Ultraviolet Aurora Research Articles

Interaction evidence between Enceladus' atmosphere and Saturn's magnetosphere

Saturn's ultraviolet (UV) auroras are highly variable, with much of the activity believed to be controlled by conditions in the solar wind. Like Jupiter, Saturn is also expected to have an electrodynamic interaction between the planet's ionosphere and its satellites' atmospheres, a process that is responsible for the moon's auroral footprint components at Jupiter. Recent observations from several instruments on the Cassini spacecraft, e.g., the UltraViolet Imaging Spectrograph (UVIS) and the Cassini Plasma Spectrometer (CAPS), show strong evidence that Enceladus makes significant plasma contributions to Saturn's magnetosphere. Modeling by Pontius and Hill (2006) suggests that Enceladus' mass loading region may be comparable in extent to Io's, whose auroral footprint emissions are clearly detected. However, the upper limits of Enceladus' magnetic footprint emissions are predicted to be only a few tenths of kilo‐Rayleighs (kR). We present results of a search for Enceladus' auroral footprint using series of UV images of Saturn's aurora taken by the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) in January 2004 and the Advanced Camera for Surveys (ACS) between February 2005 and January 2007. We detected no auroral evidence of the electromagnetic connection between Enceladus' atmosphere and Saturn's ionosphere, in agreement with modeling predictions. As our study could detect no signature of currents linking Saturn's ionosphere and Enceladus' interaction region, the implication is that the field‐aligned currents associated with mass loading at Enceladus are too weak to cause auroral emission.

A statistical analysis of the location and width of Saturn's southern auroras

Abstract. A selection of twenty-two Hubble Space Telescope images of Saturn's ultraviolet auroras obtained during 1997–2004 has been analysed to determine the median location and width of the auroral oval, and their variability. Limitations of coverage restrict the analysis to the southern hemisphere, and to local times from the post-midnight sector to just past dusk, via dawn and noon. It is found that the overall median location of the poleward and equatorward boundaries of the oval with respect to the southern pole are at ~14° and ~16° co-latitude, respectively, with a median latitudinal width of ~2°. These median values vary only modestly with local time around the oval, though the poleward boundary moves closer to the pole near noon (~12.5°) such that the oval is wider in that sector (median width ~3.5°) than it is at both dawn and dusk (~1.5°). It is also shown that the position of the auroral boundaries at Saturn are extremely variable, the poleward boundary being located between 2° and 20° co-latitude, and the equatorward boundary between 6° and 23°, this variability contrasting sharply with the essentially fixed location of the main oval at Jupiter. Comparison with Voyager plasma angular velocity data mapped magnetically from the equatorial magnetosphere into the southern ionosphere indicates that the dayside aurora lie poleward of the main upward-directed field-aligned current region associated with corotation enforcement, which maps to ~20°–24° co-latitude, while agreeing reasonably with the position of the open-closed field line boundary based on estimates of the open flux in Saturn's tail, located between ~11° and ~15°. In this case, the variability in location can be understood in terms of changes in the open flux present in the system, the changes implied by the Saturn data then matching those observed at Earth as fractions of the total planetary flux. We infer that the broad (few degrees) diffuse auroral emissions and sub-corotating auroral patches observed in the dayside sector at Saturn result from precipitation from hot plasma sub-corotating in the outer magnetosphere in a layer a few Saturn radii wide adjacent to the magnetopause, probably having been injected either by Dungey-cycle or Vasyliunas-cycle dynamics on the nightside.

Signature of Saturn's auroral cusp: Simultaneous Hubble Space Telescope FUV observations and upstream solar wind monitoring

