Proposal

Abstract

Planetary magnetospheres, i.e. the cavities which contain and are controlled by planetary magnetic fields, provide natural laboratories for the study of cosmic plasmas. Although governed by the same fundamental laws, the magnetospheres of the planets are each strikingly different, providing important and mutually illuminating case studies. Auroral emissions provide an indispensable diagnostic tool for the energetic processes occurring in planetary magnetospheres, since they are produced when energetic processes in the magnetosphere accelerate charged particles along magnetic field lines into the planet’s polar ionosphere, whereupon they impact the atmospheric particles and produce light. Observations of auroral emission across the electromagnetic spectrum from radio to X-rays thus uniquely provide a global view of the dynamics of planetary magnetospheres, and at present are revolutionising our understanding of the plasma environments of Jupiter and Saturn. Both these planets have large magnetospheres, the dynamics of which are dominated by planetary rotation; this situation contrasts markedly to the Earth’s magnetosphere which is driven by the interaction with the solar wind, although it is becoming clear that the solar wind also affects the auroras greatly on the outer planets. Auroral observations are obtained by either Earth-based remote sensing platforms, e.g. the Hubble Space Telescope (HST), or spacecraft such as Cassini, which also provide invaluable supporting in situ measurements. Several unique data sets have recently been obtained which present different facets of the auroral process of the outer planets. This International Team will bring together ultraviolet, infrared, X-ray, radio, and in situ particles and fields observations along with world-leading expertise in order to address the following topics: How are the auroral emissions of different wavelengths related? How are the auroral observations related to in situ measurements? What affects the morphology of the auroral emission? Such multi-instrument studies are crucial if we are to test the models which try and explain the dynamics of the outer planets magnetospheres, and to reach a global understanding of their manifestations. The results will have significant bearing on the wider planetary and astrophysical community.

Scientific rationale and goals

Along with the archive of high-sensitivity images of Jupiter’s and Saturn’s auroras that have been taken over the last decade, several unique data sets have recently been obtained, which provide the ideal basis for studying the auroras of the outer planets over the next two years:

HST Programs GO 11566 and 11982: These programs have very recently executed in February/March 2009, and used the Advanced Camera for Survey (ACS) to observe Saturn’s ultraviolet (UV) aurora. Saturn is currently approaching equinox, an event which occurs only once every 15 years. It has therefore not happened since the advent of high-sensitivity planetary auroral imaging and will not occur again during the expected lifetime of HST. At equinox the view from Earth is such that we are able to observe both the north and south auroral ovals simultaneously, a rare luxury even for Earth. These unique observations have thus provided the only high-sensitivity images of simultaneous conjugate auroral emission on an outer planet, and will yield vital clues as to the dynamics of Saturn’s magnetosphere.

HST Program GO 10862: This large program utilised an unprecedented 128 orbits of HST time to obtain 2000 images of Jupiter and Saturn’s ultraviolet (UV) auroras in 2007 and 2008 (see Fig. 1). The planets were observed in 4 month-long intervals in order to determine for the first time the long term behaviour of the auroras in response to changing conditions in the solar wind. They have vastly increased our archive of images of the outer planets, and although key results are already being produced as discussed below, the analysis of this substantial data set has barely begun.

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Figure 1. Example HST images of Jupiter’s (a) Saturn’s (b) UV auroras obtained in 2007. Ultraviolet (c) and IR (d) images of Saturn’s auroras obtained by UVIS and VIMS, respectively.

Ground-based IRTF observations: A program of observations of Saturn’s infrared (IR) aurora has recently been undertaken using the Infrared Telescope Facility (IRTF) telescope, timed to coincide with the 2009 HST observations of the UV aurora. Simultaneous observations of IR and UV auroral emission are extremely rare, and this campaign provides a rare opportunity to compare the behaviour of these two components of emission. Such ground-based observations also return measurements of the velocity of ionospheric plasma, a parameter closely linked with the auroral emission.

