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Monday, April 18, 2005

Aurora Watch!!!! (not)


...or 'The Solar Cycle'


Well folks, according to the lovely people over at Spaceweather.com we are about to enter what is known as a weak solar wind stream. They suggest that there is a chance of seeing aurora... if you live in Canada or Alaska (and of course anywhere else in the arctic circle but they are pretty North America-centrist). I'm afraid for everyone else there is little chance at the moment.

We are currently heading from what is known as solar maximum (lots of flares and sunspots, etc) to solar minimum (low numbers of flares and few sunspots, etc). The phrase 'solar minimum' suggests that there isn't much activity going on; however, for those at high latitude this might be the best few years for seeing consistent auroral displays.

This might take some explaining:
The sun follows a number of patterns and one of these is an 11 year periodicity called the solar cycle (sometimes the sunspot cycle). The link gives a good overview but I'll present the candid version. Basically this cycle is driven by the Sun's magnetic field; at minimum this appears something like a dipole (think bar magnet and iron filings), but then it starts to get more complicated and disordered sometimes looking like a quadrupole field. This means that there are complicated twists in the field and lots of regions where the inner field pokes out and then back in again (sunspots). At this time it is very hard to predict what sort of activity we can expect on the sun; large flares might erupt or else even bigger clouds of plasma will be thrown off from the sun as CMEs. This is the time when we might get lots of large geomagnetic storms and be able to see the aurora from mid latitude.

After this period of solar maximum (last one was in 2000-2001) the magnetic field of the sun starts to reorder itself, though this time with the magnetic field pointing in the opposite direction (the north pole becomes a south pole and vice versa) such that the 11 year cycle is really a 22 year cycle. When a dipole like shape reappears we are left with regions of magnetic field that open on one hemisphere of the sun and that close on the other. We also are left with regions of magnetic field that stretch out into space (the IMF), which allow particles from the sun to flow quickly away in the form of the solar wind. These coronal holes rotate with the same period as the sun (~27 days) and are therefore much more predictable. Plus when we enter these streams of plasma that originate from the coronal holes we are much more likely to see aurora at high latitudes. So you see, even though solar activity is low, it is still a good time to hunt for aurora if you live in the far north (or south), but not so good if you live in more temperate climes.

I should point out that big solar flares and CMEs do happen during solar minimum but much more infrequently. Besides, this is the current face of the sun. The few sunspots do not inspire me to think that a big storm is on the way...

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