Solar wind dynamic pressure and electric field as the main factors controlling Saturn's aurorae.

Abstract

The interaction of the solar wind with Earth's magnetosphere gives rise to the bright polar aurorae and to geomagnetic storms, but the relation between the solar wind and the dynamics of the outer planets' magnetospheres is poorly understood. Jupiter's magnetospheric dynamics and aurorae are dominated by processes internal to the jovian system, whereas Saturn's magnetosphere has generally been considered to have both internal and solar-wind-driven processes. This hypothesis, however, is tentative because of limited simultaneous solar wind and magnetospheric measurements. Here we report solar wind measurements, immediately upstream of Saturn, over a one-month period. When combined with simultaneous ultraviolet imaging we find that, unlike Jupiter, Saturn's aurorae respond strongly to solar wind conditions. But in contrast to Earth, the main controlling factor appears to be solar wind dynamic pressure and electric field, with the orientation of the interplanetary magnetic field playing a much more limited role. Saturn's magnetosphere is, therefore, strongly driven by the solar wind, but the solar wind conditions that drive it differ from those that drive the Earth's magnetosphere.

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Solar wind dynamic pressure and
electric field as the main factors
controlling Saturn’s aurorae
F. J. Crary1, J. T. Clarke2, M. K. Dougherty3, P. G. Hanlon3, K. C. Hansen4,
J. T. Steinberg5, B. L. Barraclough5, A. J. Coates6, J.-C. Ge ´rard7,
D. Grodent7, W. S. Kurth8, D. G. Mitchell9, A. M. Rymer6& D. T. Young1
1Southwest Research Institute, Culebra Road, San Antonio, Texas 78288, USA
2Boston University, 725 Commonwealth Avenue, Boston, Massachusetts 02215,
USA
3Blackett Laboratory, Imperial College of Science and Technology, London
SW7 2BZ, UK
4The University of Michigan, Space Research Building, Ann Arbor, Michigan
48109, USA
5Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
6University College London, Mullard Space Science Laboratory, Surrey RH5 6NT,
UK
7Universite ´ de Lie `ge, 17 avenue du 6 aout, B4000 Lie `ge, Belgium
8Department of Physics and Astronomy, The University of Iowa, Iowa City,
Iowa 52242, USA
9Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins
Road, Laurel, Maryland 20723, USA
.............................................................................................................................................................................
The interaction of the solar wind with Earth’s magnetosphere
givesrisetothebrightpolarauroraeandtogeomagneticstorms1,
but the relation between the solar wind and the dynamics of the
outer planets’ magnetospheres is poorly understood. Jupiter’s
magnetospheric dynamics and aurorae are dominated by pro-
cesses internal to the jovian system2, whereas Saturn’s magneto-
sphere has generally been considered to have both internal
and solar-wind-driven processes. This hypothesis, however, is
tentative because of limited simultaneous solar wind and mag-
netospheric measurements. Here we report solar wind measure-
ments, immediately upstream of Saturn, over a one-month
period. When combined with simultaneous ultraviolet imaging3
we find that, unlike Jupiter, Saturn’s aurorae respond strongly to
solar wind conditions. But in contrast to Earth, the main con-
trolling factor appears to be solar wind dynamic pressure and
electric field, with the orientation of the interplanetary magnetic
field playing a much more limited role. Saturn’s magnetosphere
is,therefore,stronglydrivenbythesolarwind,butthesolarwind
conditions that drive it differ from those that drive the Earth’s
magnetosphere.
In the case of Earth’s auroral activity and associated magnetic
substorms, the north–south component of the interplanetary mag-
netic field is especially important. A southward interplanetary
magnetic field allows efficient dayside reconnection between the
Earth’s and the interplanetary magnetic field, and is an important
driver of substorms, whereas a northward field results in high-
latitude reconnection and significantly weaker control of magneto-
spheric dynamics by the solar wind1. Solar wind shocks drive
auroral activity, regardless of the interplanetary magnetic field
orientation, and the magnitude of this effect correlates well with
the solar wind speed and magnetic field strength. However, the
influence of shocks is much more pronounced when the inter-
planetary magnetic field is southward4.
In contrast, Jupiter’s rapid rotation and strong magnetic field,
together with a large, internal plasma source from its satellite, Io,
drive radial, outward transport of plasma and magnetospheric
currents. These processes are the primary source of dynamics
(ref. 2, and references therein) and the aurorae5. Despite this, the
solar wind doesexert someinfluenceonJupiter’smagnetosphere6–9.
