Abstract
AbstractThe velocity of a collisionless shock (CS) is an important parameter in the determination of the spatial scales of the shock. The spatial scales of the shock determine the processes that guide the energy dissipation, which is related to the nature of the shock. During the pre‐ISEE era, estimations of relative shock‐spacecraft velocity (VSh) were based on spatial scales of the shock front regions, in particularly the foot. Multispacecraft missions allow more reliable identification of VSh. The main objective of this study is to examine the accuracy of two single spacecraft methods, which use the foot region of quasi‐perpendicular shocks in order determine VSh. This is important for observational shock studies based on a single spacecraft data such as Venus Express (VEX) and THOR, a proposed single spacecraft mission of European Space Agency. It is shown that neither method provides estimates with an accuracy comparable to multipoint measurements of VSh. In the absence of alternative techniques to identify the VSh and therefore the spatial scales of the shocks, the methods can be used to provided order of magnitude estimations for the spatial scales of the shock front. Observations of the Venusian bow shock from VEX data have been used as an illustrative example for the application of these methods to estimate the shock spatial scale and the corresponding errors of this estimation. It is shown that the spatial width of the ramp of the observed shock is L ∼ 3.4 ± 1.4c/ωpe.
Highlights
Collisionless shocks (CS) are wide spread in the universe
The main objective of this study is to examine the accuracy of two single spacecraft methods, which use the foot region of quasi-perpendicular shocks in order determine VSh
The final set of observations is small in count, compared to the initial number of shock crossings that were considered, but we believe it to be a representative sample for both methods, mainly due to the certainty of the measured VSh using two spacecraft
Summary
Collisionless shocks (CS) are wide spread in the universe. They are formed around ordinary stars, in binary systems, and are associated with gamma ray bursts. Important observational information about distant objects in the universe are received in the form of emissions. The source of these emissions is often particles thermalized or accelerated at the shock in the vicinity of the object. In spite of their common occurrence in the universe, only shocks in the heliosphere can be observed in situ. One of the most important parameters of the shock is the spatial scale of the shock front. The importance of the spatial scales of collisionless shock substructures is comprehensively discussed in Kennel et al [1985] and Balikhin et al [1995]. The knowledge of the shock velocity with respect to the spacecraft is key for the estimation of the cross-shock potential [Formisano, 1982; Dimmock et al, 2012]
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