Data levels

An exhaustive description of the APIS service can be found in the following paper, published in Astronomy and Computing in press, Arxiv pre-print. A brief description of the available science products and associated caveats is reminded below.

Data levels

The core database relies on HST FUV planetary auroral observations acquired by the STIS and ACS/SBC instruments from 1997 up to now. These consist of about 6000 individual images and spectra, obtained with different instrumental configurations (filters, slit, gratings), for each of which is derived a set of higher level data.

Imaging :

Images are displayed under three levels of data :
- level 1 (original data) : images retrieved from the STSci archive and individually re-calibrated with the most recent calibration files. These "raw" observations are displayed as fits files and jpeg plots, built in the original telescope field-of-view with instrumental units and a linear intensity scale.
- level 2 (processed data) : extracted, rotated and re-centered images displayed in the telescope field-of-view, in a planet-related frame. They are displayed as fits files and pdf, jpeg plots, built with instrumental units and a logarithmic intensity scale. The fits files contain 7 extensions including : the re-centered science image in instrumental units (extension 0), and the associated pixel planetocentric coordinates (extensions 1 to 6 : latitude, local time, zenithal solar angle and zenithal observing angle at the limb altitude, latitude and local time at the auroral peak altitude). Their header contains basic informations on the original observation and valued-added informations such as planetary and relevant spacecraft (Cassini/Galileo) ephemeris (source : IMCCE).
- level 3 (projections) :
— cylindrical projections provided in pdf and jpeg plots, built with physical units and a logarithmic intensity scale. The projections were applied to re-centered images, once a numerical model of background subtracted, and then converted into brightness units (see the note below) with a dedicated projection routine preserving the photon flux of a given emitting surface.
— polar projections provided in pdf and jpeg plots, built as cylindrical projections.

Spectroscopy :

Spectra are displayed under three levels of data :
- level 1 (original data) : first order long-slit (or slitless) STIS 2D spectra retrieved from the STSci archive and individually re-calibrated with the most recent calibration files. They are displayed as fits files and jpeg plots, built in the original HST field-of-view (slit along the y axis, wavelengths along the x axis) with physical units and a linear intensity scale.
- level 2 (processed data) : extracted 2D spectra, including a specific wavelength calibration whenever possible when not provided by STSci. They are displayed as fits files and pdf, jpeg plots, built in the original HST field-of-view with physical units and a linear intensity scale. With respect to the above mentioned paper, a supplementary processing was applied jpeg/pdf plots of small targets (i.e. out of Jupiter/Saturn) : a median spectrum was subtracted to each row of the 2D spectrum, intensities <150nm were divided by 10 in order to facilitate comparison with signal >150nm and a white arrow at the top right now indicates the body northern pole. The fits files only contain the extracted 2D spectrum. Their header contains basic informations on the original observation and valued-added informations such as planetary and relevant spacecraft (Cassini/Galileo) ephemeris (source : IMCCE).
- level 3 (1D spectra) : 1D spectra extracted from level 2 data (not available for slitless observations). Three average 1D spectra were derived from intensity occurrence levels. They are displayed as fits files and pdf, jpeg plots, built with physical units and a linear intensity scale. The header of fits files contain informations similar to those of level 2.

Notes and caveats

Needs for a specific data processing :

For some purposes concerning a limited set of observations, several improvements of the standard systematic data processing pipeline can be performed on demand :
- A semi-automatic fitting routine was systematically applied to retrieve the planet’s exact position and pixel coordinates. Afterwards, a numerical background was subtracted to re-centered images before to proceed to cylindrical and to polar projections. More accurate centering and/or background models can be built case by case.
- STIS observations with long exposure times were most of the time acquired with the time-tag mode (see the detailed informations given by the search interface). This mode records the arrival time of each photon and provides the possibility to build sub-exposures over chosen intervals. Example : Long exposure time-tagged images of Jupiter acquired in 2009 were sequenced in 21x100s long images each.
- In a near future, APIS aims at providing the total radiated and precipitated hemispheric power for each image. Such estimates can already be done on restricted regions of interest (cusps, spots).
- 1D spectra were built from simple occurrence levels applied to the original 2D spectrum. 1D spectra of specific features (auroral arcs, satellite spots/wakes etc.) can be derived case by case.

