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We analyze the local dynamics of magnetotail reconnection onset using Magnetospheric Multiscale (MMS) data. In conjunction with MMS, the macroscopic dynamics of this event were captured by a number of other ground and space‐based observatories, as is reported in a companion paper. We find that the local dynamics of the onset were characterized by the rapid thinning of the cross‐tail current sheet below the ion inertial scale, accompanied by the growth of flapping waves and the subsequent onset of electron tearing. Multiple kinetic‐scale magnetic islands were detected coincident with the growth of an initially sub‐Alfvénic, demagnetized tailward ion exhaust. The onset and rapid enhancement of parallel electron inflow at the exhaust boundary was a remote signature of the intensification of reconnection Earthward of the spacecraft. Two secondary reconnection sites are found embedded within the exhaust from a primary X‐line. The primary X‐line was designated as such on the basis that (a) while multiple jet reversals were observed in the current sheet, only one reversal of the electron inflow was observed at the high‐latitude exhaust boundary, (b) the reconnection electric field was roughly five times larger at the primary X‐line than the secondary X‐lines, and (c) energetic electron fluxes increased and transitioned from anti‐field‐aligned to isotropic during the primary X‐line crossing, indicating a change in magnetic topology. The results are consistent with the idea that a primary X‐line mediates the reconnection of lobe magnetic field lines and accelerates electrons more efficiently than its secondary X‐line counterparts. Key Points: Magnetotail reconnection onset was triggered by electron tearing during a solar wind pressure pulseOnset was characterized by the rapid collapse of the current sheet thickness and kinetic‐scale flux rope formationA primary X‐line was established within minutes of the onset [ABSTRACT FROM AUTHOR]

We analyze a magnetotail reconnection onset event on 3 July 2017 that was observed under otherwise quiescent magnetospheric conditions by a fortuitous conjunction of six space and ground‐based observatories. The study investigates the large‐scale coupling of the solar wind–magnetosphere system that precipitated the onset of the magnetotail reconnection, focusing on the processes that thinned and stretched the cross‐tail current layer in the absence of significant flux loading during a 2‐hr‐long preconditioning phase. It is demonstrated with data in the (a) upstream solar wind, (b) at the low‐latitude magnetopause, (c) in the high‐latitude polar cap, and (d) in the magnetotail that the typical picture of solar wind‐driven current sheet thinning via flux loading does not appear relevant for this particular event. We find that the current sheet thinning was, instead, initiated by a transient solar wind pressure pulse and that the current sheet thinning continued even as the magnetotail and solar wind pressures decreased. We suggest that field line curvature‐induced scattering (observed by magnetospheric multiscale) and precipitation (observed by Defense Meteorological Satellite Program) of high‐energy thermal protons may have evacuated plasma sheet thermal energy, which may require a thinning of the plasma sheet to preserve pressure equilibrium with the solar wind. Key Points: Magnetotail reconnection onset was observed during a fortuitous multiscale conjunction of the heliophysics observatoriesA transient solar wind pressure pulse triggered thinning and stretching of the cross‐tail current sheet without significant flux loadingA second solar wind pressure pulse caused the thinned current sheet to rapidly collapse and reconnect [ABSTRACT FROM AUTHOR]

