Earth's Electron Radiation Belts Research Articles

Zebra Stripe Patterns in Energetic Ion Spectra at Saturn

AbstractMagnetospheric convective electric fields that could radially transport particles adiabatically, may leave their imprints in the form of energy‐time dispersed patterns termed ”zebra stripes”. Inspired by comprehensive studies on zebra stripes in Earth's electron radiation belts, similar electron signatures were found and explained in Saturn's inner magnetosphere. Using Cassini Magnetosphere Imaging Instrument observations, we demonstrate that the zebra‐stripes in proton and water group ion spectrograms at the energy range from 30 keV to ∼1 MeV, which coexist with electron zebra‐stripes, can be traced to the same electric field, even though the associated radial flows were originally thought to be too slow to disturb the fast azimuthal energetic ion drift. The simultaneous observations of zebra stripes in electrons and ions further establish the importance of the planet's noon‐midnight electric field as a regulator of radiation belt dynamics.

Energy-dependent Boundaries of Earth's Radiation Belt Electron Slot Region

Abstract The variations in radiation belt boundaries reflect competition between acceleration and loss physical processes of energetic electrons, which is an important issue for radiation belts of planets with an internal magnetic field (e.g., Earth, Jupiter, and Saturn). Based on high-quality measurements from Van Allen Probes spanning the years 2014–2018, we develop an empirical model of the energy-dependent boundaries of Earth's electron radiation belt slot region, showing that the lower boundary follows a logarithmic function of the electron energy while the upper boundary is controlled by two competing energy-dependent processes, namely compression and recovery. The compression process relates linearly to a 15 hr averaged Kp index, while the recovery process is found to be approximately proportional to time. Detailed data-model comparisons demonstrate that our model, using only the Kp index and time epoch as inputs, reconstructs the slot region boundaries in real time for 200 keV to 2 MeV electrons under varying geomagnetic conditions. Such a data-driven empirical model is prerequisite to understanding the dynamic changes of the slot region in response to both solar and geomagnetic activities. The model can be readily incorporated into future global simulations of radiation belt electron dynamics in Earth's inner magnetosphere and provide new insights into the study of Saturn's and Jupiter's radiation belt variability.

Empirically Estimated Electron Lifetimes in the Earth's Radiation Belts: Comparison With Theory.

We compute quasilinear diffusion rates due to pitch angle scattering by various mechanisms in the Earth's electron radiation belts. The calculated theoretical lifetimes are compared with observed decay rates, and we find excellent qualitative agreement between the two. The overall structure of the observed lifetime profiles as a function of energy and L is largely due to plasmaspheric hiss and Coulomb scattering. The results also reveal a local minimum in lifetimes in the inner zone at lower energy ( ∼50 keV), attributed to enhanced scattering via ground‐based very low frequency transmitters, and a reduction in lifetimes at higher L and energy ( >1 MeV), attributed to enhanced electromagnetic ion cyclotron wave scattering. In addition, we find significant quantitative disagreement at L<3.5, where the theoretical lifetimes are typically a factor of ∼10 larger than the observed, pointing to an additional loss process that is missing from current models. We discuss potential factors that could contribute to this disagreement.

Observations and Fokker‐Planck Simulations of the L‐Shell, Energy, and Pitch Angle Structure of Earth's Electron Radiation Belts During Quiet Times

AbstractThe evolution of the radiation belts in L‐shell (L), energy (E), and equatorial pitch angle (α 0) is analyzed during the calm 11‐day interval (4–15 March) following the 1 March 2013 storm. Magnetic Electron and Ion Spectrometer (MagEIS) observations from Van Allen Probes are interpreted alongside 1D and 3D Fokker‐Planck simulations combined with consistent event‐driven scattering modeling from whistler mode hiss waves. Three (L, E, α 0) regions persist through 11 days of hiss wave scattering; the pitch angle‐dependent inner belt core (L ~ &lt;2.2 and E &lt; 700 keV), pitch angle homogeneous outer belt low‐energy core (L &gt; ~5 and E~ &lt; 100 keV), and a distinct pocket of electrons (L ~ [4.5, 5.5] and E ~ [0.7, 2] MeV). The pitch angle homogeneous outer belt is explained by the diffusion coefficients that are roughly constant for α 0 ~ &lt;60°, E &gt; 100 keV, 3.5 &lt; L &lt; L pp ~ 6. Thus, observed unidirectional flux decays can be used to estimate local pitch angle diffusion rates in that region. Top‐hat distributions are computed and observed at L ~ 3–3.5 and E = 100–300 keV.

