On Multiple Reconnection X-lines and Tripolar Perturbations of Strong Guide Magnetic Fields
Lavraud, B.; Khotyaintsev, Yu. V.; Carr, C. M.; Goldman, M. V.; Newman, D. L.; Lapenta, G.; Phan, T. D.; Gosling, J. T.; Eriksson, S.; Markidis, S.
United States, Belgium, France, Sweden, United Kingdom
Abstract
We report new multi-spacecraft Cluster observations of tripolar guide magnetic field perturbations at a solar wind reconnection exhaust in the presence of a guide field BM which is almost four times as strong as the reversing field BL. The novel tripolar field consists of two narrow regions of depressed BM, with an observed 7%-14% ΔBM magnitude relative to the external field, which are found adjacent to a wide region of enhanced BM within the exhaust. A stronger reversing field is associated with each BM depression. A kinetic reconnection simulation for realistic solar wind conditions and the observed strong guide field reveals that tripolar magnetic fields preferentially form across current sheets in the presence of multiple X-lines as magnetic islands approach one another and merge into fewer and larger islands. The simulated ΔBM/ΔXN over the normal width ΔXN between a BM minimum and the edge of the external region agree with the normalized values observed by Cluster. We propose that a tripolar guide field perturbation may be used to identify candidate regions containing multiple X-lines and interacting magnetic islands at individual solar wind current sheets with a strong guide field.