A THEMIS survey of flux ropes and traveling compression regions: Location of the near-Earth reconnection site during solar minimum

Abstract

[1] A statistical study of flux ropes and traveling compression regions (TCRs) during the Time History of Events and Macroscale Interactions during Substorms second tail season has been performed. A combined total of 135 flux ropes and TCRs in the range GSM X ∼ −14 to −31 RE were identified, many of these occurring in series of two or more events separated by a few tens of seconds. Those occurring within 10 min of each other were combined into aggregated reconnection events. For the purposes of this survey, these are most likely the products of reconnection occurring simultaneously at multiple, closely spaced x-lines as opposed to statistically independent episodes of reconnection. The 135 flux ropes and TCRs were grouped into 87 reconnection events; of these, 28 were moving tailward and 59 were moving Earthward. The average location of the near-Earth x-line determined from statistical analysis of these reconnection events is (XGSM, Y*GSM ) = (−30RE, 5RE), where Y* includes a correction for the solar aberration angle. A strong east-west asymmetry is present in the tailward events, with >80% being observed at GSM Y* > 0. Our results indicate that the Earthward flows are similarly asymmetric in the midtail region, becoming more symmetric inside −18 RE . Superposed epoch analyses indicate that the occurrence of reconnection closer to the Earth, i.e., X > −20 RE, is associated with elevated solar wind velocity and enhanced negative interplanetary magnetic field BZ. Reconnection events taking place closer to the Earth are also far more effective in producing geomagnetic activity, judged by the AL index, than reconnection initiated beyond X ∼ −25 RE.