Electromagnetic ion cyclotron (EMIC) waves can act as a loss process for both ring current ions and radiation belt electrons, and the spatial and temporal characteristics of these waves are important ...for quantifying their effects on energetic particles. Here we utilize observations from multiple spacecraft to constrain the azimuthal and radial dimensions as well as the duration of an EMIC wave event occurring on the nightside of the inner magnetosphere on 7 July 2013. These combined observations reveal waves limited to a narrow radial extent but persisting ~10+ hr and spanning ~12 hr in local time. The solar wind conditions, geomagnetic activity, and plasma environment are also examined to better understand the conditions under which persistent nightside EMIC waves can occur. Relativistic electron phase space density profiles during this event reveal local minima concurrent with the wave activity, consistent with EMIC‐driven scattering and loss of radiation belt electrons.
Plain Language Summary
Various oscillating electric and magnetic fields, or waves, can interact with high‐energy particles in near‐Earth space and cause a change in the particles' energy and/or direction of motion. Where and when these waves occur can have a significant impact on how they interact with particles. Here we combine measurements from multiple spacecraft around the Earth to study one specific wave mode. While these waves are often thought to be localized and of short durations, we observe an event on 7 July 2013 where waves persist for over 10 hr and span the entire nightside of the Earth. We explore the dynamics of the Earth's magnetic field, in response to activity on the Sun, to better understand what causes these widespread, long‐lasting waves. Changes in the energetic particle environment around the Earth are also presented to examine the effects of these waves. Events like these have the potential to cause significant effects in the particle populations around the Earth.
Key Points
Sustained, azimuthally extended EMIC waves are observed spanning the nightside of the inner magnetosphere on 7 July 2013
Wave structure lies at the inner edge of the plasma sheet overlaping the outer plasmasphere and plumes following enhanced Kp activity
Local minima in phase space density are observed concurrent with wave activity, consistent with EMIC‐driven MeV electron scattering
We conducted a statistical analysis of local phase space density (PSD) minima across a wide energy range (∼20 keVs to ∼10 MeV), using observations from the Van Allen Probes and the Geostationary ...Operational Environmental Satellite. We identified deepening minima in PSD profiles of multi‐MeV (∼5% occurrence) and of “seed” electrons (up to 15% occurrence, corresponding to ∼70–100 s keV) and compared their distribution with a 3D diffusion model using the Versatile Electron Radiation Belts (VERB) code. The comparison of the observed and modeled distributions suggests that the PSD minima of seed electrons are likely associated with hiss waves and the corresponding L‐shell dependent electron lifetimes. However, the observed distribution was not fully reproduced by the model, potentially indicating other fast loss mechanisms of seed electrons.
Plain Language Summary
This study presents a comprehensive analysis of the behavior of electrons within the Earth's radiation belts, specifically focusing on changes in the phase space density (PSD) across various energy levels. By analyzing data from the Van Allen Probes and the GOES satellite over 4 years, we observed notable reductions in PSD, particularly at very high‐energy electrons and somewhat lower energy electrons, often referred to as “seed” electrons. These reductions or minima in PSD are important for understanding the dynamics within the radiation belts. The study found that these minima can be explained by interactions with certain types of waves in space, like electromagnetic ion cyclotron or hiss waves. Furthermore, the PSD minima of the seed electrons likely depend on variations in the rate at which hiss waves can scatter electrons at various distances.
Key Points
The deepening Phase Space Density (PSD) minima are noticeably observed at μ < 300 MeV/G, with occurrence reaching 15%
The simulation can produce deepening PSD minima at μ < 60 MeV/G
The modeled seed electron deepening PSD minima are influenced by L‐dependent electron lifetimes due to interaction with hiss waves
We use the Solar, Anomalous, and Magnetospheric Particle Explorer to explore the relationship between microbursts and global flux decay of electrons from the outer Van Allen belt during the recovery ...phase of geomagnetic storms. We investigate the correlation between microbursts and global electron loss in each of the quasi‐trapped (drift loss cone), stably trapped, and untrapped electron (bounce loss cone) populations. For the quasi‐trapped electrons, we separately classify the storms as driven by coronal mass ejections or corotating interaction regions and explore their connection to microburst loss. We find that the decay lifetime of electron fluxes, that is, e‐folding times of macroscopic fluxes in the recovery phase is correlated with strong microburst activity. That is, when the microburst activity is high, global flux decay times are short, and vice versa, suggesting a cross‐scale coupling between microloss and macroloss phenomena. Furthermore, we find that the microburst to global loss coupling is predominant in the quasi‐trapped population of radiation belt electrons while having negligible influence on the untrapped and stably trapped populations. We find that microburst activity during storms driven by coronal mass ejections is coupled more strongly with global flux decay as compared with corotating interaction regions. In addition, we find that distance from the plasmapause is likely a better indicator of microburst location than L‐shell, with most microbursts occurring ~0.5–2.0 L from the model plasmapause location.
