Auroral Processes at the Giant Planets: Energy Deposition, Emission Mechanisms, Morphology and Spectra

TLDR: In this article, the ionospheric response to auroral precipitation at the giant planets is reviewed, using models and observations The emission processes for aurorae at radio, infrared, visible, ultraviolet, and X-ray wavelengths are described, and exemplified using ground-and space-based observations Comparisons between the emissions at different wavelengths are made, where possible, and interpreted in terms of precipitating particle characteristics or atmospheric conditions.

Abstract: The ionospheric response to auroral precipitation at the giant planets is reviewed, using models and observations The emission processes for aurorae at radio, infrared, visible, ultraviolet, and X-ray wavelengths are described, and exemplified using ground- and space-based observations Comparisons between the emissions at different wavelengths are made, where possible, and interpreted in terms of precipitating particle characteristics or atmospheric conditions Finally, the spatial distributions and dynamics of the various components of the aurorae (moon footprints, low-latitude, main oval, polar) are related to magnetospheric processes and boundaries, using theory, in situ, and remote observations, with the aim of distinguishing between those related to internally-driven dynamics, and those related to the solar wind interaction

Figures

Fig. 35 Selection of six UV images of Saturn’s southern aurora obtained during the interval 11 October 1997 to 30 January 2004, with the date and start time of each image shown at the top of each plot. The images are projected onto a polar grid from the pole to 30◦ co-latitude, viewed as though looking through the planet onto the southern pole. Noon is at the bottom of each plot, and dawn to the left, as indicated. The UV auroral intensity is plotted according to the colour scale shown on the right-hand side of the figure. The white crosses mark the poleward and equatorward boundaries of the auroral emissions. From Badman et al. (2006).

Fig. 2 Sketch of the key magnetospheric regions and plasma populations in the kronian magnetosphere (from Gombosi et al. (2009)).

Fig. 21 Einstein Observatory HRI (0.15–3 keV) image of Jupiter, clearly displaying the well separated emissions of the aurorae. The circle outlines the planets disk, and the equatorial plane is indicated by the two linear segments (from Metzger et al. (1983)).

Fig. 9 Flowchart of emission processes after auroral particle precipitation into the H2dominated atmosphere.

Fig. 4 Profiles in altitude (left axis) and pressure (right axis) of the volume ionization rate - that is, electron production rate profiles, as double ionization is negligible -, due to primary particles (solid line) and secondary electrons (dotted line) in the case of 10 keV electron incident beam (left) and 300 keV proton incident beam. The energy flux of the incident particles is assumed to be the same (From Rego et al. (1994)).

Fig. 8 Ionospheric Pedersen (solid line) and Hall (dashed lines) conductivities profiles as a function of pressure: (left) at Jupiter (Millward et al. 2002) for 10 keV electrons (Qprec = 10 mW m−2); (right) at Saturn for solar illumination only (78◦S, equinox, solar minimum) (thin lines) and for solar and auroral 10 keV electrons (Qprec = 0.2 mW m−2) (thick lines) (adapted from Galand et al. (2011)).

Full text

Noname manuscript No.
(will be inserted by the editor)
Auroral processes at the giant planets: aeronomy, emission
mechanisms, spectral and spatial characteristics OR
Auroral precipitation, emission processes, and dynamics at
the giant planets
Sarah V. Badman · Graziella
Branduardi-Raymont · Marina Galand ·
S´ebastien L.G. Hess · Norbert Krupp ·
Laurent Lamy · Henrik Melin · Chihiro Tao
Received: date / Accepted: date
S.V. Badman
University of Leicester, UK
E-mail: s.badman@lancaster.ac.uk Present address: Lancaster University, UK
G. Branduardi-Raymont
UCL/MSSL, UK
M. Galand
Imperial College London, UK
S.L.G. Hess
LATMOS, France
N. Krupp
MPIS, Germany
L. Lamy
Observatoire de Paris, France
H. Melin
University of Leicester, UK
C. Tao
LPP, France

