Relativistic Electron Precipitation Research Articles

Observations of Relativistic Electron Precipitation Due To Combined Scattering of Whistler‐Mode and EMIC Waves

AbstractThe two most important wave modes responsible for energetic electron scattering to the Earth's ionosphere are electromagnetic ion cyclotron (EMIC) waves and whistler‐mode waves. These wave modes operate in different energy ranges: whistler‐mode waves are mostly effective in scattering sub‐relativistic electrons, whereas EMIC waves predominately scatter relativistic electrons. In this study, we report the direct observations of energetic electron (from 50 keV to 2.5 MeV) scattering driven by the combined effect of whistler‐mode and EMIC waves using ELFIN measurements. We analyze five events showing EMIC‐driven relativistic electron precipitation accompanied by bursts of whistler‐driven precipitation over a wide energy range. These events reveal an enhancement of relativistic electron precipitation by EMIC waves during intervals of whistler‐mode precipitation compared to intervals of EMIC‐only precipitation. We discuss a possible mechanism responsible for such precipitation. We suggest that below the minimum resonance energy (Emin) of EMIC waves, the whistler‐mode wave may both scatter electrons into the loss‐cone and accelerate them to higher energy (1–3 MeV). Electrons accelerated above Emin resonate with EMIC waves that, in turn, quickly scatter those electrons into the loss‐cone. This enhances relativistic electron precipitation beyond what EMIC waves alone could achieve. We present theoretical support for this mechanism, along with observational evidence from the ELFIN mission. We discuss methodologies for further observational investigations of this combined whistler‐mode and EMIC precipitation.

Relativistic Electron Precipitation Events Driven by Solar Wind Impact on the Earth's Magnetosphere

AbstractCertain forms of solar wind transients contain significant enhancements of dynamic pressure and may effectively drive magnetosphere dynamics, including substorms and storms. An integral element of such driving is the generation of a wide range of electromagnetic waves within the inner magnetosphere, either by compressionally heated plasma or by substorm plasma sheet injections. Consequently, solar wind transient impacts are traditionally associated with energetic electron scattering and losses into the atmosphere by electromagnetic waves. In this study, we show the first direct measurements of two such transient‐driven precipitation events as measured by the low‐altitude Electron Losses and Fields Investigation CubeSats. The first event demonstrates storm‐time generated electromagnetic ion cyclotron waves efficiently precipitating sub‐relativistic and relativistic electrons from >300 keV to 2 MeV at the duskside. The second event demonstrates whistler‐mode waves leading to scattering of electrons from 50 to 700 keV on the dawnside. These observations confirm the importance of solar wind transients in driving energetic electron losses and subsequent dynamics in the ionosphere.

Characterization of Relativistic Electron Precipitation Events Observed by the CALET Experiment Using Self‐Organizing‐Maps

AbstractRelativistic electron precipitation (REP) is a relatively high‐latitude phenomenon where high‐energy electrons trapped in the outer radiation belt are lost into the Earth’s atmosphere. REP events observed at low Earth orbit show varying temporal profiles and global distributions. While the precipitation origin has been attributed to specific wave modes or scattering sources, the sorting of REP events by type or driver remains an unsolved challenge. In this study, we analyze the temporal profile of relativistic electron precipitation events observed by the CALorimetric Electron Telescope (CALET) experiment on board the International Space Station. We use an unsupervised machine learning technique called Self‐Organizing‐Maps (SOM) to automatically detect and then classify relativistic electron events observed by the two scintillator layers at the top of the apparatus, sensitive to electrons with energies >1.5 MeV and >3.4 MeV, respectively. We calculate the power spectral density (PSD) of the count rates observed by both sensors and use them as an input for the SOM. The SOM technique groups the PSDs by their similarity, resulting in a classification of relativistic electron events by the periodicity of the observed precipitation. We investigate the L‐shell and magnetic local time distribution of the resulting classification, and energy spectral index associated with the observations. Clear precipitation patterns are observed and compared to past precipitation categorization attempts as well as known distributions of various scattering mechanisms. The classification reveals features through the sorting of the variability of the rapid precipitation, allowing the identification of different precipitation populations with varying properties.

