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Henry A. Rowland
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The composite visible and ultraviolet image of Jupiter, at left (Courtesy of Dr. John T. Clarke and NASA) was created from Hubble Space Telescope (HST) data obtained with the Wide Field and Planetary Camera 2 (WF/PC2) and the Space Telescope Imaging Spectrograph (STIS). The glowing ring is the aurora, or "Northern Lights", created by the interaction of charged particles from the Jovian magnetosphere with the upper atmosphere of the planet. Recent images and spectra of the jovian aurora are available on the STScI web site. STIS has also obtained images and spectra of the aurora on Saturn. The rather featureless glow emanating from the disc of the planet is the dayglow, produced by a combination of resonant and solar-scattered fluorescence and photoelectron impact fluorescence in the tenuous upper atmosphere. Both the auroral and dayglow emissions are produced by molecular hydrogen, H2, which consists of two hydrogen atoms joined together in a dumbbell-type configuration. In the auroral regions, charged particles (electrons, protons, and some heavier nuclei) from the magnetosphere spiral along the magnetic field lines and collide with H2 in the rarefied upper atmosphere of Jupiter, bumping the electrons attached to these molecules into a state of higher energy. |
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Eventually (after an excruciating wait of 10-8 seconds or so) the electrons will fall back down to a lower energy state (the electronic ground state of H2), converting some of their energy into photons that we can detect here on earth with the proper equipment. (Actually we need to be quite a ways above the earth, due to the (fortunate) screening effect of our atmosphere at ultraviolet wavelengths.) The dayglow appears to be produced by the incoming solar radiation, which can scatter directly off an H2 molecule (resonant and fluorescent scattering, predominant at wavelengths greater than 800 Å) or alternatively produce a photoelectron (more likely at higher energies, with wavelengths less than 800 Å) which subsequently excites the molecule and produces a fluorescent photon. Quantum mechanics helps us to describe the possible ways in which this release of energy can occur; although hydrogen is a very simple molecule, the combined effects of rotation (as the dumbbell shaped molecule spins around) and vibration (as if a spring connects the two atoms which form the molecule) lead to a very complex set of emission lines, whose intensity and wavelength vary over a large portion of the ultraviolet spectrum. Click here to see a spectrum (PostScript format) of the Jovian Aurora as observed by HUT, the Hopkins Ultraviolet Telescope. The HUT Astro-2 auroral spectrum is shown in black. A model of electron-impact fluorescence incorporating the effects of self-absorption is overlaid in red; a model using optically thin branching ratios is overlaid in blue. |
| For more on astrophysical research here at Hopkins check out the following URLs: |
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| For more information on auroras and other aspects of planetary atmospheres check out these URLs: |
Questions ? Contact wolven@pha.jhu.edu
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