FROM AUTHOR]

The scale size of the plasma boundary region between the sheath and ionosphere in the Martian system is often similar to the gyro‐radii of sheath protons, ∼200 km. As a result, ion energization via kinetic structures may play an important role in modifying the ion trajectories and thus be important when evaluating the large‐scale dynamics of the Martian system. In this paper, we report observations made with the MAVEN Langmuir Probe and Waves instrument of solitary bipolar electric field structures, and assess their potential role in ion energization in the Martian system. The observed structures appear as short duration (∼0.5 ms) bipolar electric field pulses of ∼1–25 mV/m, and are frequently observed in the upstream solar wind and inside the sheath. The study presented in this paper suggests that the bipolar electric field structures observed at Mars have an average electrostatic potential drop of ∼0.07 V. The estimated upper rate at which these structures could further energize the protons is estimated, assuming the protons gain the full 0.07 eV, to be ∼0.13 eV per gyration, or a change in proton energy of ∼0.3%, and a corresponding change in the gyroradius of ∼0.3 km. These numbers imply that to first order the bipolar structures are not a significant source of ion energization in the Martian magnetosheath. Key Points: The direct observation of bipolar electric field structures in Mars' sheath and solar wind, with durations of ∼0.5 ms and amplitudes ∼1–25 mV/mThe bipolar structures have associated electric potentials of ∼0.002–2 V, ∼0.07 V on averageThe bipolar structures do not significantly impact sheath plasma through proton energization [ABSTRACT FROM AUTHOR]

Magnetospheric Multiscale (MMS) observations during an extended crossing of the Earth's dayside magnetopause show evidence of multiple reconnections at the boundary. This crossing occurred when the Interplanetary Magnetic Field (IMF) was southward and had a significant BY component. Approximately 2 hr after this crossing, the Twin Rocket Investigation of Cusp Electrodynamics‐2 rockets were launched into the northern hemisphere cusp and observed overlapping cusp ion injections. These overlapping injections are also evidence of multiple reconnections at the magnetopause. Observations more than 2 hr apart do not constitute conjunction between the spacecraft and the rockets. However, the IMF conditions during the magnetopause crossing and the cusp traversal were very similar. Therefore, had the magnetopause crossings and cusp traversals occurred at the same time, the observations would have been similar. Thus, these two events illustrate the link between multiple reconnections at the magnetopause and overlapping cusp ion injections. Plain Language Summary: Magnetic reconnection is a fundamental process in plasmas that interconnects magnetic field lines on either side of a current layer and converts magnetic energy into particle energy. It is the primary process that interconnects the magnetic field of the Sun with the magnetic field of the Earth at the current layer interface called the magnetopause. Ions and electrons from the solar wind follow these interconnected field lines into the Earth's magnetospheric cusps. Magnetic reconnection at the magnetopause occurs along long lines, called X‐lines. This paper discusses observations at the magnetopause and in the cusp that indicate that there are actually multiple X‐lines at the magnetopause and these multiple X‐lines produce a specific signature in the cusp called overlapping ion injections. Key Points: Magnetospheric multiscale observations often show evidence of multiple reconnection at the magnetopauseTwin Rocket Investigation of Cusp Electrodynamics‐2 observations of overlapping cusp ion injections are also evidence of multiple reconnections at the magnetopauseThere is a strong link between multiple reconnections at the magnetopause and overlapping cusp ion dispersions [ABSTRACT FROM AUTHOR]

Several different instrument types have been widely used to measure the density of plasma in solar system ionospheres. However, limitations in spatial and temporal coverage have prevented efforts to directly compare electron density measured by different kinds of instrument in order to assess whether systematic errors exist in such measurements. Here, we exploit the excellent observational coverage provided at Mars by the MAVEN spacecraft to compare numerous electron density measurements from two instruments, the Radio Occultation Science Experiment (ROSE, a remote sensing investigation) and Langmuir Probe and Waves (LPW, an in situ instrument). For dusk local times, we find that systematic contrasts between ROSE and LPW electron density measurements are less than 4 × 103 cm−3. As this limit is only modestly greater than the intrinsic measurement uncertainty on ROSE electron density observations, we conclude that any systematic error present in either data set is smaller than the instrumental accuracies. Differences are larger, between 8 × 103 and 1.2 × 104 cm−3, at dawn local times. From the differences, ratios, and percentage difference between the ROSE and LPW densities, we postulate that electron densities are more variable in the dawn ionosphere than in the dusk ionosphere. Key Points: Variability in electron density values is greater at dawn than at duskRadio Occultation Science Experiment (ROSE) and Langmuir Probe and Waves (LPW) electron density values agree well on duskside At dusk, differences between ROSE and LPW electron densities imply that the systematic errors of these instruments are smaller than their accuracies [ABSTRACT FROM AUTHOR]

