Polar Spacecraft Measures "Auroral Fountain" Flowing Out As Solar Wind Flows In

December 08, 1998

Residents of the far north who saw a massive display of the aurora borealis in late September were also staring through an invisible fountain of gas being accelerated into space by a powerful bubble of solar wind, which pumped about 200 gigawatts of electrical power into the Earth.

At the same time, a special space weather research satellite was taking the first measurements showing that solar events can directly affect our outer atmosphere.

"This is the first time we've been able to correlate these solar coronal mass ejections (CMEs) with enhanced ion outflows from the upper ionosphere," said Dr. James Spann of NASA's Marshall Space Flight Center. Spann is a co-investigator on the Ultraviolet Imager, one of two instruments aboard the Polar spacecraft that measured the effects of the CME as it arrived at the Earth.

Today, results from those observations will be announced at the American Geophysical Union's annual west coast conference in San Francisco. They will be discussed in a special session, Thirty Years of Ionospheric Outflow: Causes and Consequences, chaired by Dr. Thomas Moore of NASA's Goddard Space Flight Center and formerly chief of the space plasma physics branch at NASA/Marshall.

In the early 1980s, scientists at NASA/Marshall, using an instrument aboard the Dynamics Explorer-1 (DE-1) satellite, discovered that the upper ionosphere is heated by electrical currents to form a "polar plasma fountain" of oxygen and hydrogen ions.

Follow-on studies by the Thermal Ion Dynamics Experiment (TIDE) aboard the Polar spacecraft, launched in 1995, have shown that the fountain rises higher than the DE-1 could measure. Indeed, the evidence is building that Earth's magnetotail is filled with ions not from the solar wind but accelerated upward from Earth's own atmosphere.

Polar is one of several geoscience spacecraft launched by NASA and other nations in a coordinated effort to study space weather - geomagnetic substorms and other events - in the space environment around Earth.

Polar's Ultraviolet Imager (UVI), on which Spann works, uses unique filters to take pictures of the aurora borealis - even during daylight. The brightness of these images can be translated directly into how much energy is being pumped into the ionosphere, the ionized top layer of the atmosphere.

TIDE, for which Moore is the principal investigator, measures how many ions are around the spacecraft, and their direction and speed. Because sunlight electrically charges the spacecraft so it repels the low-energy ions, TIDE has to use a small plasma gun to neutralize the charge. That is done infrequently because it also disrupts other instruments.

On Sept. 23, though, nature and NASA's planners were on the same schedule. On Sept. 22, the sun belched forth a CME, a roiling bubble of plasma (electrified gases) that sailed along with the solar wind on a collision course with Earth. TIDE's plasma gun already had been scheduled to be operating, and Polar's orbital position was above the northern hemisphere, when the CME arrived.

"The amount of upwelling ions is a function of solar wind pressure or activity," Spann said.

So when the CME hit, it squeezed Earth's magnetic field, squirting stored particles from the magnetotail up the field lines towards the Earth's poles.

As UVI showed an explosion in auroral brightness, TIDE measured a significant increase in oxygen and hydrogen ions rising from the Earth.

"What we are finding is that the magnetosphere, the space environment within Earth's magnetic field, is usually indirectly driven," Spann said. "When that energy is released and it rushes forward there is a time delay.

However, "with these large pressure pulses from CMEs, we are seeing the magnetosphere respond practically instantly. It's like you hit it with a bat. Everything rings at the same time."

Solar wind pressure is routinely measured by the Wind spacecraft positioned in front of the bow shock of the magnetic field. Normally it's around 2 or 3 nanopascals, far softer than a baby's breath. (see note on units)

But when the CME arrived on Sept. 24, it jumped to 10 nanopascals.

With direct cause-and-effect evidence in hand, Spann is looking through earlier UVI images.

"January 1997 was the first big CME event that we saw," Spann said. "There have probably been three or four since then when UVI was in position to observe when the shock arrived."

Unfortunately, TIDE's plasma gun was not turned on during those events. But, the September 1998 event will allow Spann and his colleagues to look for common features and gain more insight into how the solar wind, and blasts like CMEs, affect the Earth.

Their curiosity is more than academic.

"Normal values for auroral substorms are on the gigawatt levels," Spann explained, "enough to run a large city for several days. It's a tremendous amount of energy."

The energy can be calculated from the intensity of the light from the aurora borealis. On Sept. 24, a modest amount of energy - roughly 80 gigawatts - flowed in during several smaller substorms preceding the main event which pumped 200 GW.
-end-


NASA/Marshall Space Flight Center--Space Sciences Laboratory

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