T. Majeed

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.

New Survey of Electron Impact Cross Sections for Photoelectron and Auroral Electron Energy Loss Calculations

Newly surveyed sets of energy loss cross sections are presented for N2, O2, and O. The work was motivated by a number of new electron energy loss measurements in the late 1980s and early 1990s and recent selected review articles. Each set includes a total ionization cross section and excitation cross sections that correspond to all important non-ionizing energy loss channels for that species. A total cross section for each species is constructed by summing the elastic scattering cross section with the ionization and excitation cross sections. The sum is compared to a measured total cross section obtained from electron transmission experiments. Good agreement is achieved for each of the three species. A loss function is also constructed for each species and compared with the Bethe formula above 100 eV. Good agreement is also achieved in energy loss which is dominated by ion and secondary electron production. Fluxes of photoelectrons and auroral electrons have been calculated for the new sets of energy loss...

The upper ionospheres of Jupiter and Saturn

Abstract We use a 1-D chemical diffusive model, in conjunction with the measured neutral atmospheric structure, to analyze the Voyager RSS electron density, n e , profiles for the ionospheres of Jupiter and Saturn. As with previous studies we find serious difficulties in explaining the n e measurements. The model calculates ionospheres for both Jupiter and Saturn with n e peaks of ∼ 10 times the measured peaks at altitudes which are ∼ 900–1000 km lower than the altitude of peaks in the RSS electron densities. Based on our knowledge of neutral atmospheric structure, ionization sources, and known recombination mechanisms it seems that, vibrational excitation of H 2 must play some role in the conversion of slowly radiatively recombining H + ions to the relatively more rapidly recombining H 2 + and H 3 + ions. In addition, vertical ion flow induced by horizontal neutral winds or electric fields probably also play some role in maintaining the plasma peaks observed both for Jupiter and Saturn to be at high altitudes. For the ionosphere of Saturn, the electron densities are affected by a putative influx of H 2 O molecules, Φ H 2 O , from the rings. To reproduce the RSS V2 exit n e results model requires an influx of Φ H 2 O ∼ 2 × 10 7 molecules cm −2 s −1 without invoking H 2f vibrational excitation. To maintain the model n e peak at the measured altitude vertical plasma drift maintained by meridional winds or vertical electric fields is required. The amounts of H 2 O are consistent with earlier estimates of Connerney and Waite (1984) and do not violate any observational constraints.

Jupiter Thermospheric General Circulation Model (JTGCM): Global structure and dynamics driven by auroral and Joule heating

(including auroral/Joule heating processes). The resulting JTGCM has been fully spun-up and integrated for � 40 Jupiter rotations. Results from three JTGCM cases incorporating moderate auroral heating, ion drag, and moderate to strong Joule heating processes are presented. The neutral horizontal winds at ionospheric heights vary from 0.5 km/s to 1.2 km/s, atomic hydrogen is transported equatorward, and auroral exospheric temperatures range from � 1200–1300 K to above 3000 K, depending on the magnitude of Joule heating. The equatorial temperature profiles from the JTGCM are compared with the measured temperature structure from the Galileo ASI data set. The best fit to the Galileo data implies that the major energy source for maintaining the equatorial temperatures is due to dynamical heating induced by the low-latitude convergence of the high-latitude-driven thermospheric circulation. Overall, the Jupiter thermosphere/ionosphere system is highly variable and is shown to be strongly dependent on magnetospheric coupling which regulates Joule heating.

Voyager electron density measurements on Saturn: Analysis with a time dependent ionospheric model

We have used a one-dimensional chemical diffusive model of the ionosphere, in conjunction with the measured Voyager ultraviolet spectrometer (UVS) upper atmospheric temperature and composition structure, to analyze the Voyager measurements of Saturns upper ionospheric electron densities. Electron density measurements are available from the analysis of the radio science (RSS) experiments. In addition, if interpreted as an atmospheric phenomenon the Saturn electrostatic discharges (SED), detected by the Voyager planetary radio astronomy instrument, the measurements also yield important information on electron densities at the peak. These latter data suggest a strong day-to-night variation in the peak electron densities with a maximum of ∼5 × 105 cm−3 near noon and a minimum of ∼103 cm−3 near midnight. The analysis of RSS data yields a peak of ∼104 cm−3 in the electron density profiles near the local terminators. As with simpler ionospheric models, we find serious difficulties in interpreting these electron density measurements because time constants in the model for the chemical sinks in the ionosphere, of mostly H+ ions, are >106 s much longer than a Saturnian day, suggesting little diurnal variation. We have investigated the effects of H2 vibrational excitation (characterized by a single vibrational temperature) and a gaseous H2O influx from Saturns rings, as enhancers of plasma recombination, since they result in the conversion of slowly recombining H+ ions to more rapidly recombining molecular ions. This does introduce a larger diurnal variation in the peak electron densities, but not as strong as that deduced from the analysis of SED measurements. Furthermore, it also reduces the magnitude of the electron densities at the peak well below that inferred from the SED data. Based on the model results and extrapolation of calculations by Kim and Fox [1991] from Jupiter, we suggest that SEDs may not be associated with atmospheric storm systems. In our attempt to interpret the RSS measurements of electron density profile at sunrise, we have invoked an upward field-aligned drift, H2 vibrational excitation and an influx of H2O to explain the results, and, in particular, maintain plasma peak at the measured altitude.

