Probing Ganymede's magnetosphere with field line resonances

Abstract

Abstract. We report on the spectrum of field line resonances identified in data acquired by the Galileo spacecraft within Ganymede' s magnetosphere on a relatively low latitude pass. We infer properties of the plasma distribution and its transport from the observed spectrum. The harmonic structure in the spectrum of the magnetometer data agrees very well with the freq_3uencies predicted for resonances of a dipole field. The spectrum implies a density of 2 amu cm near the equator on closed field lines of Ganymede's magnetic field inside of 2 RG (Ganymede radii). This density is significantly r3educed relative to the local density of the Jovian plasma sheet near Ganymede (_< 8 electrons cm-, or -100 amu cm -3 for an average ion mass per charge of 20 amu and average charge of 1.5 electron charges). A shadowing effect of Ganymede for the flow of particles injected at a reconnection layer on the side of the moon downstream relative to the direction of torus plasma flow accounts for the marked density depletion.Implications for conducting paths near Ganymede's surface are considered.

Excitation of the Ganymede Ultraviolet Aurora

“…in which is the Jovian corotation electric Ðeld, taken to E J be 18 mV m~1 (Volwerk et al 1999), is the equato-L * B 1.8 rial distance in Ganymede radii of the outermost closed Ðeld line, which corresponds to a surface footprint colatitude of 48¡, is the magnetopause radius, and v is b \ 2R G the mapping coefficient, i.e., the ratio between the tail Ðeld and the Jovian corotation Ðeld (Kennel & Coroniti 1975), estimated by Kivelson et al (1998) to be 0.5, which corresponds to rapid reconnection. The electric Ðeld components derived from equation (3) …”

ULF waves in Ganymede's upstream magnetosphere

“…The data are pre-whitened (i.e. we take first differences of the time series data, which effectivity multiply the spectrum by the factor f 2 forcing the power to zero at zero frequency; see also Volwerk et al, 1999), and the spectral estimates are averaged over seven harmonics. The spectral matrix is then diagonalized for the real part (see e.g.…”

“…Field line resonances (FLRs) were identified in the magnetometer data from Galileo's G8 flyby on 7 May 1997 by Volwerk et al (1999). Waves were observed during the 3 min around closest approach, 15:54:47-15:57:47 UT.…”

“…1 (right panel). The rotation period of the spinning platform of the spacecraft was approximately 20 s. For 3 min around closest approach, Galileo remained at approximately the same L-shell, similar to the situation during the G8 flyby (see Volwerk et al, 1999). We use this interval to look for the signatures of field line resonances.…”

“…The MAG data (Kivelson et al, 1992) have shown that Ganymede's internal field near the equator is oppositely directed to the Jovian field (see e.g. Volwerk et al, 1999), and particle measurements have shown characteristic signatures of magnetospheres in low-energy electrons (Frank et al, 1997) and also at high energy where trapped electrons, radiation belts and electron beams (Williams et al, 1997a(Williams et al, , 1998Williams, 2001Williams, , 2004 are identified. Similarly, plasma wave observations have shown typical magnetospheric emissions (Gurnett et al, 1996).…”