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Contents
1 Introduction: Key magnetospheric regions and interactions . . . . . . . . . . . . . . . 3
1.1 Jupiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Saturn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Response of the ionosphere to auroral forcing at the giant planets . . . . . . . . . . . 7
2.1 Energy deposition of precipitating auroral particles . . . . . . . . . . . . . . . . 8
2.1.1 Energetic electrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.1.1 Models of suprathermal electron transport . . . . . . . . . . . 8
2.1.1.2 Electron production rate . . . . . . . . . . . . . . . . . . . . . 11
2.1.2 Energetic ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1.2.1 Models of suprathermal ion transport . . . . . . . . . . . . . . 12
2.1.2.2 Comparison between electron and ion energy deposition . . . . 14
2.2 Ionospheric response to auroral forcing . . . . . . . . . . . . . . . . . . . . . . . 15
2.2.1 Electron densities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2.1.1 Observations of electron density . . . . . . . . . . . . . . . . . 15
2.2.1.2 Ionospheric models . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2.1.3 Diurnal variation . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.1.4 Latitudinal distribution . . . . . . . . . . . . . . . . . . . . . . 19
2.2.2 Ionospheric electrical conductances . . . . . . . . . . . . . . . . . . . . . 21
2.3 Auroral emission processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.3.1 UV emission processes: production and radiation transfer . . . . . . . . 26
2.3.2 Infrared emission processes: production and non-LTE effects . . . . . . . 26
2.3.3 Jupiter-Saturn and IR-UV Comparison . . . . . . . . . . . . . . . . . . 28
2.3.4 Time Variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.4 Future Developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3 Ground- and space-based observations of UV and IR aurora . . . . . . . . . . . . . . 33
3.1 UV observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.1.1 UV color ratio studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.2 Visible emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.3 Infrared emission from H
+
3
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.4 Simultaneous infrared and ultraviolet auroral observations . . . . . . . . . . . . 39
3.4.1 UV and IR altitude profiles . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.4.2 Morphology and Time variability . . . . . . . . . . . . . . . . . . . . . . 40
4 X-ray views of the outer planets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.1 Jupiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.1.1 First detection and early observations . . . . . . . . . . . . . . . . . . . 42
4.1.2 X-ray emission processes . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.1.3 Chandra and XMM-Newton reveal spatial, spectral and temporal details 44
4.1.4 Auroral morphology in simultaneous Chandra and HST STIS observations 49
4.1.5 The Galilean satellites, the Io Plasma Torus and Jupiter’s radiation belts 50
4.1.6 Open questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.2 Saturn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.2.1 Disk X-ray emission under solar control: No X-ray aurorae? . . . . . . . 52
4.2.2 X-rays from Saturn’s rings . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.2.3 Open questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.3 Uranus and Neptune . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
5 Jupiter and Saturn magnetospheric dynamics: a diagnosis from radio emissions . . . 55
5.1 Spectral and spatial properties of auroral radio emissions . . . . . . . . . . . . 55
5.1.1 Historical context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.1.2 Properties of radiated waves . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.1.3 Diagnosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
5.2 Jupiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5.2.1 Io-Jupiter: the case for moon-planet interactions . . . . . . . . . . . . . 57
5.2.1.1 Radio arcs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
5.2.1.2 Fine structures . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.2.2 Non-Io emissions and rotational dynamics . . . . . . . . . . . . . . . . . 59
5.2.3 Solar wind control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.3 Saturn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