The case of Saturn and Jupiter :

The header of Saturn and Jupiter (and their moons) processed fits files contains supplementary informations as :
- Saturn’s southern and northern SKR phases (source : Lamy, PRE7, 2011), available online.
- the position of main moons and the ephemeris of the Cassini and Galileo spacecraft (source : University of Iowa, Observatoire de Meudon).

The projections of Saturn’s images display additional specific features : white boxes in cylindrical projections indicate the expected footprint of Enceladus, and dashed-dotted (dashed) lines in polar projections indicate southern (northern) reference rotating meridians derived from the southern (northern) SKR phase system.

The projections of Jupiter’s images display markers at the footprint of the 4 galilean satellites : the arrows with decreasing thickness/length and increasing gray tone respectively indicate the footprint of Io, Europa, Ganymede and Callisto. These footprints have been modelled with the ISAaC - In-Situ and Auroral Contraints - magnetic field model (update of VIPAL, improved with the fitting of the Ganymede footprint, Hess, private communication). Supplementary magnetic boundaries of interest, such as the footprints of flux tubes crossing a constant distance at the equator, can be added on demand.

Imaging units :

We chose to display projected images in brightnesses (kilo-rayleighs, or kR) of total H2 emission over 80-170nm. Many past studies (e.g. Gérard et al., 2004, 2005, 2006 ; Grodent et al., 2001, 2003, 2005 ; Clarke et al., 2005, 2009 ; Lamy, 2008 ; Bonfond, 2010) have performed successive photometric calibrations of STIS and ACS/SBC with typical auroral spectra to convert instrumental units (counts/s) into physical brigthnesses. The derived obtained conversion factors yielded significant disparities, owing to the considered radiative species (H2 and/or H-Lya), the consideration of absorption by hydrocarbons and the selected bandpass (spectral domain of H2 emission, FUV domain or bandpass restricted to that of HST imaging filters), as discussed by (Gustin et al., 2012 et Lamy et al., 2013). Here, we used the most recent estimates of (Gustin et al., 2012) derived for the two broad filters of each instrument (with color ratios of 2.5, 1.1 and 1.04 for Jupiter, Saturn and Uranus respectively), that we completed for the other employed filters. The conversion factors of filters concerning wavelengths mainly beyond the spectrum of H2, as F165LP for ACS, were set to 0.

The case of moons :

Although fitted with ellipsoids as for the images of planets (the pixel coordinates are available within the fits files of processed data), the images of moons were neither projected nor converted into auroral kR, owing to the lack of reliable conversion factors.

STIS irregular dark current :

Recent STIS observations may suffer a significant dark current on the FUV-MAMA detector that varies unpredictably in intensity. This results in an extended glow contaminating in particular the top left quadrant of the MAMA detector. No specific processing is applied to account for this sporadic dark current (source : STSci STIS handbook).

Acknowledgments

The path from the original idea to observe a target to the value-added data delivered by APIS is long and complex, and involves a significant work from many persons and services as the principal investigators for requesting and programming the observations, the conceivers of the telescope and its instruments, the engineering team of the ESA/NASA Hubble Space Telescope for collecting the data, the Space Telescope Science Institute (STSci) and the ESA Hubble Science Archive (ESAC) for calibrating and archiving raw data and developing interactive tools for spectra (Specview, VOspec), the Institut de Mécanique Céleste et de Calcul des Ephémérides (IMCCE, Observatoire de Paris) for providing planetary ephemeris, the University of Iowa for providing ephemeris of the Cassini and Galileo spacecraft, the Centre des données de Strasbourg (CDS) for developing the Aladin interactive software and finally the APIS team for developing this service.