The Earth’s Magnetic Field - Ultra and Extremely Low Frequency Electromagnetic Fields - page 32

The opened field lines 1-6, which converge over the polar caps, can occur
simultaneously as the southward-directed interplanetary magnetic field exists for a
long time. Since the opened lines cannot serve as the magnetic trap for the charged
particles, the plasma does not pile up at these lines and hence the plasma density is
lowered in this region. Moreover, the intensity of particle precipitation at the polar
cap is much smaller than that at auroral region.
During the main phase of a magnetic storm the magnetic reconnection and
changes in the magnetosphere size are responsible for the penetration of new
particles into the magnetosphere. This tends to accelerate the magnetospheric
plasma thereby exciting the westward ring current in the magnetosphere within the
radii from 3R e to 5R e . For the quiet Sun period the ring current is caused by the
westward proton drift and eastward electron drift in the Earth's dipole magnetic
field. Recent satellite measurements have shown that the energy of trapped protons
can increase up to 10-120 keV during the main phase. The enhanced ring current
generates a magnetic field in opposition to the geomagnetic field that causes the
decreases of the geomagnetic field in accordance with ground-based observations.
The recovery phase starts with diffusion of the trapped particles in the magne-
tosphere followed by the decay of the ring current. The Coulomb scattering and
reactions of protons and neutral hydrogens of the following type H C C H !
H C H C are assumed to be the key mechanism for energy dissipation.
Now consider the changes in spacial pattern of magnetic field lines at magnetotail
during a typical substorm. Figure 1.13 illustrates the so-called near-Earth neutral
line (NENL) model of the substorm (e.g., Russell and McPherron 1973 ; Hones
1979 ; Baker et al. 1996 ). At the initial stage of substorm the “frozen in” field lines
are dragged tailward. As a result the magnetotail is also stretched more and more
along the magnetopause that gives rise to the energy accumulation at the tail. When
the open magnetic flux is transported from the dayside magnetosphere towards the
magnetotail lobes, it produces the pressure enhancement, neutral sheet sharpening,
and decrease of the neutral sheet current. This process leads to the progressive break
of the tail current and to the reconnection of the antiparallel magnetic fields of the
lobes. In response to the reconnection the most part of the magnetotail is shaped into
a huge plasmoid, which breaks away from the magnetosphere, and subsequently
escapes in antisunward direction. The result is that the length of magnetotail
decreases abruptly but it will be restored to its original size at subsequent relaxation.
The current disruption in the neutral sheet of magnetotail and generation of field-
aligned currents flowing into the ionosphere is sketched in Fig. 1.14 (McPherron
1979 ). These currents flowing along the magnetic field lines close through the
night sector of auroral oval thereby producing the so-called West auroral electrojet
(AEJ-W). Thus, the field-aligned currents connect the magnetotail with auroral
region of the ionosphere during a magnetospheric substorm. At this point the
field-aligned current amplitude increases several times while the AEJ-W reaches
a value .1-2/ 10 6 A. When the high-energy particles associated with the field-
aligned currents precipitate into the ionosphere, they dramatically increase aurora
borealis. The particle energy absorption in the atmosphere reaches a peak value