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The Warp and Woof of a Geomagnetic Storm

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The warp and woof of a geomagnetic storm

New data display shows how energy deposited by the solar wind squeezes Earth's magnetosphere

click for CME animationDec. 29, 1999: Every now and then, the sun spits out a bubble of ionized gas known as a coronal mass ejection or CME. While it's easy to envision a special effect worthy of Star Wars, a CME actually carries a fairly weak punch since its energy is spread over a large volume of space.

Part of that energy can be concentrated when the CME washes over a large object, like the Earth and its invisible shield, the magnetosphere, that is generated by the Earth's magnetic field.

Right: QuickTime simulation of a CME impact on Earth's magnetosphere. With the sun off screen to the left, the earth's magnetosphere is visualized as a series of translucent blue shells that depict the magnetic fields at 60, 75, and 85 degrees from the equator. The electrically charged CME travels through the magnetosphere in roughly one hour, compressing the magnetic field lines and inducing a buildup in the earth's auroras. After the CME passes, the field lines quickly return to their original configuration and the auroras return to their pre-impact energy levels. For visual clarity, this simulation shows the auroral increase peaking well after the CME passes.

"If the Earth happens to be in the way [of a CME], then we're looking to see how the aurora responds," said Dr. Jim Spann, a space plasma physicist at NASA's Marshall Space Flight Center.

The magnetosphere, populated with ionized gases and electrons, is like an invisible shield around the Earth. The Earth's magnetic field forces the solar wind to part and slide around it. But at the same time, a gust in the solar wind can squeeze the magnetosphere, forcing some of the magnetosphere's particles earthward along the magnetic field lines. Particles energized enough to burrow as deep as the upper atmosphere produces the dazzling aurora borealis and magnetic storms.

With a team of three satellites, scientists now can make before-and-after measurements. Wind and the Advanced Composition Explorer, circling in a halo orbit about 1 million km sunward of Earth, measure the solar wind, moving at 300 to 600 km/s (up to 1.3 million mph), about 10 to 60 minutes before it is disturbed by the magnetosphere. Imagers aboard Polar, orbiting around the Earth's north and south poles, provide TV pictures of the aurora borealis.

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By using a special set of filters, UVI rejects most of the light coming from the bright Earth and passes just light from the aurora borealis on both day and night sides of the Earth. UVI's pictures provide a direct measure of activities back in the million-kilometer-long tail of the magnetosphere. In effect, the aurora acts as a mirror that reflects activities in the tail.

Spann and his colleagues believe that by looking in this mirror, they can get a better understanding of what is happening in the tail, and get a direct measure of how much energy is pumped into the ionosphere by events such as CME and the solar wind.

Earth has two auroral ovals, each more of less centered on the magnetic poles. The size of the oval varies with the energy pumped into it by the magnetotail. It's somewhat like extending your hands in a rubber band and pulling it open. Its takes more strength to enlarge the opening.

"We have seen that the aurora responds almost immediately, principally on the day side" when a CME sweeps past the Earth, Spann said. "We are now looking at how much of that energy is deposited on the the dayside relative to the rest of the oval."

Spann is studying three different periods, a quiet magnetospheric period, a series of three pressure pulses sweeping past the Earth, and a relatively active event in September 1998.

Right: What appears to be an abstract mosaic is a carefully woven tapestry showing the evolution of a geomagnetic storm as a pressure wave sweeps past and compresses the Earth's magnetosphere. Consider the mosaic as a flattened cylinder. Roll the mosaic (from top to bottom) and envision the cylinder slowly rising from the auroral oval. This display depicts the Jan. 10, 1997, event. The technique is described in the box at the bottom of the story. Links to a 843x812-pixel, 356KB JPG. Credit: Jim Spann, NASA/Marshall.

"While you see some activity on the dayside," Spann explained, by and large most of the activity is on the nightside."

This is a work in progress, and Spann gave a midterm report this month at the American Geophysical Union's semi-annual meeting in San Francisco. He has a total of 30 events from 1997-98 to examine.

He can also look forward to increased activity during the upcoming solar maximum when sunspot numbers and solar flares are expected to peak over the next year or two. This maximum is expected to be about average, but it will be the first in which scientists have instruments like those aboard the Polar spacecraft to observe the response of the magnetosphere and aurora.

Spann said that Polar has about 2-1/2 years of useful life remaining. While the instruments continue to work well, the spacecraft is limited by its remaining supply of attitude control propellant needed to reorient it to keep sensitive instruments in the shade.

 

The images at top are from a longer sequence taken by the Ultraviolet Imager aboard the Polar spacecraft. In this plot, the Earth is divided into half-hour segments, rather than longitude, and awlays centered on noon regardless of the Earth's position. The total energy for each half-hour slice was assigned a color and plotted on a vertical strip. Then strips are added as the pulse moves by and the magnetosphere reacts, creating a moving display of the energy pumped into the aurora during an event. This display technique was developed by Keith Brittnacher of the University of Alabama in Huntsville. The top picture, showing the Oct. 23, 1997, event image links to a 1067x792-pixel, 657KB JPG. The bottom picture, showing the Sept. 24, 1998, event links to a 1113x612-pixel, 563KB JPG. Credit: Jim Spann, NASA/Marshall.

Abstract: On the relationship of interplanetary pressure enhancements and subsequent dayside auroral activity
Fall AGU Meeting, San Francisco, California, December 12-17, 1999
J. F. Spann, M. Smith, G. Germany, D. Chua, M. Brittnacher, G. Parks
Solar wind (SW) mass, momentum and energy are transported into the magnetosphere by various mechanisms and on different time scales. A correlation between electron precipitation/auroral brightening on the dayside and enhanced SW pressure/velocity have been reported as early as 1961. Global far-ultraviolet images of the aurora offer a unique opportunity to better understand and quantify what solar wind parameters control the mass, momentum and energy input to dayside ionosphere on short time scales (less than 3 minutes). The relation between the arrival of solar wind pressure enhancements at the magnetopause and dayside auroral activity is examined using over 70 cases from 1997 to 1999. From previous work, it is known that a threshold exists for SW velocity and pressure enhancement in order for dayside auroral activity to always be observed. In this study, the amount of energy deposited near local noon as determined by using global Polar/UVI auroral images, is compared to the solar wind pressure and speed. We demonstrate that there is a correlation between the amount of energy deposited in the dayside ionosphere and the strength of the magnitude of the pressure enhancement. We also present a relationship between the delay time for near-midnight auroral activity after the arrival of the pressure enhancement at the magnetopause, and the coupling of the SW to the magnetosphere.

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