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particles are somehow placed in the equatorial region of that field, most of them stay trapped, because every time their motion along the field line brings them into the strong field region, they "get mirrored" and bounce back and forth between hemispheres. Only particles whose motion is very close to parallel to the field line, with near-zero μ, avoid mirroring—and these are quickly absorbed by the atmosphere and lost. Their loss leaves a bundle of directions around the field line which is empty of particles—the "loss cone".
180:
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553:. This change in curvature makes ions advance sideways, while electrons, which gyrate in the opposite sense, advance sideways in the opposite direction. The net result, as already noted, produces the ring current, though additional effects (like non-uniform distribution of plasma density) also affect the result.
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In addition to gyrating around their guiding field lines and bouncing back and forth between mirror points, trapped particles also drift slowly around Earth, switching guiding field lines but staying at approximately the same distance (another adiabatic invariant is involved, "the second invariant").
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Magnetic mirroring makes possible the "trapping" in the dipole-like field lines near Earth of particles in the radiation belt and in the ring current. On all such lines the field is much stronger at their ends near Earth, compared to its strength when it crosses the equatorial plane. Assuming such
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is outlined by field lines. Not much plasma can cross such a stiff boundary. Its only "weak points" are the two polar cusps, the points where field lines closing at noon (-z axis GSM) get separated from those closing at midnight (+z axis GSM); at such points the field intensity on the boundary is
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is not affected by the field, because no magnetic force exists in that direction. That velocity just stays constant (as long as the field does), and adding the two motions together gives a spiral around a central guiding field line. If the field curves or changes, the motion is modified, but the
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is a constant one– straight parallel field lines and constant field intensity. In such a field, if an ion or electron enters perpendicular to the field lines, it can be shown to move in a circle (the field only needs to be constant in the region covering the circle). If q is the charge of the
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The conservation of μ is tremendously important (in laboratory plasmas as well as in space). Suppose the field line guiding a particle, the axis of its spiral path, belongs to a converging bundle of lines, so that the particle is led into an increasingly larger B. To keep μ constant,
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Because the magnetic force is perpendicular to the velocity, it performs no work and requires no energy—nor does it provide any. Thus magnetic fields (like the Earth's) can profoundly affect particle motion in them, but need no energy input to maintain their effect.
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and heat flow are also channeled by such lines — easy along them, blocked in perpendicular directions. Indeed, field lines in the magnetosphere have been likened to the grain in a log of wood, which defines an "easy" direction along which it easily gives way.
483:/2mγB) stays very nearly constant. The "very nearly" qualifier sets it apart from true constants of motion, such as energy, reducing it to merely an "adiabatic invariant." For most plasmas in the magnetosphere, the deviation from constancy is negligible.
569:", showing oxygen, helium, and hydrogen ions which gush into space from regions near the Earth's poles. The faint yellow area shown above the north pole represents gas lost from Earth into space; the green area is the
521:. The particle briefly gyrates perpendicular to its guiding field line, and then retreats back to the weaker field, the spiral unwinding again in the process. It may be noted that such motion was first derived by
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increases as Earth is approached. The gyration around the guiding field line is therefore not a perfect circle, but curves a little more tightly on the side closer to the Earth, where the larger B gives a smaller
222:. These represent the force that a north magnetic pole would experience at any given point. (Denser lines indicate a stronger force.) Plasmas exhibit more complex second-order behaviors, studied as part of
494:
However, as noted before, the total energy of a particle in a "purely magnetic" field remains constant. What therefore happens is that energy is converted, from the part associated with the parallel motion
257:
field lines manage to cross the boundary. As discussed further below, that extent depends very much on the direction of the
Interplanetary Magnetic Field, in particular on its southward or northward slant.
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The spacing between field lines is an indicator of the relative strength of the magnetic field. Where magnetic field lines converge the field grows stronger, and where they diverge, weaker.
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zero, posing no barrier to the entry of plasma. (This simple definition assumes a noon-midnight plane of symmetry, but closed fields lacking such symmetry also must have cusps, by the
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in 1895, for a charged particle in the field of a magnetic monopole, whose field lines are all straight and converge to a point. The conservation of μ was only pointed by
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The amount of solar wind energy and plasma entering the actual magnetosphere depends on how far it departs from such a "closed" configuration, i.e. the extent to which
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407:/2 is the energy associated with the perpendicular motion in electron-volts (all calculations here are non-relativistic), in a field of B nT (nanotesla), then R
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of Earth is near the top of the diagram, the South Pole near the bottom. Notice that the South Pole of that magnet is deep in Earth's interior below Earth's
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contains all the available energy, it can grow no more and no further advance into the stronger field can occur.
