Magnetopause reconnection rate estimates for Jupiter's magnetosphere based on interplanetary measurements at ~5AU

Abstract

Abstract.We make the first quantitative estimates of the magnetopause reconnection rate at Jupiter using extended in situ data sets, building on simple order of magnitude estimates made some thirty years ago by Brice and Ionannidis (1970) and Kennel andCoroniti (1975, 1977). The jovian low-latitude magnetopause (open flux production) reconnection voltage is estimated using the Jackman et al. (2004) algorithm, validated at Earth, previously applied to Saturn, and here adapted to Jupiter. The high-latitude (lobe) magnetopause reconnection voltage is similarly calculated using the related Gérard et al. (2005) algorithm, also previously used for Saturn. We employ data from the Ulysses spacecraft obtained during periods when it was located near 5 AU and within 5 • of the ecliptic plane (January to June 1992, January to August 1998, and April to October 2004), along with data from the Cassini spacecraft obtained during the Jupiter flyby in 2000/2001. We include the effect of magnetospheric compression through dynamic pressure modulation, and also examine the effect of variations in the direction of Jupiter's magnetic axis throughout the jovian day and year. The intervals of data considered represent different phases in the solar cycle, such that we are also able to examine solar cycle dependency. The overall average low-latitude reconnection voltage is estimated to be ∼230 kV, such that the average amount of open flux created over one solar rotation is ∼500 GWb. We thus estimate the average time to replenish Jupiter's magnetotail, which contains ∼300-500 GWb of open flux, to be ∼15-25 days, corresponding to a tail length of ∼3.8-6.5 AU. The average high-latitude reconnection voltage is estimated to be ∼130 kV, associated with lobe "stirring". Within these averages, however, the estimated voltages undergo considerable variation. Generally, the low-latitude reconnection voltage exhibits a "background" of ∼100 kV that is punctuated by one or two significant enhancement events during each solar rotation, in which the voltage is elevated to ∼1-3 MV. The high-latitude voltages are estimated to be about a half of these values. WeCorrespondence to: J. D. Nichols (jdn@ion.le.ac.uk) note that the peak values of order a few MV are comparable to the potential drop due to sub-corotating plasma flows in the equatorial magnetosphere between ∼20 R J and the magnetopause, such that during these periods magnetopause reconnection may have a significant effect on the otherwise rotationally dominated magnetosphere. Despite such variations during each solar rotation, however, the total amount of open flux produced during each solar rotation varies typically by less than ∼30% on either side of the overall average for that epoch. The averages over individual data epochs vary over the solar cycle from ∼600 GWb per solar rotation at solar maximum to ∼400 GWb at solar minimum. In addition we show that the IMF sector with positive clock angle is favoured for reconnection when the jovian spin axis clock angle is also positive, and vice versa, ...

Modulation of Jupiter's plasma flow, polar currents, and auroral precipitation by solar wind-induced compressions and expansions of the magnetosphere: a simple theoretical model

“…We have also considered expansions of an initially compressed system with a magnetopause radius of 45 R J outward to radii up to 90 R J . These are intended to represent the largest commonly-occurring solar wind-induced transitions that take place within the jovian system (e.g., Nichols et al, 2006). In this section we will summarise and discuss these results, beginning with the effects of solar windinduced compression.…”

“…The associated variations in the solar wind dynamic pressure are sufficient to regularly produce changes in the size of the jovian magnetosphere by factors of at least two in linear dimension (e.g., Huddleston et al, 1998). Minimum subsolar magnetopause radii are typically ∼45 R J for maximum solar wind dynamic pressures of a few tenths of a nPa, while maximum subsolar radii are typically ∼90 R J for minimum dynamic pressures of a few hundredths of a nPa (Huddleston et al, 1998;Nichols et al, 2006). The expected effect of these changes on jovian auroral emissions within the above scenario was first discussed by Southwood and Kivelson (2001) and Cowley and Bunce (2001), who noted that the basic effect should be an anti-correlation of the intensity of the main oval with the solar wind dynamic pressure.…”

“…However, a characteristic feature of the interplanetary medium at Jupiter's orbit is the extreme and rapid variations of plasma and field parameters that are due to the formation of corotating interaction regions (CIRs) in the heliosphere, and to the outflow of coronal mass ejections (CMEs) from the Sun (e.g., Gosling and Pizzo, 1999;Nichols et al, 2006). The associated variations in the solar wind dynamic pressure are sufficient to regularly produce changes in the size of the jovian magnetosphere by factors of at least two in linear dimension (e.g., Huddleston et al, 1998).…”

Significance of Dungey-cycle flows in Jupiter's and Saturn's magnetospheres, and their identification on closed equatorial field lines

“…Lowest field strengths and voltages occur during several-day CIR rarefaction regions during which the solar wind speed falls with time, while highest field strengths and voltages occur during few-day CIR compression regions when the solar wind speed increases (usually across paired shocks) and during few-day CMEs. Considering first the results for Jupiter, Nichols et al (2006) The data thus span a complete solar cycle, though the variations over the cycle were found to be relatively modest. Typical magnetopause reconnection voltages were estimated to be V DC ∼150 kV during rarefaction regions, which occur ∼90% of the time, increasing by an order of magnitude to V DC ∼1MV during CIR compressions and CMEs, which occur ∼10% of the time (i.e.…”

“…Kennel and Coroniti, 1975). Recently, however, more detailed estimates have been published, based on the growing collection of interplanetary data obtained in the vicinity of the planetary orbits by the Ulysses and Cassini spacecraft (Jackman et al, 2004;Nichols et al, 2006). These studies employed an empirical formula for the magnetopause reconnection voltage in terms of upstream interplanetary parameters, which was validated and adapted from studies at the Earth (e.g.…”