A multi-spacecraft survey of magnetic field line draping in the dayside magnetosheath

Abstract

Abstract. When the interplanetary magnetic field (IMF) encounters the Earth's magnetosphere, it is compressed and distorted. This distortion is known as draping, and plays an important role in the interaction between the IMF and the geomagnetic field. This paper considers a particular aspect of draping, namely how the orientation of the IMF in a plane perpendicular to the Sun-Earth line (the clock angle) is altered by draping in the magnetosheath close to the dayside magnetopause. The clock angle of the magnetosheath field is commonly estimated from the interplanetary magnetic field (IMF) measured by upstream monitoring spacecraft either by assuming that the draping process does not significantly alter the clock angle ("perfect draping") or that the change in clock angle is reasonably approximated by a gas dynamic model. In this paper, the magnetosheath clock angles measured during 36 crossings of the magnetopause by the Geotail and Interball-Tail spacecraft are compared to the upstream IMF clock angles measured by the Wind spacecraft. Overall, about 30% of data points exhibit perfect draping within ±10 • , and 70% are within 30 • . The differences between the IMF and magnetosheath clock angles are not, in general, well-ordered in any systematic fashion which could be accounted for by hydrodynamic draping. The draping behaviour is asymmetric with respect to the y-component of the IMF, and the form of the draping distribution function is dependent on solar wind pressure. While the average clock angle observed in the magnetosheath does reflect the orientation of the IMF to within ∼30 • or less, the assumption that the magnetosheath field direction at any particular region of the magnetopause at any instant is approximately similar to the IMF direction is not justified. This study shows that reconnection models which assume laminar draping are unlikely to accurately reflect the distribution of reconnection sites across the dayside magnetopause.

Interplanetary magnetic field rotations followed from L1 to the ground: the response of the Earth's magnetosphere as seen by multi-spacecraft and ground-based observations

“…The larger differences between Wind and the other spacecraft observed after ∼19:00 UT are caused by incorrect time shifts of the data for those structures. Our data set fits well in the 70 % as found by Coleman (2005).…”

“…The draping of the IMF over the Earth's magnetosphere was studied by Coleman (2005) using Geotail and Interball-Tail data. This was done in order to study whether "perfect draping" (i.e.…”

“…There is not direct evidence of conjugacy of the south polar station and the ground station, however, various studies have been done where the different hemispheres were compared (see e.g. Østgaard et al, 2004, 2005Parkinson et al, 2005;Laundal et al, 2010) for symmetries and asymmetries. In this case, the good correlation between the poleward motion of the aurora and the outward motion of the magnetopause as observed by Geotail, leads to the expectation that the ground stations in the Northern Hemisphere show related signatures.…”

Fractal reconnection structures on the magnetopause

“…The IMF is non-uniform, having structure on all spatial scales, and the draping process is far from that of a laminar flow approximation. It has long been recognised that the solar wind power spectrum is consistent with that of turbulence [Coleman, 2005;Goldstein and Roberts, 1999], with an inertial range extending from $1 minute to $100 hours. Other analyses show multifractal scaling of velocity and magnetic field from $1 minute to $100 minutes [Hnat et al, 2002[Hnat et al, , 2003.…”

“…Other analyses show multifractal scaling of velocity and magnetic field from $1 minute to $100 minutes [Hnat et al, 2002[Hnat et al, , 2003. Coleman [2005] showed that the clock angle of the magnetosheath field near the magnetopause can differ from the (suitably time-delayed) IMF clock angle to a significant extent: this angular difference has a standard deviation of roughly 40deg, and the global distribution of clock angles is better characterised as ''perfect draping plus noise'' than by a hydrodynamic approximation.…”