Hot flow anomalies (HFAs) result from the interaction of an interplanetary current sheet with Earth's bow shock and were discovered over a decade and a half ago. The deflected flow and hot interior of an HFA are consequences of ions reflected at the bow shock being channeled along the current sheet. Previous studies have shown that this requires a solar wind motional electric field pointing toward the current sheet on at least one side and that the current sheet must be a tangential discontinuity. Recent reports of a rapid displacement of the magnetopause by 5 Re as the result of an HFA have led us to explore the interplanetary conditions surrounding all reported HFAs. The kinetic aspects of HFA formation suggest that current sheets should pass relatively slowly along the bow shock; that is, their normals should have large cone angles. This hypothesis is confirmed. Individual multispacecraft case studies confirm that the underlying current sheets are tangential discontinuities, but most HFAs have relatively small jumps in field magnitude from before to after and thus would fail traditional identification tests as definite tangential discontinuities. The combination of our results suggests that HFAs should occur at a rate of several per day, and thus they may play a significant role in the solar-terrestrial dynamics.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999JA000320

Conditions for the formation of hot flow anomalies at Earth's bow shock

[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 RE. 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.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2009JA014828

Foreshock bubbles and their global magnetospheric impacts

The magnetosheath flow may take the form of large amplitude, yet spatially localized, transient increases in dynamic pressure, known as “magnetosheath jets” or “plasmoids” among other denominations. Here, we describe the present state of knowledge with respect to such jets, which are a very common phenomenon downstream of the quasi-parallel bow shock. We discuss their properties as determined by satellite observations (based on both case and statistical studies), their occurrence, their relation to solar wind and foreshock conditions, and their interaction with and impact on the magnetosphere. As carriers of plasma and corresponding momentum, energy, and magnetic flux, jets bear some similarities to bursty bulk flows, which they are compared to. Based on our knowledge of jets in the near Earth environment, we discuss the expectations for jets occurring in other planetary and astrophysical environments. We conclude with an outlook, in which a number of open questions are posed and future challenges in jet research are discussed.

/pdf/jets-downstream-of-collisionless-shocks-3moixq56v6.pdf

Jets Downstream of Collisionless Shocks

Hot flow anomalies near the Earth's bow shock

Results are presented from two-dimensional hybrid simulations of the interaction of a supercritical quasi-perpendicular collisionless shock wave with current sheets embedded in the upstream flow. The current sheet normals are perpendicular to the shock normal. When the motional electric field is directed toward the discontinuity, the interaction leads to the generation of a region with high ion temperature, low magnetic field, and low density, which has many properties of a class of events variously referred to as active current sheets, hot diamagnetic cavities, or hot flow anomalies (HFAs). The simulations demonstrate that the HFA results from the interaction of ions reflected at the shock with the current sheet. The interaction does not involve an instability and is not caused by the collision of the current sheet and the shock but rather is a property of a shock with an embedded current sheet. The simulations suggest that the HFAs formed in this way are always attached to the shock and extend into both the upstream and downstream regions.

Hybrid simulation of the formation of a hot flow anomaly

A study is conducted for the case of a tangential discontinuity (TD) that convects into a shock at some arbitrary angle, in order to clarify events observed in proximity to the earth's bow shock. Very different behavior is noted to result depending on the sense of ion gyration relative to the TD. The fact that particles can be injected into the TD's plane, so that they travel upstream close to the TD, implies that active current sheet (ACS) events thus far assumed to be generated by the solar wind's interaction with a large-density reflected component are actually detached from the bow shock. In other geometries, ions stay close to the shock after their TD interaction, implying that ACS events are modifications of the shock.

D. Burgess

Papers

On the effect of a tangential discontinuity on ions specularly reflected at an oblique shock