Foreshock bubbles and their global magnetospheric impacts

Abstract

[1] We employ 2.5-D electromagnetic, hybrid simulations that treat ions kinetically via particle-in-cell methods and electrons as a massless fluid to study the formation and properties of a new structure named the foreshock bubble upstream from the bow shock. This structure forms due to changes in the interplanetary magnetic field (IMF) associated with solar wind discontinuities and their interaction with the backstreaming ions in the foreshock prior to these discontinuities encountering the bow shock. The leading edge of the foreshock bubble consists of a fast magnetosonic shock and the compressed and heated plasma downstream of the shock. The leading edge surrounds the core which consists of a less-dense and hotter plasma and lower magnetic field strength. Ultra low frequency turbulence is present in both the outer and core regions of the foreshock bubbles. The size of the foreshock bubble transverse to the flow direction scales with the width of the ion foreshock and at Earth corresponds to tens of R E . The size along the flow depends on the age of the bubble and grows with time. Although they expand sunward, foreshock bubbles are carried antisunward by the solar wind, and for small IMF cone angles (angle between IMF and solar wind flow) when the foreshock lies upstream of the dayside magnetosphere they collide with the bow shock. This collision is shown to have significant magnetospheric impacts. Upon encountering the bow shock, the low pressures within the core of the bubble result in the reversal of the magnetosheath flow from antisunward to sunward direction. This in turn results in the outward motion of the magnetopause and expansion of the dayside magnetosphere. The interaction is found to noticeably impact the density and energy of trapped radiation belt ions and plasma injection into the cusp. Foreshock bubbles are found to be highly effective sites for ion reflection and acceleration to high energies via first-and second-order Fermi acceleration. The interaction of the foreshock bubble with the bow shock results in the release of energetic ions into the magnetosheath. Some of these ions are subsequently injected into the cusp.

The link between shocks, turbulence, and magnetic reconnection in collisionless plasmas

“…25,26 The solution found here differs from the example of FB in Ref. 25 in at least two important aspects. First, the structure in Fig.…”

“…In contrast, in the case of FB in Ref. 25, the cavity covers almost the entire extent of the area behind the shock. Given these differences, we have labeled this structure as "FB" to distinguish it from the previously reported FBs.…”

“…12,13 Previous studies have not reported the incidence of reconnection in simulations of fast shocks or global hybrid simulations possibly because (i) many fast shock studies were in 1-D where reconnection cannot occur; (ii) the very limited number of 2-D quasi-parallel shock simulations apparently did not check for incidence of reconnection; and (iii) the low number of particles per cell and or low grid resolution of these early simulations (e.g., Ref. 25 exhibits greater structural organization in the form of coherent structures such as flux ropes, vortices, and fast jets. Whether such reorganization into coherent structures is a consequence of a general governing principle of all systems far from equilibrium that are driven remains an intriguing speculation.…”

Dawn‐dusk asymmetry in bursty hot electron enhancements in the midtail magnetosheath

“…There are some transient perturbations at the bow shock that occur preferentially when there is a sharp IMF direction change associated with an IMF discontinuity, such as hot flow anomalies (HFAs) [e.g., Thomsen et al, 1986;Schwartz, 1995;Zhang et al, 2010] and foreshock bubbles (FBs) [e.g., Omidi et al, 2010;Turner et al, 2013]. These perturbations are believed to be created by kinetic interaction between IMF discontinuities and the bow shock or foreshock, so that our current physical understanding is mainly from hybrid simulations.…”

Combining Diffusive Shock Acceleration With Acceleration by Contracting and Reconnecting Small-Scale Flux Ropes at Heliospheric Shocks

“…It was demonstrated with 2D MHD simulations how large-scale density fluctuations interacting with an MHD shock result in a rippled shock that emits vorticity and drives a turbulent post-shock region filled with coherent structures resembling coherent magnetic filaments and islands (Giacalone & Jokipii 2007). Hybrid 2.5D simulations by Omidi et al (2010) and observations by Turner et al (2013) show that, under some conditions in the solar wind, relatively large-scale foreshock bubbles can be formed upstream of the Earth's bow shock and simultaneously smallerscale bubble-like structures can appear in the turbulent magnetosheath. Formation of magnetic islands separated by current sheets in the post-shock flow of the Earth's bow shock is also confirmed in the 2D hybrid and full kinetic simulations of Karimabadi et al (2014).…”