Message-ID: <1283647283.48716.1576179406002.JavaMail.tomcat@wiki.oulu.fi> Subject: Exported From Confluence MIME-Version: 1.0 Content-Type: multipart/related; boundary="----=_Part_48715_703723216.1576179406001" ------=_Part_48715_703723216.1576179406001 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Content-Location: file:///C:/exported.html Drift

# Drift

## Introduction

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In the presence of only uniform magnetic field B, the motion of a charge= d particle can be described with gyration about the guiding center and moti= on along the field line. This motion gets more complicated when we add, in = the first place, uniform external forces like electric fields or gravitatio= nal forces. Even more complicated situations arise with nonuniform magnetic= field configuration, or time varying electric fields. We talk about the dr= ift of the particle (guiding center motion).

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## Uniform external forces

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In the presence of both magnetic and electric fields, the equation of mo= tion for charged particle (mass m, charge q) is m dv/dt =3D q (E + v x B). = This leads to plasma drift (or E x B drift, as it is also called) with velo= city v =3D E x B / B^2 (see ionospheric convection). Because v is independent of the mass an= d sign of the charge, it is the same for negatively and positively charged = particles, and does not create electric current. However, in a plasma where= collisions between charged and neutral particles are important, an importa= nt current called the Hall current is created because ions= move slower (ion - neutral collision frequency is greater than electron - = neutral collision frequency). To give an example, the pulsating auroral patches are often seen to = drift under the E x B influence.

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The situation is not much different in the case of gravitational force, = for which we get v =3D (m/q) g x B / B^2. However, this drift is opposite f= or particles of opposite charge, and a current is created even in a collisi= onless plasma.

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## Nonuniform magnetic fields

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The gradient and curvature of the magnetic field B create drifts that ad= d up and are in opposite directions for particles of opposite signs (formin= g currents). Both drifts are perpendicular to B, and in addition the gradie= nt drift is perpendicular to the field gradient, and the curvature drift to= the plane in which the magnetic field is curved. Also, the gradient and cu= rvature drifts are proportional to the perpendicular and parallel energies = of the particle, respectively. The east to west directed ring current in the Earth 's magne= tosphere is created by the combined curvature and gradient drift.

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Closely related to the gradient drift is the fact that, when magnetic fi= eld has longitudinal variation (i.e., convergence or divergence of the fiel= d lines), both positively and negatively charged particles are accelerated = in the direction of decreasing magnetic field. This results to what is call= ed the magnetic mirror effect, where particles are reflect= ed from the region of converging magnetic field lines. This relates also to= the first adiabatic invariance, i.e., that the orbital magnetic moment is = constant.

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See Sibeck et al. (1987) for discussion on drift shell splitting.

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## References

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• Sibeck, D. G., R. W. McEntire, A. T. Y. Lui, R. E. Lopez, and S. M. Kri= migis, Magnetic field drift shell splitting: Cause of unusual dayside parti= cle pitch angle distributions during storms and substorms, J. Geophys. Res.= , 92, 13485-13497, 1987.
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