On the basis of the size of the planet’s magnetosphere and the
observed ease with which its magnetopause is pushed inward and
outward by changing solar wind conditions, it has been suggested
that dynamic pressure is the main solar wind driver, rather than the
orientation of the interplanetary magnetic field10,11. Saturn, like
Jupiter, is a rapidly rotating planet and has significant internal
plasma sources. However, its magnetic field and internal plasma
sources are weaker than Jupiter’s. Observations12by the Voyager
spacecraft showed a significant degree of solar wind control,
although the origin of these correlations was unclear. In the case
of the Voyager studies, kilometric radio emissions were used as a
proxy for magnetospheric dynamics.
Between10and30January2004,theCassinispacecraftmeasured
the solar wind upstream of Saturn’s magnetosphere while the
Hubble Space Telescope (HST) made observations of Saturn’s
ultraviolet aurorae roughly once every other day3. The Cassini
data discussed here come primarily from the CAPS instrument
(Cassini Plasma Spectrometer Subsystem, providing ion and elec-
tron data from 1eV to 50keV; ref. 13) and from the magnetometer.
During this period, Cassini solar wind ion measurements were
largely continuous (81% duty cycle) and interrupted only by
occasional downlinks of data to Earth. Cassini also made measure-
ments of energetic particle flux, local plasma waves and auroral
radio emissions from Saturn14.
During this period, the Cassini spacecraft was at a range of
(78 ^ 5) £ 106km from Saturn, over the post-dawn side of the
planet, and (31 ^ 2) £ 106km upstream. Although this distance
seems large for an upstream solar wind monitor, the Saturn–Sun–
spacecraft angle was under 3.38. Studies of Cassini and Galileo data,
obtained near Jupiter, indicate that a spacecraft may act as a viable
upstream monitor when the planet–Sun–spacecraft angle is less
than ,208 (ref. 15). Moving at nominally 500kms21, it would take
a spherically symmetric disturbance in the solar wind 17h to travel
from Cassini to Saturn.
Figure 1 shows the solar wind magnetic field, ion speed and
Figure1SolarwindconditionsmeasuredbyCassini.Thetoppanelshowsprotondensity,
the middle panel the flow speed, and the lower panel the magnetic field. The black line
showsthemagnitudeofthemagneticfield.Bxisshowninblue,ByingreenandBzinred.
Themagneticfieldisinplanetarycoordinates,aright-handedsystemwithzparalleltothe
axis of rotation (and magnetic moment), and the Sun in the þx/z half-plane.
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proton density. The solar wind speed is characterized by periods of
gradual decrease followed by shocks on 15 and 25 January.
Enhanced ion densities are associated with these shocks, as are
enhancements in energetic particle flux and low frequency plasma
waves. Energetic particle, wave and magnetic field data indicate
anothershockon1January,whenthespacecraftorientationdidnot
allow direct measurements of core solar wind protons. Following
the 25 January shock, the plasma density remained high and
variable until the end of the month, the magnetic field strength
was enhanced and several subsequent shocks were encountered.
These solar wind disturbances are consistent with corotating
interaction regions. A bimodal solar wind structure as observed
here is expected during the declining phase of the solar cycle
magnetic sector boundaries, with a reversal of the dominant By
component that occurred on 17 and 26 January. (Byindicates the y
component of the magnetic field; see Fig. 1 legend for coordinate
system.) During the month of observations, the north–south
component of the interplanetary magnetic field was weak, and the
field was primarily in the east–west direction.
We have used Cassini measurements to estimate conditions at
Saturn with a one-dimensional, numerical magneto-hydrodynamic
model16. This model assumes spherical symmetry and propagates
the measurements outward. It accounts for the fact that shocks
propagate faster than the ambient flow speed, as well as for any
evolution of the solar wind during the final 0.25 AU of its trip to
Saturn. As the model assumes spherical symmetry, the three-
dimensional structure of the solar wind may cause errors in the
modelled propagation time. We expect that our estimated arrival
times may be systematically late by up to roughly 10h, but cannot
be more exact owing to the unconstrained, three-dimensional
structure of the solar wind.
