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Auroral counterpart of magnetic field dipolarizations in Saturn's tail
Caitriona M. Jackman a,n, Nick Achilleos a, Stanley W.H. Cowley b, Emma J. Bunce b, Aikaterini Radioti c,
Denis Grodent c, Sarah V. Badman d, Michele K. Dougherty e, Wayne Pryor f,g
a
Department of Physics and Astronomy, University College London, London, UK
Department of Physics and Astronomy, University of Leicester, Leicester, UK
c
Laboratoire de Physique Atmosphérique et Planétaire, Université de Liège, Liège, Belgium
d
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA), Yoshinodai 3-1-1, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
e
Blackett Laboratory, Imperial College London, London, UK
f
Science Department, Central Arizona College, Coolidge, AZ 85128, USA
g
Space Environment Technologies, Pacific Palisades, CA 90272, USA
b
art ic l e i nf o
a b s t r a c t
Article history:
Received 24 July 2012
Received in revised form
25 February 2013
Accepted 25 March 2013
Following magnetic reconnection in a planetary magnetotail, newly closed field lines can be rapidly
accelerated back towards the planet, becoming “dipolarized” in the process. At Saturn, dipolarizations are
initially identified in magnetometer data by looking for a southward turning of the magnetic field,
indicating the transition from a radially stretched configuration to a more dipolar field topology. The highly
stretched geometry of the kronian magnetotail lobes gives rise to a tail current which flows eastward (dusk
to dawn) in the near equatorial plane across the centre of the tail. During reconnection and associated
dipolarization of the field, the inner edge of this tail current can be diverted through the ionosphere, in a
situation analogous to the substorm current wedge picture at Earth. We present a picture of the current
circuit arising from this tail reconfiguration, and outline the equations which describe the field–current
relationship. We show a new in situ example of a dipolarization identified in the Cassini magnetometer
data and use this formalism to estimate the ionospheric current density that would arise based on in situ
tail measurements of the magnetic field and the implications for corresponding auroral electron
acceleration in regions of upward directed field-aligned current. We then present a separate example of
data from the Cassini UVIS instrument where we observe small ‘spots’ of auroral emission lying near the
main oval; features suggested to be associated with dipolarizations in the tail. In the example shown, such
auroral features are the precursor to more intense activity associated with recurrent energisation via
particle injections from the tail following reconnection.
& 2013 Elsevier Ltd. All rights reserved.
Keywords:
Saturn
Dipolarization
Aurora
1. Introduction
Following magnetic reconnection in a planetary magnetotail,
newly closed field lines are rapidly accelerated back towards the
planet. Through this motion the field lines move from a stretched
configuration to a more dipolar one, and hence these events are
termed “dipolarizations”. Dipolarizations are characterised by strong
and relatively rapid changes in the north–south component (Bz) of
the field. At Earth, the change in Bz is often interpreted as the effect of
magnetic flux pileup due to the slowing-down of planetward flow in
the near-Earth tail region where the field becomes increasingly
dipolar (e.g. Hesse and Birn, 1991; Shiokawa et al., 1997; Runov
et al., 2009). This BZ enhancement is often accompanied by bursty
bulk flows (BBFs), (Angelopoulos et al., 1992). BBFs are short
(∼10 min), sporadic bursts of high speed flow (4400 km s−1) in
n
Corresponding author. Tel.: +44 20 7679 0672; fax: +44 20 7679 7153.
E-mail address: caitriona.jackman@ucl.ac.uk (C.M. Jackman).
the plasma sheet which transport magnetic flux, mass and energy.
They are often accompanied by significant energetic particle bursts.
The typical cross-tail extent of BBFs has been estimated, using multispacecraft datasets, to be between ∼3 and 7RE, with a north–south
extent of ∼5RE (1RE ¼6371 km), (Angelopolous et al., 1997; Kauristie
et al., 2000; Nakamura et al., 2004; Walsh et al., 2009). Earthwardpropagating fast bulk flows have been understood as entropydepleted flux tubes, or plasma bubbles (Pontius and Wolf, 1990).
They can propagate to Earth subject to a magnetic buoyancy force
associated with the interchange instability (Chen and Wolf, 1993),
and have been shown to stop at a location in the tail at which the
entropy of the depleted flux tube matches that of its surroundings
(Dubyagin et al., 2011).
Analogues to terrestrial BBFs and dipolarizations have been
observed at other planets also. At Jupiter, data from the Galileo
energetic particle detector have revealed evidence for both tailward and planetward plasma flows associated with magnetic
reconnection (e.g. Kronberg et al., 2008). Such flows have a limited
azimuthal extent, ∼1–2% of the tail width (Vogt et al., 2010) and
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http://dx.doi.org/10.1016/j.pss.2013.03.010
Please cite this article as: Jackman, C.M., et al., Auroral counterpart of magnetic field dipolarizations in Saturn's tail. Planetary and Space
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are thus analogous to terrestrial BBFs. At Mercury, Sundberg et al.
