Introduction
Hot flow anomalies (HFAs) are kinetic phenomena occurred near the bow shock that are marked by greatly heated solar wind plasmas and substantial flow deflection. The ion (electron) temperature inside HFAs is typically 107K (106
K). The hot tenuous plasma is usually bounded by regions of enhanced magnetic field strength and density. The outer edges of these enhancements are fast shocks and the inner edges are tangential discontinuities which separate the dense "sheath" population from the tenuous and hot plasma at the center of the HFA. Some HFAs show magnetic field and density enhancement only on the trailing edge. HFAs are produced by the interaction of certain upstream discontinuities with the bow shock. They evolve and grow with time.
Observations of a series of HFAs and their proto-HFAs by multiple THEMIS spacecraft have been reported recently. Zhang et al. (2010) have addressed the following outstanding problems: (1) How are ions and electrons heated inside HFAs? (2) How far do HFAs extend upstream from the bow shock? (3) Where do HFAs occur?
Observations
Figure 1 shows the trajectory of the THEMIS spacecraft from 0100 to 1300 UT on 19 August 2008. THEMIS B and C were upstream from the bow shock. A red triangle and a green asterisk mark the positions of B (red) and C (green). The positions of probes are 14 RE and 5 RE upstream from the bow shock at 1300 UT.
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| Figure 1. THEMIS trajectory projected in GSM THEMIS trajectory projected in GSM (left) X-Y plane and (right) X-Z plane from 0100 to 1300 UT on 19 August 2008. The positions of THEMIS B and C probes at 1300 UT are marked by a red triangle and green asterisk, respectively. During this 12 h time interval, both THEMIS B and C were upstream of the bow shock. |
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THEMIS C observed a series of seven HFAs from 0100 to 1300 UT on 19 August 2008. Figure 2 shows an overview plot of these seven HFAs. Clear flow deflections mark the seven HFAs identified by arrows in Figure 2d. As can be seen from Figure 1, THEMIS B was further upstream from the bow shock than THEMIS C. Although THEMIS B did not observe any significant deflections in the solar wind flow during this interval, it did observe 4 proto-HFAs which are characterized by density and magnetic field strength depletions and electron heating. These proto-HFAs developed into HFAs 2, 4, 5, and 6 which were observed by THEMIS C downstream.
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| Figure 2. An overview plot of THEMIS C observations of a serious of seven HFAs upstream from the bow shock. (a) Components of the magnetic field in GSM coordinate system, (b) magnetic field magnitude, (c) plasma ion density (in cm-3), (d) components of plasma ion flow, and (e) plasma ion pressure are shown. |
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Figure 3 shows HFA 4 observed by THEMIS C (right) and the corresponding proto-HFA observed by THEMIS B (left). The magnetic fields recorded by THEMIS B and C are very similar (with 110 s time shift) except during the event time interval (Figure 3a). The proto-HFA observed by THEMIS B is characterized by decreases (~50%) in the magnetic field strength and plasma density (Figure 3b and 3c), moderate ion and electron heating (Figures 3e and 3f), and turbulent but not deflected solar wind flow (Figure 3d). After 110s, this proto-HFA developed into a mature HFA observed by THEMIS C. It has a very hot core (~107K) bounded by regions of strongly enhanced magnetic field strength and density. The magnetic fields were very turbulent inside the hot core but without much depletion in the field strength and plasma density (Figures 3b and 3c). The solar wind flow was significantly deflected (Figure 3d).
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| Figure 3. Comparison of THEMIS C observations of (right) HFA 4 and (left) the corresponding proto-HFA observed by THEMIS B upstream from the bow shock. (a) Components of the magnetic field in GSM coordinate system, (b) magnetic field magnitude, (c) plasma density, (d) components of plasma ion flow, (e) plasma ion spectrum, and (f) plasma electron spectrum are shown. The ion (electron) temperature measured by THEMIS B is overplotted in black dots and using the right-hand axis in Figure 3e (Figure 3f). |
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Figure 4 (left) shows the ion distribution recorded by THEMIS B at the center of a proto-HFA. It contains two major ion populations, a solar wind beam (along the thick red line) and a reflected one (along the black line. Analysis of the density and magnetic field measurements showed that these two ion beams are coupled through the electromagnetic right-hand resonant ion beam instabilities and evolve into a single, hot, thermalized ion population as shown in the right panel of Figure 4. The original kinetic energy of the two ion beams is converted to thermal energy of ions. The lower hybrid waves are the likely source of the electron heating inside HFAs.
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| Figure 4. Ion distribution recorded by (left) THEMIS B at the center of a proto-HFA and (right) THEMIS C at the center of a mature HFA. The thick black line points toward the sun and the thick red line points in the solar wind direction. |
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Conclusions
With two spacecraft upstream from the Earth's bow shock on 19 August 2008, THEMIS offered a remarkable opportunity to track the propagation and evolution of HFAs. Observations of a series of HFAs and their proto-HFAs showed convincingly that HFAs start with two ion populations, a solar wind beam and a reflected ion distribution. These two ion beams are coupled through the electromagnetic right-hand resonant ion beam instabilities and evolve into a single, hot, thermalized ion population. The original kinetic energy of the two ion beams is converted to the thermal energy of ions. The lower hybrid waves are the likely source of the electron heating inside HFAs. The scale length of some HFAs might exceed 14. An example of an HFA that lies displaced toward the side of the TD with a quasi-parallel bow shock configuration rather than lying centered on the driving IMF discontinuity has been shown.
Source
Zhang, H., D.G. Sibeck, Q.-G. Zong, S.P. Gary, J.P. McFadden, D. Larson, K.-H. Glassmeier, and V. Angelopoulos (2010), THEMIS observations of a series of hot-flow anomaly events, J. Geophys. Res., 115, A12235, doi:10.1029/2009JA015180.
Biographical Note
Hui Zhang is an assistant professor at Physics Department, University of Alaska Fairbanks. Her research interests focus on structure and dynamics of the Earth's dayside magnetosphere including the magnetosheath, magnetopause, boundary layer, cusp; interaction of solar wind discontinuities (including shocks) with the earth's bow shock and the global effects resulting from the interaction; multiple spacecraft data analysis.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu
