UV studies of airglow and auroral emissions have been critical for our understanding of planetary upper atmospheres and their interaction with the magnetospheric plasmas and magnetic fields, directly or indirectly with the impinging solar wind. Major fundamental discoveries on the atmospheres of the Jovian planets were made in the 1980s with the UVS instrument on the Voyager spacecraft (built by researchers at the Univ. of Arizona). During this time and through the mid-1990s the International Ultraviolet Explorer (the IUE satellite) also provided key UV observations of Jupiter, Saturn and Uranus (cf, Ballester, in IUE Final Archive, ESA SP-413, 53,1998). Currently we can make remote-sensing observations of these planets at UV wavelengths in the ~1180-3200 Ang regime with the Hubble Space Telescope (HST). HST orbits the Earth above the UV absorbing, ozone-rich atmospheric layers. It has carried various UV sensitive instruments which have made many planetary discoveries and produced valuable archival datasets. The most useful instrument available at this writting (2003) is the Space Telescope Imaging Spectrograph (STIS). There are many time variable phenomena in Jovian systems which could reveal unique, key parameters of the systems. The Cassini sapcecraft will reach Saturn in the year 2004, and will provide some UV information of this system. However, the need for a remote-sensing UV telescope dedicated to planetary studies, with tailored imaging and spectroscopic capabilities, is becoming more and more pressing.
What are a magnetosphere and an aurora? The sun emits a wind of energetic magnetized plasma that escapes from its hot corona. When the
magnetic field and the plasma and ionosphere of a planet interact with
the impinging solar wind, a magnetospheric ``cavity'' is formed as the
internal and external pressures are balanced. In general, four major
aspects characterize a magnetosphere: symmetry of the system, solar
wind driven convection, corotation, and plasma sources. In a
magnetosphere, current systems and wave-particle interactions can drive
energetic particles to travel along the planetary magnetic field lines
into the upper atmosphere around the magnetic poles, thereby depositing
large amounts of energy and generating aurora. An aurora consists of
the emissions that are produced when the incoming energetic particles
(mainly electrons, but also protons and heavy ions) collide with the
atmospheric gases (atoms, molecules, ions) and excite them. When the
gas is de-excited (goes back to its ground state) it then produces an
emission. The same thing happens in a neon lamp, when a current of
electrons passes through the neon gas.
The outer planet most extensively studied has been Jupiter. Its Galilean satellite Io has the most active volcanism in the Solar System, and its atmosphere interacts electromagnetically with the surrounding medium, the Io plasma torus. This interaction is itself the source of the oxygen and sulfur ions of the torus. The plasma is picked-up by Jupiter's magnetic field into corotation with the ionosphere. Eventually, the plasma populates the entire Jovian magnetosphere. Other contributions are from the ionosphere and the solar wind. Jupiter's strong magnetic field, fast 10-hr rotation, and internal plasma sources result in an immense a magnetosphere with a strong corotational character. It is stretched and flattened by an equatorial current/plasma sheet extending from the Io/torus source at 6 Rj out to ~30 Rj where corotation breaks down. (1 Rj ~ 11 Re ~ the size of the Earth's magnetosphere!) The outermost regions are influenced by the solar wind pressure, and so the magnetopause can quickly change size by a factor of 2 following changes in the solar wind. The coupling of this giant, fast-rotating, internally supplied magnetosphere with the solar wind is complex. Even with extended Galileo in-situ measurements since 1995, we still have a lot to learn about this system. The atmosphere-magnetosphere interaction produces 10^14 Watts of auroral power, with emissions spanning from X-ray to radio wavelengths. This is 10,000 times more powerful than the Earth's aurora, and could light all cities around the world!
One major question about this fast-rotating system is to understand the importance of dependencies of the aurora and thus of the magnetosphere on corotational properties which are fixed in magnetic longitude and thus internally controlled, versus dependencies on the magnetic local time which are dominated instead by the solar wind interaction. All the auroral emissions are highly variable in brightness and morphology, and we can use these variations to learn more about the system. The WFPC2 images shown on the left figure show a partial Jovian rotation. Some spots of emission along the ovals were seeing to corotate with the planet. However, other properties were seen to depend on the magnetic local time. In particular, the brightest emissions were seen associated with a strong auroral event that was taking place and remained confined to the dawn regions. There are other, more subtle effects that we can see in the images which we are currently studying, such as the behavior of the emissions as the rotate from the morning to the afternoon.
The Galileo orbiter has toured the system since the mid 1990s, and has provided much valuable information about the Jovian magnetosphere. On December of 2000 the Cassini spacecraft flew by Jupiter on its way to Saturn, and could measure the solar wind impinging on Jupiter and dusk side of the magnetosphere. Galileo was also making its in-situ measurements of the system. HST also took a limited set of auroral images during this period. Some results have been published from this special set of coordinated observations (Science, 2002). But a full study of that STIS dataset still awaits.
Much information is still buried in the extensive HST dataset, which
started to be collected in 1994 with WFPC2. This is the basis for
our archival program at the Univ. of Arizona. In
addition, MHD simulations of the Jovian system and its
interaction with the solar wind have only recently been made, and we
are aiming at comparing the auroral observations with the MHD
simulations, magnetic field models, and simultaneous Galileo
measurements made by our colleagues at UCLA.