The Response of Earth's Electron Radiation Belts to Geomagnetic Storms: Statistics From the Van Allen Probes Era Including Effects From Different Storm Drivers

AbstractA statistical study was conducted of Earth's radiation belt electron response to geomagnetic storms using NASA's Van Allen Probes mission. Data for electrons with energies ranging from 30 keV to 6.3 MeV were included and examined as a function of L‐shell, energy, and epoch time during 110 storms with SYM‐H ≤−50 nT during September 2012 to September 2017 (inclusive). The radiation belt response revealed clear energy and L‐shell dependencies, with tens of keV electrons enhanced at all L‐shells (2.5 ≤ L ≤ 6) in all storms during the storm commencement and main phase and then quickly decaying away during the early recovery phase, low hundreds of keV electrons enhanced at lower L‐shells (~3 ≤ L ≤ ~4) in upward of 90% of all storms and then decaying gradually during the recovery phase, and relativistic electrons throughout the outer belt showing main phase dropouts with subsequent and generally unpredictable levels of replenishment during the recovery phase. Compared to prestorm levels, electrons with energies &gt;1 MeV also revealed a marked increase in likelihood of a depletion at all L‐shells through the outer belt (3.5 ≤ L ≤ 6). Additional statistics were compiled revealing the storm time morphology of the radiation belts, confirming the aforementioned qualitative behavior. Considering storm drivers in the solar wind: storms driven by coronal mass ejection (CME) shocks/sheaths and CME ejecta only are most likely to result in a depletion of &gt;1‐MeV electrons throughout the outer belt, while storms driven by full CMEs and stream interaction regions are most likely to produce an enhancement of MeV electrons at lower (L &lt; ~5) and higher (L &gt; ~4.5) L‐shells, respectively. CME sheaths intriguingly result in a distinct enhancement of ~1‐MeV electrons around L~5.5, and on average, CME sheaths and stream interaction regions result in double outer belt structures.

Reproducing the observed energy‐dependent structure of Earth's electron radiation belts during storm recovery with an event‐specific diffusion model

AbstractWe present dynamic simulations of energy‐dependent losses in the radiation belt “slot region” and the formation of the two‐belt structure for the quiet days after the 1 March storm. The simulations combine radial diffusion with a realistic scattering model, based data‐driven spatially and temporally resolved whistler‐mode hiss wave observations from the Van Allen Probes satellites. The simulations reproduce Van Allen Probes observations for all energies and L shells (2–6) including (a) the strong energy dependence to the radiation belt dynamics (b) an energy‐dependent outer boundary to the inner zone that extends to higher L shells at lower energies and (c) an “S‐shaped” energy‐dependent inner boundary to the outer zone that results from the competition between diffusive radial transport and losses. We find that the characteristic energy‐dependent structure of the radiation belts and slot region is dynamic and can be formed gradually in ~15 days, although the “S shape” can also be reproduced by assuming equilibrium conditions. The highest‐energy electrons (E &gt; 300 keV) of the inner region of the outer belt (L ~ 4–5) also constantly decay, demonstrating that hiss wave scattering affects the outer belt during times of extended plasmasphere. Through these simulations, we explain the full structure in energy and L shell of the belts and the slot formation by hiss scattering during storm recovery. We show the power and complexity of looking dynamically at the effects over all energies and L shells and the need for using data‐driven and event‐specific conditions.

Electron acceleration associated with the magnetic flux pileup regions in the near‐Earth plasma sheet: A multicase study

AbstractUsing the Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations, we study electron acceleration (&lt;30 keV) in the magnetic flux pileup regions (FPRs) in the near‐Earth plasma sheet (X ~ –10 RE). We present three cases of FRPs associated with dipolarization fronts and substorm dipolarization. Based on the characteristics of the magnetic field, we defined the magnetic field enhancement region (MFER) as the magnetic field with significant ramp that is usually observed near the dipolarization front boundary layer. On the other side, the increased magnetic field without a significant ramp is the rest of a FPR. Our results show that betatron acceleration dominates for 10–30 keV electrons inside the MFER, whereas Fermi acceleration dominates for 10–30 keV electrons inside the rest of the FPR. Betatron acceleration is caused by the enhancement of the local magnetic field, whereas Fermi acceleration is related to the shrinking length of magnetic field line. These accelerated electrons inside the FPRs in the near‐Earth tail play a potentially important role in the evolution of the Earth's electron radiation belt and substorms.

