Astronomers spend a lot of time studying what flows away from the
Sun, such as supercharged particles, hot gases, light, heat, and
other types of energy. They want to know what is coming toward the
Earth so that we can learn how to protect our astronauts, satellites,
and communications. Now scientists believe that by studying material
that flows into the Sun, they can better understand what comes out
of the Sun.
The inflowing material was not discovered until 1997, when researchers
studying images from the Large-Angle
Spectrometric Coronagraph (LASCO) onboard the SOHO
satellite noticed gas moving toward the Sun at speeds of 31 to 62
miles per second (50 to 100 km/s). The gas was somehow moving against
the powerful solar wind, which, at this distance from the Sun carries
material outward at 75 miles per second (120 km/s).
Dr. Neil Sheeley and Dr. Yi-Ming Wang at the Naval Research Laboratory,
in Washington, D.C., had seen material head toward the Sun before,
but these inflows were different. Appearing at about 1.7 million
miles (2.7 million km) above the Sun's surface, the inflows would
gain speed at first, then slow as they neared the Sun, only to stop
altogether about two solar radii [435,000 miles (700,000 km)] above
the surface. If gravity were causing the inflows to fall, they would
have sped up and traveled much faster-and they would not have stopped
To get a better look at the faint inflows, which are difficult
to notice in images showing bright features, the researchers created
special videos that show only what is moving or changing in brightness.
The videos are made by subtracting one image from another taken
a short time later. Anything that does not change in the time between
images, does not appear in the final image. If you were to use this
method on two sequential images of a stationary rock, your final
image would be blank! If a spider were crawling across that rock,
all you'd see is the spider.
Figuring it out
Drs. Sheeley and Wang then compared their videos with maps of the
Sun's magnetic fields made from images taken by the Extreme Ultraviolet
Imager Telescope (EIT) and data obtained at the National Solar Observatory.
They found that the inflows occurred at the boundaries between regions
of strong negative and positive magnetic fields. As material is
sent out from the Sun, it carries with it a magnetic field. Large
loops of magnetic field lines form, towering high above the Sun's
surface. Scientists now believe that these lines sometimes cross,
then snap to form new connections which collapse toward the Sun,
carrying with them clouds of charged particles.
The inflow regions seem to have staying power, and researchers
have been able to follow them over the course of many months. Some
of the inflows first appear on the left side of the Sun, then rotate
out of view as the Sun turns. The inflows reappear two weeks later
on the right side of the Sun, then pass out of view and reappear
on the Sun's left side after two more weeks. The number of inflows
has increased with the level of sunspot activity that has occurred
since SOHO first began observing the Sun in 1996.
A mixed up Sun
The Earth has a simple magnetic field, called a dipole, with one
north (positive) and one south (negative) magnetic pole, like a
bar magnet. Magnetic field lines run from a dipole's north pole
to its south pole. The Sun, on the other hand, has a very complex
magnetic field. During times of little sunspot activity, the Sun's
magnetic field resembles the Earth's simple dipole. However, sunspot
activity increases and decreases on an 11-year cycle. During times
of great sunspot activity, the Sun's magnetic field lines get twisted
and pulled in ways that cause regions of north and south polarity
to appear far from the geographical poles of the star. These fields
give rise to several forms of solar activity, including sunspots,
prominences, coronal mass ejections-and inflows. By studying the
inflows, scientists hope to better understand the Sun's magnetic
field and how it affects life on Earth.