3. Actitud
3.3 Formas de medir la actitud
Before discussing the results, and their in
terpretation in terms of atmospheric dynamics , it is necessary to consider the sign conventions and jargon associated with the topic. The work described in this thesis represents one of the few areas of common inter est to both ionospheric physicists and meteorologists. Unfortunately the two groups of workers have adopted differing definitions of wind direction. The meteor- ologist describes a wind by the direction from which it
63
is blowing. To ·a physicist the wind is represented by a velocity vector and is therefore described by the direct ion towards which the wind is blowing�
There is fortunately no disagreement about the sign of the vectors and their components. Figure 7.2 shows the right-handed Cartesian co-ordinate systems used in meteorology. The positive directions are east- ward, northward and upward; displacements a re respectively x, y and z.
u, V and w.
The corresponding velocity components are In Section 4, v and v were used as the X y horizontal components of the drift velocity, but were not at that time associated with any geographical dir- ection. In the new terminology vx becomes u, and vy becomes v, which makes the symbol v ambiguous - it can represent either the north-south wind component or the magnitude of the velocity. In this and succeeding sections v will be taken to represent the north-south component of wind velocity and if the magnitude of the vector is discussed it will be written/yl.
Figures 7.3(a) and (b) show the relationship between the terminology and the vector components. The meteorological terms will be used because they are in everyday use. In Figure 7.1 therefore the wind is west- erly (from the west, towards the east, u positive) below
95 km, easterly (from the east, towards the west, u negative)above 95 km.
7. 2. THE INFLUENCE OF THE GEOMAGNETIC FIELD ON THE MOVEMENT OF IONISATION.
As a first approximation , consider isolat�d electrons in a region containing a magnetic field. Any translational motion imparted to the electrons by an applied force results in an average motion along the magnetic field. The resultant motion of the electron gives no indication of the magnitude or direction of the applied force.
As an alternative first approximation, con sider the electrons in a weakly-ionised, dense gas in a magnetic field. The random collision process prevents the electrons from rotating around the magnetic field lines and the electrons follow the average motion of the neutral gas.
In the atmosphere, which at all times has the geomagnetic field passing through it , the neutral gas density decreases with height and the electron density increases with height. At the outer limits of the atmosphere , the former approximation is valid. At
65 ground level the alternative approximation is valid.
There is therefore some region of the atmosphere below which the influence of the geomagnetic field is negligible. Above this region, the geomagnetic field cannot be neg
lected.
There are various estimates of the height below which the geomagnetic field can be neglected. Clemmow , Johnson and Weekes (1955) deduced that a cylin drical cloud of charge , parallel to the geomagnetic field , would follow the motion of the neutral gas if the ratio of the collision frequency to the angular electron gyro-frequency exceeded 10-3•
+7 -1 electron gyro-frequency is 10 sec •
The angular Experimental data on the variation of collision frequency with
height in the relevant part of the atmosphere (Bjelland,
Holt, Landmark and Lied 1959; Kane 1961) gives a colli
sion frequency of 10-+4 sec-1 at 105 to 115 km. The cylindrical cloud would therefore move with the wind at altitudes of 105 km or less. It is, however, an un- realistic geometry as the nature of the diffraction drift experiment implies ionospheric irregularities spread
over a large area in a horizontal plane. The problem was reconsidered by Villars and Feshbach (1963) who extended the analysis to a weakly ionised turbulent gas. They concluded that below 100 km the geomagnetic field
has no effect on fluctuations in the ionisation density produced by turbulence. In the 110 to 120 km region the ionisation fluctuations are slightly modified by the geomagnetic field.
It is therefore reasonable to assume that the diffraction pattern drifts described in this thesis re sulted from radio waves reflected, in the atmosphere below 110 km, by electrons whose motion is dependent on the neutral gas motion, and is unaffected by the geornag-
netic field. The calculated velocities therefore refer to the neutral gas, but do not necessarily indicate the direction of the wind. This restriction on the inter- pretation of the results is discussed in the next section.
THE DIFFRACTION OF WAVES BY IRREGULARITIES IN A FLUID.
In the previous section we have seen that the radio waves are reflected from electrons which move with the neutral gas. We cannot observe directly the motion of the neutral gas, but we can interpret the results (Section
4)
as horizontal motion of the reflect- ing surface. If atmospheric waves are perturbing the medium, the observed velocity is that of the wave profile,is complicated and the point is best illustrated by an example drawn from a different field of geophysics - physical oceanography.