Search

Heavy cold ions at Mars are gravitationally bound to the planet unless some process provides energy to them. Observations show that cold (<20 eV) and dense (∼>1 cm−3) O+/O2+ ions with bulk velocities equal to energies ∼1 keV can reach deep into the nightside Martian magnetosheath. These ions are co‐located with a change of the sign of the sunward component of the magnetic field. This magnetic field topology implies the persistence of a localized planetary ions escape channel associated with draped magnetic field lines that are convecting tailward. The observed ion populations propagate approximately in the same direction as surrounding magnetosheath flow and are likely to be almost unheated ionospheric ions from low altitudes. The paper discusses planetary ion energization via Hall electric field originated from ions and electron separation associated with magnetic field curvature. Plain Language Summary: In‐situ observations above the nightside of Mars show the presence of localized dense planetary ion fluxes at altitudes exceeding 2,000 km and escaping from the planet at high energies, comparable to that of the solar wind. These fluxes are accompanied by the reversal of sunward component of magnetic field. Unlike most atmospheric escape channels, the reported phenomenon is characterized by an increase in heavy to light ions density ratio with the distance from the planet at the observed altitudes up to nearly 5,000 km, as well as an increase in overall plasma number density inside this escape channel relative to the ambient sheath environment. This behavior is consistent with acceleration process initiated by a bent magnetic flux tube. Key Points: Ions of different species gain similar energies in the Martian magnetosheath by Hall electric fields associated with magnetic curvatureA high concentration of ionospheric ions correlates with a near void of shocked solar wind protons and a magnetic field reversalNumber density at the reversal increases with distance from Mars in comparison to the surrounding sheath at least till two Martian radii [ABSTRACT FROM AUTHOR]

The time delay from an interplanetary driver arriving at the magnetopause to the response in the ionosphere has never been quantified separately for different types of storm drivers. This study investigates the delay for storms driven by high‐speed streams and associated stream interaction regions (HSS/SIR), or by interplanetary coronal mass ejection sheaths and magnetic clouds (MC). The total field‐aligned current (FAC) and SME index lag the Newell coupling function (NCF) by 40 ± 10 min during storms driven by HSS/SIR and sheaths, and by 60 ± 10 min for MCs. The correlation coefficient between FAC and NCF reaches maximum value as NCF is averaged over the preceding 80 min for sheath, 90 min for HSS/SIR, and 140 min for MC storms. Plain Language Summary: The Sun causes perturbations in the solar wind, which may drive geomagnetic storms associated with strong field‐aligned currents to the ionosphere. The solar wind drivers studied in this paper are high‐speed stream/stream interaction regions (HSS/SIR), and sheath and magnetic cloud (MC) interplanetary coronal mass ejections. The exact time from the arrival of the solar wind interplanetary driver at the magnetopause to the response in the ionospheric and field‐aligned currents (FAC) have not been quantified for different types of solar wind drivers. We study this time delay during geomagnetic storms and find that it is typically 40 min for HSS/SIR‐ and sheath‐driven storms, and 60 min for MC‐driven storms. Additionally, the total FAC best correlate with the solar wind averaged over the preceding 80 min for sheath, 90 min for HSS/SIR, and 140 min for MC‐driven storms. These results may help improve the accuracy of forecasting solar wind disturbances on the high‐latitude ionosphere. Key Points: Correlation between Newell coupling function (NCF) and total field‐aligned current (FAC) is studied for different storm driversBest correlation for sheath, high‐speed stream, and magnetic cloud storms is found by integrating NCF over 80, 90, and 140 min, respectivelySheath‐driven storms are associated with the highest values of total FAC and NCF [ABSTRACT FROM AUTHOR]

This paper reports on the standing whistler waves upstream of Mercury's quasi‐perpendicular bow shock. Using MESSENGER's magnetometer data, 36 wave events were identified during interplanetary coronal mass ejections (ICMEs). These elliptic or circular polarized waves were characterized by: (a) a constant phase with respect to the shock, (b) propagation along the normal direction to the shock surface, and (c) rapid damping over a few wave periods. We inferred the speed of Mercury's bow shock as ∼26 km/s and a shock width of 1.87 ion inertial length. These events were observed in 20% of the MESSENGER orbits during ICMEs. We conclude that standing whistler wave generations at Mercury are generic to ICME impacts and the low Alfvén Mach number (MA) collisionless shock, and are not affected by the absolute dimensions of the bow shock. Our results further support the theory that these waves are generated by the current in the shock. Plain Language Summary: The strength of planetary bow shocks varies with the planet's heliocentric distance from the Sun. Studying the bow shocks of other planets is important for extending our understanding of collisionless‐shock physics. In the solar system, the bow shocks of Mercury are unique as they are produced by low Mach numbers and low plasma beta solar wind blowing over a small magnetized body that is 1–2 orders smaller than Earth. The standing whistler waves upstream of the bow shock of Mercury were determined through statistical analyses. Similar to the observations at Earth, these waves were rapidly damping with a proportion of the wave periods; however, the damping distance at the spacecraft frame was considerably shorter at only a few kilometers upstream in the small‐scale bow shock of Mercury. The high occurrence rate of standing whistler waves suggests that Mercury's bow shock is a natural plasma laboratory, which can be used to further investigate low MA planetary shocks during the upcoming BepiColombo mission. Key Points: First survey of standing whistler waves upstream of Mercury's bow shockStanding whistler waves are common at Mercury during interplanetary coronal mass ejectionsOur results support the theory that current in shock generates standing whistler waves [ABSTRACT FROM AUTHOR]

