SM24A-01 INVITED 16:00h
Jupiter's Magnetosphere: Coupling the Equator to the Poles
The fact that the plasma in the magnetosphere of Jupiter is dominated by rotation tells us that the magnetosphere is strongly coupled to the planet. Rotation confines the plasma towards the equator, the bulk motion is predominately rotational (albeit lagging behind rigid corotation) and Jupiter's flywheel is the main source of energy. The low plasma density away from the equator, however, means that it is difficult for currents to couple the equatorial plasma to the planet's polar regions. Experience gleaned from satellites flying through the polar regions of the Earth's magnetosphere suggests that potential structures form above the ionosphere to allow the necessary currents to flow. Radio emissions from Jupiter (first detected around the date of this AGU meeting 50 years ago) are indications of electrons accelerated by such potential structures. Similarly, the intense aurora are indications of energetic particles bombarding the atmosphere. This paper reviews the current understanding of global dynamics of the jovian magnetosphere revealed by polar auroral emissions and the outstanding questions to be address, eventually, by a polar mission.
SM24A-02 INVITED 16:25h
Generation of Global Auroral Morphology at Earth and Jupiter
Global patterns of auroral morphology at Earth and Jupiter have superficial similarities but very different causes. In both cases, the brightest auroras are produced by downward-accelerated magnetospheric electrons carrying upward Birkeland (magnetic-field-aligned) currents. And in both cases, the resulting global auroral morphology is an oval. But the mechanisms that drive the upward Birkeland currents are very different in the two cases. At Earth, the upward currents are part of a complex solar-wind-magnetosphere interaction in which the upward Birkeland currents on the dusk side are part of the driving "Region-1" current system but those on the dawn side are part of the responsive "Region-2" current system. At Jupiter, the upward currents are part of a circuit that enforces partial corotation of magnetospheric plasma that is continually produced and transported outward within the magnetosphere, with no obvious solar-wind influence and hence no obvious dawn-dusk asymmetry.
SM24A-03 INVITED 16:40h
Earth{'}s Polar Magnetosphere and Aurora
High-latitude convection, magnetic field-aligned currents and particle precipitation are powered at Earth primarily by the dynamo action of the solar wind flowing across polar geomagnetic field lines. The resulting auroral luminosity provides a sensitive diagnostic of large-scale magnetospheric dynamics and structure -- features that are specific to magnetospheric processes driven by a solar wind-magnetosphere interation. At the same time, the physical processes that enable aurora, e.g., the formation of parallel electric fields and auroral particle acceleration, time-variable transmission of electromagnetic power along magnetic field lines by Alfv{\'e}n waves, and dynamic structuring of aurora resulting from impedance mismatch between the magnetospheric dynamo and ionospheric load, do not necessarily depend on the nature of either the dynamo or the load. This paper will review the large-scale electrodynamics of the terrestrial polar magnetosphere and the plasma physics and magnetohydrodynamics that enable high-latitude terrestrial aurora. It will also attempt to distinguish between terrestrial-specific processes and ones that are also likely to be operative in the Jovian polar magnetosphere and aurora.
SM24A-04 16:55h
Chandra Observations of Soft X-ray Emissions from Earth and Jupiter
Earth was observed using the High-Resolution Camera (HRC-I) aboard the Chandra X-Ray Observatory at 10 epochs between mid-December 2003 and mid-April 2004. Each observation was $\sim20$~minutes in duration, with fixed pointing so that the north polar cusp was allowed to drift through the HRC-I field-of-view ($30'\times30'$, or about $1150\times1150$~km$^2$ at the Earth as seen from Chandra apogee). The observations were performed during northern winter, so that the northern auroral region was mostly dark and fluoresced solar x-ray contamination could be avoided. The investigation was designed to compare Earth's soft x-ray auroral emissions with previous Chandra observations of Jupiter's x-ray aurora, where a pulsating x-ray "hot-spot" near the northern magnetic pole was observed which indicated a possible source due to the entry of heavy solar wind ions into the jovian cusp. The first Chandra soft (0.1-5 keV) x-ray observations of Earth's aurora show that it is highly variable (intense arcs, multiple arcs, diffuse patches, at times absent). In at least one of the observations an isolated blob of emission was seen in the expected cusp location. A fortuitously-timed overflight of a DMSP satellite (F13) provided SSJ4 particle measurements which indicate that at least one of the bright arcs we observed is produced by electron bremsstrahlung. Other preliminary results from these unique Chandra observations of Earth will be presented and discussed.
SM24A-05 17:08h
Current Carriers in the Earth's Auroral Zone: Implications for Jovian Aurora
Recent observations of X-ray emissions at Jovian polar latitudes have been cited as evidence of intense ion precipitation into the Jovian ionosphere. The ions have further been argued as being current carriers in regions of downward field-aligned current. The currents are thought to be generated either by reconnection, or by processes internal to the Jovian magnetosphere. Terrestrial aurora are also associated with field-aligned current systems. The inverted-V aurora are usually observed in regions of upward field-aligned current, and the current is carried by the precipitating electrons. Measurements by the FAST spacecraft have shown remarkably good agreement between the current as determined by the number flux of precipitating electrons and the current as inferred by the gradient of the magnetic field. There is generally less of an agreement between particle fluxes and magnetometer measurements in the return current region. This appears to be because the particle detectors do not measure electrons at sufficiently low energies. At times the electrons are accelerated upwards to high enough energy to be measured by FAST, and again reasonable agreement is found. Invoking ions as current carriers does not explain the discrepancies in downward current regions. Plasma sheet ions are usually observed at the same time as the downward currents, and they show no evidence of downward acceleration. Terrestrial observations therefore suggest that auroral currents are carried by electrons in both downward and upward current regions. Ions do not carry any significant field-aligned current. This raises questions concerning the ion current hypothesis as applied to the Jovian X-ray emissions. Processes other than acceleration in field-aligned potentials may have to be invoked to explain the ion energization at Jupiter.
