Spatially Resolved Far Ultraviolet Spectroscopy of the Jovian Aurora

doi:10.1006/icar.2001.6784

Icarus

Volume 157, Issue 1, May 2002, Pages 91-103

https://doi.org/10.1006/icar.2001.6784 Get rights and content

Abstract

Spatially resolved spectra in four 50-Å FUV spectral windows were obtained across the jovian aurora with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope. Nearly simultaneous ultraviolet imaging makes it possible to correlate the intensity variations along the STIS slit with those observed in the images and to characterize the global auroral context prevailing at the time of the observations. Spectra at ∼1-Å resolution taken in pairs included an unabsorbed window and a spectral region affected by hydrocarbon absorption. Both sets of spectra correspond to an aurora with a main oval brightness of about 130 kilorayleighs of H₂ emission. The far ultraviolet color ratios I(1550–1620 Å)/I(1230–1300 Å) are 2.3 and 5.9 for the noon and morning sectors of the main oval, respectively. We use an interactive model coupling the energy degradation of incoming energetic electrons, auroral temperature and composition, and synthetic H₂ spectra to fit the intensity distribution of the H₂ lines. It is found that the model best fitting globally the spectra has a soft energy component in addition to a 10 erg cm⁻² s⁻¹ flux of 80 keV electrons. It provides an effective H₂ temperature of 540 K. The relative intensity of temperature-sensitive H₂ lines indicates differences between the auroral main oval and polar cap emissions. The amount of methane absorption across the polar region is shown to vary in a way consistent with temperature. For the second spectral pair, the polar cap shows a higher attenuation by CH₄, indicating a harder precipitation along high-latitude magnetic field lines.

References (25)

J.M Ajello et al.
Spectroscopic evidence for high-altitude aurora at Jupiter from Galileo Extreme Ultraviolet Spectrometer and Hopkins Ultraviolet Telescope observations
J. Geophys. Res.
(2001)
V Dols et al.
Diagnostics of the jovian aurora deduced from ultraviolet spectroscopy: Model and GHRS observations
Icarus
(2000)
G.R Gladstone et al.
Hydrocarbon photochemistry in the upper atmosphere of Jupiter
Icarus
(1996)
W Harris et al.
Analysis of jovian auroral H Ly-α emission (1981–1991)
Icarus
(1996)
Y.H Kim et al.
Temperatures and altitudes of Jupiter's ultraviolet aurora inferred from GHRS observations with the Hubble Space Telescope
Icarus
(1997)
H.A Lam et al.
A baseline spectroscopic study of the infrared auroras of Jupiter
Icarus
(1997)
H Abgrall et al.
The emission continuum of electron-excited molecular hydrogen
Astrophys. J.
(1997)
J.M Ajello et al.
Galileo orbiter ultraviolet observations of Jupiter aurora
J. Geophys. Res.
(1998)
D.V Bisikalo et al.
The distribution of hot hydrogen atoms produced by electron and proton precipitation in the jovian aurora
J. Geophys. Res.
(1996)
J.T Clarke et al.
Hubble Space Telescope Goddard High-Resolution Spectrograph H₂ rotational spectra of Jupiter's aurora
Astrophys. J.
(1994)

P Drossart et al.

Line-resolved spectroscopy of the Jovian H₃⁺ auroral emission at 3.5 micrometers

Astrophys. J.

(1993)

J.-C Gérard et al.

The longitudinal variation of the color ratio of the jovian ultraviolet aurora: A geometric effect?

Geophys. Res. Lett.

(1998)

Cited by (24)

