The ups and downs of solar flares.

The ups and downs of solar flares

Even a huge solar flare is tiny compared with the mass of the whole sun, but such an outburst ought to make itself felt somehow in a simultaneous downward movement of matter. To date, scientists have published only a single example of such balanced momentum associated with a flare, based on a 1985 observation. Now a group of scientists has identified four more such events.

The team began looking after the discovery of the 1985 flare. The key to the observations is the change in the wave-length of light emitted by solar material moving toward or away from the instrument measuring it. this effect, called the Doppler shift, is the same one that causes a train's whistle to rise or fall in pitch as the train approaches or recedes.

During each of the four newly analyzed flares, which occurred in 1980, instruments aboard the Solar Maximum Mission satellite detected a wavelength decrease, or "blueshift," in X-rays emitted by ionized calcium. The shift shows that calcium in a flare was moving out toward the satellite, while the X-rays revealed its temperature to be about 15 million kelvins, typical of the flaring corona.

As Solar Max recorded each blueshift, astronomers observing the same regions of the sun from the National Solar Observatory in New Mexico recorded "redshifts" in hydrogen (H-alpha) emissions, suggesting downward movement of hydrogen. The hydrogen temperature was "only" thousands of kelvins, typical of the chromosphere, the layer beneath the corona.

In other words, flares shot up from the corona just as material from the underlying chromosphere drove downward. To see whether the movements were really balanced, a group including Richard C. Canfield of the University of Hawaii in Honolulu and Dominic M. Zarro of Applied Research Corp. in Landover, Md., combined the Doppler wavelength measurements with estimates of the mass of solar material going up and down. According to the researchers, who have submitted their report to ASTROPHYSICAL JOURNAL, the momentum of the upflowing plasma producing the X-rays equaled that of the downflowing H-alpha plasma "to within an order of magnitude" each time.

Canfield says the evidence for momentum balance is rare because of the difficulties in observing red-and blueshifted emissions at the same time and from the same location on the sun. Also, Solar Max's polychromator, which measures the blueshifted X-rays, was inoperative from late 1980 until astronauts fixed it in April 1984.

The researchers' paper discusses several solar processes that might produce simultaneous X-ray and H-alpha flows. Most, however, either seem to send both kinds in the same direction, rather than X-ray plasmas up and H-alpha down, or do not produce a balance of momentum. The authors suggest such flares result from what amounts to an explosion--"the sudden creation of a high-pressure region at the footpoint of a coronal loop." The result would be "upflowing, hot (coronal) plasma and downflowing, cool (chromospheric) plasma," with "equal and opposite momenta."