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Radio twinkling in Venusian ionosphere.

Radio Twinkling in Venusian ionosphere

Stars appear to twinkle because temperature variations create turbulence in Earth's atmosphere that changes its refractive index. Another kind of twinkling takes place in the ionosphere, the layer of charged particles at higher altitudes, where similar turbulence and other factors cause the number of electrons to fluctuate.

The result can be radio interference, and scientists now report what they call the first clear evidence that similar scintillations take place in the ionosphere of Venus.

The effect has been observed not only at Earth but also at Jupiter, Saturn and Uranus, where it has affected the radio signals of interplanetary spacecraft. All these planets have substantial magnetic fields, where interactions between spinning electrons and the magnetic field lines cause the "twinkles" in radio waves passing through their ionospheres. But it was unclear what the situation would be at Venus, whose magnetic field is extremely weak if it exists at all.

William L. Sjogren of Jet Propulsion Laboratory in Pasadena, Calif., was mapping the Venusian gravitational field by measuring the Doppler shift in radio signals from the Pioneer Venus Orbiter spacecraft, which has been there since 1978. Last year, however, in reanalyzing the data, he grew frustrated at what seemed to be "noise" that showed up sometimes in the signals. At first he suspected the solar wind (because the noise was detected during the solar-cycle maximum, when the solar wind is often strong) or electronic static caused by the spacecraft itself or its instruments.

But Richard woo of Jet Propulsion Laboratory, together with Sjogren and other colleagues, now reports in the Feb. 1 JOURNAL OF GEOPHYSICAL RESEARCH that Venus makes ionospheric twinkles on its own. They have not shown up often, Woo says; perhaps a month of twinkles occurred during the spacecraft's decade on the job. But that could be because Pioneer Venus spent most of its time orbiting too high for its Earthbound radio beam to go through the ionosphere.

The solar wind does play a part. Some other planets hold the solar wind at bay by magnetic fields, but on Venus it sometimes gets close enough to cause variations in the ionosphere's electron density, which can give a case of the twinkles to a radio beam passing through. That only happens, however, when the pressure of the incoming solar wind exceeds that of the ionospheric plasma facing it.

At Venus, the twinkling occur only in the subsolar region -- the part of the ionosphere directly facing the sun -- whereas in Earth's ionosphere they show up in both polar and equatorial regions, with additional peaks occurring during nighttime.

Another planet with little or no magnetic field but at least some atmosphere and ionosphere is Mars. (Sometimes still differ on whether the planet has an intrinsic magnetic field, and the Soviet Phobos 2 spacecraft probably will not get close enough to measure a weak one.) Is Mars likely to show the twinkles, too?

The likely answer is yes, suggests Janet G. Luhmann of the University of California, Los Angeles. In fact, she says, the day side of the Martian ionosphere may show such scintillations all the time. Mars is farther from the sum than Venus is, and it has a much thinner atmosphere, so its ionosphere is less substantial, too, and the solar wind easily gets in to stir it up.

The Venus study shows spacecraft can detect the twinkles without entering the ionosphere, as Pioneer Venus did, if they can send a radio beam through the ionosphere to the Earth. This suggests that Mars twinkles should be detectable with both the Soviet Phobos 2 mission and the upcoming U.S. Mars Observer, to be launched in 1992.
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Author:Eberhart, Jonathan
Publication:Science News
Date:Feb 25, 1989
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