Vol 2, No. 2, August 2000

*L. A. Shchepkin, G. P. Kushnarenko, and G. M. Kuznetsova*

**Institute of Solar-Terrestrial Physics, Irkutsk, Russia**

*I. A. Freizon*

**Institute of Terrestrial Magnetism, Ionosphere, and Radio
Wave Propagation, Troitsk,
Moscow Region, Russia**

At the IRI Workshop in 1996 it was
formulated that
[*Bilitza*, 1997b, p. 3]
"The future task is:
to establish an f170 model in terms of
solar zenith angle, latitude, solar sunspot number and season."
The 170-km level lies at the region of the
*F*1 ledge.
There is a minimum variability of electron density
near this level
[*Zhang and Radicella*, 1996].

The first step in the frame of this
task is to study
the main regularities in variations of N170 (or the plasma
frequence f170). This may be done with the help of the semiempirical
model describing the N170 dependence on the
thermospheric
characteristics
[*Shchepkin et al.*, 1997].
Using
the MSIS 86 model [*Hedin*, 1987]
we can
evaluate
the
N170
dependence
on latitude, longitude,
solar
activity
indices,
geomagnetic disturbances,
day of the year, and local time
(or solar zenith angle.)

The semiempirical model used
[*Shchepkin et al.*, 1997]
was based on the experimental data on
*N* at 120-200 km
with the help of the regression equation which describes the
*N* relation to the thermospheric gas characteristics.

A comparison of the model calculations with the latest version
of the International Reference Ionosphere (IRI 95)
[*Bilitza*, 1997a]
is made.

The analytical expression, which describes the relation of
*N* with
the neutral gas characteristics, was obtained from the analysis of
the
*N*(*h*) profiles calculated with the help of the aeronomical model
of the type, presented by
* Vinitsky et al.* [1982].
We
used this expression as a regression equation:

(1) |

Here
*n*_{1} is the atomic oxygen concentration at 120 km,
*n*_{2} and
*n*_{3} are the O
_{2} and N
_{2} concentrations, respectively,
*T* is the
exospheric temperature,
*F* is the 10.7-cm solar flux,
*F*_{av} is the
value of
*F* averaged over the 81-day period,
and
*c* is the solar
zenith angle.

In (1) the term with the
*a*_{1} coefficient describes the
*N* dependence
on the relation between the atomic oxygen and molecular densities.
The next term describes the dependence on
cos *c* and the relative
atomic oxygen content. The temperature term (with the
*a*_{3} coefficient) shows the
*N* response to the variation of the particle
column content related to the
*T* variations. The last three terms
describe the solar activity effect in the ionization rate. Note
that the
*F* influence on
*N* via the changes of the neutral gas is
taken into account on the basis of the
* Hedin* [1987] model.

The coefficients in (1) were obtained on the basis of
*N* from the
*N*(*h*) profiles measured on the World Regular Days at the Moscow
Observatory for several years. The thermospheric characteristics
were taken from the
* Hedin* [1987] model.

The planetary geomagnetic activity index on the particular day, on
the previous day, and two days before was taken as
*Ap*^{0},
*Ap*^{-1}, and
*Ap*^{-2}, respectively.

The models were developed for low and high solar activity levels and all the levels together.

To justify the reliability of expression (1) for various periods
and places, the residuals
*dN* between the calculated and
experimental values of
*N* for the periods of high ( *F*_{av} > 120 ) and
low ( *F*_{av} < 100 ) solar activity were considered.
The experimental
data were taken as the monthly means from the database of the
*N*(*h*) profiles in the World Data Center B2. The
*dN* values in percents are shown in
Table 1.
One can see that in the majority of the cases,
*dN* is lower
than 10% for the midlatitude stations during the 0800-1600 LT
period.

Table 2 shows the
*N* values for various latitudes and longitudes in
the following conditions:
*F*_{av} = 150, the monthly mean value of
*Ap* is 10, and the day of the year is 166 (June 15).
Data on IRI are
also
shown
for comparison.

One can see from Table 2 that
*N* decreases with latitude increase.
The maximum values of
*N* are observed at the longitudes of
60^{o}-90^{o} E, where the highest concentrations of atomic oxygen
along the longitudinal circle take place according to the MSIS 86
model
[*Hedin*, 1987].

The
*N* value at 40^{o} N latitude and 90^{o} E longitude is
higher by 17% than the value at 60^{o} N. In the American
longitudinal sector,
*N* is lower than in the Asian sector. At the
latitudes of 50 and 60^{o} N the differences are ~10%
and nearly 13%, respectively.

The IRI 95 gives also lower values of
*N* in the
American longitudinal
zone than in the Asian one.
In our model,
*N* decreases with
latitude faster than in the
IRI.

