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APIS Tutorial

Abstract
Authors
Change Log
Intro
Tutorial
References

Abstract

This is user tutorial for the APIS (Auroral Planetary Images and Spectra) database. Connection on the APIS web interface requires a login (register). The data will be loaded for analysis in Aladin and Specview.

Authors

Laurent Lamy

Edited by Baptiste Cecconi

Change Log

VersionNameNote
1Baptiste CecconiConversion from PDF version of tutorial (in french)

Introduction

Remote ultraviolet (UV) observations of giant planets and their moons are a rich source of informations on those planetary systems. Their various elements (rings, moons, atmosphere) reflect solar light and their magnetospheres produce intense atmospheric auroral emissions (Figure 1). Aurora are specifically are resulting from the collisions between charged energetic particles - mainly electrons - accelerated in the magnetosphere and guided along magnetic field lines down to the magnetic poles, and the neutral upper atmosphere. In the case of giant planets, the upper atmosphere est mostly composed of atomic and molecular Hydrogen, which electronic transitions are covering the whole UV spectral range (H2 spectral bands). In the case of moons, the exosphere is producing Oxygen emissions (OI multiplets at 1304 and 1356 A). The aurorae are thus remote proxies of active regions and dynamics of the magnetosphere, of plasma acceleration processes, and of the energy transfer to the planetary atmosphere.

Planetary UV Observations

Fig. 1: Planetary UV Observations

The APIS (Auroral Planetary Imaging and Spectroscopy) service consists in a database of planetary aurora spectro-imaging, archived at PADC (Paris Astronomical Data Centre), coupled with web interface. Fortuitously, Apis is also Egyptian god of fertility (of data, of course).

The database currently contains more then 6000 individual observations of Jupiter, Saturn, Uranus and their moons, as well as Mars. The data were acquired with the Space Telescope Imaging Spectrograph (STIS) and Advanced Camera for Surveys (ACS) instruments of the Hubble Space Telescope (HST) in the Far UV range (~1100 to 1700 Angström) since 1997. Several processing level with added value are available.

The data are accessible online at http://apis.obspm.fr. They can be easily ordered and displayed thanks to its dedicated search interface (conditional search on date, instrumental characteristics, planetary physical parameters), as showed in Part 1 of the tutorial. Once a request is done, it is also possible to directly work online with the data available in FITS format, thanks to VO (Virtual Observatory) tools, as shown in Part 2.

The goal of this tutoriel is to get familiar with the APIS features.

Tutorial

Searching for Data

Main Selection Criteria

Data can be quickly ordered and visualized thanks to the conditional search interface (Search for Data), see Figure 2. The default selection parameters are generic ones (target, telescope, instrument, observation type, instrumental mode, date of observation) and practical (observation campaign name, unique observation identifier). Once a query is done, the corresponding time interval is automatically displayed and is kept for any further queries. Use the Clear and Clear Date buttons to empty the query form fields.

Search for Data

Fig. 2: Search for Data

Training

After connecting, do a few typical searches. Check HST observations of Jupiter, Saturne, Uranus and their moons, and see how they differ.

What about Mars observations? Can we see aurora?

NB: Auroral emissions are visible around the magnetic poles, mainly as circumpolar ovals. Their morphology differs from a planet to the other, depending on the characteristics of the magnetosphere and its interaction with the solar wind.

Secondary Query Parameters

Specific query parameters (instrumental of physical) providing advanced search capabilities are available by clicking on Advanced research (bottom-right of the search interface, see Figure 2). When the secondary search interface is visible and a query has been issued, the corresponding range of integration time
is automatically displayed and is taken into account for any further queries.

Training

Identify Saturn observations when the Cassini mission was located in the Solar Wind (sub-solar distance of the magnetopause larger than 30 Rp) and was able to acquire in situ measurements.

NB: This type of combined analysis of several space plasma datasets (in situ and remote) is specifically the scope of another VO tool: AMDA developed by the CDPP (Centre de Données de Physique des Plasmas)

Processing Levels

The search results are showing several columns, corresponding to different processing levels, each available with various file format. FITS files are aimed at experienced used willing to perform his own data analysis, while graphical formats (PDF and JPEG) are directly usable (e.g., for communications).

Three processing levels are available for images:

  • Raw data [level 1]: images from the STSci (Space Telescope Science Institute) database. STSci is in charge of the initial calibration of HST observations. The FITS files contain 3 extensions including the science data (extension 0), the measurement error (extension 1), and the dead pixel map (extension 2).

  • Processed data [level 2]: images rotated and centred (corrected for the telescope inclinaison and pointing). The FITS files contain 7 extensions including the science data (extension 0) and the planetocentric coordinates of each pixel (extensions 1 to 6), namely: (1) latitude, (2) local time, (3) solar zenital angle and (4) observer's zenital angle at the cloud altitude (i.e., at limb altitude), (5) latitude and (6) local time at the auroral altitude (above cloud level).

  • Cylindrical projections [level 3]: images calibrated into physical units and projected into a cylindrical frame at the aurora altitude, after subtraction of a modeled background.

  • Polar projections [level 3]: same as above with polar projections.

