How Magnetospheres Work: Lessons from Comparative Planetology
The magnetic cavities in the solar wind that surround the planets that have intrinsic magnetic fields are dynamic, driven by external and internal forces. Within these spheres of magnetic influence, or magnetospheres, energy is stored and released, and the plasma circulates driven by the solar wind above and the ionosphere below, and possibly by forces in between. Substorms have now been reported at Mercury, Earth, Jupiter, and Saturn. Reconnection, in which oppositely directed magnetic field lines become connected, is certainly involved in all of these substorms, but they may be quite differently driven. Dayside reconnection is not expected to be equally efficient at all the planets, and plasma sources in the magnetosphere differ. By comparing the behavior of similar processes in these different settings, we obtain a deeper understanding of how magnetospheres work.
Comparative Magnetosphere of Jupiter, Saturn and Comets
Using magnetohydrodynamic models to study a wide range of planetary magnetospheres has yielded a wealth of information about how these complex systems work and about some of their similarities and differences. In this paper, I will address the similarities and differences in the magnetospheres of the two giant planets, Jupiter and Saturn, in contrast to the plasma environment of comets. At each of these objects the magnetosphere is dominated by a large source of neutrals (Io, Enceladus, the comet nucleus) that results in an extended neutral cloud that is in turn ionized resulting in a large plasma source. However, the nature of the sources is quite different and the resulting interaction of the magnetospheres with the solar wind has some very interesting differences. The size of these magnetospheres is determined by the size of the body that is an obstacle to the solar wind, however, at none of them is the body itself the obstacle. At the giant planets, the obstacle is the intrinsic magnetic field while at a comet it is the neutral cloud. Not only does the intrinsic magnetic field play a role in the size of the magnetosphere, but it also plays a key role in some of the very basic structures of the magnetosphere with the internal magnetic field of the giant planets dominating the inner magnetosphere while the lack of any significant magnetic field at comets leads to very interesting diamagnetic structure near the nucleus. We will compare and contrast these different aspects of planetary magnetospheres.
MESSENGER Observations of Mercury's Magnetosphere
MESSENGER's first and second flybys of Mercury, on 14 January and 6 October 2008, have greatly extended our knowledge of the closest planetary magnetosphere to the Sun. These very low-latitude flybys have revealed a miniature magnetosphere that is immersed in a cloud of planetary ions extending beyond the dayside bow shock. The magnetosphere and the planetary ions are highly responsive to interplanetary magnetic field (IMF) direction. Under strong northward IMF, the highest flux of high-altitude planetary ions is on the dusk flank of the tail, where they are accelerated by the fast flow in the magnetosheath and cross the magnetopause due to their large gyro-radii and the dawnward - v x B motional electric field. Evidence for Kelvin-Helmholtz boundary waves is also present in this flank region of the tail. Ultra-low-frequency (ULF) wave activity is widespread at low latitudes, possibly a result of the large loss cones for magnetospheric ions associated with the weakness of the magnetic field at the surface. Diamagnetic decreases in the magnetic field are observed close to the planet and in a "boundary layer" just inside of the forward magnetopause. Under southward IMF the high- altitude maximum in the planetary ions moves to the dawn side of the magnetosphere with the change in the -v x B motional electric field from dawnward to duskward. The intensity of the ULF waves grows for southward IMF, and the diamagnetic depressions near the planet become deeper. The boundary layer inside the magnetopause is still present, but it is less prominent against the more disturbed background magnetic field. The dawn magnetopause was threaded by a strong magnetic field normal to its surface that implies a reconnection rate ~10 times that typical at Earth. Large flux transfer events are observed in the magnetosheath for southward IMF, and plasmoids and traveling compression regions are present in the magnetotail. These observations have important implications for the dynamics of Mercury's magnetosphere relative to those of other planets.