A statistical study of the properties of ions (Of, He+ and H+) measured by the Cluster-II in cusp region as a function of the solar wind dynamic pressure and geomagnetic index Kp respectively was made during the summer and fall of 2001-2003. The main results are that: (1) O+ ion density responds in a significant way to geomagnetic index Kp, and He+ ion density is not correlated with geomagnetic Index Kp, both of them have a significant positive correlation with solar wind dynamic pressure; (2) H+ ion density is also observed to increase with solar wind dynamic pressure, and not correlated with geomagnetic index Kp. [ABSTRACT FROM AUTHOR]

Photoelectrons are produced on the dayside of Mars via solar Extreme Ultraviolet (EUV) and X‐ray ionization and are also frequently observed on the nightside as a result of field‐aligned transport. Based on the Solar Wind Electron Analyzer (SWEA) measurements made on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we show that the shape of the photoelectron energy distribution exhibits interesting cross‐terminator variations, in that the He II peak becomes less pronounced and the 10 − 50 eV smooth background becomes hardened, both more evident at lower altitudes. These observations could be understood as the outcome of "atmospheric absorption" via photoelectron collisions with ambient neutrals during day‐to‐night transport. Such a scenario could successfully interpret the diversity of the MAVEN SWEA observations such as the dawn‐dusk and north‐south asymmetries, the impacts of crustal magnetization, upstream solar wind conditions, and solar EUV and X‐ray radiation, all using the 10–50 eV spectral hardness as a diagnostic. In particular, the magnetic control of the cross‐terminator spectral reshaping is proposed as the outcome of varying day‐to‐night magnetic connectivity which naturally implies varying field‐aligned atmospheric column for "absorption." Crude estimates suggest that an "absorbing" atmospheric column of ≈1.5 × 1015 cm−2 is required for a significant photoelectron spectral reshaping beyond the EUV terminator. Therefore the observations reported here may shed light on the extension of the cross‐terminator magnetic field lines, as complementary to the day‐to‐night magnetic connectivity inferred from the occurrence of nightside photoelectrons. Plain Language Summary: Photoelectrons are an omnipresent population of the Martian ionosphere, which are produced exclusively on the dayside via photoionization and are also frequently observed on the nightside as a result of cross‐terminator transport. They are of broad interest because they make significant contributions to local ionization, heating, and airglow emission in the Martian upper atmosphere. In addition, the occurrence of nightside photoelectrons has been commonly used as a diagnostic of the day‐to‐night magnetic connectivity near Mars. Photoelectrons may also undergo significant spectral reshaping beyond the EUV terminator, an issue that has been paid little attention to but is potentially important for shedding light on the extension of the magnetic connectivity. This study is devoted to such a topic using SWEA measurements made on board the MAVEN spacecraft. We propose that collisions with atmospheric neutrals continuously modify the photoelectron energy distribution from a highly non‐thermal, structured one toward a thermal, featureless one. This scenario successfully interprets many aspects of the observed cross‐terminator variation, such as the dawn‐dusk and north‐south asymmetries, the impacts of crustal magnetization, upstream solar wind conditions, and solar activity. Key Points: The shape of the photoelectron energy distribution varies systematically across the Martian terminatorThe variation is the direct outcome of atmospheric absorption, which is more pronounced at low altitudesBoth crustal magnetization and upstream solar wind conditions affect the variation by modifying the field‐aligned atmospheric column [ABSTRACT FROM AUTHOR]