The typical subsolar stand‐off distance of Mars' bow shock is of the order of a solar wind ion convective gyroradius, making it highly non‐planar to incident ions. Using spacecraft observations and a test particle model, we illustrate the impact of the bow shock curvature on transient structures which form near the upstream edge of moving foreshocks caused by slow rotations in the interplanetary magnetic field (IMF). The structures exhibit noticeable decrease in the solar wind plasma density and the IMF strength within their core, are accompanied by a compressional shock layer, and are consistent with foreshock bubbles (FBs). Ion populations responsible for these structures include backstreaming ions that only appear within the moving foreshock and reflected ions with hybrid trajectories that straddle between the quasi‐perpendicular and quasi‐parallel bow shocks during slow IMF rotations. Both ion populations accumulate near the upstream edge of the moving foreshock which facilitates FB formation. Plain Language Summary: Planets in the solar system are continuously impacted by the solar wind, a plasma flow originating at the Sun and propagating through the interplanetary medium at high speeds. The solar wind also carries a magnetic field which at times contains twists or discontinuities. The discontinuities are associated with large scale electric currents that can have planar shapes. A planetary obstacle significantly modulate the solar wind plasma and the interaction of solar wind discontinuities with the modulated plasma upstream of the planet leads to formation of transient structures. Due to their relatively large size, these structures can significantly impact and destabilize plasma boundaries at lower altitudes closer to the surface. The results of this paper improve our understanding of solar wind interactions and formation of transient structures upstream of Mars. Key Points: Foreshock bubbles can form upstream of MarsSlow field rotations can cause foreshock bubbles while reflected ions from the quasi‐perpendicular bow shock contribute to their formationUnique ion kinetic scale processes exist around foreshock structures at Mars due to the different interaction size scale [ABSTRACT FROM AUTHOR]

We examine coupling of fluctuations in the solar wind with electromagnetic ion cyclotron (EMIC) waves in the magnetosphere using an advanced full‐wave simulation code, Petra‐M. Dipole tilt dramatically affects the coupling process. While very little wave power can reach the inner magnetosphere without tilt effects, a tilted dipole field dramatically increases the efficiency of the coupling process. Solar wind fluctuations incident at high magnetic latitude effectively reaches the ground along the field line and mode‐convert to linearly polarized field‐aligned propagating waves at the Alfvén and IIH resonances. Therefore, solar wind compressions efficiently drive linearly polarized EMIC waves when the dipole angle is tilted toward or away from the Sun‐Earth direction. Plain Language Summary: The solar wind is emitted near the ecliptic planes, and solar wind pressure is one of the critical sources of wave activities in the Earth's magnetosphere. Since Earth's magnetic field is tilted to the ecliptic plane, compressed solar wind fluctuations can be incident over a wide range of magnetic latitudes depending on seasonal and diurnal variations. When the solar wind fluctuations reach Earth's magnetosphere, incoming wave energy can be linearly transferred to another wave mode, called a mode coupling. This paper examines the role of Earth's magnetic tilt on this mode coupling between solar wind fluctuation and electromagnetic ion cyclotron (EMIC) waves using an advanced full‐wave simulation code, Petra‐M. Without magnetic tilt, solar wind fluctuations incident at a low latitude are almost totally reflected. In contrast, solar wind fluctuations incident at high latitude can propagate efficiently into the inner magnetosphere and reach the ground. Compressional fluctuations can also be converted to linearly polarized, field‐aligned propagating waves. These results suggest that solar wind compressions can drive the linearly polarized EMIC waves and that the wave occurrence can have seasonal and diurnal dependence, which is expected to maximize at the maximum tilt of Earth's dipole into the Sun‐Earth direction. Key Points: Wave mode coupling of solar wind fluctuation to electromagnetic ion cyclotron (EMIC) waves is examined using an advanced 2D‐full wave simulation code, Petra‐MWhen the dipole magnetic field is tilted, compressional waves can reach the ground near the cusp region via mode conversionThe solar wind can drive linearly polarized EMIC waves via mode conversion more efficiently when the dipole field is tilted [ABSTRACT FROM AUTHOR]

The existence of shocklets, a kind of solitary structure, has been demonstrated in the Earth's foreshock regions for decades. Their formation and evolution are believed to be controlled by ions. However, the detailed behavior of ions at them has not been well investigated observationally yet. Here, we investigate a shocklet observed by the Magnetospheric Multiscale mission in the Earth's foreshock. Analysis of ion observations reveals that the solar wind (SW) ions are bunched in gyrophase space when interacting with the whistler precursor of the shocklet, suggesting the occurrence of cyclotron resonance between them. A more detailed examination suggests that the cyclotron resonance induces a net energy flow from the whistler precursor to the SW ions. Thus, the observations presented here indicate that the cyclotron resonance between shocklet whistler precursors and the SW ions could provide a mechanism for shocklet dissipation and SW ion energization. Plain Language Summary: Shocklets, a kind of solitary structure, are thought to play a crucial role in the reformation processes of quasi‐parallel shocks. They are also suggested to be able to efficiently decelerate and heat the solar wind ions and electrons. Their formation and evolution are believed to be controlled by ion dynamics. However, the detailed behavior of ions at them has not been well investigated yet. In this paper, we study in detail a shocklet event observed by the Magnetospheric Multiscale mission in the Earth's foreshock. We especially focus on ion observations. Analysis of the data reveals that the solar wind ions are bunched in gyrophase space when interacting with the shocklet whistler precursor, indicating the occurrence of cyclotron resonance between them. Resulting from this interaction, the solar wind ions exchange energy with the whistler precursor. Viewed in the rest frame of the plasma, this energy flow is directed from the whistler precursor to the solar wind ions. Thus, the observations presented here indicate the cyclotron resonance between shocklet whistler precursors and the solar wind ions could provide a mechanism for shocklet dissipation and solar wind ion energization. Key Points: A shocklet observed by Magnetospheric Multiscale is examined in great detail to reveal the behavior of ions at shockletsGyrophase bunching indicative of cyclotron resonance is observed in the velocity distributions of the solar wind ionsCyclotron resonance could provide a mechanism for shocklet dissipation and solar wind ion energization [ABSTRACT FROM AUTHOR]