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5.3.1 Rotational dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.3.1.1 Rotational modulation . . . . . . . . . . . . . . . . . . . . . . 61
5.3.1.2 Source regions in sub-corotation . . . . . . . . . . . . . . . . . 62
5.3.2 Longer-term variations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
6 Auroral signatures of magnetospheric dynamics and boundaries at Jupiter and Saturn 63
6.1 Open-closed boundaries in Jupiter’s magnetosphere . . . . . . . . . . . . . . . . 64
6.1.1 Evidence for an open field region . . . . . . . . . . . . . . . . . . . . . . 64
6.1.1.1 In situ measurements . . . . . . . . . . . . . . . . . . . . . . . 64
6.1.1.2 Auroral observations . . . . . . . . . . . . . . . . . . . . . . . . 65
6.1.2 Auroral signatures of reconnection at the open-closed boundary . . . . . 66
6.1.3 Comparison to magnetodisk-related emissions . . . . . . . . . . . . . . . 67
6.2 Open-closed boundaries in Saturn’s magnetosphere . . . . . . . . . . . . . . . . 68
6.2.1 Characteristics of the open field region . . . . . . . . . . . . . . . . . . . 68
6.2.1.1 In situ measurements . . . . . . . . . . . . . . . . . . . . . . . 68
6.2.1.2 Auroral observations . . . . . . . . . . . . . . . . . . . . . . . . 68
6.2.2 Auroral signatures of reconnection at the OCB . . . . . . . . . . . . . . 70
6.2.3 Interpretation and differences from magnetodisk processes . . . . . . . . 71
7 Future observations and outstanding issues . . . . . . . . . . . . . . . . . . . . . . . 72
Abstract The ionospheric response to auroral precipitation at the giant planets is
reviewed, using models and observations. The emission processes for aurorae at ra-
dio, infrared, visible, ultraviolet, and X-ray wavelengths are described, and exemplified
using ground- and space-based observations. Comparisons between the emissions at
different wavelengths are made, where possible, and interpreted in terms of precipitat-
ing particle characteristics or atmospheric conditions. Finally, the spatial distributions
and dynamics of the various components of the aurorae (moon footprints, low-latitude,
main oval, polar) are related to magnetospheric processes and boundaries, using theory,
in situ, and remote observations, with the aim of distinguishing between those related
to internally-driven dynamics, and those related to the solar wind interaction.
1 Introduction: Key magnetospheric regions and interactions
The magnetospheres of the outer planets are huge plasma laboratories in space. They
are driven by the fast rotation of the planet with its strong internal magnetic field,
combined with powerful internal plasma sources (the satellites Io and Europa in the case
of Jupiter, and Enceladus at Saturn). Several comprehensive reviews of outer planet
magnetospheres and their dynamics have been published (e.g. Dessler 1983; Bagenal
et al. 2004; Dougherty et al. 2009) and in this introductory section we only briefly
overview the key magnetospheric regions and their dynamics, before describing in detail
in the subsequent sections the auroral emissions generated at different wavelengths, and
how they are utilised to diagnose the magnetospheric dynamics.
1.1 Jupiter
Our knowledge of the global configuration and dynamics of the Jovian magnetosphere
is based on measurements taken onboard spacecraft flying through the Jovian system
(Pioneer 10 and 11, Voyager 1 and 2, Ulysses, Cassini, New Horizons) and especially
from results of the orbiting spacecraft Galileo.
Figure 1 shows a sketch of the key regions and magnetospheric interactions of the
Jovian magnetosphere. Traditionally the magnetosphere is subdivided into the inner,

4
Fig. 1 Sketch of the key magnetospheric regions in the Jovian magnetosphere. Credit: Max
Plack Institute for Solar System Research.
middle and outer magnetosphere. In the inner magnetosphere orbits the volcanic moon
Io (at 6 R
J
radial distance), which is the main source of oxygen and sulphur neutrals
in the magnetosphere, and the moon Europa (at 9 R
J
radial distance) where hydrogen
and possibly oxygen originate. Both moons create a torus along their orbit around the
planet in which neutrals are ionized to form plasma tori. While the mass added to
the magnetosphere from the moons plays an important role in driving dynamics and
auroral emissions throughout the magnetosphere (described below), the moons also
have a local interaction with the jovian magnetic field, resulting in auroral footprints
at the ionospheric end of the connecting flux tubes. The interaction occurs because
the satellites form obstacles to the corotating plasma flow, which is moving faster than
their Keplerian orbital velocities. The perturbation of the plasma and field around the
moon propagates along the magnetic field as Alfv´en waves, interacting with electrons,
which finally precipitate into the ionosphere and generate aurora (e.g. Kivelson 2004).
At Jupiter the footprints of Io, Europa, and Ganymede have been identified, while the
footprint of Callisto is mostly hidden underneath the main oval (Connerney et al. 1993;
Clarke et al. 2002). The observed footprints take the form of spots (multiple spots in
the cases of Io and Ganymede) and also have trails of enhanced emissions, or ‘wakes’,
behind the footprint itself (e.g. Bonfond et al. 2008; Bonfond et al. 2013).
Due to the centrifugal force of the fast rotating planet, plasma moves radially
outward from the tori in the inner magnetosphere. The magnetic field lines frozen in to
the plasma in the middle magnetosphere are therefore continuously stretched outward
near the equator and deviate significantly from a dipole configuration. Oppositely-
directed field lines come close together, and a stable configuration can only be reached
through formation of a current sheet between the oppositely-directed fields, and an

