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News & Analysis ISRO's Anil Bhardwaj Finds Saturn and Jupiter Reflect Sun's X-Rays  
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31 May 2005 -- Scientists who study the Sun are constantly frustrated by the fact that they can only see half of it at any given moment. Sunspots that grow, die and flare up on the back side can't be monitored.
What's needed is a giant mirror on the other side of the Sun.

Turns out there are two of them, at times. Saturn reflects X-rays from solar eruptions, astronomers said today. Previous observations found the same phenomenon at Jupiter. So when either of these planets is located on the far side of the Sun, from our point of view, they could be used to monitor back-side solar activity.

"The bigger the planet and nearer to the Sun, the more solar photons it will intercept, resulting in more reflected X-rays," said study leader Anil Bhardwaj, a planetary scientist at NASA's Marshall Space Flight Center. The observations, which built upon an earlier X-ray study of Saturn, were made with NASA's Chandra X-ray Observatory. This time, researchers were able to tie the X-ray emissions at Saturn to an X-ray flare on the Sun.

Many times, solar flares are accompanied by potentially deadly proton storms, which can reach Earth in a few hours or, as another study this week found, just 15 minutes. Flares can also fuel coronal mass ejections, which are giant clouds of electrified gas that billow into the solar system and take up to a day or more to hit Earth.

Is Earth’s technology safe from the Sun’s fiery flare-ups? Go deep inside NOAA’s Space Environment Center -- the first place on Earth to feel the Sun’s wrath. Any of these dramatic storm types can be dangerous to astronauts. When one is aimed at Earth, residents of the International Space Station are sometimes ordered into a well-protected module to avoid excess radiation exposure.

If humans travel to Mars, some sort of interplanetary space weather warning system will be needed, especially when the red planet's orbit carries it to the far side of the Sun. With little atmosphere and virtually no protective magnetic field, Mars is naked to space weather. Residents would have to seek shelter during solar storms.

"These results imply we could use giant planets like Jupiter and Saturn as remote-sensing tools," Bhardwaj said. "By reflecting solar activity back to us, they could help us monitor X-ray flaring on portions of the Sun facing away from Earth's space satellites." X-rays are also emitted from Venus, Mars and the Moon.

Meanwhile, the new observations fuel an ongoing mystery.

Jupiter and Earth emit two general types of X-rays, auroral emissions from polar regions and disk emissions from low latitudes, nearer the equator. Auroral X-ray emissions from Saturn have never been firmly detected, however. "We were surprised to find no clear evidence of auroral X-ray emissions during our observations," Bhardwaj said. "There is much more we have to learn."

The research was detailed in the May 10 issue of the journal Astrophysical Journal Letters.

BACKGROUND

In a talk he gave last September on 'Remote Sensing of Solar System Bodies in Soft X-Rays,' Bhardwaj presented a comparative view of the x-ray observations on planets, comets, and moons, with emphasis on recent results.

He explained: "During the last few years several solar system bodies have been discovered to be the source of soft x-ray emissions. With these recent discoveries the solar system bodies that are now known to emit x-rays encompass a wide variety of objects. Apart from the Sun, these include planets (Venus, Earth, Mars, Jupiter, and Saturn); planetary satellites (Moon, Io, Europa, and Ganymede); minor bodies (comets - all active comets, and asteroids); extended objects (Io plasma torus, and Saturn rings), and large cavity (the Heliosphere)."

Because these X-rays provide unique information about the key physical processes operating in the atmospheric [or surface] and magnetospheric regions where they originate, they have become an important spectral regime for doing planetary remote sensing.

Bhardwaj added: "With the advent of sophisticated x-ray observatories, viz., Chandra and XMM-Newton, the field of planetary x-ray astronomy is advancing at a much faster pace. The recent data are providing a new insight into the processes of x-ray production and related phenomenal parameters, like energetics, dynamics and electrodynamics, and composition of plasmas, in planetary upper atmospheres, magnetospheres, and/or solar wind."

Dr. Anil Bhardwaj received his B.Sc. (Hons) and M.Sc. in Physics from Lucknow University and a Ph.D. in Physics (Space and Planetary Science) from the Institute of Technology, Banaras Hindu University.

He joined the Indian Space Research Organization in 1993 as a senior research scientist at the Space Physics Laboratory of the Vikram Sarabhai Space Center, Trivandrum, India. Dr. Bhardwaj is currently a NRC Senior Research Associate at NASA, MSFC.

His primary research field is the theoretical and observational studies of planetary atmospheres and ionospheres. Specifically, his areas of research interest are aurora and airglow processes in planetary atmospheres, the chemistry of planetary and cometary upper atmospheres and ionospheres, Monte Carlo simulations of charged particle acceleration and precipitation in gases, production of energetic neutral atoms (ENA) and their interaction with atmospheres, ground-based and space-based multi-spectral studies of planetary bodies, and comparative planetology.

The planetary bodies that he has studied include Jupiter, Saturn, Uranus, Neptune, Io, Europa, Ganymede, Triton, Titan, Mars, Earth, and Comets (particularly comet 1P/Halley). Dr. Bhardwaj is a PI and Co-I on GMRT, Chandra, and the HST observation programs, and a Co-I on ISRO-RESPOND project, and the Indian PI on the SARA experiment on Chandrayaan-1 (Indian Moon mission).

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