What's Up?
The Sun:101!
The sun is a moderate sized (1,400,000 km in diameter), G spectral type, variable star in a vast collection of
an estimated 200-400 billion stars in a barred spiral galaxy called the Milky Way. Though it is a smallish star,
most stars in our galaxy are smaller, and less massive. Our sun is, like most stars in our galaxy and probably
most stars, a main sequence star which means it generates its energy by thermonuclear fusion resulting in the
release of gamma ray photons during a process in the core where hydrogen nuclei (protons) are fused together
to form helium nuclei. This process, converting hydrogen to helium, driven by the enormous internal pressure
of the sun's mass (2x1030 kg or over 300,000 times the mass of the Earth!) is what makes a star a star. Since
everything forms from a roughly basic and somewhat consistent cosmic menu of elements (75% H, 24%
He,...) the variable that determines whether you get an asteroid, a planet, or a star is mass. Enough mass
creates enough pressure to start this fusion process and create a star. Not enough mass and you have a failed
star or planet (even less mass and you get an object that is not round at all). It's that simple. The sun formed
around the same time that the planets formed, from a molecular cloud - remnant of a super nova explosion.
The sun is very old, by human standards but only middle aged by stellar standards, about 4.6 billion years old
and it should be around in a more or less similar form for maybe another 5 billion years.
It might be interesting to note that considering only classical (or Newtonian) physics, the physics that you
learned in high school, the physics that defined reality for around 400 years, since Isaac Newton gave his
three laws of motion, the sun and all the stars cannot shine. Classical physics does not allow two similarly
charged protons to bond. In addition, matter - energy equivalency (E=mc2) was not known until the early
1900's. So before Einstein and before quantum physics, there was no plausible explanation for how the sun
generated its energy. Want to know more? Read "On the age of mostly everything".
The sun rotates. This was first observed by Galileo Galilei in 1610 when he observed dark "blemishes"
(sunspots) on the sun. The observed motion of these sunspots across the face of the sun was the first evidence
that the sun rotated on its axis and made it easier to imagine that the Earth rotated too, a concept foreign to the
long accepted Ptolemaic and Aristotelian models of the universe which stated that the Earth stood still while
the cosmos revolved around it. This was also contrary to church doctrine at the time which accepted the still
Earth model and asserted that the heavens were perfect, so no blemishes on the sun. Needless to say, this got
Galileo into a lot of trouble. It turns out that almost everything in space rotates, stars, planets, moons,
galaxies, clusters of galaxies, clusters of clusters of galaxies, everything rotates. And ironically, the rotation of
the sun is responsible for the sun having sunspots! Make a mental note of this for later.
Back To The Sun
The high pressure in the sun's core heats matter to very high temperatures (around 15 million K) resulting in
an abundance of charged/ionized particles (plasma). The sun's rotation combined with convection from the
heated material transports this charge around the sun. Now remembering Michael Faraday's discovery that
moving charge generates a magnetic field, this is how the sun's magnetic field is generated - electromagnetic
induction within a star. When this process operates within a star or planet, we call it a dynamo. Because the
sun is composed of plasma and not solid material, its rotation rate is not constant. Therefor, the motion of
ionized gas around the sun is not constant. The sun rotates slower at high latitudes (near the poles) than at the
equator. This differential motion causes stretching and twisting of the sun's magnetic field creating a complex
field that is the sun's magnetosphere. The complexity of the field increases from solar minimum to solar
maximum, as lines of magnetic force are wound together and distorted. As field lines, generated deep within
the sun emerge above the sun's surface or "photosphere", they create a strong local magnetic field that
prevents deeper and hotter, convecting plasma from rising to the surface. The area around this field then
appears cooler than the surrounding photosphere and so darker. These dark regions are called sunspots. Since
magnets and so magnetic fields always have a north and a south pole, the highly magnetic sunspots often
occur in pairs (e.g. north and south poles) and can be seen channeling hot plasma gases between them in
structures called prominences.
This occurs because the electrically charged particles comprising the plasma are attracted to magnetic fields
or lines of magnetic force and so are pulled along field lines from one pole to another. Other stars have
sunspots too! We have both direct visual evidence of this as well as inferred evidence from periodic increases
and decreases in a star's brightness. Other evidence includes Zeeman line splitting and Doppler imaging.
Since sunspots require a magnetic field, differential rotation, and a convective stellar envelope, we can see
visual evidence that other stars are similar to the sun. Massive stars have convective cores and radiative
envelopes. This generally produces a weaker magnetic field and fewer sunspots. Astronomers have observed
sunspots (well, starspots) ands sunspot cycles on a few other stars which helps determine the star's rotation
rate and tell us about the star's internal processes.