The intrinsic temporal nature of magnetic reconnection at the magnetopause has been an active area of research. Both temporally steady and intermittent reconnection have been reported. We examine the steadiness of reconnection using space‐ground conjunctions under quasi‐steady solar wind driving. The spacecraft suggests that reconnection is first inactive, and then activates. The radar further suggests that after activation, reconnection proceeds continuously but unsteadily. The reconnection electric field shows variations at frequencies below 10 mHz with peaks at 3 and 5 mHz. The variation amplitudes are ∼10–30 mV/m in the ionosphere, and 0.3–0.8 mV/m at the equatorial magnetopause. Such amplitudes represent 30%–60% of the peak reconnection electric field. The unsteadiness of reconnection can be plausibly explained by the fluctuating magnetic field in the turbulent magnetosheath. A comparison with a previous global hybrid simulation suggests that it is the foreshock waves that drive the magnetosheath fluctuations, and hence modulate the reconnection. Plain Language Summary: The most important process that allows the solar wind to transfer its energy to the Earth's magnetosphere is magnetic reconnection at the magnetopause, and yet it has long been debated whether the process can happen in a steady fashion or is always intermittent even under steady solar wind conditions. Here we address this question using space‐ground conjunctions under quasi‐steady solar wind driving. Such coordinated observations not only allow us to examine and compare reconnection behavior at different altitudes, but also provide an opportunity to investigate the possible driver of the variations. We find that reconnection exhibits variations in the reconnection electric field on time scales of several minutes and that the variations have significant amplitudes (30%–60%), implying continuous but unsteady reconnection. The unsteadiness can be plausibly explained by the fluctuating magnetic field in the turbulent magnetosheath. A comparison with a previous global hybrid simulation suggests that it is the foreshock waves that drive the magnetosheath fluctuations, and hence modulate the reconnection. Key Points: We study whether magnetopause reconnection can happen in a steady fashion or is always intermittent even under steady solar wind conditionsUsing space‐ground conjunction under quasi‐steady solar wind conditions, we find that reconnection electric fields vary over time by 30%–60%The continuous but unsteady reconnection can be plausibly explained by the fluctuating magnetic field in the turbulent magnetosheath [ABSTRACT FROM AUTHOR]

Due to the lower ionospheric thermal pressure and existence of the crustal magnetism at Mars, the Martian ionopause is expected to behave differently from the ionopause at Venus. We study the solar wind interaction and pressure balance at the ionopause of Mars using both in situ and remote sounding measurements from the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument on the Mars Express orbiter. We show that the magnetic pressure usually dominates the thermal pressure to hold off the solar wind at the ionopause at Mars, with only 13% of the cases where the ionospheric thermal pressure plays a more important role in pressure balance. This percentage at Venus, however, is up to 65%. We also find that the ionopause altitude at Mars decreases as the normal component of the solar wind dynamic pressure increases, similar to the altitude variation of the ionopauses at Venus. Moreover, our results show that the ionopause thickness at Mars and Venus is mainly determined by the ion gyromotion and is equivalent to about five ion gyroradii. Plain Language Summary: An ionopause is a sharp decrease in the plasma density at the top of the ionosphere, separating the ionospheric plasma from the shocked solar wind plasma. It was found to be a common feature at Venus, where its variability is well constrained by observations from the Pioneer Venus Orbiter. Past studies have shown that there are many similarities between the ionopauses at Mars and Venus. However, because the thermal pressure of the ionosphere at Mars is lower than that of Venus, the Martian ionopause is also thought to behave differently from the ionopause at Venus. We study the pressure configuration inside the ionopause at Mars and find that most of the time the magnetic pressure is greater than the thermal pressure. We also find that higher solar wind pressure pushes the ionopause downward at Mars. Moreover, we show that the thickness of the ionopauses at Mars equals a few radii of ion circular motion in the magnetic field. Our results provide insight to the process that controls the formation of the ionopause at Mars. Key Points: The ionopauses at Mars are found 13% of the time unmagnetized; this percentage at Venus, however, is up to 65%The ionopause altitude decreases with the solar wind dynamic pressure at Mars, similar to the altitude variation of the ionopauses at VenusThe ionopause thickness at Mars and Venus is mainly determined by the ion gyromotion and equivalent to about five ion gyroradii [ABSTRACT FROM AUTHOR]