Statistical study of Saturn's auroral electron properties with Cassini/UVIS FUV spectral images
2017, Icarus
Citation Excerpt :
At Saturn, only methane has been clearly detected in previous studies. The auroral emission is simulated either with a synthetic model of H2 (e.g. Gustin et al., 2002, 2009) or with a laboratory spectrum resulting from impact of 100 eV mono-energetic electrons on H2 (Dziczek et al., 2000; Ajello et al., 2005). It should be noted that this value of 100 eV simulates the energy of typical secondary electrons: the energy-dependent excitation cross-section curves of the B and C H2 electronic states are very similar for exciting electrons above 30 eV, meaning that the shape of the H2 spectrum is only moderately influenced by the electron energy variations above this value.
About 2000 FUV spectra of different regions of Saturn's aurora, obtained with Cassini/UVIS from December 2007 to October 2014 have been examined. Two methods have been employed to determine the mean energy 〈E〉 of the precipitating electrons. The first is based on the absorption of the auroral emission by hydrocarbons and the second uses the ratio between the brightness of the Lyman-α line and the H₂ total UV emission (Lyα/H₂), which is directly related to 〈E〉 via a radiative transfer formalism. In addition, two atmospheric models obtained recently from UVIS polar occultations have been employed for the first time. It is found that the atmospheric model related to North observations near 70° latitude provides the results most consistent with constraints previously published.
On a global point of view, the two methods provide comparable results, with 〈E〉 mostly in the 7–17 keV range with the hydrocarbon method and 〈E〉 in the 1–11 keV range with the Lyα/H₂ method. Since hydrocarbons have been detected on ∼20% of the auroral spectra, the Lyα/H₂ technique is more effective to describe the primary auroral electrons, as it is applicable to all spectra and allows an access to the lowest range of energies (≤5 keV), unreachable by the hydrocarbon method. The distribution of 〈E〉 is found fully compatible with independent HST/ACS constraints (emission peak in the 840–1450 km range) and FUSE findings (emission peaking at pressure level ≤0.2 µbar). In addition, 〈E〉 exhibits enhancements in the 3 LT–10 LT sector, consistent with SKR intensity measurements.
An energy flux–electron energy diagram built from all the data points strongly suggests that acceleration by field-aligned potentials as described by Knight's theory is a main mechanism responsible for electron precipitation creating the aurora. Assuming a fixed electron temperature of 0.1 keV, a best-fit equatorial electron source population density of 3 × 10³ m⁻³ is derived, which matches very well to the plasma properties observed with Cassini MAG and CAPS/ELS instruments. However, several auroral regions are characterized by relatively high 〈E〉 and low energy flux, suggesting that additional processes such as plasma injections or magnetic reconnections must be accounted for to explain the emission in these regions.
The Lyα/H₂ ratio technique can be used to build maps of 〈E〉 from single spectral images. As expected, preliminary results show that the spatial distribution of 〈E〉 is not uniform, as seen on Jupiter.
Our study reveals that a fraction of the aurora is due to very low energy electrons (<1 keV). Even in this case, comparisons between observed and modeled spectra show that 100 eV is a suitable value to represent the average energy of the secondary electrons.
Characteristics of north jovian aurora from STIS FUV spectral images
2016, Icarus
Citation Excerpt :
First, the limited number of spectral observations available has been obtained through the long 1997–2009 period and thus reflects various solar and magnetospheric–ionospheric conditions, which make it difficult to directly compare different measurements. Second, the observed spectra either cover a limited portion of the aurora due to the small size of the aperture projected onto the planet (e.g. Gladstone and Skinner, 1989 with the International Ultraviolet Explorer, Dols et al. (2000) with the Goddard High resolution spectrograph onboard HST, Gustin et al. (2002, 2004a, 2006) and Gérard et al. (2002, 2003) with the HST/STIS in the spectroscopic mode), or integrated the whole polar region without spatial resolution (e.g. observations with the Far Ultraviolet Spectroscopic Explorer by Gustin et al., 2004b). Such former spectroscopic observations thus do not allow us to draw a global picture of the processes responsible for polar auroras.
