Azimuthal extent and properties of midtail plasmoids from two‐point ARTEMIS observations at the Earth‐Moon Lagrange points

Abstract

Abstract Although distant‐tail plasmoids are perceived to extend across most of the magnetotail (~40 R E ), recent studies in the near‐Earth region ( X > −30 R E ) have revealed that near‐Earth reconnection (where plasmoids originate) is likely localized and takes place preferentially on the duskside. This discrepancy in plasmoid azimuthal extent suggests that a plasmoid may grow as it moves from near‐Earth to the distant tail. Comprehensive multipoint, midtail plasmoid observations can be used to test this hypothesis. Between October 2010 and July 2011 the ARTEMIS spacecraft (P1 and P2) at the Earth‐Moon Lagrange points (midtail, X ~ −45 to −65 R E ) provided simultaneous two‐point observations across the magnetotail for 4 days every lunar month, with a large range of spacecraft separations (0.1 to 25 R E ). We find that plasmoids near lunar orbit, like other near‐Earth reconnection‐related phenomena, occur preferentially on the duskside of the magnetotail. Two‐point ARTEMIS observations reveal that the typical plasmoid azimuthal size in our data set is about 5 to 10 R E , much smaller than expected from previous distant‐tail observations. Plasmoids with an azimuthal size greater than 9 R E also exist but only at geomagnetic activity levels higher ( AE peak > 400 nT) than typically found in our data set (median AE peak ~ 230 nT for our plasmoid data set and median AE ~ 100 nT during the entire period of ARTEMIS magnetotail observations). We conclude that plasmoids during small to moderate substorms ( AE peak < 400 nT) do not grow beyond ~10 R E until they have moved tailward of ~ −45 to −65 R E . Plasmoids during large substorms ( AE peak > 400 nT), however, either grow beyond ~10 R E before they reach lunar distance or initially extend across a large portion of the magnetotail.