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Re: more bad journalism?



On Mon, 13 May 1996, Mark Sylvester wrote:

Quote from Physics World, May 1996 p7 - a report on the Cluster
mission to study small-scale fluctuations in earth's magnetic
and electric fields.

"The solar wind is a magnetised plasma. When it reaches the
Earth's magnetic field, it is repelled, and therefore most solar
particles are excluded from the Earth's magnetosphere. However,
some particles get through, creating auroras in the sky near the
magnetic poles.....but exactly how these particles penetrate
remains a mystery."

(i) Can a plasma be magnetised??

Yes. A plasma is considered to be "magnetized" when the ratio of the
particle pressure (sum over species of nkT) to the magnetic field pressure
(B^2/8pi in cgs units) is low. The ratio is denoted "beta" and plasma
folks speak of "low beta" versus "high beta" plasmas.

(ii) I would normally deduct marks if a student wrote that a
charged particle was "repelled" by a magnetic field.

This is relatively common parlance in magnetospheric physics. The effect
of the solar wind is to compress the earth's magnetic field on the solar
side which, in turn deflects the high velocity solar wind particles away
from the earth. The two phenomena are accomplished self consistently as
can be seen by imagining what happens when high velocity charged particles
encounter an abruptly turned on magnetic field perpendicular to their
motion. Positively charged particles undergo a 180 degree deflection in
one direction at the interface and negatively charged particles do the
same in the other direction. The result is a current sheet that self
consistently accounts for the discontinuity in the magnetic field from one
side to the other.

(iii) I would expect the solar wind particles to follow helical
trajectories, tracking the magnetic field lines down to the
Earth's magnetic poles - hence the appearance of the Aurorae
at high latitudes. Surely this should not be made to sound
mysterious?

But you mustn't forget that the solar wind plasma is engaging in high
velocity motion ("supersonic," in fact, where the comparison is with the
velocity of magnetosonic waves in the plasma) which translates into
significant electric fields in the frame of the earth. It is sometimes
simpler to think of the magnetic field being "frozen in" to the plasma
and moving with it in which case the particles are "spiraling around a
moving magnetic field." In any event the whole process is complicated by
the assymetrically compressed dipole geometry of the earth's magnetic
field. In order to get solar wind particles into the earth's
magnetosphere they have to "hop" somehow from solar magnetic field lines
that are embedded in the solar wind onto the earth's magnetic field lines.

Trust me; it IS a pretty complicated business not particularly amenable
to simple descriptions, journalistic or otherwise.

John
----------------------------------------------------------------
A. John Mallinckrodt email: mallinckrodt@csupomona.edu
Professor of Physics voice: 909-869-4054
Cal Poly Pomona fax: 909-869-5090
Pomona, CA 91768 office: Building 8, Room 223
web: http://www.sci.csupomona.edu/~mallinckrodt/