A new radio component namely Saturn Anomalous Myriametric Radiation (SAM) is reported. A total of 193 SAM events have been identified by using all the Cassini Saturn orbital data. SAM emissions are L‐O mode radio emission and occasionally accompanied by a first harmonic in R‐X mode. SAM's intensities decrease with increasing distance from Saturn, suggesting a source near Saturn. SAM has a typical central frequency near 13 kHz, a bandwidth greater than 8 kHz and usually drifts in frequency over time. SAM's duration can extend to near 11 hr and even longer. These features distinguish SAM from the regular narrowband emissions observed in the nearby frequency range, hence the name anomalous. The high occurrence rate of SAM after low frequency extensions of Saturn Kilometric Radiation and the SAM cases observed during compressions of Saturn's magnetosphere suggest a special connection to solar wind dynamics and magnetospheric conditions at Saturn. Plain Language Summary: This paper reports a new type of radio emission at Saturn. We name this emission "Saturn Anomalous Myriametric Radiation" (SAM). Due to the different morphological behavior of SAM emissions in the spectrogram, compared to the regularly observed narrowband emissions, SAM emissions are identified as a new radio component. We search for SAM events by using all the available data acquired by the Cassini spacecraft. A total of 193 SAM events are found and we summarized its characteristics. The spatial distribution of SAM shows a high latitude preference in occurrence. The observed intensities of SAM increase as the spacecraft approaches Saturn, suggesting a source near Saturn. The typical central frequency of SAM is near 13 kHz and the duration of SAM can extend to near 11 hr and even longer. The majority of the 193 SAM emissions are observed after the enhancement of the dominant Saturn radio emission Saturn Kilometric Radiation (SKR), which is deemed to be closely connected to the solar wind dynamics at Saturn. Therefore, SAM emissions are also possibly related to the conditions that cause the enhancement of SKR. This newly identified SAM emission may be used to study the magnetospheric dynamics of Saturn in the future. Key Points: A new radio component namely the Saturn Anomalous Myriametric Radiation (SAM) is reportedThe characteristics of SAM are given including its spatial distribution, typical frequency, bandwidth and time durationSAM shows a possible connection to the compression of the magnetosphere [ABSTRACT FROM AUTHOR]

Solar wind protons can interact directly with the hydrogen corona of Mars through charge exchange, resulting in energetic neutral atoms (ENAs) able to penetrate deep into the upper atmosphere of Mars. ENAs can undergo multiple charge changing interactions, leading to an observable beam of penetrating protons in the upper atmosphere. We seek to characterize the behavior of these protons in the presence of magnetic fields using data collected by the Mars Atmosphere and Volatile EvolutioN spacecraft. We find that backscattered penetrating proton flux is enhanced in regions where the magnetic field strength is greater than 200 nT. We also find a strong correlation at CO2 column densities less than 5.5 × 1014 cm−2 between magnetic field strength and the observed backscattered and downward flux. We do not see significant changes in penetrating proton flux with magnetic field strengths on the order of 10 nT. Plain Language Summary: Positively charged particles in the solar wind can directly interact with hydrogen in the Martian environment. These positively charged particles can charge exchange with hydrogen atoms, becoming fast, neutral atoms that are able to navigate their way deep into the upper atmosphere of Mars. These fast neutrals can undergo further charge‐changing reactions and become positively charged once more. This paper seeks to understand how these positive particles, or penetrating protons, behave in the presence of magnetic fields using data collected by the Mars Atmosphere and Volatile EvolutioN spacecraft. We find that strong magnetic fields enhance the number of backscattered penetrating protons we observe. At low atmospheric column densities, strong magnetic fields significantly deflect these protons back to higher altitudes. At high atmospheric column densities, magnetic fields do not appear to affect these penetrating protons to the same degree as collisions with other neutral molecules. We also find that small magnetic field strengths do not affect the penetrating proton flux we observe. Key Points: Backscattered penetrating proton flux is enhanced in magnetic field regions with strengths greater than 200 nTPenetrating protons are influenced by strong magnetic fields at column densities less than 5.5 × 1014 cm−2Penetrating proton fluxes are not greatly affected by magnetic field strengths on the order of 10 nT [ABSTRACT FROM AUTHOR]

On 6–8 June 2015, the Ion and Electron Sensor on board Rosetta observed keV‐range water‐group pickup ions arriving from the solar direction. Based on magnetic field intensification and variations, the appearance of the ions was likely to have been caused by a coronal mass ejection. During the 3‐day period when Rosetta was 200 km from the comet, peak ion energy/charge (E/q) varied over a range from 50 eV to 1 keV in concert with neutral gas density variations caused by the rotation of the comet and its variable solar illumination. Thermal ion densities showed the same variations. The neutral density variations provided a unique opportunity to observe the repeated slowing of the solar wind by mass loading caused by charge exchange between energetic water‐group ions and thermal water‐group molecules. Such solar wind slowing was observed previously only by flyby missions that provided single events. Plain Language Summary: The Rosetta orbiter carried a number of instruments to measure the properties of the neutral and ionized gas surrounding the nucleus. Included in these were plasma instruments to measure the characteristics of the charged particles. The Ion and Electron Sensor was one of them. Also on board were the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, the Ion Composition Analyzer, and the Langmuir Probe. This paper discusses some of the results of measurements by these instruments and their relation to each other. It was found that the neutral gas emitted by the comet nucleus and the resulting positively charged ions interact in such a way to produce slowing down of the solar wind as a result of what is called "charge exchange", in which an electron is transferred from a neutral molecule to an ion. Key Points: Charge exchange of energetic water group pickup ions with coma water molecules is observed within the solar wind cavity of Comet 67P Churyumov‐GerasimenkoRotation of the comet produced variations in the neutral density at the spacecraft causing strong charge‐exchange deceleration of the incoming ions within the density peaksRecovery to the initial water‐group ion beam as minimum neutral densities, again appeared, was shown to result from pickup of thermal ions in the coma [ABSTRACT FROM AUTHOR]