(2012) reported several large dipolarization events, identified by a
rapid (∼1 s) increase in |BZ| followed by a slower (∼10 s) return to
pre-onset values. The events are expected to occur in spatially
limited channels like at Earth, but their timescale is much shorter
than in the terrestrial case, a feature which has been attributed to
the lack of steady field-aligned current systems at Mercury
(Sundberg et al., 2012).
The arrival of the Cassini spacecraft at Saturn in mid-2004
provided the opportunity to begin to search for in situ evidence of
reconnection-related dynamics in the tail, with most studies
focusing on a series of orbits during 2006 when the spacecraft
reached its furthest distance of ∼68 RS downtail (1 RS ¼60268 km).
At Saturn, the quasi-dipolar region extends to equatorial distances
of ∼10–16 RS from the planet (Arridge et al., 2008; Provan et al.,
2009), while beyond that the reconnection x-line has been
estimated to lie, on average, in the region of ∼20–30 RS (Mitchell
et al., 2005) based on Energetic Neutral Atom (ENA) emissions
linked to reconnection events. However, the x-line position as
scaled from Earth might be expected to be further downtail than
this, and modelling work has suggested its position is highly
sensitive to solar wind conditions (Jia et al., 2012). On the tailward
side of the reconnection “x-line”, closed loops of field and plasma
called plasmoids may form and be released down tail. At Saturn,
many examples of plasmoids have been found using in situ
magnetometer data from the Cassini spacecraft (Jackman et al.,
2007, 2008, 2011). Tailward-moving plasmoids are identified in
the first instance by a northward turning of the magnetic field
(opposite to the terrestrial field change due to the oppositely
directed planetary field). On the planetward side of the “x-line”,
dipolarizations have the opposite sign of field deflection, i.e., a
southward turning of the field. By their nature dipolarizations are
more difficult to find than plasmoids. As plasmoids are observed
out to the deepest tail orbits, their signatures often appear
as a strong deflection relative to the low background fields at
larger radial distances. Meanwhile dipolarizations are by definition
observed planetward of the reconnection x-line, and thus are on
average closer to the planet. Hence, the field deflections are harder
to spot as they lie on a higher background field. Two examples of
dipolarizations at Saturn have been mentioned in the literature
thus far. Bunce et al. (2005a) noted a reduction in field strength
and a change in orientation of the field which they linked to an
episode of compression-induced reconnection, while Russell et al.
(2008) also reported on a southward turning of the field in concert
with a reduction in the azimuthal component, attributing it to the
spacecraft's position planetward of the x-point. In this work we
present a new example of a dipolarization which was uncovered
during the tail orbit season in 2006. We use magnetometer data
only in this study as reliable plasma moments were unavailable
due to spacecraft pointing constraints.
1.1. Magnetospheric currents and the substorm current wedge
There are a number of large-scale currents that flow in the
magnetosphere including the Chapman–Ferraro currents along the
magnetopause, the ring current in the inner and middle magnetosphere, and the tail current which flows across the centre of the
tail in the equatorial plane. Tangential stress between the solar
wind and the planetary field drags the planetary field lines and
plasma antisunward, forming a long magnetic tail behind the
planet. This highly stretched tail geometry gives rise to the tail
current which is westward at Earth (dawn to dusk) and eastward
at Saturn.
During substorm onset at Earth, the magnetic field within an
azimuthally limited patch can quickly relax to a dipolar state,
while field lines adjacent to this region maintain their stretched
Fig. 1. Sketch of the terrestrial substorm current wedge (after McPherron et al.
(1973)).
configuration. This field structure leads to the formation of a
current system via Ampère's law, whereby near-radial fieldaligned currents are associated with field shear at the edges of
the dipolarised region. This phenomenon is known as the substorm current wedge (SCW), as first discussed by McPherron et al.
(1973). It has subsequently been discussed by many authors, using
multiple spacecraft in the tail, ground magnetograms, and auroral
imagers to examine all aspects of the global phenomenon
(Kauristie et al., 2000; Nakamura et al., 2001; Grocott et al.,
2004). In the famous picture introduced by McPherron (reproduced here as Fig. 1), we see the diversion of the cross-tail current
into the ionosphere, with lines representing current flow. Current
continuity requires that current flowing from east to west (dawn
to dusk) at Earth is diverted along field lines into the ionosphere at
the eastern edge of the disrupted region, flows westward in a
small ionospheric segment, and then flows outwards from the
ionosphere along the western edge. The direction of all currents is
reversed for the case of Saturn due to the oppositely-directed
planetary field. At Saturn, the tail current flows eastward (dusk to
dawn). Following dipolarization in Saturn's tail, we may expect a
specific bright auroral signature associated with the region of
upward current at the eastward end of the dipolarized region,
which is akin to the westward-travelling surge at Earth. These
bright auroral signatures take the form of spots of emission which
appear distinct from the main auroral oval. Such features, termed
“polar dawn spots” were reported by Radioti et al. (2008) for the
case of Jupiter's magnetosphere, and were postulated to be related
to magnetotail reconnection processes. In this paper we outline
the equations which describe current flow following field dipolarization in Saturn's tail and then compare our calculations of the
expected ionospheric signatures using in situ magnetometer data
and auroral images. In Section 2 we describe the theoretical
formalism. Section 3 introduces a new dipolarization case study
example, and includes calculations of the expected ionospheric
counterpart for this and two other dipolarizations. Section 4
discusses related auroral effects, Section 5 presents a discussion,
and Section 6 is a summary.
