 Scientific Justification
The aurora on Jupiter and Saturn have been studied with increasing sensitivity and resolution
in a series of HST GO programs of UV imaging and spectroscopy. These have revolutionized our
understanding of the auroral phenomenona on both planets, and the auroral emissions provide the only
remote method to study the magnetospheres of these giant planets. Much has been learned about the
auroral morphology on both planets, as described in more detail below.
Much remains to be understood about the Sun-Magnetosphere-Ionosphere coupling at both
Jupiter and Saturn, however. Our present understanding is clearly limited by the fact that the auroral
emissions on both planets vary rapidly compared with the cadence of HST observations. Missions
that study the Earth’s aurora from space observe the global auroral activity nearly continuously for
years at a time, coupled with satellite measurements of particle precipitation, ground-based
measurements of ionospheric currents, neutral winds, and local auroral emissions at high resolution.
It is through this high time coverage that the dependence of the Earth’s aurora on the solar wind has
been untangled. By comparison, on average there have been about 10 HST orbits per year of giant
planet auroral observations, while the auroral activity varies on timescales from localized flares lasting
tens of seconds to giant storms lasting hours to days. If there had been only 10 observations a year of
the Earth’s aurora, it would not have been possible to determine the controlling effects of the solar
wind on auroral storms, nor the central importance of the orientation of the interplanetary magnetic
field (IMF). We propose to take the next step toward understanding the physical processes driving the
aurora on Jupiter and Saturn, including the effects of the solar wind, through a concentrated series of
HST observations coordinated with planetary spacecraft and other earth-based observations in 2007.
A unique opportunity exists in HST cycle 15 to discover the physical principles underlying the
auroral processes on Jupiter and Saturn. This is the International Heliophysical Year (IHY), an
international program on the 50th anniversary of the 1957 International Geophysical Year to perform
intensive coordinated measurements of space physics phenomena at the Sun, the Earth, the other
planets, the solar wind, and the heliosphere. This event fortuitously overlaps with the Cassini
spacecraft orbital mission about Saturn, and the New Horizons (NH) spacecraft flyby of Jupiter. In
addition, intensive observations of both Jupiter and Saturn from ground-based observatories around
the world are planned, and especially intensive measurements of the solar wind will be carried out.
There will never be a better time to perform such a concentrated study, and the importance of these
data for understanding the physics of planetary aurora and magnetospheres cannot be overstated.
This proposal represents the cornerstone Coordinated Investigation Campaign for the
Planetary Magnetospheres part of the IHY, one of 6 sub-categories identified by the international
organizing committee as critical to the success of the IHY [http://ihy2007.org/]. The study of
planetary magnetospheres has been shown to be of critical importance to the overall IHY objective to
study Universal Physical Processes, comparing various regions within the solar system. The
Universal Processes have been defined with the knowledge that the same physical processes act at the
Sun, the Earth, and the other planets, i.e. reconnection, particle acceleration, plasma wave generation
and propagation, etc. The inclusion of auroral observations of the giant planets is a key component to
the IHY for comparison of these processes at the different locations. For example, comprehensive
observations of Jupiter and Saturn near opposition will make it possible to see the effects of the same
solar wind structure that passes the Earth on arrival at the other planets.
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Planetary Aurora – Internal Processes and Solar Wind Control: Jupiter’s aurora are the best
studied, and the most complicated, of the other planets. Jupiter sustains three independent aurora:
emissions at the magnetic footprints of three satellites, a main oval connected to the middle
magnetosphere, and polar emissions connected to the outer magnetosphere [Clarke et al. 2004].
These vary independently of each other, and have different physical origins (Figure 1). While the
main oval and polar emissions exhibit both internal control (indicated by features corotating with the
planetary magnetic field) and solar wind control (indicated by features fixed in local time) [Grodent et
al. 2003a], the satellite footprints appear not to be subject to solar wind control.
Jupiter’s main auroral oval is maps to a distance where outward drifting plasma lags behind
corotation, driving strong field-aligned currents in and out of Jupiter’s ionosphere {Grodent et al.