Model simulations by Bunce et al. (2005a) have shown that direct precipitation of electrons in Saturn's dayside cusp regions is not capable of producing significant FUV aurora. Instead, they suggested the possibility that the FUV bright emissions sometimes observed near noon are associated with reconnection occurring at the dayside magnetopause, possibly pulsed, analogous to flux transfer events seen at the Earth. Pulsed reconnection at the low‐latitude dayside magnetopause when the IMF is directed northward (antiparallel to Saturn's magnetic field lines) is expected to give rise to pulsed twin‐vortical flows in the magnetosphere and hence to bipolar field‐aligned currents centered in the vortical flows closing in ionospheric Pedersen current. In the case of southward IMF and high‐latitude lobe reconnection the model predicts that the vortical flows are displaced poleward of the open‐closed field line boundary with reversed field‐aligned currents compared with the former case. During January 2004, a unique campaign took place during which magnetic field and plasma instruments on board the Cassini‐Huygens spacecraft measured the in situ solar wind and embedded interplanetary magnetic field while the Hubble Space Telescope simultaneously observed the far ultraviolet aurora in Saturn's southern hemisphere. The IMF was highly structured during this interval. The electric potential at Cassini is estimated from solar wind magnetic field and velocity measurements for the case of low‐latitude or lobe reconnection. We show that a dayside FUV signature of intense electron precipitation is found poleward of or along the main oval during a period of minor compression period when the dayside reconnection voltage is estimated to be ∼30–100 kV. Overall, we find that the conceptual model of Bunce et al. (2005a) provides a good estimate of the UV brightness and power for the case of northward IMF but somewhat underestimates the power for the southward IMF case, except if the speed of the vortical flow is larger than its value in the nominal model.

Variable morphology of Saturn's southern ultraviolet aurora

The Space Telescope Imaging Spectrograph camera on board Hubble Space Telescope obtained 68 FUV images of Saturn's southern auroral emission between 8 and 30 January 2004, during Cassini's approach to Saturn's magnetosphere. The HST observations took place in four different solar wind regimes with a low‐field rarefaction region from 8 to 16 January, a minor compression event on 17 January, a rarefaction region with intermediate field strengths from 19 to 25 January, and a major compression region from 26 to 30 January. The images have been projected onto polar maps in order to characterize and compare the general morphology of the auroral emission. The first 20 images were obtained during a period covering about 70% of one full rotation of Saturn. They show that the bright ring of auroral emission actually consists of several arcs of different width and brightness and forming along different parallels. Overall, the auroral region is shown to rotate at ∼65% of the full planetary rotation, although the angular velocity of some isolated auroral structures constantly decrease with time, down to 20%. The strongest auroral precipitations are observed in the morning sector. The polar projections of the 48 remaining images confirm dramatic changes in morphology, characterized by different average zones. During the campaign, short intervals during which the auroral region is significantly contracted and clearly forms a spiral were followed by intervals of reinflation of the auroral region. It is suggested that the two major auroral contraction events corresponded to the arrival of the solar wind shocks observed by Cassini on 17 January and 25 January. The present analysis indicates that Saturn's auroral morphology responds to the solar wind conditions at Saturn.

Morphological differences between Saturn's ultraviolet aurorae and those of Earth and Jupiter

It has often been stated that Saturn's magnetosphere and aurorae are intermediate between those of Earth, where the dominant processes are solar wind driven, and those of Jupiter, where processes are driven by a large source of internal plasma. But this view is based on information about Saturn that is far inferior to what is now available. Here we report ultraviolet images of Saturn, which, when combined with simultaneous Cassini measurements of the solar wind and Saturn kilometric radio emission, demonstrate that its aurorae differ morphologically from those of both Earth and Jupiter. Saturn's auroral emissions vary slowly; some features appear in partial corotation whereas others are fixed to the solar wind direction; the auroral oval shifts quickly in latitude; and the aurora is often not centred on the magnetic pole nor closed on itself. In response to a large increase in solar wind dynamic pressure Saturn's aurora brightened dramatically, the brightest auroral emissions moved to higher latitudes, and the dawn side polar regions were filled with intense emissions. The brightening is reminiscent of terrestrial aurorae, but the other two variations are not. Rather than being intermediate between the Earth and Jupiter, Saturn's auroral emissions behave fundamentally differently from those at the other planets.

An Earth-like correspondence between Saturn's auroral features and radio emission

Saturn is a source of intense kilometre-wavelength radio emissions that are believed to be associated with its polar aurorae, and which provide an important remote diagnostic of its magnetospheric activity. Previous observations implied that the radio emission originated in the polar regions, and indicated a strong correlation with solar wind dynamic pressure. The radio source also appeared to be fixed near local noon and at the latitude of the ultraviolet aurora. There have, however, been no observations relating the radio emissions to detailed auroral structures. Here we report measurements of the radio emissions, which, along with high-resolution images of Saturn's ultraviolet auroral emissions, suggest that although there are differences in the global morphology of the aurorae, Saturn's radio emissions exhibit an Earth-like correspondence between bright auroral features and the radio emissions. This demonstrates the universality of the mechanism that results in emissions near the electron cyclotron frequency narrowly beamed at large angles to the magnetic field.

