S. V. Badman

Reconnection in a rotation-dominated magnetosphere and its relation to Saturn's auroral dynamics

The first extended series of observations of Saturns auroral emissions, undertaken by the Hubble Space Telescope in January 2004 in conjunction with measurements of the upstream solar wind and interplanetary magnetic field (IMF) by the Cassini spacecraft, have revealed a strong auroral response to the interplanetary medium. Following the arrival of the forward shock of a corotating interaction region compression, bright auroras were first observed to expand significantly poleward in the dawn sector such that the area of the polar cap was much reduced, following which the auroral morphology evolved into a spiral structure around the pole. We propose that these auroral effects are produced by compression-induced reconnection of a significant fraction of the open flux present in Saturns open tail lobes, as has also been observed to occur at Earth, followed by subcorotation of the newly closed flux tubes in the outer magnetosphere region due to the action of the ionospheric torque. We show that the combined action of reconnection and rotation naturally gives rise to spiral structures on newly opened and newly closed field lines, the latter being in the same sense as observed in the auroral images. The magnetospheric corollary of the dynamic scenario outlined here is that corotating interaction region-induced magnetospheric compressions and tail collapses should be accompanied by hot plasma injection into the outer magnetosphere, first in the midnight and dawn sector, and second at increasing local times via noon and dusk. We discuss how this scenario leads to a strong correlation of auroral and related disturbances at Saturn with the dynamic pressure of the solar wind, rather than to a correlation with the north-south component of the IMF as observed at Earth, even though the underlying physics is similar, related to the transport of magnetic flux to and from the tail in the Dungey cycle.

Open flux estimates in Saturn's magnetosphere during the January 2004 Cassini-HST campaign, and implications for reconnection rates

During 8–30 January 2004, a sequence of 68 UV images of Saturns southern aurora was obtained by the Hubble Space Telescope (HST), coordinated for the first time with measurements of the upstream interplanetary conditions made by the Cassini spacecraft. Using the poleward edge of the observed aurora as a proxy for the open-closed field line boundary, the open flux content of the southern polar region has been estimated. It is found to range from ∼15 to ∼50 GWb during the interval, such a large variation providing evidence of a significant magnetospheric interaction with the solar wind, in particular with the interplanetary structures associated with corotating interaction regions (CIRs). The open flux is found to decline slowly during a rarefaction region in which the interplanetary magnetic field remained very weak, while decreasing sharply in association with the onset of CIR-related solar wind compressions. Such decreases are indicative of the dominating role of open flux closure in Saturns tail during these intervals. Increases in open flux are found to occur in the higher-field compression regions after the onsets, and in a following rarefaction region of intermediate field strength. These increases are indicative of the dominating role of open flux production at Saturns magnetopause during these intervals. The rate of open flux production has been estimated from the upstream interplanetary data using an empirical formula based on experience at Earth, with typical values varying from ∼10 kV during the weak-field rarefaction region, to ∼200 kV during the strong-field compression. These values have been integrated over time between individual HST image sets to estimate the total open flux produced during these intervals. Comparison with the changes in open flux obtained from the auroral images then allows us to estimate the amount of open flux closed during these intervals, and hence the averaged tail reconnection rates. Intermittent intervals of tail reconnection at rates of ∼30–60 kV are inferred in rarefaction regions, while compression regions are characterised by rates of ∼100–200 kV, these values representing averages over the ∼2-day intervals between HST image sequences. The forms of the aurorae observed are also discussed in relation to the deduced voltage values.

Multispectral simultaneous diagnosis of Saturn's aurorae throughout a planetary rotation

From 27 to 28 January 2009, the Cassini spacecraft remotely acquired combined observations of Saturns southern aurorae at radio, ultraviolet, and infrared wavelengths, while monitoring ion injections in the middle magnetosphere from energetic neutral atoms. Simultaneous measurements included the sampling of a full planetary rotation, a relevant timescale to investigate auroral emissions driven by processes internal to the magnetosphere. In addition, this interval coincidentally matched a powerful substorm-like event in the magnetotail, which induced an overall dawnside intensification of the magnetospheric and auroral activity. We comparatively analyze this unique set of measurements to reach a comprehensive view of kronian auroral processes over the investigated timescale. We identify three source regions for the atmospheric aurorae, including a main oval associated with the bulk of Saturn Kilometric Radiation (SKR), together with polar and equatorward emissions. These observations reveal the coexistence of corotational and subcorototational dynamics of emissions associated with the main auroral oval. Precisely, we show that the atmospheric main oval hosts short-lived subcorotating isolated features together with a bright, longitudinally extended, corotating region locked at the southern SKR phase. We assign the substorm-like event to a regular, internally driven, nightside ion injection possibly triggered by a plasmoid ejection. We also investigate the total auroral energy budget, from the power input to the atmosphere, characterized by precipitating electrons up to 20 keV, to its dissipation through the various radiating processes. Finally, through simulations, we confirm the search-light nature of the SKR rotational modulation and we show that SKR arcs relate to isolated auroral spots. We characterize which radio sources are visible from the spacecraft and we estimate the fraction of visible southern power to a few percent. The resulting findings are discussed in the frame of pending questions as the persistence of a corotating field-aligned current system within a subcorotating magnetospheric cold plasma, the occurrence of plasmoid activity, and the comparison of auroral fluxes radiated at different wavelengths.