Multi‐MeV electron drift‐periodic flux oscillations observed in Earth's radiation belts indicate radial transport and energization/de‐energization of these radiation belt core populations. Using multi‐year Van Allen Probes observations, a statistical analysis is conducted to understand the characteristics of this phenomenon. The results show that most of these flux oscillations result from resonant interactions with broadband ultralow frequency (ULF) waves and are indicators of ongoing radial diffusion. The occurrence frequency of flux oscillations is higher during high solar wind speed/dynamic pressure and geomagnetically active times; however, a large number of them were still observed under mild to moderate solar wind/geomagnetic conditions. The occurrence frequency is also highest (up to ∼30%) at low L‐shells (L∗∼3−4 ${L}^{\ast }\sim 3-4$) under various geomagnetic activity, suggesting the general presence of broadband ULF waves and radial diffusion at low L‐shells even during geomagnetically quiet times and showing the critical role of the electron phase space density radial gradient in forming drift‐periodic flux oscillations. Plain Language Summary: Earth's radiation belts contain 100s of keV–10 MeV electrons which pose great threats to the avionics and humans in space. Understanding their dynamics is critical due to both scientific interests and practical needs. Periodic flux oscillations of multi‐MeV electrons at their drift frequencies have been previously observed in Earth's radiation belts. These flux oscillations are believed to be indicators of the radial transport of these electrons and contain critical information of their energization processes. Using Van Allen Probes observations, a statistical analysis is conducted to further understand this phenomenon and associated mechanisms. It is shown that the majority of observed flux oscillations result from resonant interactions between multi‐MeV electrons and broadband ultralow frequency (ULF) waves, which lead to radial diffusion, one of the most important mechanisms causing radiation belt electron acceleration/loss. These flux oscillations preferentially occur under active solar wind conditions and geomagnetic activity. They also occur more often at lower altitudes, which is due to the steeper radial gradient of the electron distribution closer to the Earth. This indicates the general presence of broadband ULF waves and radial diffusion at low altitudes and suggests the critical role of the radial gradient of the electron distribution in forming these flux oscillations. Key Points: Most multi‐MeV electron drift‐periodic flux oscillations in the outer belt result from resonant interaction with broadband ultralow frequency wavesThese flux oscillations preferentially occur during high solar wind speed/dynamic pressure and geomagnetically active timesThe occurrence rate of these flux oscillations is highest at L*∼ 3–4 due to higher phase space density radial gradients at lower L [ABSTRACT FROM AUTHOR]

We present a machine‐learning‐based model of relativistic electron fluxes >1.8 MeV using a neural network approach in the Earth's outer radiation belt. The Outer RadIation belt Electron Neural net model for Relativistic electrons (ORIENT‐R) uses only solar wind conditions and geomagnetic indices as input. For the first time, we show that the state of the outer radiation belt can be determined using only solar wind conditions and geomagnetic indices, without any initial and boundary conditions. The most important features for determining outer radiation belt dynamics are found to be AL, solar wind flow speed and density, and SYM‐H indices. ORIENT‐R reproduces out‐of‐sample relativistic electron fluxes with a correlation coefficient of 0.95 and an uncertainty factor of ∼2. ORIENT‐R reproduces radiation belt dynamics during an out‐of‐sample geomagnetic storm with good agreement to the observations. In addition, ORIENT‐R was run for a completely out‐of‐sample period between March 2018 and October 2019 when the AL index ended and was replaced with the predicted AL index (lasp.colorado.edu/home/personnel/xinlin.li). It reproduces electron fluxes with a correlation coefficient of 0.92 and an out‐of‐sample uncertainty factor of ∼3. Furthermore, ORIENT‐R captured the trend in the electron fluxes from low‐earth‐orbit (LEO) SAMPEX, which is a completely out‐of‐sample data set both temporally and spatially. In sum, the ORIENT‐R model can reproduce transport, acceleration, decay, and dropouts of the outer radiation belt anywhere from short timescales (i.e., geomagnetic storms) and very long timescales (i.e., solar cycle) variations. Plain Language Summary: The Earth's radiation belts consist of energetic particles. During periods of intense space weather, the energy and density of radiation belt particles can increase significantly and pose a danger to astronauts, spacecraft, and even technologies on the ground. This study presents a machine‐learning‐based model (ORIENT‐R) that calculates the energetic radiation belt electron fluxes. It uses solar wind observations and geomagnetic activity indices as input, without the need for boundary conditions such as other satellite measurements. The ORIENT‐R model can determine the energetic electrons with a high Pearson correlation of 0.95 and a small uncertainty factor of ∼2. Furthermore, even when geomagnetic index AL is not available, the ORIENT‐R model can still estimate with high accuracy of Pearson correlation of 0.92 and a small uncertainty factor of ∼3. Thus, the ORIENT‐R model has great value and wide application in the space physics community and space weather industry. Key Points: A neural network model was developed to forecast relativistic electron fluxes with energies >1.8 MeV in the outer radiation belt (ORIENT‐R)ORIENT‐R model reproduces the relativistic electron fluxes with high out‐of‐sample accuracy: Pearson r ∼ 0.95, uncertainty of a factor of ∼2The ORIENT‐R model reproduces short‐ and long‐term dynamics of the outer radiation belt such as transport, acceleration, decay, and dropouts [ABSTRACT FROM AUTHOR]

