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

Abstract

[1] Most models of flux transfer event (FTE) formation produce pairs of structures which in general move away from the subsolar region and give rise to signatures which can be observed in both the Northern and Southern Hemispheres. The multiple reconnection line (X line) model is unusual as a reconnection-based model that is capable of producing a single flux rope if only two X lines are present. Raeder (2006) reported the results of an MHD simulation where he studied the effect of the Earth's dipole tilt on reconnection at the dayside magnetopause for a southward IMF orientation; in his simulations, flux ropes were formed by the sequential formation of X lines, and when the dipole tilt was set to a value representative of solstice the flux ropes moved preferentially toward the winter hemisphere. Some observational evidence has previously been presented for a bias toward FTE signatures being observed in the winter hemisphere; in this paper, we present further observational evidence for this phenomenon, using an independently derived data set. Once the seasonal bias is taken into account, we find that the IMF clock angle controls the location of FTE signatures. We also find that the effective dipole tilt (combining the geomagnetic dipole tilt with the IMF tilt angle) provides no clear control of the location of FTE signatures.

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

“…The subsequent motion of most FTEs was towards the winter hemisphere. There is observational evidence from both INTER-BALL and Cluster that this effect is indeed present at the magnetopause to some degree (Korotova et al, 2008;Fear et al, 2012), although Fear et al (2012) found that in situ FTE signatures were also observed near equinox, contrary to the Raeder (2006) prediction. If this seasonal bias is present, then this introduces a limit to the use of FTE signatures as a proxy for energy transfer.…”

“…Note that more FTE signatures were observed when the IMF was southward and dawnward than when it was southward and duskward. This is in part because the IMF was more likely to have a southward/dawnward orientation than a southward/duskward orientation during the interval that was surveyed (Fear et al, 2007), but it is also because Cluster observed the post-noon Northern Hemisphere magnetopause and pre-noon Southern Hemisphere magnetopause (the most likely locations for FTE signatures when the IMF is southward and dawnward) during the local hemisphere's winter (Fear et al, 2012). The events which occur when the IMF is strongly northward are observed at the postterminator magnetopause, and are consistent with an origin at a high latitude reconnection site (Fear et al, 2005).…”

“…They found that the location of standard and reverse polarity FTE signatures was generally well ordered by the IMF orientation, but that this effect was complicated by the seasonal modulation predicted by Raeder (2006). Fear et al (2012) found that FTEs were observed preferentially in the Northern Hemisphere post-noon and Southern Hemisphere pre-noon. Since Cluster observed the post-noon magnetopause in the Northern Hemisphere winter (November 2002-February 2003 and the pre-noon magnetopause mostly during Southern Hemisphere winter (MarchJune 2003), this trend is consistent with the bias towards the winter hemisphere that was predicted by Raeder (2006).…”

“…First, we investigate the rotation of the energy transfer pattern at the magnetopause as a response to IMF B y by using the statistical occurrence of FTEs as an observational means to validate simulation results. Second, since the location of the FTE signatures appears to depend upon the dipole tilt (Raeder, 2006;Korotova et al, 2008;Fear et al, 2012), we also investigate the dipole tilt dependence of the energy transfer in the GUMICS-4 simulation which, to our knowledge, has not been examined before. The paper is organized as follows: first we describe the GUMICS-4 global MHD simulation and the FTE data set.…”

“…When the orbit crossed the magnetopause on the flanks, the magnetopause crossings occurred near apogee and so occurred near the equator; on the other hand, due to the high inclination of the Cluster spacecraft the magnetopause crossings nearer noon occurred at higher latitudes. Recently, Fear et al (2012) presented a further analysis of this dataset, examining the relationship between the location at which FTE signatures are observed and the IMF B y component. They found that the location of standard and reverse polarity FTE signatures was generally well ordered by the IMF orientation, but that this effect was complicated by the seasonal modulation predicted by Raeder (2006).…”