Necessary conditions for geosynchronous magnetopause crossings

Abstract

[1] The International Solar Terrestrial Physics database of the magnetic measurements on GOES and plasma measurements on Los Alamos National Laboratory (LANL) geosynchronous satellites is used for selection of 169 case events containing 638 geosynchronous magnetopause crossings (GMCs) in 1995 to 2001. The GMCs and magnetosheath intervals associated with them are identified using advanced methods that take into account (1) strong deviation of the magnetic field measured by GOES from the magnetospheric field, (2) high correlation between the GOES magnetic field and interplanetary magnetic field (IMF), and (3) substantial increase of the midenergy ion and electron fluxes measured by LANL. Accurate determination of the upstream solar wind conditions for the GMCs is performed using correlation of geomagnetic activity (Dst (SYM-H) index) with the upstream solar wind pressure. The location of the GMCs and associated upstream solar wind conditions are ordered in an aberrated GSM coordinate system (aGSM) with X-axis directed along the solar wind flow. In the selected data set of GMCs the solar wind total pressure Psw varies up to 100 nPa and the southward IMF Bz reaches 60 nT. We study the conditions necessary for geosynchronous magnetopause crossings using scatterplots of the GMCs in the coordinate space of Psw versus Bz. In such a representation the upstream solar wind conditions show a sharp envelope boundary beyond which no GMCs are observed. The boundary has two straight horizontal branches where Bz does not influence the magnetopause location. The first branch is located in the range of Psw = 21 nPa for large positive Bz and is associated with a regime of pressure balance. The second branch asymptotically approaches the range of Psw = 4.8 nPa under strong negative Bz, and it is associated with a regime in which the Bz influence saturates. The intermediate region of the boundary ranges from moderate negative to moderate positive IMF Bz and can be well approximated by a hyperbolic tangent function. We interpret the envelope boundary as a range of necessary upstream solar wind conditions required for the magnetopause to reach geosynchronous orbit at its closest approach to the Earth (its ''perigee'' location).

Asymmetry of magnetosheath flows and magnetopause shape during low Alfvén Mach number solar wind

“…[4] Apart from some debate related to the presence of an indentation at high latitudes near the polar cusps [Zhou and Russell, 1997;Dunlop et al, 2000;Lavraud et al, 2002Lavraud et al, , 2004Zhang et al, 2007] and dawn-dusk asymmetries [Dmitriev et al, 2004;Suvorova et al, 2005;Nishino et al, 2008], the shape of the magnetopause is often assumed to be axisymmetric about the aberrated Sun-Earth line, i.e., circular. This hypothesis has been used in most empirical models of the magnetopause [e.g., Sibeck et al, 1991;Petrinec and Russell, 1996;Shue et al, 1997;Kawano et al, 1999], although models using Artificial Neural Network techniques [Dmitriev and Suvorova, 2000] or fits to asymmetric shapes from theoretical expectations [e.g., Zhuang et al, 1981;Boardsen et al, 2000] have also been devised (cf.…”

The Shape of Strongly Disturbed Dayside Magnetopause

“…The method of magnetopause identification is described in detail by Suvorova et al (2005). A magnetopause crossing was identified when one of two requirements was satisfied: (1) the GOES magnetic field deviated significantly from the geomagnetic field with a dominant northward component in the dayside mag-netosphere and (2) the B y and B z components measured by the GOES correlated with the corresponding IMF components measured by an upstream monitor.…”

“…Stronger southward IMF results in a smaller distance to the subsolar magnetopause. However, when the magnitude of negative B z exceeds a certain threshold, the magnetopause distance no longer decreases, i.e., the southward IMF influence (reconnection) is saturated (e.g., Yang et al 2003;Suvorova et al 2005). The threshold for saturation is estimated to be about -20 nT (i.e., E ~ 10 mV m -1 ) and its magnitude increases with solar wind pressure.…”

“…The envelope boundary corresponds to minimal solar wind conditions which are necessary for GMCs. It was derived by Suvorova et al (2005) on the basis of 23 magnetopause crossings observed by 7 geosynchronous satellites during 18 storm events and using solar wind data provided by 3 different upstream monitors. Thus, four outliers observed at low solar wind pressures and B z < 0 are actually just a portion of quite large statistics.…”

“…During strong geomagnetic disturbances, the magnetopause exhibits unusual and nonlinear behavior such as reconnection saturation and prominent dawn-dusk asymmetry (e.g., Dmitriev et al 2004Dmitriev et al , 2011Suvorova et al 2005).…”