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Public Affairs
Air Force Research Laboratory

CONTACT: John Brownlee
PHONE: (505) 846-4704

July 02, 2003

VS RELEASE NO. 03-21

AIR FORCE STUDIES SOLAR PHENOMENON

HANSCOM AIR FORCE BASE, M.A. -- 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.

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 percent of the starlight and zodiacal
background. The cameras point away from Earth and "sweep out" nearly
the entire sky during each orbit (Figure 1 shows a recent, very
bright and fast limb CME observed by SMEI). 

Figure 2 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. 

In the SMEI all-sky image (Figure 2), 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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