The geomagnetic activity index Kp is widely used but is restricted by low time resolution (3‐hourly) and an upper limit. To address this, new geomagnetic activity indices, Hpo, are introduced. Similar to Kp, Hpo expresses the level of planetary geomagnetic activity in units of thirds (0o, 0+, 1−, 1o, 1+, 2−, ...) based on the magnitude of geomagnetic disturbances observed at subauroral observatories. Hpo has a higher time resolution than Kp. 30‐min (Hp30) and 60‐min (Hp60) indices are produced. The frequency distribution of Hpo is designed to be similar to that of Kp so that Hpo may be used as a higher time‐resolution alternative to Kp. Unlike Kp, which is capped at 9o, Hpo is an open‐ended index and thus can characterize severe geomagnetic storms more accurately. Hp30, Hp60 and corresponding linearly scaled ap30 and ap60 are available, in near real time, at the GFZ website (https://www.gfz-potsdam.de/en/hpo-index). Plain Language Summary: The geomagnetic activity index Kp is a measure of planetary geomagnetic activity, expressed in units of thirds (0o, 0+, 1−, 1o, 1+, 2−, ...9o). Kp is widely used in the space physics community, as it is known to be a good proxy of the solar‐wind energy input into the magnetosphere‐ionosphere‐thermosphere system. Kp has two important limitations. One is the temporal resolution. Kp is a three‐hourly index, so that temporal features within 3 hr are not resolved. The other is the upper limit of the index. Kp does not exceed a maximum value of 9o, so that under extremely disturbed conditions, geomagnetic activity is not accurately represented. We introduce a group of new geomagnetic activity indices Hpo that overcomes these limitations. Hpo is designed to represent planetary geomagnetic activity in a similar way as Kp but with higher temporal resolution and without the upper limit at 9o. This paper describes the production of 30‐min (Hp30) and 60‐min (Hp60) indices, and demonstrates their properties in comparison with Kp. Hpo indices since 1995, including near‐real‐time values, are distributed through the GFZ website (https://www.gfz-potsdam.de/en/hpo-index). Key Points: New Kp‐like planetary geomagnetic activity indices, Hpo, are presentedHourly (Hp60) and half‐hourly (Hp30) indices are available from GFZ websiteHpo indices are open‐ended without the upper limit at 9o [ABSTRACT FROM AUTHOR]

We report observations of reconnection exhausts in the Heliospheric Current Sheet (HCS) during Parker Solar Probe Encounters 08 and 07, at 16 Rs and 20 Rs, respectively. Heliospheric current sheet (HCS) reconnection accelerated protons to almost twice the solar wind speed and increased the proton core energy by a factor of ∼3, due to the Alfvén speed being comparable to the solar wind flow speed at these near‐Sun distances. Furthermore, protons were energized to super‐thermal energies. During E08, energized protons were found to have leaked out of the exhaust along separatrix field lines, appearing as field‐aligned energetic proton beams in a broad region outside the HCS. Concurrent dropouts of strahl electrons, indicating disconnection from the Sun, provide further evidence for the HCS being the source of the beams. Around the HCS in E07, there were also proton beams but without electron strahl dropouts, indicating that their origin was not the local HCS reconnection exhaust. Plain Language Summary: Magnetic reconnection in current sheets is a universal plasma process that converts magnetic energy into particle energy. The process is important in many laboratory, solar, and astrophysical plasmas. The heliospheric current sheet (HCS), which originates from the Sun and extends throughout the heliosphere, is the largest current sheet in the solar system. One of the surprises of the Parker Solar Probe mission is the finding that magnetic reconnection is almost always active in the near‐Sun HCS, despite its enormous scales. In this paper, we report direct evidence showing that reconnection in the HCS close to the Sun can be a source of energetic protons observed in the solar wind. The reason protons can be accelerated to high energies (to tens of kilo‐electronvolts) is because the available magnetic energy per particle is high close to the Sun. This finding is important because the source of energetic protons in the heliosphere is unknown. Key Points: Large available magnetic energy per particle led to significant proton acceleration by reconnection in the near‐Sun heliospheric current sheet (HCS) at 16 and 20 RsProton beams and strahl electron dropouts in separatrices are evidence for HCS being a source of energetic protons seen outside the HCSEnergetic protons beams outside the HCS also exist without strahl electron dropouts. Their origin is unlikely to be the local HCS exhaust [ABSTRACT FROM AUTHOR]

The most important parameter driving the solar wind‐magnetosphere interaction is the southward (Bz) component of the interplanetary magnetic field (IMF). While the dawn‐dusk (By) component of the IMF is also known to play an important role, its effects are usually assumed to be independent of its sign. Here we demonstrate for the first time a seasonally varying, explicit IMF By‐dependence of the ring current and Dst index. Using satellite observations and a global magnetohydrodynamic model coupled with a ring current model, we show that for a fixed level of solar wind driving the flux of energetic magnetospheric protons and the growth‐rate of the ring current are greater for By < 0 (By > 0) than for By > 0 (By < 0) in Northern Hemisphere summer (winter). While the physical mechanism of this explicit By‐effect is not yet fully understood, our results suggest that IMF By modulates magnetospheric convection and plasma transport in the inner magnetosphere. Plain Language Summary: The magnetic field carried by solar wind is the most important driver of space weather. Aurora and other phenomena in the near‐Earth space are known to be stronger when the magnetic field on solar wind points to south than north. Usually the space weather response is expected to be equally strong for eastward and westward magnetic fields in solar wind. In this paper show that this is not the case. Using satellite measurements and computer modeling we show that electric currents in the near‐Earth space are stronger for westward than eastward magnetic field in solar wind in northern hemisphere summer, while the opposite is true in northern hemisphere winter. More studies are needed for understanding the physical mechanism of this phenomenon. Key Points: We show for the first time that there is an explicit By‐dependence in the ring current/proton precipitation and in the inner magnetosphereDuring Northern Hemisphere summer (winter) the ring current fluxes/proton precipitation and the rate of change of the Dst index are stronger for By < 0 (By > 0)The By‐dependence of the ring current and energetic proton fluxes is reproduced by a global coupled MHD‐ring current model [ABSTRACT FROM AUTHOR]

This paper investigates intense energy conversion events (IECEs) at the dayside magnetopause boundary layer (MBL) using high‐resolution data from the Magnetospheric Multiscale (MMS) spacecraft. Most events were detected by MMS in a duration less than 0.5 s, corresponding to a thickness below one ion inertia length. About three‐quarters of the events occurred in local reconnecting current sheets. Interestingly, there are more electron‐only reconnections than standard reconnections. Most of the reconnection IECEs are within the exhaust of the primary reconnection and the Kelvin‐Helmholtz vortices, suggesting that secondary reconnections are important building blocks for the overall energy conversion at the magnetopause. The nonreconnection events are also primarily within the reconnection exhaust and the Kelvin‐Helmholtz vortices, highlighting the importance of reconnection and Kelvin‐Helmholtz instability in regulating the energy conversion at the MBL. Plain Language Summary: Solar wind‐magnetosphere coupling is one of the long‐standing questions in magnetosphere physics. Part of the input energy from the solar wind is converted and dissipated at the magnetopause boundary layer. Identifying where does energy conversion occurs is valuable for understanding the solar wind‐magnetosphere coupling. Here we picked up hundreds of intense energy conversion events using high precision data from Magnetospheric Multiscale at the magnetopause boundary layer to study their locations and fundamental properties. It is found that most events are associated with magnetic reconnection and are more likely located in the reconnection exhaust and Kelvin‐Helmholtz vortices. An interesting finding is that most of these intense energy conversion events are associated with reconnection in which only electrons participate, that is, without ion coupling. Key Points: Hundreds of intense energy conversion events are examined at the dayside magnetopauseThere are more electron‐only reconnections than standard reconnections in these eventsThe exhaust of the primary reconnection and Kelvin‐Helmholtz vortices are two essential regions hosting the intense energy conversion [ABSTRACT FROM AUTHOR]

