ULF-Waves on the Ground and in Space - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 59

FIELD-LINE OSCILLATION MODE CAVITY-LIKE OSCILLATION MODE

1990/8/26 (5:50~ 6:10 UT) 1991/3/24 (3:45 ~ 4 : 05UT)

1

1

64.4s
39.6s
32.3s

51.3s
34.1s
16.5s

0.5

0.5

0 0

N 40 20 0 -20 -40 S N 40 20 0 -20 -40 S

MAGNETIC LATITUDE MAGNETIC LATITUDE

Fig. 3.11. Typical latitudinal profiles of normalized power spectral densities; (left)
FLR-oscillations and (right) cavity mode-like oscillations obtained at the 210 ◦ MM
chain stations of L =1 . 4 − 2 . 13. After [112]

25 ◦
( L =1 . 2) up to the plasmapause. Outside, at high latitudes, this type of pul-
sations apparently results from FMS-wave resonance with a traveling Alfven
wave.
Energy from outer sources must be absorbed most intensively at frequency
of the cavity mode. When this frequency coincides with the FLR-frequency on
some magnetic shell, an effective energy transfer occurs from cavity to local
Alfven oscillations. Despite the considerable number of theoretical and com-
putation works devoted to cavity resonance, its existence cannot be regarded
as definitively established. Additional the questions of the Q -factor of an open
resonator and of the paths of FMS-wave propagation towards Earth, remain
to be proved.
The experimental evidence for Pc 3 , 4 cavity resonance was obtained by
Yumoto [110]. The idea of searching for cavity resonance reduces to finding
out global oscillations at discrete frequencies caused by rapid changes in solar
wind pressure. Power spectral densities of pulsation intervals at 210 ◦ magnetic
meridional chain stations [111] were compared for 20 min just before, just after,
and plus 20 min after Sc/Si (geomagnetic Sudden commencement/Sudden
pulse). Four cases of cavity mode were found, localized in the dayside sector
from 9 to 15 LT with high magnetic activity Kp

FLR in the Pc 3 , 4 range was successfully discovered from the latitudes ≈

7+.
After Sc the levels of pulsation power spectra grew by 10 times as com-
pared to the level before Sc . In pulsation spectra at different stations identical
components appeared with periods of T =64 . 4 s, 39 . 6s and 32 . 2 s. The found
high magnitude oscillations were of global character, occupying the latitude
belt from φ =

≥

46 ◦ . 7to φ =37 ◦ . 8. The intensity maximum was located at
low latitudes (Fig. 3.11).

−