According to this model, the shocks observed on 15 and 25
January reached Saturn at 12:45 UTC, 16 January, and 08:00 UTC,
26 January. Figure 2 shows the propagated plasma speed and the
magnetic field, along with four of the HST images. The first image,
32h before the arrival of the 25 January shock, is typical of other
imagestakenduringundisturbedsolarwindconditions.Thesecond
image, 11h after the shock arrival (þ11h), shows a brightened and
partiallyfilledauroraloval.Inthethirdimage,atþ42h,theovalhas
shifted to higher latitudes while remaining atypically bright. In the
final image, at þ107h, the aurora has returned to a more typical
state. The response to the 15 January shock appears similar,
although both the shock and the auroral response were weaker,
and the first image obtained after the shock was at þ40h (ref. 3).
Figure 3 shows a plot of propagated solar wind dynamic pressure
against theinput auroral power, estimated fromthe total ultraviolet
brightness3. There is avery good correlation between auroral power
and both the solar wind dynamic pressure, ru2(r ¼ 0.93), and the
convection electric field, u £ B (r ¼ 0.97). (Here r is the mass
density, u the flow velocity, r the linear correlation coefficient, u the
flow velocity vector, and B the magnetic field vector.) These high
correlation coefficients are, in part, a statistical artefact of the 25
January event, which produced the two unusually high-power
points. However,thiseventisnotthesolesourceofthecorrelations.
Using only those observations with an auroral power below
5 £ 1010W(thatis,usingonly thelower-powerhalfofthemeasure-
mentsandthusexcludingtheunusualevents),weobtaincorrelation
coefficients of 0.62 and 0.92, respectively.
Correlationswithauroralpoweralsoshowalackofcontrolbythe
direction of the interplanetary magnetic field. In the case of Earth’s
magnetosphere, the dynamics correlate wellwith a gated solar wind
electric field, u £ B cos4(v/2), where v is the projected angle
between the interplanetary magnetic field and the planet’s magnetic
moment17.Thevtermaccountsforefficient, low-latitudereconnec-
tionwhentheinterplanetary magneticfieldisparalleltotheplanet’s
magnetic moment. For the January measurements, the correlation
coefficient with this parameter is 0.74. This is a significantly lower
value than that obtained without the v term, suggesting that the
orientation of the interplanetary magnetic field is not a controlling
factor.
At the same time, the interplanetary magnetic field during
January was primarily in the y direction, while the north–south
component was weak and varying, with v ¼ 91 ^ 328. An alterna-
tiveexplanationisthatthecos4gatingfunctionisnotwell-suitedfor
theinteraction betweenthesolar windandSaturn’smagnetosphere.
ThisrelationwasderivedfortheEarthandforawidevarietyofsolar
wind magnetic field orientations, including the predominantly
southward conditions that produce a strong magnetospheric
Figure3CorrelationbetweenHST-derivedauroralinputpowerandsolarwindconditions.
Triangles show the solar wind (SW) dynamic pressure, ru2; diamonds show the solar
wind convection electric field, ju £ Bj.
© 2005 Nature Publishing Group
Figure 2 Comparison between HST images and solar wind conditions propagated to
Saturn for the period 25–30 January 2004. For details of the HST images, see ref. 3.
ThickblueverticallinesindicatethetimesofthefourHSTimagesshownabove.The solar
wind conditions are presented in the same format as Fig. 1, but density and the Bx
component are omitted for clarity. In one-dimensional, time-dependent magneto-
hydrodynamics with a uniform upstream boundary condition, the x component cannot be
accurately modelled. The modelled propagation times may be systematically long by as
much as 10h. Non-radial orientation of the solar wind structures, typical of corotating
interaction regions, could cause them to reach Saturn roughly five hours earlier than
spherically symmetric calculations would suggest. Solar rotation and the 3.38 difference
between the spacecraft’s and Saturn’s heliocentric longitude could also move arrival
times earlier by approximately 6h. Overall, the estimated arrival times may be
systematically late by up to roughly 10h.
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response.Itispossiblethatthemostappropriategatingfunctionfor
values of v near 908 differs from the cos4relation best suited for the
full range of v.
It is interesting to note that the January observations occurred
near southern summer, and that the average interplanetary mag-
netic field is almost purely azimuthal at Saturn’s distance from the
Sun. As a result, the dominant component of the field was
perpendicular to Saturn’s magnetic moment. Solar wind control
of Saturn’s magnetosphere could be significantly different near the
equinoxes, when Saturn’s axial tilt results in a 64.58 angle between
the dominant, azimuthal field component and Saturn’s magnetic
moment. This would be a manifestation of a well-documented
phenomenon at Earth18, but made more extreme owing to the
alignment between Saturn’s magnetic moment and spin axis.