Micrometeorite impacts and solar wind irradiation, the dominant space weathering (SW) processes, largely modified compositions and microtexture of soil materials on the Moon. Here, we report the SW characteristics of the Chang'e‐5 lunar soils from mid‐high latitude (43.06°N). All mineral phases exposed on the surface of a single basalt clast have a vapor deposit layer, whereas the textures of the solar wind irradiation‐damaged zone are dependent on the host mineral species. Nanophase Fe (npFe0) particles are spherical in the amorphized zone of pyroxenes, elongated in ilmenite, and irregular on the jagged surface of iron sulfide, but not found in Fe‐poor merrillite. Vesicles were found in the damaged zone of ilmenite and merrillite, but with different shapes. The observations were compared to Apollo samples and demonstrate no significant altitude‐dependent effects on the SW, which is important for decoding the reflectance spectra of the Moon. Plain Language Summary: The lunar surface has been suffering intense meteorite impacts and solar wind irradiation for billions of years, which heavily modifies its physical properties, chemical compositions and mineralogical features, and in turn, the optical reflectance spectra of the Moon. The meteorite impacts are random events, but the intensity of solar wind irradiation is latitude dependent. However, all Apollo and Luna missions landed in a narrow and low range of lunar latitude. The Chang'e‐5 (CE‐5) mission returned lunar soil samples from a middle latitude (43.06°N), providing unique samples for study of lunar space weathering (SW). In this paper, we report the SW features of various minerals from a single basaltic clast of the CE‐5 sample. Our observations reveal phase‐dependent effects on the SW. Furthermore, the CE‐5 lunar soil shows no significant differences from those of Apollo samples, suggestive of little latitude‐dependent effects on lunar SW. Key Points: The space weathering (SW) characteristics of lunar soils returned by Chang'e‐5 landing at the mid‐high latitude site are reportedMicroscopic textures of SW depend on mineral species but show no relationship with the latitude of sampling siteThe SW products by micrometeorite impacts and solar wind irradiation are distinguished [ABSTRACT FROM AUTHOR]

Magnetopause diamagnetic currents arise from density and temperature driven pressure gradients across the boundary layer. While theoretically recognized, the temperature contributions to the magnetopause current system have not yet been systematically studied. To bridge this gap, we used a database of Magnetospheric Multiscale magnetopause crossings to analyze diamagnetic current densities and their contributions across the dayside and flank magnetopause. Our results indicate that the ion temperature gradient component makes up to 37% of the ion diamagnetic current density along the magnetopause and typically opposes the classical Chapman‐Ferraro current direction, interfering destructively with the density gradient component, thus lowering the total diamagnetic current density. This effect is most pronounced on the flank magnetopause. The electron diamagnetic current was found to be 5–14 times weaker than the ion diamagnetic current on average. Plain Language Summary: The solar wind represents a continuous outflow of charged particles from the Sun's upper atmosphere into the solar system. Upon reaching Earth's magnetosphere, the solar wind's dynamic pressure is balanced by the magnetic pressure of Earth's magnetic field in a boundary layer known as the magnetopause. This boundary layer represents the entry point of the solar wind's energy into Earth's magnetosphere and upper atmosphere, playing a crucial role in energy transport throughout the interconnected system. Plasma density and temperature differences across the boundary layer generate an electric current that supports the magnetopause. In this paper, we clarify the physical mechanism of the magnetopause current by using high‐resolution data from NASA's Magnetospheric Multiscale mission. We found a significant ion temperature contribution to the magnetopause current not identified in previous studies. Our results also indicated that the plasma electrons' contribution to the magnetopause current was significantly smaller than the ion contribution. Key Points: The magnetopause diamagnetic current is composed of opposing density and temperature gradient generated componentsThe temperature gradient contributes up to 37% of the ion diamagnetic current density along the magnetopauseThe temperature component typically opposes the classical Chapman‐Ferraro current direction [ABSTRACT FROM AUTHOR]

Solitary magnetic structures known as SLAMS (short large‐amplitude magnetic structures) have been considered as essential elements of collisionless shocks with quasi‐parallel geometries. Yet the physics underlying their formation remains an open question. In this paper, we use measurements from the magnetospheric multiscale mission combined with fully kinetic simulations to study the formation of SLAMS. We find that gyro‐resonance between solar wind ions and right‐hand circularly polarized electromagnetic waves results in magnetic field amplification. Gyro‐trapping by the growing magnetic field buildup the plasma density that further enhances the current and field. The solitary nature of SLAMS stems from a beat‐like magnetic field envelope where the maximum sets the initial location for nonlinear growth. Our results present a conceptual advance on SLAMS, and may shed new light on the open question of magnetic field amplification at astrophysical shocks. Plain Language Summary: We integrate MMS measurements and fully kinetic simulations to investigate the nonlinear processes underlying ultra‐low‐frequency wave growth and evolution into solitary magnetic structures known as SLAMS. The understanding gained here presents a conceptual advance on the formation of SLAMS at planetary bow shocks, and may serve as new insight into how magnetic fields in the interstellar medium are amplified to create an environment for cosmic ray production at astrophysical shocks. Key Points: Gyro‐resonance between solar wind ions and right‐hand circularly polarized electromagnetic waves results in magnetic field amplificationGyro‐trapping by the growing magnetic field buildup the plasma density that further enhances the field growthThe solitary nature of SLAMS stems from the magnetic field envelope where the maximum sets the initial locations for nonlinear growth [ABSTRACT FROM AUTHOR]