While electromagnetic ion cyclotron (EMIC) waves have been long studied as a scattering mechanism for ultrarelativistic (megaelectron volt) electrons via cyclotron‐resonant interactions, these waves ...are also of the right frequency to resonate with the bounce motion of lower‐energy (approximately tens to hundreds of kiloelectron volts) electrons. Here we investigate the effectiveness of this bounce resonance interaction to better determine the effects of EMIC waves on subrelativistic electron populations in Earth's inner magnetosphere. Using wave and plasma parameters directly measured by the Van Allen Probes, we estimate bounce resonance diffusion coefficients for four different events, illustrative of wave and plasma parameters to be encountered in the inner magnetosphere. The range of electron energies and pitch angles affected is examined to better assess the realistic effects of EMIC‐driven bounce resonance on energetic electron populations based on actual, locally observed event‐based parameters. Significant local diffusion coefficients (~ > 10(exp −6) s(exp −1)) for 50‐ to 100‐keV electrons are achieved for both H+ band wave events as well as He+ band, with diffusion coefficients peaking for near‐90° pitch angles but remaining elevated for intermediate ones as well. Diffusion coefficients for higher‐energy 200‐keV electrons are typically multiple orders of magnitude lower (ranging from 10(exp −11) to 10(exp −6) s(exp −1)) and often peak at lower pitch angles (~20–30°). These results suggest that both H+ and He+ band EMIC waves can play a role in shaping lower‐energy electron dynamics via bounce‐resonant interactions, in addition to their role in relativistic electron loss via cyclotron resonance.
Trapped electrons in Earth's outer Van Allen radiation belt are influenced profoundly by solar phenomena such as high-speed solar wind streams, coronal mass ejections (CME), and interplanetary (IP) ...shocks. In particular, strong IP shocks compress the magnetosphere suddenly and result in rapid energization of electrons within minutes. It is believed that the electric fields induced by the rapid change in the geomagnetic field are responsible for the energization. During the latter part of March 2015, a CME impact led to the most powerful geomagnetic storm (minimum Dst = −223 nT at 17 March, 23 UT) observed not only during the Van Allen Probe era but also the entire preceding decade. Magnetospheric response in the outer radiation belt eventually resulted in elevated levels of energized electrons. The CME itself was preceded by a strong IP shock whose immediate effects vis-a-vis electron energization were observed by sensors on board the Van Allen Probes. The comprehensive and high-quality data from the Van Allen Probes enable the determination of the location of the electron injection, timescales, and spectral aspects of the energized electrons. The observations clearly show that ultrarelativistic electrons with energies E greater than 6 MeV were injected deep into the magnetosphere at L approximately equals 3 within about 2 min of the shock impact. However, electrons in the energy range of approximately equals 250 keV to approximately equals 900 keV showed no immediate response to the IP shock. Electric and magnetic fields resulting from the shock-driven compression complete the comprehensive set of observations that provide a full description of the near-instantaneous electron energization.
No instruments in the inner radiation belt are immune from the unforgiving penetration of the highly energetic protons (tens of MeV to GeV). The inner belt proton flux level, however, is relatively ...stable; thus, for any given instrument, the proton contamination often leads to a certain background noise. Measurements from the Relativistic Electron and Proton Telescope integrated little experiment on board Colorado Student Space Weather Experiment CubeSat, in a low Earth orbit, clearly demonstrate that there exist sub‐MeV electrons in the inner belt because their flux level is orders of magnitude higher than the background, while higher‐energy electron (>1.6 MeV) measurements cannot be distinguished from the background. Detailed analysis of high‐quality measurements from the Relativistic Electron and Proton Telescope on board Van Allen Probes, in a geo‐transfer‐like orbit, provides, for the first time, quantified upper limits on MeV electron fluxes in various energy ranges in the inner belt. These upper limits are rather different from flux levels in the AE8 and AE9 models, which were developed based on older data sources. For 1.7, 2.5, and 3.3 MeV electrons, the upper limits are about 1 order of magnitude lower than predicted model fluxes. The implication of this difference is profound in that unless there are extreme solar wind conditions, which have not happened yet since the launch of Van Allen Probes, significant enhancements of MeV electrons do not occur in the inner belt even though such enhancements are commonly seen in the outer belt.