We analyzed two observations obtained in Jan. 2013, consisting of spatial scans of the jovian north ultraviolet aurora with the HST Space Telescope Imaging Spectrograph (STIS) in the spectroscopic mode. The color ratio (CR) method, which relates the wavelength-dependent absorption of the FUV spectra to the mean energy of the precipitating electrons, allowed us to determine important characteristics of the entire auroral region. The results show that the spatial distribution of the precipitating electron energy is far from uniform. The morning main emission arc is associated with mean energies of around 265 keV, the afternoon main emission (kink region) has energies near 105 keV, while the ‘flare’ emissions poleward of the main oval are characterized by electrons in the 50–85 keV range. A small scale structure observed in the discontinuity region is related to electrons of 232 keV and the Ganymede footprint shows energies of 157 keV. Interestingly, each specific region shows very similar behavior for the two separate observations.
The Io footprint shows a weak but undeniable hydrocarbon absorption, which is not consistent with altitudes of the Io emission profiles (∼900 km relative to the 1 bar level) determined from HST-ACS observations. An upward shift of the hydrocarbon homopause of at least 100 km is required to reconcile the high altitude of the emission and hydrocarbon absorption.
The relationship between the energy fluxes and the electron energies has been compared to curves obtained from Knight’s theory of field-aligned currents. Assuming a fixed electron temperature of 2.5 keV, an electron source population density of ∼800 m⁻³ and ∼2400 m⁻³ is obtained for the morning main emission and kink regions, respectively. Magnetospheric electron densities are lowered for the morning main emission (∼600 m⁻³) if the relativistic version of Knight’s theory is applied.
Lyman and Werner H₂ emission profiles, resulting from secondary electrons produced by precipitation of heavy ions in the 1–2 MeV/u range, have been applied to our model. The low CR obtained from this emission suggests that heavy ions, presumably the main source of the X-ray aurora, do not significantly contribute to typical UV high latitude emission.
Characteristics of Saturn's polar atmosphere and auroral electrons derived from HST/STIS, FUSE and Cassini/UVIS spectra
2009, Icarus
Citation Excerpt :
It has been found that the mean energy of the Maxwellian electronic distributions used in the Grodent et al. (2001) model and the mono-energetic electrons found in the stopping power table provide similar H2 column values, yielding the energy deposition at the same pressure level (D. Grodent, private communication). This method has been used in a slightly different form for jovian auroral studies by Dols et al. (2000), Gustin et al. (2002, 2004a, 2004b, 2006) and Gérard et al. (2002, 2003) in order to determine electron energies from hydrocarbon absorption. It was also used by Gérard et al. (2004) to study the STIS spectra of Saturn also presented here.
Ultraviolet (UV) spectra of Saturn's aurora obtained with the Hubble Space Telescope Imaging Spectrograph (STIS), the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE) have been analyzed. Comparisons between the observed spectra and synthetic models of electron-excited H₂ have been used to determine various auroral characteristics. Far ultraviolet (FUV: 1200–1700 Å) STIS and UVIS spectra exhibit, below 1400 Å, weak absorption due to methane, with a vertical column ranging between $1.4 \times 10^{15}$ and $1.2 \times 10^{16} {cm}^{- 2}$ . Using the low-latitude Moses et al. [Moses, J.I., Bézard, B., Lellouch, E., Feuchtgruber, H., Gladstone, G.R., Allen, M., 2000. Icarus, 143, 244–298] atmospheric model of Saturn and an electron energy–H₂ column relationship, these methane columns are converted into the mean energy of the primary precipitating electrons, estimated to lie in the range 10–18 keV. This result is confirmed by the study of self-absorption with UVIS and FUSE extreme ultraviolet (EUV: 900–1200 Å) spectra. Below 1200 Å, it is seen that transitions connecting to the $v^{″} < 2$ vibrational levels of the H₂ electronic ground state are partially self-absorbed by H₂ molecules overlying the auroral emission. Because of its low spectral resolution (∼5.5 Å), the UVIS EUV spectrum we analyzed does not allow us to unequivocally determine reasonable ranges of temperatures and H₂ columns. On the other hand, the high spectral resolution (∼0.2 Å) of the FUSE LiF1a and LiF2a EUV spectra we examined resolve the H₂ rotational lines and makes it possible to determine the H₂ temperature. The modeled spectrum best fitting the FUSE LiF1a observation reveals a temperature of 500 K and self-absorption by a H₂ vertical column of $3 \times 10^{19} {cm}^{- 2}$ . When converted to energy of precipitating electrons, this H₂ column corresponds to primary electrons of ∼10 keV. The model that best fits the