THEMIS measurements of quasi‐static electric fields in the inner magnetosphere

Abstract

We use 4 years of Time History of Events and Macroscale Interactions during Substorms (THEMIS)double-probe measurements to offer, for the first time, a complete picture of the dawn-dusk electric field covering all local times and radial distances in the inner magnetosphere based on in situ equatorial observations. This study is motivated by the results from the CRRES mission, which revealed a local maximum in the electric field developing near Earth during storm times, rather than the expected enhancement at higher L shells that is shielded near Earth as suggested by the Volland-Stern model. The CRRES observations were limited to the duskside, while THEMIS provides complete local time coverage. We show strong agreement with the CRRES results on the duskside, with a local maximum near L = 4 for moderate levels of geomagnetic activity and evidence of strong electric fields inside L = 3 during the most active times. The extensive data set from THEMIS also confirms the day/night asymmetry on the duskside, where the enhancement is closest to Earth in the dusk-midnight sector, and is farther away closer to noon. A similar, but smaller in magnitude, local maximum is observed on the dawnside near L = 4. The noon sector shows the smallest average electric fields, and for more active times, the enhancement develops near L = 7 rather than L = 4. We also investigate the impact of the uncertain boom-shorting factor on the results and show that while the absolute magnitude of the electric field may be underestimated, the trends with geomagnetic activity remain intact.

Van Allen Probes investigation of the large‐scale duskward electric field and its role in ring current formation and plasmasphere erosion in the 1 June 2013 storm

“…Recent work with Time History of Events and Macroscale Interactions during Substorms electric field data by Califf et al . [] verified the results of Rowland and Wygant [], expanding the coverage to a wider set of magnetic local times. Nishimura et al .…”

“…[], and Califf et al . [], and is routinely observed during other storms in the Van Allen Probes electric field data set. A nightside/dayside asymmetry between the plasmapause location in L shell is also apparent by comparing the inbound (premidnight/night/dawn) and outbound (dayside/duskside/premidnight) densities, Figures b and b, respectively, with the plasmapause routinely located at higher L shells on the outbound passes.…”

Typical values of the electric drift E × B/B2 in the inner radiation belt and slot region as determined from Van Allen Probe measurements

“…That is, particularly, true closest to Earth where spacecraft pass through perigee with maximum speed in a strong magnetic field. As a result, analyses based on in situ measurements are traditionally restricted to L >~2+ [e.g., Matsui et al, 2004;Califf et al, 2014;Ali et al, 2016;Califf et al, 2016]. This motion-induced electric field should be subtracted from measurements to describe the electric fields in an inertial frame of reference.…”

Large‐amplitude electric fields in the inner magnetosphere: Van Allen Probes observations of subauroral polarization streams

“…Data from CRRES and THEMIS (Time History of Events and Macroscale Interactions during Substorms) show that the average dawn‐dusk electric field tends to be less than 0.6 mV/m and decreasing in magnitude below L = 3.5 during moderate storms (3 < Kp < 6). However, during the largest storms ( Kp > 6) there can be an increasing trend extending inside L = 3 with average magnitudes greater than 1.5 mV/m [ Rowland and Wygant , ; Califf et al , ].…”