Jovian Aurora Research Articles

Characteristics of north jovian aurora from STIS FUV spectral images

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 265keV, the afternoon main emission (kink region) has energies near 105keV, while the ‘flare’ emissions poleward of the main oval are characterized by electrons in the 50–85keV range. A small scale structure observed in the discontinuity region is related to electrons of 232keV and the Ganymede footprint shows energies of 157keV. 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 (∼900km relative to the 1bar level) determined from HST-ACS observations. An upward shift of the hydrocarbon homopause of at least 100km 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 2.5keV, an electron source population density of ∼800m−3 and ∼2400m−3 is obtained for the morning main emission and kink regions, respectively. Magnetospheric electron densities are lowered for the morning main emission (∼600m−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 1–2MeV/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.

The far-ultraviolet main auroral emission at Jupiter – Part 1: Dawn–dusk brightness asymmetries

Abstract. The main auroral emission at Jupiter generally appears as a quasi-closed curtain centered around the magnetic pole. This auroral feature, which accounts for approximately half of the total power emitted by the aurorae in the ultraviolet range, is related to corotation enforcement currents in the middle magnetosphere. Early models for these currents assumed axisymmetry, but significant local time variability is obvious on any image of the Jovian aurorae. Here we use far-UV images from the Hubble Space Telescope to further characterize these variations on a statistical basis. We show that the dusk side sector is ~ 3 times brighter than the dawn side in the southern hemisphere and ~ 1.1 brighter in the northern hemisphere, where the magnetic anomaly complicates the interpretation of the measurements. We suggest that such an asymmetry between the dawn and the dusk sectors could be the result of a partial ring current in the nightside magnetosphere.

RADIATION FROM ELECTRON PHASE SPACE HOLES AS A POSSIBLE SOURCE OF JOVIAN S-BURSTS

Radio-frequency short burst emissions (10–40 MHz), known as Jovian S-bursts, have been observed from the Jovian aurora for over fifty years. These emissions, associated with Io’s motion, have a rapidly declining frequency and an exceptionally narrow bandwidth. While it is widely believed that S-bursts are generated by the electron cyclotron maser instability, the mechanism responsible for the rapidly declining frequency and narrow bandwidth currently is not well established. We explore a hypothesis that electron phase space holes radiate or stimulate radiation in the Jovian aurora plasma environment as a possible source of S-burst emissions. Electron phase-space holes (EHs) are ubiquitous in an auroral environment and travel at the implied speeds (∼20,000 km s−1) of the structures creating the Jovian S-bursts. Furthermore, EHs have the proper physical size to create the observed bandwidth, have sufficient energy content, and can create an environment whereby X mode emissions can be excited. If verified, these findings imply that EHs may be an important source of radiation from strongly magnetized or relativistic astrophysical plasmas.

STELLAR WIND INDUCED SOFT X-RAY EMISSION FROM CLOSE-IN EXOPLANETS

In this paper, we estimate the X-ray emission from close-in exoplanets. We show that the Solar/Stellar Wind Charge Exchange Mechanism (SWCX) which produces soft X-ray emission is very effective for hot Jupiters. In this mechanism, X-ray photons are emitted as a result of the charge exchange between heavy ions in the solar wind and the atmospheric neutral particles. In the Solar System, comets produce X-rays mostly through the SWCX mechanism, but it has also been shown to operate in the heliosphere, in the terrestrial magnetosheath, and on Mars, Venus and Moon. Since the number of emitted photons is proportional to the solar wind mass flux, this mechanism is not very effective for the Solar system giants. Here we present a simple estimate of the X-ray emission intensity that can be produced by close-in extrasolar giant planets due to charge exchange with the heavy ions of the stellar wind. Using the example of HD~209458b, we show that this mechanism alone can be responsible for an X-ray emission of $\approx 10^{22}$~erg~s$^{-1}$, which is $10^6$ times stronger than the emission from the Jovian aurora. We discuss also the possibility to observe the predicted soft X-ray flux of hot Jupiters and show that despite high emission intensities they are unobservable with current facilities.

