Role of magnetosheath force balance in regulating the dayside reconnection potential

Abstract

[1] When the interplanetary magnetic field (IMF) is southward, most of the ionospheric potential is generated by merging between the IMF and the magnetospheric field. Typically, the ionospheric potential responds linearly to the magnitude of the southward IMF. However, when the IMF magnitude is large, the ionospheric potential saturates and it becomes relatively insensitive to further increases in the IMF magnitude. We present evidence from simulations that under purely southward IMF conditions, the value of the portion of the potential due to reconnection is controlled by the divergence of the magnetosheath flow, which determines the geoeffective length in the solar wind. Typically, the gradient in the plasma pressure controls the magnetosheath flow, so as the southward IMF increases in magnitude, the change in the magnetosheath force balance is negligible, the geoeffective length in the solar wind does not change, and the reconnection potential increases linearly with the magnitude of the IMF. However, when the IMF magnitude increases to the point where J × B becomes the dominant force in the magnetosheath, further increases in IMF magnitude do affect the overall force balance, diverting more flow away from the merging line, decreasing the geoeffective length, and limiting the global merging rate. Thus magnetosheath force balance can be seen as a single organizing factor that regulates the geoeffective length in the solar wind for the entire range of solar wind parameters.

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

“…Much further work is required to develop a full understanding of magnetosheath physics and of how magnetosheath properties affect solar wind-magnetosphere interaction in many important ways [e.g., Lavraud and Borovsky, 2008;Lopez et al, 2010Lopez et al, , 2011.…”

“…Another important parameter in solar wind-magnetosphere interaction is the solar wind Alfvén Mach number M A : the ratio of the bulk solar wind to Alfvén speeds [Lavraud and Borovsky, 2008;Borovsky, 2008]. This is because M A directly controls the bow shock compression ratio and the value of the plasma b (ratio of the thermal plasma to magnetic pressures) in the downstream magnetosheath, which in turn changes which of the magnetic or thermal plasma forces dominates the dynamics in the magnetosheath [Lavraud and Borovsky, 2008;Borovsky et al, 2009;Lopez et al, 2010Lopez et al, , 2011. As a buffer region between the solar wind and magnetosphere, the magnetosheath plays a pivotal role in the global interaction.…”

Circulation of Heavy Ions and Their Dynamical Effects in the Magnetosphere: Recent Observations and Models

“…This reduces the geoeffective lengtha scale length indicative of the amount of solar wind flow potential that is applied to the magnetosphere (Burke et al 1999). While Lopez et al (2010) used this dynamic to help explain CPCP saturation, found it applicable to the present case of non-saturated CPCP reductions when polar wind-like outflow is included in global simulations. Garcia et al (2010) presented supporting evidence in simulations that used the LFM with prescribed outflow relegated to the nightside ionosphere only.…”

“…noted that as outflow fuels the ring current, the magnetosphere expands, especially at the flanks, leading to the CPCP reduction. Lopez et al (2010) described the physical mechanisms behind this: as the magnetosphere becomes a blunter obstacle to the solar wind, the solar wind flow has to traverse a greater distance across a wider magnetosheath and hence experiences a greater deflection due to magnetosheath forces. This reduces the geoeffective lengtha scale length indicative of the amount of solar wind flow potential that is applied to the magnetosphere (Burke et al 1999).…”

Simulated ring current response during periods of dawn‐dusk oriented interplanetary magnetic field (By)

“…[4] Several studies have examined the relationship between (1) enhanced CPCP and the southward interplanetary magnetic field (IMF), and (2) enhanced ring current response and the southward IMF [e.g., Gonzalez et al, 1994;Russell et al, 2001;Siscoe et al, 2002aSiscoe et al, , 2002bSiscoe et al, 2004;Merkin et al, 2005;Ridley, 2007;Kivelson and Ridley, 2008;Lavraud and Borovsky, 2008;Lopez et al, 2009;Lopez et al, 2010]. Gonzalez et al [1994] found that ring current response grows in response to prolonged periods of southward IMF in data.…”