Interplanetary shocks and foreshocks observed by STEREO during 2007–2010

Abstract

Interplanetary shocks in the heliosphere modify the solar wind through which they pass. In particular, shocks play an important role in particle acceleration. During the extended solar minimum (2007–2010) STEREO observed 65 forward shocks driven by stream interactions (SI), with magnetosonic Mach numbers Mms ≈ 1.1–4.0 and shock normal angles θBN ~ 20–87°. We analyze the waves associated with these shocks and find that the region upstream can be permeated by whistler waves (f ~ 1 Hz) and/or ultra low frequency (ULF) waves (f ~ 10−2–10−1 Hz). While whistlers appear to be generated at the shock, the origin of ULF waves is most probably associated with local kinetic ion instabilities. We find that when the Mach number (Mms) is low and the shock is quasi‐perpendicular ( θBN > 45°) whistler waves remain close to the shock. As Mms increases, the shock profile changes and can develop a foot and overshoot associated with ion reflection and gyration. Whistler precursors can be superposed on the foot region, so that some quasi‐perpendicular shocks have characteristics of both subcritical and supercritical shocks. When the shock is quasi‐parallel ( θBN < 45°) a large foreshock with suprathermal ions and waves can form. Upstream, there are whistler trains at higher frequencies whose characteristics can be slightly modified probably by reflected and/or leaked ions and by almost circularly polarized waves at lower frequencies that may be locally generated by ion instabilities. In contrast with planetary bow shocks, most of the upstream waves studied here are mainly transverse and no steepening occurs. Some quasi‐perpendicular shocks (45° < θBN < 60°) are preceded by ULF waves and ion foreshocks. Fluctuations downstream of quasi‐parallel shocks tend to have larger amplitudes than waves in the sheath of quasi‐perpendicular shocks. We compare SI‐driven shock properties with those of shocks generated by interplanetary coronal mass ejections (ICMEs). During the same years, STEREO observed 20 ICME‐driven shocks with Mms ≈ 1.2–4.0 and θBN ~ 38–85°. We find that shocks driven by ICMEs tend to have larger proton foreshocks (dr ~ 0.1 AU) than shocks driven by stream interactions (dr ≤ 0.05 AU). This difference of ion foreshock size should be linked to shock age: ICME‐driven shocks form at shorter distances to the Sun and therefore can energize particles for longer times as they propagate to 1 AU, while stream interaction shocks form closer to Earth's orbit and have been accelerating ions for a shorter interval of time.

Statistical Study of Low‐Frequency Electromagnetic Cyclotron Waves in the Solar Wind at 1 AU

“…The presence of shocks in the solar wind may affect the results in this paper because the shocks can produce various types of waves (e.g., Blanco‐Cano et al, ; Kajdič et al, ; Lee, ; Wilson, ; Wilson et al, , ). We restudy our results by trying to exclude shock regions.…”

Jets Downstream of Collisionless Shocks

“…This raises a question whether observations in such cases correspond to ion foreshocks similar to those observed at planetary bow shocks, these particles are part of Solar Energetic Particle (SEP, e.g., Schwenn 2006;Reames 1996) events in the suprathermal energy range (between ∼1 keV and several hundred keV), or a combination of both. Although Blanco-Cano et al (2016) could not look at any ion distributions, in the past literature ion distributions upstream of IP shocks were reported almost exclusively to be isotropic (e.g., Gosling 1983). Since such distributions are typical of SEPs and of planetary foreshocks upstream of almost parallel bow-shocks, it is not possible to say what kind of ion foreshocks were actually observed by Blanco-Cano et al (2016).…”

“…Although Blanco-Cano et al (2016) could not look at any ion distributions, in the past literature ion distributions upstream of IP shocks were reported almost exclusively to be isotropic (e.g., Gosling 1983). Since such distributions are typical of SEPs and of planetary foreshocks upstream of almost parallel bow-shocks, it is not possible to say what kind of ion foreshocks were actually observed by Blanco-Cano et al (2016). There are only three papers reporting other kinds of ion distributions upstream of IP shocks, namely Tokar et al (2000), Viñas et al (1984) and .…”

“…It seems then that IP particle foreshocks should be fairly easy to observe, although the reflected particle fluxes are expected to be less intense due to shocks' low Mach numbers. Blanco-Cano et al (2016) found suprathermal ions to be a common feature upstream of IP shocks. However most of the ICME driven foreshocks reported by the authors lasted of the order of half a day or more in the spacecraft data and were estimated to have typical extensions of 0.1 AU.…”

“…IP shocks differ from their planetary counterparts in that they have much larger curvature radii and usually they exhibit smaller Mach numbers than, for example, Earth's bow shock (e.g., Gosling 1983). Statistical studies (e.g., Blanco-Cano et al 2016;Kilpua et al 2015) show that IP shocks tend to exhibit magnetosonic Mach numbers less than ∼4 but typically between 1 and 2. Kajdič et al (2014) showed that about 50 % of IP shocks are supercritical, meaning that they reflect particles to some degree.…”