2007-Jun-26, 05:44 PM
As per my Solar Weather thread about Hg cloud patterns on Alpha Andromedae (abbreviated AA herein), I have a question regarding solar magnetic fields. The article in 'New Scientist' that talked about the above-cited subject stated that the subject star had no magnetic field! :confused::confused:
How is this possible?? AA is a rotating ball of ionized gas! How could it not possess a magnetic field?
2007-Jun-27, 01:06 AM
The strength of a stellar magnetic field depends on the strength of convection inside the star. It is well known that hot stars with weak convection have weak global magnetic fields, or none at all. It is also well known that small cool stars, which are strongly convective, have strong magnetic fields, and exhibit more flares than larger, hotter stars (i.e, Wolf 359 (http://www.solstation.com/stars/wolf359.htm)).
In the case of alpha-Andromedae (Alpheratz (http://www.astro.uiuc.edu/~kaler/sow/alpheratz.html)), the star was shown to have no detectable global magnetic field in Wade, et al., 2006 (http://adsabs.harvard.edu/abs/2006A%26A...451..293W). But their results are subject to some interpretation, as not all of their own data are entirely inconsistent with a global magnetic field. However, they also note that a magnetic field of higher spatial frequency, and below about 100 Gauss in strength, would have gone undetected by their measurements. Earth's surface magnetic field is typically less than 1G, so we are still talking about a "random" (i.e., non-global, non-dipole) magnetic field in the stellar atmosphere that is at least 100 times as strong as Earth's magnetic field.
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