An official website of the United States government
Abstract Deep penetration of energetic electrons (10s–100s of keV) to lowL‐shells (L < 4), as an important source of inner belt electrons, is commonly observed during geomagnetically active times. However, such deep penetration is not observed as frequently for similar energy protons, for which underlying mechanisms are not fully understood. To study their differential deep penetration, we conducted a statistical analysis using phase space densities (PSDs) ofµ = 10–50 MeV/G,K = 0.14 G1/2Re electrons and protons from multiyear Van Allen Probes observations. The results suggest systematic differences in electron and proton deep penetration: electron PSD enhancements at lowL‐shells occur more frequently, deeply, and faster than protons. Forµ = 10–50 MeV/G electrons, the occurrence rate of deep penetration events (defined as daily‐averaged PSD enhanced by at least a factor of 2 within a day atL < 4) is ∼2–3 events/month. For protons, only ∼1 event/month was observed forµ = 10 MeV/G, and much fewer events were identified forµ > 20 MeV/G. Leveraging dual‐Probe configurations, fast electron deep penetrations atL < 4 are revealed: ∼70% of electron deep penetration events occurred within ∼9 hr; ∼8%–13% occurred even within 3 hr, with lower‐µelectrons penetrating faster than higher‐µelectrons. These results suggest nondiffusive radial transport as the main mechanism of electron deep penetrations. In comparison, proton deep penetration happens at a slower pace. Statistics also show that the electron PSD radial gradient is much steeper than protons prior to deep penetration events, which can be responsible for these differential behaviors of electron and proton deep penetrations.
more »
« less
Observations of Relativistic Electron Enhancement and Butterfly Pitch Angle Distributions at Low L (<3)
Abstract Electrons in Earth's outer radiation belt are highly dynamic, with fluxes changing by up to orders of magnitude. The penetration of electrons from the outer belt to the inner belt is one such change observed during geomagnetic storms and was previously observed in electrons up to 1 MeV for some strong storms observed by the Van Allen Probes. We analyze pulse height analysis data from the Relativistic Electric and Proton Telescope (REPT) on the Van Allen Probes to produce electron flux measurements with lower minimum energy and significantly improved resolution compared to the standard REPT data and show that electron penetrations into the inner belt (L ≤ 2) extend to at least 1.3 MeV and penetrations into the slot region (2 < L < 2.8) extend to at least 1.5 MeV during certain geomagnetic storms. We also demonstrate that these penetrations are associated with butterfly pitch angle distributions from 1 to 1.3 MeV.
more »
« less
- PAR ID:
- 10485611
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 51
- Issue:
- 2
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Deep penetration of outer radiation belt electrons to lowL(<3.5) has long been recognized as an energy‐dependent phenomenon but with limited understanding. The Van Allen Probes measurements have clearly shown energy‐dependent electron penetration during geomagnetically active times, with lower energy electrons penetrating to lowerL. This study aims to improve our ability to model this phenomenon by quantitatively considering radial transport due to large‐scale azimuthal electric fields (E‐fields) as an energy‐dependent convection term added to a radial diffusion Fokker‐Planck equation. We use a modified Volland‐Stern model to represent the enhanced convection field at lowerLto match the observations of storm time values ofE‐field. We model 10–400 MeV/G electron phase space density with an energy‐dependent radial diffusion coefficient and this convection term and show that the model reproduces the observed deep penetrations well, suggesting that time‐variant azimuthalE‐fields contribute preferentially to the deep penetration of lower‐energy electrons.more » « less
-
Abstract We report on observations of electromagnetic ion cyclotron (EMIC) waves and their interactions with injected ring current particles and high energy radiation belt electrons. The magnetic field experiment aboard the twin Van Allen Probes spacecraft measured EMIC waves nearL = 5.5–6. Particle data from the spacecraft show that the waves were associated with particle injections. The wave activity was also observed by a ground‐based magnetometer near the spacecraft geomagnetic footprint over a more extensive temporal range. Phase space density profiles, calculated from directional differential electron flux data from Van Allen Probes, show that there was a significant energy‐dependent relativistic electron dropout over a limitedL‐shell range during and after the EMIC wave activity. In addition, the NOAA spacecraft observed relativistic electron precipitation associated with the EMIC waves near the footprint of the Van Allen Probes spacecraft. The observations suggest EMIC wave‐induced relativistic electron loss in the radiation belt.more » « less
-
Abstract We perform a comprehensive investigation of the statistical distribution of outer belt electron acceleration events over energies from 300 keV to ∼10 MeV regardless of storm activity using 6‐years of observations from Van Allen Probes. We find that the statistical properties of acceleration events are consistent with the characteristic energies of combined local acceleration by chorus waves and inward radial diffusion. While electron acceleration events frequently occur both at <2 MeV atL < 4.0 and at multi‐MeV atL > 4.5, significant acceleration events are confined toL > ∼4.0. By performing superposed epoch analysis of acceleration events during storm and non/weak storm events and comparing their geomagnetic conditions, we reveal the strong correlation (>0.8) between accumulated impacts of substorms as measured by time‐integrated AL (Int(AL)) and the upper flux limit of electron acceleration. While intense storms can provide favorable conditions for efficient acceleration, they are not necessarily required to produce large maximum fluxes.more » « less
-
Abstract Discovery of the Earth's Van Allen radiation belts by instruments flown on Explorer 1 in 1958 was the first major discovery of the Space Age. The observation of distinct inner and outer zones of trapped megaelectron volt (MeV) particles, primarily protons at low altitude and electrons at high altitude, led to early models for source and loss mechanisms including Cosmic Ray Albedo Neutron Decay for inner zone protons, radial diffusion for outer zone electrons and loss to the atmosphere due to pitch angle scattering. This scattering lowers the mirror altitude for particles in their bounce motion parallel to the Earth's magnetic field until they suffer collisional loss. A view of the belts as quasi‐static inner and outer zones of energetic particles with different sources was modified by observations made during the Solar Cycle 22 maximum in solar activity over 1989–1991. The dynamic variability of outer zone electrons was measured by the Combined Radiation Release and Effects Satellite launched in July 1990. This variability is caused by distinct types of heliospheric structure that vary with the solar cycle. The launch of the twin Van Allen Probes in August 2012 has provided much longer and more comprehensive measurements during the declining phase of Solar Cycle 24. Roughly half of moderate geomagnetic storms, determined by intensity of the ring current carried mostly by protons at hundreds of kiloelectron volts, produce an increase in trapped relativistic electron flux in the outer zone. Mechanisms for accelerating electrons of hundreds of electron volts stored in the tail region of the magnetosphere to MeVenergies in the trapping region are described in this review: prompt and diffusive radial transport and local acceleration driven by magnetospheric waves. Such waves also produce pitch angle scattering loss, as does outward radial transport, enhanced when the magnetosphere is compressed. While quasilinear simulations have been used to successfully reproduce many essential features of the radiation belt particle dynamics, nonlinear wave‐particle interactions are found to be potentially important for causing more rapid particle acceleration or precipitation. The findings on the fundamental physics of the Van Allen radiation belts potentially provide insights into understanding energetic particle dynamics at other magnetized planets in the solar system, exoplanets throughout the universe, and in astrophysical and laboratory plasmas. Computational radiation belt models have improved dramatically, particularly in the Van Allen Probes era, and assimilative forecasting of the state of the radiation belts has become more feasible. Moreover, machine learning techniques have been developed to specify and predict the state of the Van Allen radiation belts. Given the potential Space Weather impact of radiation belt variability on technological systems, these new radiation belt models are expected to play a critical role in our technological society in the future as much as meteorological models do today.more » « less