Key Factors Determining Nightside Energetic Electron Losses Driven by Whistler‐Mode Waves

AbstractEnergetic electron losses by pitch‐angle scattering and precipitation to the atmosphere from the radiation belts are controlled, to a great extent, by resonant wave particle interactions with whistler‐mode waves. The efficacy of such precipitation is primarily modulated by wave intensity, although its relative importance, compared to other wave and plasma parameters, remains unclear. Precipitation spectra from the low‐altitude, polar‐orbiting ELFIN mission have previously been demonstrated to be consistent with energetic precipitation modeling derived from empirical models of field‐aligned wave power across a wide swath of local‐time sectors. However, such modeling could not explain the intense, relativistic electron precipitation observed on the nightside. Therefore, this study aims to additionally consider the contributions of three modifications—wave obliquity, frequency spectrum, and local plasma density—to explain this discrepancy on the nightside. By incorporating these effects into both test particle simulations and quasi‐linear diffusion modeling, we find that realistic implementations of each individual modification result in only slight changes to the electron precipitation spectrum. However, these modifications, when combined, enable more accurate modeling of ELFIN‐observed spectra. In particular, a significant reduction in plasma density enables lower frequency waves, oblique, or even quasi field‐aligned waves to resonate with near ∼1 MeV electrons closer to the equator. We demonstrate that the levels of modification required to accurately reproduce the nightside spectra of whistler‐mode wave‐driven relativistic electron precipitation match empirical expectations and should therefore be included in future radiation belt modeling.

On the Spatial and Temporal Evolution of EMIC Wave‐Driven Relativistic Electron Precipitation: Magnetically Conjugate Observations From the Van Allen Probes and CALET

AbstractElectromagnetic ion cyclotron (EMIC) waves have been shown to be able to drive strong electron precipitation, particularly at MeV energies. However, the spatio‐temporal evolution of both the waves and the resulting precipitation is still not well understood. Here we investigate the evolution of relativistic electron precipitation driven by EMIC waves through combined observations from the Van Allen Probes and the CALorimetric Electron Telescope experiment onboard the International Space Station. Two case studies are examined where EMIC waves near the magnetic equator and precipitation at low altitude were detected in close magnetic conjunction, both of which were confined to narrow radial regions but persisted multiple hours. These observations, combined with quasilinear calculations, confirm that long‐lived EMIC waves can drive hours‐long MeV electron precipitation loss. However, the magnitude of the precipitation varied significantly during one of the events, as resonance conditions, particularly plasma density, evolved.

Energetic Particle Precipitation in Sub‐Auroral Polarization Streams

AbstractSub‐auroral polarization streams (SAPS) are one of the most intense manifestations of magnetosphere‐ionosphere coupling. Magnetospheric energy transport to the ionosphere within SAPS is associated with Poynting flux and the precipitation of thermal energy (0.03–30 keV) plasma sheet particles. However, much less is known about the precipitation of high‐energy (≥50 keV) ions and electrons and their contribution to the low‐altitude SAPS physics. This study examines precipitation within one SAPS event using a combination of equatorial THEMIS and low‐altitude DMSP and ELFIN observations, which, jointly, cover from a few eV up to a few MeV energy range. Observed SAPS are embedding the ion isotropy boundary, which includes strong 300–1,000 keV ion precipitation. SAPS are associated with intense precipitation of relativistic electrons (≤3 MeV), well equatorward of the electron isotropy boundary. Such relativistic electron precipitation is likely due to electron scattering by electromagnetic ion cyclotron waves at the equator.

Relativistic Electron Precipitation Driven by Mesoscale Transients, Inferred From Ground and Multi‐Spacecraft Platforms