2003b]. These currents in turn give rise to field-aligned potentials in the topside ionosphere,
accelerating the auroral particles to 10’s of keV. Since plasma is continuously released near Io (about
1 ton/sec) and drifts outwards, the main oval aurora is always present, with a brightness and specific
distribution determined by other factors. For example, while individual features in the main oval
rotate with the planet, “dawn storms” are observed which last several hours and are fixed in magnetic
local time [Clarke et al. 1998]. These events grow to several MegaRayleighs (MR) in brightness,
much more intense than the brightest auroral emissions at the Earth [Gérard et al. 1994]. Their
controlling factors are not known, but due to their local time character they are thought to be related to
changes in the solar wind interaction. Knowledge of the solar wind conditions during more than one
dawn storm, as well as control periods when no storms take place, will be needed to understand this.
Jupiter’s polar aurora are the most highly variable of the emissions, constantly changing but
with some specific patterns identified from past HST images [Grodent et al. 2003a]. The polar
“flares” appearing in the so-called active region are the most dramatic auroral storms in the solar
system. They rise from a background level of about 1 kR to ten’s of MR on time scales of tens of
seconds, then fade over a period of a few minutes. The energy input to Jupiter’s upper atmosphere is
1000’s of mW/m2, sufficient to drive a supersonic expansion, and bursts of soft X-rays are often seen
as well as thermal IR emissions [Gladstone et al. 2002]. The other polar emissions reveal statistical
patterns that are fixed in local time, consistent with driving forces in the solar wind interaction.
Again, lacking solar wind measurements at most times in the past, the connections of any of these
processes with solar wind conditions are not known. Since the polar emissions map along the Jovian
magnetic field to large distances near the magnetopause boundary, they are most likely to be
controlled by the interaction with the solar wind. In addition, short-lived isolated emissions near local
midnight are thought to be produced by reconnection in plasma clumps which have been detected
traveling downstream in the Jovian magnetotail [Grodent et al. 2004]. A lack of continuous
measurements of plasma conditions in the magnetotail has prevented us from making this association
to date. Following closest approach, the NH trajectory takes it far down the magnetotail, the path by
which magnetospheric plasma is ultimately lost from the Jovian system. It is known that the 1 ton/sec
of plasma from Io drifts slowly outward with a complex behavior with local time due to Jupiter’s
rapid rotation. While this plasma exhibits a slow “drizzle” down the tail, Galileo measurements have
shown large organized clouds of plasma that break free and drift down the tail (like coronal mass
ejections from the Sun). These clouds, formed during tail reconnection events, are sufficiently large
to empty much of the near-Jupiter environment, with potentially dramatic effects that can be observed
in the aurora [Grodent et al, 2004].
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One prior attempt has been made to discover Jovian auroral variations tied to solar wind
conditions during the approach of Cassini to Jupiter in late 2000. Continuous Cassini measurements
of the solar wind were not possible, due to competing measurements requiring a different spacecraft
orientation. Since no instrument on Cassini could image the aurora, HST observations were
scheduled with a spacing of several days to a week. With this separation, no large solar wind
variations were sampled by HST images, but much was learned about the distinction between
corotational and local time effects, specifically in the polar regions. The science return was quite
extraordinary, with an initial group of 7 papers published in Nature and a host of more detailed
investigations published since then. In an earlier set of Cassini UVIS observations while the visible
camera mapped the planet, the aurora were seen to double in overall intensity during a solar wind
pressure increase, but these lower resolution data did not indicate which part of the aurora brightened
nor the location or distribution of the “storm” [Pryor et al. 2005]. A similar campaign in parallel with
the NH flyby of Jupiter during Cycle 15 is sorely needed to address these questions.
Saturn’s aurora behave very differently from those at the Earth and Jupiter, and the
comparison of these planets is a chief goal of the field of comparative planetary magnetospheres.