Cassini UVIS and VIMS auroral observations: Similarly, since 2004 the Cassini spacecraft has been obtaining images of the UV and IR auroral emission using the Ultraviolet Imaging Spectrometer (UVIS) and the Visual and Infrared Mapping Spectrometer (VIMS) during the HST observing period. Analysing these images in conjunction with the Earth-based observations will enhance their diagnostic power, not least because they can observe the nightside aurora much better than Earth-based platforms.

Spacecraft in situ measurements: The Cassini spacecraft has also been obtaining radio, plasma and magnetic field measurements since orbit insertion, providing vital evidence as to the processes which cause and modify the auroral emission at Saturn. At Jupiter, the 2007 New Horizons flyby revealed the conditions in the solar wind at the same time as HST images of its auroras were being obtained. The Jupiter polar orbiter Juno is currently in science planning stage, with launch scheduled for 2011.

The purpose of this International Team is to bring together the above various data sets and world-leading expertise in order to provide a global view of the processes which drive the auroral emission on the outer planets. The focused meetings will provide new perspectives on each data set and spark off new collaborations, as well as plan for future observations. Specifically, we will address the following topics:

Topic 1: How are the auroral emissions of different wavelengths related?

The outer planets exhibit auroral emission across the electromagnetic spectrum [see e.g. the reviews by Bhardwaj and Gladstone , 2000; Clarke et al., 2004]. The most widely studied is the UV component, originally observed by Pioneer and Voyager spacecraft but recently imaged by the highly sensitive instruments onboard HST. Jupiter exhibits three UV components: the moon footprints, [e.g. Bonfond et al., 2008], the main auroral oval [Grodent et al., 2003a], and the variable high latitude auroras [Grodent et al., 2003b]. Saturn’s UV auroral emission generally consists of a spiral ﬁxed in local time, but including individual corotating components [Gérard et al., 2004; Grodent et al., 2005], and which occasionally expands poleward on the dawn side in response to solar wind compression regions [Badman et al., 2005; Clarke et al., 2005]. Recent Cassini UVIS observations have also shown high latitude auroral activity independent of the main emission. For both planets the IR aurora, observed using ground-based platforms, e.g. the IRTF [Stallard et al., 2007; Clarke et al., 2004] and spacecraft instruments, e.g. VIMS [Stallard et al., 2008] appears to be roughly similar to the UV component, but exhibit distinct differences which will be explored by this Team. No auroral X-ray emission has yet been detected from Saturn, but for Jupiter the hard X-ray emission appears to coincide with the brighter UV components [e.g. Branduardi-Raymont et al., 2008]. Although these diﬀerent auroral components are known to be broadly similar, truly simultaneous images of diﬀerent wavelengths are extremely rare, such that we remain ignorant of how well these components really complement each other. Finally, Saturn’s kilometric radio (SKR) emissions are thought to be related to the UV auroral emission, on the basis that both have been observed to increase in tandem [Crary et al., 2005; Kurth et al., 2005] and that the magnetic footprints of the radio sources have been shown to be roughly co-located with the brightest UV emission [Lamy et al., 2009]. In addition, SKR modelling by Lamy et al. [2008] indicated the similarity between the sub-corotating UV auroral emission and SKR sources. However, the correlation between the UV and the radio emitted powers is not always high [Clarke et al., 2009], and the UV auroras have not been shown to ﬂash regularly like the SKR, such that the true relation of these emissions is unknown. This ISSI International Team will bring together the data sets discussed above in order to determine whether and possibly how the diﬀerent auroral components on each planet are related.

Topic 2: How are the auroral observations related to in situ measurements?

