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Phil. Trans. R. Soc. A (2012) 370, 5213–5224
doi:10.1098/rsta.2012.0028
Temperature changes and energy inputs in
giant planet atmospheres: what we are
learning from H+
3
BY TOM S. STALLARD1, *,† , HENRIK MELIN1,† , STEVE MILLER2 ,
JAMES O’DONOGHUE1 , STAN W. H. COWLEY1 , SARAH V. BADMAN3,† ,
ALBERTO ADRIANI4 , ROBERT H. BROWN5 AND KEVIN H. BAINES6
1 Department
of Physics and Astronomy, University of Leicester, Leicester, UK
of Physics and Astronomy, University College London,
London, UK
3 Institute of Space and Astronautical Science, Japan Aerospace Exploration
Agency, Kanagawa 252-5210, Japan
4 IFSI-INAF, Rome, Italy
5 Lunar and Planetary Laboratory and Steward Observatory, University of
Arizona, Tucson, AZ 85721-0092, USA
6 Space Science and Engineering Center, University of Wisconsin-Madison,
Madison, WI 53706, USA
2 Department
Since its discovery at Jupiter in 1988, emission from H+
3 has been used as a valuable
diagnostic tool in our understanding of the upper atmospheres of the giant planets. One
of the lasting questions we have about the giant planets is why the measured upper
atmosphere temperatures are always consistently hotter than the temperatures expected
from solar heating alone. Here, we describe how H+
3 forms across each of the planetary
disks of Jupiter, Saturn and Uranus, presenting the first observations of equatorial H+
3
at Saturn and the first profile of H+
3 emission at Uranus not significantly distorted by
the effects of the Earth’s atmosphere. We also review past observations of variations in
temperature measured at Uranus and Jupiter over a wide variety of time scales. To this,
we add new observations of temperature changes at Saturn, using observations by Cassini.
We conclude that the causes of the significant level of thermal variability observed over
all three planets is not only an important question in itself, but that explaining these
variations could be the key to answering the more general question of why giant planet
upper atmospheres are so hot.
Keywords: aurora; aeronomy; Jupiter; Saturn; Uranus
*Author for correspondence ([email protected]).
† Visiting astronomer at the Infrared Telescope Facility, which is operated by the University of
Hawaii under Cooperative Agreement no. NNX-08AE38A with the National Aeronautics and Space
Administration, Science Mission Directorate, Planetary Astronomy Program.
This article is based on observations collected at the European Organization for Astronomical
Research in the Southern Hemisphere, Chile: 080.C-0120(A).
One contribution of 21 to a Theo Murphy Meeting Issue ‘Chemistry, astronomy and physics of H+
3 ’.
5213
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5214
T. S. Stallard et al.
1. H+
3 in the giant planets
Since H+
3 was first discovered in the upper atmosphere of Jupiter in 1988
[1], our understanding of H+
3 emission from the giant planets has advanced
significantly. Observations of H+
3 emission are now used not only as a direct
measure of the auroral morphology of these planets, but also as a probe investigating both the magnetospheric conditions that drive the aurora and the
ionospheric and thermospheric effects that they cause. Measurements of the H+
3
temperature, which directly relates to the thermal conditions within the
surrounding atmosphere, have allowed us to produce insights into the way energy
is dissipated within the upper atmospheres of these planets [2].
(a) Aurorae in the giant planets
For both Jupiter and Saturn, a combination of interaction with the solar
wind and internal processes drives currents from the magnetosphere down into
the planet forming aurorae near the poles. At Jupiter, solar wind interaction
results in a wide polar region of sporadic, highly variable and structured
emission, suggesting that the polar regions are controlled either directly by
field lines that are open to the solar wind [3] or indirectly through viscous
interactions along the flanks of the magnetosphere [4]. At Saturn, interactions
with the solar wind result in the formation of a main auroral oval, produced
by upward-directed field-aligned currents flowing at and equatorward of the
open–closed field line boundary [5]. As a result, the main oval is strongly
controlled by changes in the solar wind, with compression in the solar wind
brightening the dawn aurora, forming it into a spiral morphology and, at
extreme compressions, filling the entire dawn side of the polar region with bright
emission [6,7].
More equatorward aurorae are formed through currents that close within the
magnetosphere. Neutral material lost from the volcanic moons Io and Enceladus
is ionized to form a plasma torus. At Jupiter, this plasma torus is accelerated into
co-rotation by the magnetic field and is centrifugally driven outwards, until the
increasing momentum and decreasing magnetic field strength leads to a sudden
breakdown in co-rotation at approximately 20 RJ . In turn, this breakdown sets
up a strong current system that closes through the ionosphere along the magnetic
field lines, driving the powerful and continual ‘main oval’ aurora at Jupiter [8].
