Relativistic electron precipitation (REP) in the atmosphere can contribute significantly to electron loss from the outer radiation belts. In order to estimate the contribution to this loss, it is important to estimate the spatial extent of the precipitation region. We observed REP with the zenith pointing (0°) Medium Energy Proton Electron Detector (MEPED) on board Polar Orbiting Environmental Satellites (POES), for 15 years (2000–2014) and used both single‐satellite and multisatellite measurements to estimate an average extent of the region of precipitation in L shell and magnetic local time (MLT). In the duration of 15 years (2000–2014), 31,035 REP events were found in this study. Events were found to split into two classes; one class of events coincided with proton precipitation in the P1 channel (30–80 keV), were located in the dusk and early morning sector, and were more localized in L shell (dL < 0.5), whereas the other class of events did not coincide with proton precipitation, were located mostly in the midnight sector, and were wider in L shell (dL ∼ 1–2.5). Both classes were highly localized in MLT (dMLT ≤ 3 h), occurring mostly during the declining phase of the solar cycle and geomagnetically active times. The events located in the midnight sector for both classes were found to be associated with tail magnetic field stretching which could be due to the fact that they tend to occur mostly during geomagnetically active times or could imply that precipitation is caused by current sheet scattering.

Electromagnetic ion cyclotron (EMIC) waves have been proposed to cause relativistic electron precipitation (REP). In our study, we carry out 4 years of analysis from 2013 to 2016, with REP spikes obtained from POES satellites and EMIC waves observation from Van Allen Probes. Among the 473 coincidence events when POES satellites go through the region conjugate to EMIC wave activity, only 127 are associated with REP. Additionally, the coincidence occurrence rate is about 10% higher than the random coincidence occurrence rate, indicating that EMIC waves and relativistic electrons can be statistically related, but the link is weaker than expected. H+ band EMIC waves have been regarded as less important than He+ band EMIC waves for the precipitation of relativistic electrons. We demonstrate that the proportion of H+ band EMIC wave events that are associated with REP (22% to 32%) is slightly higher than for He+ band EMIC wave activity (18% to 27%). An even greater proportion (25% to 40%) of EMIC waves are accompanied by REP events when H+ band and He+ band EMIC waves occur simultaneously.

Prior studies of microburst precipitation have largely relied on estimates of the spatial scale and temporal duration of the microburst region in order to determine the radiation belt loss rate of relativistic electrons. These estimates have often relied on the statistical distribution of microburst events. However, few studies have directly observed the spatial and temporal evolution of a single microburst event. In this study, we combine Balloon Array for Radiation belt Relativistic Electron Losses balloon‐borne X‐ray measurements with Focused Investigations of Relativistic Electron Burst: Intensity, Range, and Dynamics II and AeroCube‐6 CubeSat electron measurements to determine the spatial and temporal evolution of a microburst region in the morning MLT sector on 13 August 2015. The microburst region is found to extend across at least 4 h in local time in the morning sector, from 09:00 to 13:00 MLT, and from L of 5 out to 10. The microburst event lasts for nearly 9 h. Smaller scale structure is investigated using the dual AeroCube‐6 CubeSats, and is found to be consistent with the spatial size of whistler mode chorus wave observations near the equatorial plane.

Relativistic electron precipitation (REP) is an important loss mechanism of the Earth's outer radiation belt electrons (Li & Hudson, 2019 and references therein), as well as a source of energy input into the Earth's atmosphere. It is widely accepted that electron precipitation is caused by wave-particle interactions that occur in the Earth's magnetosphere (e.g., Millan & Thorne, 2007;Thorne, 2010); however, sufficient stretching of magnetic field lines is another potential driver of electron and proton precipitation (e.g.,

The mechanisms that drive relativistic electron precipitation (REP) from the radiation belts can be better understood with a better knowledge of the particle energies involved. National Oceanic and Atmospheric Administration Polar Operational Environmental Satellites, being a network of multiple satellites, can provide multiple point spectral data over a long time period, including the Van Allen Probe's era. The number of energy channels is limited, but the particle detectors on Polar Operational Environmental Satellites have a narrow field of view allowing an investigation of bounce loss cone particles. We use the ratio of count rates in the E3 (>300 keV) and the P6 (>700 keV) channels as a parameter to define spectral hardness. Using this parameter, the spatial variation of spectral hardness of REP events was investigated. It was found that very soft events were mostly found in the dusk‐midnight‐early morning magnetic local time sectors and L∼ 5–7 while the hardest events were located in the postnoon sector peaking at L∼ 4–5. The hardest events peaked at lower L shells, and less than 20% were coincident with low‐energy (30–80 keV) proton precipitation. Further, around 70% of nightside REP coincident with proton precipitation was associated with stretched magnetic field lines indicating that curvature scattering may have been an important driver. Around 62% of nightside REP coincident with proton precipitation associated with relaxed magnetic field lines, suggesting a mechanism other than magnetic field curvature scattering, was highly energetic.

