ICTP'97 IRI Task Force Activity

The 1997 International Reference Ionosphere Task Force Activity
at the International Center for Theoretical Physics took place
from June 4 to 10 (the week after the IRI'97 Workshop). The main 
topic was "The Global Variation of Profile Shape Parameters" with 
five specific tasks: (1) A description of the global and temporal 
variation of the electron density at 170 km altitude; (2) An 
improved table of B0 parameters for the IRI bottomside profile 
and also a new table for the B1 parameters; (3) The representation 
of the F1 region in IRI using the new probablity function 
established during last year's Task Force Activity and using also 
the N170 anchor point; (4) New approaches to modelling the electron 
density topside for IRI; (5) A discussion of data sources for 
topside modelling including also Total Electron Content (TEC) data 
and the deduction of topside shape parameters from TEC data.

The main result of the week-long activity are new tables for the B0
and B1 parameters that describe the IRI bottomside profile. These
tables describe the variation of B0 and B1 in terms of modified
dip latitude, local time, season and solar activity; an interpolation
scheme is used to provide a continous variation with all the variables.
The new B0, B1 parameters are based on averages obtained from several
ionosonde stations using the ARTIST as well as the POLAN ionogram
inversion programs. Both of these programs now provide directly the B0 and
B1 parameters. The most notable improvement with these new tables is
found at equatorial latitudes where the IRI B0 table did so fare
not have any values at all. This is a very important improvement
since the largest bottomside thicknesses (B0) are found in the
equatorial ionosphere. The corresponding IRI subroutine was
modified for inclusion of the new equatorial B0 values. In addition 
to the improved B0 table a table of values was also procuded for the
parameter B1 which describes the shape of the bottomside profile.
So fare the IRI program assumed a constant value of 3 for B1; B1
was corrected to 3.5, 4, 4.5 and so on to allow the merging of the
bottomside profile from the top with the E-valley function from the
bottom. Using the new B1 table values and interpolation scheme the
seasonal and diurnal changes in the shape of the bottomside profile
are better represented. 

Future improvements of the B0, B1 tables are expected with more
data for high solar activity conditions especially at equatorial
latitudes. Efforts will be undertaken to provide more data for
these conditions for the 1998 IRI Task Force Activity.

Several models where presented for the N170 parameter. But since
these models depend on additional profile parameters, e.g. foF2
and neutral densities, it was decided to attempt a modelling
approach for N170 similar to the Ducharme et al formula for foF1.
This means the electron density at 170 km will be represented
in terms of geomagnetic (or dip) latitude, solar zenith angle,
season and solar activity. For this modelling effort the ground-based 
(ionosonde) and satellite (in-situ) data base for N170 will be combined
and than approximated with suitable functions.

At the IRI'97 workshop in Kuehlungsborn it had been decided that the
next edition of the IRI model will include the F1 probablity
function that was established during the 1996 IRI/ICTP Task Force
Activity (TFA). During this year's TFA discussions continued on the
subject of how to include a new F1 region profile in IRI. In addition
to the anchor point at 170 km an important parameter is the height
of the F1 feature (hmF1). An effort will be undertaken to evaluate
the Radicella et al. formula for hmF1 with newer data. A particular
problem for modelling this part of the IRI profile is the deep valley
between the F and E region during nighttime. The satellite data
show a large variability of N170 during nighttime. 

For the topside three new modelling appraoches were presented. The
GSFC group is using a modified Bent/IRI formalism in conjunction
with a large Alouette/ISIS data base. The main advanges compared
to the earlier Bent and IRI models are a subdivision in more
altitudinal segments with dynamical segment boundaries and more
thna doubling the number of profiles used in determing the model
parameters. The Graz group is using a new magnetic field coordinate
to better represent the variations of the topside scale height.
First results where presented using IRI at 600 km as normalization
point. The ICTP group is using a Epstein function to represent
incoherent scatter profiles from Malvern, U.K. and EISCAT. Very
interesting is also the quality parameter epsilon that was introduced 
by the ICTP group to measure the goodness of a fit to an
experimental topside profile. 

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