Solar Wind
Figure 10: Bow shock, about half a light-year across, created from the wind from the star L.L. Orionis colliding
with the Orion Nebula flow
The solar wind is the stream of charged (mostly protons and electrons) and neutral particles and associated
electric and magnetic fields that moves outward from the sun at high velocities and populates and defines the
Interplanetary Magnetic Field or IMF and the heliosphere. A good definition of the heliosphere might be;
"that region of space dominated by the IMF". These particles are believed to be accelerated to very high
speeds (~500 to 2,000 km/s) by sudden releases of energy associated with a process called magnetic
reconnection around highly magnetic sunspot regions. The magnetic field of the solar wind is constantly
pushing on the Earth's magnetic field, moving, compressing, and distorting the field and creating electrical
currents Solar wind velocities and magnetic field strength vary with the solar cycle, the sun's magnetic
latitude, and with radial distance from the sun. At solar maximum, the solar wind velocity is greatest at the
sun's equator and less at the poles. Overall, the sun's magnetic field strength is low and polar coronal holes
extend to low latitudes. This trend reverses at solar minimum; the solar wind slows down, the sun's magnetic
field strength strengthens, and polar coronal holes retreat or disappear altogether.
The Heliosphere
The realm of the sun's influence extends out beyond the orbits of the planets to the far reaches of the
heliosphere, the vast bubble of charged solar wind particles and associated electric and magnetic fields that
define the limits of our solar system and interplanetary space. The extent of the heliosphere is not well known
but is believed to extend to on the order of 150 A.U. or so from the sun. Expanding out from the sun,
energetic particles carrying the sun's magnetic field become dispersed. As their density decreases with
increasing distance from the sun, they are not able to push as strongly against particles from interstellar
winds. The point where the two pressures balance is called the heliopause. As solar wind particles encounter
interstellar particles near this boundary, they slow down transforming their kinetic velocities into thermal
energy. The accumulation of these particles near the edge of the heliosphere (think of traffic backups during
rush hour) coupled with their deceleration energies creates a sort of shockwave called the termination shock.
The two Voyager spacecraft, launched in 1977, are now very near the outer edge of the heliosphere at
distances of about 123 AU and 101 AU respectively (Feb 1, 2013) and have already pierced the termination
shock at 94 and 84 AU. Beyond the termination shock is the heliosheath, perhaps 10's of AU thick and
bounded at its outer point by the heliopause, the final point of influence of the sun's magnetic field. Beyond
the heliopause, the Inter Stellar Medium (ISM) populates the vast spaces between the stars. Each of these
regions is characterized by the density, strength, and velocities of its fields and particles. The two Voyager
apcecraft will sample these regions with a suite of field and particle instruments until about 2025 when their
nuclear power sources will run out. Scientists anticipate that the Voyagers will have passed into interstellar
space by that time.
The shape of the heliosphere is defined by the opposing pressures of the IMF and ISM. And, the IMF pressure
is a function of the speed and direction of the solar wind which is in turn impacted by the solar cycle.
Solar Max And The Effects Of Solar Variability
As previously mentioned, our sun goes through cycles of activity. The most prominent of these cycles is the
11 year sunspot cycle though there are other cycles (The Hallstatt solar cycle (2300 years) and the Gleissberg
solar cycle (80-90 years), etc.). The number of sunspots in a given year varies predictably over a roughly 11
year period. This year, 2013, is projected to be the peak of the sunspot cycle called SOLAR MAXIMUM
(Solar Max). Other changes in the sun can be observed throughout the solar cycle. During Solar Max the
frequency of solar storms is at a peak. Solar storms are huge releases of electromagnetic and particle energy
driven by magnetic reconnection events in the sun's corona. These storms in the form of solar flares and
Coronal Mass Ejections (CMSs) hurl large amounts of high energy electromagnetic and particle radiation into
the solar system carrying with it a portion of the sun's magnetic field. As these storms impact solar system
bodies, they modify the electrical, magnetic, and chemical environments of planets, moons, and smaller
bodies (asteroids and comets). Solar storms as well as the more constant solar wind cause changes in
atmospheric chemical composition of planets and moons (e.g. Titan) due to both photo and particle
disassociation. Changes to surface properties such as the beautiful reds, and yellows in the surface of Io are a
direct result of high energy radiation modifying surface properties. The shapes of the magnetospheres of the
gas giants - Jupiter, Saturn, Uranus, and Neptune are modified by changes in solar wind and solar storm
pressure interacting with the planet's magnetic dynamo. This also produces auroras observed in the UV in all
the outer planets.
There is strong evidence from orbiting spacecraft and surface rover observations that Mars once had a global
magnetic field (magnetosphere), warmer temperatures, oceans of liquid water, and much higher atmospheric
pressures. Today, Mars is a cold, arid planet with no global magnetic field and a rarified atmosphere by
Earthly standards. What happened? It is generally believed that Mars lost its magnetosphere as its internal
dynamo cooled and froze out. This gave the solar wind open access to Mars' atmosphere resulting in
atmospheric erosion from solar wind particles and the eventual loss of water and most of the rest of the
atmosphere. So, the solar wind is in part responsible for the enormous climate change that Mars has
experienced over the last few billion years. Other contributors to this process include meteor collisions and
absorption of CO2 by the Martian surface.
