arXiv subjects

Darrell F. Strobel

Publications and source records attributed to Darrell F. Strobel.

15 recordsLinked to original sources

arXiv2022

Alternating north-south brightness ratio of Ganymede's auroral ovals: Hubble Space Telescope observations around the Juno PJ34 flyby

We report results of Hubble Space Telescope observations from Ganymede's orbitally trailing side which were taken around the flyby of the Juno spacecraft on June 7, 2021. We find that Ganymede's northern and southern auroral ovals alternate in brightness such that the oval facing Jupiter's magnetospheric plasma sheet is brighter than the other one. This suggests that the generator that powers Ganymede's aurora is the momentum of the Jovian plasma sheet north and south of Ganymede's magnetosphere. Magnetic coupling of Ganymede to the plasma sheet above and below the moon causes asymmetric magnetic stresses and electromagnetic energy fluxes ultimately powering the auroral acceleration process. No clear statistically significant time variability of the auroral emission on short time scales of 100s could be resolved. We show that electron energy fluxes of several tens of mW m$^{-2}$ are required for its OI 1356 Å$\;$ emission making Ganymede a very poor auroral emitter.

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arXiv2022

Mapping the Brightness of Ganymede's Ultraviolet Aurora using Hubble Space Telescope Observations

We analyze Hubble Space Telescope (HST) observations of Ganymede made with the Space Telescope Imaging Spectrograph (STIS) between 1998 and 2017 to generate a brightness map of Ganymede's oxygen emission at 1356 A. Our Mercator projected map demonstrates that the brightness along Ganymede's northern and southern auroral ovals strongly varies with longitude. To quantify this variation around Ganymede, we investigate the brightness averaged over 36$^{\circ}$-wide longitude corridors centered around the sub-Jovian (0$^{\circ}$ W), leading (90$^{\circ}$ W), anti-Jovian (180$^{\circ}$ W), and trailing (270$^{\circ}$ W) central longitudes. In the northern hemisphere, the brightness of the auroral oval is 3.7 $\pm$ 0.4 times lower in the sub-Jovian and anti-Jovian corridors compared to the trailing and leading corridors. The southern oval is overall brighter than the northern oval, and only 2.5 $\pm$ 0.2 times fainter on the sub- and anti-Jovian corridors compared to the trailing and leading corridors. This demonstrates that Ganymede's auroral ovals are strongly structured in auroral crescents on the leading side (plasma downstream side) and on the trailing side (plasma upstream side). We also find that the brightness is not symmetric with respect to the 270$^\circ$ meridian, but shifted by $\sim$20$^\circ$ towards the Jovian-facing hemisphere. Our map will be useful for subsequent studies to understand the processes that generate the aurora in Ganymede's non-rotationally driven, sub-Alfvénic magnetosphere.

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arXiv2021

Brown dwarfs as ideal candidates for detecting UV aurora outside the Solar System: Hubble Space Telescope observations of 2MASS J1237+6526

Context: Observations of auroral emissions are powerful means to remotely sense the space plasma environment around planetary bodies and ultracool dwarfs. Therefore successful searches and characterization of aurorae outside the Solar System will open new avenues in the area of extrasolar space physics. Aims: We aim to demonstrate that brown dwarfs are ideal objects to search for UV aurora outside the Solar System. We specifically search for UV aurora on the late-type T6.5 brown dwarf 2MASS J12373919+6526148 (in the following 2MASS J1237+6526). Methods: Introducing a parameter referred to as auroral power potential, we derive scaling models for auroral powers for rotationally driven aurora applicable to a broad range of wavelengths. We also analyze Hubble Space Telescope observations obtained with the STIS camera at near-UV, far-UV, and Ly-$α$ wavelengths of 2MASS J1237+6526. Results: We show that brown dwarfs, due to their typically strong surface magnetic fields and fast rotation, can produce auroral UV powers on the order of 10$^{19}$ watt or more. Considering their negligible thermal UV emission, their potentially powerful auroral emissions make brown dwarfs ideal candidates for detecting extrasolar aurorae. We find possible emission from 2MASS J1237+6526, but cannot conclusively attribute it to the brown dwarf due to low signal-to-noise values in combination with nonsystematic trends in the background fluxes. The observations provide upper limits for the emission at various UV wavelength bands. The upper limits for the emission correspond to a UV luminosity of $\sim$1 $\times$ 10$^{19}$ watt, which lies in the range of the theoretically expected values. Conclusions: The possible auroral emission from the dwarf could be produced by a close-in companion and/or magnetospheric transport processes.

