Saturday, December 11, 2010

Update for Sun, Dec 12, 2010

Still no luck with being able to launch the rocket. We have been seeing lots of aurora, but generally too far to the north for us to be able to reach it with the rocket. This is frustrating for us, of course. When we plan the rocket campaigns, which is something that begins 2 or 3 years in advance, one of the things we do is select a launch site. Depending on what kind of aurora we want to study (and there are many, many different types -- different colors, different shapes) we choose a site that it at the right latitude. The problem is that these things depend on how active the sun is -- which is quite complicated. More about solar activity and solar cycles later (maybe tomorrow).

More about aurora: aurora is something you see nearly all the time at high latitudes like this. In fact, there is usually a ring of aurora around the north and south poles. This is called the "auroral oval" and you can see pictures at NASA's website , taken by a satellite very high above the north pole with a camera pointed back down to Earth. You may be wondering what the auroral oval looks like in the southern hemisphere -- it turns out that you usually see the same shape and brightness in both hemispheres!

I should probably explain a bit more about this. The first point is that the auroral oval is really centered around the magnetic poles of Earth, not the geographic pole. The magnetic pole is where a compass needle points and is not in the same location at the geographic pole. Again, there are many, many different types of aurora including pulsating aurora, flickering aurora, tall rays, vortices, enhanced aurora and on and on. Each of these different types is associated with a different process that drives it - and so, each type can tell us more about the space far above it. Not surprisingly, you find these different types in different locations and at different times.

Why does aurora occur as a ring around the poles? Tough question, but the basic answer goes back to the fact that aurora is caused by electrically charged particles (electrons and ions) that fall to Earth, but that do so by flowing along magnetic field lines. Of course, in order to cause aurora, these particles have to somehow get ON those field lines. It turns out that the field lines that are anchored (to Earth) in the regions of the auroral oval provide easier access to these charged particles. Since it is easier for these particles to get attached to these field lines, more can travel down to our upper atmosphere.

Take another look at the movie from the other day, below, that shows how Earth's magnetic field shields us from the solar wind. If you look closely at the northern polar region, you can see that it shows where aurora is produced as a result of  the interaction of the solar wind and our magnetic field. What you cannot see in the movie is that the size of the auroral oval depends on the energy contained in the solar wind. When the sun is very active, the solar wind carries a lot of energy to Earth and causes lots of bright aurora. Under these conditions, the auroral oval expands, delivering aurora to cities further south. These days, though, the solar activity has been very low, keeping the auroral oval small -- and keeping the aurora here too far to the north for us to be able to hit with the rocket.

One last comment - the stuff I am describing here is part of a new field of research, called space weather. The idea of space weather forecasting is to be able to predict magnetic storms -- events that carry huge amounts of energy and that can seriously damage satellites orbiting around Earth, or that drive extremely large electrical currents in our electrical power supply (destroying transformers), etc. We are now also learning that airplanes flying over the north pole (say, from the US to China) can sometimes expose passengers to higher levels of radiation than expected. This is probably not a problem for passengers (who fly occasionally), but is more of a concern for the pilots and flight attendants who make these flights on a regular basis.

No comments:

Post a Comment