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The Response of Jupiter's Magnetosphere to an Outburst on Io

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The Response of Jupiter's Magnetosphere to an Outburst on Io The Resp onse of Jupiters Magnetosphere to an Outburst on Io Michael E Brown and Antonin H Bouchez Division of Geological and Planetary Sciences California Institute of Technology Pasadena California submitted to Science April BrownBouchez Abstract Loss of material from the satellite Io provides the ma jor source of material for Jupiters magnetosphere b ombardmentby particles in Jupiters magnetosphere is the ma jor cause of loss of material from Io Though this basic outline is clear the tightly coupled interaction b etween Io and Jupiters magnetosphere has long resisted understanding Wehave conducted a sixmonthlong campaign of intensive monitoring of the Io plasma torus and neutral so dium clouds to use the timevariable b ehavior of the system to determine the characteristics of the interaction During the observations a large outburst of material from Io whichwehyp othesize to b e caused by the eruption of a massivevolcanic plume on the satellite caused a transient increase in the neutral cloud and plasma torus masses The resp onse of the plasma torus to this outburst clearly shows that the interaction b etween Io and Jupiters magnetosphere is stabilized by a feedbackmechanism in which increases in the plasma torus mass cause a nonlinear increase in loss from the plasma torus limiting plasma buildup Jupiters magnetosphere is lled with plasma mostly derived from Io the innermost of the large satellites of Jupiter and the most volcanically activebody in the solar system The magnetospheric plasma and the volcanos on Io are coupled in a complex interaction the volcanos feed material primarily sulfur and oxygen atoms and comp ounds to the atmosphere and surface frosts of Io b ombardmentby magnetospheric plasma removes the atmosphere and frosts into an extended cloud of gas surrounding the satellite this gas is ionized by collisions with the magnetospheric plasma b ecomes incorp orated into the Io plasma torus the inner dense p ortion of the Jovian magnetosphere and returns to further b ombard Io in a tightly coupled lo op Although the situation as describ ed ab ove sounds inherently unstable ob servations haveshown that the plasma density in the Io plasma torus remained roughly constantover at least a year p erio d even though volcanic activity on Io is presumably sp oradic Stability of the system in the face of such varying volcanic input must b e achieved through some feedbackmechanism either through the regulation of supply to the extended gas clouds surrounding Io or through regulation of loss from the plasma torus We will refer to these twotyp es of feedbackmechanisms as supplylimited and losslimited resp ectively The typ e of stabilizing mechanism op erating is determined by the detailed characteristics of the interaction b etween Io and the plasma torus and of the plasma transp ort pro cesses in the Jovian magnetosphere a knowledge of the stabilitymechanism thus provides insightinto these two p o orly understo o d phenomena The system will b e supplylimited if supply to the extended neu tral gas clouds do es not increase linearly with increasing plasma b ombardment Two atmospheric mo dels which lead to supplylimitation include an ionospheric BrownBouchez buering mo del where an increase in plasma b ombardment increases Ios iono sphere which then b egins to deect the plasma b ombardment and reduce the supply of material to the neutral clouds and a constantsource mo del where supply to the neutral clouds is a characteristic solely of Ios atmosphere and do es not change with changing plasma b ombardment We currently lack sucientknowledge of Ios atmosphere to b e able to evaluate these mo dels The system will b e losslimited if loss from the plasma torus dep ends non linearly on plasma mass such that an increase in plasma mass causes an even larger increase in plasma loss The only plasma transp ort mo del currently de velop ed whichwould lead to losslimitation hyp othesize that depletion of the plasma torus is driven bycentrifugallydriven diusion a nonlinear diusion caused by the fast rotation of the Jovian magnetosphere In such a system the diusion rate is prop ortional to the square of the plasma density rather than to the rst p ower of the density for linear diusion so the diusion rate and thus mass loss dep end nonlinearly on mass Another p ossible losslimiting mechanism could b e plasma transp ort initiated by largescale plasma instability where the onset and growth of the instabilitywould lead to a nonlinear plasma loss Currently few observational constraints on the plasma transp ort mechanism are available The resp onse of the system to variable volcanic input dep ends strongly on the