In this study we examine the ability of protons of solar origin to access the near‐equatorial inner magnetosphere. Here we examine four distinct solar proton events from 20–200 MeV, concurrent with both quiet time and storm time conditions using proton data from the ACE satellite in the solar wind upstream of Earth and data from the Relativistic Electron Proton Telescope (REPT) instrument aboard Van Allen Probes. We examine the direct flux correspondence between interplanetary space and the inner magnetosphere. Small substructures in interplanetary space are observable in the REPT flux profiles, which can penetrate down to L values of ≤4. Furthermore, there are orbit‐to‐orbit variations in the west‐to‐east anisotropic flux ratios. The anisotropic flux ratios are used as a proxy for cutoff energies and display cutoff variations with L shell and energy. The dependence of the anisotropic flux ratio on Dst values is shown. The results paint a picture of highly dynamic spatial and temporal proton cutoff rigidities in the near‐equatorial inner magnetosphere. Key Points: Small flux substructures in interplanetary space are observed in the inner magnetosphereWest‐to‐east flux ratios are used as a proxy for cutoff location and energyWest‐to‐east flux ratios are highly dynamic from orbit to orbit and respond quickly to magnetospheric changes [ABSTRACT FROM AUTHOR]

We present detailed analysis of the global relativistic electron dynamics as measured by total radiation belt content (RBC) during coronal mass ejection (CME) and corotating interaction region (CIR)‐driven geomagnetic storms. Recent work has demonstrated that the response of the outer radiation belt is consistent and repeatable during geomagnetic storms. Here we build on this work to show that radiation belt dynamics can be divided into two sequential phases, which have different solar wind dependencies and which when analyzed separately reveal that the radiation belt responds more predictably than if the overall storm response is analyzed as a whole. In terms of RBC, in every storm we analyzed, a phase dominated by loss is followed by a phase dominated by acceleration. Analysis of the RBC during each of these phases demonstrates that they both respond coherently to solar wind and magnetospheric driving. However, the response is independent of whether the storm response is associated with either a CME or CIR. Our analysis shows that during the initial phase, radiation belt loss is organized by the location of the magnetopause and the strength of Dst and ultralow frequency wave power. During the second phase, radiation belt enhancements are well organized by the amplitude of ultralow frequency waves, the auroral electroject index, and solar wind energy input. Overall, our results demonstrate that storm time dynamics of the RBC is repeatable and well characterized by solar wind and geomagnetic driving, albeit with different dependencies during the two phases of a storm. Key Points: The radiation belt responds to both CME‐ and CIR‐driven geomagnetic storms in a consistent and repeatable wayTwo storm phases, dominated by loss and then acceleration, have strong albeit different correlations to solar wind and geomagnetic driversThese results provide an overarching framework that provides a coherent description of storm time radiation belt dynamics [ABSTRACT FROM AUTHOR]

