The Upper Atmosphere during the eclipse

When the upper atmosphere is eclipsed by the sun, most of the radiation causing ionisation and heating is blocked by the moon. As a result, the atmosphere cools and as it does so, it contracts downwards. This contraction draws in air from around the eclipsed region and as a result, the winds are expected to converge on this region. Recent modelling work carried out by the Atmospheric Physics Group at UCL suggests that these wind changes will be measurable along with their effect on the F region ionisation, which will differ depending on the location.

Unlike the neutral air, the ionised particles are not free to move horizontally, as they are confined by the earth's magnetic field. As a result, any movement of the neutral air in the north-south direction will blow ionisation along the magnetic field. To the north, the increased southerly wind will blow ionisation up the field line, to higher altitudes where the lifetime of ionisation is longer. To the south the increased northerly wind will blow ionisation down the magnetic field line to lower altitudes where the ionisation is more quickly destroyed.

If this phenomenon is observed with two ionosondes, we would expect to see a higher, more dense F-layer to the north of the eclipsed region, while to the south, the F-layer would be expected to be lower in both altitude and density.

Under the path of the eclipse itself, the behaviour of the F peak is highly dependent on the resulting wind. Indeed, previous ionospheric observations during eclipses have observed the peak layer density to increase, decrease or remain unchanged. Not one of the previous campaigns however had the capability to measure the winds causing these effects, and the behaviour of the peak remains the subject of theory.

A study of ionospheric loss rate

Previous observations of eclipses have attempted to measure the ionospheric loss rate, as the production of ionisation by solar radiation is interrupted during the eclipse. With no accurate measurements of the sun, the assumption had to be made that the ionising radiation was uniformly emitted across the solar disk. With the advent of satellite technology, this assumption has proved to be woefully inadequate. This eclipse will again be unique in that for the first time, comprehensive in-situ measurements of solar radiation will be made by the SOHO satellite. With two instruments on board capable of measuring wavelengths in the Extreme Ultra Violet, it will be possible to determine the active regions on the solar disk and in the lower solar atmosphere to an accuracy well in excess (0.5 arcsec) of that required by any comparison with the ionosphere. By identifying the active regions, and calculating when they are eclipsed, their effect on the decay rate of the ionisation in the E and F1 regions can be studied. At totality, it is expected that some ionising radiation from the solar corona will still be incident on the earth's atmosphere and the effect of this radiation will be seen in the ionosphere. Ionosphere/Thermosphere coupling RAL and the UCL have a long standing and successful partnership in the science of Thermosphere/Ionosphere interactions. This collaboration, started in 198? has mainly concentrated on the high latitude region around the EISCAT radar. This summer marks the expansion of this science to mid-latitudes with the installation of a Fabry-Perot Interferometer at RAL. This instrument is measuring thermospheric winds and temperatures at around 250 km by detecting the 630 nm airglow emission from atomic oxygen. We are using this instrument to investigate the technique of inferring neutral winds by the motion of the ionospheric F layer (Rishbeth 19??). RAL lies outside the total eclipse region, and will only see a 95% partial eclipse. Its location, to the north of the eclipse region however, puts it in a very useful position to measure the change in the winds that are expected as a result of the thermospheric cooling in the eclipse region itself.
02/10/97 Chris Davis