At Saturn, however, sub-rotation is initiated by plasma production and transport
near the inner edge of the Enceladus plasma torus, near 3–4 RS [9], resulting in
a relatively weak mid-latitude auroral oval [7,10].
In addition to these aurorae, each moon also has currents that flow through
the perturbed plasma in the moon’s vicinity, producing discrete spots of auroral
emission on the planet [11,12]. Jupiter also has a mid-to-low latitude emission
that varies between approximately 1 per cent and 40 per cent of the peak auroral
emission; the source for this emission remains unexplained [13].
Thus, although the brightest aurorae at Jupiter and Saturn have very different
magnetospheric origins, both planets experience a similar range of interactions,
and it is only in the relative strength of these different current systems that the
two planets differ.
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H3+ in gas giant atmospheres
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The situation at Uranus is considerably more complicated. The rotational pole
is offset 97◦ from the normal to the ecliptic plane and the magnetic field is
inclined to the rotational axis at 58.6◦ , with unusually large quadrupole and
octopole moments [14]. The effects these characteristics have upon the formation
of both internal current systems and the interaction with the solar wind remain
poorly understood and probably depend strongly upon season. However, an
auroral signature was detected by Voyager, showing emission in the ultraviolet
concentrated around the magnetic poles [15], and recent Hubble Space Telescope
observations have shown bright spots of auroral emission [16].
Short-term studies have shown that it is H+
3 produced by solar extreme
ultraviolet (EUV) ionization that typically dominates across the disc of
Uranus, with variability associated with auroral emission being measured at
approximately 20 per cent. This correlates well with images of the emission,
which appear to show the aurora spread across an extended disc [2], though
the significant effect of seeing at Uranus (where typical high-quality seeing has a
half-width of 0.5 RU ) has limited the spatial details available for this planet.
(b) Extreme ultraviolet ionization at the giant planets
Figure 1 shows the normalized H+
3 intensity profiles for each planet, summing
across a spectrograph slit aligned with the rotational axis. The Jovian profile
was produced from a number of IRTF-SpeX [17] spectra, taken on 8 February
2001, as the Jovian central meridian longitude varied between 150 and 170.
The Saturn profile, published by Stallard et al. [9], was produced by summing
IRTF-CSHELL [18] spectra over the period of 7–10 February 2010. The Uranus
profile, previously unpublished, was produced by co-adding adaptive opticcorrected VLT-CRIRES [19] spectra from 28–30 October 2007, resulting in an
unprecedented view of Uranus with significantly improved spatial resolution.
At Jupiter and Saturn, the brightest emission is clearly produced by the auroral
processes discussed earlier. Aurorae associated with both solar wind and internal
processes are concentrated near the poles, enhanced along the line of sight. For
Jupiter, it has previously been shown that there is also significant H+
3 emission
across the disc of the planet. While this includes a significant component of
unexplained emission at mid-to-low latitudes [20], at the equator it is produced
entirely through EUV ionization, and is measured at approximately 1–2% of the
auroral peak emission [13].
Here, we present the first measurement of equatorial H+
3 emission from Saturn.
+
Previous profiles of Saturn’s H3 emission have not shown significant emission at
latitudes below the auroral region [21]. Figure 1 shows Saturn’s H+
3 emission
profile, aligned along the central meridian. While the main profile has been
selectively filtered to improve the spatial resolution (see Stallared et al. [9]), the
equatorial profile uses all the spectra from this period, increasing the number of
spectra used by a factor of 25. This shows that there is a significant non-auroral
component within the H+
3 emission at Saturn, where the emission at the equator
is approximately 2.5(±0.4)% of the peak auroral emission. This suggests that the
relative contribution of EUV ionization and particle precipitation to the global
H+
3 emission is similar for both Jupiter and Saturn. It is also possible that there
is an equivalent to the Jovian mid-to-low latitude emissions at Saturn, but the
resolution of the data is too limited to determine this with any certainty.
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5216
(a)
1.0
T. S. Stallard et al.
–70°–50°
–30°
latitude
–10° 0° 10°
30°
50° 70°90°
0.8
90
70
50
30
0.6
10
0
–10
0.4
–30
–50
–70
0.2
0
–20
(b)
1.0
–10
–70°–50° –30°
0
arcsec
latitude
–10° 0° 10°
10
30°
20
50° 70°90°
90 70
50
30
0.8
0.6
10
0
–10
0.4
–30
–50
–70
0.2
0
–10
–5
0
arcsec
5
10
(c)
1.0
90 70
50
30
0.8
0.6
10
0
–10
0.4
0.2
0
–70
–2
–1
0
arcsec
1
–30
–50
2
Figure 1. Profiles of H+
3 emission aligned with the rotational axes of (a) Jupiter, (b) Saturn and
(c) Uranus, taken on 8 February 2001, 7–10 February 2010 and 28–30 October 2007, respectively.