Abstract: Geomagnetic activity depends on a variety of factors including solar zenith angle, solar UV, strength of the interplanetary magnetic field, speed and density of the solar wind, orientation of the Earth’s dipole, distance of the Earth from Sun, occurrence of CMEs and CIRs, and possibly other parameters. We have investigated some of these using state-dependant linear prediction filters. For a given state a prediction filter transforms a coupling function such as rectified solar wind electric field (VBs) to an output like the auroral electrojet index (AL). The area of this filter calculated from the sum of the filter coefficients measures the strength of the coupling. When the input and output are steady for a time longer than the duration of the filter the ratio of output to input is equal to this area. We find coupling strength defined in this way for Es=VBs to AL (and AU) is weakest at solar maximum and strongest at solar minimum. AL coupling displays a semiannual variation being weakest at the solstices and strongest at the equinoxes. AU coupling has only an annual variation being strongest at summer solstice. AL and AU coupling also vary with time relative to a stream interface. Es coupling is weaker after the interface, but ULF coupling is stronger. Total prediction efficiency remains about constant at the interface. The change in coupling strength with the solar cycle can be explained as an effect of more frequent saturation of the polar cap potential causing a smaller ratio of AL to Es. Stronger AL coupling at the equinoxes possibly indicates some process that makes magnetic reconnection less efficient when the dipole axis is tilted along the Earth–Sun line. Strong AU coupling at summer solstice is likely due to high conductivity in northern summer. Coupling changes at a stream interface are correlated with the presence of strong wave activity in ground and satellite measurements and may be an artifact of the method by which solar wind data are propagated. [Copyright &y& Elsevier]

Abstract: The second and third flybys of Mercury by the MESSENGER spacecraft occurred, respectively, on 6 October 2008 and on 29 September 2009. In order to provide contextual information about the solar wind properties and the interplanetary magnetic field (IMF) near the planet at those times, we have used an empirical modeling technique combined with a numerical physics-based solar wind model. The Wang–Sheeley–Arge (WSA) method uses solar photospheric magnetic field observations (from Earth-based instruments) in order to estimate the inner heliospheric radial flow speed and radial magnetic field out to 21.5 solar radii from the Sun. This information is then used as input to the global numerical magnetohydrodynamic model, ENLIL, which calculates solar wind velocity, density, temperature, and magnetic field strength and polarity throughout the inner heliosphere. WSA-ENLIL calculations are presented for the several-week period encompassing the second and third flybys. This information, in conjunction with available MESSENGER data, aid in understanding the Mercury flyby observations and provide a basis for global magnetospheric modeling. We find that during both flybys, the solar wind conditions were very quiescent and would have provided only modest dynamic driving forces for Mercury''s magnetospheric system. [Copyright &y& Elsevier]

Abstract: Within the CISM project empirical models serve as baselines against which to monitor the increase in predictive skill of physics-based numerical codes as they advance. Empirical models also establish a suite of forecast models from which to evolve toward greater forecast accuracy by incorporating numerical codes as they advance enough to increase forecast skill. Establishing a suite of forecast models allows the CISM project to contribute results of use to operational space weather forecasting earlier than might otherwise be possible. Developing a suite of empirical models allows CISM to address issues of data ingestion early in the project under relatively simple conditions. Out of the data-ingestion effort has come a technique of probabilistic forecasts, which allows one to ingest information on IMF from solar data instead of from measurements taken only at the Lagrangian L1 point. To implement the evolution from empirical to numerical forecast models, CISM has adopted a coupled two-line system of model development, a science-model line interacting with a forecast-model line. [Copyright &y& Elsevier]

We test the ability of a data-derived model of geomagnetic activity, originally optimized to have a high prediction efficiency (PE), for its ability to predict only large geomagnetic disturbances. Correlation-based metrics, such as prediction efficiency, are often used as a measure of model performance. This metric puts equal weight on prediction of both large and small measurements. However, for space weather purposes, one is often interested in knowing only if a large disturbance event will occur so less emphasis should be placed on small measurements. If only large events are of interest, then a correlation metric is not the best measure of model performance. In this work, we determine how well a data-derived model, originally optimized to have a high prediction efficiency; predicts large geomagnetic events. The ratio of the number of correct to false alarm forecasts, RF, is used as an event-predictor metric. It is shown that In the electrojet regions the data-derived model that predicts the north-south component of the ground magnetic field B∞ has a spatial RF profile similar to that of the prediction efficiency. Maximal values of RF = 4 are found at 0300 MLT when an event is defined as an excursion in the hourly-averaged north-south component of the ground magnetic field below -400 nT. Whereas the local time profile of PE(B∞) is similar to RF (B∞), the profile of PE(&varbar;dB∞/dt&varbar;) differs substantially from ELF (&varbar;dB∞/dt&varbar;) in the noon sector. Epoch analysis shows that the poor performance in the noon sector is a result of pre-event levels of &varbar;dB∞/d&varbar; not being clearly separated from post-event levels. [ABSTRACT FROM AUTHOR]

