Solar wind find may help space weather forecasting
Posted: May 28, 2001

Scientists have confirmed the existence of and imaged "free spirit" atoms -- atoms not subject to magnetic forces because they are electrically neutral -- in the solar wind for the first time. They hope to use the observations to better estimate the arrival time of solar storms and to estimate the amount of dust left over from the solar system's birth.

The scientific team used the Low Energy Neutral Atom imager (LENA) instrument on NASA's Imager for Magnetopause to Aurora Global Exploration (IMAGE) spacecraft to make the detection and produce the images.

This is a picture of the electrically neutral atoms in the solar wind as detected by NASA's IMAGE spacecraft on June 8, 2000. Two views are shown, one made between 8:14 and 9:14 a.m., and one taken between 9:15 and 10:15 a.m. The innermost white circle at the bottom center of each photo represents the Earth to scale, and the surrounding traces represent the Earth's magnetic field lines at four local times. The false colors represent the intensity of neutral atoms entering the LENA instrument, with red as the highest intensity and blue the lowest. Photo: NASA
"Our LENA instrument was designed to see neutral atoms coming from our own planet. It just amazes me that it is also taking us outside the Earth's magnetic field to study interplanetary gas and dust, and even outside the solar system to study the interstellar gas cloud," said Dr. Thomas Moore, Project Scientist for IMAGE at Goddard Space Flight Center, Greenbelt, Md. Moore is lead author of a paper describing the neutral atom detection published in Geophysical Research Letters.

The solar wind is a stream of electrified and magnetic gas that blows constantly from the Sun at speeds of about 250 miles per second. The gas is electrified because heat and radiation from the Sun remove electrons that normally surround the gas atoms and balance their electric charges. Scientists thought that within this gale of electrified atoms, a few might manage to obtain a complete complement of electrons and thus be rendered electrically neutral (the number of surrounding electrons, which are negatively charged, balances the number of positively charged protons at the center of the atom). The detection by IMAGE is the first confirmation of this idea, and the surprising result is that such atoms may be up to ten times more common than previously thought.

Electrically neutral atoms become the "free spirits" of the solar wind because they do not feel magnetic forces, and thus can chart their own paths through the solar system, ignoring the contours of the ubiquitous magnetic fields in interplanetary space that their electrically charged cousins must follow. Like free spirits everywhere, neutral atoms are rare; for every 10,000 electrified solar wind atoms, there exist no more than 10 neutral atoms. Nevertheless, this faint whisper of particles snubs the Earth's magnetic field that blocks the electrified solar wind, offering a rare glimpse of solar wind activity from within our magnetic shield.

Solar scientists have proposed that neutral atoms may help to better estimate the arrival time of solar storms, which occasionally disrupt high-technology systems at Earth. Some solar storms are caused by the impact of billion-ton clouds of electrified gas with the Earth's magnetic field. Such clouds, called Coronal Mass Ejections (CME's), are blasted from near the visible surface of the Sun at speeds of up to 4.5 million miles per hour.

According to the theory, the CME clouds should carry a higher population of neutral atoms than the solar wind because they come from near the solar surface, where it is much cooler than the solar atmosphere, which is the source of the solar wind. With less heat, fewer atoms will lose their electrons, and thus more neutral atoms are carried within a CME cloud. As it races through space, the CME cloud slows down because it plows into the slower solar wind and its magnetic field. Because they can't feel magnetic forces, the CME neutral atoms ignore what the electrically charged majority is doing and continue to travel at the original speed of the CME.

This leads some solar scientists to propose that there should be a burst of neutral atoms at Earth shortly before the arrival of an Earth-directed CME cloud (about 2 - 3 hours before impact, on average). If further observations with IMAGE show this to be correct, it could greatly benefit space weather forecasts, because it is difficult to gauge the distance and speed of CME clouds when they are heading directly towards us.

However, most solar wind neutral atoms are made, not born; that is, they become neutral by taking electrons from another atom that is electrically neutral. Like an atomic version of a purse snatching, a fast electrified atom in the solar wind passes near an existing neutral atom and steals an electron from it, then races on, blissfully unaware of the magnetic field that once constrained its motion.

There are three sources of original neutral atoms encountered by the solar wind as it flows toward Earth: neutrals that are released from the dust grains left over from the cloud that formed the solar system, neutrals from other stars (in interstellar gas clouds that the solar system passes through), and neutrals from the Earth's upper atmosphere.

The 11-year cycle of violent solar activity will change the amount of neutrals from Earth, while the amount of interstellar neutrals will vary as Earth progresses in its orbit around the Sun (from upwind to downwind in the interstellar gas flow through the solar system). This will cause the solar wind neutrals detected by IMAGE to vary accordingly. By making long-term observations and subtracting these variations out, researchers using IMAGE can estimate the neutral atoms contributed by the solar system dust (which is constant), and thus get a better estimate on how much dust remains.

"Estimates on the amount of primordial dust in the inner solar system vary by a factor of 10,000, so this will really help us get a better picture of how much dust there is inside of Earth's orbit," said Dr. Michael Collier, LENA experiment scientist and co-author of the neutral atom detection paper.

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