Prompt acceleration of magnetospheric electrons to ultrarelativistic energies by the 17 March 2015 interplanetary shock

Abstract

Trapped electrons in Earth's outer Van Allen radiation belt are influenced profoundly by solar phenomena such as high‐speed solar wind streams, coronal mass ejections (CME), and interplanetary (IP) shocks. In particular, strong IP shocks compress the magnetosphere suddenly and result in rapid energization of electrons within minutes. It is believed that the electric fields induced by the rapid change in the geomagnetic field are responsible for the energization. During the latter part of March 2015, a CME impact led to the most powerful geomagnetic storm (minimum Dst = −223 nT at 17 March, 23 UT) observed not only during the Van Allen Probe era but also the entire preceding decade. Magnetospheric response in the outer radiation belt eventually resulted in elevated levels of energized electrons. The CME itself was preceded by a strong IP shock whose immediate effects vis‐a‐vis electron energization were observed by sensors on board the Van Allen Probes. The comprehensive and high‐quality data from the Van Allen Probes enable the determination of the location of the electron injection, timescales, and spectral aspects of the energized electrons. The observations clearly show that ultrarelativistic electrons with energies E > 6 MeV were injected deep into the magnetosphere at L ≈ 3 within about 2 min of the shock impact. However, electrons in the energy range of ≈250 keV to ≈900 keV showed no immediate response to the IP shock. Electric and magnetic fields resulting from the shock‐driven compression complete the comprehensive set of observations that provide a full description of the near‐instantaneous electron energization.

Simulated Prompt Acceleration of Multi‐MeV Electrons by the 17 March 2015 Interplanetary Shock

“…A dropout in flux centered on 90° pitch angle is the first indication of the IP shock arrival prior to the first drift echo. Note by contrast that the flux peaked at 90° in measurements from Probe A at L = 3 at the time of IP shock arrival, as reported by Kanekal et al () and Baker et al (), where drift shell splitting (Hudson et al, ; Sibeck et al ) and magnetopause shadowing are expected to have negligible effect, also evident in the low L value portion of Figure , left.…”

“…Location of the Van Allen Probes and GOES 13 and 15 spacecraft at the time of the interplanetary shock arrival on 17 March 2015 ∼ 0445 UT at the magnetopause (Kanekal et al, ).…”

The interaction of ultra-low-frequency pc3-5 waves with charged particles in Earth’s magnetosphere

“…It has become clear that ultra-relativistic electrons with energies greater than 6 MeV can be injected deep into the inner magnetosphere (L * 3) within a few minutes after IP shock impact (Kanekal et al 2016). Observationally, there is no obvious immediate response to IP shocks for electrons with energies between 250 and 900 keV.…”

Coronal mass ejections and their sheath regions in interplanetary space

“…First, the impact of the shock to the magnetosphere will launch a magnetosonic wave that propagates in the magnetosphere and the associated induced electric field can rapidly accelerate electrons to MeV energies (e.g., Foster et al 2015;Kanekal et al 2016). In particular, if the drift velocity of an electron matches the speed of the magnetosonic wave, the electron can "surf" on the wave and attain very high energies.…”