The Io UV footprint: Location, inter-spot distances and tail vertical extent

in Journal of Geophysical Research. Space Physics (2009), 114

The Io footprint (IFP) consists of one or several spots observed in both jovian hemispheres and is related to the electromagnetic interaction between Io and the magnetosphere. These spots are followed by ... [more ▼]

The Io footprint (IFP) consists of one or several spots observed in both jovian hemispheres and is related to the electromagnetic interaction between Io and the magnetosphere. These spots are followed by an auroral curtain, called the tail, extending more than 90° longitude in the direction of planetary rotation. We use recent Hubble Space Telescope images of Jupiter to analyze the location of the footprint spots and tail as a function of Io's location in the jovian magnetic field. We present here a new IFP reference contour---the locus of all possible IFP positions---with an unprecedented accuracy, especially in previously poorly covered sectors. We also demonstrate that the lead angle - the longitudinal shift between Io and the actual IFP position - is not a reliable quantity for validation of the interaction models. Instead, the evolution of the inter-spot distances appears to be a better diagnosis of the Io-Jupiter interaction. Moreover, we present observations of the tail vertical profiles as seen above the limb. The emission peak altitude is ~900 km and remains relatively constant with the distance from the main spot. The altitudinal extent of the vertical emission profiles is not compatible with precipitation of a mono-energetic electron population. The best fit is obtained for a kappa distribution with a characteristic energy of ~70 eV and a spectral index of 2.3. The broadness of the inferred electron energy spectrum gives insight into the physics of the electron acceleration mechanism at play above the IFP tail. [less ▲]

Diagnostics of the jovian aurora deduced from ultraviolet spectroscopy: Model and HST/GHRS observations

in Icarus: International Journal of Solar System Studies (2000), 147(1), 251-266

A model coupling an electron energy degradation code with a detailed synthetic spectrum of the H-2 Lyman and Werner band system is used to calculate the emerging auroral ultraviolet spectra from Jupiter's ... [more ▼]

A model coupling an electron energy degradation code with a detailed synthetic spectrum of the H-2 Lyman and Werner band system is used to calculate the emerging auroral ultraviolet spectra from Jupiter's atmosphere excited by electrons with different initial energy distributions. The atmospheric model is adapted from the vertical P-T profile measured by the Galileo probe and midlatitude model hydrocarbon photochemistry. Each altitude layer, with its own gas temperature, contributes to the emergent ultraviolet spectrum and the absorbers are vertically distributed within the source region of the auroral emissions. Examples of the calculated spectra are shown to validate the synthetic spectrum and to illustrate the importance of the electron energy distribution and the vertical structure. The model is then applied to the analysis of seven HST/GHRS spectra of the 1200-1700 Angstrom region obtained with 5-Angstrom resolution at various locations in the north and south Jovian aurora. These spectra have different color ratios which characterize the energy of the precipitated electrons, although they do not have a high enough spectral resolution to permit a determination of the H-2 temperature. We find that the characteristic energy of the assumed initial Maxwellian distribution ranges between 17 and 40 keV. A clear signature of acetylene absorption is observed near 1520, 1480, and 1440 Angstrom where the C2H2 cross section shows strong absorption peaks. The acetylene column abundance overlying the emission peak varies from 0.02 to 0.2 of the methane column. A better fit is obtained for some spectra when ethane absorption is added. The C2H6 column abundance varies from 0 to 0.5 of the methane column. These changes relative to methane are presumably the result of perturbations by heat released by the fast electron thermalization and/or perturbations to the hydrocarbon chemistry resulting from the production of H atoms by the aurora, A spectrum of the Io flux tube footprint and its trailing tail shows an ultraviolet color and hydrocarbon absorption quite similar to some of the main oval spectra, This observation suggests that the electrons of the Io flux tube are energized to a few tens of keV, similar to the electron precipitated in the main ovals and polar caps. Echelle spectra between 1216 and 1220 Angstrom at 0.07 Angstrom resolution are also compared with the model fitting best the mid-resolution spectra. It is found that the effective H-2 rovibrational temperature associated with the echelle spectra are significantly higher than predicted by the mid-latitude model. A large vertical temperature gradient just above the methane homopause due to large heating by auroral precipitation is a plausible explanation for this difference. (C) 2000 Academic Press. [less ▲]