2. Theoretical formalism to derive auroral counterpart of
dipolarizations
In this paper, we employ data from the Cassini magnetometer
(Dougherty et al., 2004) to show an example of a field dipolarization. This data is in the Kronocentric Radial Theta Phi (KRTP) coordinate system. In this spherical polar system, referenced to
Saturn's spin and magnetic axes, the radial component (Br) is
positive outward from Saturn, the theta component (Bθ) is positive
southward, and the azimuthal component (Bφ) is positive in the
Please cite this article as: Jackman, C.M., et al., Auroral counterpart of magnetic field dipolarizations in Saturn's tail. Planetary and Space
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direction of planetary corotation (in a prograde direction). When
Saturn's magnetotail lobes are in a stretched configuration, the
magnetometer on Cassini will measure a significant radial component associated with this geometry. However, as introduced
above, the stretched field lines may reconnect, following which
newly-closed (dipolarized) field lines travel rapidly back toward
the planet. This results in a southward turning of the field
(enhanced positive Bθ). The effect of this reconnection in the tail
and motion of field lines is that a shear is generated between the
dipolarized field in the wedge region and the stretched field on
either side, which is associated with a near-radial current via
Ampère's law. Fig. 2(a) is a view of Saturn's equatorial plane from
the north with the Sun to the left, showing the currents that would
arise following a dipolarization of the magnetic field in the tail.
The near-radial current acts to divert the cross-tail current, thus
reducing its value in the dipolarized region. As outlined above, we
may expect a specific bright auroral signature associated with the
region of upward current at the eastward end of the dipolarized
region, and it is the size and strength of this signature that we
wish to derive, based on some reasonable assumptions. Fig. 2(b) is
a cut through the tail cross-section, looking towards Saturn, with
the central plasma sheet shown in red. The bulge in the plasma
sheet represents the newly dipolarized region, and the reconfiguration of the field lines leads to the east–west gradient in Bθ,
which in turn requires negative radial current (jr) to the west of
the bulge and positive jr to the east. We note that jr o0 (i.e. an
inward radial current) corresponds to a downward field-aligned
current in the ionosphere and vice versa. The radial current
flowing at a given distance in the tail will not generally be the
total flowing in the ionosphere if some has already closed in the
plasma sheet on either side of the dipolarized region at a closer
distance, as indicated in Fig. 2(a). Thus, the current that one
detects at a specific distance down-tail has the character
of a lower limit on the total, and we will discuss this further in
Sections 3 and 4.
The blue box in Fig. 2(b) denotes the path integral we apply to
calculate the current. The box is intended to show that it
encompasses all of the radial current flowing at that distance in
one hemisphere. In the case of Fig. 2(b), we have placed this box in
the northern hemisphere, but for the purposes of this work,
we assume symmetry with the southern hemisphere. The north–
south extent of the path integral, D, is half the thickness of the
plasma sheet, and the azimuthal extent, W, may be anything up to
half the width of the dipolarized region depending on how
distributed in azimuth the radial current is. The other “half” of
the width contains the current flowing in the opposite direction.
We discuss appropriate values for this width in Sections 3 and 4.
Assuming Bθ inside the bulge is then much larger than Bθ outside,
the principal contribution to the path integral is the ‘downward’
3
segment within the bulge, and from Ampère's law, we obtain
Bθ ðin bulgeÞ D ¼ μ0 I r
ð1Þ
where Ir is the total outward (and inward) radial current in one
hemisphere at that radial distance.
We can then calculate the radial current density, jr from Ir as:
jr ¼ I r =ðW DÞ ¼ Bθ =ðμ0 WÞ
ð2Þ
Thirdly, we wish to calculate the associated field-aligned
current density in the conjugate ionosphere and thus examine
the required electron acceleration, and the size of the consequent
region of precipitation. From current continuity, the ionospheric
current density is given by:
ji ¼ jr ðBi =Bt Þ
ð3Þ
We note that Bt is the total field strength within the layer of the
tail where the radial current is carried within the tail plasma sheet,
and this may reasonably be estimated as half the total lobe field
strength in this region, Blobe taken above. Thus, Bt≈Blobe/2. Bi, the
ionospheric field strength, is taken as 50,000 nT.
Lastly, the size of the region of auroral precipitation is given by
conservation of magnetic flux, i.e.,
Br W D ¼ Bi Ai
ð4Þ
where Ai is the area of the auroral spot. For simplicity, we assume a
square region, with length of each side Li, and hence the ionospheric spatial scale is
Li ¼ √Ai
ð5Þ
Thus, through observation of a magnetic field deflection by a
spacecraft at a given radial distance, we can apply the above
equations and some simple assumptions to derive the total ionospheric current density that would arise from such a reconfiguration of the tail and to estimate the size of the resulting auroral
feature.