AbstractPrecipitation of relativistic electrons into the Earth's atmosphere regulates the outer radiation belt fluxes and contributes to magnetosphere‐atmosphere coupling. One of the main drivers of such precipitation is electron scattering by whistler‐mode waves. Such waves typically originate at the equator, where they can resonate with and scatter sub‐relativistic (tens to a few hundred keV) electrons. However, they can occasionally propagate far away from the equator along field lines, reaching middle latitudes, where they can resonate with and scatter relativistic (>500 keV) electrons. Such a propagation is typical for the dayside, but statistically has not been found on the nightside where the waves are quickly damped along their propagation due to Landau damping. Here we explore two events of relativistic electron precipitation from low‐altitude observations on the nightside. Combining measurements of whistler‐mode waves from ground observatories, relativistic electron precipitation from low‐altitude satellites, total electron content maps from GPS receivers, and magnetic field and electron flux from equatorial satellites, we show wave ducting by plasma density gradients is the possible channel that allows the waves to reach middle latitudes and scatter relativistic electrons. We suggest that both whistler‐mode wave generation and ducting can be driven by equatorial mesoscale (with spatial scales of about one Earth radius) transient structures during nightside injections. We also compare these nightside events with observations of ducted waves and relativistic electron precipitation at the dayside, where wave generation and ducting are driven by ultra‐low‐frequency waves. This study demonstrates the potential importance of mesoscale transients in relativistic electron precipitation, but does not however unequivocally establish that ducted whistler‐mode waves are the primary cause of the observed electron precipitation.

Studying the Impact of the Geospace Environment on Solar Lithosphere Coupling and Earthquake Activity

In solar–terrestrial physics, there is an open question: does a geomagnetic storm affect earthquakes? We expand research in this direction, analyzing the seismic situation after geomagnetic storms (GMs) accompanied by the precipitation of relativistic electrons from the outer radiation belt to form an additional radiation belt (RB) around lower geomagnetic lines. We consider four widely discussed cases in the literature for long-lived (weeks, months) RBs due to GMs and revealed that the 1/GMs 24 March 1991 with a new RB at L~2.6 was followed by an M7.0 earthquake in Alaska, 30 May 1991, near footprint L = 2.69; the 2/GMs 29 October 2003 (Ap = 204) with new RB first in the slot region at L = 2–2.5 cases followed by an M7.8 earthquake on 17 November 2003 at the Aleutian Islands near footprint L = 2.1, and after forming an RB at L~1.5 which lasted for ~26 months, two mega quakes, M9.1 in 2004 and M8.6 in 2005, occurred at the globe; the 3/GMs 3 September 2012 with a new RB at L= 3.0–3.5 was followed by an M7.8 earthquake in Canada near footprint L = 3.2; and the 4/GMs 21 June 2015 with a new RB at L = 1.5–1.8 was followed by an M6.3 earthquake on 7 September 2015 in New Zealand, near footprint L = 1.58. The obtained results suggest that (1) major earthquakes occur near the footprints of geomagnetic lines filled with relativistic electrons precipitating from the outer radiation belt due to geomagnetic storms, and (2) the time delay between geomagnetic storm onset and earthquake occurrence may vary from several weeks to several months. The results may expand the framework for developing mathematical magnetosphere–ionosphere coupling models.

Dispersed Relativistic Electron Precipitation Patterns Between the Ion and Electron Isotropy Boundaries

AbstractRelativistic electron precipitation to the Earth's atmosphere is an important loss mechanism of inner magnetosphere electrons, contributing significantly to the dynamics of the radiation belts. Such precipitation may be driven by electron resonant scattering by middle‐latitude whistler‐mode waves at dawn to noon; by electromagnetic ion cyclotron (EMIC) waves at dusk; or by curvature scattering at the isotropy boundary (at the inner edge of the electron plasma sheet anywhere on the nightside, from dusk to dawn). Using low‐altitude ELFIN and near‐equatorial THEMIS measurements, we report on a new type of relativistic electron precipitation that shares some properties with the traditional curvature scattering mechanism (occurring on the nightside and often having a clear energy/L‐shell dispersion). However, it is less common than the typical electron isotropy boundary and it is observed most often during substorms. It is seen equatorward of (and well separated from) the electron isotropy boundary and around or poleward of the ion isotropy boundary (the inner edge of the ion plasma sheet). It may be due to one or more of the following mechanisms: EMIC waves in the presence of a specific radial profile of the cold plasma density; a regional suppression of the magnetic field enhancing curvature scattering locally; and/or electron resonant scattering by kinetic Alfvén waves.

