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# #18-B Interplanetary Magnetic Field Lines

### An optional activity, to draw the expected shapes of interplanetary magnetic field lines.

(Files in red–history)

Index

16. The Sun

16H. Schwabe, 1843

16a. Schwabe paper

16b. Carrington, 1859

17. The Corona

18. Solar Wind

18H.Solar Wind obs.

18A. Interplan. Field

18B. Heliosphere.

19. Magnetopause

19H.Chapman, 1930

20.Global Structure

21. Lagrangian pts.

22. "Wind" s/c

23. The Tail

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 (or "interplanetary lines of force"), we believe this field comes from the Sun, carried by magnetic field lines dragged out by the solar wind.

 Here this "dragging" process will be explained, and it will be used to obtain the expected shape of those field lines.

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 two or more ions start out located on the same field line, they will always share the same field line. If they then manage to move, the field line gets deformed: it is as if the magnetic field is "frozen" into the plasma.

(For any space physicist reading this: the deformation process also involves electric fields.)

## Postscript, 17 November 1999

As 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.

Questions from Users:   Does the Earth's magnetic field rotate?
***         Do interplanetary field lines guide the solar wind back sunwards?

Back to the Solar Wind:   #18    TheSolar Wind

Solar Wind History:   #18H    TheSolar Wind--History

Last updated 25 November 2001