Saturn's equinoctial auroras

Received 23 October 2009; accepted 24 November 2009; published 23 December 2009. [1] We present the first images of Saturn’s conjugate equinoctial auroras, obtained in early 2009 using the Hubble Space Telescope. We show that the radius of the northern auroral oval is � 1.5 smaller than the southern, indicating that Saturn’s polar ionospheric magnetic field, measured for the first time in the ionosphere, is � 17% larger in the north than the south. Despite this, the total emitted UV power is on average � 17% larger in the north than the south, suggesting that field-aligned currents (FACs) are responsible for the emission. Finally, we show that individual auroral features can exhibit distinct hemispheric asymmetries. These observations will provide important context for Cassini observations as Saturn moves from southern to northern summer. Citation: Nichols, J. D., et al. (2009), Saturn’s equinoctial auroras, Geophys. Res. Lett., 36, L24102, doi:10.1029/2009GL041491.

Cassini observations of ion and electron beams at Saturn and their relationship to infrared auroral arcs

We present Cassini Visual and Infrared Mapping Spectrometer observations of infrared auroral emissions from the noon sector of Saturns ionosphere revealing multiple intense auroral arcs separated by dark regions poleward of the main oval. The arcs are interpreted as the ionospheric signatures of bursts of reconnection occurring at the dayside magnetopause. The auroral arcs were associated with upward field-aligned currents, the magnetic signatures of which were detected by Cassini at high planetary latitudes. Magnetic field and particle observations in the adjacent downward current regions showed upward bursts of 100–360 keV light ions in addition to energetic (hundreds of keV) electrons, which may have been scattered from upward accelerated beams carrying the downward currents. Broadband, upward propagating whistler waves were detected simultaneously with the ion beams. The acceleration of the light ions from low altitudes is attributed to wave-particle interactions in the downward current regions. Energetic (600 keV) oxygen ions were also detected, suggesting the presence of ambient oxygen at altitudes within the acceleration region. These simultaneous in situ and remote observations reveal the highly energetic magnetospheric dynamics driving some of Saturns unusual auroral features. This is the first in situ identification of transient reconnection events at regions magnetically conjugate to Saturns magnetopause.

Transient internally driven aurora at Jupiter discovered by Hisaki and the Hubble Space Telescope

Jupiters auroral emissions reveal energy transport and dissipation through the planets giant magnetosphere. While the main auroral emission is internally driven by planetary rotation in the steady state, transient brightenings are generally thought to be triggered by compression by the external solar wind. Here we present evidence provided by the new Hisaki spacecraft and the Hubble Space Telescope that shows that such brightening of Jupiters aurora can in fact be internally driven. The brightening has an excess power up to similar to 550 GW. Intense emission appears from the polar cap region down to latitudes around Ios footprint aurora, suggesting a rapid energy input into the polar region by the internal plasma circulation process.

Comment on "Jupiter: A fundamentally different magnetospheric interaction with the solar wind" by D. J. McComas and F. Bagenal