Three processing levels are available for spectra:

  • Raw data [level 1]: 2D spectra from the STSci archive. The X dimension of the spectral axis. The Y dimension is the spatial axis, along the slit direction. The FITS files contain the same three extensions as for raw images.

  • Processed data [level 2]: 2D spectra extracted and calibrated in wavelength.

  • 1D spectra [level 3]: typical 1D spectra (with 3 occurrence levels) extracted from the 2D processed spectra.

Training

Click on a thumbnail image to display a higher resolution image, and compare with neighboring observation with the <-- and --> keys.

Click on the JPG or PDF link to get the full resolution image.

Interactive Processing with VO Tools

Thanks to the underlying VO-compliant database system, another specific feature of APIS is to allow direct online processing or visualisation, without having to download the data.

Reading and Analyzing Images

In the image search results, the FITS formatted processing levels (levels 1 and

  1. can be displayed using the Aladin link below the thumbnail image. This launches the Aladin tool in another browser window, and displays the image directly from APIS. The connection is made through the SAMP (Simple Application Messaging Protocol), materialized by a yellow target icon in the menu bar.
training

Examples of processing in Aladin:

  • Change image contrast with a right mouse button click and move vertically to adjust the contrast, or horizontally to adjust the cuts;

  • The image contrast can also be modified via the Image/Pixel Constrast menu item. Various transfer functions can also be select (linear, logarithmic, etc);

  • Display the histogram of pixel intensity (pix option), the full distribution is available clicking on all values. Identify the different components of the image (background, planet, aurora) moving the cursor on the color table;

  • Superimpose intensity iso-contours (cont option);

  • Plot the intensity distribution along a cut going through the auroral oval (dist option).

Aladin also allows you to load several images and compare them.

Training

Load several processed images (e.g., 4 to 6 images from the January 2004 Saturn campaing, between January 25th and 30th, during the Saturn approach phase of Cassini).

Have a look to the various extensions of 1 of those images, and then display all the images simultaneously (multiview option).

Notice the apparition of intense emissions on the dawn side (left-hand side) on January 28th. In order to quantitatively identify the location of active regions, superimpose iso-contours for each image and then look for the pixel locations corresponding to bright regions on the last 2 extensions.

Check that the bright emissions on January 28th are located at higher latitude than the other days.

NB: Saturn auroral emissions are very sensitive to the Solar Wind conditions. The event observed on January 28th 2004 illustrates the effect of an incoming interplanetary shock at Saturn, which translate into a strong auroral forcing on the dawn side.

Reading and Analyzing Spectra

Aladin

2D spectra can be read the same way as images with Aladin.

When a 2D spectrum is acquired just before or just after an image, the telescope usually stays aligned and the spectrum is simply obtained adding a slit (and a grating) on the optical path, along the Y axis of the detector and centered in the middle of the X axis. The 2D spectrum hence gives a spatial information along the slit, and a spectral information perpendicularly to the slit. Figure 3 illustrated the link between an image and a 2D spectrum in the case of Saturn. It is possible to identify in the 2D spectrum the contribution of the sky (almost no signal), of the rings and the planetary disc (emissions at large wavelength corresponds to the reflected solar light), and of wide band signatures corresponding H and H2 auroral emissions.

Images and 2D Spectra

Fig. 3: Images and 2D spectra corresponding to Saturn in december 2000.

Training

Load a 2D spectrum with Aladin (select the spectrum o6baa7xkq of the 2000-2001 Jupiter campaign, acquired right after an image).

Adjust the contrast to enhance the wide band emissions.

In the Frame drop-down menu (top-right corner the the image panel), select XY Fits. The (X,Y) pixel coordinates are then displayed while moving the pointer over the image. Find the Y values of the four main wide band emission boundaries.

With the help of the image (as on Figure 3), you can visually identify which place correspond to the auroral oval.

NB: The location of the slit is generally selected so that it intercepts several regions of emissions (cuts of the main auroral oval, polar emissions, Galilean moon controlled emissions in the case of Jupiter, etc).

Specview

Specview is a spectra analysis tool. It provides an graphical interface to plot and superimpose 1D spectra - intensity versus wavelength - extracted from 2D or 1D spectra (Figure 4). Contrarily to Aladin, you have to manually start up Specview before clicking on the Specview link below the spectra available in FITS format in the search results of spectra (columns 1 and 3).

The data are then transfered from APIS to Specview via SAMP.

1D Spectra in Specview

Fig. 4: Extracted 1D spectra in 2 bright emission regions of the o43ba1bnq 2D spectrum of Jupiter. The vertical bar at 1216 A indicates H-Lyman α line.

Training

Load the same 2D spectrum in Specview. There are several ways to extract a 1D spectrum:

  • First plot the sum of all lines of the 2D spectrum with the Go button on the right-hand side of All lines. Each line Y(X), i.e. each pixel along the slit, corresponds to an individuel 1D spectrum).

  • Then plot successively the integrated spectrum on each bright emission region as identified previously, using the Ymin and Ymax values in the Top and Bottom fields and then click on Go.

