Other
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmsm43b..03v&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #SM43B-03
Other
2409 Current Systems (2708), 2708 Current Systems (2409), 2740 Magnetospheric Configuration And Dynamics, 2756 Planetary Magnetospheres (5443, 5737, 6030)
Scientific paper
Maxwell's equations allow the magnetic field B to be calculated if the electric current density J is assumed to be completely known as a function of space and time. The charged particles that constitute the current, however, are subject to Newton's laws as well, and J can be changed by forces acting on charged particles. Particularly in plasmas, where the concentration of charged particles is high, the effect of the electromagnetic field calculated from a given J on J itself cannot be ignored. Whereas in ordinary laboratory physics one is accustomed to take J as primary and B as derived from J, in plasmas B may be viewed as primary and J as derived from B simply as (c/4π )∇ × B -- a view proposed by Cowling (1957) and Dungey (1958) and in recent years strongly argued by Parker. Here I investigate the relation between ∇ × B and J in the same terms and by the same method as previously applied to the MHD relation between electric field and plasma bulk flow [Vasyliūnas, Geophys. Res. Lett., 28, 2177--2180, 2001]: assume that one but not the other is present initially, and calculate what happens. The result is that, for all configurations with spatial scales much larger than the electron inertial length, (1) a given ∇ × B produces the corresponding J, (2) a given J does not produce any ∇ × B but disappears instead. The result can be understood by noting that ∇ × B ≠q (4π /c)J implies a time-varying electric field (displacement current) which acts to change both terms (in order to bring them toward equality); the changes of the two terms, however, proceed on different time scales, light travel time for B and electron plasma period for J, and clearly the term changing much more slowly is the one that survives. (The electron inertial length, by definition, is where the two time scales are equal.) Some illustrative simple examples will be discussed, together with implications for determining magnetic fields in space plasmas.
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