The Kelvin‐Helmholtz instability at the magnetospheric boundary plays a crucial role in solar wind‐magnetosphere‐ionosphere coupling, particle entry, and energization. The full extent of its impact has remained an open question due, in part, to global models without sufficient resolution to capture waves at higher latitudes. Using global magnetohydrodynamic simulations, we investigate an event when the Magnetospheric Multiscale (MMS) mission observed periodic low‐frequency waves at the dawn‐flank, high‐latitude boundary layer. We show the layer to be unstable, even though the slow solar wind with the draped interplanetary magnetic field is seemingly unfavorable for wave generation. The simulated velocity shear at the boundary is thin (∼0.65RE) and requires commensurately high spatial resolution. These results, together with MMS observations, confirm for the first time in fully three‐dimensional global geometry that KH waves can grow in this region and thus can be an important process for energetic particle acceleration, dynamics, and transport. Plain Language Summary: The boundary separating magnetospheric plasma and the solar wind can become unstable due to the Kelvin‐Helmholtz instability, forming waves that can facilitate mass, momentum, and energy transfer into the magnetosphere. How prevalent Kelvin‐Helmholtz waves are along the magnetopause is therefore a fundamental question to understanding the magnetospheric response to the solar wind. Determining when and where these waves occur on the boundary has remained a challenge. The lower latitudes have been extensively studied while the Cluster mission has provided observations of KH at high‐latitudes. No global models have been capable of resolving the high‐latitude boundary layer, preventing numerical studies of waves within this region. We present a simulation that captures Kelvin‐Helmholtz waves at the high‐latitude boundary for the first time. The instability formed despite the magnetosphere being immersed in pristine slow wind, which reduced the velocity drop across the shear layer at the equator. The simulated period took advantage of an opportunity when the Magnetospheric Multiscale mission was located at the high‐latitude boundary and observed boundary oscillations. Together with the observations, we confirm that Kelvin‐Helmholtz waves can grow at high‐latitudes and thus be able to contribute to particle entry and energization in this region. Key Points: Global magnetohydrodynamic simulation of the magnetosphere captures unstable Kelvin‐Helmholtz waves at the high‐latitude boundary layer for the first timeThe growth of the surface waves occurs despite the stabilizing slow solar wind and draped magnetic field near the high‐latitude cuspThe fastest growing wave mode resolved by the simulation is consistent with Magnetospheric Multiscale mission observations of the same event [ABSTRACT FROM AUTHOR]

The ultra‐low frequency wave‐particle drift‐bounce resonance in the inner magnetosphere has been studied in detail, due to its important role in particle energization. However, it remains an open question how drift‐bounce resonance manifests in the dayside outer magnetosphere, where particles' orbits show bifurcations because of off‐equatorial magnetic field minima. In this study, we investigate this question, by analyzing Magnetospheric Multiscale observations of the January 20, 2017 event. A test‐particle simulation is conducted to help us understand the observations. The observed pitch angle‐time spectrograms show "pawtrack‐like" structures. We find there are more than two resonant pitch angles at fixed energy, since off‐equatorial minima change the relationship between the bounce (drift) frequency and pitch angle from unimodal function to trimodal function. These results reveal a new drift‐bounce acceleration mechanism in the dayside outer magnetosphere, which potentially affects the efficiency of particle energization during geomagnetic activities like geomagnetic storms. Plain Language Summary: The sun continuously ejects ionized gases called solar winds toward the Earth, affecting Earth's geomagnetic field. When the solar activity is strong, the geomagnetic field is severely disturbed, influencing the safety of power transmissions and space activities. This phenomenon is connected with particle acceleration in the geomagnetic field. The low frequency oscillation of geomagnetic field lines could efficiently transfer energy to charged particles in space, which plays an important role in particle acceleration. This study analyzes special observations which are different from the conventional pattern. A simulation is conducted to help interpret the observations. We find this low frequency magnetic field oscillation interacts with particles differently near the dayside boundary of the geomagnetic field, as the compression of the geomagnetic field by solar winds affects the motion of charged particles nearby. These results help us better understand the formation of geomagnetic activities. Key Points: The effects of off‐equatorial magnetic field minima on ultra‐low frequency wave‐ion interaction have been studied in this letterOff‐equatorial minima lead to more than two resonant pitch angles in drift‐bounce resonanceThe consequent pitch angle distribution shows "pawtrack‐like" structures [ABSTRACT FROM AUTHOR]