Far ultraviolet Observations of Jovian low latitude regions with HST/STIS

in Bulletin of the American Astronomical Society (1999, September 01), 30(11),

Far ultraviolet observations of the Jovian disk were made at low and mid-latitudes with FUV MAMA/STIS on board HST in January 1999 both in the imaging and spectroscopic modes. An image was obtained with ... [more ▼]

Far ultraviolet observations of the Jovian disk were made at low and mid-latitudes with FUV MAMA/STIS on board HST in January 1999 both in the imaging and spectroscopic modes. An image was obtained with the Lyalpha filter in the hydrogen bulge region for comparison with the expected Lyman-alpha brightness distribution for Ly-alpha resonance scattering. Other images in the 1200-1700 { Angstroms} region show band structures parallel to the equator with fading contrast toward the center and the limb. Spectroscopic observations were made in the 1200-1700 { Angstroms} (G140L) and 1245-1298 { Angstroms} (G140M) regions at ~ 5 { Angstroms} resolution to map the H_2 airglow and the UV absorbents along the STIS slit. Preliminary results indicate that a C_2H_2 absorption signature is clearly observed in the solar ultraviolet reflected spectrum. The ethylene absorption may be mapped to derive variations of the acetylene abundance. The H_2 FUV airglow shows both the fluorescence and the electron impact components. Its spatial variation is described and compared with the expected airglow distribution. We acknowledge funding by NASA and by the PRODEX program of the European space agency. [less ▲]

FUV spectroscopy of the H_2 emission in the Jovian aurora: model update and results

in Bulletin of the American Astronomical Society (1998, September 01)

The Jovian aurora exhibits bright emissions mainly due the bright Lyman-alpha line and to radiation from excited singlet states to the ground electronic state extending from 800 to 1600 Angstroms. Above ... [more ▼]

The Jovian aurora exhibits bright emissions mainly due the bright Lyman-alpha line and to radiation from excited singlet states to the ground electronic state extending from 800 to 1600 Angstroms. Above 1200 Angstroms, the molecular spectrum is dominated by the Lyman (B-X) bands and continuum and the Werner (C-X) bands. These transitions have been observed with the Goddard High Resolution Spectrometer (GHRS) and the Space Telescope Imaging Spectrometer (STIS) at various spectral and spatial resolutions. To analyze these spectra, a model synthetic auroral spectrum has been constructed and applied to the analysis of the ro-vibrational temperature of H_2 and a search for non-H_2 emission features. It has been recently updated to include the latest singlet state excitation and cascading cross sections, self-absorption in optically thick lines and to account for the energy distribution of the secondary electrons which cause additional excitation. We illustrate these effects in a few study cases and apply the model to high resolution (0.5 Angstroms) GHRS spectra and low resolution STIS spectra of the entire H_2 spectrum at wavelengths longer than Lyman-alpha. We determine the methane column overlying the auroral emission peak and find that a better fit is obtained with additional absorption by acetylene. [less ▲]

HST spectra of the Jovian ultraviolet aurora: Search for heavy ion precipitation

in Astrophysical Journal (1998), 507(2), 955-967

Ultraviolet spectra using Hubble Space Telescope sampled between 1250 and 1680 Angstrom, at spectral resolution less than or equal to 0.57 Angstrom are reported for characteristically bright regions of ... [more ▼]