2.1. Auroral acceleration
We next wish to calculate various auroral emission parameters
from the derived ionospheric current density. For this, we employ
the kinetic theory of Knight (1973), applied using the simplifying
assumption appropriate to the hot plasma sheet population that
the source plasma parameters are almost independent of position
along each tail field line. Estimates based on the parameters
presented by McAndrews et al. (2009) of the degree to which
the plasma sheet population is equatorially confined due to
centrifugal action on the sub-corotating field lines shows that this
is a negligible effect in Saturn's tail. We begin by comparing the
derived ionospheric current density with the maximum upwarddirected current density that can be carried by magnetospheric
Fig. 2. Schematic sketches of the currents that would arise in Saturn's tail following dipolarization of the magnetic field in the tail. (a) Equatorial plane view, (b) cut through
tail cross-section. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article as: Jackman, C.M., et al., Auroral counterpart of magnetic field dipolarizations in Saturn's tail. Planetary and Space
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4
Fig. 3. Cassini magnetometer data in KRTP co-ordinates for 2006, day 220, 03:00–04:00 UT. Vertical dashed line marks the centre of a southward turning of the field,
indicating a dipolarization of the field lines in Saturn's magnetotail following reconnection. Vertical dot–dashed lines mark the beginning and end of the event, while
horizontal dot–dashed lines mark the total change in the Bθ component over the event. Tickmarks are every 5 min.
electrons without field-aligned acceleration. This is given by:
jjj0 ¼ eN
W th
2π me
1=2
;
ð6Þ
where the magnetospheric electron population has been assumed
to be an isotropic Maxwellian of density N and thermal energy Wth
(equal to kBT, where T is temperature and kB is Boltzmann's
constant), and e and me are the electron charge and mass
respectively. This expression corresponds to the case of a full
downward-going loss-cone, and an empty upward-going losscone.
If the current density that we calculate for our examples is
higher than this limiting value, then according to Knight's (1973)
kinetic theory, a field-aligned voltage must exist along the field
lines which accelerates magnetospheric electrons into the ionosphere to produce the required current. The minimum value of this
voltage is given by
Φjj ¼
W th
e
"
ji
jjj0
!
#
−1 ;
ð7Þ
this value being appropriate if the ‘top’ of the voltage drop is
located at a radial distance well above the minimum value given
by
!1=3
r min
j
≈ i
:
ð8Þ
Ri
jjj0
In Eq. (8) we have assumed as a sufficient approximation that
the field strength drops as the inverse cube of the radial distance
along the polar field lines, corresponding to the planetary dipole
field. Eq. (7) also assumes that the voltage drop is sufficiently
compact along the field lines that no electrons mirror before they
have experienced the full voltage drop. Following Lundin and
Sandahl (1978), the enhanced precipitating energy flux of the
electrons is then given by
2
3
!2
Ef 0
j
4 i
Ef ¼
þ 1 5;
2
jjj0
ð9Þ
where Ef 0 is the unaccelerated electron energy flux corresponding
to Eq. (6), given by
W th 1=2
:
ð10Þ
Ef 0 ¼ 2NW th
2π me
These expressions will be used below to estimate the fieldaligned voltages, energy fluxes, and consequent UV auroral luminosities associated with the upward-directed currents which arise
following dipolarizations in Saturn's magnetotail. The auroral
intensity (kR) is obtained from Ef by applying the result that
1 mW m−2 of electron precipitation produces a UV emission of
approximately 10 kR (Grodent et al., 2001).
3. Case study example: 2006 day 220
Here, we present a new in situ observation of a magnetotail
dipolarization by the Cassini magnetometer in 2006. The magnetic
field data in KRTP co-ordinates for 2006, day 220 03:00–04:00 are
shown in Fig. 3. The key feature of interest is the smooth decrease
followed by sharp, rapid increase (southward turning) of the Bθ
component, centred around ∼03:24 UT, when the spacecraft was
∼46 RS downtail. This southward turning is interpreted as a
dipolarization, and the timing is marked with a vertical dashed
line through all panels. Dot–dashed lines mark the start and end of
the southward turning, identified as the local extrema of the Bθ
component. The total change in the north–south component Bθ,
during this central 5 min is 0.8 nT. This change in Bθ is coincident
with a negative excursion in the radial component, indicating that
the current sheet centre crossed over the spacecraft for several
minutes before Cassini returned to the northern lobe. The latitude
Please cite this article as: Jackman, C.M., et al., Auroral counterpart of magnetic field dipolarizations in Saturn's tail. Planetary and Space
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of the spacecraft was steady at ∼8.51 throughout the interval
shown and thus we might expect it to have been above the current
sheet. The fact that Cassini is below the current sheet centre for a
short time during this interval points to a dramatic change in the
current sheet position, linked to the dynamic behaviour associated
with the field dipolarization. The variation of the azimuthal
component of the field during this event is seen to largely anticorrelate with Br, likely reflecting “sweep-back” of the tail field
lines associated with sub-corotation of the plasma (McAndrews
et al., 2009).
