Friday, 27 August 2010

Night One of our Jupiter observations, Mauna Kea Observatory

It's 4am Hawaii Standard Time, and we are 2/3 of the way through our six-hour shift. And what a great shift it has been. The sky is perfectly clear, there is almost no wind and the "seeing conditions" - the blurring of our images due to the Earth's atmosphere (a useful thing for breathing, but it gets in the way for astronomy) - is very slight.

We are simultaneously measuring Jupiter's spectrum along its noon meridian and taking images of the whole planet, using an instrument on NASA's Infrared Telescope Facility called SpeX. That means we can properly locate the spectra we are taking on the disk of the planet. We take the images at a shorter exposure than the spectra. So, by the end of the night, we will have taken a total of 250 images and 150 spectra. That's a PhD's worth of data in one night, and we have three more to go.

As I'm writing this, the Northern Aurora of Jupiter is coming into full view on the disk of the planet. The Southern Aurora has pretty much set, and all that we can see looks like the smile on the Cheshire Cat. These aurorae are the most powerful in the Solar System - 100 times brighter than anything we see on Earth. They are testimony to the dynamic coupling between the planet's atmosphere and its enormous magnetosphere, the region of space controlled by Jupiter's magnetic field.

To give some idea of the scale: Earth's magnetosphere stretches about 60,000 km in the direction of the Sun, and about 600,000 km in the direction away from the Sun, its tail dragged out by the stream of particles thrown out by the Sun called the Solar Wind. Jupiter's magnetosphere extends 2 million km in the sunward direction, and some 750 million km downstream. In fact, it reaches all the way to the orbit of Saturn.

Lit up, if it could be, Jupiter's magnetosphere would appear in the sky to be more twice as large as the Moon. It is this giant structure that heats the top of Jupiter's atmosphere, by throwing energetic particles at it and generating enormous currents and electric winds. We are probing this by looking at the spectrum of the H3+ molecular ion I talked about in my last blog.

We have analysed one of the spectra which us a temperature in region of 950 K. This is much higher than sunlight alone can produce. It clearly shows how much Jupiter's magnetosphere is responsible for the conditions that occur in the upper atmosphere.

But it is not all one way traffic. As Jupiter rotates (once every 9 hours 55 minutes), so does its magnetic field. And that field drags the charged gas that fills the magnetosphere, known as plasma, along with it. Indeed, this mutual interaction spins up the magnetosphere, heats up the atmosphere and - since there s not such thing as a free lunch - actually slows down the planet as it spins on its axis. No need to worry, though. At the present rate, Jupiter will carry on spinning for at least ten times the lifetime of the Solar System.

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