A complete description of Sun-Earth connections requires a deep understanding of the atmospheric processes that amplify in complex ways the effects of the ever-changing magnetospheric and solar energy inputs. The physical effects of these inputs are not well understood, but must entail nonlinear feedbacks that act to couple all the layers of the Earth's atmosphere. This ultimately shapes the global system in which we live. Central to this problem is determining the atmospheric response to energetic particle precipitation (EPP) both on regional and global scales. This is a last great frontier in solar-terrestrial research that forms the backdrop upon which long-term human-induced atmospheric change is superimposed. Particles deposited into Earth's atmospheric layers lose energy by ionizing constituent molecules. Where and with what strength such ionization occurs is controlled by solar wind forcing and the subsequent levels of geomagnetic activity. Quantifying EPP variations is crucial to understanding the production of key reactive atmospheric chemical constituents, namely reactive odd nitrogen¬ (NOx) and odd hydrogen (HOx). NOx and HOx participate in ozone (O3) catalytic cycles, with significant consequences for the temperature structure of the atmosphere. Changes in temperature gradients subsequently affect winds and thus atmospheric wave propagation. After a 6-year run, a Nagoya University team has compiled temporal variation of daily-averaged NO column density and found that the amplitude of seasonal variation is remarkably smaller in 2014, by almost a factor of 2, than those of 2013 and 2015. At the same time, our high-energy electron flux measurements obtained by Van Allen Probes (E≳1 MeV) for L~6.0 which is close to that of Syowa Station (L~6.1) also shows significant absence of energetic electrons during the southern summer and winter of 2014. This correlation of NO and electron flux is suggestive evidence that energetic electrons not only cause sporadic enhancement but also influence the long variability of mesospheric and lower thermospheric NO with a timescale of several months. To properly quantify these effects into the future, the incoming precipitating particle spectrum must be known with high accuracy in absolute intensity across the electron range from 10 keV to multiple MeV energies. This accuracy will allow requisite determination of ionization rates at altitudes from 100 km down to below 40 km. There have not yet been, over the foregoing decades of the Space Age, measurements sufficient to characterize the full energy spectrum of EPP over space and time. This presentation discusses a mission concepts that would address with great care the impact and coupling of energetic particles in Earth's atmospheric system. [ABSTRACT FROM AUTHOR]

Abstract: The MESSENGER spacecraft flyby of Mercury on 14 January 2008 provided a new opportunity to study the intrinsic magnetic field of the innermost planet and its interaction with the solar wind. The model presented in this paper is based on the solution of the three-dimensional, bi-fluid equations for solar wind protons and electrons in the absence of mass loading. In this study we provide new estimates of Mercury’s intrinsic magnetic field and the solar wind conditions that prevailed at the time of the flyby. We show that the location of the boundary layers and the strength of the magnetic field along the spacecraft trajectory can be reproduced with a solar wind ram pressure P sw =6.8nPa and a planetary magnetic dipole having a magnitude of 210 R M 3 −nT and an offset of 0.18 R M to the north of the equator, where R M is Mercury’s radius. Analysis of the plasma flow reveals the existence of a stable drift belt around the planet; such a belt can account for the locations of diamagnetic decreases observed by the MESSENGER Magnetometer. Moreover, we determine that the ion impact rate at the northern cusp was four times higher than at the southern cusp, a result that provides a possible explanation for the observed north–south asymmetry in exospheric sodium in the neutral tail. [ABSTRACT FROM AUTHOR]