Bow shock specularly reflected ions in the presence of low-frequency electromagnetic waves: a case study

Abstract

Abstract. An energetic ion (E≤40 keV) event observed by the CLUSTER/CIS experiment upstream of the Earth's bow shock is studied in detail. The ion event is observed in association with quasi-monochromatic ULF MHD-like waves, which we show modulate the ion fluxes. According to three statistical bow shock position models, the Cluster spacecrafts are located at ∼0.5 R E from the shock and the averaged bow shock θ Bn0 is about ∼30 • . The analysis of the threedimensional angular distribution indicates that ions propagating roughly along the magnetic field direction are observed at the onset of the event. Later on, the angular distribution is gyrophase-bunched and the pitch-angle distribution is peaked at α 0 ∼θ Bn0 , consistent with the specular reflection production mechanism. The analysis of the waves shows that they are left-handed in the spacecraft frame of reference (right-handed in the solar wind frame) and propagate roughly along the ambient magnetic field; we have found that they are in cyclotron-resonance with the field-aligned beam observed just upstream. Using properties of the waves and particles, we explain the observed particle flux-modulation in the context of θ Bn changes at the shock caused by the convected ULF waves. We have found that the high count rates coincide with particles leaving the shock when θ Bn angles are less than ∼40 • , consistent with the specular reflection hypothesis as the production mechanism of ions.

Comparison of accelerated ion populations observed upstream of the bow shocks at Venus and Mars

“…7-9. Both populations are gyrophase bunched and similar to those observed in the shock foot region (Meziane et al, 2004a). Paschmann et al (1981) and Sckopke et al (1983) showed that the ion distribution in the upstream foot region of a quasi-perpendicular bow shock is consistent with a partial ring distribution that originates from the reflected solar wind at the bow shock.…”

“…The flow directions of both populations point away from the subsolar region. The field-aligned tailward flow with substantial gyromotion of both populations is similar to the reflected-gyrating component in the terrestrial bow shock foot region (Paschmann et al, 1981;Sckopke et al, 1990;Meziane, 2004a). It is worth reminding ourselves that the elevation-azimuth pattern of the gyrating ions as shown in Fig.…”

“…10, specular reflection means a shift of guiding center in the direction of the −V × B electric field (Alfvén and Fälthammar, 1963), and also means that a portion of the ion energy is converted from gyrating motion into field-aligned motion as illustrated in left panels of Fig. 10 (Gosling et al, 1982;Meziane et al, 2004a). However, the latter type of the energy conversion is facilitated only at the bow shock flank where solar wind ions enter the shock obliquely to its surface (Fig.…”

“…10a). To overcome this difficulty at the subsolar bow shock, a ripple structure of the shock (Meziane et al, 2004a) due to, e.g., ion cyclotron wave excitation , has been proposed, although such a structure has only been examined remotely (Miao et al, 2009) but not by in-situ observations.…”

“…Both the field-aligned beams and the intermediate component have lower alpha/proton ratios than that in the original solar wind or in the diffuse component (Fuselier and Thomsen, 1992), and their sources are considered to be either the reflected solar wind (e.g., Gosling et al, 1978;Paschmann et al, 1980;Mazelle et al, 2003;Kucharek et al, 2004;Meziane et al, 2004a), or heated magnetosheath ions that leak through the bow shock along the magnetic field (Tanaka et al, 1983). The leakage scenario involves ion heating by electromagnetic waves downstream of the shock (Tanaka et al, 1983), and the reflection scenario normally employs a simplified assumption of specular reflection of the solar wind by a quasi-perpendicular shock (Paschmann et al, 1980).…”