The shape of the sun's corona changes during this cycle. The shape of the corona is determined by the sun's
magnetic field. High energy galactic cosmic rays (GCRs) from violent cosmic events such as super nova
explosions are scattered by solar wind particles. Since the solar wind is emphasized during periods of high
solar activity, fewer cosmic rays enter the inner solar system and so our atmosphere. Interactions between
cosmic rays and Earth's atmosphere produce increased levels of Carbon-14 (14C). This increase in Carbon-14
provides historic records of solar cycles dating back many thousands of years. These records indicate that we
are now in a relatively high period of overall solar activity and that many disruptions in the solar cycle have
happened in the past. For reasons still not well understood, these disruptions (e.g. Maunder Minimum) have in
some cases been correlated with decreases in global temperatures and even mini ice ages.
The amount of solar ultraviolet radiation varies by as much as 400% over the course of the solar cycle, being
highest during solar max. However, increased UV also modifies the chemistry in the stratosphere to produce
more ozone (O3) which is an effective UV absorber. So, the net result to us hear on the surface is about zero.
Auroras
Variations in solar activity play important roles in our daily lives. We already know that the interactions
between the magnetic field of the solar wind and the Earth's magnetosphere result in beautiful auroras seen in
high northern and southern latitudes. These aurora (Aurora Borialis and Aurora Australis) are roughly
symmetrical and are driven by similar processes that drive auroras on the outer planets. Auroras have been
observed locally in the atmospheres of Mars and Venus as well. Neither planet having a global magnetic field.
Auroras on Earth are seen in the presence of the uniform solar wind. Solar storms, flares and CMEs, intensify
these sightings and extend them to much lower latitudes. In fact, newspaper reports of the 1859 super storm
record auroral sightings in the northern hemisphere as far south as South America!
Radiation Exposure
We are protected on Earth from the hazardous space radiation environment by our atmosphere which absorbs
almost all high energy radiation (UV, X-Ray, Gamma Ray) and by our magnetosphere which deflects most of
the sun's high energy solar wind particles. However, astronauts in space are exposed to much higher levels of
both EM and particle radiation. Astronauts on extended missions to the moon, planets, or asteroids would be
at risk of much greater exposure due to the length of the mission as well as extensive exposure outside the
Earth's protective magnetosphere. Solar storms during these missions pose even greater risks to astronaut
safety. This is one of the biggest problems that must be solved if we intend to send humans into the solar
system.
Even at altitudes of around 35,000 feet where commercial airlines fly, pilots and frequent flyers are exposed
to increased doses of hazardous radiation, as much as 50 to 100 times as much as we experience on the
ground. Increased exposure to high energy cosmic rays can result in health problems including cancer. Flights
over the poles increase this radiation risk. This is why some airlines prohibit pregnant airline attendants from
flying.
Weather And Climate Connection?
The sun's output is remarkably constant over periods of thousands of years. The "Solar Constant" which is a
measure of flux density or the average amount EM radiation at all wavelengths per unit area received by the
Earth at its upper atmosphere is measured my Earth orbiting satellites (since 1978) to be 1,361 (kW/m2) and
is now known to vary by only 0.1% over a solar cycle which is equal to about 2 watts per square meter, not
enough to strongly influence climate. Though small, the solar constant does vary periodically with the 11 year
sunspot cycle as well as solar cycles of 88 (Gleisberg Cycle), 208 (DeVries Cycle), and 1,000 years (Eddy
Cycle). On much longer time scales, the sun has increased its luminosity by about 30% over its 4.6 billion
year life time due to lessoning outward-directed pressure and thus contraction and heating of the core through
its thermonuclear fusion process.
Efforts to connect the 11 year sunspot cycle with weather or climate change have been explored in various ways
since William Herschel suggested a transient weather connection with solar activity in 1801, noting the
Maunder Minimum (1645 - 1715) in sunspot count as one piece of evidence. Many theories predicting Global
Climate Change (GCC) over this time frame have been proposed and abandoned. Examples of this are
Daansgard's cycles, and a 22 year drought cycle in the western US, but to date no clear relationship with climate
has been found. There is some evidence that increased cosmic ray flux creating more aerosol condensation seed
nuclei during periods of low solar activity (and so weaker solar magnetic field or IMF) may result in an increase
in low altitude cloud cover, particularly at high latitudes though no weather or climate connection has been
correlated with these results. It is far more likely that human induced greenhouse gas emission, volcanic
activity, and natural changes (internal forcing) are responsible for a rise in global temperatures.