Within Earth's magnetosphere lie the Van Allen radiation belts, surrounding the Earth with energetic, charged particles. Under typical geomagnetic conditions, there are two such belts-a relatively stable inner belt, and a dynamic outer belt composed of electrons and low-energy protons, typically confined within 3 ≤ L ≤ 8 (e.g., Millan & Baker, 2012;Millan & Thorne, 2007). Outer belt electrons with relativistic energies often remain trapped by Earth's magnetic field for less than a day before either escaping across the magnetopause or precipitating into Earth's atmosphere (Thorne et al., 2005). Precipitating electrons can degrade shortwave radio signals (Evans & Greer, 2000), damage satellite instrumentation (Horne et al., 2013), and produce compounds capable of destroying mesospheric and stratospheric ozone (e.g., Brasseur & Solomon, 2005;Randall et al., 2005). Quantifying the size of the region over which relativistic electron precipitation (REP) occurs will improve understanding of the dynamics in the outer radiation belt, informing future radiation belt and climate models.If a particle's equatorial pitch angle is small enough that its mirror point lies within Earth's atmosphere (<∼100 km above the surface), the particle will likely collide with atmospheric particles and precipitate (Millan & Baker, 2012). Particles are defined to be in the bounce loss cone (BLC) if they will enter the atmosphere within a single bounce period (Selesnick, 2006). Mechanisms such as wave-particle interactions can cause pitch angle scattering, changing affected particle pitch angles, and causing particles that were previously trapped to instead enter the BLC (Millan & Thorne, 2007). Waves capable of pitch angle scattering relativistic electrons include whistler-mode plasmaspheric hiss and chorus, and electromagnetic ion-cyclotron (EMIC) waves (

In order to observe and study the magnetic fields near Earth, high-quality magnetic field measurements are necessary. However, the spacecraft carrying magnetic field sensors are often magnetically noisy-they produce stray magnetic fields that contaminate the measurements. One dominant noise source on many spacecrafts are the systems that control the orientation of the spacecraft. This manuscript presents a novel approach to the suppression of magnetic interference caused by these systems, improving the quality of acquired data. Specifically, a technique for the simultaneous separation of multiple measurements into physically meaningful components is combined with an automated component selection technique. This allows for a high-quality estimate of the spacecraft noise to be generated and subsequently removed from magnetic field measurements, greatly improving the data quality. For example, an interval of data captured by the CASSIOPE/Swarm-Echo satellite was shown to have its local interference reduced by an average of 89%.

Direct comparisons between RBSP (Van Allen Probes or Radiation Belt Storm Probes) and TWINS (Two Wide‐angle Imaging Neutral‐atom Spectrometers) for the main phase of two storms, March 17 and October 7, 2015, showed agreement between the in situ ion measurements and the ion spectra from the deconvolved energetic neutral‐atom (ENA) measurements, except when O+ ions were significant. Spatial evolution of individual energy peaks in the ion spectra is studied using TWINS data. O+ ions are seen to result in intense peaks at 5–10 keV/amu in the TWINS ion spectra. These ion populations are confined to low L shells (L < 5) and localized in the premidnight sector. When H+ ions are significant, the low energy peaks (<25 keV/amu) are found to be less intense than the high energy peaks (>25 keV/amu), located at L > 4 and localized within the premidnight sector. During times of rapidly varying AE indices, two spatially distinct peaks, between 3–5RE and 6–8RE, are observed for the ions with energies >25 keV/amu. The outer peak appears for a few hours and fades while the inner peak is more stable. These structures are found to be consistent with particle injections observed in the RBSP data. When double peaked structures are swept off, low energy ions accumulate in the premidnight to midnight sectors whereas high energy ions are located premidnight to postmidnight sectors. Faster drift orbits of >25 keV/amu ions may cause this kind of distribution.