Simultaneous Measurements of Substorm‐Related Electron Energization in the Ionosphere and the Plasma Sheet

Abstract

On 26 March 2008, simultaneous measurements of a large substorm were made using the Poker Flat Incoherent Scatter Radar, Time History of Events and Macroscale Interactions during Substorm (THEMIS) spacecraft, and all sky cameras. After the onset, electron precipitation reached energies ≳100 keV leading to intense D region ionization. Identifying the source of energetic precipitation has been a challenge because of lack of quantitative and magnetically conjugate measurements of loss cone electrons. In this study, we use the maximum entropy inversion technique to invert altitude profiles of ionization measured by the radar to estimate the loss cone energy spectra of primary electrons. By comparing them with magnetically conjugate measurements from THEMIS‐D spacecraft in the nightside plasma sheet, we constrain the source location and acceleration mechanism of precipitating electrons of different energy ranges. Our analysis suggests that the observed electrons ≳100 keV are a result of pitch angle scattering of electrons originating from or tailward of the inner plasma sheet at ~9RE, possibly through interaction with electromagnetic ion cyclotron waves. The electrons of energy 10–100 keV are produced by pitch angle scattering due to a potential drop of ≲10 kV in the auroral acceleration region (AAR) as well as wave–particle interactions in and tailward of the AAR. This work demonstrates the utility of magnetically conjugate ground‐ and space‐based measurements in constraining the source of energetic electron precipitation. Unlike in situ spacecraft measurements, ground‐based incoherent scatter radars combined with an appropriate inversion technique can be used to provide remote and continuous‐time estimates of loss cone electrons in the plasma sheet.

Optical Signatures of the Outer Radiation Belt Boundary

“…The pixelated trapezoidal structure is a map of electron energy flux of 100-keV electrons overlaid on to the DASC image. The uncertainties and caveats of the energy flux estimation are explained in detail in earlier works (Semeter & Kamalabadi, 2005;Sivadas et al, 2017). Figure 2a shows line plots of electron energy flux estimated from NOAA-17 measurements of electrons <1 keV, 1-20 keV, >30 keV, the total energy flux, and white-light emission counts at its northern magnetic foot point from MCGR all-sky camera.…”

Self‐Consistent Modeling of Electron Precipitation and Responses in the Ionosphere: Application to Low‐Altitude Energization During Substorms

“…There have been extensive reports that the Geophysical Research Letters 10.1029/2018GL078828 energetic electrons in the ring current/radiation belts and plasma sheet can precipitate into the ionosphere through various mechanisms (e.g., Fu et al, 2011;Horne et al, 2003;Jordanova et al, 2008;Li et al, 2017Li et al, , 2013Newell et al, 2009;Ni et al, 2008;Su et al, 2017;Thorne et al, 2010). For example, Sivadas et al (2017) analyzed an intense transient energetic ionization layer in the D region (<70 km) after substorm onset. In order to understand the low-altitude structural variations and the magnetospheric drivers, it is natural to investigate simultaneous and magnetically conjugate measurements in both ionosphere and magnetosphere.…”

“…Transient structures in the ionosphere, including sudden enhancement of ionization in the low-altitude D region (e.g., Cresswell-Moorcock et al, 2013;Rodger et al, 2012), have important implications to terrestrial atmosphere dynamics, including ozone loss (e.g., Seppälä et al, 2015). The low-altitude structures can be attributed to solar proton events between 1 and 100 MeV in energy (e.g., Clilverd et al, 2005;Reagan & Watt, 1976) or energetic electron precipitation from the magnetosphere (e.g., Beharrell et al, 2015;McGranaghan, Knipp, Matsuo, et al, 2015;Oyama et al, 2017;Sivadas et al, 2017). There have been extensive reports that the Geophysical Research Letters 10.1029/2018GL078828 energetic electrons in the ring current/radiation belts and plasma sheet can precipitate into the ionosphere through various mechanisms (e.g., Fu et al, 2011;Horne et al, 2003;Jordanova et al, 2008;Li et al, 2017Li et al, , 2013Newell et al, 2009;Ni et al, 2008;Su et al, 2017;Thorne et al, 2010).…”

“…In order to understand the low-altitude structural variations and the magnetospheric drivers, it is natural to investigate simultaneous and magnetically conjugate measurements in both ionosphere and magnetosphere. For example, Sivadas et al (2017) analyzed an intense transient energetic ionization layer in the D region (<70 km) after substorm onset. With simultaneous measurements from ground-based incoherent scatter radar (ISR) and in situ Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft, they identified >100-keV loss cone electrons accelerated from the distant plasma sheet as the source for the low-altitude ionization in the ionosphere.…”

A New Framework to Incorporate High‐Latitude Input for Mesoscale Electrodynamics

“…Semeter and Kamalabadi (2005) have demonstrated a technique for inverting ISR measurements to determine the electron energy spectrum, and later studies successfully applied the same inversion technique (Sivadas et al, 2017). As shown by this study, the crucial next development step for HIME is including the mesoscale particle precipitation information.…”