On the left, the H+
3 emission is shown (bold), along with the profile scaled up by a factor of 10
(dotted line). Calculated errors are shown in grey. The mean intensity on the body of the planet
is also shown (solid horizontal line; s.e. is shown by the dashed surrounding lines). The calculated
position of the aurora of each planet is marked; these aurorae are produced through interaction
with the solar wind (most poleward, solid vertical line) and internally driven current systems (more
equatorward, dot–dash vertical line; the origin is unclear for Uranus). Where the emission is diffuse,
an extended region of emission is shown (in grey). For Saturn, the location of the rings is also shown
(vertical dashed); the rings of Jupiter and Uranus cannot be seen within the spectral wavelengths
used. On the right, matching colours are used to show each planet’s orientation within the slit used
to make the measurements (vertical black lines). A grid of latitude and longitude is shown in steps
of 10◦ and 30◦ degrees, respectively.
A new profile of the pole-to-pole H+
3 emission profile at Uranus is also shown
in figure 1. Because this profile was produced from data using adaptive optics, it
has the highest spatial detail ever measured for such a profile on Uranus. What it
clearly shows is that, averaged over three nights of data, the typical auroral profile
is relatively flat, with no obvious variability caused by auroral brightening. The
region in which auroral features were observed by Voyager is only approximately
10(±4)% brighter than the rest of the planet (though the current rotational phase
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H3+ in gas giant atmospheres
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of Uranus is now unknown), suggesting that EUV ionization generally dominates
as a source of H+
3 at Uranus. Similarly, there is no strong evidence either of line
of sight brightening at the limb or of the equatorial bulge observed by Trafton et
al. [22].
2. Heat excess in the thermospheres of the giant planets
One of the leading questions that remains unanswered about the atmospheres
of the giant planets is: why are the thermospheres of these planets so hot?
Measurements of the upper atmosphere temperatures for each of these planets
are consistently 300–700 K hotter than the temperatures predicted through solar
heating alone [23]. This suggests that the measured temperatures are the result
of a significant additional heating source.
One obvious suggestion for such heating would be that it is generated in
the auroral regions by particle precipitation, Joule heating and ion drag. These
processes produce heating of between a few (for Saturn, see Smith et al. [24]) and
a few hundred (for Jupiter, see Melin et al. [25]) terawatts. These heating rates are
one or two orders of magnitude greater than that produced by the absorption of
solar EUV radiation by the upper atmosphere, and would easily fill the ‘energy
gap’ if distributed globally. However, models of ionospheric and thermospheric
interactions at these planets (e.g. Smith & Aylward [26] for Jupiter; MüllerWodarg et al. [27] for Saturn) show that such heating is confined to high latitudes
by the extreme Coriolis forces. Alternative localized sources for the heating in the
equatorial regions have been proposed, such as propagation of heat from lower in
the atmosphere through upward-flowing gravity waves or lower latitude particle
precipitation, but there remains no definite evidence that these cause the required
heating (see Matcheva & Strobel [28]).
For Uranus, Melin et al. [29] have combined data taken by numerous
observatories and instruments and, having re-analysed the spectra, have used
them to produce a measure of the variability in both temperature and column
density between 1992 and 2009. These observations show that the column density
generally remains relatively constant, independent of solar cycle or season, but
that, in some cases, dramatic auroral intensifications drive up the H+
3 density by
a factor of 3–4, perhaps indicating the effects of compressions by the solar wind,
or some other aurora-inducing event. Within the temperature measurements,
however, there has clearly been a cooling trend for the past 15 years. Although
temperatures measured over this period appear to be somewhat variable, with a
typical short-term variability of 100 K during each observing run, the overall trend
shows a steady decrease in more than 150 K with time. This downward trend has
occurred over the period between the last solstice (in 1986) and equinox (in 2007)
on Uranus.
