The location of reconnection at the magnetopause: Testing the maximum magnetic shear model with THEMIS observations

Abstract

[1] Reconnection at the Earth's magnetopause is the mechanism by which magnetic fields in different regions change topology to create open magnetic field lines that allow energy and momentum to flow into the magnetosphere. One of the long-standing open questions about magnetic reconnection is the location of the reconnection line. There are two reconnection scenarios discussed in the literature: (1) antiparallel reconnection where shear angles between the magnetospheric field and the interplanetary magnetic field (IMF) are near 180°and (2) component reconnection where a tilted reconnection line which crosses the magnetopause in the subsolar region at shear angles not near 180°. Early satellite observations were limited to the detection of accelerated ion beams in the magnetopause boundary layer to determine the general direction of the reconnection line location with respect to the satellite. An improved view of the reconnection location at the magnetopause was determined from ionospheric emissions observed by polar-orbiting imagers which revealed that both scenarios occur. The time-of-flight effect of precipitating ions in the cusp in connection with the low-velocity cutoff method pinpointed reconnection locations and their dependency on IMF conditions. These results are summarized by the maximum magnetic shear model. This study uses confirmed magnetic reconnection locations from the THEMIS mission to test the predictions of this reconnection location model. The results reveal that the maximum magnetic shear model predicts the observed reconnection locations for dominant IMF B Y conditions very well but needs further improvement and modifications for dominant southward IMF conditions.

Magnetospheric Multiscale Science Mission Profile and Operations

“…Although no statistical survey of reconnection region encounters has been performed, the predicted magnetopause crossings are an indication of the probability that MMS will encounter an electron diffusion region. The encounter probabilities of a few percent given in Table 1 (see also Griffiths et al 2011) are consistent with the relatively rare encounters of the reconnection line by missions such as Cluster and THEMIS (e.g., Trattner et al 2012).…”

“…The location of reconnection is empirically related to regions of high and intermediate shear. The empirical model has been validated in several independent tests Dunlop et al 2011;Trattner et al 2012). The model predicts the location of the reconnection line using the interplanetary magnetic field (IMF) orientation.…”

Theory and Modeling for the Magnetospheric Multiscale Mission

“…While all the models predict the nearly equatorial reconnection line observed during intervals of strongly southward IMF orientations (e.g., Phan et al 2000), the maximum shear model has proven remarkably successful in predicted the locations where reconnection is observed in situ (Trattner et al 2012;Dunlop et al 2011aDunlop et al , 2011b or inferred from remote observations for other IMF orientations (Trattner et al 2007). The distribution of bipolar magnetic field signatures and streaming suprathermal electron anisotropies within FTEs is consistent with reconnection along a tilted component reconnection line passing through the vicinity of the subsolar point on the magnetopause (Daly et al 1984).…”

Swarm and ESR observations of the ionospheric response to a field‐aligned current system in the high‐latitude midnight sector

“…The Z GSM component of the interplanetary magnetic field (IMF), which plays a crucial role in the solar wind-magnetosphere coupling, remains slightly positive during the whole time. It has to be noted, however, that the Y GSM component measured around À5 nT but decreased to À15 nT after the arrival of the discontinuity and may thus act as a reconnecting component and actually couple effectively the IMF and the Earth's magnetic field [e.g., Trattner et al, 2012]. The X component remains between À5 nT and 0 nT throughout the event.…”