Discovering the existence of magnetic fields on the sun
        *Zeeman discovers that magnetic fields split spectral lines
        *Hale uses Zeeman technique on solar spectrum and discovers strong magnetic fields on the solar surface.
        *Astronomers use satellites and spacecraft to analyze the sun's magnetic fields
Magnetic fields, solar plasma, and radiation
        *Nature of plasma (ionized gas, free electrons)
        *Magnetic field lines created by plasma motion in the solar interior.
        *Mutual annihilation of magnetic fields produce intense heat and energy
        *The solar corona heated by tangled  magnetic fields,  coronal holes.
Reversed polarities and the magnetic field
        *The sun's polarity changes every eleven years
        *Differential rotation of the sun creates tangled magnetic lines
Unraveling the magnetic fields: possible explanations of the inexplicable
        *Rebirth of the solar magnetic field every eleven years
        *Toroidal fields are sloughed away and magnetic reconnection restores the poloidal field
        *Possible explanations for observed solar phenomena
 
 
 
 

Discovering solar magnetic fields

    In 1896 physicist Pieter Zeeman discovered a special relationship between spectral emission lines and magnetic fields.(Kippenhahn, R.; pg. 98)  Spectral emission lines are  brightly colored  lines on a spectrum that correspond to the energy released from excited electrons of certain elemental gases.  The most commonly recognized emission line is the hydrogen alpha line in the red area.  This is most likely because hydrogen is the most common element in the universe.  Zeeman discovered that if a gaseous element was heated enough to exite the electrons in the presence of a strong magnetic field, the resulting emission line would be split in two or more segments.  He also determined that the strength of the magnetic field is directly proportional to the width of the split in the emission line.
    About ten years later a young physicist by the name of George Ellery Hale discovered the Zeeman effect on emission lines of the solar spectra. (Lang, K.R., pg. 77, 1997)  Hale was responsible for the invention of the spectroheliograph, an instrument used to measure and analyze the solar spectra, as well as the development of the first modern solar observatory at Mt. Wilson, California.  He found that when sunspots were observed through the spectroheliograph, the emission lines were widely split; evidence of a strong magnetic field.  In fact, the results implied that the Sun produced magnetic fields that were almost three thousand times stronger then the magnetic field of the Earth.  These discoveries were the very beginning of an entirely new branch of physical and astronomical research.
    Careful observations and tracking of solar features led to many theories and models of the inner workings of the Sun and its magnetic fields.  Labs tried to recreate, on a much smaller scale, the interactions between solar plasma, magnetic fields, and radiative transfer through the Sun.  Only in the past five years, with the implementation of two spacecraft, Ulysses and SOHO, have scientists been able to make significant progress in their understanding of the Sun's inner workings.
    Launched in 1990, carried by the space shuttle Discovery, Ulysses was designed to collect data from a unique orbit around the Sun.  Using Jupiter's powerful gravity field as a slingshot, Ulysses was swung out of the plane of the orbiting planets and into an orbit around the Sun from which it would be able to observe and collect data from the north and south poles.(Wang, Yi-Ming, et al; pg. 464)  As it takes a long time to travel out to Jupiter and then to reach a position high above the solar south pole,  data from the polar regions did not start to reach Earth until 1994.  Approximately one year later on December 1995, SOHO (Solar and Heliospheric Observatory) was launched into orbit around the Sun.   SOHO maintains a stationary orbit between the Earth and the Sun, at a distance of about one million miles from the planet.(Lang, K. R.; Scien. Am., pg. 40)  Complimentary data from these two satellites is helping solar physicists around the world to gain a much better comprehension of what is really going on inside of our sun.

