Minima-lly Invasive Technique for Sounding the Sun

Our pesky Sun has been diabolically withholding its sunspots for the past couple of years, causing solar physicists to wonder what’s going on with this most minimum of sunspot minima.  The Sun goes through an eleven-year cycle of magnetic activity related to the appearance of sunspots, solar flares, and disturbances of the interplanetary environment called space weather.  We’re at a low in solar activity right now, and it’s been just a LITTLE too quite on the Sun.

All this lack of activity has scientists wondering just what’s going on inside our star that has kept its normally sunny complexion clear. They can’t exactly go out to the Sun and stick some instruments into the solar surface to diagnose the problem. But, they can do the next best thing — study it remotely with the Global Oscillation Network Group facility. GONG and the orbiting SOHO/MDI instrument measure sound waves on the surface of the Sun. Those sound waves are a very good probe of what’s going on inside the Sun — similar to the way a sonogram tells a doctor what’s happening inside your body. GONG and SOHO/MDI are vanguard instruments in the science of helioseismology.

What they are showing scientists is a clear “sonogram” of a jet stream of material that flows from east to west just beneath the Sun’s visible surface.  It’s called a “torsional oscillation” and a new one gets generated near the solar poles every 11 years. Over a period of 17 years,  the stream migrates slowly to the solar equator.  The most telling point abou these streams is that they are associated with the production of sunspots once they reach a critical latitude of 22 degrees.

Two scientists, Rachel Howe and Frank Hill of the National Solar Observatory, have found that the stream associated with the new solar cycle has been moving rather lazily — taking three years to cover a 10 degree range in latitude compared to two years for the last solar cycle. Since the current minimum is now one year longer than usual, Howe and Hill conclude that the extended solar minimum phase may have resulted from the slower migration of the flow.

Now that the stream has finally reached the critical 22 degree latitude, we should be seeing some more solar activity ramping up as time goes by.  It’s not clear why this torsional oscillation slowed down, but the good news is that the Sun’s magnetic dynamo continues to operate, and we’re probably seeing the beginnings of a new solar cycle.