Lecture 6

 The Solar "Atmosphere" and the Solar Wind

I. Modeling the Magnetic Field:  The Rubber Band Analogy

A.  In order to understand solar activity, much of which is driven or affected by the solar magnetic field, it is necessary to have a basic knowledge of the genesis and behavior of magnetic fields; magnetic forces are the "flip side" of electrical forces and are considered a relativistic effect of moving charge, force fields such as the magnetic field have long been a way of describing those forces that produce an action-at-a-distance such as the electromagnetic force or the gravity force
1.  magnetic fields are produced by moving charges (currents) and they affect other moving charges; a plasma is a gas of moving charged particles (like the gas which makes up the Sun) as such a plasma produces and affects magnetic fields; likewise, a changing magnetic field can affect or "stir up" a plasma
2.  the properties of a magnetic field may be visualized using the idea of magnetic field lines; small bar magnets (such as a compass) line up parallel to the lines, where lines are drawn closer together the magnetic field is stronger, where they are further apart the magnetic field is weaker; one special property of magnetic field lines is that they have no source or sink, but rather always return to a given point as a continuous loop
B.  Magnetic field lines, the rubber band analogy:  magnetic field lines behave as if they were ideal rubber bands which never touch, may be stretched almost without limit and if they should snap, they instantaneously reconnect to the nearest lose end so they remain continuous loops
1.  in the rubber band analogy, a plasma acts like a viscous fluid such that a moving plasma can stretch or drag magnetic field lines like flowing water might stretch or twist rubber bands, likewise changing or "moving" rubber bands could stir up the fluid or could be changed as they pass through the fluid just as magnetic field lines affect a plasma
2.  many rubber bands together could produce stronger forces and as they are stretched can store more and more energy, just as many magnetic field lines together represent a stronger magnetic field and a larger volume filled with magnetic field lines means more energy is stored in the magnetic field
3.  if these rubber bands are stretched and twisted severely enough they can "knot-up" or even snap releasing a large amount of energy in a short period of time; real magnetic field lines can't "snap", but sometimes they dramatically reconfigure themselves in a very short period of time releasing large amounts of energy in the process; this is called a reconnection event
C.  Sunspots and the magnetic cycle of the Sun, revisited using the rubber band analogy; at a minium in the sunspot cycle, the Sun's magnetic field looks roughly like that of a bar magnet (dipole field) with magnetic field lines running north to south and passing through the plasma of the convection zone and photosphere
1.  the Sun's differential rotation causes the plasma near the equator to undergo many more rotations over time than the plasma near the poles; the plasma then drags the magnetic field lines "sideways" much stronger at the equator than the higher latitudes causing the original dipole field to be wound around and around by the differential rotation
2.  as the process continues, the convection cells twist and roll the highly stretched field lines even more until "knots" appear in the field lines which erupt through the surface of the Sun impeding convection and cooling the photosphere at the points where the strong lines pierce the surface, giving rise to sunspot groups
3.  finally, as magnetic field lines of opposite polarity from opposite hemispheres meet at the equator they "cancel out" (not part of the rubber band analogy) in effect "untangeling" the axial component of the field lines and leaving the original dipole field, but now reversed

Animation of Solar Magnetic Lines Reversal
From SOHO (ESA & NASA): http://sohowww.nascom.nasa.gov/gallery/Movies/animations.html

II.  The Solar "Atmosphere" and the Solar Wind

A.  Chromosphere:  "sphere of color", so known because the hot (10,000 K) low pressure gas glows in the light of hydrogen emission lines giving it a pinkish purple color that can only be seen from Earth during a total solar eclipse when the glare of the photosphere is blocked out by the disk of the Moon; the chromosphere begins just above the photosphere and on averge reachs several thousand kilometers outward where a relatively narrow transition region gives way to the million K plasma of the corona, flame-like spikes of material called spicules continuously shoot tens of thousands of kilometers above the photosphere and then dissipate back into the chromosphere
1.  solar prominences occur when strong magnetic fields (typically associated with sunspots) expand outward into the corona lifting material from the chromosphere with them which then usually falls back towards the surface along the field lines, occassionally a slightly more energtic event called a mass ejection can occur which accelerates the chomospheric material away from the Sun
2.  the most energtic events of all occur when the strong magnetic field lines associated with sunspots become so twisted and knotted that they undergo a reconnection event (see I.B.3 above) releasing vast amounts of energy over a timescale of a few minutes to a few hours, these solar flares heat the surface and surrounding plasma to 100 million K and produce energtic photons and high energy charged particles accelerated to almost the speed of light (the high energy particles from a solar flare could kill an unprotected astronaut in just a few minutes anywhere in the inner solar system)
B.  Corona:  the hot (million K) outer atmosphere of the Sun seen as a pearly-white halo of light during total solar eclipses; the corona is so hot that it glows mainly in the X-ray band of the spectra, dark regions of the corona that show up in X-ray images of the Sun are regions where magnetic field lines from the Sun open out into the solar system allowing coronal plasma to escape as a component of the solar wind (see below)
1.  both the chromosphere and the corona are much hotter than the photosphere, since heat flows naturally only from a hot region to a cooler region the atmosphere of the Sun must have energy "pumped" into it by some mechanism analogous to a house heat pump which uses input electrical energy to transfer heat energy from the cooler outside to the warmer inside
2.  while the details are not known, the suspected mechanism is related to the continuous creation and dissipation of magnetic field lines arising from the churning plasma of the convective zone; energy is injected into the chromosphere and corona by the changing magnetic field lines that "stir up" the plasma above the surface of the Sun heating it to the high temperatures we observe
C.  Solar wind:  a continuous (but not necessarily steady) outflow of charged particles (mostly protons and electrons) that stream outward at up to several million km/h; one source of the solar wind are the so called coronal holes where the Sun's magnetic field lines extend out into the solar system (perhaps eventually combining with the galactic field); a second source recently identified are numerous individual "open" field lines which weave their way outward through the closed loops of the rest of the solar magnetic field allowing particles to escape the Sun in places other than coronal holes