The various plots are kept in memory. It is possible to superimpose them with the Coplot menu: select the spectra to be compared and then Plot (see Figure 4). The Process button is used to apply extra processings, such as a multiplying factor to one of the spectra.

Normalize the spectra from each of the four regions identified around de 1550 A and superimpose them. What difference appears at small wavelength?

NB: *When auroral emissions are produced under an organic atmospheric layer (such as CH4), they are absorbed below 1500 A (Figure 5). This absorption is used to measure the penetration depth, and thus the energy of the electrons.

Level 3 data are directly providing three typical 1D spectra, which are respectively the mean spectra computed on (i) all, (ii) 50% or (iii) 10% most intense 1D spectra (lines) of the 2D spectrum.

Superimposing transitions and theoretical spectra

Specview can also query online or local catalogues of chemical transition lines in order to identify chemical species observed in the studied spectra. As aurora are atmospheric emissions, their spectra directly provides the atmospheric composition. For the giant planet, we mostly expect (1) the intense Lyman α of Hydrogen at 1216 A and (2) the Werner and Lyman bands of H2 between 800 and 1700 A (Figure 5).

Synthetic Lyman alpha spectrum

Fig. 5: Synthetic spectrum of the H-Lyα line of H2 bands, together with the absorption cross-section of CH4 (secondary atmospheric species capable of absorbing auroral emission below 1400 A).

(1) H: We suspect that the bright line around 1220 A is the H-Lyα line. We will check this now.

The Line IDs button at the top-right corner of the Specview window is popping up a new window where it is possible to look for chemical transitions, either locally (top part, with pre-recorded lines) or online (bottom part, with online catalogues).

Training

Set up a local query in the observed spectral range. Select the Lyα line and display it on the spectrum with Draw. If the observed Lyα line does not show up at the right place, this means that the 2D spectrum was not calibrated in wavelength. It is easy to check with the same operation on the corresponding 1D spectra, which have been correctly calibrated.

NB: Atomic Hydrogen is not only present in the observed atmosphere but also in the geocorona. When the geocorona portion intercepting the HST line of sight is lit by the Sun, the Terrestrial Hydrogen scatters back the solar light on the H-Lyα transition. This explains that this line is so bright all along the slit and not only at the place of the aurora. Furthermore, although that line is very thin, it is showing a rather large profile on the 2D spectrum, imposed by the width of the slit. This can be checked looking at spectra obtained with slits of various width (filter or aperture criterion, then G140L / 52 X 2 , which corresponds to an aperture of 52 x 2 arcsec). Other lines from the geocorona (such as that of the Oxygen at 1305 A) can also contaminate the observation.

(2) H2: We now want to check that the rest of the observed spectrum corresponds to H2 emissions.

It is possible to apply the same method as described in (1) to find the individual transitions of the molecule.

Training

Example:

Once in Line IDs, choose the online catalogue named SESAM in the Configuration menu and select the Search linelists using VAMDC infrastructure option: check the Molecule species and specify H2, check the Radiative process and select a limited spectral interval such as 1550-1600 A. Display a few theoretical lines on the observed spectrum.

It is clear that this approach is useful for identifying a specific transition in a limited spectral interval, with a width of a few A, but it is not very appropriate for a larger interval. H2 produced a large number of lines spanning over the observed UV range, forming band spectra (Werner and Lyman). In order to check that the observed spectrum corresponds to the H2 bands, it is easier to compare directly with a theoretical spectrum combining all individual H2 transitions. We will use here the synthetic spectrum [H2_spectra_Menager_1keV.fits](http://voparis-srv.obspm.fr/vo/planeto/apis/dataset/Bastet/H2_spectra_ Menager_1keV.fits) in a local access mode.

Training

Load the theoretical spectrum.

Then load your observed 2D spectrum (do not use the 1D spectra here, as their format doed not allow Specview to perfom the operations hereafter described). Zoom on the Y axis (not on X) to adjust the intensity scale to maximum signal observed at large wavelength. Double-click on the spectral peak between 1500 and 1600 A, then click in the new window on the theoretical spectrum previously loaded. It will then be normalized at the place of the du doucle-click and overplotted to observed spectrum.

The observed wide band spectrum is showing the general aspect of the H2 bands.

NB: Two noticeable differences can be observed:

  • the relative intensity between the observed and theoretical spectra could be fainter below 1400 A, due to signal absorption by atmospheric CH4 (see Figure 5).

  • an excess of absorbed signal above 1600 A, if the observed spectrum includes a contribution a solar light reflected by the atmosphere.

Interoperability

As APIS has been built using VO standards, it is easy to interface this database to interoperable tools.

The VESPA and CDPP/AMDA portals can be used to query APIS remotely.

Training

Example:

Search for Uranus data in VESPA. Display APIS data (display resuts) and visualize the available search parameters (show columns). Test a conditional search on one or several os those criteria, using the advanced query form button next to the display results button on the previous page.

References

The Auroral Planetary Imaging and Spectroscopy (APIS) service (2015) L. Lamy, R. Prangé, F. Henry, P. Le Sidaner, Astron. and Computing, arXiv, DOI.

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