Transcript Lomonosov Moscow State University Auroral oval size at

Possible mechanisms of
Saturn's aurora
generation
E.S. Belenkaya1, S.W.H. Cowley2, J.D. Nichols2,
V.V. Kalegaev1, and M.S. Blokhina1
1Institute
of Nuclear Physics, Moscow State University, Vorob’evy Gory,
119992 Moscow, Russia
2Department of Physics & Astronomy, University of Leicester, Leicester LE1
7RH, UK
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Abstract
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UV images of the southern dayside oval were obtained by the Hubble Space
Telescope (HST) in February 2008.
Simultaneously Cassini observed IMF upstream of Saturn’s dayside bow shock.
Using a global paraboloid model of the magnetospheric magnetic field we
mapped Saturn’s auroras into the magnetosphere.
The model took into account the observed high solar wind dynamic pressure
and IMF measured by Cassini and suitably lagged (~7h: ~6 h for the ionospheric
reaction to solar wind conditions and 1 h for light propagation to Earth).
UV images were examined for northward and southward IMF.
The magnetospheric field structure was very different in these cases, however,
the dayside UV oval has a consistent location relative to the field structure in each
case.
The poleward boundary of the oval is located close to the open-closed field line
boundary and thus maps to the vicinity of the magnetopause.
The equatorward boundary of the oval maps typically near the outer boundary
of the equatorial ring current appropriate to the compressed conditions prevailing.
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Interaction of the solar wind
with Saturn’s magnetosphere
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Although the magnitude of the IMF (~0.3 nT) is significantly
lower than magnetic field of Saturn (BS0=21160) nT, the reconnection
is very significant in forming the global structure and dynamics of the
kronian magnetosphere.
We study an interval, unique to date, in which HST imaging was
coordinated with simultaneous interplanetary observations by Cassini
located immediately upstream of Saturn’s bow shock.
While the dynamics of Saturn’s magnetosphere is driven mainly
by the planet’s rotation (e.g. Badman and Cowley, 2007), the auroras
and related radio emissions are also respond to increases in solar
wind dynamic pressure (Clarke et al., 2005, 2009; Crary et al., 2005;
Kurth et al., 2005; Jackman et al., 2005; Bunce et al., 2006; Badman
et al., 2008).
Changes in the IMF lead to variations in the size and position of
the open field region (calculated in the paraboloid model) that are
reflected in the aurora (Belenkaya et al., 2007, 2008, 2010).
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Magnetospheric magnetic field in the
paraboloid model (Alexeev et al., 2006)
Magnetopause: x/Rss = (y2+z2)/2Rss2
 Bm = Bd(BS0,RS) + Bsd(BS0,RS,Rss) + BTS(Rss,R2,Bt) +
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+ Brc(Brc1,Rrc1,Rrc2) + Bsrc(Brc1,Rrc1,Rrc2,Rss) + b(kS,BIMF).
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Paraboloid model includes planetary magnetic field, magnetic field of magnetospheric current
systems shielded by magnetopause currents, and an IMF partially penetrated into the
magnetosphere.
divB=0; divj=0.
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Time-dependent input model parameters:
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Rss is magnetopause subsolar distance;
R2
is distance to the inner edge of the tail current sheet;
Bt /0 is field at the inner edge of the tail current sheet;
0 = (1 + 2 R2/Rss)1/2;
Brc1 is radial component of the field at the outer edge of the ring current;
Rrc1 is distance to the outer edge of the ring current;
Rrc2 is distance to the inner edge of the ring current;
BIMF is IMF;
kS is coefficient of IMF penetration into magnetosphere.
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Saturn
The angular velocity of Saturn: S=1.638·10−4 s−1
 Magnetic moment of Saturn: МS=4.6·1013 G·km3=0.2G·RS3
 Magnetic field at Saturn’s equator: BS0=21160 nT
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Model parameters
Compressed magnetosphere (Pioneer-11) psw~0.08 nPa (Belenkaya et al., 2006)
Rss=17.5RS, Rrc1=12.5RS , Rrc2=6.5RS, Brc1 =3.62nT, R2=14RS, Bt=8.7nT
Intermediate magnetosphere (Voyager-1) psw~0.03 nPa (Belenkaya et al., 2008)
Rss=22RS, Rrc1=15RS , Rrc2=6.5RS, Brc1 =3nT, R2=18RS, Bt=7nT
Expanded magnetosphere (Cassini SOI orbit) psw~0.01 nPa (Alexeev et al., 2006)
Rss=28RS, Rrc1=24.5RS , Rrc2=6.5RS, Brc1 =2.2nT, R2=22.5RS, Bt=5.3nT
Dependence of Rss on Psw
 For Earth: Rss~ Psw-1/6 (e.g., Shue et al., 1997),
 for Jupiter: Rss~ Psw-1/4 – Psw-1/5 (Huddleston et al., 1998),
 for Saturn: Rss~ Psw-1/4.3 (Arridge et al., 2006)
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Trajectory of the Cassini spacecraft in KSM
coordinates from near the periapsis of Rev 58 to
near the periapsis of Rev 59
The three panels show the Cassini trajectory
projected onto the X-Y, X-Z, and Y-Z planes.