The properties of cold, dense, low energy (<150 eV) ions within Earth's magnetosphere between 6 and 14 RE distance are examined using data sampled by Time History of Events and Macroscale Interactions during Substorms spacecraft during a new low‐energy plasma mode that operated from June 2016 to July 2017. These ions are a persistent feature of the magnetosphere during enhanced solar wind dynamic pressure and/or magnetospheric activity. These ions have densities ranging from 0.5 to tens of cm−3, with a mean of ∼1 cm−3 and temperatures of a few to tens of eV, with a mean of ∼13 eV. These yield cold to hot ion density and temperature ratios that are 4.4 and 4×10−3, respectively. Comparisons reveal that the cold ion densities are positively correlated with solar wind dynamic pressure. These ions are organizable, according to their pitch‐angle distribution, as being transverse/convection dominated (interpreted as plume plasma) or magnetic field‐aligned (FAL) (uni‐ or bi‐directional characteristic of ion outflow or cloak plasma). Transverse ions preferentially occur in the prenoon to dusk sectors during sustained active magnetospheric conditions driven by enhanced solar wind dynamic pressure under southward Bz and westward By IMF orientations. Transverse ion velocities (reaching several tens of km/s) have a westward directed tendency with a slight radially outward preference. In contrast FAL ions preferentially occur from morning to noon during northward IMF orientations, enhanced solar wind dynamic pressure, and quiet magnetospheric conditions within several hours after moderate to strong activity. The FAL ions also have bulk velocities ≲30 km/s, with an eastward and radially outward tendency. Key Points: Dense low energy ions are a persistent feature of the magnetosphere during enhanced solar wind dynamic pressure and/or KpLow energy transverse ions tend to occur in duskside magnetosphere under southward Bz and westward By IMF conditions, with westward flowLow energy magnetic field‐aligned (uni‐ or bi‐directional) ions tend to occur in the dawnside under northward IMF, with an eastward flow [ABSTRACT FROM AUTHOR]

Wakes behind spacecraft caused by supersonic drifting positive ions are common in plasmas and disturb in situ measurements. We review the impact of wakes on observations by the Electric Field and Wave double‐probe instruments on the Cluster satellites. In the solar wind, the equivalent spacecraft charging is small compared to the ion drift energy and the wake effects are caused by the spacecraft body and can be compensated for. We present statistics of the direction, width, and electrostatic potential of wakes, and we compare with an analytical model. In the low‐density magnetospheric lobes, the equivalent positive spacecraft charging is large compared to the ion drift energy and an enhanced wake forms. In this case observations of the geophysical electric field with the double‐probe technique becomes extremely challenging. Rather, the wake can be used to estimate the flux of cold (eV) positive ions. For an intermediate range of parameters, when the equivalent charging of the spacecraft is similar to the drift energy of the ions, also the charged wire booms of a double‐probe instrument must be taken into account. We discuss an example of these effects from the MMS spacecraft near the magnetopause. We find that many observed wake characteristics provide information that can be used for scientific studies. An important example is the enhanced wakes used to estimate the outflow of ionospheric origin in the magnetospheric lobes to about 1026 cold (eV) ions/s, constituting a large fraction of the mass outflow from planet Earth. Plain Language Summary: Wakes caused by spacecraft motion or drifting plasma are common behind spacecraft with scientific instruments and disturb in situ observations of space plasmas. We review the impact of wakes on observations by the Electric Field and Wave double‐probe instruments on the Cluster satellites. In the solar wind, the wake behind a Cluster spacecraft is caused by the spacecraft body, is narrow, and can partly be compensated for when analyzing data. In the regions above the Earth's polar regions, the wake behind a Cluster spacecraft is caused by an electrostatic structure around the positively charged spacecraft, causing an enhanced wake. The charging stops positive ions from reaching the spacecraft. Rather, this wake can be used to estimate the flux of cold (eV) positive ions escaping from the ionosphere. Above the poles the flux is about 1026 ions/s, constituting a large fraction of the mass outflow from planet Earth. For an intermediate range of parameters, when the drift energy of the ions is comparable to the equivalent charge of the spacecraft, also the charged wire booms of a double‐probe instrument must be taken into account. We discuss an example from the MMS spacecraft near the magnetopause. Key Points: Plasma wakes are common behind scientific spacecraftWakes in the solar wind can be compensated for in data analysisEnhanced wakes in the polar lobes can be used to detect cold outflowing ions [ABSTRACT FROM AUTHOR]