5
associated plasmasheet. An equatorially confined magnetodisc is formed, which wobbles
up and down with respect to the equator due to the 9.6
◦
tilt between Jupiter’s magnetic
dipole axis and the planetary rotation axis. The magnetodisc is relatively thin in the
dawn sector (2 R
J
half thickness) and thicker on the dusk side (7.6 R
J
half thickness)
(Khurana et al. 2004).
As the plasma moves outward through the magnetosphere, it also slows. This means
that the magnetic field frozen in to the plasma in the magnetodisk is sub-corotating,
yet these field lines have their ends fixed in the ionosphere, where collisions between
atmospheric neutrals rotating with the planet and ions can occur. The planet therefore
supplies angular momentum to the magnetosphere, attempting to spin the field and
plasma back up to corotation. The angular momentum is transferred by a field aligned
current system, which is directed upward from the ionosphere, radially outward in the
equatorial middle magnetosphere (such that the j × B force acts in the direction of
planetary rotation), returning downward to the ionosphere at higher latitudes, and
closing through an equatorward ionospheric current. The portion of the current di-
rected upward from the ionosphere, carried by down-going electrons, is responsible for
Jupiter’s main auroral oval (Cowley and Bunce 2001; Hill 2001).
The radial distance where the plasma begins to depart from rigid corotation, i.e.
where the ionosphere can no longer impart sufficient angular momentum, seems to be
dependent on local time. It is further out in the pre-dawn sector, at 40 R
J
, compared to
20–25 R
J
in the dusk sector, which may be related to the distribution of mass-loading
and loss in the magnetosphere (Vasyliunas 1983; Krupp et al. 2001; Woch et al. 2004).
Therefore, while Jupiter’s main emission is relatively stable over time, its intensity and
location can be affected by the location and magnitude of corotation breakdown in the
magnetosphere, which in turn can be affected by, e.g. volcanic activity at Io or solar
wind compression of the magnetosphere. These processes are discussed in more detail
in Section 3.1.
In the outer magnetosphere the field lines are stretched and sub-corotating. When
the current sheet becomes particularly thin, reconnection can occur between oppositely-
directed field lines. This ultimately results in the release of a plasmoid downtail and the
contraction of the newly-reconnected field line back toward the planet. Reconnection
in the magnetotail could occur only on closed, stretched field lines, or continue onto
open, lobe field lines (Vasyliunas 1983; Cowley et al. 2003). In situ measurements
show that reconnection preferentially occurs at radial distances of 60–80 R
J
and its
signatures are sometimes observed with a periodicity of 2–3 days (Krupp et al. 1998;
Woch et al. 1998; Louarn et al. 1998; McComas and Bagenal 2007; Hill et al. 2009;
Vogt et al. 2010). One possible scenario to explain the periodicity, involving a cycle of
mass loading and unloading, was first pointed out by Krupp et al. (1998) and Woch
et al. (1998). They suggested that, after reconnection, the emptied field lines take
approximately a day to snap back radially inwards towards the planet, and azimuthally
in the direction of planetary rotation, before the mass-loading cycle starts again. The
field lines moving radially inward after reconnection can have auroral signatures in
the ionosphere, poleward of the main oval, related to field-aligned currents linking the
dipolarised field line to the ionosphere (Grodent et al. 2004; Kasahara et al. 2011).
Even though the solar wind interaction at Jupiter does not play the most impor-
tant role in terms of dynamics, compared to rotationally-driven dynamics, evidence of
solar wind driving and auroral signatures have been identified in the high latitude and
outermost regions (see Section 6.1). Currently two basic scenarios are discussed: i) an
open magnetosphere where magnetic flux opened during reconnection at the dayside

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Auroral processes at the giant planets: aeronomy, emission mechanisms, spectral and spatial characteristics or auroral precipitation, emission processes, and dynamics at the giant planets" ?

Comparisons between the emissions at different wavelengths are made, where possible, and interpreted in terms of precipitating particle characteristics or atmospheric conditions. Finally, the spatial distributions and dynamics of the various components of the aurorae ( moon footprints, low-latitude, main oval, polar ) are related to magnetospheric processes and boundaries, using theory, in situ, and remote observations, with the aim of distinguishing between those related to internally-driven dynamics, and those related to the solar wind interaction. 

Q2. What are the future works mentioned in the paper "Auroral processes at the giant planets: aeronomy, emission mechanisms, spectral and spatial characteristics or auroral precipitation, emission processes, and dynamics at the giant planets" ?

