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.
Home
02/10/97 Chris Davis