Outer Van Allen Radiation Belt Response to Interacting Interplanetary Coronal Mass Ejections

Abstract

We study the response of the outer Van Allen radiation belt during an intense magnetic storm on 15-22 February 2014. Four interplanetary coronal mass ejections (ICMEs) arrived at Earth, of which the three last ones were interacting. Using data from the Van Allen Probes, we report the first detailed investigation of electron fluxes from source (tens of kiloelectron volts) to core (megaelectron volts) energies and possible loss and acceleration mechanisms as a response to substructures (shock, sheath and ejecta, and regions of shock-compressed ejecta) in multiple interacting ICMEs. After an initial enhancement induced by a shock compression of the magnetosphere, core fluxes strongly depleted and stayed low for 4 days. This sustained depletion can be related to a sequence of ICME substructures and their conditions that influenced the Earth's magnetosphere. In particular, the main depletions occurred during a high-dynamic pressure sheath and shock-compressed southward ejecta fields. These structures compressed/eroded the magnetopause close to geostationary orbit and induced intense and diverse wave activity in the inner magnetosphere (ULF Pc5, electromagnetic ion cyclotron, and hiss) facilitating both effective magnetopause shadowing and precipitation losses. Seed and source electrons in turn experienced stronger variations throughout the studied interval. The core fluxes recovered during the last ICME that made a glancing blow to Earth. This period was characterized by a concurrent lack of losses and sustained acceleration by chorus and Pc5 waves. Our study highlights that the seemingly complex behavior of the outer belt during interacting ICMEs can be understood by the knowledge of electron dynamics during different substructures.Electrons in the outer belt are usually divided to source (a few tens of kiloelectron volts), seed (a few hundreds of kiloelectron volts), and core (megaelectron volts) populations. While orbiting the Earth, these

Solar Wind Properties and Geospace Impact of Coronal Mass Ejection‐Driven Sheath Regions: Variation and Driver Dependence

“…We found that for these subsets the majority of events compressed the magnetopause significantly, beyond <8 R E , and typically this occurred at least 50% time of the sheath duration, while clearly a smaller fraction of events led to a compression beyond geostationary orbit, and it occurred typically only for relatively short fraction of sheath duration. We however note that even a relatively short period of compression beyond or close to geostationary orbit can empty quickly in particular the outer parts of the outer Van Allen radiation belts (e.g., Kilpua et al, ). As the fast‐ and strong‐ejecta sheaths have also typically strong magnetic field fluctuations, high dyn pessure, and moderate to intense ring current response, they are expected to cause particularly strong and deep losses of relativistic electrons from the outer radiation belt due to effective magnetopause shadowing (Hietala et al, ; Kilpua, Hietala, et al, ; Kilpua et al, ; Turner et al, ).…”

“…We however note that even a relatively short period of compression beyond or close to geostationary orbit can empty quickly in particular the outer parts of the outer Van Allen radiation belts (e.g., Kilpua et al, ). As the fast‐ and strong‐ejecta sheaths have also typically strong magnetic field fluctuations, high dyn pessure, and moderate to intense ring current response, they are expected to cause particularly strong and deep losses of relativistic electrons from the outer radiation belt due to effective magnetopause shadowing (Hietala et al, ; Kilpua, Hietala, et al, ; Kilpua et al, ; Turner et al, ). Our study also suggests that sheaths of slow ejecta can also compress/erode the subsolar magnetopause significantly, but very rarely beyond the geostationary orbit and also compression/erosion beyond <8 R E occurs typically less than 50% of the sheath duration.…”

“…Furthermore, sheaths are found to result in particularly large geomagnetically induced currents (Huttunen et al, ) as well as intense low‐energy particle precipitation to the upper atmosphere that consequently affect the thermosphere neutral density response and enhance significantly nitric oxide production (Knipp et al, ; ). Sheaths also compress significantly the dayside magnetopause location earthward (e.g., Hietala et al, ; Kilpua, Hietala, et al, ; Lugaz et al, ), and they have distinct effects on the outer Van Allen radiation belts, typically causing deep and sustained depletion of relativistic electrons fluxes (e.g., Alves et al, ; Hietala et al, ; Kilpua, Hietala, et al, ; Kilpua et al, ; Lugaz et al, ; Turner et al, ).…”

Cosmic noise absorption signature of particle precipitation during ICME sheaths and ejecta

“…The effectiveness of sheaths to deplete the belts is partly related to their strong dynamic pressure that can effectively enhance magnetopause shadowing. On the other hand, the average ULF Pc5 and EMIC wave powers in the inner magnetosphere are higher during sheaths than during ejecta (Kalliokoski et al, 2019;Kilpua et al, 2019b).…”

“…As a consequence, it is expected that the inner magnetosphere responds differently to sheath and ejecta. Recent studies (e.g., Kilpua et al, 2015;Lugaz et al, 2015;Turner et al, 2019;Kilpua et al, 2019b) have indeed highlighted that sheaths and ejecta distinct responses in energetic electron fluxes in the radiation belts. A key characteristic of sheaths is that they cause long and sustained depletions of electron fluxes at wide range of energies and L-shells.…”