typ e of feedback op erating Fig thus determination of the timevariable behavior of the system would provide signicant insightinto p o orly understo o d asp ects of Io its atmosphere the Jovian magnetosphere and their interactions To determine this timevariable b ehavior we monitored emission from the Io plasma torus and from the extended neutral cloud of material surrounding Io for months in A ma jor p erturbation to the system o ccurred in March We rep ort here on this p erturbation most likely due to a massivevolcanic outburst on Io and on the implications for Ios atmosphere and volcanos and for the interaction of Io with the Jovian magnetosphere Observations We observed the Io plasma torus and the extended neutral cloud on clear nights b etween Decemb er and June using the Lick Observatory meter coude auxiliary telescop e connected to the Hamilton echelle sp ectrograph Light from a slit arcminutes long by arcseconds wide was passed through the sp ectrograph and disp ersed to a resolution of of ab out A The slit was centered on the p osition of Jupiter and aligned with either the plasma torus plane for observations of plasma torus ion emissions or the Io orbital plane for observations of the extended neutral cloud The arcminute extent of the slit gave a spatial resolution of ab out R for J a distance of ab out R on the east dawn and R on the west dusk side J J of Jupiter An R isaJovian radius km Each of the almost J observations consisted of a minute CCD integration centered at a wavelength of either A to record the plasma torus S emission lines at and A or A to record the extended neutral cloud Na lines at and A BrownBouchez Figure Mo dels of the resp onse of the extended neutral clouds of Io and the Io plasma torus to a volcanic outburst on Io The dotted line indicates the volcanic input including a twostep day outburst The dashed line shows the neutral cloud mass and the solid line shows the plasma torus mass In b oth mo dels the neutral cloud mass resp onds quickly to the outburst b ecause the lifetime in the neutral clouds is less than a day The plasma torus mass resp onds more slowly b ecause the plasma lifetime is of the order of days a The supplylimited case In this mo del the stabilityoftheinteraction is maintained b ecause an increase in the plasma density reduces the supply of material to the neutral clouds by for example increasing the ionosphere on Io and deecting the plasma After the volcanic outburst ends the high plasma torus mass causes the supply rate to the neutral clouds to drop b elow its preoutburst value The decrease in supply to the plasma torus causes the magnetospheric mass to slowly decline until the equilibrium value is reached Note that whenever the neutral cloud mass is greater than its equilibrium value the mass of the plasma torus must increase b The losslimited case In this mo del stabilityismaintained b ecause the plasma loss rate dep ends non linearly on the plasma mass as exp ected for centrifugallydriven diusion for example Note the fast decay of the plasma torus p erturbation compared to the supplylimited case whichbeginseven as the volcanic input is ab oveits preoutburst value BrownBouchez From these sp ectra we extract emission intensity as a function of distance from Jupiter along the entire slit These two sp ecies were chosen b ecause they have the brightness emission intensities in the visible wavelength range for a plasma torus ion and neutral cloud atom resp ectively So dium is approximately times less abundant than sulfur or oxygen in the neutral clouds but we will attempt to use it as a tracer of the ma jor sp ecies though its eectiveness as such a tracer has yet to b e demonstrated The plasma torus sp ectra and neutral cloud sp ectra are analyzed separately To examine the longterm b ehavior of the plasma torus intensities werstre move shortterm p erio dic mo dulations The twoobserved mo dulations in the data o ccur at p erio ds of hours the System I I I rotational p erio d of Jupiter and hours an unexplained mo dulation termed System IV The measured intensities are divided by the slidingaverage of the mo dulation at these p erio ds to removethevariations of ab out and for System I I I and IV resp ectively The corrected plasma torus intensityversus Julian date for a point R the approximate orbital distance of Io on the dawn side of Jupiter J is plotted in Figure a The longterm intensity b ehavior at other lo cations in the plasma torus is qualitatively similar The emission intensity is prop ortional to the pro duct of the electron and ionized sulfur densities approximately pro p ortional to the square of the S density as the emission is due to the electron impact excitation of the sulfur Analysis of the emission intensities from the neutral so dium cloud suers the complication that the emission seen is from the
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