Energetic particle precipitation (EPP) is one of the fundamental drivers of space weather in the coupled atmosphere-ionosphere-magnetosphere (AIM) system. These electrons and ions from the sun or the terrestrial magnetosphere, ranging in energy from hundreds of eV to GeV, precipitate into the atmosphere in response to enhanced topside (solar and magnetosphere) driving. They deposit their energy at a wide range of altitudes, enhancing ionization, and changing neutral temperature, density, and winds. During times of prolonged driving the resulting changes can adversely affect anthropogenic systems including disruption of communication and power systems, and increased satellite drag leading to orbital decay. In addition to its effects on space weather, EPP has been recognized as an important component of climate via its ability to indirectly destroy ozone, modifying local radiative balance in the middle and upper atmosphere. Despite the recognized importance of EPP to the AIM system, the way in which these two-way coupled systems interact is highly complex and remains poorly understood and constrained. Measurements from our current observational fleet are not able to fully capture EPP-driven AIM dynamics. As a result, we lack a fundamental understanding of many aspects of this coupled system, and models cannot be validated and are inhibited in their ability to forecast space weather. To compound this situation, different aspects of the AIM system are studied by the different communities with insufficient cross-community cooperation. Properly studying AIM dynamics, a societal level priority, requires a global systems science (holistic) approach to data collection, analysis, and modeling. This Chapman conference will bring together participants from the AIM communities to focus efforts on identifying and communicating outstanding issues, how models can bridge knowledge gaps, promising techniques for enhanced analysis, and required new types of observations.

Our study presents the first simultaneous multi-point observations of the local-time asymmetry of 10's-100's of keV energy particle pitch angle distributions (PADs), and associated wave activity during the main phase of the geomagnetic storm that occurred on 7 September 2017. This study is a companion to Pandya et al. (2025), who identified a rare conjunction in which RBSP-A and RBSP-B were located in the evening and morning sectors, respectively, enabling the comparison of ion populations across local time. Building on that work, we identify an additional rare conjunction during the same interval in which RBSP-A and MMS-1 were co-located in the evening sector for similar to 15 min (2,325-2340 UT) within Delta L < 0.2, Delta MLT < 2 min and Delta MLAT similar to 20 degrees. Our analysis shows strong local time asymmetries. Protons in the evening sector exhibited freshly injected, enhanced fluxes with nearly isotropic PADs at >60 keV energies and pancake PADs at <60 keV energies, whereas dawn-noon proton fluxes retained pancake PADs characteristic of older populations. In contrast, electrons showed a significant flux decrease in the evening sector and isotropic enhancement in the morning sector, consistent with their eastward gradient-B and curvature drifts. Concurrent wave observations revealed strong ULF, magnetosonic and hiss waves in the evening sector, while hiss waves dominated in the morning sector. Our results demonstrate that storm-time magnetic field reconfiguration, rather than wave-particle interactions, primarily governs keV particle PAD asymmetries. The findings highlight the importance of coordinated multi-point measurements for advancing our understanding of ring current asymmetries and storm-time particle transport. Plain Language Summary During geomagnetic storms, particles trapped in Earth's magnetic field are strongly energized and redistributed differently at different magnetic local times (MLTs). In this study, we investigate the 7 September 2017 geomagnetic storm using the multi-spacecraft conjunction. Previous work (Pandya et al., 2025) reported a unique event in which the two RBSP observatories were located on opposite sides of Earth, allowing direct comparison of particle behavior across local time. Building on that study, we identify a second rare conjunction in which the RBSP-A and MMS-1 spacecraft were located in nearly the same region of the evening sector at the same time. These coordinated measurements show that protons and electrons responded very differently at different local times during the storm. Protons were enhanced and nearly isotropic in the evening sector but weakened in the dawn sector, while electrons showed the opposite trend. These asymmetries were accompanied by different wave activity on the dawn and dusk sides of the magnetosphere. Our results suggest that the reconfiguration of the Earth's magnetic field during the storm, rather than wave-particle interactions, was the dominant factor shaping these particle distributions. This study highlights the importance of multi-point observations for understanding the Earth's near-space environment.