[1] McComas and Bagenal [2007] (hereinafter referred to as MB) have presented a discussion of the reconnectionmediated interaction of the Jovian magnetosphere with the interplanetary medium, which they suggest to be significantly different to that at Earth. In the latter case, it is well established that ‘open’ flux is produced at the magnetopause when the interplanetary magnetic field (IMF) is directed opposite to the equatorial planetary field, is transported to the tail by the solar wind, and returns as closed flux via plasma sheet reconnection preferentially during substorms, thus forming the Dungey cycle of flux transport [e.g., Dungey, 1961]. MB propose that the consequences of open flux production at Jupiter are different, however, due to a suggested difficulty of closed flux tube return from the tail against a substantial down-tail flow of iogenic plasma. They suggest instead that open flux is effectively removed by two-lobe reconnection when the IMF has the opposite polarity, such that the open flux in the system remains small. Two-lobe reconnection has been discussed theoretically for many years [e.g., Dungey, 1963; Cowley, 1981], though convincing evidence for its occurrence at Earth has only recently been found [e.g., Imber et al., 2006, 2007]. Here, however, we question both aspects of MB’s discussion. [2] With regard to the return of tail flux by plasma sheet reconnection, MB characterise the process as requiring closed field contraction over distances of 1500–2000 RJ at speeds of 40 km s , thus requiring 30–40 days. They suggest this to be unlikely given the surrounding fast down-tail flow of iogenic plasma. However, we regard this scenario as being unduly pessimistic, first because the estimate of the distance to the tail reconnection site is unrealistically large, and second because the closed flux tube contraction speed is unrealistically small, both contributing to unrealistically large estimates of the transport time. MB’s estimate of the distance to the tail reconnection site is essentially the length of the entire tail of open field lines, obtained by multiplying the solar wind speed by the residence time of open flux tubes in the lobe. This time is estimated to be 3–4 days on the basis that open field lines flow toward the plasma sheet at 10% of the solar wind speed ( 40 km s ) for 10% reconnection efficiency with the IMF, leading to a tail length of 1500–2000 RJ as indicated above. In fact, this significantly underestimates the length of the Jovian tail, since the lobe flow speed is slowed relative to MB’s estimate by the ratio of the lobe and IMF field strengths, i.e. factors of two to three, while an overall magnetopause reconnection efficiency of 10% seems optimistic. A more realistic residence time is 10– 20 days [Nichols et al., 2006], leading to tail lengths of 5000–10000 RJ in agreement with Lepping et al. [1983]. [3] The main point to emphasise, however, is not the inaccuracy of MB’s tail length estimate, but that such estimates provide no information about the location of the tail reconnection sites, other than an upper limit. For Earth, for example, similar estimates produce tail lengths of 1000 RE [e.g., Milan, 2004], while substorm-related reconnection is typically initiated at down-tail distances of 20–30 RE [e.g., Nagai and Machida, 1998]. While flux return from the distant tail may be unlikely as MB suggest, a reasonable conclusion is that open flux will then accumulate until reconnection occurs substorm-like sufficiently close to the planet that the closed flux is indeed able to return. The return flow speeds are then expected to be comparable to the lobe Alfven speed [e.g., Badman and Cowley, 2007], at least an order of magnitude faster than the return speeds employed in MB’s estimate. [4] Significant evidence indeed exists for sporadic reconnection in the Jovian nightside plasma sheet at distances of 100 RJ, resulting in ion jets directed both toward and away from the planet [e.g., Woch et al., 2002]. These dynamics are generally assumed to relate to pinch-off of distended closed field lines and the down-tail release of iogenic plasma occurring as part of the Vasyliunas cycle [Vasyliunas, 1983]. However, supposing that after plasmoid release the reconnection continues into the tail lobe, then closed flux is generated that will clearly flow back to the planet unencumbered by surrounding down-tail flow, whether the combined reconnection is envisaged as largescale [Cowley et al., 2003], or occurs more sporadically and multiply on smaller scales [Kivelson and Southwood, 2005]. While Dungeyand Vasyliunas-cycle tail reconnection need not be coherently related in this way, the argument is sufficient to show that open flux return from the Jovian tail by plasma sheet reconnection is not as problematic as MB suggest. [5] We now turn to MB’s second argument, that open flux can instead be effectively removed from the tail by two-lobe reconnection poleward of the cusp, such that the amount of open flux in the system remains small. This requires the open flux removal rate by two-lobe reconnection for southward-directed IMF, averaged over typical GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L10101, doi:10.1029/2007GL032645, 2008

Simultaneous Cassini VIMS and UVIS observations of Saturn's southern aurora: Comparing emissions from H, H2 and H3+ at a high spatial resolution

Here, for the first time, temporally coincident and spatially overlapping Cassini VIMS and UVIS observations of Saturns southern aurora are presented. Ultraviolet auroral H and H2 emissions from UVIS are compared to infrared H3+ emission from VIMS. The auroral emission is structured into three arcs – H, H2 and H3+ are morphologically identical in the bright main auroral oval (∼73°S), but there is an equatorward arc that is seen predominantly in H (∼70°S), and a poleward arc (∼74°S) that is seen mainly in H2 and H3+. These observations indicate that, for the main auroral oval, UV emission is a good proxy for the infrared H3+ morphology (and vice versa), but for emission either poleward or equatorward this is no longer true. Hence, simultaneous UV/IR observations are crucial for completing the picture of how the atmosphere interacts with the magnetosphere.

Magnetosonic Mach number dependence of the efficiency of reconnection between planetary and interplanetary magnetic fields

We present a statistical investigation into the magnetosonic Mach number dependence of the efficiency of reconnection at the Earths dayside magnetopause. We use the transpolar voltage V PC, derived from radar observations of the ionospheric electric field, as a proxy for the dayside reconnection voltage. Our results show that the IMF clock angle dependence of V PC is closely approximated by the function f(

Asymmetric distribution of reconnection jet fronts in the Jovian nightside magnetosphere

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