Mapping the electron energy in Jupiter's aurora: Hubble spectral observations

AbstractFar ultraviolet spectral observations have been made with the Hubble Space Telescope in the time‐tag mode using the Space Telescope Imaging Spectrograph (STIS) long slit. The telescope was slewed in such a way that the slit projection scanned from above the polar limb down to midlatitudes, allowing us to build up the first spectral maps of the FUV Jovian aurora. The shorter wavelengths are partly absorbed by the methane layer overlying part of the auroral emission layer. The long‐wavelength intensity directly reflects the precipitated energy flux carried by the auroral electrons. Maps of the intensity ratio of the two spectral regions have been obtained by combining spectral emissions in two wavelength ranges. They show that the amount of absorption by methane varies significantly between the different components of the aurora and inside the main emission region. Some of the polar emissions are associated with the hardest precipitation, although the auroral regions of strong electron precipitation do not necessarily coincide with the highest electron energies. Outputs from an electron transport model are used to create maps of the distribution of the characteristic electron energies. Using model atmospheres adapted to auroral conditions, we conclude that electron energies range between a few tens to several hundred keV. Comparisons of derived energies are in general agreement with those calculated from magnetosphere‐ionosphere coupling models, with values locally exceeding the standard model predictions. These results will provide useful input for three‐dimensional modeling of the distribution of particle heat sources into the high‐latitude Jovian upper atmosphere.

JIRAM, the Jovian Infrared Auroral Mapper

JIRAM is an imager/spectrometer on board the Juno spacecraft bound for a polar orbit around Jupiter. JIRAM is composed of IR imager and spectrometer channels. Its scientific goals are to explore the Jovian aurorae and the planet’s atmospheric structure, dynamics and composition. This paper explains the characteristics and functionalities of the instrument and reports on the results of ground calibrations. It discusses the main subsystems to the extent needed to understand how the instrument is sequenced and used, the purpose of the calibrations necessary to determine instrument performance, the process for generating the commanding sequences, the main elements of the observational strategy, and the format of the scientific data that JIRAM will produce.

Effects of methane on giant planet’s UV emissions and implications for the auroral characteristics

This study reviews methods used to determine important characteristics of giant planet’s UV aurora (brightness, energy of the precipitating particles, altitude of the emission peak,…), based on the absorbing properties of methane and other hydrocarbons. Ultraviolet aurorae on giant planets are mostly caused by inelastic collisions between energetic magnetospheric electrons and the ambient atmospheric H2 molecules. The auroral emission is situated close to a hydrocarbon layer and may be attenuated by methane (CH4), ethane (C2H6) and acetylene (C2H2) at selected wavelengths. As methane is the most abundant hydrocarbon, it is the main UV absorber and attenuates the auroral emission shorward of 1350Å. The level of absorption is used to situate the altitude/pressure level of the aurora, hence the energy of the precipitated electrons, whose penetration depth is directly related to their mean energy. Several techniques are used to determine these characteristics, from the color ratio method which measures the level of absorption from the ratio between an absorbed and an unabsorbed portion of the observed auroral spectrum, to more realistic methods which combine theoretical distributions of the precipitating electrons with altitude dependent atmospheric models. The latter models are coupled with synthetic or laboratory H2 spectra and the simulated emergent spectra are compared to observations to determine the best auroral characteristics.Although auroral characteristics may be very variable with time and locations, several typical properties may be highlighted from these methods: the Jovian aurora is the most powerful, with brightness around 120kR produced by electrons of mean energy ∼100keV and an emission situated near the 1μbar level (∼250km above the 1bar level) while Saturn’s aurora is fainter (∼10kR), produced by electrons less than 20keV and situated near the 0.2μbar level (∼1100km).

Reconnection sites in Jupiter’s magnetotail and relation to Jovian auroras

The Galileo spacecraft explored Jupiter’s magnetotail in a low-inclination orbit, where it detected the signatures of tail reconnection. In this paper, we examine and classify the tail reconnection signatures into four types: dipolarizations, strong northward B θ excursions, tailward-moving plasmoids and planetward-moving plasmoids. The distribution of these four types of events is used to infer the most probable location of the Jovian tail reconnection site to be near 0200 LT at a planetocentric distance of 80 Jovian radii. Dipolarizations are mainly observed planetward of this point, and strong northward B θ excursions and plasmoids are found mostly tailward. The observations also suggest that the Jovian tail reconnection starts at a point (neutral point), a localized region in the tail, instead of along an extended azimuthal line (X-line). Using the updated Khurana’s Jupiter’s magnetospheric model, which includes the external field and the effects of the swept-back configuration of tail field lines, we map the signatures of Jovian tail reconnection into the Jupiter’s ionosphere. We confirm that the dawn auroral storms or the polar dawn spots observed by the Hubble Space Telescope (HST) are located close to the extrapolated footpoints of tail dipolarizations and could be the auroral signatures of tail reconnection.

H Lymanαline in Jovian aurorae: electron transport and radiative transfer coupled modelling

Aims. The aim of this work is to characterize the impact of the solar UV flux and of the electron precipitation on Jupiter's atmosphere and to study the H Lyman α intensity and spectral profile in Jupiter's aurorae. In particular we characterize the sensitivity of the line to the relevant input parameters.Methods. We use a multi-stream electron transport code solving the 1D Boltzmann equation and a Feautrier technique radiative transfer code.Results. We calculate ionization rate profiles of the main species of Jupiter's ionosphere and the heating rate due to electron precipitation, as a function of the energy of the precipitation. We also calculate the emission rate of Lyman α photons under auroral electron precipitation and the ensuing line profile. The profile exhibits a centre reversal which could be used as a diagnostic of Jupiter's auroral low-energy electron precipitation.

Search for mid-IR rotational and ν1→ ν2 difference band [formula omitted] emission in Jupiter’s northern aurora

Using TEXES at the IRTF telescope, we obtained high-resolution, mid-infrared, longslit spectra of Jupiter’s northern auroral “hot spot” near System III longitude 180° to search for selected pure-rotational and ν 1→ ν 2 difference band lines of H 3 + . Because these lines have not been investigated in the laboratory, we used theoretically predicted frequencies and line intensities to guide this search. No pure rotational H 3 + line emission was detected near the predicted frequencies. However, two metastable ν 1→ ν 2 difference band lines appear to have been marginally detected at the 68% confidence level (1 − σ). The non-detection of the pure rotational line sets a 1 − σ upper limit to the vertical column abundance of H 3 + in the northern jovian aurora of 8.4 × 10 12 cm −2, which is independent of any assumption concerning the departure of the vibrational H 3 + energy level populations from thermal equilibrium. This is consistent with the column 6 × 10 12 cm −2 inferred by Melin et al. [Melin, H., Miller, S., Stallard, T., Grodent, D., 2005. Icarus 178, 97–103] from their non-LTE H 3 + emission model, in which the fundamental band ro-vib levels are underpopulated by a factor of 6–10 relative to LTE. We find that the IR-inactive ν 1 levels of H 3 + , from which the jovian ν 1→ ν 2 difference band emission originates, are populated in thermal equilibrium. The difference band lines thus serve as proxies for the rotational lines in establishing the total auroral H 3 + column. As a result, the marginally detected 943.953 cm −1 ν 1→ ν 2 difference band line implies a vertical H 3 + column abundance in the range (4.5 ± 3.1) × 10 12 cm −2, consistent with the upper limit from the rotational line. The difference band line constrains the vibrational relaxation of the IR-active ν 2 fundamental band in Jupiter’s aurora to a factor of 4.5–7.5, with an uncertainty ±68%, which supports the model-dependent result of Melin et al.

Modeling of the Jovian Auroral Environment and Its Effects on Spacecraft Charging

A well-known concern for polar orbiting spacecraft at the Earth is spacecraft charging due to the aurora. Studies of Jupiter reveal the presence of a variety of similar auroral phenomena. In particular, three regions have been identified-a narrow auroral zone at high latitudes, a complex and variable environment over the poles, and auroralike features associated with the main jovian moons and their magnetic flux tubes. These auroral structures are, like their earthly counterparts, expected to be sources of charging. This paper reviews the observational data and models of the jovian aurora zone and polar regions. In combination with models of the jovian plasma environment, the ambient charging currents are then computed with the intention of providing realistic estimates of spacecraft potentials at Jupiter. These are of particular importance to the Juno mission as it is a Jupiter polar orbiter and will pass through the jovian auroral zones near the planet. Of special concern, unlike previous missions to Jupiter, Juno will utilize large solar arrays. While previous missions (e.g., the Voyagers and Galileo) utilized RTGs and were carefully designed to avoid differential surface charging, the Juno mission's solar arrays may make it more sensitive to surface charging. Although the jovian aurora is a possibly serious threat to Juno and similar missions, as will be discussed, an understanding of the environment and proper mitigation techniques should limit their effects.

Spectral morphology of the X‐ray emission from Jupiter's aurorae

Simultaneous Chandra X‐ray and Hubble Space Telescope FUV observations of Jupiter's aurorae carried out in February 2003 have been re‐examined to investigate the spatial morphology of the X‐ray events in different energy bands. The data clearly show that in the Northern auroral region (in the main auroral oval and the polar cap) events with energy > 2 keV are located at the periphery of those with energy < 2 keV and coincide with FUV bright features. In addition, X‐ray spectra extracted from the areas where the two event distributions are concentrated possess different shapes. We associate the > 2 keV events (∼45 MW emitted power) with the electron bremsstrahlung component recently revealed by XMM‐Newton in the spectra of Jupiter's aurorae, and the < 2 keV emission (∼230 MW) with the product of ion charge exchange, now established as the likely mechanism responsible for the soft X‐ray Jovian aurora. We suggest that the same population of energetic electrons may be responsible for both, the X‐ray bremsstrahlung and the FUV emission of Jupiter's aurorae. Comparison of the > 2 keV X‐ray and FUV (340 GW) powers measured during the observations shows that they are broadly consistent with the predicted emissions from a population of energetic electrons precipitating in the planet's atmosphere, thus supporting our interpretation.

Energy‐flux relationship in the FUV Jovian aurora deduced from HST‐STIS spectral observations

Far ultraviolet spectral observations of the Jovian aurora have been made since 1997 with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope at low spectral resolution. The combination of the spectral resolution with the intensity variation along the STIS slit provides information on the latitudinal variation of the precipitating auroral electron energy flux and the mean electron energy, from which the electron current density at the top of the atmosphere can also be deduced. It is found that the mean electron energies associated with the main oval lie in the range 30–200 keV and show a tendency to increase with the precipitating energy flux. The current densities lie in the range ∼0.04–0.4 μA m−2, consistent with previous estimates, and are also positively correlated with the energy flux. The observed relationship between the auroral time‐integrated energy fluxes and the electron energies in the main oval is compatible with that expected from Knight's theory of field‐aligned currents. The best agreement between the observed data and the Knight curves is obtained for an electron temperature of Te = 2.5 keV and a source density N = 0.003 cm−3, that is within the range of values observed in the equatorial plane during the Voyager flybys. No systematic dependence of the electron energy with magnetic local time is found, but the morning sector around 0800 MLT shows greater variability than other regions of the oval. Analysis of time‐tagged data shows that the main oval energy flux usually varies steadily over the several minute intervals of observation and that the mean electron energy usually undergoes correlated variations such that the current density remains relatively constant. It is shown that these overall properties are also consistent with Knight's theory of auroral electron acceleration associated with field‐aligned current flow, from which it is inferred that the temporal variations observed are often due to slow changes in the magnetospheric “source” electron parameters in the presence of near‐steady magnetosphere‐ionosphere coupling currents. By contrast, time‐integrated emissions in the polar region are found to be associated with similar mean electron energies to the main oval but with typically smaller energy fluxes and current densities. Pressure balance arguments are advanced, which indicate that the brighter of these emissions must be associated with an auroral acceleration mechanism perhaps similar to that operative in the main oval, while it remains possible that the weaker emissions could result from precipitation from a quasi‐isotropic hot magnetospheric electron source.

The Cassini–Huygens flyby of Jupiter

The Cassini–Huygens spacecraft flew by Jupiter on December 30, 2000. The instruments aboard the spacecraft started making scientific observations three months earlier. Joint, collaborative observations were carried out with the teams of other spacecraft, notably Galileo, and with Earth-based observers. An operational overview of the flyby is presented and attention drawn to contributions of the eleven papers of this series which follow. Prime achievements of this campaign have been to better define the present state of fundamental elements of the jovian system, confirming many previously tentative conclusions. Particularly noteworthy is that the interactions between the solar wind and the jovian magnetosphere have been explored far deeper than before, along with the link to the morphology and dynamics of the jovian aurora.

Jovian auroral spectroscopy with FUSE: analysis of self-absorption and implications for electron precipitation

High-resolution ( ∼ 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 H 2 spectra and the spectral range allows the study of self-absorption. Below 1100 Å, transitions connecting to the v ″ ⩽ 2 levels of the H 2 ground state are partially or totally absorbed by the overlying H 2 molecules. The FUSE spectra provide information on the overlying H 2 column and on the vibrational distribution of H 2. Transitions from high-energy H 2 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 H 2 gas temperature and 1.3 × 10 18 , 1.5 × 10 20 , and 1.3 × 10 20 cm − 2 for the H 2 self-absorbing vertical column respectively. Comparison between the H 2 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 H 2 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 ∼ 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 H 2 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.