This suggests that there is a strong seasonal variation in the upper atmosphere
temperature at Uranus, with the entire northern hemisphere (under Uranus
longitude system coordinates) continually lit during the solstice period, and each
hemisphere seeing a full day and night cycle at equinox. However, because direct
heating from the Sun cannot explain the degree of change in the temperatures,
this seasonal effect must be indirectly driving the temperature changes. Melin
et al. [29] have suggested that this heating is the result of changes in the
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T. S. Stallard et al.
magnetospheric interaction between the solar wind and the planet. We suggest
an alternative source for this long-term change in temperature. Seasonal solar
heating changes are limited at high altitude, but deeper in the atmosphere
the changing solar illumination will drive stronger changes in temperature. It
is possible that energy is being transported vertically into the thermosphere–
ionosphere from lower layers, perhaps through gravitational wave propagation
(as has been proposed for Jupiter; see Yelle & Miller [23]). If this is the case, it
has highly significant implications in our understanding of Jupiter and Saturn.
At Jupiter, our best understanding of sources of auroral heating comes from
the modelling of an auroral event by Melin et al. [25]. This event, originally
observed by Stallard et al. [30], occurred between 8 and 11 September 1998,
resulting in an average temperature increase in 100 K over that period. This
heating event occurred at the same time as a marked increase in the ion subrotation on the main auroral oval. Melin et al. [25] modelled this heating event
by scaling a one-dimensional vertical profile of the ionosphere until it agreed with
those conditions measured on 8 and 11 September. This showed that heating
caused directly by particle precipitation was counteracted by increased cooling
caused by the resultant auroral emission, but that the flow of the current through
the atmosphere also resulted in significant Joule heating and ion drag heating,
and it was these processes that drove the observed increases in temperature.
The auroral event observed was clearly accompanied by an increase in the
electric field imposed on the ionosphere by coupling to the middle magnetosphere,
probably caused by a change in conditions, either as a result of changes
in the magnetodisc, perhaps through volcanism on Io, or as a result of
changes in the solar wind strength. At Jupiter, the majority of the enhanced
current driving this heating lies beneath the homopause, carried by ions and
electrons within the hydrocarbon-rich lower thermosphere, suggesting that the H+
3
temperatures are at least partially driven by variability deeper in the atmosphere.
Another significant finding from Melin et al. [25] was that, during the
period of this auroral event, the upper atmosphere had a significant excess
of heating. In order to explain the moderate temperature increases seen, it
suggests that there must have been an additional loss process not accounted
for in the one-dimensional model. The most likely explanation for this would be
through meridional winds transporting heat equatorward, despite what current
atmospheric models suggest.
At Saturn, the comparative weakness of H+
3 emission has meant that
reliable temperature measurements have proved difficult to produce. Individual
temperatures have been measured for the entire auroral region, such as 380 ± 70 K
on 17 September 1999 and 420 ± 70 K on 2 February 2004, measured using the
UKIRT-CGS4 instrument, suggesting Saturn’s thermosphere is best fit with a
general temperature of approximately 400 K [31]. More recently, the temperature
of a specific segment of the auroral oval was calculated from Cassini-VIMS
observations [32], using very high spatial resolution images, resulting in a
temperature of 440 ± 50 K [33].
3. Cassini measurements of the aurora of Saturn
Here we present new measurements of the temperature within the auroral region
of Saturn using the Cassini-VIMS instrument. While Cassini has the distinct
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advantage of observing Saturn from a close distance, where the instrument is
able to collect significantly more photons and the signal is not affected by the
Earth’s atmosphere, the spectral resolution of the VIMS instrument (approx.
400) is significantly poorer than telescopic spectrometers, a particular challenge
as reflected sunlight and thermal emission from the interior of Saturn are as
bright, if not brighter, than the H+
3 emission across the majority of the VIMS’s
wavelength coverage. Here, we have effectively reduced these limitations by using
emissions only from above the limb of the planet, away from the background
contributions of Saturn’s disc. However, this limits the observations we can use
to those made where the bright aurora lies above the limb.
For each image, the specific location of each pixel was calculated using the
NASA SPICE code. This provided a calculated height for each pixel above the
1 bar limb, in the plane perpendicular to the line of sight. From this, it was
possible to average the intensity in a specified region above the limb for every
wavelength bin within each image, in this case 300–1100 km above the limb. An
additional background value was calculated by averaging pixels above 3000 km
and was subtracted from this value.
This spectrum was then fitted with a theoretical H+
3 emission spectrum,
produced by combining theoretical emission lines from Neale et al. [34], with
a partition function from Miller et al. [35], which were then convolved to the
measured spectral characteristics for each wavelength bin within VIMS. This was
repeated across a wide temperature range and these values were used to produce
a minimum residual fit to the auroral spectrum.
We have analysed the emission from specific images (in figure 2a–f ), taken on 9
June 2007. These images were taken during a period of bright and highly variable
emission from the southern aurorae, including regions of dawn brightening and
polar infilling, as described in Stallard et al. [7,10]. These images are spaced in
time into three pairs, with the images in each pair separated by approximately
45 min and each pair of images by approximately 4 h, as described in table 1,
providing an opportunity to attempt short-term thermal variability in Saturn’s
main oval.