Saturn's magnetosphere is large and fast (10-hr) rotating like that of
Jupiter. It also has internal plasma and neutral sources in the icy
satellites and the rings, both with ephemeral atmospheres, and also
Titan, which has a dense atmosphere. However, unlike Io, Titan orbits
in the outskirts of the system, sometimes inside of it and sometimes
"outside" of it and directly exposed to the solar wind. The Kronian and
Jovian magnetospheres therefore have dominant or at least substantial
internal mass and rotational energy sources against an external solar
wind control, in contrast to the case of the Earth.
In the 1980-1981 Voyager encounters, Saturn's magnetic field was found
to be surprisingly aligned with the spin axis, and presents a simpler
system than Jupiter's because the field non-dipolar terms are much
smaller. Saturn's aurora was also positively identified in UV emission
and seemed to emanate from high latitude ovals thought to map to the
magnetopause, although, as for Jupiter, the models and mapping are
uncertain. Saturn' system may thus be controlled mainly by the solar
wind interaction, rather than by internal forces. However, Voyager
also revealed kilometric radio emissions (SKR) consisting of sychrotron
radiation by precipitating auroral electrons. The SKR and
some ring spokes showed some enhancements at a given magnetic
longitude, thus indicating that longitudinal (and thus corotational)
effects are at play as well. HST imaging will be key for untangling
the behavior of Saturn's aurora and magnetosphere.
Saturn's aurora was known to be bursty from both Voyager and contemporaneous IUE observations, and we think that Voyager may have observed on such morning auroral event. Such events seem to be typical of fast-rotating magnetospheres. (On Jupiter the events are fixed at dawn rather than at 8 AM in magnetic local time.) More information on the temporal behavior of Saturn's aurora can be obtained from analysis of the newer STIS data, because these data was obtained in the STIS time-tag mode. Coordinated observations should be made in the years to come with the Cassini mission in-situ solar wind and magnetospheric measurements.
Because the rotational period of Saturn is highly uncertain, we do not know if the bursts of UV aurora enhancements are constrained in magnetic longitude. With HST imaging observations, made over extended periods and combined with Cassini in-situ data, we may be able to resolve this issue. The STIS images can also be used to resolve corotational versus magnetic local time effects from observing, like for Jupiter, throughout a good part of a planet rotation, as the features rotate from the morning to the afternoon.
Of the planets with atmosphere-magnetosphere interactions, Uranus has
been studied very little. This system is quite interesting, however,
like that of Neptune (but Neptune is too far to be observed with HST).
Uranus has a highly asymmetric magnetosphere because its magnetic
dipole is largely tilted with respect to the spin axis, which in turn
has a high obliquity. During the Voyager encounter in 1986 the spin axis was
basically aligned with the direction of the incoming solar wind. In a
17.24 hr planetary rotation, the magnetic field rotated around this
direction, but the magnetic north pole remained facing the wind.
Uranus is now in a new season (spring equinox), and the orientation of
the magnetic field changes largely during a planetary rotation, so it
is quite different to that of 1986. The magnetic field alternates
between pointing towards and away from the Sun. For Neptune, the
plasma re-organized from a single to a double plasma sheet structure
during a single planet rotation. Uranus may have a more consistent
double-lobed structure, or could remain more closed.
In the Voyager 2 flyby of Uranus in 1986 the UVS experiment was able to detect aurora on Uranus in the molecular hydrogen emissions. This is shown in the figure above. The aurora did not show continuous emissions along ovals. Instead, the aurora were seen as enhanced regions of emissions mapping to the magnetotail. They fall along the predicted north and south auroral ovals mapping to 5 Uranus radii (overplotted in the figure). The north/south brightness ratio of the emissions agreed with diffuse precipitation (scattering of particle into the loss cones) as the relevant mechanism.
Since Uranus has been visited only once by spacecraft, remote sensing studies of airglow and auroral emissions are quite critical for this planet. Uranus relatively low gravity and high (750 K) upper atmospheric temperature produces a highly extended atomic hydrogen corona, and solar and interplanetary Lyman alpha emission is scattered by the upper atmosphere. The far distance from Earth, combined with complex rotational and magnetic field geometries, produce an intrinsically weak Lyman-alpha emission that has been difficult to observe and most enigmatic to interpret (cf, Ballester 1998; Herbert and Sandel 1999).
In the 1980's IUE had detected large time variations in Uranus dayside
H Lyman-alpha emission, which were interpreted as the first indicators
of the planet's auroral activity (and magnetic field). However,
Voyager could not resolve auroral H Lyman-alpha in the sunlit
hemisphere, while it did find localized H2 aurora (as shown above).
The Voyager data were time-averaged throughout the whole encounter, so
they could not yield temporal information. Was the time variability
observed by IUE real? Due to bursty, localized aurora?
The first images resolving H Lyman-alpha on Uranus were taken by STIS
in June and September of 1998. The average of the latter observations
are shown above. Unfortunately, STIS detected weak disk-averaged
Lyman-alpha signal that was at the lowest levels observed by IUE. STIS
did not detect significant time variations nor localized aurora. New
STIS observations of Uranus have been approved for 2010-2011, to search
for changes in the FUV spectrum from enhanced hydrocarbon absorption
due to strong upwelling during the spring equinox, and also to image H