|
07/07/2004
Stormy Weather
Last week I started to discuss a proposed “hydrogen economy”
and fuel cells but was diverted when I found that hydrogen was
not guilty in the Hindenburg disaster. I planned to discuss that
hydrogen economy this week but got diverted again, this time by
some mind-boggling pictures. No, not those pictures of Saturn’s
rings relayed by the Cassini spacecraft, but pictures of the Sun in
an article and insert in the July issue of National Geographic.
The insert includes one very large photograph of a region of the
Sun containing a number of Sunspots, one large enough to
swallow our Earth. The detailed patterns of swirling gases and
cell-like structures around those dark Sunspots are truly stunning.
Actually, we’re already living in the ultimate “hydrogen
economy”. Hydrogen has been fueling the Sun for billions of
years and we’ve known for over 60 years that its heat and light
come from the nuclear fusion of hydrogen to form helium, as in
the hydrogen bomb. Fortunately, 90 percent of the atoms in the
Sun are still hydrogen so there’s plenty of fuel to keep it shining
for billions of years to come. The Geographic article, “The Sun.
Living with a Stormy Star” by Curt Suplee, quotes John Harvey
of the National Solar Observatory: “The Sun is the only
astronomical object that critically matters to mankind.”
While our lives depend on the heat and light from the Sun, it also
sends our way a steady solar wind of radiation and particles, with
occasional bursts of stuff that can be very troublesome. With
talk of colonizing the moon and even going to Mars, it’s vital
that we have advance warnings of and protection of our
astronauts and their electronic equipment from these powerful
blasts of radiation and matter. In other words, we need to be able
to become expert forecasters of “space weather”.
To forecast the weather in space, we must understand the Sun’s
composition and structure. First, we know it’s composed of
gaseous hydrogen and helium, with a smattering of other
elements. But is not your ordinary kind of gas. Depending on
where you are in the Sun, temperatures range from roughly 4000
to 15 million degrees Centigrade. At these temperatures, the
hydrogen atom’s electron is stripped off leaving behind its
nucleus, a proton. The gas is actually a “plasma”, a mixture of
protons with their positive charges and electrons with their
negative charges. (The atoms of helium and other elements in
the Sun are also stripped of their electrons in this plasma.) This
plasma, with all its charged particles, is electrically conducting.
With all those electrical charges swirling around, the Sun is one
big ball of magnetism.
The core of the Sun is the nuclear furnace in which the hydrogen
protons fuse to form helium. According to a NASA Web site,
the density of the Sun’s core is about 150 times the density of
water. At the surface, the density drops to only 2 ten millionths
of the density of water. This wild variation averages out to an
overall density of the Sun of about 1 gram per cc, the density of
water. But I digress. Back to the core, key products of this
nuclear furnace are high-energy photons known as gamma-rays.
These gamma-rays from the core take a long time, maybe
200,000 years or so, to get to the surface of the Sun. By the time
they reach the surface, they’ve been scattered, absorbed and re-
emitted zillions of times and have lost so much energy that they
are degraded into low-energy photons of visible, ultraviolet or
infrared light. After their long journey to the surface, these
photons take only 8 minutes to get to Earth!
In traveling from the core to the Sun’s surface, the photons pass
through a “radiation” zone surrounding the core into a so-called
“convection” zone. It’s in this cooler convection zone, which
extends out to the surface that things get messy. Here, bunches
of plasma swirl around in all manner of patterns made more
complex by the fact that different regions of the Sun rotate at
different speeds. As a result, the magnetic lines of force are
distorted - twisting, breaking and reconnecting.
Many years ago, I remember that pictures of the Sun taken
during eclipses showed only flame-like flares in the corona of the
Sun. With modern advances in astronomical techniques, we
don’t have to wait for eclipses to see the outer features of the
Sun. Now we see not only flares but also spectacular loops of
plasma extending far above the surface. These loops are plasma
following stretching magnetic lines of force, like iron filings line
up when dumped on a sheet of paper over a magnet. The loops
emerge (or exit) from (into) pairs of sunspots. The magnetic
energy contained in these stretched out loops of plasma can be
enormous. Sort of like rubber band that is stretched; if you’re
holding a stretched rubber band and it snaps, it can give you
quite a sting.