VIMS spectral bins are often relatively noisy owing to instrumental defects,
and before individual bins can be used to produce a spectrum, they need to be
individually checked for linear aberrations. This has been carried out on all the
bins with strong auroral signatures, as well as those bins with obviously incorrect
brightness values.
Having fitted these spectra, shown in figure 3, we can see that the
fitted temperatures during this period vary between approximately 560 and
approximately 620 K, as indicated in table 1. We have assigned an estimated
uncertainty of ±30 K, based upon the error within the fit. These temperatures
are significantly higher than temperatures measured in the past, suggesting that
the significant variability in the H+
3 aurora during this period is likely to have
resulted in significant heating, in a similar way to the energy deposition measured
during auroral events at Jupiter.
There is clearly a drop in the measured temperature between the second and
third pair of images. Combining the measured temperatures for each pair of
images, weighted according to the integration time of the observation (either
by 44 or 22 min), allows us to reduce the error further. This results in an average
temperature of 611 K, 611 K and 567 K (±20) and a re-normalized brightness of
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5220
T. S. Stallard et al.
(a)
(b)
(c)
(d)
(e)
(f)
Figure 2. A sequence of images, taken by the Cassini-VIMS instrument on 9 June 2009, during
a period of significant auroral variation. Each image is produced by combining a number of
wavelength bins with H+
3 emission lines within them, and subtracting non-auroral bins to remove
reflected sunlight. Latitude and longitude (local time measured in hours) are shown as a grid, in
steps of 10◦ and 3 h, respectively. The region of emission combined in figure 3, 300–1100 km above
the limb of the planet, is delineated (grey lines above the limb). (Online version in colour.)
(a)
(b)
1.2
1.0
0.8
0.6
0.4
0.2
0
614 K
(d )
1.2
1.0
0.8
0.6
0.4
0.2
0
(c)
630 K
626 K
(f)
(e)
600 K
3.4
3.6
564 K
3.8
4.0
4.2
3.4
3.6
558 K
3.8
4.0
4.2
3.4
3.6
3.8
4.0
4.2
Figure 3. Spectra of H+
3 above the limb for each image shown in figure 2, with normalized intensity
plotted against wavelength in microns. The H+
3 spectra (bold) were produced by combining the
total intensity measured between 300 and 1100 km above the limb of the planet, and subtracting
a background intensity from more than 3000 km. Each spectrum has been fitted with a modelled
H+
3 to produce a best-fit temperature (dashed line). (Online version in colour.)
0.934, 1.00 and 0.498 for each pair (a and b), (c and d) and (e and f ), respectively.
These indicate that the temperature and auroral brightness of Saturn’s northern
hemisphere were essentially constant for the period between 04.35 and 08.50 (a
slight increase in emission strength occurred), but that the temperature fell by
44 K in the 4 h and 14 min between the second and third pairs of images.
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Table 1. The observation time and integration time for each of the images used in figures 2 and 3,
and together with the fitted temperature and normalized intensity for each image within the region
300–1100 km above the limb.
(a)
(b)
(c)
(d)
(e)
(f )
time of
observation (UT)
total integration time
(time (ms) per pixel)
temperature (K)
normalized
emission
04.16
05.04
08.28
09.22
12.40
13.38
44 min
22 min
44 min
22 min
44 min
22 min
605
624
618
596
563
576
0.833
0.998
1.000
0.852
0.485
0.451
(640)
(320)
(640)
(320)
(640)
(320)
(±30)
(±30)
(±30)
(±30)
(±30)
(±30)
The temperature variability is also matched by changes in auroral brightness.
The halving in auroral brightness within the last pair of spectra is consistent
with the decrease in emission strength that would be expected for a temperature
drop from 611 to 567 K, assuming that the H+
3 column density remained constant.
This leads to the conclusion that VIMS observed a significant cooling in the upper
atmosphere of Saturn during this period.
Recently, Miller and co-workers (unpublished calculations) have been
calculating the changes in total energy content of Saturn’s auroral/polar upper
atmosphere that temperature changes might represent. By using atmospheric
profiles generated by Galand et al. [36] they concluded that, depending on whether
the H+
3 ‘temperature’ was sensitive to a temperature change in the whole column
of air above the homopause or just that above the main H+
3 emission altitude,
a change in temperature of 1 K s−1 represented heating or cooling of either
2.7 × 1015 or 1.1 × 1015 W, respectively. The drop of 44 K in 254 min represents a
temperature change of 2.887 mK s−1 . In turn, this would correspond to a cooling
rate of 7.8 × 1012 W for the whole column above the homopause, or 3.2 × 1012 W
for the column above the main H+
3 emission layer. This is a highly significant rate,
and corresponds to the kind of heating rates produced in Saturn’s polar upper
atmosphere by Joule heating and ion drag [24].