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arXiv2020

Pluto's Ultraviolet Spectrum, Surface Reflectance, and Airglow Emissions

During the New Horizons spacecraft's encounter with Pluto, the Alice ultraviolet spectrograph conducted a series of observations that detected emissions from both the interplanetary medium (IPM) and Pluto. In the direction of Pluto, the IPM was found to be 133.4$\pm$0.6R at Lyman $α$, 0.24$\pm$0.02R at Lyman $β$, and <0.10R at He I 584Å. We analyzed 3,900s of data obtained shortly before closest approach to Pluto and detect airglow emissions from H I, N I, N II, N$_2$, and CO above the disk of Pluto. We find Pluto's brightness at Lyman $α$ to be $29.3\pm1.9$R, in good agreement with pre-encounter estimates. The detection of the N II multiplet at 1085Å marks the first direct detection of ions in Pluto's atmosphere. We do not detect any emissions from noble gasses and place a 3$σ$ upper limit of 0.14 R on the brightness of the Ar I 1048Å line. We compare pre-encounter model predictions and predictions from our own airglow model, based on atmospheric profiles derived from the solar occultation observed by New Horizons, to the observed brightness of Pluto's airglow. Although completely opaque at Lyman $α$, Pluto's atmosphere is optically thin at wavelengths longer than 1425Å. Consequently, a significant amount of solar FUV light reaches the surface, where it can participate in space weathering processes. From the brightness of sunlight reflected from Pluto, we find the surface has a reflectance factor (I/F) of 17% between 1400-1850Å. We also report the first detection of an C$_3$ hydrocarbon molecule, methylacetylene, in absorption, at a column density of ~5$\times10^{15}$ cm$^{-2}$, corresponding to a column-integrated mixing ratio of $1.6\times10^{-6}$.

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arXiv2018

The Lyman-α Sky Background as Observed by New Horizons

Recent observations of interplanetary medium (IPM) atomic hydrogen Lyman-α (Lyα) emission in the outer solar system, made with the Alice ultraviolet spectrograph on New Horizons (NH), are presented. The observations include regularly spaced great-circle scans of the sky and pointed observations near the downstream and upstream flow directions of interstellar H atoms. The NH Alice data agree very well with the much earlier Voyager UVS results, after these are reduced by a factor of 2.4 in brightness, in accordance with recent re-analyses. In particular, the falloff of IPM Lyα brightness in the upstream-looking direction as a function of spacecraft distance from the Sun is well-matched by an expected 1/r dependence, but with an added constant brightness of ~40 Rayleighs. This additional brightness is a possible signature of the hydrogen wall at the heliopause or of a more distant background. Ongoing observations are planned at a cadence of roughly twice per year.

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arXiv2018

The UV spectrum of the Ultracool Dwarf LSR J1835+3259 observed with the Hubble Space Telescope

An interesting question about ultracool dwarfs recently raised in the literature is whether their emission is purely internally driven or partially powered by external processes similar to planetary aurora known from the solar system. In this work we present Hubble Space Telescope observations of the energy fluxes of the M8.5 ultracool dwarf LSR J1835+3259 throughout the UV. The obtained spectra reveal that the object is generally UV-fainter compared to other earlier-type dwarfs. We detect the \ion{Mg}{2} doublet at 2800 \A and constrain an average flux throughout the Near-UV. In the Far-UV without Lyman alpha, the ultracool dwarf is extremely faint with an energy output at least a factor of 1000 smaller as expected from auroral emission physically similar to that on Jupiter. We also detect the red wing of the Lyman alpha emission. Our overall finding is that the observed UV spectrum of LSR J1835+3259 resembles the spectrum of mid/late-type M-dwarf stars relatively well, but it is distinct from a spectrum expected from Jupiter-like auroral processes.