Well, if one of these loops of plasma snaps, the energy released
can be huge, equivalent to many billions of atom bombs going
off at once. The result is a “coronal mass ejection”, or CME. A
CME is a blast of billions of tons of plasma into space at millions
of miles an hour and if it’s heading our way it can mean big
trouble. (A solar flare can also emit potentially damaging
radiation.) Although the density of the plasma in a CME is so
low that in a lab we’d consider it quite a good vacuum, for a
spacewalking astronaut it could be fatal.
With its plasma of charged particles, the CME carries its own
magnetic field. Fortunately, here on Earth, we have our own
magnetic field and our “magnetosphere” does a great job of
protecting us from damaging radiation. It let’s the uncharged
heat and light photons go through into our atmosphere, which
filters out the most damaging UV and X-rays. However, a
magnetic field deflects the charged particles in the CME plasma.
If the polarity of the CME magnetic field matches the polarity of
the magnetosphere, that’s good. If it’s the opposite, that’s bad.
I imagine that it’s similar to the situation where you have two
magnets. Try to push the two north poles together, and the
magnets repel each other. If the magnets are strong and you put
their north and south poles facing each other, the magnets will
slam together. If the CME polarity is opposite to the Earth’s, the
CME slams into our magnetosphere and distorts our magnetic
field, causing millions of amperes of current to circulate around
our planet. This can shut down an entire power grid as happened
in Canada in 1989 when millions of people around Montreal
were without power.
Obviously, we would love to be able to predict when CMEs and
significant flare bursts will occur. In the past 20 years or so,
scientists have made great strides towards the prediction of space
weather by seeing what’s going on inside the sun. We’ve also
positioned satellites out a million or so miles in space to give us
at least minutes or hours warning of approaching bursts.
How do we see what’s going on inside the Sun? To see into our
earth, we use seismology, the study of earthquakes and the waves
they generate. By following the speeds, the intensities and the
paths of these earthquake waves, seismologists can figure out a
lot about the structure and composition of the innards of our
planet. The past 20 years or so has seen great strides in
“helioseismology”, the seismology of the Sun. Key players in
this relatively new science are the SOHO (Solar and Heliospheric
Observatory) satellite, a joint effort of NASA and the European
Space Agency, and GONG (Global Oscillation Network Group),
an international network of observatories linked to provide
continuous observation of the sun on a 24-hour basis.
You say the sun is made of gas and doesn’t have earthquakes, so
how can you do seismology? It turns out that gas is a good
conductor of sound waves and, sure enough, the Sun is loaded
with all kinds of sound waves. For example, look at the surface
of the Sun with an appropriate detector and you see a multitude
of cells of gas that pulse up and down in cycles with a period of
only five minutes. You can follow this up and down motion by
using the familiar Doppler effect. As the cell moves towards
you, the light becomes bluer; as the cell moves away from you it
becomes redder.
I marvel that, using extensive observations of these and other
types of waves in conjunction with appropriate computer data
crunching and modeling, researchers can literally look into the
Sun’s interior. They can measure the time it takes for waves to
hit the opposite side of the sun and make their ways back to the
surface closest to us. Amazingly, they can spot potential CMEs
on the other side of the Sun and thus have advance warning of
possible danger when that particular CME comes around and
points to the Earth.
The helioseismologists have actually been able to map the
weather inside the Sun thousands of miles deep in the convection
zone. The Geographic article shows a computer model showing
the equivalent of our jet streams with big fronts of plasma
marching around the equator and cyclone-like storms at the
higher and lower latitudes. Who knows? Some day, they may
well achieve the goal of predicting “space weather” from their
data on internal “solar weather”. If so, our intrepid astronauts on
the Moon or Mars would be forever in their debt!
Allen F. Bortrum
|