Saturn’s aurora are largely confined to a main oval, which is only approximately centered on the
magnetic pole and exhibits large latitudinal motions [Gérard et al. 2004; Clarke et al. 2005; Grodent
et al. 2005]. While there is a general trend to brighter emissions on the dawn side, individual auroral
features rotate with the planet, indicating a combination of corotational (internally driven) and local
time (solar wind controlled) physical processes. The distance to which the auroral emissions map
within the magnetosphere is not known. Saturn’s aurora have been studied far less than Jupiter’s by
HST, but more concentrated observations have been made since the Cassini spacecraft arrived at
Saturn. One outstanding result of the HST images taken as Cassini approached Saturn was the
measurement of the response of the aurora to a large solar wind disturbance (Figure 2) [Gérard et al.
2005]. Saturn’s auroral oval brightened dramatically, moved to higher latitudes and adopted a smaller
radius, and the dawn side (but not the dusk side) of the oval filled in with emission. From the solar
wind data, these changes correlate well with the dynamic pressure but not with the direction of the
interplanetary magnetic field (IMF) [Crary et al. 2005]. This contrasts with the Earth, where a
southward turning of the IMF commonly initiates auroral substorms. It has been suggested that the
IMF may also affect Saturn, but its tilted orientation at that time caused the IMF to not align with the
magnetic axis of Saturn. Near equinox, there may still be a strong IMF influence on Saturn’s aurora.
In addition, there is preliminary evidence for auroral emission from the magnetic footprint of
Enceladus on Saturn from ACS images in Feb. 2005. All HST images in which this feature may
appear are being examined, and we are cautiously optimistic that it will provide a mapping of the
magnetic field geometry and a tracer of the Enceladus interaction, as we have for Io at Jupiter. In this
event, the series of Saturn images and a few complete rotations of the planet will greatly improve our
understanding of Saturn’s magnetic field and of the main source of plasma and E ring particles.
The earlier HST Saturn campaign in Jan. 2004 during the Cassini approach successfully
demonstrated that increases in solar wind density cause auroral storms, while no effect of the IMF was
seen. This is believed to be due to the large tilt of Saturn, causing misalignment with the IMF. Closer
to Saturn equinox in Jan. 2007, the better alignment of the IMF with Saturn’s magnetic field will
provide a test of this theory. In addition, Cowley and colleagues have compared the observed radius
of the auroral oval with the power in the solar wind subtended by the corresponding distance for an
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estimate of the cross-tail potential available to drive the auroral emissions. This general technique can
be applied to either Jupiter or Saturn. In addition to the solar wind data, it will be possible to study the
correlations of auroral emissions with Saturn Kilometric Radiation (SKR), the nonthermal radio
emissions associated with Saturn’s aurora [Kurth et al. 2005]. SKR measurements will be made
continuously by Cassini over the period of proposed HST observations. The combination of Cassini
field and plasma measurements inside Saturn’s magnetosphere, solar wind conditions extrapolated to
Saturn, and high resolution HST auroral images provides a powerful combination.
Using a 1D MHD model one can extrapolate solar wind measurements made near the Earth
out to Jupiter and Saturn. The extrapolation model (led by K.C. Hansen) is very high resolution in
radius, with an assumed spherical symmetry of the solar corona and solar wind. If the Sun is in a
highly dynamic state (for example, launching a coronal mass ejection) then the propagation is accurate
near the time when the Sun-Spacecraft-Planet are in a line. We have tested the model under these
conditions by using ISEE3 data propagated to Pioneer 11 and Voyagers 1 & 2 and ACE data
compared with Cassini measurements both at Jupiter and Saturn. These 10 comparisons indicate that
the model works well within several weeks of the linear alignment [Hanlon et al. 2004a,b]. The
model predicts the velocity structure of the solar wind and the arrival time of shock discontinuities
and magnetic sector boundaries to accuracies better than one day and sometimes as good at six hours
[Pryor et al. 2005]. The general orientation of the IMF can also be predicted with good accuracy on a
time scale of a day, while the absolute magnitude of the IMF is relatively less certain. In addition, a
new 3D simulation (the ENLIL code) is being developed, and is expected to contribute. In addition,
while the ACE spacecraft near the Earth is the prime candidate for local solar wind data, there are
other spacecraft which are or will be measuring the solar wind during the proposed observation
periods. A prime example is STEREO, with multiple spacecraft moving steadily away from the Earth
(to be launched in spring 2006). Extrapolating data outward from these spacecraft will greatly increase
the angular coverage, and provide correlative information about the evolution of disturbances. In
addition, the Solar corona normally displays a well-ordered rigidly rotating sector structure at solar
minimum (Figure 3), increasing the accuracy of the extrapolation away from the linear alignment
period. We believe that we can have meaningful data at the outer planets within +/- 30 days of the
linear alignment, or +/- slightly more than one solar rotation.