Ionospheric ions (mainly H+, He+, and O+) escape from the ionosphere and populate the Earth's magnetosphere. Their thermal energies are usually low when they first escape the ionosphere, typically a few electron volt to tens of electron volt, but they are energized in their journey through the magnetosphere. The ionospheric population is variable, and it makes significant contributions to the magnetospheric mass density in key regions where magnetic reconnection is at work. Solar wind--magnetosphere coupling occurs primarily via magnetic reconnection, a key plasma process that enables transfer of mass and energy into the near-Earth space environment. Reconnection leads to the triggering of magnetospheric storms, auroras, energetic particle precipitation and a host of other magnetospheric phenomena. Several works in the last decades have attempted to statistically quantify the amount of ionospheric plasma supplied to the magnetosphere, including the two key regions where magnetic reconnection occurs: the dayside magnetopause and the magnetotail. Recent in situ observations by the Magnetospheric Multiscale spacecraft and associated modeling have advanced our current understanding of how ionospheric ions alter the magnetic reconnection process, including its onset and efficiency. This article compiles the current understanding of the ionospheric plasma supply to the magnetosphere. It reviews both the quantification of these sources and their effects on the process of magnetic reconnection. It also provides a global description of how the ionospheric ion contribution modifies the way the solar wind couples to the Earth's magnetosphere and how these ions modify the global dynamics of the near-Earth space environment. [ABSTRACT FROM AUTHOR]

Based on global hybrid simulation results, we predict that foreshock turbulence can reach the magnetopause and lead to reconnection as well as Earth-sized indents. Both the interplanetary magnetic field (IMF) and solar wind are constant in our simulation, and hence, all dynamics are generated by foreshock instabilities. The IMF in the simulation is mostly Sun-Earth aligned with a weak northward and zero dawn-dusk component, such that subsolar magnetopause reconnection is not expected without foreshock turbulence modifying the magnetosheath fields. We show a reconnection example to illustrate that the turbulence can create large magnetic shear angles across the magnetopause to induce local bursty reconnection. Magnetopause reconnection and indents developed from the impact of foreshock turbulence can potentially contribute to dayside loss of planetary plasmas. Based on global hybrid simulation results, we predict that foreshock turbulence can reach the magnetopause and lead to reconnection as well as Earth-sized indents. Both the interplanetary magnetic field (IMF) and solar wind are constant in our simulation, and hence, all dynamics are generated by foreshock instabilities. The IMF in the simulation is mostly Sun-Earth aligned with a weak northward and zero dawn-dusk component, such that subsolar magnetopause reconnection is not expected without foreshock turbulence modifying the magnetosheath fields. We show a reconnection example to illustrate that the turbulence can create large magnetic shear angles across the magnetopause to induce local bursty reconnection. Magnetopause reconnection and indents developed from the impact of foreshock turbulence can potentially contribute to dayside loss of planetary plasmas. [ABSTRACT FROM AUTHOR]

The magnetospheric cusp plays an essential role in mass and energy transportation from the solar wind to the geo-space environment. Recent development of the X-ray imaging technique makes it possible to study the large scale properties of the cusp region from a global view. As the detected X-ray signals will be integrated along the line-of-sight (LOS), it is a challenging task to derive the information about the 3D cusp region from a 2D X-ray image. Based on global magnetohydrodynamic (MHD) code, this paper simulated and analyzed the X-ray image of cusp. An approach was developed to present the following aspects related to the morphology of cusp from the X-ray image: (1) The tangent direction of the cusp boundary is the direction with appreciable increase of local standard deviation in X-ray intensity, and (2) the direction running through the cusp center is the direction with peak X-ray intensity. Lack of knowledge on radial distribution of X-ray signal along the line-of-sight, exact position of cusp cannot be further derived without assumptions. Nevertheless, under special solar wind conditions such as zero Interplanetary Magnetic Field (IMF) By, it is reasonable to assume that the cusp center is in the noonmidnight meridian plane. Therefore, position of the cusp center can be reconstructed. Based on MHD simulations, the above conclusions have been validated with different viewing geometries and solar wind conditions. With reasonable assumptions, the large-scale cusp features in response to the solar wind variations can be clearly revealed by analyzing X-ray images. [ABSTRACT FROM AUTHOR]