We note that the length of time between extrema in Bθ is about
5 min, which we take to represent the duration of the passage of
the dipolarized field lines over the spacecraft. This is commensurate with typical timescales at Earth, but much longer than those
at Mercury, where dipolarizations manifest themselves over timescales of seconds. We note however that while Cassini is situated
at 46 RS downtail, it catches but a glimpse of the propagating field
disturbance associated with the dipolarization. The effects of the
dipolarization on the surrounding field may be felt for much
longer than this (and indeed the auroral counterpart may persist
for significantly longer as current is diverted from regions of field
reconfiguration).
In order to estimate the auroral signature that would be
associated with such a dipolarization, we employ the equations
set out in Section 2. Firstly, to obtain the total radial current
flowing around the circuit, we assume that the “height” of the
region is approximately the half-width of the plasma sheet, i.e.,
D ¼2 RS (for a full plasma sheet width of 4 RS (Kellett et al., 2009;
Sergis et al., 2011; Arridge et al., 2011)). Using these numbers, and
rearranging Eq. (1), Ir ¼Bθ D/μ0, we find that Ir ¼ 7.7 104 A.
We note that this current is only that part which flows past
Cassini at 46 RS: the total current may indeed be somewhat
greater than this. Specifically, if the dipolarised region extends
further inwards toward the planet we may expect the total current
flowing to the ionosphere to be larger. For comparison, the largescale currents which flow in Saturn's magnetosphere driven by
magnetosphere-ionosphere coupling have typical values of the
order of several MA (Talboys et al., 2011). Thus the current we
derive here, while significant, is not dominant.
In order to calculate jr we must assume a value for the
azimuthal extent, W. Here we draw on analogy with Earth, where
for the case of narrow localised channels (like polar boundary
filaments related to bursty bulk flows at Earth (Angelopoulos et al.,
1992)), a value of W¼ 4 RS represents a good starting point, giving
the cross-tail width of the dipolarized region as at least twice the
5
plasma sheet thickness and ∼10% of the expected tail width at the
radial distance of this observation. Employing this value, Eq. (2)
gives jr ¼2.6 10–12 A m−2.
The ionospheric current density ji in turn can be obtained from
Eq. (3) upon choosing a suitable value for Bt. As mentioned above,
Bt is given as half the lobe field strength in this region. The lobe
field strength either side of this event reaches up to ∼1.5 nT, and
thus we assume here that Bt ¼0.75 nT. These numbers yield a total
current density, ji ¼ 176.1 nA m−2 from Eq. (3). We note that this
value is comparable with but a factor of ∼two–three smaller than
the current density associated with the large-scale currents mentioned above (Bunce et al., 2008a).
Lastly we want to estimate the size of the auroral feature that
would be produced, and using Eqs. (4) and (5), we calculate the
ionospheric spatial scale, Li ¼660 km, which translates into ∼0.71
latitude.
We wish to compare this current density with the maximum
current that can be carried without field-aligned acceleration, j‖0,
as given by Eq. (6). Thus we need to assume values for the density
and temperature of the plasma sheet electron populations, as it is
these particles that will carry the current from the dipolarized
region to the ionosphere. We draw on the work of Arridge et al.
(2009), who derived average properties for Saturn's nightside and
pre-dawn plasma sheet from Cassini electron spectrometer data.
They showed that the plasma sheet exists in two states: quiescent
and disturbed, where the disturbed state corresponds to intervals
where magnetotail reconnection is ongoing. We take their values
of electron density and temperature for the central plasma sheet
of 104 m−3 and 130 eV respectively. Substituting into Eq. (6),
these parameters yield a small limiting current density, j‖0 of
∼3.1 nA m−2 for the plasma sheet source. The ionospheric current
density which we derive from the in situ dipolarization example is
well in excess of this limiting current density, and so field-aligned
acceleration of electrons is necessary. Table 1 lists the auroral
parameters that follow.
We note that the predicted brightness of the spot associated with
this dipolarization is 13.2 kR. This compares to the average brightness
of the main auroral oval of ∼20–30 kR (Grodent et al., 2005). Thus, we
expect the feature to be visible well above the instrument detection
threshold, but to be fainter than the main oval emission. The statistical
width of the southern UV aurora is ∼21 (Badman et al., 2006). Thus, a
feature which is ∼0.71 latitude 0.71 longitude should be clearly
visible close to the main oval.
