You Served Well

Back when I was in graduate school (in the last century, and I can say that without feeling TOO old), I worked on a team that used data from the Ulysses spacecraft to study the effects of the solar wind on comet plasma tails. Ulysses has been following an orbit that took it over the poles of the Sun, allowing the spacecraft’s instruments to sample every “regime” of the solar wind.

Now, unless you study the Sun, you probably don’t think about this, but it doesn’t blow its wind evenly in all directions. The speed and density (think of it as “loadedness” because it carries particles along) are not the same at each latitude.  In the Sun’s “midsection” (its mid-latitudes) the wind is heavily loaded (it’s more dense) and blows at an average speed of 450 km/sec. (970,000 miles per hour).  At higher latitudes (the closer you get to the poles) the wind is much less dense and blows out at around 750 km/sec. (1.6 million miles per hour). And of course, there are locally gusty conditions where winds can temporarily go faster.

What we were trying to do was correlate the appearance of a comet’s plasma tail to what latitude of the solar wind it was experiencing. A plasma tail is very sensitive to the solar wind and responds almost instantly to any changes. So, what we found (with the help of the Ulysses spacecraft) was that at equatorial latitudes, the plasma tail of a comet experiencing this part of the solar wind has a distinctly disturbed appearance. It varies over time and actually experiences what we called “disconnection events” (where the plasma tail breaks off and floats away) as it experiences reversals of the magnetic field at those latitudes.

When the same comet travels through the polar latitudes, it encounters that steady, less-dense and faster solar wind. It has smaller variations in speed and density, no magnetic field reversals, and no disconnection events. A plasma tail looks pretty boring at the higher latitudes (by contrast to its often disrupted, kinky appearance at mid-latitudes).  What we found was that the comet, by acting as a “solar wind sock,” can be used to map the conditions in different latitudes of the solar wind. And Ulysses gave us exquisite readings of wind conditions from its vantage point in the solar wind — which then allowed us almost to “predict” what the comet would look like when it encountered those conditions. Or, to take it another way, we could look at a picture of a comet and just about tell you where it was in the solar wind and we used Ulysses data to back it up.

It was fascinating work, and Ulysses performed well for us — even though our project was just one small one out of many that took advantage of this spacecraft’s work. It’s been a good run for Ulysses, and the news today that it will be switched off on July 1 is sad, but inevitable. The spacecraft has held up four times longer than originally expected, but its onboard generators are starting to fail (along with some other issues). It will likely be taking data right up to the end, and I think that’s a fitting tribute for a well-run, high-productivity mission.