In this study, we investigate ionospheric responses, including currents and aurorae, to solar wind dynamic pressure (SW Pdyn) sudden increases, which are critical for understanding solar windmagnetosphere-ionosphere coupling. We focus on two similar SW Pdyn pulse events that occurred on January 24, 2012 and November 12, 2010. In both cases, equivalent ionospheric currents (EIC) vortices were generated within about 10 min after the pressure pulse arrival, with a counter-clockwise rotating vortex (viewed from above) observed on the dusk side in the former case, and a clockwise vortex observed on the dawn side in the latter. Simultaneous ground-based All-Sky Imager (ASI) observations in the vicinity of the observed EIC vortex in each case showed that aurorae intensified on the dusk side and diminished on the dawn side. These observations provide direct evidence of the scenario proposed by Shi et al. (2014) that magnetospheric flow vortices generated by a solar wind pressure pulse carry fieldaligned currents into the ionosphere and thereby modulate auroral activity. The dawn/dusk asymmetry in the auroral intensification is a direct result of the opposite sense of vortex rotation on the dawn and dusk sides, which generate oppositely directed field-aligned currents into/out of the ionosphere [ABSTRACT FROM AUTHOR]

Direction of the upstream interplanetary magnetic field (IMF) significantly changes the magnetospheric configuration, influencing the atmospheric escape mechanism. This paper investigates effects of IMF on the ion escape mechanism from a Mars-like planet that has a weak dipole magnetic field directing northward on the equatorial surface. The northward (parallel to the dipole at subsolar), southward (antiparallel), and Parker-spiral IMFs under present solar wind conditions are compared based on multispecies magnetohydrodynamics simulations. In the northward IMF case, molecular ions escape from the high-latitude lobe reconnection region, where ionospheric ions are transported upward along open field lines. Atomic oxygen ions originating either in the ionosphere or oxygen corona escape through a broader ring-shaped region. In the southward IMF case, the escape flux of heavy ions increases significantly and has peaks around the equatorial dawn and dusk flanks. The draped IMF can penetrate into the subsolar ionosphere by erosion, and the IMF becomes mass-loaded as it is transported through the dayside ionosphere. The mass-loaded draped IMF is carried to the tail, contributing to ion escape. The escape channels in the northward and southward IMF cases are different from those in the Parker-spiral IMF case. The escape rate is the lowest in the northward IMF case and comparable in the Parker-spiral and southward IMF cases. In the northward IMF case, a weak intrinsic dipole forms a magnetosphere configuration similar to that of Earth, quenching the escape rate, while the Parker-spiral and southward IMFs cause reconnection and erosion, promoting ion escape from the upper atmosphere. [ABSTRACT FROM AUTHOR]

The present‐day Venusian atmosphere is dry, yet, in its earlier history a significant amount of water evidently existed. One important water loss process comes from the energy and momentum transfer from the solar wind to the atmospheric particles. Here, we used measurements from the Ion Mass Analyzer onboard Venus Express to derive a relation between the power in the upstream solar wind and the power leaving the atmosphere through oxygen ion escape in the Venusian magnetotail. We find that on average 0.01% of the available power is transferred, and that the percentage decreases as the available energy increases. For Mars the trend is similar, but the efficiency is higher. At Earth, the ion escape does not behave similarly, as the ion escape only increases after a threshold in the available energy is reached. These results indicate that the Venusian induced magnetosphere efficiently screens the atmosphere from the solar wind. Plain Language Summary: Today, there is barely any water on Venus, but presumably a large amount existed in its earlier history. Therefore, the water must have been lost over the course of the Venusian history. An important process for removal of water is escape to space, induced by the interaction between the Venusian atmosphere and the solar wind (a fast stream of particles ejected from the Sun). In this study, we investigated the connection between the energy available in the upstream solar wind and the energy leaving Venus in the form of oxygen ion escape. By characterizing this relation, we investigate how the Venusian atmosphere reacts to changes in the upstream solar wind and how well it protects itself from atmospheric loss caused by the solar wind. We find that the energy transfer decreases as the available upstream energy increases, a trend that is very similar to that found at Mars. However, the Venusian atmosphere seems to absorb less energy from the solar wind than Mars. This indicates that the Venusian induced magnetosphere efficiently screens the atmosphere from the solar wind. This is important for the understanding of the effect of the solar wind on the Venusian atmospheric evolution. Key Points: The total escaping power from Venus does not increase linearly with increasing available power in the solar windThe coupling coefficient, that is, ratio between power out and in, decreases with increasing solar wind energy fluxThe trend of the coupling coefficient with the upstream parameters for Venus is similar to that of Mars but different from that of Earth [ABSTRACT FROM AUTHOR]