Solitary Waves Across Supercritical Quasi‐Perpendicular Shocks

Abstract

We consider intense electrostatic solitary waves (ESW) observed in a supercritical quasi‐perpendicular Earth's bow shock crossing by the Magnetospheric Multiscale Mission. The ESW have spatial scales of a few tens of meters (a few Debye lengths) and propagate oblique to a local quasi‐static magnetic field with velocities from a few tens to a few hundred kilometers per second in the spacecraft frame. Because the ESW spatial scales are comparable to the separation between voltage‐sensitive probes, correction factors are used to compute the ESW electric fields. The ESW have electric fields with amplitudes exceeding 600 mV/m (oriented oblique to the local magnetic field) and negative electrostatic potentials with amplitudes of a few tenths of the electron temperature. The negative electrostatic potentials indicate that the ESW are not electron phase space holes, while interpretation in terms of ions phase space holes is also questionable. Whatever is their nature, we show that due to the oblique electric field orientation the ESW are capable of efficient pitch‐angle scattering and isotropization of thermal electrons. Due to the negative electrostatic potentials the ESW Fermi reflects a significant fraction of the thermal electrons streaming from upstream (downstream) back to upstream (downstream) region, thereby affecting the shock dynamics. The role of the ESW in electron heating is discussed.

Electrostatic Turbulence and Debye-scale Structures in Collisionless Shocks

“…This feature of the occurrence of bipolar structures in collisionless shocks is reported for Figure 3. The analysis of properties of a particular bipolar structure measured aboard MMS4 that is based on voltage signals induced on six voltage-sensitive probes by the electric field of the bipolar structure (see Vasko et al 2018, for methodology details): (a, b) voltage signals V 1 vs. −V 2 and V 3 vs. −V 4 of the opposing probes mounted on 60-m antennas in the spacecraft spin plane; (c) voltage signals V 5 vs. −V 6 of the opposing probes mounted on 15-m axial antennas along the spin axis; the time delays between voltage signals of the opposing probes are used to compute the direction of propagation k and velocity Vs of the bipolar structure; (d) the electric field components E 12 , E 34 and E 56 along the antenna directions that were computed using the voltage signals of the opposing probes, E ij ∝ (V i − V j )/(2l ij ), where l 12 = l 34 = 60 m and l 56 = 15 m are antenna lengths; (e) the electric field E l of the bipolar structure (black) oriented a few degrees off the axial antenna (as one can infer from similar bipolar profiles in panel (d)); the electrostatic potential of the bipolar structure (blue) is computed as Φ = E · k Vs dt. In all panels dots represent measured quantities, while solid lines correspond to spline interpolated quantities.…”

Multispacecraft Analysis of Electron Holes

“…The key to unlocking accurate information regarding the properties of EHs, and subsequently their interaction with the surrounding plasma, is to accurately determine their velocity. Without the velocity, we cannot distinguish between EHs and ESWs of negative potential (e.g., McFadden et al, 2003;Vasko et al, 2018), and we are unable to determine their parallel (to the magnetic field B) length scale l ∥ . In order to get as accurate velocity estimates as possible, the distance between the two points that time the structures should be as large as possible.…”

“…Later, Matsumoto et al (1994) were able to resolve this noise in the time domain, revealing the source to be bipolar electrostatic structures. ESWs have since been found in most space plasmas, including the auroral region (Temerin et al, 1982), the inner magnetosphere (Mozer et al, 2015), the flow breaking region (Ergun et al, 2014), the magnetopause , the bow shock (Bale et al, 1998;Vasko et al, 2018), the solar wind (Malaspina et al, 2013), and near Saturn's magnetosphere (Williams et al, 2006). ESWs have since been found in most space plasmas, including the auroral region (Temerin et al, 1982), the inner magnetosphere (Mozer et al, 2015), the flow breaking region (Ergun et al, 2014), the magnetopause , the bow shock (Bale et al, 1998;Vasko et al, 2018), the solar wind (Malaspina et al, 2013), and near Saturn's magnetosphere (Williams et al, 2006).…”

“…Because of the isolated occurrence of these structures, they were named Electrostatic Solitary Waves (ESWs). ESWs have since been found in most space plasmas, including the auroral region (Temerin et al, 1982), the inner magnetosphere (Mozer et al, 2015), the flow breaking region (Ergun et al, 2014), the magnetopause , the bow shock (Bale et al, 1998;Vasko et al, 2018), the solar wind (Malaspina et al, 2013), and near Saturn's magnetosphere (Williams et al, 2006). Simulations have shown that ESWs can be formed by different plasma instabilities, such as the Buneman instability (Che et al, 2010;Norgren, André, Graham, et al, 2015) and various electron beam instabilities (Omura et al, 1996).…”

Shock Drift Acceleration of Ions in an Interplanetary Shock Observed by MMS

“…Both wavelets indicate significant activity near the shock: large-scale whistler wave precursors (electromagnetic) just upstream and electrostatic wave activity in the ramp. The electrostatic waves most likely represent some combination of ion-acoustic waves, lower hybrid waves, and bipolar structures with wavelengths on the order of a few Debye lengths (Gurnett 1985;Hull et al 2006;Vasko et al 2018). Although the electrostatic waves may contribute to the pitch angle scattering of electrons (Vasko et al 2018), they are highly inefficient for scattering or acceleration protons.…”