麻豆传媒

For the past couple of years, our sun has been at the minimum of its 11-year activity cycle. Its face has been virtually spotless for months on end, and there鈥檝e been no dire alerts of titanic solar storms about to slam into Earth.

The problem is that this 鈥渜uiet sun鈥� has continued far too long 鈥� two years ago, a special task force predicted that the transition from the just-ended Cycle 23 to the upcoming Cycle 24 would come around March 2008. It didn鈥檛. (To be fair, there was sharp disagreement within the group at that time.)

Much fanfare accompanied the appearance of a tiny high-latitude sunspot in early 2008, supposedly heralding Cycle 24鈥檚 arrival. Yet for months and months afterward the sun鈥檚 face remained spotless.

Knowing when the upturn in solar activity begins and, more importantly, how strong it will get at maximum has grown in importance over the years. When the sun gets agitated, it buffets our planet with huge 鈥渟torms鈥� of high-speed plasma (ionised gas), punctuated by threatening flares of relativistic protons (see Space storm alert: 90 seconds from catastrophe).

Rhythmic pulsations

But despite centuries-long records of sunspot counts and 50 years of mapping the sun鈥檚 magnetic fields, scientists still don鈥檛 understand what makes one cycle strong and another weak. The same task force that was bullish on Cycle 24 two years ago now believes the forthcoming activity could be the weakest in a century.

This week, two groups of researchers offered hope that we鈥檙e finally understanding the sun鈥檚 complex workings a little better.

The first comes from Rachel Howe and Frank Hill of the National Solar Observatory, who now believe that sunspots are linked to slow, eastward-moving 鈥渏et streams鈥� about 7000 kilometres below the sun鈥檚 visible surface, or photosphere. They鈥檝e analysed 15 years of observations made using helioseismology, which monitors rhythmic pulsations at the surface created by pressure waves bouncing around the solar interior.

鈥淭hink of the sun as a musical instrument,鈥� Hill explains. 鈥淎 piano has 88 keys, but the sun has five million 鈥榥otes鈥� or modes of oscillation.鈥�

Deep-seated currents

Howe and Hill find that a pair of deep-seated currents migrate from the solar poles toward the equator during each cycle, and that the migration has been unusually sluggish of late.

They took three years to shift 10掳 towards the equator, and only now have they reached a solar latitude of 22掳, the point at which activity perks up and sunspots start to appear. They can鈥檛 yet tell whether the flow somehow causes sunspots, only that the two phenomena appear to be related.

鈥淗ad this analysis been available two years ago, we鈥檇 have seen the delayed onset of Cycle 24 coming,鈥� notes solar physicist Dean Pernell of NASA鈥檚 Goddard Space Flight Center in Maryland.

The new findings were presented this week at a meeting of the American Astronomical Society鈥檚 Solar Physics Division, which had a special session on Cycle 24.

Sunspot model

The second announcement concerns sunspots themselves and the arrangement of the intense magnetic tangles within them. Writing in Science Express, Matthias Rempel of the National Center for Atmospheric Research and three colleagues used a supercomputer grinding out 76 trillion calculations per second to create the first comprehensive, 3D model of these mysterious dark patches鈥� inner workings.

The simulations reveal in detail how superheated gas streams along magnetic filaments from a spot鈥檚 dark, central umbra to the lighter penumbra surrounding it.

Solar physicists first recognised this outward flow about 100 years ago. But, as Rempel鈥檚 team notes, 鈥淭he onset of these flows is closely related to the magnetic field inclination鈥� and that outflows occur whenever the magnetic field is inclined more than 45掳 from vertical.

The hope is that a better understanding of sunspots will allow scientists to predict their behaviour more accurately and, in particular, to identify the ones most likely to trigger dangerous solar flares.

Courtesy of magazine