Space weather impacts on satellites and forecasting the Earth's electron radiation belts with SPACECAST

Satellites can be damaged by high energy charged particles in the Earth's radiation belts and during solar energetic particle (SEP) events. Here we review the growing reliance on satellite services, new vulnerabilities to space weather, and previous events that have led to loss of service. We describe a new European system to forecast the radiation belts up to 3 h ahead, which has three unique features: first, it uses physics‐based models, which include wave‐particle interactions; second, it provides a forecast for the whole outer radiation belt including geostationary, medium, and slot region orbits; third, it is a truly international effort including Europe, United States, and Japan. During the 8–9 March 2012 storm and SEP event, the models were able to forecast the &gt;800 keV electron flux to within a factor of 2 initially, and later to within a factor of 10 of the GOES data. Although ACE and GOES data became unreliable during the SEP event, the system continued forecasting without interruption using ground‐based magnetometers. A forecast of the 24 h electron fluence &gt;2 MeV is used to provide a risk index for satellite operators. We show that including wave‐particle interactions for L* &gt; 6.5 improves the agreement with GOES data substantially and that a fast inward motion of the magnetopause to L* &lt; 8 is related to rapid loss of relativistic electrons at geostationary orbit. Thus, we suggest that better wave‐particle models and better coupling between the solar wind and the models of the magnetopause and radiation belts should lead to better forecasting.

Modeling the evolution of chorus waves into plasmaspheric hiss

[1] Plasmaspheric hiss (PH) is a band-limited, incoherent whistler mode emission found predominantly in the plasmasphere or high-density plasma regions in the near-Earth space environment. Since its discovery in the late 1960s, PH has been recognized as playing an important role in shaping the structure and dynamics of the Earth's electron radiation belts and creating the slot region that separates the inner and outer belts. However, the origin of PH has been a topic of intense debate for over four decades. Here we present a model for the origin of PH that involves the evolution of chorus waves into the PH spectrum. We perform extensive ray tracing using the HOTRAY code and calculate Landau damping using newly developed suprathermal flux maps from THEMIS observations, that are L and magnetic local time dependent, for both inside and outside the plasmasphere. Our results show remarkable consistency with the observed statistical characteristics of hiss, including the day/night asymmetry in wave power, frequency spectrum, geomagnetic control of PH, quasi-parallel equatorial wave normal angles, and confinement within the plasmasphere. Our model also reproduces ancillary features such as exohiss and extremely low frequency (ELF) hiss and might be related to a previously reported phenomenon called lower hybrid resonance duct trapping in the ionosphere. A detailed analysis of ray morphologies shows a separation into four distinct groups, which correspond to (1) rays that are trapped at the plasmapause, (2) PH rays, (3) ELF hiss rays, and (4) rays that represent the bulk of the chorus ray power.

Magnetospheric VLF response to the atmospheric infrasonic waves

We consider the influence of atmospheric infrasonic waves on VLF whistler features in the Earth electron radiation belts. An infrasonic wave produces variations of plasma concentration in the ionosphere and VLF wave coefficient of reflection from the ionosphere. The coefficient of whistler reflection and its variation were calculated both analytically for simplified models and numerically for more realistic models of ionospheric altitude electron distributions. The frequency of infrasonic waves is close to the eigen-frequency of the wave - particle interaction processes in the radiation belts. Hence the magnetospheric resonator Q-factor modulation can switch the stationary VLF electromagnetic emission generation mode (hiss emissions) to a quasi-periodic or self-oscillation mode. The spatio-temporal processes in a disturbed magnetosphere were studied by an original code. The obtained results can explain the causes of the modulation of energetic electron precipitation fluxes and whistler wave intensity with a time scale of 10 to 100 sec in the morning and in the forenoon part of the radiation belts. Such periodical and quasiperiodical VLF emissions are often observed in the subauroral and auroral Earth's magnetosphere.

Model of the slot region of earth's electron radiation belt depending on the heliospheric parameters

Relativistic electrons in the slot region of Earth's electron radiation belt are studied using CORONAS-I satellite data obtained in March–May 1994. The strong shifts of the slot latitudinal location (from L≈3 to L≈2) were found. These shifts are associated with the Earth crossing sector boundaries formed by sector corotating structures of the solar wind and interplanetary magnetic field (IMF). The quantitative analysis of the relationship between properties of the slot region and values of the solar wind and IMF parameters was undertaken. The empirical model of the slot region dynamics was developed by means of Artificial Neural Network (ANN).

The outer boundary of the Earth's electron radiation belt: Dependence upon L, energy, and equatorial pitch angle

From near‐equatorial pitch angle measurements made with fine angular resolution on the CRRES and SCATHA satellites an investigation has been made of the location of the nightside outer boundary of the Earth's electron radiation belt and its dependence upon energy and mirror point altitude. The energy versus L dependence of the position of the trapping boundary is important for comparisons with models of the mechanisms to explain the loss of trapping at the outer edge of the radiation belt. Often the satellites passed through the trapping boundary very slowly, permitting detailed fine‐scale measurements. With fine spatial resolution the cutoff L values were frequently found to be almost independent of energy for E ≤ 300 keV and were nearly independent of equatorial pitch angle up to 30°. In the radiation belt near the outer edge sometimes the flux dropped to only intermediate values or to background level just for short times or equivalently narrow L intervals, suggesting time variations or the irregular onset of the mechanism(s) responsible for the trapping boundary.