Magnetic reconnection at the dayside magnetopause in global Lyon-Fedder-Mobarry simulations

We present a survey of magnetic reconnection at the dayside magnetopause using the Lyon-Fedder-Mobarry global magnetospheric simulation code. We explore simulations of the magnetospheric response under solar wind conditions with interplanetary magnetic field (IMF) clock angles of 0, 45, 90, 135, and 180 degrees. Defining the dayside reconnection rate as the total amount of magnetic flux per unit time that changes magnetic topology, we find the reconnection rate increases linearly from 45 to 135 degrees with more modest increases from 0 to 45 degrees and 135 to 180 degrees. Similar results are obtained using two other related definitions of the reconnection rate. We show the increase in the reconnection rate with greater IMF clock angles follows a trend in the orientation of the separator line relative to the solar wind magnetic field, a quantity that scales approximately as sin($theta$IMF/2), as noted by Kan and Lee (1979). For northward IMF conditions, we apply a similar argument based on the width of weak ∣B∣ regions at the magnetopause. We find the simulations exhibit two qualitatively different reconnection processes. The first process we identify as three-dimensional topology merging along a separator line, in which solar wind and dipolar field lines reconnect, creating field lines with one foot point on the Earth and the other in the solar wind. For IMF angles of 45 and 90 degrees this topology merging is a predominantly antiparallel reconnection process concentrated near nulls in the magnetic cusps, while for larger IMF clock angles of 135 or 180 degrees the reconnection extends across belt-like regions spanning the subsolar point. For clock angles of 45 and 90 degrees we also identify a second type of reconnection process that is a three-dimensional form of guide field reconnection and typically occurs in regions along the separator line closer to the subsolar point. This process can mediate the reconnection of solar wind field lines with each other or can couple to the topology merging, leading to a subset of field lines that exhibit reconnection in multiple locations at the same moment of time.