Magnetic fields, solar plasma, and radiation

    Solar magnetic fields have profound effects on the manner in which the Sun radiates heat and energy into the solar system.  It is the continuous process of the creation, merging, and destruction of these magnetic fields that can determine how solar matter is flung out into space, the speeds of the solar winds, and the pathways that charged gases leaving the Sun will follow.   Scientists also believe that these twisting and chaotic fields are responsible for heating the solar corona. (anon., Sky & Telescope; pg. 16; Lang, K. R., Scien. Am. pg 40)
    Although solar researchers do not understand exactly how magnetic fields are created, they are certain that the movement and flow of solar plasma in the convection zone and the photosphere play an important role.  Plasma is hot, ionized gas where the electrons are stripped from the nuclei of the gas, creating an opposing charges in the plasma.  The electrons, moving independently of the nuclei, are pulled to one side in the fast flowing motion of plasma. (e- e-     + +)  This creates the charge imbalance and in an attempt to regain neutrality, the positive charge of the nuclei exerts a powerful force on the electrons pulling them to the other extreme.  The resulting oscillation of charge sets up an electrical current that in turn creates the magnetic field. The magnetic field lines passing through the plasma are frozen in the plasma and are stretched in the direction that the plasma moves.  Likewise, magnetic field lines cannot penetrate any plasma that did not contain them as they formed.   (Kippenhahn, Rudolf pg.115)
    Current belief holds that magnetic field lines are created near the base of the convection zone.  This is at odds with Babcock's dynamo theory, (Lang, K. R.  pgs. 90-92), which (in its simplest form) states that the solar magnetic cycles are caused by the differential rotation of the Sun pulling the initially poloidal magnetic lines into toroidal, or east-west lines.  Babcock determined that the Sun's internal magnetic lines lie just beneath the surface and when they are twisted into the toroidal field, kinks and loops break through the surface forming flares, spots, and prominences.  However, as Lang points out:
        "Any dynamo generated in the convection zone would therefore propagate radially outward, rather than
          creating a symmetric, churning motion from mid latitudes toward the equator."  (Lang, K. R.; pg 90-92)
Recent data collected from SOHO also seems to support the theory that the fields are generated deeper in the solar interior than Babcock originally supposed.  In the same manner that seismologists can infer activity far down in the earth's interior, helioseismologist are making new discoveries about the interior activities and structure of the Sun, by analyzing sound wave oscillations beneath the solar surface.  They have been able to determine that the plasma currents observed in the photosphere actually run much deeper than previously hypothesized. (Graham, D.;  Tech. Rev.  pg. 14)  SOHO's observations  show not only that patterns found in earth's winds are evident in solar plasma flows, but that :
         "...great bands of plasma slightly warmer than neighboring solar gases, dive deep into the solar interior,
          --then flow back toward the equator, creating a circular gyre reminiscent of earth's great ocean currents."
            (Thompson, Dick; pg. 68)
fig. 1fig. 2

http://wwwssl.msfc.nasa.gov/ssl/pad/solar/3dmovie.htm

    Deep in the convection zone, in places where these currents meet and converge, they have found pockets of strong magnetic energy.  Lang believes that is the tremendous force of the "churning" ionized plasmas that force weaker magnetic fields to merge into stronger concentrations of magnetic energy. (Lang, K. R.; Scienc. Amer. pg. 40) The newly formed magnetic flux tubes are then carried up to the surface by rising pockets of hot plasma in which they are frozen.  These pockets are now thought to resemble pancakes, rather then the spherical bubbles observed in boiling water.  Their shape is caused by friction between the different rotational velocity of the faster convection zone against the outer layer of the radiative zone, which is slower.   (Graham, D.; Tech. Rev. pg. 14:  Glanz, J.; Science, Vol. 273 pg.179) All of these modern observations and theories about the inner movements beneath the solar surface have helped  begin to solve a puzzle that has plagued solar researchers for years.
    In 1869, two solar researchers were observing the spectrum of the corona during a solar eclipse. Both of them noticed a bright green emission line in the coronal spectra.  Nobody could identify the element that would cause the emission line until 1941, when Swedish astronomer Bengt Edlen proved that the emission line was caused by ordinary iron atoms stripped of ten to fifteen electrons.  The only way that was possible was if the temperature of the corona was in the millions of degrees kelvin.  (Lang, pg.104 and 105)  These results were very difficult to accept.  Nobody could understand how energy traveling from a surface with a temperature of five to six thousand degrees kelvin, could climb into the millions of degrees, especially in a gas that was much less dense than the lower layers of the solar atmosphere. (Lang, K. R.; Scientific American, pg. 40 ) Further explorations into the Sun's magnetic field, lead scientists to the conclusion that somehow the magnetic activity on the Sun must be responsible for heating the solar corona.  However,  strong magnetic activity could only really be observed on the solar surface during sunspot maximum on the eleven year cycle.  The coronal temperatures remain relatively constant regardless of the eleven year cycle.

Reversed polarities and the magnetic field
    For the past one hundred and fifty years,  solar observers have tracked times when sunspots, solar flares and prominences appear in greater numbers and then drop down to almost no activity at all.   In a regular and repeated pattern, the Sun's cycle of activity and inactivity exhibits itself in unvarying eleven year periods.   The first clues to the nature of the magnetic Sun came from careful analyzing of the solar cycle.  Babcock's model of the winding of poloidal magnetic field lines was based on these cycles and the polarities exhibited by sunspots.  With new data transmitting all the time, theories must be recreated almost yearly, but some mysteries still remain unsolved.
    The end of each eleven year solar activity cycle is marked by a reversal in the polarities of the sun's mean magnetic field.  This remarkable and unvarying pattern of solar activity has been observed since the mid-eighteen hundreds and known to be determined by the magnetic cycle since the early twentieth century.  What is unknown and so far inexplicable is how the seemingly random  magnetic field generation can give rise to such a predictable pattern.  Even Babcock's simple model of poloidal fields being wound up by the differential rotation of the sun cannot answer why the polarities are reversed.(Kaufman, W. J.;  pg. 213; Kippenhahn, R. pg. )
    Close to the solar surface, magnetic fields lines are constantly being ripped apart and forced together by mutual attraction, converging plasma currents, and solar storms.  The strength of the magnetic field near the surface can bend the magnetic "streamers" extending  from the poles in toward the equator.  This concentration of magnetic strength serves to keep hot plasma closer to the equatorial region, creating an extremely hot belt of gas around the solar equator. (Lang, K. R.; Science. Amer., pg. 40)  The coronal holes in the polar regions are also the source of the extraordinarily fast solar winds.  Just as the winds carry the magnetic fields outward, their path is also determined by magnetic fields and they are pulled down toward the equatorial region by the strength of the fields on the surface.