LiF2a spectrum is characterized by a temperature of 400 K and is not self-absorbed, making this segment ideal to determine the H₂ temperature at the altitude of the auroral emission. The latter value is in agreement with temperatures obtained from $H_{3}^{+}$ infrared polar spectra. Self-absorption is detectable in the LiF2a segment for H₂ columns exceeding $6 \times 10^{19} {cm}^{- 2}$ , which sets the maximum mean energy determined from the FUSE observations to ∼15 keV. The total electron energy range of 10–18 keV deduced from FUV and EUV observations places the auroral emission peak between the 0.1 and 0.3 μbar pressure levels. These values should be seen as an upper limit, since most of the Voyager UVS spectra of Saturn's aurora examined by Sandel et al. [Sandel, B.R., Shemansky, D.E., Broadfoot, A.L., Holberg, J.B., Smith, G.R., 1982. Science 215, 548] do not exhibit methane absorption. The auroral H₂ emission is thus likely located above but close to the methane homopause. The H₂ auroral brightness in the 800–1700 Å bandwidth varies from 2.9 kR to 139 kR, comparable to values derived from FUV Faint Object Camera (FOC) and STIS images.
Cassini UVIS observations of Jupiter's auroral variability
2005, Icarus
Citation Excerpt :
An optically thin auroral spectral component due to soft electrons is required in the fit to this spectrum at EUV wavelengths. A recent electron transport model by Grodent et al. (2001) with a flux of weak (0.1 keV), soft (∼3 keV) and hard (15 keV) electrons is capable of modeling Cassini (Ajello et al., 2005), Far-Ultraviolet Spectrographic Explorer (FUSE) and HUT Jupiter auroral observations (Grodent et al., 2003; Gustin et al., 2002, 2004). Thus, there is evidence in auroral spectra from Galileo, HUT, FUSE and now Cassini for significant emission at high altitudes from a soft electron source, in addition to the main auroral component deep in the atmosphere.
The Cassini spacecraft Ultraviolet Imaging Spectrograph (UVIS) obtained observations of Jupiter's auroral emissions in H₂ band systems and H Lyman-α from day 275 of 2000 (October 1), to day 81 of 2001 (March 22). Much of the globally integrated auroral variability measured with UVIS can be explained simply in terms of the rotation of Jupiter's main auroral arcs with the planet. These arcs were also imaged by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope (HST). However, several brightening events were seen by UVIS in which the global auroral output increased by a factor of 2–4. These events persisted over a number of hours and in one case can clearly be tied to a large solar coronal mass ejection event. The auroral UV emissions from these bursts also correspond to hectometric radio emission (0.5–16 MHz) increases reported by the Galileo Plasma Wave Spectrometer (PWS) and Cassini Radio and Plasma Wave Spectrometer (RPWS) experiments. In general, the hectometric radio data vary differently with longitude than the UV data because of radio wave beaming effects. The 2 largest events in the UVIS data were on 2000 day 280 (October 6) and on 2000 days 325–326 (November 20–21). The global brightening events on November 20–21 are compared with corresponding data on the interplanetary magnetic field, solar wind conditions, and energetic particle environment. ACE (Advanced Composition Explorer) solar wind data was numerically propagated from the Earth to Jupiter with an MHD code and compared to the observed event. A second class of brief auroral brightening events seen in HST (and probably UVIS) data that last for ∼2 min is associated with auroral flares inside the main auroral ovals. On January 8, 2001, from 18:45–19:35 UT UVIS H₂ band emissions from the north polar region varied quasiperiodically. The varying emissions, probably due to auroral flares inside the main auroral oval, are correlated with low-frequency quasiperiodic radio bursts in the 0.6–5 kHz Galileo PWS data.
The Cassini Campaign observations of the Jupiter aurora by the Ultraviolet Imaging Spectrograph and the Space Telescope Imaging Spectrograph
2005, Icarus
Citation Excerpt :
Moreover, this enormous power input into the auroral zone presumably controls thermosphere global dynamics (Emerich et al., 1996) and atmospheric chemistry (Perry et al., 1999; Wong et al., 2000). The high spatial resolution capability (∼200 km) of STIS with the two-dimensional multi-anode array MAMA detectors provides FUV images that resolve the polar cap and main auroral oval (Gustin et al., 2002; Grodent et al., 2003a; 2003b). The observations for the Jupiter aurora by UVIS during the CC began in October 2000.