We can also approach these calculations another way: with
knowledge of the source population, we can consider the width of
Table 1
Table of auroral parameters derived from in situ magnetic field observations of dipolarizations in Saturn's tail
2006 day 220
(this work)
2006 day 201
(Russell et al., 2008)
2004 day 184
(Bunce et al. 2005a)
Radial distance of observation (RS)
ΔBθ (nT)
Duration of dipolarization (min)
Bt (nT)
ji (nA m−2)
IR (A)
46
0.8
∼5
0.75
176.1
7.7 104
30
2.15
∼10
2.2
161.3
2.1 105
15
3.9
∼60
4.75
135.5
3.7 105
Plasma sheet source (Arridge et al., 2009)
(N¼104 m−3;Wth ¼130 eV)
Max unaccelerated j‖0(nA m−2)
Unaccelerated electron energy flux Ef0 (μW m−2)
3.1
0.8
3.1
0.8
3.1
0.8
Auroral parameters
Φ‖ (kV)
rmin/ri
Ef (m Wm−2)
Auroral intensities (kR) 1 mW m−2 ¼10 kR (Grodent et al., 2001)
Size of spot
7.37
3.86
1.32
13.2
0.61 0.61
6.74
3.75
1.11
11.1
1.11 1.11
5.65
3.54
0.78
7.8
1.61 1.61
Please cite this article as: Jackman, C.M., et al., Auroral counterpart of magnetic field dipolarizations in Saturn's tail. Planetary and Space
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6
the channels through which particles would precipitate, and
calculate the limiting values on channel width in order to require
acceleration of electrons leading to an observable auroral feature.
Sufficiently narrow channels will lead to sufficiently high ionospheric current densities and acceleration will be required. Firstly,
let us assume that the ionospheric current density is 3.1 nA m−2
(the limiting current for a plasma sheet source population).
Working back using Eqs. (1)–(3), we find a value of 230.8 RS for
W, the channel width, and from Eqs. (4) and (5), we calculate an
auroral spot size of 5.021. We note that this value for W is more
than the diameter of Saturn's tail, and thus is clearly unrealistic.
Thus we conclude that for essentially any realistic value of W,
acceleration must be required.
Unfortunately there are no corresponding auroral images from
the UVIS instrument for this interval. It is not possible for the
spacecraft to be situated in the near-equatorial region sampling
the reconfiguration of the plasma sheet while at the same time
obtaining a good view of the full auroral oval from the necessary
high latitude vantage point. Thus, observations of each are
mutually exclusive with a single spacecraft at Saturn. However,
the numbers detailed above give us an idea of what we might
expect to see for a dipolarization of this nature.
As mentioned in the introduction, two further examples of
dipolarizations have also been reported in Saturn literature. We
employed the method outlined above to calculate the expected
currents based on these in situ field signatures, and list these along
with the calculated spot size and intensity in Table 1. The
durations given are defined as the times between the extrema in
Bθ, giving a snapshot of the propagation of the front across the
spacecraft. The first additional example, from Russell et al. (2008)
was observed on day 201 of 2006 ∼30 RS downtail. From this
example we calculate a total current in one hemisphere of
2.1 105 A, an expected auroral intensity of ∼11.1 kR and a spot
size of ∼1.11 1.11. The second example, from Bunce et al. (2005a),
was observed at a radial distance of 15 RS during the outbound
pass of Saturn Orbit Insertion. In this case, the calculations yield a
total current in one hemisphere of 3.7 105 A, an intensity of
∼7.8 kR and a spot size of ∼1.61 1.61. Thus from our sample of
three dipolarizations, we obtain a range of expected intensities of
∼7.8–13.2 kR with spot sizes from ∼0.6–1.61 square. We note again
that these numbers have the character of lower limits as they are
based on observations from a single spacecraft sampling a small
portion of the region where current is diverted. We note that the
total current, IR, is smallest for the example furthest from the
planet, at 46 RS, in agreement with our statement above that a
portion of the current may close on either side of the dipolarized
region closer to the planet. The total current observed in the
example at 15 RS is the largest.
In Section 4 we show a sequence of auroral images for a
different interval and describe a feature which we believe could
be the auroral counterpart of dipolarizations such as those
discussed here.
4. Related auroral effects
The main auroral oval emission at Saturn has been postulated
to originate near the boundary between open and closed field
lines (Cowley et al., 2004) and subsequent high-latitude dayside
observations have found this to be a good first approximation
(Bunce et al., 2008a; Talboys et al., 2009; Belenkaya et al., 2011).
Thus flux closure through magnetotail reconnection results in a
change in the position of the main auroral oval (Badman et al.,
2005). Similarly, consecutive reconnection events at Saturn's
magnetopause expand the main emission to lower latitudes and
distort its shape at noon by creating transient auroral features
characterised as bifurcations of the main auroral emission (Radioti
et al., 2011; Badman et al., 2012). Additionally, observations
(Gérard et al., 2005) and theoretical studies (Bunce et al., 2005b)
showed that bright emissions observed occasionally near noon are
probably associated with reconnection occurring at the dayside
magnetopause, similar to the “lobe cusp spot” at Earth (Milan
et al., 2000). Transient small scale auroral spots close to the main
oval post-noon have been previously suggested to be linked to
magnetospheric injections (Radioti et al., 2009). In the present
study we concentrate on small scale auroral features close to
midnight which may be associated with the dipolarization of
magnetic field lines following reconnection and the ensuing
current system as described above. Such features would form near
the poleward boundary of the main auroral oval.
