MHD After-Effects of a Sound Impact - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 405

the ionosphere in about 6 min, causing tangible perturbations of the electron
concentrations in the E -and F -layers. On its path, the pulse became much
broader: from 1 s at the distance where the shock wave was transformed into
an acoustic wave to 1 min at the altitudes of the F -layer.
The entry of the acoustic wave into the ionosphere was accompanied by
a noisy electromagnetic signal in the frequency band of tens of kHz, which
traveled along the geomagnetic field-lines into the magnetosphere. At the same
time, relatively thin current jets arose in the magnetosphere, inside which a
magnetic disturbance of tens of nT was formed. At the time of entry of the
acoustic wave, a perturbation arose in the ionosphere which traveled with a
speed of 1-10 km/s (depending on the direction relative to the geomagnetic
field); this produced crescent-shaped features on the ionograms of stations
situated hundreds of kilometers from the explosion.
At this time, the magnetic field variations in the middle latitudes attained
magnitudes of 10 nT. Against this background, no clear variations of the

geomagnetic field that could be reliably linked with the blast have been ob-
served.
It is assumed that the generation of electromagnetic effects in the magne-
tosphere is determined by electrodynamic processes in the E-layer. Here the
motion of neutral particles, which is dependent on acoustical waves, should
cause a local generation of an electrical field and currents. Thus, near the
explosion's field tube, longitudinal currents may be created and plasma noise
may be generated.
With this goal in mind, the time of the MASSA experiment blast was cho-
sen such that at the moment when the acoustic wave would reach the altitude
of the E-layer (100-120 km), the AUREOL-3 satellite would be located at a
minimum distance from the magnetic field tube (see Fig.15.4).
As a result, the satellite detected a spot of increased electromagnetic noise
intensity over a wide range of frequencies in the explosion's field tube in
the ionosphere ( [5], [6]). Figure 15.5 shows the results of EZ -component
measurements (in a direction making a 30 ◦ angle with the main magnetic
field) and the transverse EH -component of the electrical field in a range of
10-20, 100-200, 200-450, and 450-1000 Hz in relative percent (an approxi-
mate logarithmic scale). There is a pronounced rise in the noise level above
the background level (the long and short dotted lines) in the vicinity of the
explosion's field tube (at distances

∼

200 km) in the frequency range 0.1-2
kHz, and especially with respect to the EZ -component. The magnetic field
of these oscillations is insignificant and may be they are almost electrostatic
ones.
There was also a significant rise in the range of 15 kHz in intensity on this
same segment of the trajectory (cf. left and right panels). The polarization
of the electrical oscillations was transverse, and the magnetic component was
also small. The ratio of the field components enable us to assume that these
were swift waves, propagated near the resonant cone. An increase in the noise
intensity can be also noticed in the range of 4.5-16 kHz (Fig. 15.5).

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