William Wilkinson

The Earth's quasi-parallel bow shock: Review of observations and perspectives for Cluster

Abstract Spacecraft crossings of the Earths quasi-parallel bow shock under typical high Mach number conditions are characterized by the presence of large-amplitude, compressive, low-frequency magnetic pulsations. Thus, instead of the fairly abrupt jump from upstream to downstream states one associates with their quasi-perpendicular counterparts, quasi-parallel shocks are more aptly described in terms of an extended transition from upstream to downstream conditions. The complex, turbulent appearance of the quasi-parallel shock diverted much of the research efforts in the field towards the quasi-perpendicular regime until about 15 years ago. This paper reviews what we have learnt about the quasi-parallel shock transition zone from spacecraft observations at the Earths bow shock in the last decade and a half. Field and particle measurements suggest a prominent role is played by short (∼10 s ) large-amplitude ( |δ B |/| B |>2 , typically 3 or more) magnetic structures that evolve out of the upstream wave field and are convected towards the shock by the supersonic solar wind. As these structures grow in amplitude, they steepen, slow down in the shock rest frame and begin to merge with similar structures, leading to the downstream state. A number of questions about the nature of the quasi-parallel shock transition remain and the possible contribution Cluster may make towards their resolution is highlighted.

Ion heating within the ramp of quasi-perpendicular subcritical collisionless shocks

The heating of the bulk of the incident ions at low Mach number quasi-perpendicular collisionless shocks is observed to take place very rapidly, within the shock ramp. In this paper we show that the trajectories of ions passing through the shock ramp lead to a dispersion in phase space resulting in an effective increase in ion temperature. The main heating is predicted to take place in the second half of the ramp. We present results of self-consistent computer simulations which confirm this prediction.

Ramp thickness and ion heating in 1D simulations of laminar quasi-perpendicular shocks

One-dimensional hybrid simulations are used to investigate the effect of the shock ramps width on the heating of the transmitted ions at low Mach number nearly perpendicular shocks. The suitability of the hybrid code for such a study is established by considering the correlation between precursor wavelength and ramp thickness as a function of θ Bn , the angle between the upstream magnetic field and upstream-pointing shock normal. The simulated shocks are found to preserve their dispersive character at geometries close to perpendicular, becoming rapidly narrower as θ Bn approaches 90°. The effect of shock thickness on the transmitted ions is examined by comparing their heating at three shocks with θ Bn = 83°, 86° and 88°, respectively, which are successively thinner by a factor of 2. The results are discussed in the light of recent theoretical studies on the subject.

Multiple encounters of a specularly reflected ion with planar quasi-perpendicular shocks

Abstract We investigate the details of the trajectory of ions that are initially specularly reflected at a planar quasi-perpendicular (where the angle θBn between the upstream magnetic field and shock normal directions exceeds 45°) collisionless shock. In the first of two separate studies we examine the possibility of multiple reflections at the shock front. The number of times an ion re-encounters the shock increases as the shock geometry becomes more perpendicular. However, the length of time spent by the ion interacting with the shock, a parameter of relevance to the effectiveness of the particles acceleration by the shock, does not increase monotonically with θBn due to the details of the particles trajectory. The interaction time is also a strong function of the particles initial normal speed. In a second study, the trajectories of ions able to overcome the shock potential are followed downstream. For θBn ⩽ 60°, ions which cross the potential jump at their first re-encounter with the shock eventually escape upstream after two, four or six shock traversals, depending on the value of θBn. The particles interaction time with the shock rises rapidly with the number of re-encounters and generally, though not always, with θBn. The magnitude of the electrostatic potential and the Mach number of the upstream flow have a very minor effect on particle trajectories. For θBn > 60°, particles that cross the shock remain downstream unless they traverse the shock after having undergone further specular reflections. Multiple shock crossings can lead to a significant increase in the upstream extent of an ions trajectory relative to that reached after the initial specular reflection.

EISCAT measurements of the solar wind: Observations of interaction regions

Abstract EISCAT can make measurements of corotating interaction regions (CIRs) at distances of less than 30 solar radii to more than 120 solar radii from the Sun. These regions are characterised by velocities intermediate between those of undisturbed fast and slow flow and by increased levels of scintillation. The long baselines between the EISCAT sites make it possible to resolve two components of plasma velocity present in the IPS line of sight, while comparisons with white-light coronagraph measurements and IMP-8 velocity measurements allow regions of compressional interaction to be identified. In this paper we present results of a study of corotating interaction regions using EISCAT data taken between 1990 and 1996. We also discuss observations of transient events (Coronal Mass Ejections) in the solar wind.

Upstream parameter dependence of anisotropies in the ion distributions downstream of quasi-perpendicular shocks

Abstract The reflection of part of the incident ions at quasi-perpendicular collisionless shocks results in highly structured and nonthermal ion distributions immediately downstream of such shocks. These distributions are unstable and lead to the generation of waves which in turn act to smooth out the distributions and thermalize the ions. Precisely which instabilities dominate the downstream turbulence depends sensitively on a number of parameters, such as the temperature anisotropy of the ion distributions and the velocity drift of the reflected ions relative to the directly transmitted/core ions. In this paper we conduct a series of one-dimensional hybrid simulations in order to estimate the values of these parameters immediately downstream of quasi-perpendicular shocks, and characterize their dependence on upstream conditions.