The Auroras of Saturn

This month marks the twenty-fifth anniversary of the launch of the Hubble Space Telescope. The Hubble Space Telescope (HST) was launched with the Space Shuttle Discovery on April 24th, 1990. Unfortunately, it returned blurry images due to an issue with the mirror. That was corrected in December of 1993, and Hubble has since been associated with mind-blowing images, like the Pillars of CreationV838 Mon, and Eta Carinae.
An "ultra deep field" Hubble image, showing galaxies much farther away than most human minds can really perceive. Image from Astronomy Picture of the Day, June 5th 2014. Further image credit to NASA/ESA.
An “ultra deep field” Hubble image, showing galaxies much farther away than most human minds can really perceive. Image from Astronomy Picture of the Day, June 5th 2014. Further image credit to NASA/ESA.
Hubble, however, is not just capturing images of very distant objects. It is also looking much closer to home–at our own solar system! That brings us to today’s paper, “Survey of Saturn auroral storms observed by the Hubble Space Telescope: Implications for storm time scales“, written by C.J. Meredith, S.W.H. Cowley, and J.D. Nichols. Translated into more everyday English, the title means that a survey of multiple auroras were used to figure out how long the storms last. The paper was published in the Journal of Geophysical Research: Space Physics in 2014.
Saturn has aurora. They are surprisingly similar to those on Earth, though they would appear in purples and reds instead of the greens and reds generally seen on Earth. Saturn’s aurora give off light not just in the visible spectrum, but also in infrared and especially in ultraviolet light, making them also visible to scientific instruments, including instruments on the Hubble Space Telescope.
As just mentioned, the process for creating aurora on Earth and Saturn are quite similar. Both planets have a magnetosphere–a magnetic field encircling the planet that shields it from the stream of charged particles from the sun known as the solar wind. These particles can be harmful to people or spacecraft, so the magnetosphere is a huge help for the inhabitants of Earth. For both the Earth and Saturn, the magnetosphere also has a so-called magnetotail, a stream of plasma trailing towards the sun.
The sun is an active thing, with a magnetic field of its own, sunspots, and solar flares. The solar wind is therefore also changing. Under certain conditions, the solar wind can compress the magenetotail, reconnecting it to the rest of the magnetosphere. The energy of this leads some charged particles to interact with the atmosphere, giving off a visible glow; aurora.
Saturn's aurora in ultraviolet, as seen by Hubble. Image credit NASA/ESA.
Saturn’s aurora in ultraviolet, as seen by Hubble. Image credit NASA/ESA.
These storms have been looked at in literature besides today’s paper, but those often looked at one storm or another specifically. Instead, this paper looked at all of the storms seen on Saturn by Hubble, as long as they fit the conditions of brightness and size that the authors set forth. In 2060 images taken from 1997 to 2013, with a total of 74.4 hours of exposure time, 12 such storms were identified. These storms are not observed from beginning to end, as they are intermittent and last a fair amount of time. However, combining the information of all of the twelve auroral storms provides insights that looking at just one storm cannot provide.
These storms were observed in 143 separate orbits of the Hubble Space Telescope, each observation around forty-five minutes long. While the Cassini spacecraft orbiting Saturn also observes these aurora, it does so at different angles and different distances. Hubble provides a certain consistency that provides better information for this particular study.
The Hubble Space Telescope, as seen with Earth as a backdrop. Photo credit ESA/NASA.
The Hubble Space Telescope, as seen with Earth as a backdrop. Photo credit ESA/NASA.
While twelve storms is a rather small data set from which to derive statistics, is seems that the storms last between eleven and twenty-one hours, usually about sixteen hours (which happens to be about one and a half rotations of Saturn). They seem to recur every four to seven days, generally about five and a half days.
In order to likely see one of these aurora in full from start to end, you would need to observe Saturn for about thirty-two hours, imaging every three hours or so. Right now, there is no instrument that can do this. Even so, a fair amount of information has been gleaned from looking at just a dozen storms. And although the Hubble cannot look at everything, it can view quite a bit, and has been an enabler of discovery and learning (as well as amazing photographs!) for twenty-five years–hopefully with more to come! 

 

Bibliography
Sources used to directly write this paper. 
American Geophysical Union. “New Study Provides ‘smoking gun’ Evidence That Saturn’s Collapsing Magnetic Tail Causes Auroras.” N.p., 19 May 2014.
An AGU Press Release discussing Saturn’s aurora and how they form. Also the source of the image collage of Hubble views of the aurora.
Astronomy Picture of the Day. “2004 March 5 – V838 Mon: Echoes from the Edge.” N.p., n.d. Web.
One of the images I linked to within the post, and one of my favorite Hubble pictures.
Astronomy Picture of the Day “2012 December 30 – Doomed Star Eta Carinae.” N.p., n.d. Web.
Another image I linked to within the post.
Astronomy Picture of the Day. “2014 June 5 – Hubble Ultra Deep Field 2014.” N.p., n.d. Web.
The Hubble deep field image that appears in the post.
Astronomy Picture of the Day “2015 January 7 – Hubble 25th Anniversary: Pillars of Creation.” N.p., n.d. Web.
The Pillars of Creation, probably one of the most famous Hubble images, is linked to within the post.
Bennett, Jeffrey et al. The Cosmic Perspective: The Solar System. 7th ed. San Francisco: Addison-Wesley. Print.
A textbook used for reference about how aurora work on Earth.
Cook, Jia-Rui. “NASA Spacecraft Get a 360-Degree View of Saturn’s Auroras.” N.p., 11 Feb. 2014.
An article about Saturn’s aurora and the investigations into them.
Hubble Space Telescope. “Hubble Space Telescope Begins ‘Two-Gyro’ Science Operations.” N.p., n.d. Web.
The page I used for the image of the Hubble Space Telescope that appears in the post.
Hubble Space Telescope. “Timeline: Hubble Space Telescope.” N.p., n.d. Web.
A page used for reference on the timing of important Hubble events.
Meredith, C. J., S. W. H. Cowley, and J. D. Nichols. “Survey of Saturn auroral storms observed by the Hubble Space Telescope: Implications for storm time scales.” Journal of Geophysical Research: Space Physics (2014).
The main paper for today.
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