Observations of the Io footprint: morphology, variability and Io – plasma interactions: The
HST discovery of bright FUV auroral emissions from the magnetic footprint of Io opened a new
window to remotely sense the interaction between Io, the Jovian magnetic field and Jupiter’s upper
atmosphere. The Io Flux Tube (IFT) emission was observed to remain at the magnetic footprint of Io,
and STIS-MAMA images (Figure 1) also show a long tail extending downstream along the magnetic
footprint of Io’s orbit over more than 100° in longitude [Clarke et al. 2002]. The electrodynamic
interaction near Io was initially explained by a unipolar inductor model, where the motion of Io across
the Jovian magnetic field lines generates a ~500 keV electric potential driving currents along Jupiter’s
field lines and closing in Jupiter’s ionosphere. Another model involves the generation of Alfvenic
wave propagating from Io to Jupiter. A hybrid model by Crary and Bagenal [1997] suggested that the
interaction begins as an Alfvén disturbance near Io, evolving into a steady current loop downstream.
Multiple Io footprints, presumably resulting from Alfvén wave reflections, have been observed in
STIS images (Figure 1) [Clarke et al. 2002; Gérard et al. 2006]. The number of spots and their
separation were found to increase when Io was positioned closest to the torus outer edge (Figure 2).
At this point, the observed variation of the FUV spot structure with Io’s position appears inconsistent
4
with models where reflections of Alfvén wings occur between the torus boundary and Jupiter’s
ionosphere. Multiple reflections of Alfvén waves have been observed, however, as arc structures in
radio decametric (DAM) emissions observed from the ground [Queinnec and Zarka 1998] and
Voyager. Crary and Bagenal [1997] suggest the waves are trapped between the Jovian ionosphere and
the outer edge of the torus, leading to 0.14° to 0.7° of longitudinal separation, a shorter distance than
observed between the FUV spots. When Io is close to the centrifugal equator, two or more spots
appear in the south but only one in the north, suggesting a N/S asymmetry. Additional observational
constraints are now clearly needed to elucidate the nature of the interaction between Io, the torus
plasma and the Jovian magnetic field. In particular, we plan to address the following key questions:



Are asymmetries present when Io is near the torus center and when the two hemispheres
are observed (quasi-) simultaneously?
For a given Io position near the torus edge, are N/S asymmetries observed?
Is the brightness distribution between the multiple spots similar when Io is facing the
north and the south Jovian ionospheres?
To solve these questions, we propose (i) to cover the range of Io longitudes not previously
observed with STIS or ACS images, and (ii) to observe consecutively the north and south high-latitude
regions during single HST orbits, to minimize the role of longitudinal gradients in the plasma torus
and, (iii) to optimize the program, short exposures will be taken to examine short-term (min.)
variations in the structure and brightness of the multiple spots. As described before, the link between
the multiple FUV spots and the fringes observed in the decametric radio emissions needs to be
determined. Several major questions remain open:



Are the two features signatures of the same phenomenon?
Are the separations between spots and radio fringes identical ?
What is the nature of the link between radio DAM fringes and UV multiple spots?
To address these fundamental questions, simultaneous observations from HST and the DAM
emissions at the Nançay radio telescope are needed. Efforts to find parallel observations in the
existing databases have revealed that such measurements need to be planned to be successful. We
therefore propose to carefully plan joint HST / DAM measurements, based on our recently acquired
knowledge of the relationship between Io’s position and the multiplicity of the Io footprints.