Cooling of the polar regions can be affected either by the lateral transfer of
heat by winds to lower latitudes or by vertical transfer, mainly by conduction, to
lower altitudes, below the homopause. It is hard to imagine downward conduction
being sufficiently rapid to account for such a large temperature drop in so short a
period: scaling the figures calculated in Melin et al. [25] for Saturn’s atmospheric
density and temperature profile shows that cooling by downward conduction,
when integrated across Saturn’s auroral region, is unlikely to exceed 0.3 TW.
This is an order of magnitude too weak to account for the change. Steady-state
modelling, as has already been noted, shows that horizontal transfer is difficult
since coriolis forces turn equatorial winds westwards, so that they become zonal
rather than meridional. But these VIMS observations indicate that Saturn is
far from being in ‘steady state’, and it remains a challenge to the modelling
community to see what effect conditions that vary on fairly rapid time scales
will produce.
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T. S. Stallard et al.
4. Conclusion
As we have seen, the H+
3 temperatures measured at Jupiter, Saturn and
Uranus have all been found to undergo significant variability. These changes in
temperature have been observed on a number of different time scales.
Uranus provides us with a unique view into a world where the auroral
contributions appear to be somewhat limited and temperature variations occur
on both short and long time scales. This potentially provides us with a view of
how changes in heating within the underlying homosphere are driving conditions
within the upper atmosphere, a form of ionospheric heating that has, in the past,
been relatively ignored.
Recent observations of the apparently variable rotation rate of Saturn have
re-ignited the debate about ionospheric–magnetospheric interactions. One of the
leading theories to explain the changes in the magnetospheric rotation rate is
that these changes are created somehow in the lower atmosphere and propagate
into the magnetosphere through the ionosphere [37]. These changes in rotational
period have also been associated with seasonal variation, though there is an
apparent lag between the change in season and the rotation rate [38]. In this
context, the results from Uranus suggest that energy exchange between the
lower atmosphere and the ionosphere may be far more important than we have
previously thought.
Short-term variations in the temperatures of all three planets appear to result
from changes within the thermosphere itself, either within the lower hydrocarbonrich auroral regions on Jupiter or even directly at the peak H+
3 emission, as we
have now measured at Saturn. These dramatic changes in temperature are one
potential source for the elevated equatorial temperatures measured in all the
giant planets. Most atmospheric models are run under the assumption that the
atmospheres are in a steady state, and the effects of changes in temperature are
included within a balanced system. What the observations we have presented here
show is that all these planets are each far from achieving thermal equilibrium.
Continual changes in temperature mean that energy is continually flowing
through the auroral regions. This dynamism could easily lead to the formation
of waves within the upper atmosphere, in turn driving heating away from the
auroral regions and down to the equator.
Thus, in our future studies attempting to answer why the auroral temperatures
within giant planets vary so greatly, we may, at the same time, make significant
strides into explaining why the upper atmospheres are so hot.
This work was supported by a RCUK Fellowship for T.S. and by the UK STFC for H.M.
References
1 Drossart, P. et al. 1989 Detection of H+
3 on Jupiter. Nature 340, 539–541. (doi:10.1038/
340539a0)
2 Miller, S., Stallard, T., Smith, C., Millward, G., Melin, H., Lystrup, M. & Aylward, A. 2006 H+
3:
the driver of giant planet atmospheres. Phil. Trans. R. Soc. A 364, 3121–3137. (doi:10.1098/
rsta.2006.1877)
3 Cowley, S. W. H., Bunce, E. J., Stallard, T. S. & Miller, S. 2003 Jupiter’s polar ionospheric
flows: theoretical interpretation. Geophys. Res. Lett. 30, 1220. (doi:10.1029/2002GL016030)
Phil. Trans. R. Soc. A (2012)