The storm‐time ring current is formed by the inward convection of the near‐earth plasma sheet, so understanding the changing source of the plasma sheet is key for understanding ring current development. The ionospheric and solar wind sources can be distinguished by the charge state of the heavy ions; solar wind ions are highly iononized, while ionospheric ions are predominantly singly ionized. AMPTE/CHEM measurements are used to track the changing composition in the 6–9 Re plasma sheet as a storm develops in order to determine the fraction of the population that comes from each source. We find that prior to the storm, the solar wind source dominates. During the storm main phase, the solar wind contribution decreases, while the ionospheric contribution increases, making the source predominantly ionospheric. The source returns to predominantly solar wind plasma during the recovery phase. Plain Language Summary: The ions trapped in the earth's magnetic field have two possible sources: the sun or the earth's ionosphere. A stream of ionized particles from the sun called the solar wind is constantly moving outward, flowing by the earth. These particles can enter the earth's magnetosphere and become trapped. Ions can also be accelerated out of the ionosphere into the magnetosphere. The interplay between these two sources is not well understood. In this paper, we test how the source changes as a large geomagnetic storm develops. We find that the source switches from being dominantly solar wind, to dominantly ionospheric during the peak of the storm. Key Points: Ionospheric and solar wind sources to the near‐earth plasma sheet are tracked during one storm using the AMPTE/CHEM instrumentThere is a sharp change from a predominantly solar wind to ionospheric source during both intensifications in the storm's main phaseThe source changes back to predominantly solar wind during the recovery phase [ABSTRACT FROM AUTHOR]

Flux transfer events (FTEs), as flux ropes (FRs), are considered key agents for solar wind energy to enter the terrestrial magnetosphere. Recent observations identify entangled flux tubes that collide and pull against each other. Reconnection occurs to disentangle and produce a new pair of FRs with different connectivity. In this paper, we examine the entanglement interface and how such an entanglement process evolves in time by comparing 17 entanglements observed by the Magnetospheric Multiscale (MMS) mission. The By‐dominated interplanetary magnetic field (IMF) distribution of the 17 events agrees with previous findings. We have identified three evolutionary stages characterized by the magnetic field and pressure enhancement. Our study confirms the FR nature of these events and explains how a disparate pair of ropes is formed from two entangled flux tubes, each initially connected to a different hemisphere of the magnetosphere. Plain Language Summary: The Earth's intrinsic magnetic field deflects the solar wind flow at a boundary called the magnetopause. Near this boundary, twisted flux tubes are found. Such tubes may become entangled if they move toward each other, say, from the Southern and Northern Hemispheres. This study analyzes 17 such events and identifies three evolutionary stages as the entanglement proceeds. The results improve our understanding of not only the complex coupling between solar wind and the Earth's magnetic field but other similar processes in space plasma. Key Points: IMF clock angles associated with 17 flux tube entanglement events present peaks near horizontal directionThe entanglement process evolves in three distinguishable stagesMagnetic tension force contributes to the pressure balance at the entanglement interface [ABSTRACT FROM AUTHOR]

Electron inflow and outflow velocities during magnetic reconnection at and near the dayside magnetopause are measured using satellites from NASA's Magnetospheric Multiscale (MMS) mission. A case study is examined in detail, and three other events with similar behavior are shown, with one of them being a recently published electron‐only reconnection event in the magnetosheath. The measured inflow speeds of 200–400 km/s imply dimensionless reconnection rates of 0.05–0.25 when normalized to the relevant electron Alfvén speed, which are within the range of expectations. The outflow speeds are about 1.5–3 times the inflow speeds, which is consistent with theoretical predictions of the aspect ratio of the inner electron diffusion region. A reconnection rate of 0.04 ± 25% was obtained for the case study event using the reconnection electric field as compared to the 0.12 ± 20% rate determined from the inflow velocity. Key Points: Electron inflow velocities are determined for reconnection at the magnetopause and in the magnetosheathFor four events inflow velocities of 200–400 km/s imply normalized reconnection rates of 0.05–0.25Reconnection rates using electron inflow velocities (0.12) and the reconnection electric field (0.04) are compared for one event Plain Language Summary: When the solar wind impacts the Earth's magnetosphere, an explosive energy conversion process called magnetic reconnection opens the door for solar wind energy to enter the magnetosphere by interconnection of the magnetic fields of the solar wind and of Earth. In this process, magnetic energy is converted to charged‐particle energy. Magnetic reconnection is fairly well understood at large scales and even down to the ion scale. However, the breaking and linking of field lines and the acceleration of electrons occur at much smaller scales, which are only recently being accessed by the NASA Magnetospheric Multiscale mission. This paper analyzes the speed at which electrons flow into and out of reconnection sites. The inflow speeds are crucial because they provide a measurement of the rate at which reconnection proceeds. [ABSTRACT FROM AUTHOR]

Flux transfer events are transient magnetized plasma structures that are self‐balancing, rope‐like phenomena that can appear at the Earth's magnetopause when the interplanetary magnetic field is southward. The formation of this type of flux rope has been unclear. Using measurements of particles and magnetic fields on the MMS spacecraft, we find that rope‐like structures containing magnetospheric energetic electrons have exactly the same occurrence rate as those without magnetospheric electrons, independent of solar wind properties and location of observation. This equality is consistent with a pair of flux bundles being generated at the same time but displaced from each other as an outcome of their collision and resultant entanglement and reconnection. This entanglement reconnection produces one flux rope with two ends connected to the magnetosphere and the other connected on both ends to the shocked solar wind. Away from the subsolar entanglement zone, reconnection transports magnetic flux from the "closed" magnetosphere to the "open" tail lobes, as predicted by Dungey (1961, https://doi.org/10.1103/PhysRevLett.6.47). Plain Language Summary: The Earth's intrinsic magnetic field deflects the solar wind flow at a boundary called the magnetopause. Near this boundary, twisted flux tubes are found when the external field in the solar wind is directed nearly oppositely to that of the Earth. These tubes are found to be of two types: devoid of and containing energetic electrons. Under all conditions, there are equal numbers of these two types of magnetic structures. This paper suggests a possible mechanism of how these pairs of structures can be generated. It does not change our understanding of how newly opened magnetic flux is transferred by the solar wind from the Earth's magnetosphere to its magnetotail but does explain why flux ropes are found across the dayside magnetopause. Key Points: An inventory of twisted self‐balancing flux ropes at the Earth's magnetopause indicates they are produced in pairsThe extent of the magnetopause simultaneously undergoing reconnection leads to entangled magnetic flux pulling in opposite directionsWhere flux tubes pull against each other becomes a site of reconnection, producing one flux tube doubly connected, and one disconnected [ABSTRACT FROM AUTHOR]

