Active Cloud Releases and MHD-Emission - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 379

Table 14.1. The main parameters of the 'Trigger' experiment

place: Kiruna (67 . 9 ◦ N, 21 . 1 ◦ E ) Height: 164 km

Total mass: 12 kg, 6% of Cs + 10 − 3 Degree of ionization 1 . 6 × at τ

Number of molecules: 1 . 7 × 10 25 Conductivity for N e =3 . 1 × 10 12 m − 3

σ P =5 × 10 − 3 S/m Initial phase of the emission: τ ≈ 0 . 6s

σ H =5 . 7 × 10 − 3 S/m

Cloud radius R 0 =1 . 2km at τ

Number density N versus radius r : Integral ionospheric conductivity:

N ( r )= N ( r =0)exp −r 2 /R 0

Σ P =18S, Σ H =20S

pierces the cloud. Generation and propagation of a current pulse j and the

pulse's roles in electron precipitations are described in [9].
The parameters of the 'Trigger' experiment (11.02.1977) (see Table 14.1
after [7]) can be used in order to estimate the ULF-frequency range electro-
magnetic effects caused by a plasma release. In this experiment, a cesium
cloud (Cs + ) was released at a height of 164 km.

14.2 MHD-Pulse Initiation

The eciency of MHD-wave excitation at various stages of scattering into the
ionosphere depends on the amount of mobile neutral component. It is also
dependant on how the gas components were emitted. The most convenient
way to simplify this rather complicated problem is to suppose an instant
emission. Further, it is convenient to consider the movement of neutral cloud
components such as expansion of gas into vacuum. As the gas slows down it
rakes up ionospheric plasma and almost stops when the energy of the displaced
background plasma is equal to the release energy.

Neutral Components Dynamics

The injected neutral component mobilizes the electron and ion gases which
have arisen at the initial stage (in the container itself), and at later stages
under the influence of solar UV radiation. Due to the weak ionization 1 s after
emission, the feedback action of ionized components on a neutral motion can
be neglected.
Let us suppose that the expanding cloud of neutral gas is a sphere of
radius R n ( t ). Let the radial component v n of the neutral velocity in a cloud
be linearly dependent on the running radius r , i.e.

r
R n ( t ) ,

v n 0 ( t )= dR n ( t )
dt

v n ( t, r )= v n 0 ( t ) . (14.1)

Let the concentration distribution N n ( r, t ) on the time-scale of the process

be: