At the inclined surfaces of such ripples, solar-wind plasma may be less decelerated and heated, yet still compressed and focused, yielding coherent high-speed jets within slower ambient plasma in the downstream magnetosheath region ( Hietala et al., 2009, 2012). As a result, the quasi-parallel shock may be regarded as undulated or rippled. The interaction region, called foreshock, exhibits localized magnetic-field and plasma structures (e.g., short large-amplitude magnetic structures SLAMS) and waves that are convected back to the shock and which merge into it and thus continuously form and reform it (e.g., Schwartz and Burgess, 1991 Omidi et al., 2005 Blanco-Cano et al., 2006 a, b). Under these conditions, shock-reflected particles are able to propagate along the IMF into the region upstream of the shock, where the particles then interact with the solar wind.
In the subsolar magnetosheath, their occurrence is, hence, enhanced when the interplanetary magnetic field (IMF) points in a quasi-radial direction, i.e., when the angle between the IMF and the Earth–Sun line – the IMF cone angle – is low. Jets are known to occur more often downstream of the quasi-parallel shock ( Archer and Horbury, 2013 Plaschke et al., 2013, 2016). Thus, jets are highly distinctive phenomena in the subsolar magnetosheath. Within those jets, the dynamic pressure can easily exceed values measured in the pristine solar wind, and a significant fraction of jets even feature super-magnetosonic plasma velocities ( Savin et al., 2008, 2014 Hietala et al., 2009 Plaschke et al., 2013). The region downstream of the Earth's bow shock, the magnetosheath, is oftentimes permeated by localized plasma entities of significantly enhanced dynamic pressure, so-called magnetosheath jets (for a recent review, see Plaschke et al., 2018). This variability is commonly somewhat larger within jets than outside them, masking the systematic decrease in ϕ at core regions of individual jets. They usually exhibit large variations over the entire range of ϕ: 0 to 90 ∘.
#Field angle omniweb series
Furthermore, time series of ϕ pertaining to individual jets significantly deviate from the superposed epoch analysis results. However, while the alignment is more significant for faster than for slower jets, and for jets observed close to the bow shock, the overall effect is small: typically, reductions in ϕ of around 10 ∘ are observed at jet core regions, where the jets' velocities are largest. Based on Magnetospheric Multiscale (MMS) jet observations and corresponding superposed epoch analyses of the angles ϕ between the velocity and magnetic fields, we can confirm that this suggestion is correct. Simulations and case study results suggest that jets also modify the magnetic field in the magnetosheath on their passage, aligning it more with their velocity. Due to their excess velocity with respect to their environment, they push slower ambient plasma out of their way, creating a vortical plasma motion in and around them. Jets in the subsolar magnetosheath are localized enhancements in dynamic pressure that are able to propagate all the way from the bow shock to the magnetopause.
0 kommentar(er)