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The Cassini position is marked by star at the
beginning of each day.
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The intersections of the magnetopause and
bow shock with these planes are shown by the
solid and dashed red lines, respectively,
obtained from the models of Kanani et al.
(2010) and Masters et al. (2008) for a psw=0.1
nPa.
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The blue segment of the trajectory
corresponds to the interval (12-15 Febr 2008)
for which Cassini measured IMF (Belenkaya at
al., 2010).
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Magnetic field for DOY 43 (12 Febr) to 46
(15 Febr) of 2008
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Three
components
of
the
magnetic field in KSM coordinates
(Bx, By, Bz), and the field
magnitude|B| in nT.
Green - magnetosphere, red magnetosheath, and blue - solar
wind. The blue vertical stripes
labelled “A” to “D” correspond to
the suitably lagged times of HST
images.
The spacecraft was located in
the solar wind just upstream of
Saturn’s dayside bow shock from
the middle of DOY 43 (12 Febr) to
the end of DOY 45 (14 Febr). For
this interval we have simultaneous
observations of the IMF (1nT) and
UV images of southern aurora.
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Cassini data from 12-15 February 2008.
Propagation and response time effects
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It was found (Belenkaya et al.,
2010) that for the studied cases the
delay from solar wind observation to
ionosphere is ~7 h 10 min.
This time includes three time
delays:
1. The one-way light travel time
between Saturn at the time of
emission and the HST position at the
time the image was obtained (~1 h
10 min).
2. The solar wind propagation time
between Cassini and the reconnection
regions on Saturn’s magnetopause
(30 min).
3. The auroral and flow response
time in Saturn’s ionosphere resulting
from changes in the IMF following
their arrival at the magnetopause
(330 min) (Belenkaya et al., 2010).
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We have averaged the IMF data
over a 1 h interval centred on this
time in order to obtain a reasonable
representative value of the IMF
relating to each image (Belenkaya
et al., 2010).
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Saturn’s magnetosphere was
compressed by the solar wind, with
the dynamic pressure peaking at
0.1 nPa on DOY 44 and 45.
The corresponding model
parameters are:
 Rss = 17.5 RS, Rrc1 = 12.5 RS,
 Rrc2 = 6.5 RS, Brc1 = 3.62 nT,
 R2 =14 RS, Bt= 8.7 nT; = -8.4о
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Field lines emerging from Saturn’s ionosphere in
the noon-midnight meridian for IMF vectors
corresponding to HST images A, B, C, and D
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For ks = 0.2, the 2D meridianal noonmidnight magnetic field structure is shown.
Field lines on the nightside terminate
where they intersect the magnetopause.
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For southward and northward IMF the
magnetospheric magnetic field structures
are
quite
different
even
for
IMF
magnitudes less than 1 nT.
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Bz<0
Bz>0
As a consequence, corresponding
changes in the shape and size of the open
field line region in the ionosphere should
also be expected.
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A and C UV images of Saturn’s
southern auroras
The poleward and equatorward boundaries of
the emission are shown by red crosses.
Solid orange line shows the open field line
region boundary for ks = 0.2.
A: IMF={0.20, -0.85, -0.24} nT
 С: IMF={-0.11, 0.28, 0.25} nT
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HST UV images of Saturn’s southern auroras
are projected onto a spheroidal surface 1100
km above the 1 bar level. At the top are given
DOY and the start time of the 20 min
combined exposure time.
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Open field line «window» at the southern
magnetopause for northward IMF (image C)
IMF={-0.11, 0.28, 0.25} nT
Intersection of the open magnetic field lines with the
southern magnetopause for the case C with ks = 0.2.
Solid curves show projections along the field lines of
constant ionospheric latitude from -74° to -90° at steps of
4°, while dashed lines show projections of lines of
constant LT (with step 1h).
The principal meridians (noon, dusk, dawn, midnight)
are shown by the green lines.