Ultraviolet spectra using Hubble Space Telescope sampled between 1250 and 1680 Angstrom, at spectral resolution less than or equal to 0.57 Angstrom are reported for characteristically bright regions of Jupiter's morning and afternoon northern aurora. Several observed spectra exhibit sharply enhanced resolution. We interpret this as bright auroral emission foreshortened on the morning limb with a maximum intensity at least as high as 2000 kR. We have searched for evidence that the primary precipitating particles exciting the aurora include the heavy ions known to exist in Jupiter's plasma torus and magnetosphere. We have also searched for such ambient heavy ions and neutrals at rest in the auroral ionosphere, the end products of previous precipitation, excited by the auroral cascade. We argue that primary emission would be characterized by a dramatically Doppler-broadened (similar to 10-15 Angstrom) and redshifted line profile resulting from the cascade process and the angle between the line of sight and the magnetic field lines in the atmosphere. In contrast, ambient emission would be distinguished by narrow emission lines. We have modeled the theoretical sulfur and oxygen line shapes for ion precipitation and conclude that electron precipitation is responsible for most of the H-2 emissions. O ions contributed <13% of the precipitating energy flux, and S ions contributed < 50%. This dominance suggests that field-aligned magnetospheric currents are more important than energetization of energetic ions and subsequent scattering by plasma waves as a mechanism for generating the Jovian aurora. We set an upper limit over our spectra of 35-43 R to the emission from ambient oxygen and sulfur ions and their neutrals, except that for the S II 1256 triplet, the upper limit for the nominally brightest line, at 1260 Angstrom, is 74 R. Hence, we find no evidence for the accumulation of sulfur in the auroral ionosphere. A single narrow emission line from an unidentified ambient specie near 1254 Angstrom may be detected at the 4 sigma level, introducing the possibility of complex auroral aeronomy. Differences were observed in the auroral spectral hydrocarbon absorption at different locations, which cannot be interpreted without ambiguity between auroral and atmospheric structural causes. We have found that the brighter emission in an auroral sector consistently shows more spectral hydrocarbon absorption than the dimmer emission. We suggest two alternative physical explanations for this phenomenon. [less ▲]

Observations of short time scale variability of the Jovian UV aurora and simulation of morphological patterns

in Bulletin of the American Astronomical Society (1997, July 01)

A database of far ultraviolet auroral images collected with the Faint Object Camera and Wide Field Planetary Camera (WFPC2) on board the Hubble Space Telescope has been constructed over the last five ... [more ▼]

A database of far ultraviolet auroral images collected with the Faint Object Camera and Wide Field Planetary Camera (WFPC2) on board the Hubble Space Telescope has been constructed over the last five years. Some morphological features are recurrent but significant time variations are also observed. A set of WFPC2 images obtained in May 1997 shows that, within a relatively stable general morphology, variations occur inside the polar cap in 4 minutes or less, implying short timescale acceleration processes. A model simulating Earth view of auroral arcs and diffuse emissions in the north polar region has been developed. Simple geometric cases are described to illustrate the dependence on the altitude, atmospheric scale height and central meridian planetary longitude of an idealized auroral morphology seen from Earth orbit. The numerical simulation makes it possible to assess the importance of limb brightening and the contribution from high altitude auroral emission located behind the planetary limb. As an application, four images obtained with WFPC2 are used to determine the characteristics of their auroral (discrete and diffuse) structures. The apparent brightness distribution along the arcs may only be reproduced if intrinsic longitudinal (or local time) variations are introduced, in addition to the path length effects of the viewing geometry. [less ▲]

Characterization and Numerical Simulation of the Jovian UV Aurora Observed with the HST Cameras

in Science with the Hubble Space Telescope - II (1996)

Not Available

The morphology and energetics of the Jovian and Saturnian UV aurora: HST observations and modelling

Conference (1995, July 02)

SIMULTANEOUS OBSERVATIONS OF THE SATURNIAN AURORA AND POLAR HAZE WITH THE HST/FOC

in Geophysical Research Letters (1995), 22(20), 2685-2688

Near simultaneous observations of the Saturnian H-2 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 ... [more ▼]

Near simultaneous observations of the Saturnian H-2 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 degrees N with a brightness of about 150 kR of total H-2 emission. The maximum vertical optical depth at 210 nm is found to be located similar to 5 degrees equatorward of the auroral emission zone. The haze particles are presumably formed by hydrocarbon aerosols initiated by H-2(+) auroral production. In this case, the 3 x 10(10) W of H-2 emission observed with the FOG, 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. [less ▲]

Ultraviolet imaging of temporal variation of the north Jovian aurora with the Hubble Space Telescope

Conference (1994, April 25)