Energetic electrons in the inner part of the Jovian magnetosphere and their relation to auroral emissions

The energetic particle distribution in the magnetosphere of Jupiter changes significantly between the inner and the middle magnetosphere. One of the most prominent changes is a transition of the electron pitch angle distribution (PAD) from a pancake to a bidirectional distribution. The transition is a persistent and localized feature defining a distinct spatial boundary between 10 and 17 RJ. We discuss the possible relation between the PAD boundary and some of the observed structures in the Jovian aurora. A comparison between the Hubble Space Telescope observations and the predicted ionospheric footprints of the PAD boundary indicates a good correlation, with a discrete belt of emissions equatorward of the main auroral oval. Furthermore, the precipitation energy flux associated with the energetic electron distribution at the PAD boundary is compatible with the brightness range of these auroral emissions.

Driving mechanisms of magnetospheric dynamics of planets and satellites

We review basic mechanisms of magnetospheric dynamics taking the cases of Ganymede, Mercury, and Jupiter. The topics include; (1) Whether or not possible effect of the intrinsic time variability of the reconnection rate can be seen in Ganymede's magnetosphere which is embedded in an almost fixed magnetic field of Jupiter. (2) How well the occurrence of reconnection is observationally founded for Mercury's magnetosphere. (3) In what ways various types of Jovian aurora are related to specific features of magnetospheric dynamics. (4) By what mechanisms solar wind influences the activity of the Jovian magnetosphere. (5) How the energetic particles are supplied to the boundary region of the Jovian magnetosphere and escape to the interplanetary space keeping high energies.

Driving mechanisms of magnetospheric dynamics of planets and satellites

We review basic mechanisms of magnetospheric dynamics taking the cases of Ganymede, Mercury, and Jupiter. The topics include; (1) Whether or not possible effect of the intrinsic time variability of the reconnection rate can be seen in Ganymede's magnetosphere which is embedded in an almost fixed magnetic field of Jupiter. (2) How well the occurrence of reconnection is observationally founded for Mercury's magnetosphere. (3) In what ways various types of Jovian aurora are related to specific features of magnetospheric dynamics. (4) By what mechanisms solar wind influences the activity of the Jovian magnetosphere. (5) How the energetic particles are supplied to the boundary region of the Jovian magnetosphere and escape to the interplanetary space keeping high energies.

The structure and dynamics of the Jovian energetic particle distribution

We review the recent progress made in unravelling the properties of the energetic particle population in the magnetosphere of Jupiter. The importance of the findings with respect to mechanisms driving the dynamics of the Jovian system is addressed. We concentrate on the implications of phase space density variations for particle loss and source mechanism. Systematic local time and radial dependencies observed in the characteristics of the energetic ion and electron distributions, specifically the particle pitch angle distributions and particle flow pattern are discussed. They possibly bear important information for disentangling those mechanisms responsible for driving the Jovian aurora and for identifying the magnetospheric source populations. Furthermore, we discuss transient particle events in the tail which point to the importance of reconnection for the tail dynamics.

Spectral observations of transient features in the FUV Jovian polar aurora

Images of the Jovian FUV aurora show several morphologically and dynamically different regions. The main oval is usually fairly steady while the high‐latitude emission inside the oval can vary over timescales of <1 min. We describe time and spatially resolved spectral observations of the Jovian aurora made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. Time‐tagged spectra have been used to determine variations of the amount of hydrocarbon absorption above the auroral source layer. Rapid brightenings are observed in the high‐latitude emission with typical lengths of 50–100 s. They generally are not correlated with enhancements of the main oval in the same longitude sector, indicating decoupled source regions in the magnetosphere. The location of the high‐latitude spots and their variability suggest that, unlike the main auroral oval, the polar cap brightenings are connected to short‐term variations of the solar wind parameters. Results from a two‐stream electron transport model are used to convert observed spectral color ratios into mean electron energies 〈E〉. Electron energies during these brightenings typically range from 40 to 120 keV, close to the values found in the main oval. The time evolution of 〈E〉 generally shows little correlation or anticorrelation with the energy flux precipitated during these transient events. This feature suggests that the mechanism responsible for rapid transient brightenings does not increase the energy of the precipitated electrons, but it enhances their number flux. Pitch angle scattering into the loss cone by magnetic pumping of energy perpendicular to the field lines following magnetic field compression is a possible process.