We now discuss a sequence of auroral images taken in the
ultra-violet (UV) wavelength band by the Ultraviolet Imaging
Spectrograph (UVIS) instrument on Cassini (Esposito et al.,
2004). This instrument consists of a slit through which the auroral
image is scanned via continuous slews. The images are referred to
as “pseudo-images” because the final image is reconstructed from
many rows taken throughout the slew, and thus shows different
parts of the auroral region at different times rather than an
instantaneous snapshot (Grodent et al., 2011). In the case of the
images shown here, the slew time is 12 min, which means that the
last row of the image was recorded 12 min after the first row. This
delay introduces some artefacts to the images due to the motion of
the spacecraft and the rotation of auroral features during the time
of the image accumulation. However, we do not expect major
morphological changes of the features of interest on timescales of
less than 12 min and so these images are perfectly appropriate for
our purposes. The FUV channel (111–191 nm) was used in conjunction with the low-resolution slit which provides 64 spatial
pixels of 1 mrad (along the slit) by 1.5 mrad (across the slit). The
spacecraft slews across the region of interest are performed in the
direction perpendicular to the long axis of the slit.
Fig. 4 shows eight pseudo-images, polar projections of Saturn's
northern polar regions, taken on day 129 of 2008 during Cassini's
revolution 67. All images have the Sun to the bottom and dawn to
the left, and the grid is plotted every 101 latitude and 401 longitude. The projections are performed assuming that the emission is
produced in an infinitesimally thin layer at 1000 km above the
1 bar reference ellipsoid. The region of interest (the auroral zone)
of each frame consists of 64 90. The start time of the slew for
each image is marked in each of the panels. During the interval
shown in this figure the spacecraft altitude above the surface of
the planet ranges from 16.07 RS (image 1) to 15.55 RS (image 8),
and the sub-spacecraft latitude ranges from 46.091 to 47.411. Such
a vantage point made it possible to observe both the dayside and
nightside sectors of Saturn's northern polar region.
Beginning at image 1, taken from 07:53 UT, we see a faint spot
post-midnight, highlighted with the yellow circle. The maximum
brightness of this spot is ∼16.5 kR, somewhat fainter than the
main oval emission, which in this case peaks at ∼36 kR. The size of
the spot is ∼3–3.51 latitude 8–101 longitude, centred on ∼72.41
latitude and ∼1881 longitude. Using the magnetic field model of
Bunce et al. (2008b), which includes the three-term planetary field
and the ring current, this location maps to between ∼10 and
∼13 RS in the equatorial plane, depending on the state of expansion of the magnetosphere. We note that this mapping does not
include the day-night asymmetry due to magnetopause and tail
currents. Over the subsequent four images, the brightness of this
spot increases, reaching a peak value of 34.7 kR in image number 3,
taken from 08:23, and with a latitude of ∼72.61 and longitude of
∼2131. Here, we consider that the spot indicated in images 1 and
2 is the same bright one in image 3. We note that the central spot
moves only in longitude and not in latitude, at a rate of 251 in
Please cite this article as: Jackman, C.M., et al., Auroral counterpart of magnetic field dipolarizations in Saturn's tail. Planetary and Space
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Fig. 4. UVIS pseudo-images of Saturn's northern polar region for 2008, day 129. All images are aligned with noon to the bottom, dawn to the left and dusk to the right. The
start time of each 12-min slew is marked on each image. The spot suggested to be linked to a dipolarization of the tail is circled in yellow. The colour bar gives a
correspondence between the colour table and the emission brightness in kiloRayleighs (kR) of H2, where 1 kR ¼ 109 photons cm−2 s−1 emitted in 4π sr by H2 molecules in the
EUV+FUV range (excluding Ly-α) and assuming no absorption by methane (Gustin et al., 2009). The polar projection procedure does not preserve photometry; therefore, the
colour table may only be used as a proxy for the projected emission brightness. (For interpretation of the references to colour in this figure legend, the reader is referred to
the web version of this article.)
30 min, thus indicating that it corresponds to a magnetospheric
feature whose motion switches from radial to azimuthal. Assuming that the feature indicated on images 1, 2, and 3 is the same
one, it extends somewhat poleward of the main ring of emission, a
possible indication that these signatures are linked to closure of
open flux (thus reducing the area of the dark region inside the
polar cap) (Cowley et al., 2005). However, we cannot confirm this
in the absence of in situ measurements in the magnetotail.
We term the auroral feature in the early images a ‘spot’, but note
that in images 6–8 this feature further intensifies and evolves into
an arc-like brightening extending throughout much of the dawnside oval. The intense feature under study should not be confused
with the small scale structures (∼0.51) commonly observed on
the main auroral emission between pre-noon and dusk (Grodent
et al., 2011) and associated with patterns of upward field aligned
currents resulting from non-uniform plasma flow in the equatorial
plane, possibly due to flow perturbations by magnetopause
Kelvin–Helmholtz waves.