We have analyzed the Cassini Ultraviolet Imaging Spectrometer (UVIS) observations of the Jupiter aurora with an auroral atmosphere two-stream electron transport code. The observations of Jupiter by UVIS took place during the Cassini Campaign. The Cassini Campaign included support spectral and imaging observations by the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS). A major result for the UVIS observations was the identification of a large color variation between the far ultraviolet (FUV: 1100–1700 Å) and extreme ultraviolet (EUV: 800–1100 Å) spectral regions. This change probably occurs because of a large variation in the ratio of the soft electron flux (10–3000 eV) responsible for the EUV aurora to the hard electron flux (∼15–22 keV) responsible for the FUV aurora. On the basis of this result a new color ratio for integrated intensities for EUV and FUV was defined ( $4 π I_{1550 – 1620 Å} / 4 π I_{1030 – 1150 Å}$ ) which varied by approximately a factor of 6. The FUV color ratio ( $4 π I_{1550 – 1620 Å} / 4 π I_{1230 – 1300 Å}$ ) was more stable with a variation of less than 50% for the observations studied. The medium resolution (0.9 Å FWHM, G140M grating) FUV observations (1295–1345 Å and 1495–1540 Å) by STIS on 13 January 2001, on the other hand, were analyzed by a spectral modeling technique using a recently developed high-spectral resolution model for the electron-excited H₂ rotational lines. The STIS FUV data were analyzed with a model that considered the Lyman band spectrum (B ${}^{1}Σ_{u}^{+} \to X {}^{1}Σ_{g}^{+}$ ) as composed of an allowed direct excitation component (X ${}^{1}Σ_{g}^{+} \to B {}^{1}Σ_{u}^{+}$ ) and an optically forbidden component (X ${}^{1}Σ_{g}^{+} \to EF, GK, H \bar{H}, \dots {}^{1}Σ_{g}^{+}$ followed by the cascade transition ${}^{1}Σ_{g}^{+} \to B {}^{1}Σ_{u}^{+}$ ). The medium-resolution spectral regions for the Jupiter aurora were carefully chosen to emphasize the cascade component. The ratio of the two components is a direct measurement of the mean secondary electron energy of the aurora. The mean secondary electron energy of the aurora varies between 50 and 200 eV for the polar cap, limb and auroral oval observations. We examine a long time base of Galileo Ultraviolet Spectrometer color ratios from the standard mission (1996–1998) and compare them to Cassini UVIS, HST, and International Ultraviolet Explorer (IUE) observations.
Jovian auroral spectroscopy with FUSE: Analysis of self-absorption and implications for electron precipitation
2004, Icarus
High-resolution ( $\sim 0.22 Å$ ) spectra of the north jovian aurora were obtained in the 905–1180 Å window with the Far Ultraviolet Spectroscopic Explorer (FUSE) on October 28, 2000. The FUSE instrument resolves the rotational structure of the H₂ spectra and the spectral range allows the study of self-absorption. Below 1100 Å, transitions connecting to the $v^{″} ⩽ 2$ levels of the H₂ ground state are partially or totally absorbed by the overlying H₂ molecules. The FUSE spectra provide information on the overlying H₂ column and on the vibrational distribution of H₂. Transitions from high-energy H₂ Rydberg states and treatment of self-absorption are considered in our synthetic spectral generator. We show comparisons between synthetic and observed spectra in the 920–970, 1030–1080, and 1090–1180 Å spectral windows. In a first approach (single-layer model ), the synthetic spectra are generated in a thin emitting layer and the emerging photons are absorbed by a layer located above the source. It is found that the parameters of the single-layer model best fitting the three spectral windows are 850, 800, and 800 K respectively for the H₂ gas temperature and $1.3 \times 10^{18}$ , $1.5 \times 10^{20}$ , and $1.3 \times 10^{20} {cm}^{- 2}$ for the H₂ self-absorbing vertical column respectively. Comparison between the H₂ column and a 1-D atmospheric model indicates that the short-wavelength FUV auroral emission originates from just above the homopause. This is confirmed by the high H₂ rovibrational temperatures, close to those deduced from spectral analyses of H⁺₃ auroral emission. In a second approach, the synthetic spectral generator is coupled with a vertically distributed energy degradation model, where the only input is the energy distribution of incoming electrons (multi-layer model ). The model that best fits globally the three FUSE spectra is a sum of Maxwellian functions, with characteristic energies ranging from 1 to 100 keV, giving rise to an emission peak located at 5 μbar, that is $\sim 100 km$ below the methane homopause. This multi-layer model is also applied to a re-analysis of the Hopkins Ultraviolet Telescope (HUT) auroral spectrum and accounts for the H₂ self-absorption as well as the methane absorption. It is found that no additional discrete soft electron precipitation is necessary to fit either the FUSE or the HUT observations.

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