Microbursts are impulsive injections of energetic (few keV to >MeV) electrons into the atmosphere, primarily caused by nonlinear scattering driven by whistler mode chorus waves. While the relative importance of microburst precipitation as a loss process has not been fully quantified, many studies have shown microbursts may play a significant role in the loss of outer radiation belt electrons. We present a multi-platform statistical analysis of chorus and energetic electron precipitation in an attempt to constrain the azimuthal spatial extent (triangle $\mathit{{\increment}}$MLT) of the microburst precipitation region and determine how this extent varies with geomagnetic activity. Statistical upper bounds of this azimuthal extent are determined with observations of general energetic electron precipitation that can include direct microburst detections, while statistical lower bounds determination requires direct microburst detections. The resulting distributions of both upper and lower bounds azimuthal extent suggest that microbursts may frequently constitute an important source of electron loss from the outer radiation belt. We find that 36% of upper bound events in the dawn sector span more than 5 hr in MLT. This azimuthal extent increases with geomagnetic activity, particularly in the dawn and noon MLT sectors.

The Imaging Microburst Precipitation with Atmospheric X-ray emissions (IMPAX) CubeSat was recently selected by NASA’s H-FORT program, with a planned launch date of late 2027. The overarching goal of IMPAX is to quantify relativistic electron microburst precipitation as a radiation belt loss mechanism.

Cold plasma of ionospheric origin has recently been found to be a much larger contributor to the magnetosphere of Earth than expected1, 2-3. Numerous competing mechanisms have been postulated to drive ion escape to space, including heating and acceleration by wave-particle interactions4 and a global electrostatic field between the ionosphere and space (called the ambipolar or polarization field)5,6. Observations of heated O+ ions in the magnetosphere are consistent with resonant wave-particle interactions7. By contrast, observations of cold supersonic H+ flowing out of the polar ionosphere8,9 (called the polar wind) suggest the presence of an electrostatic field. Here we report the existence of a +0.55 +/- 0.09 V electric potential drop between 250 km and 768 km from a planetary electrostatic field (E parallel to circle plus = 1.09 +/- 0.17 mu V m-1) generated exclusively by the outward pressure of ionospheric electrons. We experimentally demonstrate that the ambipolar field of Earth controls the structure of the polar ionosphere, boosting the scale height by 271%. We infer that this increases the supply of cold O+ ions to the magnetosphere by more than 3,800%, in which other mechanisms such as wave-particle interactions can heat and further accelerate them to escape velocity. The electrostatic field of Earth is strong enough by itself to drive the polar wind9,10 and is probably the origin of the cold H+ ion population1 that dominates much of the magnetosphere2,3.

Energetic particle precipitation (EPP) is one of the fundamental drivers of space weather in the coupled atmosphere-ionosphere-magnetosphere (AIM) system. These electrons and ions from the sun or the terrestrial magnetosphere, ranging in energy from hundreds of eV to GeV, precipitate into the atmosphere in response to enhanced topside (solar and magnetosphere) driving. They deposit their energy at a wide range of altitudes, enhancing ionization, and changing neutral temperature, density, and winds. During times of prolonged driving the resulting changes can adversely affect anthropogenic systems including disruption of communication and power systems, and increased satellite drag leading to orbital decay. In addition to its effects on space weather, EPP has been recognized as an important component of climate via its ability to indirectly destroy ozone, modifying local radiative balance in the middle and upper atmosphere. Despite the recognized importance of EPP to the AIM system, the way in which these two-way coupled systems interact is highly complex and remains poorly understood and constrained. Measurements from our current observational fleet are not able to fully capture EPP-driven AIM dynamics. As a result, we lack a fundamental understanding of many aspects of this coupled system, and models cannot be validated and are inhibited in their ability to forecast space weather. To compound this situation, different aspects of the AIM system are studied by the different communities with insufficient cross-community cooperation. Properly studying AIM dynamics, a societal level priority, requires a global systems science (holistic) approach to data collection, analysis, and modeling. This Chapman conference will bring together participants from the AIM communities to focus efforts on identifying and communicating outstanding issues, how models can bridge knowledge gaps, promising techniques for enhanced analysis, and required new types of observations.

Abstract On 7 January 2014, a solar storm erupted, which eventually compressed the Earth's magnetosphere leading to the generation of chorus waves. These waves enhanced local wave‐particle interactions and led to the precipitation of electrons from 10 s eV to 100 s keV. This paper shows observations of a low energy cutoff in the precipitation spectrum from Van Allen Probe B Helium Oxygen Proton Electron measurements. This low energy cutoff is well replicated by the predicted loss calculated from pitch angle diffusion coefficients from wave and plasma observations on Probe B. To our knowledge, this is the first time a single spacecraft has been used to demonstrate an accurate theoretical prediction for chorus wave‐induced precipitation and its low energy cutoff. The specific properties of the precipitating soft electron spectrum have implications for ionospheric activity, with the lowest energies mainly contributing to thermospheric and ionospheric upwelling, which influences satellite drag and ionospheric outflow.

