S-1. Sunlight & Earth
S-1B. Global Climate
S-3.The Magnetic Sun
S-4. Colors of Sunlight
S-5.Waves & Photons
Optional: Quantum Physics
Q3. Energy Levels
The Law of Field Line Preservation
When a spacecraft breaks away from the influence of the Earth's magnetic field into interplanetary space, it finds there a weak magnetic field. The field may be weak, but it extends over huge distances, and can have important effects. From the observed direction of interplanetary magnetic field lines, we believe this field comes from the Sun, carried by magnetic field lines dragged out by the solar wind.
When some process moves plasma inside a magnetic field, what happens depends of the relative strength of the two. If the magnetic field is strong--as happens in the corona, close to the Sun--then it dominates, and determines where the plasma can or cannot go. That is why magnetic field-line loops tend to keep back the solar wind, unlike the outward-bound lines in the "coronal holes" between them.
But if the field is weak, then the plasma rules and pushes the field lines around. A rule which is fairly well obeyed states then that:
If they then manage to move, the field line gets deformed: it is as if the magnetic field is "frozen" into the plasma.
Drawing Interplanetary Field lines
Using this "law of field line preservation", we will now derive the shape of interplanetary magnetic field lines.
On that line mark points at distances 15/16", 2" and 3 3/8" on both sides of the y-axis, then draw radial lines from the center of the Sun through those points, extending them until they are 1/2" from the sides of the sheet or 1" from the top.
For those used to metric units, let the radius of the Sun be 1 cm (diameter=2cm), the pencil line follows y=10 cm and the marks on it are at distances of approximately 23.7, 50.2 and 83.9 millimeters from the y-axis. Extend the lines until they reach within 1 cm of the sides or 3 cm of the top.)
Marking the Spokes
Spiral field lines
Postscript, 17 November 1999As noted at the beginning, two extreme modes exist in the interaction between a plasma and a magnetic field. If the plasma is rarefied, even if its particles have high energy, its motion is guided and channelled by magnetic field lines. On the other hand, if the plasma is dense and the magnetic field relatively weak--the situation in most of interplanetary space--instead of the magnetic field deforming the plasma's motion, that motion deforms the magnetic field.
It was also noted that with increasing distance from the Sun, the spiral shape of interplanetary magnetic field lines becomes more and more tightly wound, until their shape differs little from circles.
Both points were well illustrated by the phenomena that followed intense solar activity in April-May 1998, reported by Robert Decker of the Applied Physics Lab of the Johns Hopkins University in Maryland. That activity created a disturbance in the solar wind, as well as a flow of protons with energies about 1000 times that of the solar wind, and these were observed by a number of spacecraft--ACE at the L1 Lagrangian point (near Earth, distance from the Sun about 1 AU), by Ulysses (5 AU), and by Voyagers 1-2--Voyager 2 at 56 AU and Voyager 1 at 72 AU.
The solar wind disturbance arrived at Voyager 1 about 7.5 months later, propagating radially at the velocity of the solar wind flow in which it was embedded. The protons, on the other hand, although they moved much faster, were relatively few in number, which forced them to spiral along field lines. They were observed by Voyager 1 after 6 months--1.5 months before the disturbance in the solar wind reached that distance--and Dr. Decker calculated that their spiral path took them 10 times around the Sun, a total distance of about 2000 AU.