An unexpected solar effect on Venus.

An unexpected solar effect on Venus

Pioneer Venus, a spacecraft that has orbited Venus for more than 11 years, revealed early in its mission that the number of free electrons in the planet's ionosphere often varies from one orbit to the next. The reason for this phenomenon, however, appears different from what scientists have assumed.

According to Larry H. Brace of NASA's Goddard Space Flight Center in Greenbelt, Md., two kinds of solar variations may affect the number of electrons in Venus' ionosphere: the sun's extreme ultraviolet (EUV) brightness, and the speed and density of the solar wind. Many researchers working on the problem have assumed that the number of electrons in Venus' ionosphere rises and falls with the amount of solar EUV light, which can free electrons by ionizing neutral atoms in Venus' sun-facing upper atmosphere. However, Brace and his colleagues report in the recently released April 1 JOURNAL OF GEOPHYSICAL RESEARCH that the density of the solar wind appears to influence the up-and-down electron population measured by Pioneer Venus to a far greater extent than does the sun's EUV. Sometimes, the authors suggest, the solar wind just transports the electrons around to the planet's night side. But on other occasions, when the solar wind contains a greater number of charged particles, it may "blow" the electrons completely past the ionosphere and into space.

Most of the difference in the number of Venus ionospheric electrons shows up in the "ionotail," a term Brace coined in 1987 to describe the portion of the ionosphere that the solar wind pushes out into space so that it resembles the tail of a comet. Unfortunately, even the 11 years' worth of data gathered by Pioneer Venus have not provided enough evidence to show conclusively what makes the number of electrons change.

But the craft has shown that the height of Venus' ionosphere apparently depends on the sun's cycle of energy output, which averages about 11 years. Data gathered near a time of minimum solar activity revealed very large changes in the number of electrons detected well down the ionotail, about 1,400 to 2,500 kilometers from Venus. However, Brace says, near solar minimum the variations in the sun's EUV output are smaller and their effect on electron production is difficult to judge.

Some measurements made at solar maximum showed that the sun was radiating less EUV light at that time and that Venus' ionosphere was lower, with the craft encountering electrons only 150 to 600 km above the planet. This time, the data showed that with the sun's EUV output varying widely, the number of electrons went up and down only a little.

As for the solar cycle's peak, "we don't have sufficient data for solar maximum yet," says Brace. "But we have enough data at times of moderate solar activity -- on either side of minimum -- to see that the ionosphere's electron densities do follow the solar-cycle variations." Brace's group is now looking at the craft's data for signs that some free electrons are being accelerated enough by the solar wind to leave the ionosphere.