SM24A-06 17:23h
Solar Wind Driven and Inertial Reconnection in the Jovian Magnetotail
Reconnection in the terrestrial magnetosphere is largely controlled by the interplanetary magnetic field (IMF). For a southward IMF magnetic flux removed from the dayside magnetopause to the tail lobes is returned to the dayside by magnetotail reconnection. In simulation studies for a steady southward IMF, a single X-line O-line pair forms in the magnetotail. With time the O-line moves tailward but the X-line stays in the near-Earth magnetotail. In an analogous simulation at Jupiter a northward IMF leads to dayside reconnection and the formation of an X-O pair in the magnetotail. However, in the rapidly rotating Jovian magnetosphere both neutral lines exit the tail and a second X-O pair form in the near-Jupiter tail. This process repeats with a period of 30 to 70 hours [Fukazawa et al., GRL submitted, 2004]. However, reconnection can occur in the Jovian magnetotail even in the absence of dayside reconnection. On the dayside rotating magnetic flux tubes filled with outflowing Iogenic plasma are constrained by the dayside magnetopause. However on the nightside they are no longer constrained and can stretch far into the tail. In the early morning quadrant these stretched closed flux tubes can reconnect. We have carried out a series of global magnetohydrodynamic simulations of the interaction of the Jovian magnetosphere with the solar wind to study the evolution of these inertially reconnecting flux tubes. In this study we have examined the changes in the Jovian magnetotail following a decrease in the solar wind dynamic pressure in the absence of an IMF. We find an X-O line pair form on closed field lines in the dawn quadrant as expected. The O-line moves tailward and toward the dawn magnetopause where it exits the system. Like tail reconnection at Earth, the X-line does not move. When we further reduced the dynamic pressure the location at which the X-O pair formed moved tailward. At Earth the field lines from the tail reconnection map to the polar edge of the auroral oval. At Jupiter the oval is thought to lie much closer to the planet. We have used calculations of the energy flux to the ionosphere as a proxy for auroral emissions and will present the auroral signatures of both driven and inertial reconnection.
SM24A-07 17:36h
Application of a linear gyrofluid model to the Jupiter-Io interaction
One of the most interesting of the Jovian auroral processes is highly sporadic radio emissions from Jupiter's magnetosphere that extend from 10 kHz to 33 MHz while most of the powerful decametric emissions observed between 15 and 27 MHz occur within two ranges of Central Meridian Longitude (CML). The duration of these pulses varies from 200 ms to 1 ms, hence, they referred to as S-bursts. An Alfv$\acute e$n eigenmode, called the ionospheric Alfv$\acute e$n resonator, can develop between the conducting boundary of Jupiter and the exponential increase in Alfv$\acute e$n velocity on the topside of ionosphere. A Vlasov code is used to provide density profiles along the Jupiter-Io flux tube while a gyrofluid model will be used to study the propagation of Alfv$\acute e$n waves. With this effort, we are attempting to examine whether the Alfv$\acute e$n resonator in Jupiter's ionosphere may be associated with the periodicity of S-bursts. Alfv$\acute e$nic fluctuations may be partially (or mostly) reflected by the large density gradient near Io's torus. We will calculate the percentage of the wave power that reaches high latitudes of Jupiter through the use of our gyrofluid code.
SM24A-08 17:51h
Dynamical Consequences of Two Modes of Centrifugal Instability in Jupiter's Outer Magnetosphere
The global scale structure of the Jovian magnetosphere has been established through analysis of data from seven spacecraft whose collective passes cover all local times, mainly in the near-equatorial region. As contrasted with Earth's magnetosphere and auroral ionosphere, where the day-night asymmetries dominate the local time variation of structure, the Jovian magnetosphere and auroral ionosphere also reveal strong dawn-dusk asymmetries. Those asymmetries illuminate the effects of plasma rotation in the presence of externally imposed forces at the magnetopause. Analysis of the system leads to a description of the physical processes that produce the asymmetry and gives insight into unique aspects of the plasma transport in a magnetosphere dominated by rotation. The most dynamic portion of the magnetosphere is the plasma disk. At some local times, its outer edge is marginally stable and at others strongly unstable. The most unstable sector is localized on the night side, where an outflow of material down tail thins the plasma disk. The same outflow evacuates portions of the flux tubes, causing the outer portions to break off and leave closed depleted flux tubes behind. As the depleted flux tubes move on to the dayside at large radial distance on the morning side, they form a distinct plasma/magnetic regime. The residual sheet thickens as flux tubes rotate to noon. During this rotation, there is some evidence of weak loss of plasma. A massive change takes place in the afternoon sector where the sheet energizes and thickens as it moves from noon to dusk and assimilates the empty flux tubes of the outer magnetosphere. It is probable that this assimilation is accomplished through a short perpendicular scale centrifugally driven ballooning leading to Bohm diffusion of plasma to refill the previously emptied tubes. The well-known dawn dusk asymmetry in the UV aurora is very likely associated with the vastly larger tapping of the ionospheric `flywheel' in the afternoon sector that the model proposes.