Self-consistent Planetary Dipole and Paraboloidal Model Parameters Fitting by the Mercury's Magnetic Field after MESSENGER's First and Second Flybys
The paraboloidal model of Mercury's magnetospheric magnetic field is used to determine best-fit magnetospheric current system and internal dipole parameters from magnetic field measurements taken during the first and second MESSENGER flybys of Mercury on 14 January and 6 October 2008. Together with magnetic field measurements taken during the 29 March 1974 and 16 March 1975 Mariner 10 flybys, there exist three low-latitude traversals separated in longitude and one high-latitude encounter. Modeling of these magnetic field measurements using the paraboloidal parameterization provides not only values for the magnitude of the planetary dipole and its location within the planet, but also the mean values and variation in the magnetopause and tail current systems during these four flybys. The structure of Mercury's magnetic field and magnetospheric current systems obtained with the paraboloidal model fitting compares well with that provided by other modeling frameworks. The dipole offset (about 460 km) northward direction relative to the origin give smaller rms deviation about 11 nT.
The Enigmatic Dayside Boundary Layer at Mercury
The two MESSENGER flybys of Mercury revealed the presence of a pronounced boundary layer structure just inside the dayside magnetopause boundary in the morning sector. During both flybys a step decrease of 25 to 30 percent in the magnetic field magnitude occurred on the outbound trajectory about 5 minutes prior to the magnetopause crossing. Corresponding step-wise changes in the distributions of protons in the energy range 0.1 to 10 keV were also observed. Interpreted in terms of spatial structure these observations correspond to a boundary layer approximately 2000 km thick, which is appreciable compared with the small size of Mercury's magnetosphere. Significantly, comparable boundary layers were seen on the outbound legs of both MESSENGER flybys, the first occurring for northward interplanetary magnetic field (IMF) and the second for southward IMF. Signatures of intense magnetopause and tail reconnection during the second flyby imply that rapid convection was then occurring. Thus, whatever process generated the boundary layer seen during the second flyby must act rapidly and be continuously effective even in the presence of strong convective flows. Several possibilities for the generation of the layer include cusp entry and subsequent drift of solar wind plasma, mass loading of ions originating from the planetary surface or the exosphere, and processes associated with reconnection and viscous mixing known to occur at Earth. Detailed examination of the signatures of the layer in magnetic field and ion observations permit an assessment of whether the same generation process acted during both encounters as well as the identification of key signatures that can be used to discriminate among different layer formation processes.
Fundamental Plasma Physics in the Martian Magnetotail and Boundary Layer
We expect many of the same plasma physics processes observed in the terrestrial magnetosphere, including collisionless reconnection and viscous boundary layer processes, to also operate in other planetary environments. We now present observations from Mars Global Surveyor (MGS) that may provide evidence for the occurrence of these processes around Mars. While MGS lacks the full plasma instrumentation and multi- point measurements that have proven crucial in understanding these processes at the Earth, it does have magnetic field and electron measurements from the Magnetometer/Electron Reflectometer instrument. We discuss MAG/ER observations of flux ropes, bipolar magnetic field signatures, and field-aligned electron flows possibly indicative of reconnection in the Martian magnetotail current sheet. We also describe quasi-periodic fluctuations in electron flux and magnetic field above the Martian dayside suggestive of boundary layer processes such as the Kelvin-Helmholtz instability. Magnetotail and boundary layer processes such as these may contribute to the loss of ionized planetary gases from the Martian atmosphere, one of the primary science goals of the MAVEN Mars Scout mission currently in development.
Mars and Venus ion cyclotron waves and their implications
Mars and Venus do not have appreciable global magnetic fields to shield their neutral atmospheres from erosion by the solar wind. When their atmospheric hydrogen atoms are ionized and picked up by the solar wind, proton cyclotron waves are created from the free energy of the ring-beam distribution of the pick-up ions. At Mars, proton cyclotron waves observed by Mars Global Surveyor extend from the magnetosheath to over 12 Mars radii, with intermittent occurrence and amplitudes slowly varying with distance. The wave occurrence pattern indicates that the Martian hydrogen exosphere cannot be spherically symmetric but is rather disk- shaped with asymmetry in the direction of the interplanetary electric field. In order to travel across the magnetic field the picked up ions must be neutralized near where they are picked up and these fast neutrals are transported to distant regions where they get re-ionized and produce waves far downstream. Thus the top of Mars exosphere appears to extend in a disk to high altitude, with its orientation controlled by the interplanetary magnetic field. At Venus, proton cyclotron waves were first observed in the magnetosheath by Pioneer Venus Orbiter and later in the solar wind around Venus by Venus Express at a distance as far as 7 Venus radii. This paper compares the properties of the Mars and Venus waves and discusses the implication of these observations.