The location of the open-closed magnetic field line boundary in the dawn sector auroral ionosphere

Abstract. As a measure of the degree of coupling between the solar wind-magnetosphere-ionosphere systems, the rate at which the size of the polar cap (the region corresponding to ionospheric termini of open magnetic flux tubes) varies is of prime importance. However, a reliable technique by which the extent of the polar cap might be routinely monitored has yet to be developed. Current techniques provide particularly ambiguous indications of the polar cap boundary in the dawn sector. We present a case study of space- and ground-based observations of the dawn-sector auroral zone and attempt to determine the location of the polar cap boundary using multi-wavelength observations of the ultraviolet aurora (made by the IMAGE FUV imager), precipitating particle measurements (recorded by the FAST, DMSP, and Cluster 1 and 3 satellites), and SuperDARN HF radar observations of the ionospheric Doppler spectral width boundary. We conclude that in the dawn sector, during the interval presented, neither the poleward edge of the wideband auroral UV emission (140-180nm) nor the Doppler spectral width boundary were trustworthy indicators of the polar cap boundary location, while narrow band UV emissions in the range 130-140nm appear to be much more reliable.

Implications of Jovian X‐ray emission for magnetosphere‐ionosphere coupling

The first observations of Jupiter made by the Chandra X‐Ray Observatory revealed a powerful X‐ray aurora located in the polar caps. The X‐ray emission exhibited a 40‐min periodicity. Such 40‐min periodicities have previously been seen in energetic particle fluxes and in Jovian radio emission. This paper develops scenarios in which the X‐ray emission is produced by energetic heavy ion precipitation, either on open field lines connecting to the solar wind or on closed field lines reaching to the outer magnetosphere. In order to produce enough X‐ray power, both scenarios require the existence of field‐aligned electric fields located somewhere between the ionosphere and the magnetosphere, most likely at a radial distance of a few Jovian radii. The potential needed for solar wind ions to produce the observed X‐rays is about 200 kV and the potential needed for the magnetospheric ions is at least 8 MV. Protons and helium ions are also accelerated by the potential and should produce an intense ultraviolet aurora. Downward electrical current is carried by the precipitating ions and also by upwardly accelerated secondary electrons produced by the primary ion precipitation. The estimated downward Birkeland current is about 1000 MA for the solar wind case and is about 10 MA for the magnetospheric case. For the magnetosphere scenario, this observed current represents part of the “return” current portion of the magnetospheric circuit associated with the departure of the mass‐loaded magnetospheric plasma from corotation. The auroral X‐ray emission maps at least part of this return current in the polar cap, whereas the main oval, produced by electron precipitation, is thought to map the region of upward Birkeland currents. The accelerated secondary electrons could be responsible for the periodic radio emission (i.e., QP‐40 bursts).

Corotation-driven magnetosphere-ionosphere coupling currents in Saturn’s magnetosphere and their relation to the auroras

Abstract. We calculate the latitude profile of the equatorward-directed ionospheric Pedersen currents that are driven in Saturn’s ionosphere by partial corotation of the magnetospheric plasma. The calculation incorporates the flattened figure of the planet, a model of Saturn’s magnetic field derived from spacecraft flyby data, and angular velocity models derived from Voyager plasma data. We also employ an effective height-integrated ionospheric Pedersen conductivity of 1 mho, suggested by a related analysis of Voyager magnetic field data. The Voyager plasma data suggest that on the largest spatial scales, the plasma angular velocity declines from near-rigid corotation with the planet in the inner magnetosphere, to values of about half of rigid corotation at the outer boundary of the region considered. The latter extends to ~ 15–20 Saturn radii (RS) in the equatorial plane, mapping along magnetic field lines to ~ 15° co-latitude in the ionosphere. We find in this case that the ionospheric Pedersen current peaks near the poleward (outer) boundary of this region, and falls toward zero over ~ 5°–10° equator-ward of the boundary as the plasma approaches rigid corotation. The peak current near the poleward boundary, integrated in azimuth, is ~ 6 MA. The field-aligned current required for continuity is directed out of the ionosphere into the magnetosphere essentially throughout the region, with the current density peaking at ~ 10 nA m-2 at ~ 20° co-latitude. We estimate that such current densities are well below the limit requiring field-aligned acceleration of magnetospheric electrons in Saturn’s environment ( ~ 70 nAm-2), so that no significant auroral features associated with this ring of upward current is anticipated. The observed ultraviolet auroras at Saturn are also found to occur significantly closer to the pole (at ~ 10°–15° co-latitude), and show considerable temporal and local time variability, contrary to expectations for corotation-related currents. We thus conclude that Saturn’s ‘main oval’ auroras are not associated with corotation-enforcing currents as they are at Jupiter, but instead are most probably associated with coupling to the solar wind as at Earth. At the same time, the Voyager flow observations also suggest the presence of radially localized ‘dips’ in the plasma angular velocity associated with the moons Dione and Rhea, which are ~ 1–2 RS in radial extent in the equatorial plane. The presence of such small-scale flow features, assumed to be azimuthally extended, results in localized several-MA enhancements in the ionospheric Pedersen current, and narrow bi-polar signatures in the field-aligned currents which peak at values an order of magnitude larger than those associated with the large-scale currents. Narrow auroral rings (or partial rings) ~ 0.25° co-latitude wide with intensities ~ 1 kiloRayleigh may be formed in the regions of upward field-aligned current under favourable circumstances, located at co-latitudes between ~ 17° and ~ 20° in the north, and ~ 19° and ~22° in the south.Key words. Magnetospheric physics (current systems; magnetosphere-ionosphere interactions; planetary magnetospheres)

