Chorus wave emissions are pervasive electromagnetic waves observed throughout the inner magnetosphere, and can intensify significantly during enhanced geomagnetic activity. Chorus waves have been ...shown to drive a particular type of precipitation that is composed of ring current and radiation belt losses and results in the quasi-periodic optical ionospheric phenomenon called pulsating aurora. Understanding how chorus waves evolve over time following periods of generation and rapid growth (i.e. injection-driven) and how they evolve as they propagate from equatorial sources to higher magnetic latitudes will lead to insights about how they drive energetic electron precipitation.
The Diffuse Auroral Eraser Troyer, Riley N; Jaynes, Allison N; Jones, Sarah L ...
Earth and Space Science Open Archive ESSOAr,
07/2020
Paper
Open access
The source of diffuse aurora has been widely studied and linked to electron cyclotron harmonic (ECH) and upper-band chorus (UBC) waves. It is known that these waves scatter 100s of eV to 10s of keV ...electrons from the plasma sheet, but the relative contribution of each wave type is still an open question. In this paper, we report on a new structured diffuse aurora feature observed on March 15, 2002 that could help further our understanding. This feature is characterized by four phases: (1) the initial phase exhibiting regular diffuse aurora, (2) the brightening phase, where a stripe of diffuse aurora rapidly brightens, (3) the eraser phase, where the stripe dims to below its initial state, and (4) the recovery phase, where the diffuse aurora returns to its original brightness. Using a superposed epoch analysis of 22 events, we calculate the average recovery phase time to be 20 seconds, although this varies widely between events. We hypothesize that the process responsible for these auroral eraser events could be an interaction between ECH and chorus waves.
Lower-band whistler-mode chorus waves are important to the dynamics of Earth's radiation belts, playing a key role in accelerating seed population electrons (100's of keV) to relativistic ($>$ 1 MeV) ...energies, and in scattering electrons such that they precipitate into the atmosphere. When constructing and using statistical models of lower-band whistler-mode chorus wave power, it is commonly assumed that wave power is spatially distributed with respect to magnetic L-shell. At the same time, these waves are known to drop in power at the plasmapause, a cold plasma boundary which is dynamic in time and space relative to L-shell. This study organizes wave power and propagation direction data with respect to distance from the plasmapause location to evaluate what role the location of the plasmapause may play in defining the spatial distribution of lower band whistler-mode chorus wave power. It is found that characteristics of the statistical spatial distribution of equatorial lower band whistler mode chorus are determined by L-shell, and are largely independent of plasmapause location. The primary physical importance of the plasmapause is to act as an Earthward boundary to lower band whistler mode chorus wave activity. This behavior is consistent with an equatorial lower band whistler mode chorus wave power spatial distribution that follows the L-shell organization of the particles driving wave growth.
Radiation belt electrons undergo frequent acceleration, transport, and loss processes under various physical mechanisms. One of the most prevalent mechanisms is radial diffusion, caused by the ...resonant interactions between energetic electrons and ULF waves in the Pc4‐5 band. An indication of this resonant interaction is believed to be the appearance of periodic flux oscillations. In this study, we report long‐lasting, drift‐periodic flux oscillations of relativistic and ultrarelativistic electrons with energies up to ∼7.7 MeV in the outer radiation belt, observed by the Van Allen Probes mission. During this March 2017 event, multi‐MeV electron flux oscillations at the electron drift frequency appeared coincidently with enhanced Pc5 ULF wave activity and lasted for over 10 h in the center of the outer belt. The amplitude of such flux oscillations is well correlated with the radial gradient of electron phase space density (PSD), with almost no oscillation observed near the PSD peak. The temporal evolution of the PSD radial profile also suggests the dominant role of radial diffusion in multi‐MeV electron dynamics during this event. By combining these observations, we conclude that these multi‐MeV electron flux oscillations are caused by the resonant interactions between electrons and broadband Pc5 ULF waves and are an indicator of the ongoing radial diffusion process during this event. They contain essential information of radial diffusion and have the potential to be further used to quantify the radial diffusion effects and aid in a better understanding of this prevailing mechanism.
Key Points
Long‐lasting, drift‐periodic flux oscillations of 1.8–7.7 MeV electrons in the radiation belt are reported using Van Allen Probes data
Flux oscillations appeared coincidently with enhanced Pc5 ULF waves with amplitude well correlated with phase space density radial gradient
These flux oscillations indicate the resonant interactions between electrons and broadband Pc5 ULF waves
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Multi‐MeV electron drift‐periodic flux oscillations observed in Earth's radiation belts indicate radial transport and energization/de‐energization of these radiation belt core populations. Using ...multi‐year Van Allen Probes observations, a statistical analysis is conducted to understand the characteristics of this phenomenon. The results show that most of these flux oscillations result from resonant interactions with broadband ultralow frequency (ULF) waves and are indicators of ongoing radial diffusion. The occurrence frequency of flux oscillations is higher during high solar wind speed/dynamic pressure and geomagnetically active times; however, a large number of them were still observed under mild to moderate solar wind/geomagnetic conditions. The occurrence frequency is also highest (up to ∼30%) at low L‐shells (L∗∼3−4 ${L}^{\ast }\sim 3-4$) under various geomagnetic activity, suggesting the general presence of broadband ULF waves and radial diffusion at low L‐shells even during geomagnetically quiet times and showing the critical role of the electron phase space density radial gradient in forming drift‐periodic flux oscillations.
