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Different Transient Phenomena at the Edges of Traveling Foreshocks

Primoz Kajdic, Xóchitl Blanco-Cano, Diana Rojas-Castllo, Nojan Omidi

TL;DR

The paper investigates transient structures at the edges of traveling foreshocks (TFs) upstream of Earth's bow shock, focusing on the occurrence of HFAs and HFA-like foreshock compressional boundaries (FCBs). Using multispacecraft data from Cluster and MMS, it identifies two TF events with HFAs at edges—one detected only by the bow-shock–proximate spacecraft and another by all four—plus two MMS events where HFA-like FCBs appear at TF edges without heating of the solar wind beam. The results show that HFAs can replace FCBs at TF edges under favorable convection-electric-field conditions, while HFA-like FCBs form when IMF rotations are too thick to heat the solar wind beam, with the cores dominated by suprathermal ions. The study also discusses multi-TUMS events where TFs co-exist with several upstream structures, highlighting complex upstream-downstream coupling and suggesting implications for bow-shock dynamics and magnetosheath perturbations, consistent with prior simulations of FCB/HFA dynamics.

Abstract

Past kinetic simulations and spacecraft observations have shown that traveling foreshocks (TFs) are bounded by either foreshock compressional boundaries (FCBs) or foreshock bubbles (FBs). Here we present four TFs with a different kind of structure appearing at one of their edges. Two of them, observed by the Cluster mission, are bounded by a hot flow anomaly (HFA). In one case, the HFA was observed only by the spacecraft closest to the bow shock, while the other three probes observed an FCB. In addition, two other TFs were observed by the MMS spacecraft to be delimited by a structure that we call HFA-like FCB. In the spacecraft data, these structures present signatures similar to those of HFAs: dips in magnetic field magnitude and solar wind density, decelerated and deflected plasma flow and increased temperature. However, a detailed inspection of these events reveals the absence of heating of the SW beam. Instead, the beam almost disappears inside these events and the plasma moments are strongly influenced by the suprathermal particles. We suggest that HFA-like FCBs are related to the evolution and structure of the directional discontinuities of the interplanetary magnetic field whose thickness is larger than the gyroradious of suprathermal ions. We also show that individual TFs may appear together with several different types of transient upstream mesoscale structures, which brings up a question about their combined effect on regions downstream of the bow shock.

Different Transient Phenomena at the Edges of Traveling Foreshocks

TL;DR

The paper investigates transient structures at the edges of traveling foreshocks (TFs) upstream of Earth's bow shock, focusing on the occurrence of HFAs and HFA-like foreshock compressional boundaries (FCBs). Using multispacecraft data from Cluster and MMS, it identifies two TF events with HFAs at edges—one detected only by the bow-shock–proximate spacecraft and another by all four—plus two MMS events where HFA-like FCBs appear at TF edges without heating of the solar wind beam. The results show that HFAs can replace FCBs at TF edges under favorable convection-electric-field conditions, while HFA-like FCBs form when IMF rotations are too thick to heat the solar wind beam, with the cores dominated by suprathermal ions. The study also discusses multi-TUMS events where TFs co-exist with several upstream structures, highlighting complex upstream-downstream coupling and suggesting implications for bow-shock dynamics and magnetosheath perturbations, consistent with prior simulations of FCB/HFA dynamics.

Abstract

Past kinetic simulations and spacecraft observations have shown that traveling foreshocks (TFs) are bounded by either foreshock compressional boundaries (FCBs) or foreshock bubbles (FBs). Here we present four TFs with a different kind of structure appearing at one of their edges. Two of them, observed by the Cluster mission, are bounded by a hot flow anomaly (HFA). In one case, the HFA was observed only by the spacecraft closest to the bow shock, while the other three probes observed an FCB. In addition, two other TFs were observed by the MMS spacecraft to be delimited by a structure that we call HFA-like FCB. In the spacecraft data, these structures present signatures similar to those of HFAs: dips in magnetic field magnitude and solar wind density, decelerated and deflected plasma flow and increased temperature. However, a detailed inspection of these events reveals the absence of heating of the SW beam. Instead, the beam almost disappears inside these events and the plasma moments are strongly influenced by the suprathermal particles. We suggest that HFA-like FCBs are related to the evolution and structure of the directional discontinuities of the interplanetary magnetic field whose thickness is larger than the gyroradious of suprathermal ions. We also show that individual TFs may appear together with several different types of transient upstream mesoscale structures, which brings up a question about their combined effect on regions downstream of the bow shock.
Paper Structure (13 sections, 1 equation, 10 figures)

This paper contains 13 sections, 1 equation, 10 figures.

Table of Contents

  1. Introduction
  2. Instruments and Data Sets
  3. HFAs at the edges of traveling foreshocks
  4. 5 January 2005 event
  5. 12 January 2005 event
  6. HFA-like foreshock compressional boundaries
  7. 6 February 2022 event
  8. 30 January 2022 event
  9. Discussion
  10. Multi-TUMS events
  11. HFA-like FCBs at the edges of traveling foreshocks
  12. Conclusions
  13. Acknowledgments

Figures (10)

  • Figure 1: (A) Traveling foreshock observed on 5 January 2005 by the Cluster 1 spacecraft. Two events with major solar wind flow deceleration are associated to it. The first is a SHFA which was detected at 10:47 UT deep inside the traveling foreshock (red shading). The second event is an HFA detected at $\sim$10:50 UT at the edge of the traveling foreshock (purple shading). (B) A shorter time interval centered on the HFA. Panels in Figures (A) and (B) exhibit (from top to bottom) IMF magnitude, IMF components in GSE coordinate system, plasma density, perpendicular (blue) and parallel (red) temperatures, SW bulk speed, SW velocity components in GSE coordinate system, dynamic pressure and ion spectrum. Additionally, panel Ai exhibits a Morlet wavelet spectrum of the B$_{X,GSE}$ component. (C) Magnetic field profiles of the four Cluster spaceceraft during the HFA. (D) B$_y$ profiles of the IMF directional discontinuity at all spacecraft. The arrows mark the times used by the timing analysis.
  • Figure 2: Locations of the Cluster probes during the detection of the IMF directional discontinuity on 5 January 2005. The brown arrow represents the vector normal to the directional discontinuity while the magenta arrow indicates the direction of the SW velocity.
  • Figure 3: (A) From top to bottom: magnetic field magnitude, ion fluxes in different energy ranges and ion spectra during the time period when the traveling foreshock on 5 January 2005 was observed. (B) Ion distribution functions at selected times (marked in Figure \ref{['fig:flux20050105']}Af).
  • Figure 4: (A) Cluster 1 observations of a traveling foreshock with an HFA on its rear edge observed on 12 January 2005. (B) B$y$ profiles of the rear edge of the traveling foreshock. The arrows mark the times used in the timing analysis. (C) Positions of the Cluster probes during the detection of the IMF directional. The brown arrow represents the vector normal to the directional discontinuity while the magenta arrow indicates the direction of the SW velocity.
  • Figure 5: (A) Ion fluxes around the time of the 12 January 2005 event. (B) Ion distribution functions at selected times (marked in Figure \ref{['fig:flux20050112']}Af).
  • ...and 5 more figures