5. Discussion
We now discuss our interpretation of the magnetospheric
dynamics which would be related to the series of auroral images
presented in Fig. 4. We suggest that initially, reconnection has
occurred in the magnetotail, resulting in the dipolarization of field
lines. These newly closed field lines then travel rapidly back
toward the planet, leading to heating and compression of the
plasma planetward of the reconnection site. This plasma transport
can proceed at the Alfvén speed, which is of order several hundred
kilometres per second in the plasma sheet, and can reach
44000 km s−1 in the lobes (Arridge et al., 2009). For example,
assuming a reconnection site at ∼45 RS (the radial distance of the
spacecraft for the first case study shown here), Alfvén waves
travelling at 500 km s−1 would take ∼58 min to travel inward to
∼16 RS. This position then marks the approximate outer boundary
of the quasi-dipolar region of the magnetosphere (as mentioned in
the introduction), inside of which planetward transport would be
slowed considerably and the primary direction of motion can
switch from radial to azimuthal. Such azimuthal motion following
injection of energetic particles at Saturn has been studied using
the Cassini Magnetospheric Imaging Instrument (MIMI) (Mauk
et al., 2005; Brandt et al., 2008), and indeed the events later in the
day on 2008, day 129, have been specifically analysed by Mitchell
et al. (2009).
In the Mitchell et al. (2009) work, the authors compared the
energetic neutral atom (ENA) and Saturn kilometric radiation
(SKR) data, along with UVIS images from 2008, day 129. They
reported the initiation of several recurrent acceleration events in
the midnight to dawn quadrant at radial distances of ∼15–20 RS,
and they associated these events with reconnection in the tail. We
note that it is entirely possible that reconnection was initiated at
larger radial distances down-tail and that the 15–20 RS radial
range was merely the region where ENA production occurred.
They show a spectrogram from the Radio and Plasma Wave
Science (RPWS) instrument which reveals the beginning of an
enhancement in SKR at ∼09:00 UT. Intensification and low frequency extension of radio emission at Saturn such as observed in
relation to this event has been shown to be correlated with
magnetotail reconnection and energy release (Jackman et al.,
2009). The start of this SKR enhancement matches closely with
the timing of image 5, which is the first strong brightening to
occur in the UVIS images. We note that the brightened feature in
image 5 is seen to grow out of previous lower-level activity at the
same local time (seen most clearly from image 2 onwards). Thus
we suggest that the SKR enhancement and auroral brightening at
∼09:00 UT is the primary signature of the dipolarization, with the
lower-level auroral activity beforehand representing precursorlike behaviour.
Some ∼45 min later Mitchell et al. (2009) report the first
intensification in the ENA images (noting here that ENA images are
themselves integrated over specific time intervals as opposed to
instantaneous snapshots). The UVIS images shown here in Fig. 4
(images 7–8) show increasingly intense emission on the dawn side
around this time. The timing of the ENA brightening and continuing
intensification of the aurora thereafter fits well with our estimation
of Alfvén travel times above, in that it comes ∼1 h after the
observation of the main bright discrete auroral spots associated with
the field dipolarization (image 5). We do not show here any UVIS
images beyond 10:10 UT, but this subsequent interval is covered in
detail by Mitchell et al. (2009), who note the recurrent appearance of
ENA brightenings and SKR enhancements thereafter.
Please cite this article as: Jackman, C.M., et al., Auroral counterpart of magnetic field dipolarizations in Saturn's tail. Planetary and Space
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8
6. Summary
References
Reconnection in the magnetotail can lead to rapid motion of
newly closed field lines planetward and the diversion of the crosstail current through the ionosphere, resulting in discrete auroral
emission. In this paper we have, by analogy with the terrestrial
substorm current wedge phenomenon (McPherron et al., 1973),
described the physical mechanism by which dipolarization of field
lines in Saturn's magnetotail following reconnection can cause an
associated auroral signature. We present a new in situ example of
a dipolarization seen in the Cassini magnetic field data. From this
example and two others, and based on our assumptions for the
north–south and azimuthal extent of the path integral, we
calculate the expected size and intensity of associated auroral
signatures, and find a range in size from 0.61–1.61 square, with
brightness ranging from 7.8–13.2 kR.
Due to the observational constraints of a single spacecraft, it
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and to simultaneously image the aurora from Cassini. Thus we
have instead shown a separate example of auroral images which
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maximum brightness of ∼16.51–34.7 kR. This spot is a precursor to
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While the size and intensity of the observed auroral feature is
larger than our theoretical predictions, we note that the currents
which we derive have the character of a lower limit and the
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Thus we have demonstrated in this work that dipolarizations in
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their occurrence and range of spot sizes and intensities will be the
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Acknowledgements
CMJ was funded by a Leverhulme Trust Early Career Fellowship
and a Royal Astronomical Society Fellowship. This work was
discussed at the International Space Science Institute International
Team 195 “Investigating the Dynamics of Planetary Magnetotails”.
A.R. and D.G. are funded by the Belgian Fund for Scientific
Research (FNRS) and by the PRODEX Program, managed by the
European Space Agency in collaboration with the Belgian Federal
Science Policy Office. E.J.B. was supported by STFC grant PP/
E001130/1. SWHC was supported by STFC grant ST/H002480/1.
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Science (2013), http://dx.doi.org/10.1016/j.pss.2013.03.010i
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Please cite this article as: Jackman, C.M., et al., Auroral counterpart of magnetic field dipolarizations in Saturn's tail. Planetary and Space
Science (2013), http://dx.doi.org/10.1016/j.pss.2013.03.010i