|
Solar minimum is
looking strangely like Solar Max.
+
Play Audio | +
Download Audio | +
Historia en Español | +
Email to a friend | + Join
mailing list
September
15, 2005: Just one week ago, on Sept. 7th, a huge
sunspot rounded the sun's eastern limb. As soon as it
appeared, it exploded, producing one of the brightest x-ray
solar flares of the Space Age. In the days that followed, the
growing spot exploded eight more times. Each powerful
"X-flare" caused a shortwave radio blackout on Earth and
pumped new energy into a radiation storm around our planet.
The blasts hurled magnetic clouds toward Earth, and when they
hit, on Sept 10th and 11th, ruby-red auroras were seen as far
south as Arizona.
 So this is solar minimum?
Right: An X-flare photographed on
Sept. 9th by Birgit Kremer of Marbella, Spain. [movie]
Actually, solar minimum, the lowest point of the sun's
11-year activity cycle, isn't due until 2006, but forecasters
expected 2005, the eve of solar minimum, to be a quiet year on
the sun.
It has not been quiet. 2005 began with an X-flare on
New Year's Day--a sign of things to come. Since then we've
experienced 4 severe geomagnetic storms and 14 more
X-flares.
"That's a lot of activity," says solar physicist David
Hathaway of the National Space Science and Technology Center
in Huntsville, Alabama.
Compare 2005 to the most recent Solar Max: "In the year
2000," he recalls, "there were 3 severe geomagnetic storms and
17 X-flares." 2005 registers about the same in both
categories. Solar minimum is looking strangely like Solar
Max.
Scientists like Hathaway track the 11-year solar cycle
by counting sunspots. When sunspot numbers peak, that's Solar
Max, and when they ebb, that's solar minimum. This is supposed
to work because sunspots are the main sources of solar
activity: Sunspot magnetic fields become unstable and explode.
The explosion produces a flash of electromagnetic radiation--a
solar flare. It can also hurl a billion-ton cloud of
magnetized gas into space--a coronal mass ejection or "CME."
When the CME reaches Earth, it sparks a geomagnetic storm and
we see auroras. CMEs can also propel protons toward Earth,
producing a radiation storm dangerous to astronauts and
satellites. All these things come from sunspots.
Above: Ruby-colored Northern Lights
over Payson, Arizona, on Sept. 11, 2005. Photo credit: Chris
Schur. [gallery]
As expected, sunspot numbers have declined since 2000,
yet solar activity persists. How can this be?
Hathaway answers: "The sunspots of 2005, while fewer,
have done more than their share of exploding." Consider
sunspot 798/808, the source of the Sept 7th superflare and
eight lesser X-flares. All by itself, this sunspot has made
Sept. 2005 the most active month on the sun since March 1991.
Weird? Much about the sun's activity cycle remains
unknown, Hathaway points out. "X-ray observations of flares by
NOAA's Earth-orbiting satellites began in 1975, and CMEs were
discovered only a few years earlier by the 7th Orbiting Solar
Observatory. Before the 1970s, our records are
spotty."
This means we don't know what is typical. Scientists
have monitored only three complete solar cycles using
satellite technology. "It's risky to draw conclusions" from
such a short span of data, he says.
Above: Sunspot counts and X-flares
during the last three solar cycles. Note how solar activity
continues even during solar minimum. Credit: David Hathaway,
NASA/NSSTC.
Hathaway offers a cautionary tale: Before 2005, the
last solar minimum was due in 1996 and the sun, at the time,
seemed to be behaving perfectly: From late-1992 until
mid-1996, sunspots began to disappear and there were precisely
zero X-flares during those long years. It was a time
of quiet. Then, in 1996 when sunspot counts finally reached
their lowest value—bang!—an X-flare erupted.
"The sun can be very unpredictable," says Hathaway,
which is something NASA planners must take into account when
they send humans back to the Moon and on to Mars.
Returning to 2005: is this year an aberration--or a
normal rush to the bottom of the solar cycle? "We need to
observe more solar cycles to answer that question," says
Hathaway. "And because each cycle lasts 11 years, observing
takes time."
Meanwhile, Hathaway is waiting for 2006 when solar
minimum finally arrives. Who knows what the Sun will do
then?
SEND
THIS STORY TO A FRIEND
Author: Dr. Tony Phillips
| Production Editor: Dr. Tony Phillips
| Credit: http://science.nasa.gov/
| 
.gif){kind=spacer}