We report an earthward moving ion‐scale flux rope embedded within the trailing edge of a hot flow anomaly (HFA) observed by the Magnetospheric Multiscale satellite constellation on 17 December 2016 upstream of Earth's quasi‐parallel bow shock. The driver of the HFA, a tangential discontinuity, was observed by the Wind spacecraft without flux rope signatures around it in the solar wind. This suggests that the earthward moving flux rope was generated inside the HFA. This ion‐scale flux rope is not a force free structure and expands due to a strong magnetic pressure gradient force. Solar wind ions are decelerated inside the flux rope by the static electric field likely caused by the charge separation of solar wind particles. Our observations imply that magnetic reconnection may have occurred inside the HFA. Reconnection and flux ropes may play a role in particle acceleration/heating inside foreshock transients. Plain Language Summary: Energetic particles are often observed inside the foreshock transients or in the foreshock region. The acceleration mechanisms of these energetic particles remain an open question. Possible candidates responsible for the acceleration have been put forward, such as Fermi acceleration, electron firehose, and lower hybrid drift instabilities and magnetic reconnection. However, to date, magnetic reconnection is only found in hybrid simulations during the generation of foreshock transients, but never reported by in situ observations. In this paper, we report an ion‐scale flux rope observed at the trailing edge of a hot flow anomaly, which could be generated during the magnetic reconnection. Our observations indicate that reconnection could occur locally within foreshock transients and contribute to their particle acceleration. Key Points: An ion‐scale flux rope with 6.1–7.5 ion inertial length is observed at the trailing edge of a hot flow anomalySolar wind ions were decelerated inside the flux rope, and the kinetic energy of solar wind ions was likely converted to the magnetic energyThe flux rope is close to a one‐dimensional structure and expands due to the strong magnetic pressure gradient force [ABSTRACT FROM AUTHOR]

The most important driver of geomagnetic activity is the north–south (Bz) component of the interplanetary magnetic field (IMF), which dominates the solar wind‐magnetosphere coupling and all solar wind coupling functions. While the east–west (By) IMF component is also included in most coupling functions, its effect is always assumed to be symmetric with respect of its sign. However, recent studies have shown that, for a fixed value of any coupling function, geomagnetic activity is stronger for By>0 than for By<0 in Northern Hemisphere winter. In Southern Hemisphere winter, the dependence on the By sign is reversed. In this paper, we use measurements of National Oceanic and Atmospheric Administration Polar‐Orbiting Operational Environmental Satellites to show that the flux of magnetospheric electrons precipitating into the atmosphere also exhibits an explicit By dependence. This By dependence is strong in the midnight and dawn sectors where it causes a related By effect in the westward electrojet and geomagnetic activity by modulating ionospheric conductivity. Key Points: Fluxes of energetic electrons precipitating into atmosphere are strongly dependent on the sign and amplitude of IMF ByThis IMF By effect on electron fluxes is strong around midnight and at dawn but weak at duskThis IMF By effect is expected to modulate ionospheric conductivity and to affect a similar effect in geomagnetic activity [ABSTRACT FROM AUTHOR]

The most important driver of geomagnetic activity is the north–south (Bz) component of the interplanetary magnetic field (IMF), which dominates the solar wind‐magnetosphere coupling and all solar wind coupling functions. While the east–west (By) IMF component is also included in most coupling functions, its effect is always assumed to be symmetric with respect of its sign. However, recent studies have shown that, for a fixed value of any coupling function, geomagnetic activity is stronger for By>0 than for By<0 in Northern Hemisphere winter. In Southern Hemisphere winter, the dependence on the By sign is reversed. In this paper, we use measurements of National Oceanic and Atmospheric Administration Polar‐Orbiting Operational Environmental Satellites to show that the flux of magnetospheric electrons precipitating into the atmosphere also exhibits an explicit By dependence. This By dependence is strong in the midnight and dawn sectors where it causes a related By effect in the westward electrojet and geomagnetic activity by modulating ionospheric conductivity. Key Points: Fluxes of energetic electrons precipitating into atmosphere are strongly dependent on the sign and amplitude of IMF ByThis IMF By effect on electron fluxes is strong around midnight and at dawn but weak at duskThis IMF By effect is expected to modulate ionospheric conductivity and to affect a similar effect in geomagnetic activity [ABSTRACT FROM AUTHOR]

Fast interplanetary shocks occurrence, their parameters, and drivers near Jupiter's orbit are determined in this paper. It was found that 70% of the fast shocks are forward (FS) and 30% are reverse (RS). Interplanetary coronal mass ejection‐driven FS occur more frequently in all solar cycle phases except in the declining phase, when corotating interaction region‐driven shocks predominate. Most of the shocks were quasi‐perpendicularly (65° to 70°) propagating relative to the ambient interplanetary magnetic field. The average shock magnetosonic Mach number is slightly higher for FS (2.6) than for RS (2.4), which in turn are stronger than shocks near 1 and 10 AU reported in previous works. This occurs because of the full development of corotating interaction region shocks and higher occurrence of strong interplanetary coronal mass ejections near 5 AU and that the magnetosonic speed at 5 AU has only 60% of its value at 1 AU. Plan Language Summary: Interplanetary shock waves are sudden variations in solar wind plasma and magnetic field parameters. They propagate in the interplanetary space and have important effects on planetary magnetospheres. For instance, shocks cause expansions and compressions of Jupiter's magnetosphere and trigger its auroral emissions. In this work plasma and magnetic field data from several spacecraft that crossed interplanetary space near Jupiter's orbit space are analyzed. A list of interplanetary shocks is compiled, and their parameters and drivers are determined. It was found that shocks at 5 AU have preferentially a quasi‐perpendicular propagation to the magnetic field direction and are on average stronger than shocks observed near Earth's orbit. Key Points: Interplanetary shock occurrence, parameters, and drivers near Jupiter's orbit are determinedThere are about 70% fast forward and 30% reverse shocks; about 90% of RS are CIR driven, while 45% of FS are CIR driven and 55% are ICME drivenShocks at 5 AU are mostly quasi‐perpendicularly propagating and stronger on average than shocks at 1 AU [ABSTRACT FROM AUTHOR]