Vibrationally excited H2 in the outer planets thermosphere: Fluorescence in the Lyman and Werner bands

Abstract We have considered the impact of fluorescence of ground state H 2 on the distribution of the vibrational levels of Ha in the upper atmospheres of Jupiter and Saturn for non-auroral latitudes. For v ⩾ 3, for the conditions studied, this is the most important source of vibrationally excited H 2 compared with other sources, such as photoelectron induced fluorescence, dissociative recombination of H + 3 ions, and direct vibrational excitation of H 2 by photoelectron impact. Combining the Voyager limb observations of H 2 band emissions on Saturn, theoretical calculations of the H 2 fluoresence distribution, and column constraints of Jovian H 2 airglow, we estimate that some of the higher vibrational levels may have effective temperatures > 3000 K on both Jupiter and Saturn. In turn, the vibrational population of v ⩾ 4 levels are sufficiently increased by the fluorescence source that the chemical sink for the ionization is enhanced. As a result, ionospheric densities may be greatly affected. We also show that the vertical ion flows induced by horizontal neutral winds or dynamo electric fields must play some role in maintaining the plasma peaks at higher altitudes.

Discovery of Soft X-Ray Emission from Io, Europa and the Io Plasma Torus

We report the discovery of soft (0.25-2 keV) X-ray emission from the Galilean satellites Io and Europa, probably Ganymede, and from the Io Plasma Torus (IPT). Bombardment by energetic (greater than 10 keV) H, O, and S ions from the region of the IPT seems to be the likely source of the X-ray emission from the Galilean satellites. According to our estimates, fluorescent X-ray emission excited by solar X-rays, even during flares from the active Sun, charge-exchange processes, previously invoked to explain Jupiters X-ray aurora and cometary X-ray emission, and ion stripping by dust grains fail to account for the observed emission. On the other hand, bremsstrahlung emission of soft X-rays from nonthermal electrons in the few hundred to few thousand eV range may account for a substantial fraction of the observed X-ray flux from the IPT.

Multispectral observations of Jupiter's Aurora

Remote sensing of Jupiters aurora from x-ray to radio wavelengths has revealed much about the nature of the jovian aurora and about the impact of ionosphere-magnetosphere coupling on the upper atmosphere of Jupiter. As indicated by the combination of x-ray and ultraviolet observations, both energetic heavy ions and electrons energized in the outer magnetosphere contribute to auroral excitation. Imaging with the Hubble Space Telescope in the ultraviolet and with the InfraRed Telescope Facility at infrared wavelengths shows several distinct regions of interaction: 1) a dusk sector where turbulent auroral patterns extend well into the polar cap; 2) a morning sector generally characterized by a single spatially confined auroral arc originating in the outer or middle magnetosphere of Jupiter; 3) diffuse emissions associated with the Io plasma - spectroscopy has provided important information about the thermal structure of Jupiters auroral atmosphere and the altitude distribution of auroral particle energy deposition, while Lyman alpha line profiles yield clues to the nature of thermospheric dynamical effects. Galileo observations at visible wavelengths on the nightside offer a new view of the jovian aurora with unprecedented spatial information. Infrared observations have added much to the understanding of thermal structure at all latitudes, the dynamics of the thermospheric wind system, and auroral morphology, and may hold the key to understanding the role of Joule heating in Jupiters thermosphere. ROSAT observations have revealed soft x-ray emissions from Jupiters lower latitudes as well as from the auroral zones, implying that energetic particle precipitation also occurs at low latitudes in regions magnetically linked to the inner radiation belts. In this review, multispectral observations of jovian auroral emissions are presented within a theoretical/modeling framework that is intended to provide some insight into magnetosphere-ionosphere coupling and its effects on the upper 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.

Analytical representation of g factors for rapid, accurate calculation of excitation rates in the dayside thermosphere

An algorithm has been developed that rapidly and accurately calculates altitude profiles of volume excitation rates for many species of aeronomic interest. These rates may be specified as functions of solar activity (specific to the Hinteregger et al. [1981] model in this phase of the work) and solar zenith angle (SZA). The algorithm relies on an analytical expression for the g factor as a function of total vertical column density. The g factor for a given state and excitation process can be defined as the direct volume excitation rate for this process divided by the density of the species involved in the direct excitation. Modeling results from this work show that g, when expressed as a function of total vertical column density, is independent of model atmosphere within a small error illustrated in the text. The dependence within g then reduces to solar activity (characterized by F 10.7 ) and SZA. Extensive photoelectron transport calculations have been performed that provide numerical g factors for 22 excitation processes, 3 F 10.7 values (75, 150, and 250), and 8 solar zenith angles (0°, 45°, 60°, 70°, 80°, 85°, 87°, and 90°). The results have been fitted with a 10 coefficient analytical expression comprised of a function based on an analytical solution to a simplified photoelectron transport equation and two Gaussian functions needed for structure because of soft X ray energy deposition. The resulting coefficients are used to calculate g factors for user-specified pairs of F 10.7 and SZA by two-dimensional interpolation. By inputting a model atmosphere the corresponding volume excitation rates may also be specified. For photoionization excitation the analytic expression for the g factor is much simpler. Results presented herein include comparisons between numerical g factors and their fits, along with examples of both g factors and corresponding volume excitation rates for the purpose of showing the effects of changing solar activity and SZA. Tables are included that identify excitation processes, identify additional processes that must be considered to derive emission rates, and provide cross-section information for the electron impact processes addressed in this work.