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arXiv2017

Structure and Composition of Pluto's atmosphere from the New Horizons Solar Ultraviolet Occultation

The Alice instrument on NASA's New Horizons spacecraft observed an ultraviolet solar occultation by Pluto's atmosphere on 2015 July 14. The transmission vs. altitude was sensitive to the presence of N2, CH4, C2H2, C2H4, C2H6, and haze. We derived line-of-sight abundances and local number densities for the 5 molecular species, and line-of-sight optical depth and extinction coefficients for the haze. We found the following major conclusions: 1) We confirmed temperatures in Pluto's upper atmosphere that were colder than expected before the New Horizons flyby, with upper atmospheric temperatures near 65-68 K. The inferred enhanced Jeans escape rates were (3e22-7e22) N2/s and (4e25-8e25) CH4/s at the exobase (at a radius of ~2900 km, or an altitude of ~1710 km). 2) We measured CH4 abundances from 80 to 1200 km above the surface. A joint analysis of the Alice CH4 and Alice and REX N2 measurements implied a very stable lower atmosphere with a small eddy diffusion coefficient, most likely between 550 and 4000 cm2/s. Such a small eddy diffusion coefficient placed the homopause within 12 km of the surface, giving Pluto a small planetary boundary layer. The inferred CH4 surface mixing ratio was ~0.28-0.35%. 3) The abundance profiles of the C2Hx hydrocarbons (C2H2, C2H4, C2H6) were not simply exponential with altitude. We detected local maxima in line-of-sight abundance near 410 km altitude for C2H4, near 320 km for C2H2, and an inflection point or the suggestion of a local maximum at 260 km for C2H6. We also detected local minima near 200 km altitude for C2H4, near 170 km for C2H2, and an inflection point or minimum near 170-200 km for C2H6. These compared favorably with models for hydrocarbon production near 300-400 km and haze condensation near 200 km, especially for C2H2 and C2H4 (Wong et al. 2017). 4) We found haze that had an extinction coefficient approximately proportional to N2 density.

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arXiv2017

Haze in Pluto's Atmosphere

Haze in Pluto's atmosphere was detected in images by both the Long Range Reconnaissance Imager (LORRI) and the Multispectral Visible Imaging Camera (MVIC) on New Horizons. LORRI observed haze up to altitudes of at least 200 km above Pluto's surface at solar phase angles from ~20° to ~169°. The haze is structured with about ~20 layers, and the extinction due to haze is greater in the northern hemisphere than at equatorial or southern latitudes. However, more haze layers are discerned at equatorial latitudes. A search for temporal variations found no evidence for motions of haze layers (temporal changes in layer altitudes) on time scales of 2 to 5 hours, but did find evidence of changes in haze scale height above 100 km altitude. An ultraviolet extinction attributable to the atmospheric haze was also detected by the ALICE ultraviolet spectrograph on New Horizons. The haze particles are strongly forward-scattering in the visible, and a microphysical model of haze is presented which reproduces the visible phase function just above the surface with 0.5 μm spherical particles, but also invokes fractal aggregate particles to fit the visible phase function at 45 km altitude and account for UV extinction. A model of haze layer generation by orographic excitation of gravity waves is presented. This model accounts for the observed layer thickness and distribution with altitude. Haze particles settle out of the atmosphere and onto Pluto's surface, at a rate sufficient to alter surface optical properties on seasonal time scales. Pluto's regional scale albedo contrasts may be preserved in the face of the haze deposition by atmospheric collapse.

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arXiv2016

Saturn Variable Thermosphere

Our knowledge of Saturns neutral thermosphere is far superior to that of the other giant planets due to Cassini Ultraviolet Imaging Spectrograph (UVIS) observations of 15 solar occultations and 26 stellar occultations analyzed to date. These measurements yield H2 as the dominant species with an upper limit on the H mole fraction of 5 %. Inferred temperatures near the lower boundary are ~ 150 K, rising to an asymptotic value of ~ 400K at equatorial latitudes and increasing with latitude to polar values in the range of 550-600 K. The latter is consistent with a total estimated auroral power input of ~ 10TW generating Joule and energetic particle heating of ~ 5-6TW that is more than an order of magnitude greater than solar EUV/FUV heating. This auroral heating would be sufficient to solve the energy crisis of Saturns thermospheric heating, if it can be efficiently redistributed to low latitudes. The inferred structure of the thermosphere yields poleward directed pressure gradients on equipotential surfaces consistent with auroral heating and poleward increasing temperatures. A gradient wind balance aloft with these pressure gradients implies westward, retrograde winds ~ 500 m/s or Mach number ~ 0.3 at mid-latitudes. The occultations reveal an expansion of the thermosphere peaking at or slightly after equinox, anti-correlated with solar activity, and apparently driven by lower thermospheric heating of unknown cause. The He mole fraction remains unconstrained as no Cassini UVIS He 58.4 nm airglow measurements have been published.