Correlative study of ultraviolet aurora and low‐latitude Pi2 pulsations

We study the relationship between auroras observed by the ultraviolet imager (UVI) on the Polar spacecraft and Pi2 pulsations detected by the Circum‐pan Pacific Magnetometer Network at magnetic latitudes lower than 55°. Instead of focusing on the few minutes around auroral breakups, we treat both auroral power and pulsation parameters as continuous time series in an effort to quantify the relationship between the two phenomena. A 3‐hour interval on 3 April 1996 is selected for the present study because it includes several episodes of auroral intensification. The auroral power is represented by electron precipitation energy (PUVI) that is inferred from the observed 170‐nm photon emission over the 2000–0200 magnetic local time sector in 36‐s time windows separated by ∼5 min. Parameters characterizing Pi2 signals in the magnetometer data are computed in a moving 5‐min time window using the autoregressive spectral analysis technique. The parameters include the amplitude APi2 in the Pi2 band (7–25 mHz), the frequency, the ellipticity, and azimuth of the major axis of polarization for oscillations with a well‐defined spectral peak. Overall, there is a high degree of similarity between the temporal profiles of PUVI and APi2. The pulsation amplitude varied from ∼0.1 nT for a PUVI change of <1 GW to ∼1 nT for a PUVI change of >20 GW; although due to different decay timescales, ∼10 min for APi2 and ∼80 min for PUVI, the linear cross‐correlation coefficient between the two parameters remained at a modest level (∼0.75). In one instance, APi2 was enhanced without auroral intensification, and for this pulsation the azimuth of the major axis of polarization was oriented in the east‐west direction instead of the north‐south direction that is the norm for Pi2 pulsations associated with auroral intensifications. Examination of solar wind plasma data from the Geotail spacecraft suggests that this particular event was directly driven by changes in the solar wind dynamic pressure. We conclude that low‐latitude magnetic pulsations are a sensitive indicator of changes of auroral intensity and that pulsations that are not associated with aurora could be distinguished by examining their polarization properties.

Jupiter’s Aurora: Solar Wind and Rotational Influences

AbstractJovian auroral emissions are observed at infrared, visible, ultraviolet, and x-ray wavelengths. As at Earth, pitch-angle scattering of energetic particles into the atmospheric loss cone and the acceleration of current-carrying electrons in field-aligned currents both play a role in exciting the auroral emissions. The x-ray aurora is believed to result principally from heavy ion precipitation, while the ultraviolet aurora is produced predominantly by precipitating energetic electrons. The magnetospheric processes responsible for the aurora are driven primarily by planetary rotation. Acceleration of Iogenic plasma by rotationally-induced electric fields results in both the formation of the energetic ions that are scattered and the formation of strong, field-aligned currents that communicate the torques from the ionosphere. In addition to rotation-driven processes, solar-wind-modulated processes in the outer magnetosphere may lead to highly, time-dependent acceleration and thus also contribute to jovian auroral activity. Observational evidence for both sources will be presented. See Waite et al. (2001, Nat., 410, 787).