Ionospheric Plasma Density Gradients Associated With Night‐Side Energetic Electron Precipitation

AbstractEnergetic electron precipitation from the equatorial magnetosphere into the atmosphere plays an important role in magnetosphere‐ionosphere coupling: precipitating electrons alter ionospheric properties, whereas ionospheric outflows modify equatorial plasma conditions affecting electromagnetic wave generation and energetic electron scattering. However, ionospheric measurements cannot be directly related to wave and energetic electron properties measured by high‐altitude, near‐equatorial spacecraft, due to large mapping uncertainties. We aim to resolve this by projecting low‐altitude measurements of energetic electron precipitation by ELFIN CubeSats onto total electron content (TEC) maps serving as a proxy for ionospheric density structures. We examine three types of precipitation on the nightside: precipitation of <200 keV electrons in the plasma sheet, bursty precipitation of <500 keV electrons by whistler‐mode waves, and relativistic (>500 keV) electron precipitation by EMIC waves. All three types of precipitation show distinct features in TEC horizontal gradients, and we discuss possible implications of these features.

Full-wave modeling of EMIC wave packets: ducted propagation and reflected waves

Electromagnetic ion cyclotron (EMIC) waves can scatter radiation belt electrons with energies of a few hundred keV and higher. To accurately predict this scattering and the resulting precipitation of these relativistic electrons on short time scales, we need detailed knowledge of the wave field’s spatio-temporal evolution, which cannot be obtained from single spacecraft measurements. Our study presents EMIC wave models obtained from two-dimensional (2D) finite-difference time-domain (FDTD) simulations in the Earth’s dipole magnetic field. We study cases of hydrogen band and helium band wave propagation, rising-tone emissions, packets with amplitude modulations, and ducted waves. We analyze the wave propagation properties in the time domain, enabling comparison with in situ observations. We show that cold plasma density gradients can keep the wave vector quasiparallel, guide the wave energy efficiently, and have a profound effect on mode conversion and reflections. The wave normal angle of unducted waves increases rapidly with latitude, resulting in reflection on the ion hybrid frequency, which prohibits propagation to low altitudes. The modeled wave fields can serve as an input for test-particle analysis of scattering and precipitation of relativistic electrons and energetic ions.

Investigating Whistler‐Mode Wave Intensity Along Field Lines Using Electron Precipitation Measurements

AbstractElectron fluxes in Earth's radiation belts are significantly affected by their resonant interaction with whistler‐mode waves. This wave‐particle interaction often occurs via first cyclotron resonance and, when intense and nonlinear, can accelerate subrelativistic electrons to relativistic energies while also scattering them into the atmospheric loss cone. Here, we model Electron Losses and Fields INvestgation’s (ELFIN) low‐altitude satellite measurements of precipitating electron spectra with a wave‐electron interaction model to infer the profiles of whistler‐mode intensity along magnetic latitude assuming realistic waveforms and statistical models of plasma density. We then compare these profiles with a wave power spatial distribution along field lines from an empirical model. We find that this empirical model is consistent with observations of subrelativistic (<200 keV) electron precipitation events, but deviates significantly for relativistic (>200 keV) electron precipitation events at all MLTs, especially on the nightside. This may be due to the sparse coverage of wave measurements at mid‐to‐high latitudes which causes statistically averaged wave power to be likely underestimated in current empirical wave models. As a result, this discrepancy suggests that intense waves likely do propagate to higher latitudes, although further investigation is required to quantify how well this high‐latitude population can account for the observed relativistic electron precipitation.

Влияние высыпающихся релятивистских электронов на характеристики сигналов системы РСДН-20 во время события 28 марта 2017

Based on the experimental data of the EISCAT radar and numerical modeling methods, the influence of the fluxes of subrelativistic (relativistic) electrons of the Earth's outer radiation belt on the operation of the RSDN-20 long-range navigation system operating at frequencies of 11.905 kHz, 12.649 kHz and 14.881 kHz has been studied. As a result of computational experiments, a slight decrease in attenuation under perturbed conditions was obtained at a frequency of 14.881 kHz with a slight, less than an order of magnitude increase in the electron concentration in the D- and E-layers of the ionosphere. At lower frequencies and in the case of an increase in concentration in the reflection region, an increase in signal attenuation is observed. It is shown that the violation of monotonicity in the frequency dependence of signal attenuation is associated with the position of the boundary between regions with different types of signal attenuation and the height of the signal reflection from the ionosphere. In the case when the intersection point of the high-altitude profile of the electron collision frequency υe with the profile of the doubled plasma frequency 2ωe is inside the signal reflection region, a nonlinear type of attenuation dependence on frequency is observed. The phases of RSDN-20 signals are weakly affected by the precipitation of relativistic energy electrons into the atmosphere. A comparison of the amplitudes of the signals obtained in computational experiments and the signals recorded at the observatories of the Polar Geophysical Institute Lovozero and Barentsburg showed partial agreement in most of the changes recorded during the precipitation of subrelativistic electrons in the polar ionosphere. The presented estimates can be used on short radio paths of 100–200 km in the high latitude region to monitor the state of the ionosphere.