Plain Language Summary
Earth's radiation belts contain 100s of keV–10 MeV electrons which pose great threats to the avionics and humans in space. Understanding their dynamics is critical due to both scientific interests and practical needs. Periodic flux oscillations of multi‐MeV electrons at their drift frequencies have been previously observed in Earth's radiation belts. These flux oscillations are believed to be indicators of the radial transport of these electrons and contain critical information of their energization processes. Using Van Allen Probes observations, a statistical analysis is conducted to further understand this phenomenon and associated mechanisms. It is shown that the majority of observed flux oscillations result from resonant interactions between multi‐MeV electrons and broadband ultralow frequency (ULF) waves, which lead to radial diffusion, one of the most important mechanisms causing radiation belt electron acceleration/loss. These flux oscillations preferentially occur under active solar wind conditions and geomagnetic activity. They also occur more often at lower altitudes, which is due to the steeper radial gradient of the electron distribution closer to the Earth. This indicates the general presence of broadband ULF waves and radial diffusion at low altitudes and suggests the critical role of the radial gradient of the electron distribution in forming these flux oscillations.
Key Points
Most multi‐MeV electron drift‐periodic flux oscillations in the outer belt result from resonant interaction with broadband ultralow frequency waves
These flux oscillations preferentially occur during high solar wind speed/dynamic pressure and geomagnetically active times
The occurrence rate of these flux oscillations is highest at L*∼ 3–4 due to higher phase space density radial gradients at lower L
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37.
The Origin and Shape of Diffuse Auroral Patches Rae, Kyle; Donovan, Eric; Liang, Jun ...
NASA Center for AeroSpace Information (CASI). Conference Proceedings,
12/2011
Conference Proceeding
Patchy pulsating aurora occurs commonly in the post-midnight sector. Recent studies have moved us significantly closer to understanding the mechanisms responsible for pitch angle scattering of the ...Central Plasma Sheet (CPS) electrons that produce these aurora. However, there is not yet an adequate explanation of what physical process gives rise to the patchy nature of the aurora. These patches last for minutes up to tens of minutes, with sizes that do not change significantly over their life time, and remain more or less stationary relative to the ground. In this paper, we use THEMIS and NORSTAR ASI observations of these auroral features to explore the shape of these patches. Based on our results, we conclude that the patches are the ionospheric counterpart of structures in cold plasma near the magnetospheric equator.
Small-Scale Features in Pulsating Aurora Jones, Sarah; Jaynes, Allison N; Knudsen, David J ...
NASA Center for AeroSpace Information (CASI). Conference Proceedings,
12/2011
Conference Proceeding
A field study was conducted from March 12-16, 2002 using a narrow-field intensified CCD camera installed at Churchill, Manitoba. The camera was oriented along the local magnetic zenith where ...small-scale black auroral forms are often visible. This analysis focuses on such forms occurring within a region of pulsating aurora. The observations show black forms with irregular shape and nonuniform drift with respect to the relatively stationary pulsating patches. The pulsating patches occur within a diffuse auroral background as a modulation of the auroral brightness in a localized region. The images analyzed show a decrease in the brightness of the diffuse background in the region of the pulsating patch at the beginning of the offphase of the modulation. Throughout the off phase the brightness of the diffuse aurora gradually increases back to the average intensity. The time constant for this increase is measured as the first step toward determining the physical process.
A variety of dynamic behavior of multi‐MeV electrons near the inner edge of the outer radiation belt has been revealed by detailed analysis of Van Allen Probes data. This study presents multi‐MeV ...electron flux and phase space density (PSD) using Van Allen Probes data during two strong geomagnetic storms to reveal their energy‐dependent dynamics in the outer belt, with dynamics occurring on timescales of hours to ∼1 week. Enhancements are shown down to L = 2.6, where ∼3 MeV electron populations are enhanced by an order of magnitude during one storm of study. Study of a second comparable storm shows rapid depletion of electron populations up to ∼7 MeV in the region 2.6 ≤ L ≤ 3. We also identify a local electron PSD peak at L ≈ 3 that slowly accumulates during quiet time and is rapidly depleted during an intense storm. Possible contributors to these dynamics are discussed.
Plain Language Summary
Electrons in Earth's magnetic field, or magnetosphere, form into two bands known as the Van Allen radiation belts. The Van Allen Probes mission consisted of two satellites to measure energetic particle flux in these belts. Based on early studies of the first 2 years of data from the mission, it was first believed that along the inner edge of the outer belt, there was a barrier to multi‐MeV electrons, with physical mechanisms of this barrier yet to be understood. Further studies have revealed that multi‐MeV electrons can be dynamic more Earthward than originally thought. This study shows that multi‐MeV electrons along the inner edge of the outer belt can be highly dynamic on rapid timescales, with a variety of behavior including enhancements or depletions that appear to be influenced by preconditioning of the Van Allen radiation belts. The strength of these dynamics is generally diminished with increased electron energy and decreased distance toward Earth. Additionally, a localized region of multi‐MeV electrons has been newly found by analyzing electron populations in coordinate systems mapped to the particles' motion. This population is then rapidly depleted during strong modification to the magnetosphere. The physical mechanisms influencing electron dynamics in the magnetosphere are discussed.
Key Points
Multiple orders of magnitude dynamics of multi‐MeV electron flux and phase space density are shown near the inner edge of the outer belt
These dynamics, and their timescales, appear to be energy‐dependent, with variations on the timescales of hours to one week within L ⪅ 3
Quiet‐time formation of a local peak at L ≈ 3 of ⪆6 MeV electron phase space density and its rapid decay during storms are shown
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