G. R. Gladstone

A pulsating auroral X-ray hot spot on Jupiter

Jupiters X-ray aurora has been thought to be excited by energetic sulphur and oxygen ions precipitating from the inner magnetosphere into the planets polar regions. Here we report high-spatial-resolution observations that demonstrate that most of Jupiters northern auroral X-rays come from a ‘hot spot’ located significantly poleward of the latitudes connected to the inner magnetosphere. The hot spot seems to be fixed in magnetic latitude and longitude and occurs in a region where anomalous infrared and ultraviolet emissions have also been observed. We infer from the data that the particles that excite the aurora originate in the outer magnetosphere. The hot spot X-rays pulsate with an approximately 45-min period, a period similar to that reported for high-latitude radio and energetic electron bursts observed by near-Jupiter spacecraft. These results invalidate the idea that jovian auroral X-ray emissions are mainly excited by steady precipitation of energetic heavy ions from the inner magnetosphere. Instead, the X-rays seem to result from currently unexplained processes in the outer magnetosphere that produce highly localized and highly variable emissions over an extremely wide range of wavelengths.

ROSAT observations of the Jupiter aurora

Rontgen satellite (ROSAT) high-resolution imager (HRI) and position sensitive proportional counter (PSPC) observations of Jupiter obtained in April 1991 and May 1992 reveal soft X ray emissions apparently associated with Jupiters aurora and similar to X ray emissions observed earlier by the Einstein Observatory. The HRI images show emission mainly from Jupiters northern hemisphere at all Jovian longitudes observed, and there is some indication of a longitudinal modulation of the emission in phase with the well-known ultraviolet modulation of the northern aurora. The PSPC data reveal a very soft spectrum. Comparison of the observed spectrum with models for both electron bremsstrahlung radiation and line emission from S and O ions indicates that the line spectrum gives a much better statistical fit to the observed spectrum. The X ray observations presented here therefore support the hypothesis that ion precipitation is the most likely cause of the Jovian X ray emissions, a result first suggested by the Einstein results [Metzger et al., 1983].

First observation of Jupiter by XMM-Newton

We present the first X-ray observation of Jupiter by XMM-Newton. Images taken with the EPIC cameras show prominent emission, essentially all confined to the 0.2−2.0 keV band, from the planets auroral spots; their spectra can be modelled with a combination of unresolved emission lines of highly ionised oxygen (OVII and OVIII), and a pseudo-continuum which may also be due to the superposition of many weak lines. A 2.8� enhancement in the RGS spectrum at 21−22 A (∼0.57 keV) is consistent with an OVII identification. Our spectral analysis supports the hypothesis that Jupiters auroral em issions originate from the capture and acceleration of solar wind ions in the planets magnetosphere, followed by X-ray product ion by charge exchange. The X-ray flux of the North spot is modulat ed at Jupiters rotation period. We do not detect evidence fo r the∼45 min X-ray oscillations observed by Chandra more than two years earlier. Emission from the equatorial regions of the planets disk is also observed. Its spectrum is consistent w ith that of scattered solar X-rays.

Evidence for Supersonic Turbulence in the Upper Atmosphere of Jupiter

Spectra of the hydrogen Lyman α (Ly-α) emission line profiles of the jovian dayglow, obtained by the Goddard High Resolution Spectrograph on the Hubble Space Telescope, appear complex and variable on time scales of a few minutes. Dramatic changes occur in the Ly-α bulge region at low latitudes, where the line profiles exhibit structures that correspond to supersonic velocities of the order of several to tens of kilometers per second. This behavior, unexpected in a planetary atmosphere, is evidence for the particularly stormy jovian upper atmosphere, not unlike a stars atmosphere.

A model analysis of Galileo electron densities on Jupiter

A one-dimensional chemical-diffusive model of the Jovian ionosphere, in conjunction with measured upper atmospheric temperatures, is used to analyze the upper ionospheric electron densities on Jupiter measured by the Galileo RSS instrument on December 8, 1995. The analyses of these measurements have yielded quite different ionospheric properties at ingress and egress in terms of both the magnitude and the altitude of the peak electron density (n e ). At ingress, the peak n e was ∼ 10 5 cm -3 at an altitude of ∼ 900 km. However, at egress the n e peak was ∼ 5 times smaller than at ingress and was located at ∼ 1800 km altitude. As with our previous studies, we find it necessary to invoke a combination of vibrationally excited H 2 and vertical plasma flow to explain the measured ionospheric structure. The most interesting conclusion of this study is that a downward drift of plasma is required to fit the peak altitude of the ingress n e profile. The direction of the vertical flow is most likely determined by the horisontal neutral wind. At egress, the situation is quite different because a strong field-aligned drift of 90 m/s, most likely caused by the meridional component of the neutral wind, is required to maintain the peak n e at the measured altitude. The role of vibrationally excited H 2 in determining the magnitude of the measured n e appears to be less important at ingress than at egress.

Estimates of atomic deuterium abundance and Lyman-alpha airglow in the thermosphere of Jupiter

We have made calculations of the atomic D distribution in the thermosphere of Jupiter. The principal reactions determining the D abundance appear to be generation by reaction of H with vibrationally hot HD and loss by reaction of D with H2(υ=0,1) and CH3. The H and CH3 distributions have been calculated using a 1-D photochemical-diffusion model with the column H constrained using the Lyman-α airglow. For H2 effective vibrational temperatures, Tυ, between 1 and 4 times kinetic we find D columns between 4×1011 and 2×1013 atoms cm−2. HD can be vibrationally excited due to VV energy transfer from H2(υ=1). Using a radiative transfer model with coupling of the H and D Lyman-α lines we have calculated line profiles and total intensities across the Jovian disk and on the limb. For the above D columns and a H column ∼3.5×1017 cm−2, compatible with equatorial Lyman-α airglow observations, the disk D intensity varies from 80 to 600 R for overhead Sun and viewing, whereas on the terminator the D maximum total intensity is ∼60 R at ∼860 km above the 1 bar level for the maximum D column.

Far-UV emissions from the SL9 impacts with Jupiter

Observations with the International Ultraviolet Explorer (IUE) during the impacts of the fragments of comet D/Shoemaker-Levy 9 with Jupiter show far-UV emissions from the impact sites within a ∼10 min time scale. Positive detections of H2 Lyman and Werner band (1230–1620 A) and H-Lyα emissions are made for impacts K and S, and marginally for P2. No thermal continuum is observed. The radiated far-UV output was >1021 ergs. The H2 spectrum is consistent with electron collisional excitation if significant CH4 absorption is included. Such emissions could result from plasma processes generated by the impacts. Non-thermal excitation by the high altitude entry and explosion shocks may also be relevant. Emissions by Al+ (1671 A) and C (1657 A) of cometary origin are tentatively identified.