Prompt Appearance of Large‐Amplitude EMIC Waves Induced by Solar Wind Dynamic Pressure Enhancement and the Subsequent Relativistic Electron Precipitation

AbstractElectromagnetic ion cyclotron (EMIC) waves play an important role in relativistic electron dynamics. In this study, we find a large‐amplitude EMIC wave event induced by the prompt enhancement of solar wind dynamic pressure on 6 November 2015. These large‐amplitude EMIC waves are simultaneously observed by multiple satellites over 13 hr in magnetic local time (MLT) with a peak amplitude of ∼4 nT. Satellites at different locations observed different bands of EMIC waves, implying the importance of background plasma density in EMIC wave generation. Electron pitch angle distributions show obvious responses to EMIC wave activities. During EMIC wave appearance, the fluxes of relativistic electrons with pitch angles around 90° increase, while the fluxes of field‐aligned relativistic electrons decrease, showing distinct “bite‐out” signatures, indicating pitch angle scattering by EMIC waves, and the scattering efficiency depends on the amplitude and polarization of EMIC waves. Combined with phase space density profiles of electrons that are nearly constant at energies below the minimum resonant energy of electrons (Emin) but show dropout at energies above the Emin after EMIC wave activities, we conclude that large‐amplitude EMIC waves can cause rapid electron loss down to several hundred keV. In addition, simultaneous observations of hundreds of keV electron precipitation and tens of keV proton precipitation by Polar Operational Environmental Satellites near the region where EMIC waves are observed, provide direct evidence of relativistic electron precipitation caused by the large‐amplitude EMIC waves, ultimately driven by solar wind structures.

Time‐Dependent Inversion of Energetic Electron Precipitation Spectra From Ground‐Based Incoherent Scatter Radar Measurements

AbstractIn the D‐region, the ionization rate cannot be detected directly with any known measurement technique, therefore it must be estimated. Starting from space‐based measurements of precipitating particle flux, we estimate the ionization rate in the atmosphere using the Electron Precipitation Monte Carlo transport method. This ionization rate is used to calculate the expected electron density in the D‐region with the Glukhov‐Pasko‐Inan five species (GPI5) atmospheric chemistry model. We then compare the simulated electron density with that measured by the Poker Flat Incoherent Scatter Radar (PFISR). From ground‐based radar measurements of electron density enhancements due to sub‐relativistic and relativistic electron precipitation, we present a method to extract the ionization rate altitude profiles using inverse theory. We use this estimation of ionization rate to find the energy distribution of the precipitating particles. With this inverse method, we are able to link ground measurements of electron density to the precipitating flux in a time dependent manner and with uncertainty in the inverted parameters. The method was tested on synthetic data and applied to specific PFISR data sets. The method is able to retrieve the ionization rate altitude profiles that, when forward modeled, return the expected electron densities within ∼7% error as compared to the PFISR data. For the case presented here, the arbitrary energy distribution inversion results are comparable in magnitude and shape to those presented in Turunen et al. (2016, https://doi.org/10.1002/2016jd025015) for the inversion of a single event of pulsating aurora observed by EISCAT.

PHYSICAL EFFECTS OF THE POWERFUL TONGA VOLCANO EXPLOSION IN THE EARTH – ATMOSPHERE – IONOSPHERE – MAGNETOSPHERE SYSTEM ON JANUARY 15, 2022

PHYSICAL EFFECTS OF THE POWERFUL TONGA VOLCANO EXPLOSION IN THE EARTH – ATMOSPHERE – IONOSPHERE – MAGNETOSPHERE SYSTEM ON JANUARY 15, 2022

Pitch Angle Isotropy of Relativistic Electron Microbursts as Observed by SAMPEX/HILT: Statistical and Storm‐Time Properties

