Henry Rishbeth, Department of
Physics, University of Southampton, Southampton SO17 1BJ, U.K.
The ionosphere is an important component of the upper atmosphere
and of the solar-terrestrial system generally. Ionospheric observations,
both ground-based and in-situ, give data relevant to many STP
topics. Ground-based observations - ionosondes, coherent and incoherent
scatter radars, plus more specialized instruments - have a vital
role, largely because of the geographic coverage and long data
sequences that they provide.
Here are some personal ideas on the "frontier topics",
which I sent to Moscow in September 1994, in response to a request
from Sasha Feldstein of WDC-B. I thought they may be of interest
to the INAG readership, so for this article I have edited out
the specifically Russian aspects of the original.
"The upper atmosphere as a dynamic system" (or "Vast
heat engine") i.e. Global dynamics of the upper atmosphere.
The upper atmosphere circulation, locally and globally, is strongly
influenced by the ionospheric plasma and the geomagnetic field.
From well-established ionospheric theory, neutral-air parameters
[such as pressure and meridional wind speed] can be estimated
from ionosonde and other data, with sufficient accuracy to be
useful in studies of energy inputs - solar radiation; particles;
magnetospheric electric fields, tidal forcing, momentum sources
and sinks - and the global energy balance in general.
Coupling with the magnetosphere and solar wind. Ionospheric
phenomena are a valuable indicator of magnetospheric processes.
This is primarily a matter of high-latitude observations, but
not entirely; data from lower latitudes are needed too.
Coupling with the lower atmosphere. Possible effects on STP
parameters of meteorological storms and weather systems; also
seismic and volcanic activity. "Sun-weather relationships"
may be controversial, but possible physical mechanisms are being
found. Short-term and long-term weather forecasts based on STP
data are claiming some success, and may have economic importance.
Medium-scale structure in the ionosphere (scale size 10-1000
km). What causes it? How is it related to magnetospheric, lower
atmospheric, surface and topographical features, and what can
we learn about them by studying the ionospheric structure (e.g.
by radio tomography)? The scientific literature contains many
questions but few answers on the subject.
"The ionosphere as a plasma physics laboratory"
i.e. study of small-scale structure" scale size 1 km - 1
m); its structure and morphology at high latitudes (auroral and
polar), also at equatorial and (to some extent) middle latitudes.
"The ionosphere as a communications medium". Despite
the use of satellites and cables, the ionosphere retains much
of its practical importance; even satellite and cable communications
are not immune from ionospheric and magnetospheric effects. This
topic requires good ionospheric modelling (both "empirical"
and "physical" models) and also involves both long-term
"predictions" and short-term "forecasting"
of ionospheric parameters.
"Predictability of the ionosphere" - at least three
aspects:
Long-term predictions of seasonal/solar cycle effects etc.,
based on solar observations and on comprehensive knowledge and
modelling of ionospheric structure and behaviour;
Forecasts of impending ionospheric storms, essentially depending
on solar and interplanetary data;
Once a storm has begun: Short-term "nowcasting"
of the subsequent progress of ionospheric disturbances, and of
their effects on communications etc.
Long-term global change. Ionospheric datasets extend over
several decades. If continued and maintained, they contain information
not only on obvious solar, geomagnetic, and seasonal variations,
but also on more subtle long-term changes, both natural and man-made.
So they are potentially valuable for studying effects of global
warming/cooling, ozone depletion, and industrial pollution. It
should be noted that other processes, at present unknown, may
become apparent in the future. There are many cases in which old
datasets suddenly became important for investigating a newly-discovered
phenomenon.
Data sets, especially those that go back to the 1930s or even
the 1960s, are valuable resources that must be preserved. Though
it may be difficult to maintain STP monitoring instruments in
many countries, it is important to national and international
science to do so. Thus priorities should be set in each country
for defining a basic long-term program: see Willis et al., J.
Atmos. Terr. Phys. 56, 871 (1994). Experience suggests that these
programs only survive if they are seen to meet national needs;
but there are educational and cultural aspects to be considered,
as well as industrial and commercial benefits.
Not every ionosonde, magnetometer, radar or other instrument
can be kept going, nor every research institute or group. Selectivity
is essential. There should be a mixture of "research"
instruments and "monitoring" instruments (though these
categories are not sharply demarcated).
Data have value, but the trap of regarding STP data as a commercial
commodity, to be bought and sold, must be avoided. No country,
however large, can do good STP / geophysical / environmental studies
(whether short-term or long-term) entirely with its own native
data. Exchange with other countries is essential, even for basic
items as solar-geophysical indices. Any country that puts a commercial
price on STP data can expect to find itself at the losing end
of the exchange when data from abroad are required.
In connection with 6, it would be very useful to quantify
the present-day practical use of the ionosphere, country by country.
Could INAG members or the national Commission G reps take on this
task ? They are well qualified to do so.