We report Magnetospheric Multiscale observations of reconnection in a thin current sheet at the interface of interlinked flux tubes carried by converging reconnection jets at Earth's magnetopause. The ion skin depth‐scale width of the interface current sheet and the non‐frozen‐in ions indicate that Magnetospheric Multiscale crossed the reconnection layer near the X‐line, through the ion diffusion region. Significant pileup of the reconnecting component of the magnetic field in this and three other events on approach to the interface current sheet was accompanied by an increase in magnetic shear and decrease in Δβ, leading to conditions favorable for reconnection at the interface current sheet. The pileup also led to enhanced available magnetic energy per particle and strong electron heating. The observations shed light on the evolution and energy release in 3‐D systems with multiple reconnection sites. Plain Language Summary: The Earth and the solar wind magnetic fields interconnect through a process called magnetic reconnection. The newly reconnected magnetic field lines are strongly bent and accelerate particles, similar to a rubber band in a slingshot. In this paper we have used observations from NASA's Magnetospheric MultiScale spacecraft to investigate what happens when two of these slingshot‐like magnetic field lines move toward each other and get tangled up. We found that the two bent magnetic field lines tend to orient themselves perpendicular to each other as they become interlinked and stretched, similar to what rubber bands would do. This reorientation allows the interlinked magnetic fields to reconnect again, releasing part of the built‐up magnetic energy as strong electron heating. The results are important because they show how interlinked magnetic fields, which occur in many solar and astrophysics contexts, reconnect and produce enhanced electron heating, something that was not understood before. Key Points: Magnetic flux pileup observed upstream of reconnecting current sheet at the interface of converging reconnection jetsMagnetic flux pileup was accompanied by increase in magnetic shear and decrease in Δβ, leading to conditions favorable for reconnectionMagnetic flux pileup leads to enhanced available magnetic energy per particle and strong electron heating [ABSTRACT FROM AUTHOR]

This paper reports two unique auroral features: postmidnight purple auroral rays and global Pc1 geomagnetic pulsations, observed before the onset of the corotating interaction region (CIR) storm of 21 March 2017, at the beginning of the first campaign of the new Particles and Waves in the Inner magnetosphere using Ground‐based network observation (PWING) longitudinal ground network with the Arase satellite. The purple auroral rays were observed from ~0315 to 0430 UT (~03–04 magnetic local time) in the northeastern sky at Husafell, Iceland (magnetic latitude: 64.9°N). We newly propose that the entry of high‐density CIR plasma into the magnetotail created purple auroral rays in the sunlit ionosphere. Pc1 geomagnetic pulsations at frequencies of 0–0.5 Hz were observed after ~00 UT over a wide local time range, of 13 hr, from midnight to afternoon sectors at subauroral latitudes associated with CIR arrival. These results indicate preconditioning of the magnetosphere due to crossing of a CIR. Plain Language Summary: We report auroral ray structures, which show a unique purple color, and global geomagnetic pulsations observed on 17 March 2017. The purple auroral rays were observed in the northeastern sky at Husafell, Iceland. The geomagnetic pulsations at frequencies of below 0.5 Hz were observed over a wide longitudinal range extending from midnight through morning to afternoon sectors at subauroral latitudes. These phenomena took place associated with the arrival of the so‐called corotating interaction region (CIR)" in the interplanetary space, which is characterized by high‐density high‐speed solar wind plasma. The CIR is one of the phenomena frequently occurring during the minimum phase of the 11‐year solar cycle. We suggest that the CIR can cause these unique purple auroras and global geomagnetic pulsations. The present observations also suggest a possible mechanism for dropout of radiation belt electrons due to electromagnetic ion cyclotron waves (=geomagnetic pulsations) associated with the CIR arrival. Key Points: Unique postmidnight purple auroral rays and global Pc1/EMIC waves were observed during a CIR‐driven solar wind density enhancementPc1/EMIC waves were found over a wide longitudinal range extending from midnight through morning to the afternoon due to CIR arrivalEntry of high‐density solar wind plasma into the magnetotail may have created tall purple auroral rays in the sunlit ionosphere [ABSTRACT FROM AUTHOR]

We present the first observation of fast plasma flows in Mercury's magnetotail. Mercury experiences substorm activity phenomenologically similar to Earth's; however, field‐of‐view limitations of the Fast Imaging Plasma Spectrometer (FIPS) prevent the instrument from detecting fast flows in the plasma sheet. Although FIPS measures incomplete plasma distributions, subsonic flows impart an asymmetry on the partial plasma distribution, even if the flow directions are outside the field of view. We combine FIPS observations from 387 intervals containing magnetic field dipolarizations to mitigate these instrument limitations. By taking advantage of variations in spacecraft pointing during these intervals, we construct composite plasma distributions from which mean flows are determined. We find that dipolarizations at Mercury are embedded within fast sunward flows with an averaged speed of ~300 km/s compared to a typical background flow of ~50 km/s. Plain Language Summary: Similar to Earth, Mercury has a global magnetic field that forms a protective cavity, known as the magnetosphere, within the solar wind. The solar wind compresses the dayside magnetosphere, while stretching the nightside magnetosphere behind the planet. Variations within the solar wind cause dynamic activity within Mercury's magnetosphere, with a process known as magnetic reconnection mediating the interaction. Magnetic reconnection changes the topology of magnetic field lines and transfers energy and momentum from the magnetic field to the plasma within it. At Earth, magnetic reconnection in the nightside magnetosphere drives fast flows of plasma toward the planet, which when nearing the planet are slowed and diverted. These flows cannot be identified directly at Mercury because of limitations of the MESSENGER spacecraft measurements collected there. This research paper develops a new statistical technique to identify and characterize these fast flows at Mercury. Key Points: Multiple FIPS plasma observations from the MESSENGER spacecraft have been combined statistically to determine average flowsObservations collected during dipolarizations produce an average plasma flow of ~300 km/s compared to ~50 km/s during background intervalsSeveral dipolarizations are required to unload Mercury's magnetotail during a substorm, and some flows may reach the planet's surface [ABSTRACT FROM AUTHOR]