Ganymede’s UV footprint and the Jovian magnetic anomaly: Repeated imaging of Jupiter's
aurora has shown that the northern main oval has a distorted shape, forming a 'kink' in the longitude
range of 90-140º. Its characteristics suggest that the auroral kink is a signature of a localized magnetic
anomaly in Jupiter’s northern hemisphere. Existing images demonstrate that the Io and Europa
footprint locations are also affected by a possible northern magnetic anomaly, resulting in a deviation
from the position predicted by magnetic field models. If this anomaly influences the location of both
the main oval and the Io footprint, one should observe a similar deviation of the Europa and
Ganymede footprints [Clarke et al. 2002]. Unfortunately, so far Ganymede has been imaged in a
sector confined to longitudes ranging from 150° to 250°, that is completely off the kink sector. The
pending questions are, with a concentration on the kink region in the longitude range 90°-140°:

Is the footprint of Ganymede equally deviated in the kink sector?
5



What is the effective range of the potential magnetic anomaly?
Is Ganymede's footprint followed by an emission tail, like Io and Europa?
What is the effect of Ganymede's internal magnetic field on the interaction with Jupiter's
magnetospheric plasma?
The proposed observing program will consist of 128 orbits total, with the detailed scheduling
discussed below. The number of orbits for each visit is determined by the time spacing permitted and
period of time needed to determine correlations with the measured variations in the solar wind. To
enhance the participation of a large number of investigators, we specify no proprietary period for the
HST data, and we will further provide an online data set of reduced auroral images, with all standard
reduction steps taken and converted to brightness in kR per pixel.
Table 1: The investigators in this program cover all fields of expertise needed to define the
observations, process the images, and interpret their results. The PI and CoIs have wide experience in
the fields of planetary magnetospheres and aurora, as shown by over 100 published papers involving
HST observations. A partial list of tools for image analysis and modeling by team members is:
Code
- Mapping and projection of HST images
- Generation of synthetic images of giant
planets' aurora seen from Earth orbit
- Auroral energy deposition and thermal structure
of auroral thermosphere
- Radiative transfer in planetary atmospheres
- Modeling of magnetospheric currents and
acceleration processes in Jupiter and Saturn’s aurora
Description
Clarke et al. (2002), Grodent et al. (2005)
Grodent et al. (2003a), Gérard et al. (2004)
Grodent et al. (2001)
Gladstone et al. (2002)
Cowley & Bunce (2003), Cowley et al. (2005)
References:
Clarke, J.T., et al., J. Geophys. Res., 103(E9), 20,217, (1998).
Clarke J.T., et al., Nature, 415, 997 (2002).
Clarke, J.T., et al., in Jupiter, Planet, Satellites, and Magnetosphere, CUP, (2004).
Clarke, J.T., et al., Nature, 433, 717, (2005).
Cowley, S. and E. Bunce, Ann. Geophysicae, 21, 1691 (2003).
Cowley, S. et al., J. Geophys. Res., 110, doi:10.1029/2004JA010796 (2005).
Crary, F.J., et al., Nature, 433, 720, (2005).
Gérard, J.-C., et al., Science, 266, 1675, (1994).
Gérard, J.-C., et al., J. Geophys. Res., 109, doi:10.1029/2004JA010513, (2004).
Gérard, J.-C., et al., J. Geophys. Res., 110, doi:10.1029/2005JA011094, (2005).
Gladstone, G.R., et al., Nature, 415, 1000, (2002).
Grodent, D. et al., J. Geophys. Res., 106, 12, 933 (2001).
Grodent, D., et al., J. Geophys. Res., 108, 1389, doi:10.1029/2003JA009921, (2003a).
Grodent, D., et al., J. Geophys. Res., 108, 1366, doi:10.1029/2003JA010017, (2003b).
Grodent, J., et al., J. Geophys. Res., 109, doi:10.1029/2003JA010341, (2004).
Grodent, D., et al., J. Geophys. Res., 110, doi:10.1029/2004JA010983, (2005).
Hanlon, P. et al., J. Geophys. Res., doi:10.1029/2003JA010116, 2004a.
6
Hanlon, P.G., J. Geophys. Res., doi:10.1029/2003JA010112, 2004b.
Kurth, W.S., et al., Nature, 433, 722, (2005).
Pryor, W.R., et al., Icarus, 178, 312, (2005).