Key Points
Quantified upper limit of MeV electrons in the inner belt
Actual MeV electron intensity likely much lower than the upper limit
More detailed understanding of relativistic electrons in the magnetosphere
We conduct a statistical study on the sudden response of outer radiation belt electrons due to interplanetary (IP) shocks during the Van Allen Probes era, i.e., 2012 to 2015. Data from the ...Relativistic Electron-Proton Telescope instrument on board Van Allen Probes are used to investigate the highly relativistic electron response (E greater than 1.8 MeV) within the first few minutes after shock impact. We investigate the relationship of IP shock parameters, such as Mach number, with the highly relativistic electron response, including spectral properties and radial location of the shock-induced injection. We find that the driving solar wind structure of the shock does not affect occurrence for enhancement events, 25% of IP shocks are associated with prompt energization, and 14% are associated with MeV electron depletion. Parameters that represent IP shock strength are found to correlate best with highest levels of energization, suggesting that shock strength may play a key role in the severity of the enhancements. However, not every shock results in an enhancement, indicating that magnetospheric preconditioning may be required.
Early observations indicated that the Earth's Van Allen radiation belts could be separated into an inner zone dominated by high-energy protons and an outer zone dominated by high-energy electrons. ...Subsequent studies showed that electrons of moderate energy (less than about one megaelectronvolt) often populate both zones, with a deep 'slot' region largely devoid of particles between them. There is a region of dense cold plasma around the Earth known as the plasmasphere, the outer boundary of which is called the plasmapause. The two-belt radiation structure was explained as arising from strong electron interactions with plasmaspheric hiss just inside the plasmapause boundary, with the inner edge of the outer radiation zone corresponding to the minimum plasmapause location. Recent observations have revealed unexpected radiation belt morphology, especially at ultrarelativistic kinetic energies (more than five megaelectronvolts). Here we analyse an extended data set that reveals an exceedingly sharp inner boundary for the ultrarelativistic electrons. Additional, concurrently measured data reveal that this barrier to inward electron radial transport does not arise because of a physical boundary within the Earth's intrinsic magnetic field, and that inward radial diffusion is unlikely to be inhibited by scattering by electromagnetic transmitter wave fields. Rather, we suggest that exceptionally slow natural inward radial diffusion combined with weak, but persistent, wave-particle pitch angle scattering deep inside the Earth's plasmasphere can combine to create an almost impenetrable barrier through which the most energetic Van Allen belt electrons cannot migrate.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Pitch angle distributions (PADs) in the radiation belts are well characterized with sinn(α). By tracking the exponent “n,” termed pitch angle index, we are able to observe persistent and cross‐energy ...changes in PADs of Van Allen radiation belt electrons using Van Allen Probes particle observations. The PAD measurements are well fit down to a single satellite spin, and therefore can track spatially and temporally confined changes to determine the connection between particles and waves. With this method, we study long‐lasting and high‐energy anisotropic electron PADs during a quiet period over 2 days, 26 and 27 June 2013. One potential driver for these changes is electromagnetic ion cyclotron wave interaction with the particles. We use several ground magnetometer stations from Canadian Array for Realtime Investigations of Magnetic Activity and Finnish pulsation magnetometer network of Sodankylä Geophysical Observatory to observe waves during the 2 days of interest. The connection between the waves and particles is inconclusive, although there is some temporal overlap of the phenomenon.
Plain Language Summary
The Van Allen Radiation belts can trap energetic electrons in the magnetic fields surrounding Earth. Often, these particles are studied during time periods of strong geomagnetic activity. However, quiet times can also result in interesting distributions and interactions in the radiation belts. We study one such geomagnetically quiet time where electron pitch angle distributions are suddenly and persistently anisotropic, as observed by instruments on the Van Allen Probes twin satellites. Neither probes observed electromagnetic waves that could cause anisotropic pitch angle distributions, but other satellites and ground measurements indicated wave activity during this time period. The connection between these two phenomenons is inconclusive.
Key Points
Persistent pitch angle distribution anisotropy is observed over 2 days associated with electromagnetic ion cyclotron (EMIC) waves
EMIC waves during geomagnetically quiet periods can have long lasting effect on radiation belts
Multipoint measurements can help pinpoint important mechanisms in radiation belt dynamics
Relativistic electron precipitation into the atmosphere can contribute significant losses to the outer radiation belt. In particular, rapid narrow precipitation features termed precipitation bands ...have been hypothesized to be an integral contributor to relativistic electron precipitation loss, but quantification of their net effect is still needed. Here we investigate precipitation bands as measured at low earth orbit by the Colorado Student Space Weather Experiment (CSSWE) CubeSat. Two precipitation bands of MeV electrons were observed on 18–19 January 2013, concurrent with precipitation seen by the 2013 Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) campaign. The newly available conjugate measurements allow for a detailed estimate of the temporal and spatial features of precipitation bands for the first time. We estimate the net electron loss due to the precipitation bands and find that ~20 such events could empty the entire outer belt. This study suggests that precipitation bands play a critical role in radiation belt losses.
Key Points
MeV e‐precipitation is measured simultaneously by CubeSat and balloons
Conjunctive measurements enable detailed quantification of rapid MeV e‐loss
Precipitation bands contribute significant losses to the outer radiation belt
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