Interplanetary magnetic field By dependence of the relative position of the dayside ultraviolet auroral oval and the HF radar cusp

Comparative analyses of CUTLASS Super Dual Auroral Radar Network HF radar data and images of the dayside ultraviolet aurora from the Polar far ultraviolet imager and the Visible Imaging System Earth Camera have revealed that the GSM y component of the interplanetary magnetic field (By) affects the relative positions of ultraviolet auroral emissions and the HF radar signature of the cusp in the postnoon sector during episodes of flux transfer at the magnetopause. When By is positive, the region of strong auroral emissions is found to be at the same latitude as the radar footprint of the cusp. When By is negative, the radar cusp is found to be poleward of the strong auroral emissions. These results are consistent with models of the ionospheric convection and field‐aligned current response to reconnection at the magnetopause under positive and negative By conditions. We extend the models to predict the auroral and convection responses in the morning sector.

The dayside ultraviolet aurora and convection responses to a southward turning of the interplanetary magnetic field

Abstract. We examine the large-scale ultraviolet aurora and convection responses to a series of flux transfer events that immediately followed a sharp and isolated southward turning of the IMF. During the interval of interest, SuperDARN was monitoring the plasma convection in the dayside northern ionosphere, while the VIS Earth Camera and the Far Ul-traviolet Imager (UVI) were monitoring the northern hemisphere’s ultraviolet aurora. Reconnection signatures were seen in the SuperDARN HF radar data in the postnoon sector following a sharp southward turning of the IMF. The presence of flux transfer events is supported by measurements of a classic dispersed ion signature in the low-altitude cusp from the DMSP spacecraft. Subsequent to the onset of reconnection, the postnoon convection and ultraviolet aurora expanded in concert, reaching 18 MLT in half an hour. The auroral oval was found to move equatorward at the convection speed in the 16–18 MLT sector, implying that it was related directly to an adiaroic magnetospheric boundary. In the present study, we have estimated the field-aligned current response to magnetic reconnection in terms of the vorticity of the ionospheric plasma convection velocity. The convection velocities were obtained using two methods: (a) direct reconstruction of the full vector velocities from bistatic measurements of the convection by the SuperDARN HF radars in a relatively small region of the auroral zone, and (b) from global-scale spherical harmonic fits to the SuperDARN velocities deduced from the map potential model. Regions of high vorticity, which were predicted to be an estimate of a component of the total field-aligned current, agree extremely well with the images of the dayside UV aurora, indicating that, in this case, the plasma vorticity is an excellent estimator of the morphology of dayside field-aligned currents (FACs). The morphology of the aurora and ionospheric electric field in the postnoon sector supports the existence of a dayside current wedge induced in response to dayside reconnection.Key words. Magnetospheric physics (auroral phenomena; magnetosphere-ionosphere interactions; solar wind magne-tosphere interactions)

Excitation of the Ganymede Ultraviolet Aurora

We analyze the ultraviolet aurorae observed on Ganymede by means of the Hubble Space Telescope and compare them to similar phenomena on Earth. We find that the tenuous nature of Ganymede's atmosphere precludes excitation of the aurora by high-energy electrons and requires a local acceleration mechanism. We propose the following as plausible mechanisms for generating both the continuous background emission and the intense auroral bright spots:

Io's ultraviolet aurora: Remote sensing of Io's interaction

With the recent observations of Io's very striking equatorial aurora by Roesler et al. [1999], it is now possible to remotely sense Io's local interaction with the Io plasma torus. Here we present an explanation for the formation of Io's aurora which implies that Io's electron flow is strongly rotated and shielded around Io. The Hall effect close to Io breaks the interaction symmetry to produce a brighter anti‐Jovian radiation spot, and it also results in rotated Alfvén wings. Observations of Io's aurora also impose new constraints on Io's atmosphere. We find, among other things, an atomic oxygen to sulfur dioxide ratio of ∼20%.

Auroral emissions of the giant planets

Auroras are (generally) high‐latitude atmospheric emissions that result from the precipitation of energetic charged particles from a planet's magnetosphere. Auroral emissions from the giant planets have been observed from ground‐based observatories, Earth‐orbiting satellites (e.g., International Ultraviolet Explorer (IUE), Hubble Space Telescope (HST), and Röentgensatellit (ROSAT)), flyby spacecraft (e.g., Voyager 1 and 2), and orbiting spacecraft platforms (e.g., Galileo) at X‐ray, ultraviolet (UV), visible, infrared (IR), and radio wavelengths. UV, visible, and IR auroras are atmospheric emissions, produced or initiated when ambient atmospheric species are excited through collisions with the precipitating particles, while radio and X‐ray auroras are beam emissions, produced by the precipitating species themselves. The emissions at different wavelengths provide unique and complementary information, accessible to remote sensing, about the key physical processes operating in the atmospheric and magnetospheric regions where they originate. This paper reviews the development of our current understanding of auroral emissions from Jupiter, Saturn, Uranus, and Neptune, as revealed through multispectral observations and supplemented by plasma measurements.

