Magnetopause energy transfer dependence on the interplanetary magnetic field and the Earth's magnetic dipole axis orientation

Abstract

Abstract. We examine the spatial variation of magnetospheric energy transfer using a global magnetohydrodynamic (MHD) simulation (GUMICS-4) and a large data set of flux transfer events (FTEs) observed by the Cluster spacecraft. Our main purpose is to investigate whether it is possible to validate previous results on the spatial energy transfer variation from the GUMICS-4 simulation using the statistical occurrence of FTEs, which are manifestations of magnetospheric energy transfer. Previous simulation results have suggested that the energy transfer pattern at the magnetopause rotates according to the interplanetary magnetic field (IMF) orientation, and here we investigate whether a similar rotation is seen in the locations at which FTE signatures are observed. We find that there is qualitative agreement between the simulation and observed statistics, as the peaks in both distributions rotate as a function of the IMF clock angle. However, it is necessary to take into account the modulation of the statistical distribution that is caused by a bias towards in situ FTE signatures being observed in the winter hemisphere (an effect that has previously been predicted and observed in this data set). Taking this seasonal effect into account, the FTE locations support the previous simulation results and confirm the earlier prediction that the energy transfers in the plane of the IMF. In addition, we investigate the effect of the dipole orientation (both the dipole tilt angle and its orientation in the plane perpendicular to the solar wind flow) on the energy transfer spatial distribution. We find that the energy transfer occurs mainly in the summer hemisphere, and that the dayside reconnection region is located asymmetrically about the subsolar position. Finally, we find that the energy transfer is 10 % larger at equinox conditions than at solstice, contributing to the discussion concerning the semiannual variation of magnetospheric dynamics (known as "the Russell-McPherron effect").

The effects of seasonal and diurnal variations in the Earth's magnetic dipole orientation on solar wind–magnetosphere‐ionosphere coupling

“…Variations in the cross-polar cap potential thus seem to be largely (70-90%) due to variations in solar wind-magnetosphere coupling. A recent study by Palmroth et al [2012] also demonstrated that there is a seasonal difference in this coupling. Their simulations showed a $10% larger energy transfer from the solar wind to the magnetosphere at equinox compared to solstice.…”

“…While this difference is smaller than what we report here, it agrees qualitatively with our results. Palmroth et al [2012] also noted that the energy transfer at solstice takes place preferably in the summer hemisphere. This is again consistent with our results, as we find that the separator line is tilted within the GSM x-z plane, such that the subsolar point on the separator line is in the summer hemisphere, and its tailward end in the winter hemisphere, although this is difficult to see in Figure 4.…”

Seasonal and clock angle control of the location of flux transfer event signatures at the magnetopause

“…If, for some reason, the reconnection site regularly formed significantly northward of the magnetosheath stagnation point in the Northern Hemisphere winter, and southward of the stagnation point in the Southern Hemisphere winter, then the single X line reconnection process could give rise to a seasonal bias in the same sense as the Raeder [2006] model predicts, and was observed by Korotova et al [2008] and in this study. Possible north–south asymmetries in the location of the reconnection line are still under investigation [ Palmroth et al , 2012], but unless a mechanism is found that consistently moves a single X line toward the winter hemisphere then we suggest that the observed seasonal bias is more likely to be caused by sequential multiple X line reconnection as modeled by Raeder [2006].…”

Spatial variation of energy conversion at the Earth's magnetopause: Statistics from Cluster observations

“…Presentation of these magnetopause crossings on a single magnetopause surface is problematic because the wide range of IMF and solar wind conditions indicate different sizes of the magnetopause surface. Furthermore, the Earth's dipole tilt angle has an effect on the location of the load and generator [ Palmroth et al ., ]. Hence, energy conversion during a crossing may indicate a load, but the location of a crossing may fall within a generator region if the dipole tilt angle is not taken into account.…”