Spot remover. A solar telescope watched as this small sunspot (arrow) vanished within an hour in the wake of a moderate solar flare.

A Vanishing Sunspot

The seething surface of the sun has conjured a disappearing act. A continent-sized sunspot vanished within an hour of a solar flare earlier this year, marking the first time scientists have seen a spot fade so quickly. The event will force theorists to explain how flares can unleash energy in such rapid bursts.

Sunspots mark the roots of giant loops of magnetic field that arc above the sun's visible surface. Powerful electric currents flow in twisted patterns along these arcs. According to models, solar flares erupt when the currents collide and snap suddenly into new alignments. That "reconnection" flings high-energy particles into the solar system, sparking auroras and threatening satellites and astronauts. Researchers have a poor grasp of the details of this process because they haven't caught the outbursts of magnetic energy that drive flares.

The latest images, reported in the 1 December issue of Astrophysical Journal Letters, have changed that luck. A team led by solar physicist Haimin Wang of the New Jersey Institute of Technology in Newark tracked a batch of sunspots on 20 February with a telescope at the Big Bear Solar Observatory near San Bernardino, California. A new instrument gave the team minute-by-minute updates on the strengths of magnetic fields in the spots. During the observations, a sunspot 5000 kilometers wide--considered small for our sun--disappeared within an hour. "We've been looking for this evidence, because theorists didn't believe the magnetic fields could change so rapidly," Wang says. At the same time, the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) satellite observed x-rays from a moderate flare at the same location.

This serendipitous detection could help reveal the workings of flares, according to solar physicist Alexander Kosovichev of Stanford University in Palo Alto, California. Combining the rate at which the magnetic field drained away with details of the flare's energy from RHESSI provides a "significant" new constraint for models of how flares are triggered, Kosovichev says. Wang's team is now analyzing sunspot images in conjunction with about 100 other recent flares to search for similar events.

Related sites
Haimin Wang's home page
RHESSI mission
Sunspot basics