AbstractObservations of relativistic electron precipitation events from Earth's inner magnetosphere to the upper atmosphere provide essential information about the source/loss processes and dynamics of Earth's radiation belts. Equatorial electron pitch angle isotropy can be an indicator of electron precipitation into the atmosphere, with isotropic populations resulting in a full bounce loss cone (BLC) and anisotropic populations resulting in an emptier BLC. Here we study electron microbursts, which are ˜100 millisecond duration streams of relativistic electrons thought to be scattered into the BLC by particle‐wave scattering. By measuring the ratios of >1 MeV electron flux across different detector rows in the Heavy Ion Large Telescope (HILT) instrument onboard the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite, we quantify the pitch angle isotropy of an electron population. Applying this calculation to microburst events, we have determined the dependence of microburst pitch angle distributions on burst magnitude, magnetic local time (MLT), L‐Shell, and storm phase. We found that high flux magnitude microbursts are typically highly isotropic, while lower flux magnitude microbursts have varying degrees of anisotropy. Furthermore, microbursts are most isotropic as compared to the background population in the afternoon/evening sector as well as initial and main phases of geomagnetic storms. These results have important implications for microburst scattering mechanisms as well as quantification of precipitation driven by this process.

Localized mesospheric ozone destruction corresponding to isolated proton aurora coming from Earth’s radiation belt

Relativistic electron precipitation (REP) from the Earth’s radiation belt plays an important role in mesospheric ozone loss as a connection between space weather and the climate system. However, the rapid (tens of minutes) destruction of mesospheric ozone directly caused by REP has remained poorly understood due to the difficulty of recognizing its location and duration. Here we show a compelling rapid correspondence between localized REP and ozone destruction during a specific auroral phenomenon, the called an isolated proton aurora (IPA). The IPA from the Earth’s radiation belt becomes an important spatial and temporal proxy of REP, distinct from other auroral phenomena, and allowing visualizing micro-ozone holes. We found ozone destruction of as much as 10–60% within 1.5 h of the initiation of IPA. Electromagnetic ion cyclotron waves in the oxygen ion band observed as the driver of REP likely affect through resonance with mainly ultra-relativistic (> 2 mega-electron-volts) energy electrons. The rapid REP impact demonstrates its crucial role and direct effect on regulating the atmospheric chemical balance.

Nonresonant Scattering of Relativistic Electrons by Electromagnetic Ion Cyclotron Waves in Earth's Radiation Belts.

Electromagnetic ion cyclotron waves are expected to pitch-angle scatter and cause atmospheric precipitation of relativistic (>1 MeV) electrons under typical conditions in Earth's radiation belts. However, it has been a long-standing mystery how relativistic electrons in the hundreds of keV range (but <1 MeV), which are not resonant with these waves, precipitate simultaneously with those >1 MeV. We demonstrate that, when the wave packets are short, nonresonant interactions enable such scattering of hundred-keV electrons by introducing a spread in wave number space. We generalize the quasilinear diffusion model to include nonresonant effects. The resultant model exhibits an exponential decay of the scattering rates extending below the minimum resonant energy depending on the shortness of the wave packets. This generalized model naturally explains observed nonresonant electron precipitation in the hundreds of keV concurrent with >1 MeV precipitation.

Relativistic Electron Precipitation by EMIC Waves: Importance of Nonlinear Resonant Effects

AbstractRelativistic electron losses in Earth's radiation belts are usually attributed to electron resonant scattering by electromagnetic waves. One of the most important wave modes for such scattering is the electromagnetic ion cyclotron (EMIC) mode. Within the quasi‐linear diffusion framework, the cyclotron resonance of relativistic electrons with EMIC waves results in very fast electron precipitation to the atmosphere. However, wave intensities often exceed the threshold for nonlinear resonant interaction, and such intense EMIC waves have been shown to transport electrons away from the loss cone due to the force bunching effect. In this study we investigate if this transport can block electron precipitation. We combine test particle simulations, low‐altitude observations of EMIC‐driven electron precipitation by the Electron Losses and Fields Investigations mission, and ground‐based EMIC observations. Comparing simulations and observations, we show that, despite the low pitch‐angle electrons being transported away from the loss cone, the scattering at higher pitch angles results in the loss cone filling and electron precipitation.