DOC PREVIEW
Statistics of Depleted Fux Tubes

This preview shows page 1-2 out of 7 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 7 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 7 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 7 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

Statistics of depleted flux tubes in the jovian magnetosphereIntroductionA model for depleted flux tubesOccurrenceFlux tubes and surrounding plasmaDiscussion and conclusionsAcknowledgmentReferencesPlanetary and Space Science 53 (2005) 937–943Statistics of depleted flux tubes in the jovian magnetosphereC.T. Russell, M.G. Kivelson, K.K. KhuranaDepartment of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USAReceived 25 September 2003; received in revised form 23 March 2005; accepted 6 April 2005Available online 2 June 2005AbstractOn many of its passes through the Io torus the Galileo spacecraft has detected the presence of what appear to be thin magneticflux tubes with fields somewhat higher than their surroundings. On these flux tubes the magnetic pressure is sufficiently above thepressure of neighboring tubes that it is possible the plasma contributions to the pressure within these tubes are depleted. Due to theirshort duration, they are only detectable in high time-resolution magnetometer data. Herein we survey all high time-resolution datathat are available over the full Galileo mission and present a final statistical study. These tubes occupy 0.32% of the torus outside theorbit of Io. None are found inside. Their strength indicates that the ratio of the thermal pressure to magnetic pressure in the outertorus is about 2%. Comparison of the observed electron density in the neighborhood of these tubes indicates that the iontemperature is in the range 30–100 eV, consistent with other estimates. The amount of magnetic flux transported by these thin tubescould supply the amount of magnetic flux mass-loaded and transported to the magnetotail if the inward velocity is about 300 timesthat of the outward transport. Finally, the thin flux tubes are found in clusters, as they would occur if they resulted from the breakupof larger flux tubes.r 2005 Elsevier Ltd. All rights reserved.1. IntroductionThe exploration of the jovian system by the Galileospacecraft has been one of both excitement andchallenge. Many new and unexpected phenomena havebeen discovered and a much longer time has been spentin orbit than ever envisioned by mission planners. At thesame time Galileo has been communications challenged.Its high-gain antenna failed to deploy. While someinstruments, at least in portions of the orbit, could takeuseful data at a very low rate, commensurate with thebandwidth of the communication system (about 20 bitsper second), many instruments could only acquire theirprime data by operating at a high data rate, storing themeasurements to tape and transmitting the contents ofthe tape recorder slowly.The Galileo magnetometer can provide data at thevery lowest rates, with measurements averaged overperiods as long as hours, up to much higher ratesof several samples per second (upper limit dependenton mission phase). The low-sample-rate data haveallowed coverage of the dynamics of the entire magneto-sphere, while the high-rate data have enabled microscaleprocesses such as ion-cyclotron waves to be probed.One of the phenomena, observable only in high-ratedata in the Io torus, is the occurrence of what appearto be thin flux tubes of slightly elevated magneticfield strength (Kivelson et al., 1997). The magneticfield strength profile often has a flat top with sharpedges. The difference in magnetic field strengthis equivalent to a transverse pressure differenceequivalent to the plasma pressure expected in thisportion of the jovian magnetosphere, leading to theconclusion that these flux tubes are depleted in theirthermal plasma. We know of no mechanism in thejovian magnetosphere that could generate propagatingwaves of this nature. Thus, it is most reasonable toassume that these features are convecting, time-station-ary flux tubes.ARTICLE IN PRESSwww.elsevier.com/locate/pss0032-0633/$ - see front matter r 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.pss.2005.04.007Corresponding author.E-mail address: [email protected] (C.T. Russell).The initial paper discussing these events (Kivelson etal., 1997) did not distinguish the isolated elevated fluxtubes from those occurring at the outer edge of thewarm plasma torus at about 7.6RJ. The outer edge ofthe torus appears to be marked by an enhancement ofplasma beta and a reduced field strength (see Kivelson etal. (1997, Fig. 3) and Russell (2001, Fig. 21). The fluxtubes here transition with increasing radial distancefrom being mainly enhanced with a few dropouts tobeing mainly depressed with a few enhancements. This isthe classic signature of crossing a boundary and thatboundary appeared, as the paper correctly noted, to beundergoing the interchange instability. There is alsostrong plasma heating in this region (Frank andPatterson, 1999). The other phenomenon, which wehave referred to as depleted flux tubes, occurs in aquieter environment and does not appear to beconnected with a boundary. In contrast to the boundaryflux tubes that are almost too numerous to identifyseparately, the depleted flux tubes are relatively rare. Itis this relatively rare phenomenon usually found deep inthe Io torus that we study herein.Intermittent interchange of flux tubes had beenexpected in the Io torus as the means by which themass added to the magnetosphere at Io in the form ofheavy ions could be transported outward and a ‘‘steady-state’’ mass profile of the magnetosphere maintained(Pontius et al., 1986; Southwood and Kivelson, 1987).Thorne et al. (1997), following this model, usedarguments based on the associated signature of thesestructures in energetic particle data to estimate that theymoved inward at about 100 km/s. A different way ofestimating their velocity is magnetic flux conservation. Ifthey are carrying magnetic flux inward to replace thattransported outward with the plasma, then theirobserved occurrence rate allows a velocity estimate tobe calculated. Previous reports on these depleted fluxtubes (Russell et al., 2000b, 2001a) and the statisticsreported below place this occurrence rate near 0.3%.Thus, these tubes must be flowing inward on averageabout 300 times faster than the bulk of the Io torus ismoving outward. This outward motion in the hot Iotorus outside the orbit of Io is expected to be tens ofmeters per second (Russell, 2001), based on the observeddensity profile (Bagenal, 1994) and the canonical1000 kg s1added at Io


Statistics of Depleted Fux Tubes

Download Statistics of Depleted Fux Tubes
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Statistics of Depleted Fux Tubes and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Statistics of Depleted Fux Tubes 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?