U.S. Air Force studies solar phenomenon
Posted: July 14, 2003

The Air Force Research Laboratory (AFRL) experiment to study solar activity from Earth orbit, the Solar Mass Ejection Imager (SMEI), launched in January 2003 from Vandenberg AFB on the Air Force Space Test Program's Coriolis mission spacecraft, has accomplished a major milestone -- observing its first Earth-directed ("halo") coronal mass ejection in May.

A very bright and fast (~2000 km/s) mass ejection observed by SMEI on May 31. The ejection is observed within the field of the sunward camera out to a distance of 45 deg., about 1/2 AU, from the sun. Credit: Air Force
Built as a proof-of-concept imaging experiment and designed specifically to detect, track and forecast the arrival at Earth of coronal mass ejections (CMEs), SMEI has "seen" over a dozen spaceward-pointing ("limb") CMEs since launch. With this major step of its mission achieved, SMEI is helping scientists better understand, and predict with longer lead-times, the harmful solar effects on spacecraft.

The most important source of space weather at Earth, CMEs are vast clouds of plasma (ionized gas) and magnetic fields that periodically erupt from the sun. CMEs trigger geomagnetic storms potentially injurious to satellite electronics, disrupt communications, increase radiation doses for astronauts and high-flying aircraft, and damage ground-based power-generating equipment. If CMEs are more fully understood and accurately forecast, steps can be taken to mitigate their effects on space assets.

SMEI orbits 830 km above the Earth along the day-night terminator. It uses three CCD carefully baffled cameras that reject unwanted light. To image CMEs from 90 solar radii to beyond Earth orbit, the system must see objects as dim as 1% of the starlight and zodiacal background. The cameras point away from Earth and "sweep out" nearly the entire sky during each orbit (above image shows a recent, very bright and fast limb CME observed by SMEI).

The image below shows the halo event observed by SMEI that caused a geomagnetic storm (at Earth). This erupted from the sun following two very bright (X-class) flares from a newly active region (NOAA 365) near sun center on May 27-28. The two ejections likely merged together as one halo seen moving outward through the 7 deg. field of view of the SOHO LASCO C3 coronagraph.

SMEI all-sky image of the halo CME on May 29. Halo appears as a broad, bright ring centered on the sun (white cross). The halo is brightest in the north (top), but can be traced over a 150-degree arc. This image is built up over a full 100-minute orbit. The previous orbital image has been subtracted to enhance faint structures. Blacked out areas include masked bright sunlight and the experiment's 20 degree radius exclusion zone around the sun. Credit: Air Force
In the SMEI all-sky image, the halo appears as a broad, bright ring with the sun location at its center, moving outward after May 29, ~06:00. Although brightest to the north, the halo is visible as an arc over ~150 deg. of sky. Much of the sunward camera is blocked out by bright sunlight, so the halo is best seen with the middle camera (#2).

The CME reaches 1 AU (Earth orbit) and apparently passes over the Earth late on May 29 and early on May 30. In a movie of this event, the sky tends to brighten followed by a broad, diffuse band passing from Camera 2 into the third camera which views the night side, suggesting that SMEI observed the CME pass over the Earth's terminator and into the night side behind Earth.

The halo front trailed two interplanetary (IP) shocks that hit the ACE spacecraft at L1 (1 million miles in front of Earth) on May 29 at 11:55 and 18:30 UT, yielding speeds of 1160 and 992 km/s, respectively. A strong geomagnetic storm commenced on May 29, ~12:00 reaching the maximum Kp index until May 30, 03:00. The halo arrival coincided with a strong IP magnetic field that caused the storm. From the LASCO data the speed of the halo near the sun was ~1170 km/s, consistent with the halo speed in SMEI of ~1000 km/s and with the IP shock speeds.

The SMEI instrument was designed and built by a team of scientists and engineers from the Air Force Research Laboratory, University of California at San Diego, University of Birmingham (UK), Boston College and Boston University. It was supported by the Air Force, the University of Birmingham, UK, and NASA. The SMEI team looks forward to 3 to 5 years operation of the SMEI instrument resulting in greatly improved forecasts of geomagnetic storms.

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