In the solar wind density, we often observe periodic fluctuations on time scales ranging from a few minutes to a few hours which we refer to as Periodic Density Structures (PDSs). The PDSs belong to the class of “meso-scale structures” with radial length scales greater than or equal to the size of the Earth’s dayside magnetosphere. The periodic character of these transients (≈0.2-4.0 mHz) can determine periodic compressional fluctuations of the Earth’s magnetic field at similar frequencies (“forced breathing” mode). The corresponding time scales overlap with the frequency range of Pc5 Ultra Low Frequency (ULF) waves (≈1.7-6.7 mHz). The compressional “forced breathing” fluctuations are often global and impact the entire Earth’s magnetosphere system/dynamics. Using a recently developed spectral analysis approach applied to magnetic field observations at satellites and ground stations, we were able to differentiate directly driven magnetic field oscillations from Pc5 ULF waves triggered by other sources. Here, we discuss clear examples of such a directly driven process also showing effects on radiation belt electron dynamics and loss.

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&lt;p&gt;In the solar wind density, we often observe periodic fluctuations on time scales ranging from a few minutes to a few hours which we refer to as Periodic Density Structures (PDSs). The PDSs belong to the class of &amp;#8220;meso-scale structures&amp;#8221; with radial length scales greater than or equal to the size of the Earth&amp;#8217;s dayside magnetosphere. The periodic character of these transients (&amp;#8776;0.2-4.0 mHz) can determine periodic compressional fluctuations of the Earth&amp;#8217;s magnetic field at similar frequencies (&amp;#8220;forced breathing&amp;#8221; mode). The corresponding time scales overlap with the frequency range of Pc5 Ultra Low Frequency (ULF) waves (&amp;#8776;1.7-6.7 mHz). The compressional &amp;#8220;forced breathing&amp;#8221; fluctuations are often global and impact the entire Earth&amp;#8217;s magnetosphere system/dynamics.&amp;#160; Using a recently developed spectral analysis approach applied to magnetic field observations at satellites and ground stations, we were able to differentiate directly driven magnetic field oscillations from Pc5 ULF waves triggered by other sources. Here, we discuss clear examples of such a directly driven process also showing effects on radiation belt electron dynamics and loss.&lt;/p&gt;

Waves which couple to energetic electrons are particularly important in space weather, as they drive rapid changes in the topology and intensity of Earth’s outer radiation belt during geomagnetic storms. This includes Ultra Low Frequency (ULF) waves that interact with electrons via radial diffusion which can lead to electron dropouts and rapid acceleration and inward transport of electrons during. In radiation belt simulations, the strength of this interaction is specified by ULF wave radial diffusion coefficients. In this paper we detail the development of new models of electric and magnetic radial diffusion coefficients derived from in-situ observations of the azimuthal electric field and compressional magnetic field. The new models use L* as it accounts for adiabatic changes due to the dynamic magnetic field coupled with an optimized set of four components of solar wind and geomagnetic activity, Bz, V, Pdyn and Sym-H, as independent variables (inputs). These independent variables are known drivers of ULF waves and offer the ability to calculate diffusion coefficients at a higher cadence then existing models based on Kp. We investigate the performance of the new models by characterizing the model residuals as a function of each independent variable and by comparing to existing radial diffusion models during a quiet geomagnetic period and through a geomagnetic storm. We find that the models developed here perform well under varying levels of activity and have a larger slope or steeper gradient as a function of L* as compared to existing models (higher radial diffusion at higher L* values).

Abstract Understanding intense electron precipitation is crucial for characterizing radiation belt loss and assessing related impacts on the atmosphere. We investigate the evolution of electron flux during an ensemble of 70 geomagnetic storms, focusing on equatorial and low‐Earth orbit observations of trapped and precipitating ∼30–100 keV energy electrons. We reveal that the most intense electron precipitation is associated with equatorial flux capping through self‐limiting processes, for example, as described theoretically by Kennel and Petschek (1966, https://doi.org/10.1029/jz071i001p00001). Our results indicate that the most intense electron precipitation is caused by electron injections associated with self‐limiting processes. Dawn side injections are observed to have fluxes that exceed the Kennel‐Petschek limit, consistent with the excitation of strong chorus waves and resulting in intense precipitation and return of the trapped flux to the Kennel‐Petschek limit. Our results clearly demonstrate the important role of self‐limiting processes in affecting the dynamics of newly injected electrons and driving intense electron precipitation.