Observations of auroras are augmented significantly by magnetic field and plasma measurements in the magnetosphere itself. For example, Bunce et al. [2008] recently showed using joint HST and Cassini data that Saturn’s auroral oval was co-located with a sheet of upward field-aligned current at the outer reaches of the magnetosphere, just inside the magnetopause boundary. Similarly, Radioti et al. [2009] linked transient auroral features observed with HST with quasi-simultaneous signatures of energetic particle injections detected with Cassini. Recently, Cassini has occupied a high inclination orbit, such that it has repeatedly ﬂown through flux tubes connected to the high latitude auroral region while observations of the aurora were being obtained. This presents the ideal opportunity to observe the plasma populations and currents associated with the auroras, and reveal the overall dynamics of Saturn’s magnetosphere. In addition to determining the causes of the auroral emissions, the collaborations proposed here will help shed light on the cause of the ubiquitous near-planetary period oscillations observed throughout Saturn’s magnetosphere, since the location of the southern auroral oval has also been observed to oscillate [Nichols et al., 2008]. For example, it will be very interesting to determine if the northern oval also oscillates in phase with other magnetospheric phenomena. This ISSI Team will allow the requisite sharing of data between particles and fields and auroral imaging teams.

Topic 3: What affects the morphology of the auroral emission?

Both Jupiter’s and Saturn’s auroras have been shown to brighten and expand considerably in response to impinging solar wind compression regions [Clarke et al., 2004, 2009; Nichols et al., 2007; Pryor et al., 2005]. In the case of Jupiter’s main auroral oval this is unexpected, based on simple theoretical reasoning [Cowley et al., 2007]. It is thus of interest to examine how the different auroral components react to changes in the solar wind conditions by comparing auroral images with spacecraft solar wind data, such as those obtained during the New Horizons Jupiter flyby and during intervals when Cassini is in the solar wind. Our team have expertise in auroral modelling, and will develop time-dependent simulations as the important next step in understanding the observations. However, Gérard et al. [2006] showed that Saturn’s auroral emission is variable even under steady, quiet solar wind conditions, and Clarke et al. [2009] showed that the intense brightening of Jupiter’s dawnside main auroral oval appeared to occur at intervals independent of solar wind conditions. In addition, Radioti et al. [2008] reported quasi- periodic auroral signatures of internally driven reconnection processes occurring in Jupiter’s magnetotail, and Grodent et al. [2008] confirmed the presence of a magnetic anomaly in the northern hemisphere of Jupiter which affects the auroral morphology. The HST images obtained during early 2009 will be of great help in addressing this topic, since they were obtained near Saturn’s equinox and the resulting apparent tilt of its axis will result in the interaction with the solar wind varying greatly throughout each solar rotation. It will be interesting to determine whether the various auroral components (both north and south) are affected as a result. There is much we do not understand as to what affects the auroral morphology of the outer planets, and the multi-instrument approach proposed here will allow us to full address this topic.

The value of ISSI and expected output

The forum provided by ISSI will facilitate the joint analysis of data from many different space- and Earth-based instruments in a manner not possible during large conferences or individual travel. It will allow experts from around the globe to share ideas and data, and plan for future planetary auroral studies. In this respect ISSI will be of great help putting a global perspective on the auroras of the outer planets, and we expect that this ISSI Team will produce a series of papers published in international journals based on these collaborations.

Schedule, timeliness of the project, and facilities required

We anticipate two 5-day meetings, one in early 2010, and one a year later in 2011. This proposal is primarily timed to make maximum use of the recently-obtained data sets discussed above. With Cassini now in its Extended Mission and HST's last servicing mission imminent, now is the ideal time to make the most of the data obtained from these instruments while they are active. Also, with science planning for the Juno mission currently underway with launch scheduled for 2011, meeting at ISSI will be the ideal forum to discuss future observations of the auroras of the outer planets. The proposed meetings of this ISSI International Team will also plug the gap between two consecutive Magnetospheres of the Outer Planets (MOP) meetings. We will require a meeting room with a data projector, plenty of plug sockets for laptops and wireless internet access.

Financial support

We request from ISSI support at the rate of 15 members x 5 days x 2 meetings = 150 per diems, plus the travel costs of the team leader.

References

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