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arXiv2016

The Atmosphere of Pluto as Observed by New Horizons

Observations made during the New Horizons flyby provide a detailed snapshot of the current state of Pluto's atmosphere. While the lower atmosphere (at altitudes <200 km) is consistent with ground-based stellar occultations, the upper atmosphere is much colder and more compact than indicated by pre-encounter models. Molecular nitrogen (N$_2$) dominates the atmosphere (at altitudes <1800 km or so), while methane (CH$_4$), acetylene (C$_2$H$_2$), ethylene (C$_2$H$_4$), and ethane (C$_2$H$_6$) are abundant minor species, and likely feed the production of an extensive haze which encompasses Pluto. The cold upper atmosphere shuts off the anticipated enhanced-Jeans, hydrodynamic-like escape of Pluto's atmosphere to space. It is unclear whether the current state of Pluto's atmosphere is representative of its average state--over seasonal or geologic time scales.

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arXiv2011

HST/ACS Observations of Europa's Atmospheric UV Emission at Eastern Elongation

We report results of a Hubble Space Telescope (HST) campaign with the Advanced Camera for Surveys to observe Europa at eastern elongation, i.e. Europa's leading side, on 2008 June 29. With five consecutive HST orbits, we constrain Europa's atmospheric \ion{O}{1} 1304 \A and \ion{O}{1} 1356 \A emissions using the prism PR130L. The total emissions of both oxygen multiplets range between 132 $\pm$ 14 and 226 $\pm$ 14 Rayleigh. An additional systematic error with values on the same order as the statistical errors may be due to uncertainties in modelling the reflected light from Europa's surface. The total emission also shows a clear dependence of Europa's position with respect to Jupiter's magnetospheric plasma sheet. We derive a lower limit for the O$_2$ column density of 6 $\times$ 10$^{18}$ m$^{-2}$. Previous observations of Europa's atmosphere with STIS in 1999 of Europa's trailing side show an enigmatic surplus of radiation on the anti-Jovian side within the disk of Europa. With emission from a radially symmetric atmosphere as a reference, we searched for an anti-Jovian vs sub-Jovian asymmetry with respect to the central meridian on the leading side, and found none. Likewise, we searched for departures from a radially symmetric atmospheric emission and found an emission surplus centered around 90 degree west longitude, for which plausible mechanisms exist. Previous work about the possibility of plumes on Europa due to tidally-driven shear heating found longitudes with strongest local strain rates which might be consistent with the longitudes of maximum UV emissions. Alternatively, asymmetries in Europa's UV emission can also be caused by inhomogeneous surface properties, inhomogeneous solar illuminations, and/or by Europa's complex plasma interaction with Jupiter's magnetosphere.

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arXiv2011

Observations and modeling of H_2 fluorescence with partial frequency redistribution in giant planet atmospheres

Partial frequency redistribution (PRD), describing the formation of the line profile, has negligible observational effects for optical depths smaller than ~10^3, at the resolving power of most current instruments. However, when the spectral resolution is sufficiently high, PRD modeling becomes essential in interpreting the line shapes and determining the total line fluxes. We demonstrate the effects of PRD on the H_2 line profiles observed at high spectral resolution by the Far-Ultraviolet Spectroscopic Explorer (FUSE) in the atmospheres of Jupiter and Saturn. In these spectra, the asymmetric shapes of the lines in the Lyman (v"- 6) progression pumped by the solar Ly-beta are explained by coherent scattering of the photons in the line wings. We introduce a simple computational approximation to mitigate the numerical difficulties of radiative transfer with PRD, and show that it reproduces the exact radiative transfer solution to better than 10%. The lines predicted by our radiative transfer model with PRD, including the H_2 density and temperature distribution as a function of height in the atmosphere, are in agreement with the line profiles observed by FUSE. We discuss the observational consequences of PRD, and show that this computational method also allows us to include PRD in modeling the continuum pumped H_2 fluorescence, treating about 4000 lines simultaneously.