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Mappings of field lines into the KSM equatorial plane
for image C with ks = 0.2
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IMF={-0.11, 0.28, 0.25} nT
Equatorial
projection
of
the
equatorward auroral oval boundary is
shown by the blue squares and of the
poleward boundary by the purple
squares. The black two circles show the
inner and outer boundaries of the
equatorial ring current. The black curve
across the tail indicates the inner edge
of the tail current sheet. The pale blue
lines show projections of the fixed
southern
ionospheric
latitudes
o
o
o
of -70 , -74 , and -78 . The red curve
corresponds to the boundary of the
closed field lines. ks = 0.2.
The dayside auroral oval maps
from
the dayside magnetopause
(i.e. close to the boundary between
open and closed field lines), to close
to the outer edge of the ring current
at ~12 RS.
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Open field line «windows» at the southern and
northern magnetopause for southward IMF
(image A)
b)
IMF={0.20, -0.85, -0.24 } nT
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Mappings of field lines into the KSM equatorial plane for
image A with ks = 0.2
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Top panel shows the near-planet region, while
bottom panel shows a larger region extending into the
magnetospheric tail.
The closed field line region is contained between the
red and green lines, where the former is projected
from the southern hemisphere and the latter from the
northern.
The boundaries of the auroral oval are shown by the
blue and purple squares for the equatorward and
poleward boundaries, respectively. The black two
circles show the inner and outer boundaries of the
equatorial ring current, while the black curve across
the tail indicates the inner edge of the tail current
sheet. The pale blue curves show projections of fixed
southern ionospheric latitudes of -70o, -74o, -78o,
and -82o; ks = 0.2.
The oval maps between the outer region of
the ring current and the open-closed field
boundary.
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Earth’s magnetosphere for southward and northward IMF
Southward IMF. Open field
lines do not intersect equatorial
plane. The last closed field lines
form a single curve for the
southern and the northern polar
caps.
Northward IMF. Open field
lines of the southern and the
northern polar caps intersect
the equatorial plane forming
two different curves
representing open-closed
boundary.
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Results of mapping in the Saturn’s paraboloid model
of the observed by the HST dayside UV auroral oval
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For southward and northward IMF, measured by Cassini, the mapping along
magnetic field lines of the observed dayside UV oval and calculated open-closed
boundary was performed to the equatorial plane and magnetopause.
It occurs that even a rather moderate IMF (<1 nT) is very significant for the
magnetospheric magnetic field structure of Saturn possessing strong intrinsic
magnetic field (BS0=21160 nT) and located at 9.5 AU from the Sun.
To date a few simultaneous data sets of the UV oval and IMF exist for Saturn,
one obtained in January 2004 during Cassini approach at distances from the
planet of ~1300 RS, resulting in significant IMF propagation time uncertainties,
the second in February 2008 when four UV image were obtained when Cassini
was in the solar wind just upstream from the dayside bow shock.
Calculations were undertaken using the paraboloid magnetosphere model
appropriate to the compressed magnetospheric conditions prevailing and
suitably lagged and averaged Cassini IMF data.
Mapping in the parabolod model of the UV oval boundaries allowed us to
understand the connection between the southern polar ionosphere and domains
within the equatorial magnetosphere and/or magnetopause, thus helping to
clarify the relationship between the observed emissions and the main
mechanisms of auroral generation (Hill, 2005; Sittler et al., 2006; Cowley et al.,
2004).
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Alternative mechanisms of the auroral
generation at Saturn
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1. Field-aligned currents and auroras could be associated with
the corotation breakdown in the inner and middle magnetosphere
(Hill, 2005).
2. The precipitating particles (~10 keV) reside in the outer
boundary of the dayside plasma sheet and ring current, where the
plasma is highly turbulent due to the enhanced wave activity.
This mechanism of the auroral generation is referred to as the
centrifugal instability model (Sittler et al., 2006).
3. A ring of upward-directed currents should flow in the vicinity
of the boundary between open and closed field lines due to the
sheer in the azimuthal velocity (Cowley et al., 2004).
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Conclusions
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1. The corotation breakdown at Saturn occurs in the Enceladus torus at 3 4 RS, mapping to about -62° in the southern ionosphere. The corresponding
field-aligned currents (Hill, 2005) could be associated with an auroral oval
in IR emission at Saturn (a ‘secondary oval’), lying equatorward of the
main UV oval investigated here (Stallard et al., 2008).
2. The equatorward boundary of the oval was found to map typically to
distances of ~12 RS in the equatorial plane, near the outer boundary of the
model ring current appropriate to the compressed conditions prevailing.
Thus, the mechanism suggested by Sittler et al. (2006) could act in this
region.
3. The poleward edge of the UV oval, was found typically to be located
near the boundary between open and closed field lines, in agreement with
the mechanism suggested by Cowley et al. (2004a,b) and with the previous
conclusions of Belenkaya et al. (2006, 2007, 2008, 2010).
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