Stallard, T.S. et al., Ap. J., 521, L149 (1999).
Figure 1: HST STIS image of UV
auroral emissions from Jupiter’s north
polar region, showing the three
different emission processes.
Figure 2: Mosaic of three UV images of Saturn’s
aurora in Jan. 2004, overlaid on visible wavelength
HST images for artistic effect (this figure was
produced for the cover of Nature on 17 Feb. 2005).
The series covers the period of time of a major
solar wind disturbance arriving at Saturn, and the
auroral response (images taken 24, 26, and 28 Jan.
2004 from lower left to upper right).
Figure 3: Samples of rectified traces of Io southern
footprint emission ordered by Io distance from the torus
plane. The scales in degrees of longitude and latitude are
shown at the bottom. The color scale indicates the emission
brightness.
7
Figure 4: Measurements of the solar
wind dynamic pressure near Saturn by
the Cassini spacecraft near solar
minimum. Note the repeating maxima
separated by one solar rotation (~24
days). Vertical lines are plotted at one
day intervals, from which it can be seen
that daily observations will be needed to
sample several points in the narrower
maxima.
 Description of the Observations
Jupiter during New Horizons flyby (42 orbits): This period of observations will encompass
the New Horizons (NH) approach and flyby of Jupiter. On approach, NH will measure the
solar wind plasma density starting at closest approach (C/A) - 39 days. HST observations
under two-gyro mode will be possible from 9 Feb. 2007 according to the All Sky Window
Information online tool. Initial HST observations are requested for 1 orbit/day to sample the
auroral emissions with temporal resolution of the auroral response to solar wind disturbances
(see Figure 4). For a NH closest approach on 28 Feb. 2007 (from the post-launch trajectory),
these observations will continue until C/A – 1 day (27 Feb.) for 17 orbits in this phase. During
C/A +/- 1 day, we request 5 HST orbits/day for the most intensive coverage of the aurora
during this most intensive portion of the NH measurements, for 15 orbits in this phase. Five
orbits/day is the most allowed for ACS SBC MAMA observations outside of the SAA.
Following C/A, we request 1 orbit/day for 10 days for comparison with the NH measurements
of plasma blobs in the magnetotail region (10 orbits). The daily schedule is needed to resolve
the period of these bursts, established to be ~3 days by Galileo measurements, and the 10 days
will follow NH to a distance of ~250 RJ down the tail.
Jupiter pre-opposition (38 orbits): This period will cover measurements of the solar wind
from near 1 AU extrapolated to Jupiter, with information about both the IMF and solar wind
density. One orbit/day is requested for a period of 30 days pre-opposition (from 10 May to 9
June 2007), when the extrapolation is most accurate. For two of those days, spaced 1/2 solar
rotation apart for the variation in solar wind conditions, we request 5 orbits/day to observe the
auroral variations with Jupiter rotation. This is needed to separate corotational motions
(indicating internally driven processes) from local time effects (indicating externally driven
processes, i.e. the interaction with the solar wind). The pre-opposition period is needed in
addition to the NH flyby to determine the sensitivity of the auroral and magnetospheric
variations with IMF, which NH cannot measure.
Saturn pre-opposition (38 orbits): As for the preceding Jupiter pre-opposition observations,
this period will cover measurements of the solar wind from near 1 AU extrapolated to Saturn.
One orbit/day is requested for a period of 30 days pre-opposition (from 11 Jan to 10 Feb
2007), when the extrapolation is most accurate. For two of those days, spaced 1/2 solar
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rotation apart for a variation in solar wind conditions, we request 5 orbits/day to observe the
auroral variations with Saturn rotation for the same reasons as at Jupiter. The earlier HST
campaign on Saturn in Jan. 2004 during the Cassini approach was successful in determining
that increases in solar wind density cause auroral storm events, while no effect of the IMF was
seen. This is believed to be due to the large tilt of Saturn, causing misalignment with the IMF.
Closer to Saturn equinox in Jan. 2007, the closer alignment of the IMF with Saturn’s magnetic
field will provide a test of this theory. The combination of field and plasma measurements by
Cassini inside Saturn’s magnetosphere, solar wind conditions extrapolated to Saturn, and high
resolution auroral images from HST provide a powerful combination.
