EISCAT observations of the E-region semidiurnal tide

Abstract

Many observers have reported a semidiurnal tide in the E-region of the ionosphere^1,7. Theory demonstrates the possibility of several tidal modes with the relative strength of each mode depending on the efficiency of generation in the stratosphere, the attenuation suffered in propagating through the mesosphere and mode-coupling in regions of nonlinearity. To determine which modes actually occur it is important to observe the tide at all heights in the E-region and at a range of latitudes. EISCAT, the European incoherent scatter radar^2,3, is an ideal facility to make such observations at 69.5° N. At this latitude, measurements are usually perturbed by strong auroral heating, but near sunspot minimum auroral disturbances are sometimes very small and the underlying pattern of the semidiurnal tides can be observed. Such conditions occurred in 1985 and here we report observations on five different days that illustrated the dominance of the (2,4) mode at high latitudes.