Overall,theJanuaryCassiniandHSTmeasurementsindicatethat
Saturn’s magnetosphere, although similar in some respects to those
of the Earth and Jupiter, is not simply an intermediate case. It
resembles Earth’s magnetosphere in that its auroral dynamics are
strongly driven by solar wind conditions. But it differs from Earth’s
interaction with the solar wind by virtue of an evidently weak
influence of the direction of the interplanetary magnetic field. This
may either be a characteristic of Saturn’s magnetosphere, or be a
result of differences between the solar wind at 10 AU and 1 AU
(specifically, the weaker north–south component of the solar
wind magnetic field). The latter possibility suggests that Saturn’s
magnetospheric response to the solar wind maybe primarily driven
by solar wind shocks, a process that is observed at the Earth2but
generally overshadowed by the strong response of Earth’s magneto-
sphere to the orientation of the interplanetary magnetic field. This
apparent insensitivity to the magnetic field orientation and sensi-
tivity to solar wind dynamic pressure and/or motional electric field
make Saturn’s magnetosphere resemble that of Jupiter more than
that of Earth. In contrast to Jupiter, where the influence of the solar
wind is weak and the aurorae are largely due to internal processes,
the solar wind plays a controlling role in Saturn’s auroral
dynamics.
A
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other NASA/JPL contracts; P.G.H. was supported by a PPARC-UK quota studentship; and J.C.G.
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Competing interests statement The authors declare that they have no competing financial
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Correspondence and requests for materials should be addressed to F.J.C. (fcrary@swri.edu).
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An Earth-like correspondence
between Saturn’s auroral
features and radio emission
W. S. Kurth1, D. A. Gurnett1, J. T. Clarke2, P. Zarka3, M. D. Desch4,
M. L. Kaiser4, B. Cecconi1, A. Lecacheux3, W. M. Farrell4, P. Galopeau5,
J.-C. Ge ´rard6, D. Grodent6, R. Prange ´3, M. K. Dougherty7& F. J. Crary8
1Department of Physics and Astronomy, The University of Iowa, Iowa City,
Iowa 52242, USA
2Boston University, 725 Commonwealth Avenue, Boston, Massachusetts 02215,
USA
3Space Research Department, Observatoire de Paris, 92195 Meudon, France
4NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
5CETP/UVSQ, 78140 Velizy, France
6LPAP, Universite ´ de Lie `ge, alle ´e du 6 aout, 17, B-4000 - Lie `ge, Belgium
7Blackett Laboratory, Imperial College of Science and Technology, London
SW7 2BZ, UK
8Southwest Research Institute, Culebra Road, San Antonio, Texas 78288, USA
.............................................................................................................................................................................
Saturn is a source of intense kilometre-wavelength radio emis-
sions that are believed to be associated with its polar aurorae1,2,
and which provide an important remote diagnostic of its mag-
netospheric activity. Previous observations implied that the
radio emission originated in the polar regions, and indicated a
strong correlation with solar wind dynamic pressure1,3–7. The
radio source also appeared to be fixed near local noon and at the
latitude of the ultraviolet aurora1,2. There have,however, been no
observations relating the radio emissions to detailed auroral
structures. Here we report measurements of the radio emissions,
which, along with high-resolution images of Saturn’s ultraviolet
auroral emissions8, suggest that although there are differences in
the global morphology of the aurorae, Saturn’s radio emissions
exhibit an Earth-like correspondence between bright auroral
features and the radio emissions. This demonstrates the univer-
sality of the mechanism that results in emissions near the
electron cyclotron frequency narrowly beamed at large angles
to the magnetic field9,10.
The first studies of Saturn’s primary radio emission (Saturn
kilometric radiation, SKR) were by the Voyager spacecraft in the
early 1980s1,3. This emission had some features that were more like
those of Earth than of Jupiter: the SKR source appears to be fixed in
local time, favouring a region near local late morning or noon, but
varyingwithsourcelatitude2.Itisalsostronglycorrelatedwithsolar
wind parameters, such as the dynamic pressure7. However, the SKR
also has some features that are more like those of Jupiter than of
Earth: it was found to be strongly modulated by planetary rotation,
despite the fact that models of Saturn’s magnetic field are axisym-
metric11. This has suggested an as-yet-unobserved ‘active sector’ or
possible ‘magnetic anomaly’ rotating with the planet12,13.
The measurements shown here are primarily from the radio
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