16 Jun 2011

 

 

May be no sunspot cycle 25!

 

This is news that will leave many people open-mouthed, because the effects of even slight, long term variations on our star could have serious consequences here on Earth.
Three independent works, based on three different indicators of the solar cycle, have reached the same (not unexpected but still disquieting) conclusions: the next solar maximum, of the current cycle 24, predicted to occur in 2013, will be practically non-existent, while the next cycle 25, due in 2020, may never arrive!
The first of the three studies, all presented this week at the annual meeting of the Solar Physics Division of the American Astronomical Society (underway at New Mexico State University in Las Cruces) is by a team of researchers led by Frank Hill of the National Solar Observatory. Using the instruments of the Global Oscillation Network Group, they modeled the internal plasma flows that cause surface oscillations, identifying an east-west current that begins at intermediate latitudes and moves slowly towards the solar equator. This flow is tightly related to the formation of new sunspots, and its behaviour in the recent past led to the correct prediction of the late arrival of cycle 24.
When one solar cycle is underway, this flow of plasma re-organises itself for the next cycle, as indicated by the yellow-orange structures in the above diagram. But in the last few years there has been no sign of the new flow that would eventually give life to cycle 25!
The second study, carried out by Matt Penn and William Livingston, uses the strength of the magnetic field in sunspots as an index for the solar cycle. Spots typically form in magnetic fields of 2500-3500 gauss, while they don't appear at all at field strengths below 1500 gauss. The data collected during the last 13 years confirms a long term weakening of the sunspot magnetic fields, a trend confirmed by the increasing temperature of the spots: this actually tends to be inversely proportional to their magnetic fields.
In cycles 23 and 24 the mean magnetic field strength in sunspots decreased by 50 gauss per year, a trend that if it were to continue (and it look like it will) will make the average field intensity fall below 1500 gauss, making the appearance of sunspots a very rare phenomenon in the next decades.
The third study, led by Richard Altrock (Air Force's coronal research program at NSO's Sunspot), looked at the migration toward the solar poles of active regions in the corona, that for all their delicate beauty, actually trace intense magnetic structures rooted in the solar interior. Using highly ionised iron present in the corona (where temperatures exceed 2 million degrees Celsius) as a tracer and examining 40 years of observations, Altrock found that the speed of migration of the active regions, typically appearing between 70 and 80 degrees latitude, has been much slower in cycle 24.
As a consequence, the structures from cycle 23, still present beyond 80 degrees, may last much longer than normal, in some sense hindering the appearance the active regions of the next cycle.

 

by Michele Ferrara & Marcel Clemens

credit: National Solar Observatory, New Mexico State University