∗ Primary Author 1 Astrophysical and Planetary Sciences Department, University of Colorado, Boulder, CO 2 Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 3 Southwest Research Institute, San Antonio, TX 4 Space Science Research Institute, Boulder, CO 5 NASA Marshall Spaceflight Research Center, Huntsville, AL 6 Los Alamos National Laboratory, Los Alamos, NM 7 Space Sciences Laboratory, University of California, Berkeley, Berkeley, CA 8 College of Engineering, Boston University, Boston, MA

Abstract Interactions between whistler mode chorus waves and electrons are a dominant mechanism for particle acceleration and loss in the outer radiation belt. One form of this loss is electron microburst precipitation: a sub‐second intense burst of electrons. Despite previous investigations, details regarding the microburst‐chorus scattering mechanism—such as dominant resonance harmonic—are largely unconstrained. One way to observationally probe this is via the time‐of‐flight energy dispersion. If a single cyclotron resonance is dominant, then higher energy electrons will resonate at higher magnetic latitudes: sometimes resulting in an inverse time‐of‐flight dispersion with lower‐energy electrons leading. Here we present a clear example of this phenomena, observed by a FIREBIRD‐II CubeSat on 27 August 2015, that shows good agreement with the Miyoshi‐Saito time‐of‐flight model. When constrained by this observation, the Miyoshi‐Saito model predicts that a relatively narrowband chorus wave with a ∼0.2 of the equatorial electron gyrofrequency scattered the microburst.

Precipitation losses of Earth's ring current and radiation belt particles represent a major loss process that ultimately helps balance the intensity levels of the radiation environment in near-Earth space.Energetic particle precipitation (EPP) involves particles that are quasi-trapped along magnetic field lines venturing to very high magnetic latitudes, where their mirror points in the dipole-like magnetic trap start to move to altitudes ≤ ~100 km, where collisions with atmospheric neutrals and ionospheric particles becomes significant.Such collisions result in radiation belt particles depositing their energy into the ionosphere and atmosphere and being lost from the system, but the altitude at which a precipitating energetic particle deposits the majority of its energy is highly dependent on the particle's energy and pitch angle.The highest energy, several MeV electrons from Earth's outer radiation belt deposit most of their energy at altitudes < 40 km, within the stratosphere.Many outstanding questions remain concerning the nature of radiation belt precipitation losses into the atmosphere and the impacts of such energy input on atmospheric heating and chemistry and the bottom-side ionosphere.Implications of these unknowns are broad ranging and cross-divisional, spanning Earth and Atmospheric Sciences, such as sudden stratospheric warming events and terrestrial climate, to Heliophysics, such as ionospheric energy budgets, radiation belt physics, and space weather.We encourage the Decadal Survey Committee to consider the following science objectives to be of high priority for dedicated missions and research over the next decade: Prioritize new missions to be developed that adequately instrument and populate a constellation of satellites at various altitudes in LEO with energetic particle instrumentation plus ionospheric and atmospheric remote sensing capabilities.Such a constellation should explore and establish the consequences of and causal role between energetic particle precipitation (i.e., ring current, radiation belt, and solar energetic particles) and: i) transient events of mesospheric NOx and HOx production, ozone depletion, and stratospheric and tropospheric warming and ii) localized, transient structures and sporadic intensification of the D-and E-region ionospheres.

The Van Allen Probes mission operations materialized through a distributed model in which operational responsibility was divided between the Mission Operations Center (MOC) and separate instrument specific SOCs. The sole MOC handled all aspects of telemetering and receiving tasks as well as certain scientifically relevant ancillary tasks. Each instrument science team developed individual instrument specific SOCs proficient in unique capabilities in support of science data acquisition, data processing, instrument performance, and tools for the instrument team scientists. In parallel activities, project scientists took on the task of providing a significant modeling tool base usable by the instrument science teams and the larger scientific community. With a mission as complex as Van Allen Probes, scientific inquiry occurred due to constant and significant collaboration between the SOCs and in concert with the project science team. Planned cross-instrument coordinated observations resulted in critical discoveries during the seven-year mission. Instrument cross-calibration activities elucidated a more seamless set of data products. Specific topics include post-launch changes and enhancements to the SOCs, discussion of coordination activities between the SOCs, SOC specific analysis software, modeling software provided by the Van Allen Probes project, and a section on lessons learned. One of the most significant lessons learned was the importance of the original decision to implement individual team SOCs providing timely and well-documented instrument data for the NASA Van Allen Probes Mission scientists and the larger magnetospheric and radiation belt scientific community.