Downloaded from http://rsta.royalsocietypublishing.org/ on June 17, 2017
H3+ in gas giant atmospheres
5223
4 Delamere, P. A. & Bagenal, F. 2010 Solar wind interaction with Jupiter’s magnetosphere.
J. Geophys. Res. 115, A10201. (doi: 10.1029/2010JA015347)
5 Bunce, E. J. et al. 2008 Origin of Saturn’s aurora: simultaneous observations by Cassini and
the Hubble Space Telescope. J. Geophys. Res. 113, A09209. (doi:10.1029/2008JA013257)
6 Grodent, D., Gérard, J.-C., Cowley, S. W. H., Bunce, E. J. & Clarke, J. T. 2005 Variable
morphology of Saturn’s southern ultraviolet aurora. J. Geophys. Res. 110, A07215. (doi:10.1029/
2004JA010983)
7 Stallard, T. et al. 2008 Complex structure within Saturn’s infrared aurora. Nature 456, 214–217.
(doi: 10.1038/nature07440)
8 Cowley, S. W. H. & Bunce, E. J. 2001 Origin of the main auroral oval in Jupiter’s coupled
magnetosphere–ionosphere system. Planet. Space Sci. 49, 1067–1088. (doi:10.1016/S0032-0633
(00)00167-7)
9 Stallard, T., Melin, H., Cowley, S. W. H., Miller, S. & Lystrup, M. B. 2010 Location and
magnetospheric mapping of Saturn’s mid-latitude infrared auroral oval. Astrophys. J. Lett.
722, L85–L89. (doi:10.1088/2041-8205/722/1/L85)
10 Stallard, T., Miller, S., Melin, H., Lystrup, M., Cowley, S. W. H., Bunce, E. J., Achilleos, N. &
Dougherty, M. 2008 Jovian-like aurorae on Saturn. Nature 453, 1083–1085. (doi:10.1038/
nature07440)
11 Clarke, J. T., Ben Jaffel, L. & Gérard, J.-C. 1998 Hubble Space Telescope imaging of Jupiter’s
UV aurora during the Galileo orbiter mission. J. Geophys. Res. 103, 202 17–202 36. (doi:10.1029/
98JE01130)
12 Pryor, W. R. et al. 2011 The auroral footprint of Enceladus on Saturn. Nature 472, 331–333.
(doi:10.1038/nature09928)
13 Rego, D., Miller, S., Achilleos, N., Prangé, R. & Joseph, R. D. 2000 Latitudinal profiles of the
Jovian IR emissions of H+
3 at 4 mm with the NASA infrared telescope facility: energy inputs
and thermal balance. Icarus 147, 366–385. (doi:10.1006/icar.2000.6444)
14 Connerney, J. E. P., Acuna, M. H. & Ness, N. F. 1987 The magnetic field of Uranus. J. Geophys.
Res. 92, 15 329–15 336. (doi:10.1029/JA092iA13p15329)
15 Herbert, F. & Sandel, B. R. 1994 The Uranian aurora and its relationship to the magnetosphere.
J. Geophys. Res. 99, 4143–4160. (doi:10.1029/93JA02673)
16 Lamy, L. et al. 2012 Earth-based detection of Uranus’ aurorae. Geophys. Res. Lett. 39, L07105.
(doi:10.1029/2012GL051312)
17 Rayner, J. T., Toomey, D. W., Onaka, P. M., Denault, A. J., Stahlberger, W. E., Vacca, W. D.,
Cushing, M. C. & Wang, S. 2003 SpeX: a medium-resolution 0.8–5.5 micron spectrograph
and imager for the NASA infrared telescope facility. Publ. Astron. Soc. Pac. 115, 362–382,
(doi:10.1086/367745)
18 Greene, T. P., Tokunaga, A. T., Toomey, D. W. & Carr, J. B. 1993 CSHELL: a high spectral
resolution 1–5 mm cryogenic echelle spectrograph for the IRTF. In Infrared detectors and
instrumentation (ed. A. M. Fowler), pp. 313–324. Proc. SPIE 1946. Bellingham, WA: SPIE.
19 Käufl, H.-U. et al. 2004 CRIRES: a high-resolution infrared spectrograph for ESO’s VLT. Proc.
SPIE 5492, 1218–1227. (doi:10.1117/12.551480)
20 Miller, S., Achilleos, N., Ballester, G. E., Lam, H. A., Tennyson, J., Geballe, T. R. & Trafton,
L. M. 1997 Mid-to-low latitude H+
3 emission from Jupiter. Icarus 130, 57–67. (doi:10.1006/
icar.1997.5813)
21 Stallard, T., Miller, S., Ballester, G. E., Rego, D., Joseph, R. D. & Trafton, L. M. 1999 The
H+
3 latitudinal profile of Saturn. Astrophys. J. Lett. 521, L149–L152. (doi: 10.1086/312189)
22 Trafton, L. M., Miller, S., Geballe, T. R., Tennyson, J. & Ballester, G. E. 1999 H2 Quadrupole
and H+
3 emission from uranus: the uranian thermosphere, ionosphere, and aurora. Astrophys.
