Sunday, January 10, 2016

Jupiter's Pulsing Aurora

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Image showing bright patches glowing associated with Jupiter's ultraviolet aurora - observed by the Hubble Space Telescope.

In an earlier post (Nov. 21, 2015) I noted the work of two University of South Florida astronomers (Ray S. Clary and James H. Hunter Jr.), on the attempted detection of auroral activity on Jupiter ('Hydrogen Alpha Auroral Activity on Jupiter') and published  in The Astrophysical Journal (Vol. 199, p. 517).  They noted correctly that given the correlation between Jupiter's radio emissions and solar outbursts nonthermal, visible emissions from the planet ought to have been detected. Most importantly, they pointed out that:

"At least the polar areas should be visible since aurorae should be most likely there"

They carried out their observations from April through November, 1972 "usually with Jupiter 2 hours or less  from the meridian".

However, their results were mixed, e.g. in  'Results and Conclusions' the authors wrote:

"Most of the pictures (> 90 percent) taken in H-alpha during the experiment showed no significant detail on the 'surface' of Jupiter."

They added that:

"Equatorial features should have been resolved if they were visible. Haze and /or clouds reduced the resolution and brightness of images and it was frequently necessary to discontinue an observing run."
One of their interpretations was:

"Apparent variations in Jupiter's emissions were observed but the effects must be deemed spurious doe to conditions at the observing site"

Decades later, and using appropriate space telescopes, those variations in emission don't seem to be spurious at all. Only recently Tao et al (Journal of Geophysical Research -Space Physics, Oct. 2015, p. 1002), using the Japanese Hisaki Space Telescope, measured variations in the brightness of the Jovian aurorae. They reported observing two kinds of auroral pulses. In one, the aurora brightened for up to several days at a time, and the authors attributed this to the solar wind. Thus, as it 'washes over' the planet, the charged particles are buffeted and compress Jupiter's magnetic field. This is very similar to what happens on Earth.

A major difference is that unlike Earth's auroras, those on Jupiter are nearly continuous - driven by the planet's rapid rotation and its volcanic moon (Io),  which spews out sulfur and oxygen ions as well as electrons into space, The latter speed along the planet's magnetic field lines and - if powerful enough - slam into the atmosphere causing its particles to glow.

The team also observed much more rapid variations, with pulses typically lasting less than ten hours. By comparing the Hisaki images with images taken simultaneously by the Hubble Space Telescope, Tao et al could see these variations arose from the aurora brightening at lower altitudes, at the bottom of the auroral arc and as reported by Kimura at al (op. cit.)

Using Hisaki's onboard spectrometer, the Tao team was also able to estimate how fast the electrons were traveling on the basis of how deep in the atmosphere the light originated. They found that when the aurora flares up it's not because faster, more energetic electrons are penetrating deeper into the atmosphere. It is instead due to the overall increase in the number of electrons.

All of this indicates that Jupiter's most intense auroras occur when plasma is suddenly injected into its magnetic field, most likely from Io.

We have to await further work to confirm these pulses and also more in-depth research into their causes.

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