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Using data collected by NASA’s STEREO
spacecraft, researchers at Southwest Research Institute and the National Solar
Observatory have developed the first detailed images of solar wind structures
as plasma and other particles from a coronal mass ejection (CME) traveled 93
million miles and impacted Earth.
The images from a December 2008 CME
event reveal an array of dynamic interactions as the solar wind, traveling at
speeds up to a million miles per hour, shifts and changes on its three-day
journey to Earth, guided by the magnetic field lines that spiral out from the
Sun’s surface. Observed structures include the solar wind piling up at the
leading edge of a CME, voids in the interior, long thread-like structures, and
rear cusps. Quiet periods show a magnetic disconnection phenomenon called a
plasmoid, “puffs” that correlate with in-situ density fluctuations, and V-shaped structures
centered on the current sheet—a heliospheric structure in which the polarity of
the Sun’s magnetic field changes from north to south.
“For the first time, we can see
directly the larger scale structures that cause blips in the solar wind
impacting our spacecraft and Earth,” says SwRI’s Craig DeForest, lead
author of an Astrophysical Journal
article. “There is still a great deal to be learned from these data, but
they are already changing the way we think about the solar wind.”
“For 30 years,” says coauthor
Tim Howard, also of SwRI, “we have been trying to understand basic anatomy
of CMEs and magnetic clouds, and how they correspond to their source structures
in the solar corona. By tracking these features through the image data we can
establish what parts of a space weather storm came from which parts of the
solar corona, and why.”
The team used a combination of image
processing techniques to generate the images over a distance of more than 1 AU
(astronomical unit), overcoming the greatest challenge in heliospheric imaging,
that of extracting faint signals amid far brighter foreground and background
signals. Small “blobs” of solar wind tracked by the team were more
than 10 billion times fainter than the surface of the full Moon and 10 thousand
times fainter than the starfield behind them.
“These data are like the first
demonstration weather satellite images that revolutionized meteorology on
Earth,” says DeForest. “At a glance it is possible to see things from
a satellite that cannot be extracted from the very best weather stations on the
ground. But both types of data are required to understand how storms
develop.”
In particular, the new images reveal the shape and density of Jupiter-sized
clouds of material in the so-called empty space between planets; in contrast, in-situ probes such as the WIND and
ACE spacecraft reveal immense detail about the solar wind, at a single point in
space.
