CDAW 8 observations of plasmoid signatures in the geomagnetic tail: An assessment

Abstract

A primary objective of the Coordinated Data Analysis Workshop 8 (CDAW 8) was the critical assessment of the plasmoid hypothesis. In this study, various types of magnetic phenomena, including closed loops and flux ropes, were considered as candidates for producing the north‐then‐south magnetic field perturbations characteristic of the “plasmoids” observed in the magnetotail by ISEE 3 during the CDAW 8 A and G events. For these two events the principal axis analyses of the magnetic field data and supporting energetic particle and plasma measurements are found to be consistent with the closed‐loop model of plasmoids for which the plane of rotation for B lies near the GSE X‐Z plane and only small field components lie in the minimum variance direction oriented largely along the Y direction. While small pitch angle spiraling of the field lines within these structures such as suggested by Hughes and Sibeck (1987) and Birn et al. (1989) cannot be definitively measured using single‐spacecraft magnetometer observations, the ISEE 3 measurements are not consistent with moderate strength or Venus‐type, strong core field flux ropes. The event A and G plasmoids were preceded by smoothly draped lobe field lines and followed by strong southward fields in the postplasmoid plasma sheet as predicted by the reconnection model of substorms. This latter feature of the observations is consistent with plasmoids moving down the tail, at least in part, due to the Maxwell stress exerted by lobe field lines which were disconnected at the near‐tail neutral line and have draped about the earthward side of the plasmoid. Calculations, based upon typical plasmoid and tail parameters, are presented which indicate that the J×B force associated with these disconnected lobe field lines may be sufficient to accelerate plasmoids up to the speeds observed by ISEE 3. A new characteristic of the plasmoid signature identified in this study is the presence of enhanced internal magnetic fields which can exceed the magnitude of the adjacent lobe fields by as much as 10–20%. While the internal structure of plasmoids remains poorly understood, an extension of the traveling compression region model (Slavin et al., 1984) is proposed to explain these high field regions. Finally, the relationship between the nature of individual substorms and the characteristics of the plasmoids observed in the tail by ISEE 3 is considered. The event A observations suggest that during isolated substorms the magnetotail may undergo an interval of intense reconnection which results in the creation of a single large plasmoid. Extended substorm intervals, exemplified by event G, may produce numerous smaller plasmoids as X lines form in the cislunar tail and retreat tailward while the magnetosphere continues to draw energy from the solar wind. Overall, the energy expended in accelerating the plasmoid down the tail appears comparable to that dissipated in the inner magnetosphere and ionosphere. Additional studies of plasmoid structure and dynamics as a function of substorm activity and distance down the tail should be conducted to determine the general validity of the conclusions reached on the basis of the CDAW 8 events.