Magnetospheric plasma density diagnosis from gradient measurements of geomagnetic pulsations

It is shown that gradient measurements of geomagnetic pulsation fields along a meridian can be used as a method of magnetospheric plasma density diagnosis. The gradient method is applied to observations of Pc4 pulsations at five stations spaced about 100 km apart on a meridian through the British Isles. Values are calculated for equatorial plasma density which compare favourably with those obtained from whistler measurements with the advantage that there is no requirement to calculate the L-shell of the field line for the top of which the plasma density is found. (author)