These facts mean, by necessity, that the authors are entering an era where ground-based infrared observations of H+3 will be the main tool with which to study the magnetosphere-ionosphere-thermosphere interaction at the gas giants. Coordinated observations are required to study the full thermosphere-ionospheremagnetosphere coupled system, including Io ( or Enceladus at Saturn ) activity and solar wind conditions. To study the solar wind variation, the observing interval should be at least one week to resolve the time scale for magnetospheric compression and the following expansion phase. The results of such campaigns will provide significant advances in their understanding of the relative contributions of solar wind and magnetodisk driving processes at the giant planets. 

Q3. What is the main advantage of radio observations?

The main advantage of radio observations relies on the capability for long-term, quasi-continuous, remote measurements at high spectral and temporal resolution. 

Q4. What is the effect of the neutral beam on the ion intensity?

As neutral species are not affected by the magnetic field, the neutral beam spreads spatially (in particular latitudinally), which may result in an attenuation of the ion intensity at the centre of the beam (e.g. Lorentzen 2000). 

Q5. What are the observable parameters required to study the io torus?

The observable parameters required are Io’s volcano activity, the Io torus, the IR and UV aurora, radio and X-ray emissions, and the solar wind (ideally in-situ near the planet or at least propagated from near-Earth measurements). 

Q6. What are the mechanisms that have been introduced to match the observed peak electron densities?

Loss mechanisms have been introduced in order to match the observed peak electron densities: (1) charge-exchange of H+ with vibrationally-excited H2 (e.g. McConnell et al. 

Q7. What is the main source of oxygen and sulphur neutrals in the magnetosphere?

In the inner magnetosphere orbits the volcanic moon Io (at 6 RJ radial distance), which is the main source of oxygen and sulphur neutrals in the magnetosphere, and the moon Europa (at 9 RJ radial distance) where hydrogen and possibly oxygen originate. 

Q8. What are the parameters that can be derived from imaging or spectral observations?

The parameters that can be derived from either imaging or spectral observations depend on the spectral resolution, wavelength coverage, and the signal-to-noise ratio (SNR). 

Q9. How did Connerney and his colleagues (1998) constrained the existing models of the magnetic field?

Using narrowband IRTF NSFCAM images of H+3 emission, including emission from the Io footprint aurora, Connerney et al. (1998) constrained the existing magnetic field models of Jupiter, adding important constraints to the morphology of the magnetic field at magnetic latitudes equatorward of the main auroral oval. 

Q10. What is the main contributor to the electron density peak magnitude?

Galand et al. (2011) showed that in the auroral regions where the dip angle is large, the main contributor is the vertical component of the thermospheric wind, which decreases the electron density peak magnitude by as much as 75%. 

Q11. How can the authors get a more complete view of the auroral processes?

By analyzing observations obtained in two or more wavelength bands that are both spatially overlapping and temporally simultaneous, it is possible to get a more complete view of the auroral processes. 

Q12. What is the role of particle precipitation in the thermosphereionosphere?

In other words, particle precipitation, which can be traced via auroral emissions, plays a critical role in the thermosphereionosphere system and its coupling to the magnetosphere. 

Q13. What is the way to account for oblique beaming?

A possible way to account for oblique and variable beaming from shell-driven (quasi-perpendicular) emission relies on refraction close and far from the source. 

Q14. What instrument has provided high spatial resolution observations of Saturn’s IR aurora?

Recently the Visual and Infrared Mapping Spectrometer (VIMS, Brown et al. (2004)) instrument on Cassini has provided high spatial resolution observations of Saturn’s IR aurora. 

Q15. What is the net momentum transferred from the atmosphere to the magnetosphere?

There is a net momentum transferred from the atmosphere to the magnetosphere, while energy through, for instance particle precipitation, is deposited from the magnetosphere to the atmosphere (e.g. Hill 1979, 2001; Cowley and Bunce 2001). 

Q16. How many pixels are normalized to the peak in the main emission brightness profile?

Figure 2 displays the normalized to the peak in the main emission brightness profile of the aurora, as a function of angular distance measured from the morphological center of the main emission for profile a. 

Q17. What is the fraction of H atoms given by Equation 6a?

As the former becomes increasingly dominant towards lower energies, the fraction FH of H atoms - given by Equation 6a - increases as well.