The total HST program request is 128 orbits. This total includes 10 dedicated orbits
for specific observing geometries, which are combinations of the planetary longitude and
satellite orbital longitude. The long list of possible observing times is beyond the page limit of
this section. Since the scheduling of these geometries involves trade-offs between many
parameters, we will work with STScI personnel to schedule the best possible times. To the
extent that the footprint observations can be scheduled as part of the daily series, all these extra
orbits may not be needed. We will work with the schedulers to determine this, but in this
proposal we request the full number of orbits with no overlap. More details on the coordinated
measurements are given below, after a short discussion of the HST scheduling. With the
restrictions of two-gyro mode, these observations must be carried out pre-opposition when
each planet is more than approximately 60 deg from the Sun. In cycle 15, these periods will
be early Feb. to early June 2007 for Jupiter and early Oct. to early Feb. 2007 for Saturn. For
the best extrapolation of solar wind conditions from 1 AU to each planet, the observations
need to be made within +/- 30 days of opposition.
 Special Requirements
The main special requirement for this program will be the time critical nature of the
observations. Having scheduled smaller observing sets like these in the past, it has generally
not proven difficult to find one orbit each day when a given planet can be observed (i.e. not
limited by guide stars or bright objects). Specifically, the observations will consist of single
orbits each day over a prescribed period of days, with occasional days when we schedule 5
orbits. The 5 orbits per day number is set by the limit that the MAMA detector can be
operated outside the SAA. We have checked the times allowed by two-gyro tracking for each
planet for the periods requested, and at times may need to skip a day or two due to lunar
avoidance. We have considerable flexibility in choosing the days for the 5 orbit series. One
requirement will be to ensure that the requested observing geometries for the Jovian auroral
footprints are achieved at some time during the Jupiter observations. Since there is no
requirement about which day a particular geometry is observed, we are optimistic that we can
easily schedule the great majority of these observations. The 10 orbits for specific footprint
geometries, discussed previously, may be schedule at any time during cycle 15, but if these
geometries can be included in the other observations the additional orbits will not be needed.
In the case of Saturn, the ACS SBC field of view (FOV) images both poles, and we
will point at the planet center. For Jupiter, only one pole is imaged at a time, and when we
learn the times of the observations we can select an offset to the pole whose auroral emission
9
region is facing the Earth at that time. In terms of roll angle, we have no constraints beyond a
preference to align the N/S planet line close to the diagonal of the FOV to increase the
coverage of the limb (Jupiter) or the rings (Saturn). Since this orientation is close to the
nominal roll angle for planets, this has not limited our ability to schedule observations in the
past. Finally, the count rates from each planet with each proposed filter have been measured in
prior observations, and are well within the permitted limits.
 Coordinated Observations
Solar Wind Coordinated Measurements: The distribution of HST auroral images is driven
by the time scale for solar wind changes at distances of several AU. Near solar minimum (as
in 2007), the solar wind has a well-ordered sector structure with the solar rotation period of
about 24 days. While the disturbances found near solar minimum are smaller than those near
solar maximum, there are still large variations, with a real advantage for this program in the
well-ordered structure for the extrapolation across several AU. Figure 4 shows measurements
of the solar wind dynamic pressure measured by Cassini near Saturn in 2004 at solar
minimum. It can be seen that coverage of one complete solar rotation is critical to ensure that
large variations will be sampled, and observations separated by one day are sufficient to
provide several points of the response of the aurora to a given passing disturbance. A
discussion of the specific solar wind measurements has been given in the Scientific
Justification. We plan to use data from the ACE spacecraft near the Earth, plus other
spacecraft more distributed in solar longitude (such as STEREO) which become available.
The use of multiple spacecraft will improve the accuracy of the extrapolation. K.C. Hansen
will lead the modeling effort to extrapolate solar wind conditions using his existing code.