Saturn's hydrogen aurora: Wide field and planetary camera 2 imaging from the Hubble Space Telescope

Wide field and planetary camera 2/Hubble Space Telescope (WFPC2/HST) images of Saturn's far ultraviolet aurora reveal emissions confined to a narrow band of latitudes near Saturn's north and south poles. The aurorae are most prominent in the morning sector with patterns that appear fixed in local time. The geographic distribution and vertical extent of the auroral emissions seen in these images provide the first new observational insights into the physical mechanisms that power Saturn's aurora since the Voyager encounters 17 years ago.

Temperatures and Altitudes of Jupiter's Ultraviolet Aurora Inferred from GHRS Observations with theHubble Space Telescope

We observed the jovian UV auroral regions with the Goddard high resolution spectrograph (GHRS) on board the Hubble Space Telescope (HST) on Apr. 29, May 2, and June 10, 1995. Observations of target areas were made in pairs in the two wavelength ranges 1257–1293 Å and 1587–1621 Å. Spectra in the long wavelength range are dominated by emissions of the H2Lyman band system and show well separated rotational features, which we have used to determine the temperatures of the auroral emission regions. Spectra in the short wavelength range are mostly due to emission in the H2Lyman and Werner band systems, but their intensities are reduced by hydrocarbon absorption. The brightest spectral pair was observed toward an area with longitude 155° and jovicentric latitude 58° when the central meridian longitudes (CMLs) were 191° and 203°. This area was found to be bright in our previous HST observations in 1993 and in HST faint object camera images. Assuming that electron impact excitation is the major source of the jovian aurora, we estimate total emission rates in the Lyman band system of about 270 and 46 kR for the long and short wavelength spectra of the pair, respectively. The attenuation of emission rate in the short wavelength spectrum implies a methane column density of about 3 × 1016cm−2, and a temperature of about 450 K is inferred from the long wavelength spectrum of the brightest pair. For all six pairs of observed spectra, we estimate methane column densities in the range (1–7) × 1016cm−2, which, when compared to a standard mid-latitude model, corresponds to a pressure range from a few μbar to a few tens of μbar. The temperatures derived are in the range 400–850 K with a possible tendency toward lower temperatures for higher methane column densities. This tendency and the uncertainty in the temperatures derived may indicate that the temperatures increases rapidly with altitude around the methane homopause in the auroral regions.

Simultaneous observations of the Saturnian aurora and polar haze with the HST/FOC

Near simultaneous observations of the Saturnian H2 north ultraviolet aurora and the polar haze were made at 153 nm and 210 nm respectively with the Faint Object Camera on board the post‐COSTAR Hubble Space Telescope. The auroral observations cover a complete rotation of the planet and, when co‐added, they reveal the presence of an auroral emission near 80°N with a brightness of about 150 kR of total H2 emission. The maximum vertical optical depth at 210 nm is found to be located ∼5° equatorward of the auroral emission zone. The haze particles are presumably formed by hydrocarbon aerosols initiated by H2+ auroral production. In this case, the 3 × 1010 W of H2 emission observed with the FOC, combined with the deduced haze optical depth requires an efficiency of aerosol formation of about 7%. This result indicates that auroral production of hydrocarbon aerosols is a viable source of high‐latitude haze.

ROSAT Observations of X-ray Emissions from Jupiter During the Impact of Comet Shoemaker-Levy 9

Röntgensatellit (ROSAT) observations made shortly before and during the collision of comet Shoemaker-Levy 9 with Jupiter show enhanced x-ray emissions from the planet's northern high latitudes. These emissions, which occur at System III longitudes where intensity enhancements have previously been observed in Jupiter's ultraviolet aurora, appear to be associated with the comet fragment impacts in Jupiter's southern hemisphere and may represent brightenings of the jovian x-ray aurora caused either by the fragment impacts themselves or by the passage of the fragments and associated dust clouds through Jupiter's inner magnetosphere.