Electron Acceleration by ICME-driven Shocks at 1 au

Abstract

We present a comprehensive study of in situ electron acceleration during 74 shocks driven by interplanetary coronal mass ejections (ICMEs) with good suprathermal electron observations by the Wind 3DP instrument at 1 au from 1995 through 2014. Among the selected 59 quasi-perpendicular (15 quasi-parallel) shock cases, ∼86% (∼60%), ∼62% (∼36%), and ∼17% (∼7%) show significant electron flux enhancements of J D /J A > 1.5 across the shock, respectively at 0.43, 1.95, and 40 keV, where J D and J A are the electron flux in the shock’s downstream and the preceding ambient solar wind. For significantly shocked suprathermal electrons, the differential flux J D positively correlates most with the magnetosonic Mach number M s , while the flux enhancement J D /J A positively correlates most with the magnetic compression ratio r B , among the shock parameters. Both J D and J A generally fit well to a double-power-law spectrum at ∼0.4–100 keV, J ∝ E −β , with an index of β 1 ∼ 2–6 below a break energy of E br (which is typically ∼2 keV) and an index of β 2 ∼ 2.0–3.2 at energies above. is similar to in all the shock cases, while is similar to (larger than) in ∼60% (∼40%) of the shock cases with significant electron enhancements. Furthermore, J D /J A mostly peaks in the directions perpendicular to the interplanetary magnetic field at ∼0.4–50 keV. These results suggest that both quasi-parallel and quasi-perpendicular shocks accelerate electrons in situ at 1 au mainly via shock drift acceleration, with an acceleration efficiency probably affected by the induced electric field at the shock surface.

Statistical Results for Solar Energetic Electron Spectra Observed over 12 yr with STEREO/SEPT

“…The very gradual events showing rise times of 1000 minutes further challenge the shock acceleration scenario as the shock would have to efficiently accelerate the NR electrons over distances of about half an astronomical unit, which is not expected based on insitu observations of shock-crossings at 1 au where the shock is very inefficient in accelerating NR electrons (e.g. Dresing et al 2016;Yang et al 2019). However, a few of the very long-rise time events ( 1.5 days) of our sample are accompanied by a CME driving a shock where the time scale of the shock propagation to 1 au roughly fits the electron event rise time.…”

“…While magnetic reconnection in solar flares is generally regarded as the mechanism that accelerates solar energetic electrons up to relativistic energies (e.g. Mann Dresing et al 2016;Yang et al 2019). Other cases of shock-accelerated electrons, e.g.…”

Unusual enhancement of ~ 30 MeV proton flux in an ICME sheath region

“…We immediately spot the arrival of a shock wave around 20:06 UT on 4 June at the Wind satellite as indicated by the vertical line labeled 3. Yang et al ( 2019 ), in a statistical study of electron acceleration at IP shocks detected by Wind, described this shock in detail, showing the enhancement of 0.1–4 MeV protons as well as 0.2–40 keV electrons across the shock front. This IP shock had quasi-perpendicular geometry, and the electron energy spectra were interpreted with shock drift acceleration (Yang et al 2019 ).…”

Pan‐Spectrum Fitting Formula for Suprathermal Particles

“…The fitted is around 6 and 2, respectively, for the representative SEP electron and proton spectra shown in Figure 4. Figures 5a-5g show the fitting results to the representative spectra of ESP Fe ions (Desai et al, 2004), ACR He ions (Cummings et al, 2002), SEP 3 He ions (Mason et al, 2002), bow-shocked electrons (Liu et al, 2000), an "upstream electron event" near the bow shock (Liu et al, 2000), HXRs (Krucker et al, 2007), and solar wind suprathermal electrons at ∼0.4-200 keV (Yang et al, 2019), using the PS (red), CDPL (purple), ER (blue), and BD (green) formulae. Table 1 lists the fitted parameters of these spectra.…”

“…All the spacecraft data used in this manuscript are available through Krucker et al (2009), Mewaldt et al (2012), Desai et al (2004), Cummings et al (2002), Mason et al (2002), Liu et al (2000), Krucker et al (2007), Yang et al (2019), and Tao et al (2016).…”