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If the initial velocity of the particle has a different direction, one only needs resolve it into a component
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about 50 years later, and the connection to adiabatic invariant was only made afterwards.
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decreases, the angle between v and B then increases, until it reaches 90°. At that point W
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Now, it can be shown that in the motion of gyrating particles, the "magnetic moment" μ = W
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which is responsible for many of the particle motion in the magnetosphere. Furthermore,
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A simulation of a charged particle being deflected from the Earth by the magnetosphere.
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general character of spiraling around a central field line persists: hence the name "
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Schematic view of the different current systems which shape the Earth's magnetosphere
194:. Earth's magnetic field is produced in the outer liquid part of its core due to a
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658:" Tool dedicated to the 3d simulation of charged particles in the magnetosphere..
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Piddington, J. H. (1979). "The Closed Model of the Earth's
Magnetosphere".
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representing the source of Earth's magnetic field as a magnet The
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was mentioned earlier in connection with the ring current.
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Solar Wind
Squeezes Some of Earth's Atmosphere into Space
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parallel to B, and replace v in the above formula with v
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Thus in the "closed" model of the magnetosphere, the
573:-or plasma energy pouring back into the atmosphere.
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46:. Unsourced material may be challenged and removed.
655:3D Earth Magnetic Field Charged-Particle Simulator
544:One reason for the drift is that the intensity of
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319:must equal the magnetic force qvB. One gets
241:boundary between the magnetosphere and the
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167:Learn how and when to remove this message
106:Learn how and when to remove this message
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198:that produce electrical currents there.
468:Magnetic Mirroring and Magnetic Drift
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44:adding citations to reliable sources
446:The velocity parallel to the field
383:perpendicular to B and a component
184:A sketch of Earth's magnetic field
14:
985:Sura Ionospheric Heating Facility
504:to the perpendicular part. As v
144:Layout of mathematical formulas.
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601:Journal of Geophysical Research
55:"Magnetosphere particle motion"
31:needs additional citations for
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1:
841:Interplanetary magnetic field
783:Magnetosphere particle motion
367:{\displaystyle R_{g}=mv/(qB)}
255:Interplanetary Magnetic Field
302:The simplest magnetic field
290:Motion of charged particles
142:. The specific problem is:
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846:Heliospheric current sheet
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479:/B (or relativistically, p
311:the radius of the circle (
138:to meet Knowledge (XXG)'s
798:Van Allen radiation belt
778:Magnetosphere chronology
707:Atmospheric circulation
621:10.1029/ja084ia01p00093
517:The result is known as
717:Earth's magnetic field
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216:Earth's magnetic field
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214:interacting with the
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994:Other magnetospheres
856:Solar particle event
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224:magnetohydrodynamics
220:magnetic field lines
149:improve this article
40:improve this article
613:1979JGR....84...93P
248:fixed point theorem
192:North Magnetic Pole
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519:magnetic mirroring
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283:Birkeland currents
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951:Research projects
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748:Birkeland current
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633:Plasma fountain
411:in kilometers is
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140:quality standards
131:This article may
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51:Find sources:
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29:This article
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1067:Ring systems
1062:Lunar swirls
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793:Ring current
788:Plasmasphere
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763:Magnetopause
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434:= (3.37/B)
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275:ring current
269:Trapping of
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239:magnetopause
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147:Please help
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38:Please help
33:verification
30:
973:Unwin Radar
899:Double Star
836:Heliosphere
826:Solar flare
539:This motion
418:= (144/B)
151:if you can.
1104:Categories
1021:Ganymedian
894:Cluster II
875:Satellites
851:Heliopause
808:Solar wind
758:Ionosphere
732:Polar wind
727:Jet stream
587:References
243:solar wind
188:North Pole
157:March 2018
66:newspapers
1057:Gas torus
1052:Flux tube
1036:Neptunian
1026:Saturnian
980:SuperDARN
881:Full list
753:Bow shock
722:Geosphere
460:motion."
208:electrons
96:June 2016
133:require
1087:Neptune
1072:Jupiter
1031:Uranian
1011:Martian
1001:Hermian
904:Geotail
609:Bibcode
436:√
420:√
135:cleanup
80:scholar
1082:Uranus
1077:Saturn
1016:Jovian
958:EISCAT
930:THEMIS
918:(2015)
916:
889:(2016)
887:
712:Aurora
635:Source
527:Alfvén
271:plasma
212:plasma
196:dynamo
82:
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1006:Lunar
968:SHARE
963:HAARP
924:Polar
909:IMAGE
885:Arase
210:of a
87:JSTOR
73:books
941:Wind
403:=m v
399:If W
206:and
204:ions
202:The
59:news
914:MMS
617:doi
250:.)
42:by
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