Untangling the magnetic fields:  possible explanations of the inexplicable
    In an attempt to understand what causes the magnetic poles to flip and how the poloidal field is recreated from the tangled lines, solar researchers have built plasma models and analyzed computer simulations of magnetic movement to help create theories to explain the Sun's behavior.  As new data from our solar satellites is looked at and analyzed many of these theories must be modified or new ones invented.
    It is generally well accepted that the Sun does not have a primordial magnetic field, but that the magnetic field is recreated each cycle.(Brummell, N. et. al.; pg. 1370)  Differential rotation can well explain how magnetic fields become tangled and how they can create periods of extreme solar activity, but that is where the theory begins to have problems.  How can increased rotation lead to a simple poloidal field and low activity?  David Rust of John Hopkins University believes that in order for the cycle to begin again, the sun must shed the toroidal field like a snake shedding its skin so that the new simple field can emerge.  He bases his theories on twenty year studies of magnetic fields and plasma models.(Glantz, J. pg. 1517)  The shedding of the toroidal field occurs because of helicity or twist in the magnetic field caused by the churning currents of plasma where the fields are created. The fields are twisted into a helix like a rubber band that has been wound up.  In his early  model, the arches of magnetized field lines erupt through the surface and reconnect, forming the simple dipole field. Excess magnetic field lines break away and dissipate in the solar atmosphere. Due to the fact that it would take longer than the time given in the solar cycle for this to happen,  he and other physicists were not happy with the theory. When  measurements of the spiraling magnetic field lines extending out from the Sun were taken, they discovered that the spirals were ten percent greater than could be accounted for by solar rotation; "-just the amount expected if the sun were sloughing off its toroidal field in the solar wind rather than dissipating it." (Glantz, J.; Science, vol. 269#5230, pg. 1517)
    This new theory still relies on the helicity of the magnetic fields, but  instead of breaking apart and dissipating, Rust hypothesizes that helicity is conserved.  As pieces of the twisted magnetic fields go unstable, they kink and break off, reconnecting with other pieces forming an expanding toroidal field away from the sun. (Glantz, J.; Science, vol. 269#5230, pg. 40)  Eugene Parker of the University of Chicago remarks that the theory is "rather interesting, but I don't think they have proved their point yet."(Glantz, J.; pg. 1517)
    There are many mysteries left to answer about the sun's cycles and activity. Research in this field continues to be important due to the strong effects the solar cycle can have on the earth's global climate.  With our technology in space and here on earth, it is important to know how the sun may be affecting those power sources and furthermore, it is important to be able to predict when magnetic storms may occur and effect our technology.  Hope is that as Ulysses and SOHO continue to be our eyes in space, their observations will help make those answers clear.
 
 

1.)  Brummell, Nichols; Cattaneo, Fausto; Toomre, Juri;  Science  Vol. 269#5229 pg 1370  (1995)
                   "Turbulent dynamics in the solar convection zone."

2.)  Glanz, James;  Science  Vol. 273#(5272) pg 179  (1996)
                     "Unruly sun emerges in solar observatory's first results"

3.) Glanz, James; Science  Vol. 269#5230 pg 1517 (1995)
                     "Does magnetic twist crank up the sun's outburst"

4.)  Graham, David;  Technology Review  Vol. 100#9 pg 14  Jan/Feb. 1998
                    "Blueprint of a Star: Innovations from the World of Technology"

5.)  Kaufman William J.; Discovering the Universe;  1996, W. H. Freeman and Co.
 

6.)  Kippenhahn, Rudolf;   Discovering the Secrets of the Sun; 1994, John Wiley & Sons

7.)  Lang, Kenneth R.;  Scientific American  Vol. 276 #3, pg. 40 (1997)
                       "SOHO Reveals the Secrets of the Sun"

8.)  Lang, Kenneth R.;  Sun, Earth and Sky;  1997, Springer - Verlag, Berlin, New York

9.)  Reiner, M.J.;  Fainberg, J; Stone, R.G.;  Science  Vol. 270#5235 pg 461 (1995)
                     "Large-scale interplanetary magnetic field configuration revealed by solar radio bursts.

10.)  Thompson, Dick;  Time  Vol. 150#10 pg. 68 (1997)
                     "Eye on the Storm-Tossed Sun"

11.) Wang, Yi-Ming; Hawley, Scott H.; Sheeley, Neil R Jr.;  Science  Vol. 271#5248  pg 464 (1996)
                     "The Magnetic Nature of Coronal Holes"

12.) anonymous; Sky & Telescope, Vol. 95 Issue #2, pg 16   February 1998
                       "SOHO sheds light on solar corona"