J. 524, 1059–1083. (doi:10.1086/307838)
23 Yelle, R. V. & Miller, S. 2004 The thermosphere and ionosphere of Jupiter. In Jupiter. The
planet, satellites and magnetosphere (eds F. Bagenal, T. E. Dowling & W. B. McKinnon), pp
185–218. Cambridge Planetary Science, vol. 1. Cambridge, UK: Cambridge University Press.
24 Smith, C. G. A., Aylward, A. D., Miller, S. & Müller-Wodarg, I. C. F. 2005 Polar heating in
Saturn’s thermosphere. Ann. Geophys. 23, 2465–2477. (doi:10.5194/angeo-23-2465-2005)
Phil. Trans. R. Soc. A (2012)
Downloaded from http://rsta.royalsocietypublishing.org/ on June 17, 2017
5224
T. S. Stallard et al.
25 Melin, H., Miller, S., Stallard, T., Smith, C. & Grodent, D. 2006 Estimated energy balance
in the jovian upper atmosphere during an auroral heating event. Icarus 181, 256–265.
(doi:10.1016/j.icarus.2005.11.004)
26 Smith, C. G. A. & Aylward, A. D. 2009 Coupled rotational dynamics of Jupiter’s thermosphere
and magnetosphere. Ann. Geophys. 27, 199–230. (doi:10.5194/angeo-27-199-2009)
27 Müller-Wodarg, I. C. F., Mendillo, M., Yelle, R. V. & Aylward, A. D. 2006 A global circulation
model of Saturn’s thermosphere. Icarus 180, 147–160. (doi:10.1016/j.icarus.2005.09.002)
28 Matcheva, K. I. & Strobel, D. F. 1999 Heating of Jupiter’s thermosphere by dissipation
of gravity waves due to molecular viscosity and heat conduction. Icarus 140, 328–340.
(doi:10.1006/icar.1999.6151)
29 Melin, H., Stallard, T., Miller, S., Trafton, L. M., Encrenaz, Th. & Geballe, T. R. 2011 Seasonal
variability in the ionosphere of uranus. Astrophys. J. 729, 134. (doi:10.1088/0004-637X/
729/2/134)
30 Stallard, T., Miller, S., Millward, G. & Joseph, R. D. 2002 On the dynamics of the Jovian
ionosphere and thermosphere. II. The measurement of H+
3 vibrational temperature, column
density, and total emission. Icarus 156, 498–514. (doi:10.1006/icar.2001.6793)
31 Melin, H., Miller, S., Stallard, T., Trafton, L. M. & Geballe, T. R. 2007 Variability in the H+
3
emission of Saturn: consequences for ionisation rates and temperature. Icarus 186, 234–241.
(doi:10.1016/j.icarus.2006.08.014)
32 Brown, R. H. et al. 2004 The Cassini visual and infrared mapping spectrometer (VIMS)
investigation. Space Sci. Rev. 115, 111–168. (doi:10.1007/s11214-004-1453-x)
33 Melin, H. et al. 2011 Simultaneous Cassini VIMS and UVIS observations of Saturn’s southern
aurora: comparing emissions from H, H2 and H+
3 at a high spatial resolution. Geophys. Res.
Lett. 38, L15203. (doi:10.1029/2011GL048457)
34 Neale, L., Miller, S. & Tennyson, J. 1996 Spectroscopic properties of the H+
3 molecule: a new
calculated line list. Astrophys. J. 464, 516. (doi:10.1086/177341)
35 Miller, S., Stallard, T., Melin, H. & Tennyson, J. 2010 H+
3 cooling in planetary atmospheres.
Faraday Discuss. 147, 283–291. (doi:10.1039/c004152c)
36 Galand, M., Moore, L., Mueller-Wodarg, I., Mendillo, M. & Miller, S. 2011 Response of
Saturn’s auroral ionosphere to electron precipitation: electron density, electron temperature,
and electrical conductivity. J. Geophys. Res. 116, A09306. (doi:10.1029/2010JA016412)
37 Smith, C. G. A. 2011 A Saturnian cam current system driven by asymmetric thermospheric
heating. Mon. Not. R. Astron. Soc. 410, 2315–2328. (doi:10.1111/j.1365-2966.2010.17602.x)
38 Andrews, D. J., Cowley, S. W. H., Dougherty, M. K., Lamy, L., Provan, G. & Southwood,
D. J. 2012. Planetary period oscillations in Saturn’s magnetosphere: evolution of magnetic
oscillation properties from southern summer to post-equinox. J. Geophys. Res. 117, A04224.
(doi:10.1029/2011JA017444)
Phil. Trans. R. Soc. A (2012)