Magnetic reconnection in Saturn's magnetotail: A comprehensive magnetic field survey

Abstract

Reconnection within planetary magnetotails is responsible for locally energizing particles and changing the magnetic topology. Its role in terms of global magnetospheric dynamics can involve changing the mass and flux content of the magnetosphere. We have identified reconnection related events in spacecraft magnetometer data recorded during Cassini's exploration of Saturn's magnetotail. The events are identified from deflections in the north‐south component of the magnetic field, significant above a background level. Data were selected to provide full tail coverage, encompassing the dawn and dusk flanks as well as the deepest midnight orbits. Overall 2094 reconnection related events were identified, with an average rate of 5.0 events per day. The majority of events occur in clusters (within 3 h of other events). We examine changes in this rate in terms of local time and latitude coverage, taking seasonal effects into account. The observed reconnection rate peaks postmidnight with more infrequent but steady loss seen on the dusk flank. We estimate the mass loss from the event catalog and find it to be insufficient to balance the input from the moon Enceladus. Several reasons for this discrepancy are discussed. The reconnection X line location appears to be highly variable, though a statistical separation between events tailward and planetward of the X line is observed at a radial distance of between 20 and 30RS downtail. The small sample size at dawn prevents comprehensive statistical comparison with the dusk flank observations in terms of flux closure.

Dipolarization Fronts With Associated Energized Electrons in Saturn's Magnetotail

“…A key signature of a dipolarization is the sharp rotation of the magnetic field in the north‐south meridian (an increase in the B θ component): highlighted in Figure b between the vertical dashed lines. Smith et al () recently developed a technique to locate magnetic field deflections greater than background fluctuations of the field. Smith et al () set the threshold such that deflections must be greater than (root‐mean‐square) of the field, calculated for an hour interval surrounding the deflection.…”

“…In total 712 significant ( ) southward magnetic field deflections (termed MAG detections) were located within the data set (Smith et al, ). In contrast, only 58 intervals of electron energization (termed ELS detections) were identified independently within the same interval of data.…”

“…Dipolarizations can be seen to occur across most of the magnetotail, though the majority occur postmidnight ( 22 / 28 ). Figure b shows the 28 detections highlighted in blue if they occurred within 3 hr of another event (Jackman et al, ; Smith et al, ) and red if they were isolated. The 3 hr time interval was selected by scaling a 30 min timescale previously used at the Earth (Slavin et al, ).…”

“…As Saturn's magnetic field has the opposite orientation to the Earth, the deflections associated with Kronian dipolarization fronts are southward (as opposed to typical terrestrial northward turn). More recent Kronian studies have located dipolarization fronts either from their southward rotation of the magnetic field (Jackman et al, , ; Russell et al, ; Smith et al, ; Yao, Grodent, et al, ) or from the presence of an inflowing (Thomsen et al, ) or heated plasma population (Thomsen, Mitchell et al, ). In contrast to the Earth, the Saturn equivalent of bursty bulk flows may not be expected to show a dominant planetward component; any radial inflow must combine with the strong azimuthal corotational flow (Mcandrews et al, ; Thomsen et al, ).…”

Automated force‐free flux rope identification

“…To investigate whether the deflection of the field is significant, the method then searches for local maxima and minima of B Z . Defining the event using the local north‐south maxima of the field attempts to find the duration that would be chosen by eye, following the work of Slavin et al [], and more recently Jackman et al [], Vogt et al [], DiBraccio et al [], and Smith et al []. A period extending 1.25 s either side of the crossing (vertical grey bars in Figure ) is examined to find these local extrema.…”