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arXiv2004

Detection of Atomic Chlorine in Io's Atmosphere with HST/GHRS

We report the detection of atomic chlorine emissions in the atmosphere of Io using Hubble Space Telescope observations with the Goddard High Resolution Spectrograph (GHRS). The Cl I 1349 A dipole allowed and Cl I] 1386 A forbidden transition multiplets are detected at a signal to noise ratio (SNR) of 6 and 10, respectively, in a combined GHRS spectrum acquired from 1994 through 1996. Oxygen and sulfur emissions are simultaneously detected with the chlorine which allows for self-consistent abundance ratios of chlorine to these other atmospheric species. The disk averaged ratios are: Cl/O = 0.017 +/- 0.008, Cl/S = 0.10 +/- 0.05, and S/O = 0.18 +/- 0.08. We also derive a geometric albedo of 1.0 +/- 0.4 % for Io at 1335 A assuming an SO2 atmospheric column density of 1x10^{16} cm^{-2}.

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arXiv2003

The Far Ultraviolet Spectrum of the Io Plasma Torus

The spectrum of the Io plasma torus in the range 905 - 1187 A was recorded at 0.26 A resolution by the Far Ultraviolet Spectroscopic Explorer (FUSE) on 2001 January 14. Five orbits of data were obtained with the west ansa of the torus centered and tracked in the 30" x 30" apertures of the FUSE spectrographs for a total observation time of 9740 seconds. This region of the spectrum is dominated by transitions of S II, S III and S IV, whose multiplet structure is nearly completely resolved. We confirm our earlier detection of emission from resonance multiplets of Cl III and Cl II and derive an abundance of Cl^{+2} of 2.1% relative to S^{2+}, leading to an overall chlorine ion abundance in the torus of slightly less than 1%. A number of features near 990 A remain unidentified, and C III at 977 A is detected in two independent channels at the 3-sigma level. The inferred relative ion abundance of C^{2+} relative to S^{2+} is 3.7 x 10^{-4}. We also present spectra at 0.085 A resolution taken on 2001 October 19 and 21 with the 4" x 20" aperture. In these spectra the observed lines are resolved and their widths correspond to ion temperatures of 60 - 70 eV for all three sulfur ions.

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arXiv2000

HST/STIS Ultraviolet Imaging of Polar Aurora on Ganymede

We report new observations of the spectrum of Ganymede in the spectral range 1160 - 1720 A made with the Space Telescope Imaging Spectrograph (STIS) on HST on 1998 October 30. The observations were undertaken to locate the regions of the atomic oxygen emissions at 1304 and 1356 A, previously observed with the GHRS on HST, that Hall et al. (1998) claimed indicated the presence of polar aurorae on Ganymede. The use of the 2" wide STIS slit, slightly wider than the disk diameter of Ganymede, produced objective spectra with images of the two oxygen emissions clearly separated. The OI emissions appear in both hemispheres, at latitudes above 40 degrees, in accordance with recent Galileo magnetometer data that indicate the presence of an intrinsic magnetic field such that Jovian magnetic field lines are linked to the surface of Ganymede only at high latitudes. Both the brightness and relative north-south intensity of the emissions varied considerably over the four contiguous orbits (5.5 hours) of observation, presumably due to the changing Jovian plasma environment at Ganymede. However, the observed longitudinal non-uniformity in the emission brightness at high latitudes, particularly in the southern hemisphere, and the lack of pronounced limb brightening near the poles are difficult to understand with current models. In addition to observed solar HI Lyman-alpha reflected from the disk, extended Lyman-alpha emission resonantly scattered from a hydrogen exosphere is detected out to beyond two Ganymede radii from the limb, and its brightness is consistent with the Galileo UVS measurements of Barth et al. (1997).

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