New Horizons Measurements: The New Horizons mission to Pluto & Charon, launched on
19 Jan. 2006, will execute a gravity assist at Jupiter with a closest approach (C/A) on 28
February 2007. The NH spacecraft will pass Jupiter on the dusk side on 28 Feb. 2007 at a
distance of ~32 RJ, about 3 times closer than Cassini got to Jupiter. The next opportunity for
in-situ observations will not occur until the Juno mission, currently planned for arrival at
Jupiter in 2016. Starting 38 days before closest approach (C/A-38d), the NH particle
instrument SWAP will measure the solar wind proton density, while the NH ultraviolet
spectrograph ALICE simultaneously measures the total emission from Jupiter’s aurora (but
without spatially resolving the different emission regions). The solar wind measurements will
be nearly continuous until C/A, with only occasional telemetry and trajectory correction
maneuvers. The high resolution ACS SBC observations are required to determine the relative
contributions from the various auroral regions (e.g., main oval, polar, and satellite footprints,
in each hemisphere). These observations will concentrate on the effect of the solar wind
dynamic pressure on Jupiter’s aurora and magnetosphere. The later May/June series will
concentrate on the effect of the IMF on Jupiter’s aurora and magnetosphere.
Near closest approach, ALICE and the LEISA near-IR spectral imager on NH will
simultaneously map the auroral emissions from Jupiter, and the LORRI panchromatic imager
will be used just after C/A to image the nightside aurora as it crosses the limb of the planet to
accurately measure the altitude of peak emission. ACS SBC observations will be required to
provide high spatial resolution and a time history for the morphology of the aurora in this
10
period. The SWAP and PEPSSI instruments will then study plasma flows as NH flies down
the magnetotail, but as seen from NH Jupiter will be too close to the Sun for ALICE or LORRI
to view nightside aurora. ACS SBC observations are of vital importance to making maximum
use of this particle data. The spacing of these plasma clouds in the magnetotail is of the order
of 3 days, we will observe the aurora daily for another 10 days, at which point NH will be 250
RJ down the tail, and dynamical events observed by NH will be correlated with large-scale
changes in the auroral brightness and morphology. CO-I R. Gladstone of SwRI will
coordinate the data sets from NH and HST.
Cassini at Saturn: During the Saturn observing period the Cassini spacecraft will be orbiting
Saturn, covering a broad range of sun angles and distances, with near-continuous
measurements of the magnetic field and particle fluxes. The comparison of these data with the
HST auroral images will be led by the Cassini instrument team members F. Crary, M.
Dougherty, W Kurth, and D. Mitchell.
Ground-based near-IR and Non-thermal Radio Observations: Once the HST time
allocation for this program is known, complementary infrared ground-based spectral
observations of Jupiter and Saturn will be organized. The University College team (Co-I T.
Stallard and S. Miller) will apply for observing time with the CSHELL high resolution
spectrometer on NASA's InfraRed Telescope Facility in Hawaii to obtain spectra of Doppler
shifted H3+ emissions. These simultaneous observations will make it possible to relate the UV
auroral morphology with the ionospheric wind system associated with the magnetospheric
convection patterns, i.e. the Dungey and Vasyliunas cycles, and therefore give
important/additional information on the origin of the auroral particles and the induced motion
in the ionosphere. Previous IR measurements suggest that the ion convection patterns are
quite different on the two planets. At this point, the observations of the ion motion in Saturn's
ionosphere are lacking the global context that will be provided by the HST UV auroral images.
Measurements of the decametric emissions from Jupiter are made continuously from the array
at Nançay France, and measurements of the Saturn kilometric emissions will be measured
continuously over the period of HST observations by the Plasma Wave experiment on Cassini
(these emissions cannot be observed from the ground). Statistics based on DAM emissions
observed from Nancay indicate a large probability of observing emission from the Io footprint
during approximately 20 hours per month in each hemisphere.
 Justify Duplications
While there have been prior observing periods which attempted to correlate auroral
emissions with the solar wind, the overlap of cycle 15 with the IHY presents a unique
opportunity to have the most concentrated series of coordinated measurements of planetary
aurora with solar wind conditions, nonthermal radio emissions, and near-IR emissions. Please
see the IHY web site [http://ihy2007.org] for more detail on the vast number of space physics
measurements that will take place in 2007.
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