Comprehensive study of the magnetospheric response to a hot flow anomaly

Abstract

Abstract. We present a comprehensive observational study of the magnetospheric response to an interplanetary magnetic field (IMF) tangential discontinuity, which first struck the postnoon bow shock and magnetopause and then swept past the prenoon bow shock and magnetopause on July 24, 1996. Although unaccompanied by any significant plasma variation, the discontinuity interacted with the bow shock to form a hot flow anomaly (HFA), which was observed by Interball-1 just upstream from the prenoon bow shock. Pressures within and Earthward of the HFA were depressed by an order of magnitude, which allowed the magnetopause to briefly (-7 min) move outward some 5 R E beyond its nominal position and engulf Interball-1.A timing study employing nearby Interball-1 and Magion-4 observations demonstrates that this motion corresponded to an antisunward and northward moving wave on the magnetopause. The same wave then engulfed Geotail, which was nominally located downstream in the outer dawn magnetosheath. Despite its large amplitude, the wave produced only minor effects in GOES-8 geosynchronous observations near local dawn. Polar Ultraviolet Imager (UVI) observed a sudden brightening of the afternoon aurora, followed by an even more intense transient brightening of the morning aurora. Consistent with this asymmetry, the discontinuity produced only weak near-simultaneous perturbations in highlatitude postnoon ground magnetometers but a transient convection vortex in the prenoon Greenland ground magnetograms. The results of this study indicate that the solar wind interaction with the bow shock is far more dynamic than previously imagined and far more significant to the solar wind-magnetosphere interaction.

Occurrence statistics of magnetic impulsive events

“…In contrast, found no preference for near-zero IMF cone angle for their events. We note, however, that both Konik et al (1994) and reported strong evidence for events to be associated with fluctuations (directional changes) in the IMF, something for which, unfortunately, we have not been able to obtain reliable results based on the hourly averages Evidence from several case studies (Sitar et al, 1998;Sibeck et al, 1999;Moretto et al, 2002;Murr et al, 2002) together with the statistical study of Murr and Hughes (2003) seem to confirm the possibility that dynamic pressure changes associated with cavities in the foreshock (created by back-streaming ions reflected at the bow shock) could be the main source of MIEs (e.g. Sibeck, 1995).…”

“…Early suggestions included bursty reconnection at the magnetopause (Glassmeier et al, 1984;Lanzerotti et al, 1986) and magnetopause motion driven by pressure pulses or abrupt changes in the interplanetary magnetic field (IMF) (Friis-Christensen et al, 1988;Sibeck et al, 1989). Currently, evidence seems to be mounting in support of foreshock generated density variations in the solar wind as an important source of TCVs (Sibeck, 1995;Sitar et al, 1998;Sibeck et al, 1999;Zesta et al, 1999;Moretto et al, 2002;Murr et al, 2002;Murr and Hughes, 2003). Particularly, Murr and Hughes (2003) very convincingly demonstrate one-to-one correspondence between TCV events and foreshock cavity-like signatures for 30 out of their 31 events.…”

Transient production of F-region irregularities associated with TCV passage

“…The time evolution of the current systems of the TCV events on 22 May and 24 July have been described by Kataoaka et al (2002) and Sitar et al (1998), respectively. The solar wind origins of the 22 May and 24 July TCV events were identified as interplanetary tangential discontinuities by Kataoka et al (2002) and Sibeck et al (1999), respectively. The equivalent convection velocities derived from magnetometer data are shown by arrows in Figs.…”

Impact of solar wind depression on the dayside magnetosphere under northward interplanetary magnetic field

“…We interpreted this reversal as a result of a gradient in the dynamic pressure from the MP toward the gap region that induced a sunward oriented mechanical force, which stopped these tiny clouds and then reversed their direction of motion (Mishin, 1993;Baraka and Ben-Jaffel, 2007). We also proposed some similarities between our gap's effect and hot flow anomalies (HFA), particularly plasma flow out of the Sun-Earth line (Sibeck et al, , 1999. Furthermore, the orientation of the cusps was found to be highly affected by the depression in the solar wind flow, while lobes were flared out when B z = 0 due to the applied depression in the solar wind flow.…”

“…Magnetic reconnection north and south of cusp regions may induce disturbances that could also distort the MP shape. Other processes may trigger magnetopause motion Earthward even when the dynamic pressure is constant, like erosion (Fairfield, 1971;Sibeck, 1991Sibeck, , 1994Tsyganenko and Sibeck, 1994) or expansion of the MP due to HFA (Sibeck, 1999).…”