The steepness of the magnetic shear angle “saddle”: A parameter for constraining the location of dayside magnetic reconnection?

Abstract

AbstractObservations of cutoff velocities in plasma distribution functions within the cusps have been used to remotely map reconnection sites on the dayside magnetopause. From these observations, a model of the location of magnetic reconnection along the dayside magnetopause has been developed during conditions of southward interplanetary magnetic field. The reconnection merging region runs along a crest of maximum magnetic shear across the magnetopause, either along two separate antiparallel lines or along a single continuous line. The single, continuous line also follows along a crest of a “saddle” topology in the magnetic shear angle, crossing local noon at low to middle latitudes. In this study the steepness of the saddle at local noon as a function of the solar wind condition and the dipole tilt angle of the Earth is examined. In addition, the locations of dayside low to middle latitude magnetopause crossings by spacecraft for which plasma velocity signatures suggest that the reconnection X line passed over the spacecraft are compared to the expected location of the model X line and as a function of the steepness of the saddle. The comparisons between model and observations are used to help determine to what extent the steepness of the saddle constrains the location of the X line.