The
*N* values in a diurnal cycle have a maximum around noon in
agreement with the experimental data. One can see it in Figure 1,
where the experimental data on the plasma frequency
*f*_{N} [*Reinish and Huang*, 1996]
at a height of 170 km are compared with the
calculations for 2 months of 1990 at Millstone Hill
[*Reinisch and Huang*, 1996].
The modeled values of the plasma frequency are in a good
agreement with the digisonde measurements of Millstone Hill.

The diurnal variation of
*N* is almost symmetrical relative to
noon. Table 3
shows the relations
*R* = *N* (0800 LT)
/*N* (1600 LT) for the
European-Asian and American longitudinal sectors for three latitudes.
One can see that
*R* deviates from 1 by
< 8%. It may be noted
that there is a tendency for
*N* to increase in the morning hours at
relatively low latitudes.

The
*N* variations during a year are characterized by the maximum in
May and the minimum in December. This is illustrated in Table 4.

At latitude of 50^{o} N and longitude of 90^{o} E the relation
is true:
*N* (May)
/*N* (December)
= 1.46 for
*F*_{av} = 150 and
*Ap* = 10.

The IRI gives different seasonal
variation of
*N* :
the
*N* maximum is in June, and the
*N* minimum is in January. The
ratio
*N* (June)/
*N* (January)
= 2.2.

The differences between the values of
*N* in our
model and IRI 95
are visually seen
in Table 4.
In summer it is
< 7%. The
strongest differences are in winter (40%).

The considered
*N* (summer)/
*N* (winter)
ratio
in the IRI is too large
as compared with the experimental data.
For example, at Slough
(latitude of 51.5^{o} N)
the
electron density
ratio,
for July and January of 1950 when the
*F*_{av} values
were almost
150 and 151, respectively) was 1.32.
Adak (latitude of 52^{o} N) in 1959 provides another
example:
*N* (July,
*F*_{av} = 213)/*N* (December,
*F*_{av} = 183) = 1.58.
One can see that this ratio is significantly
lower than in the IRI in spite of higher
*F*_{av} in July than in December.

It is worth noting that we have little differences between
the modeled and experimental values of
*N* in the
cases
considered
(see Table 1).

The value of
*N* increases with an increase of the solar activity
level. This can be seen in Table 5, where the
*N* values for various
*F* (when
*F* = *F*_{av}) are shown for the following conditions:
*j* = 50 ^{o} N,
*l* = 90 ^{o} E,
*D* = 166, and
*Ap* = 10.
The electron density
*N* increases by a factor of 1.15
when
*F* changes
from 70 to 150. At the same time,
the
*N* increase in the IRI for
similar conditions is by a factor of 1.25.

The value of
*N* decreases when
*Ap* grows. The
*N*(*Ap*) dependence is
better pronounced in the range
4 < Ap < 30. For these conditions,
*N*(*Ap*=4)/*N*(*Ap*=30) = 1.1. The same
*N* relation to
*F* is true for
*Ap* = 30 and 170. The
*N*(*Ap*) dependence is almost linear for
*Ap* > 30.
Then
*dN*/*Ap* = 0.023 10^{10} m^{-3}
per one unit of
*Ap* change.

The main features of the electron density variations at the 170-km
altitude with the local time, season, solar and geomagnetic
activity, latitude (in the range 30^{o}-70^{o} N), and the longitude
are discussed on the basis of the semiempirical model calculations.

There is a general coincidence of the
*N* values
calculated
with the help of the
model
discussed
and IRI 95.
In summer the difference between the
two models is
< 10%.
In winter it reaches 40% with
lower
*N* values in the IRI 95.
The latter gives larger amplitudes of the
seasonal variation.
Similar spatial distribution patterns in summer are seen
in both cases.

Bilitza, D.,
The International Reference Ionosphere status
1995/96,
* Adv. Space Res., 20* (9), 1751,
1997a.

Bilitza, D., Summary,
in * Proceedings of the IRI Task Force Activity 1996*,
edited by S. M. Radicella,
IC/IR/97, 11, p. 1,
Trieste,
Italy,
1997b.

Hedin, A. E., MSIS 86 Thermospheric Model,
* J. Geophys. Res., 92* (A5), 4649, 1987.

Reinisch, B. W., and X. Huang, The
*F*1 region at 170 km,
* Adv. Space Res., 18* (6), 53, 1996.

Shchepkin, L. A., G. P. Kushnarenko, I. A. Freizon, and
G. M. Kuznetsova,
The connection of the middle ionosphere electron
density with the thermospheric state,
* Geomagn. Aeron. (In Russian), 37*, 5, 106,
1997.

Vinitsky, A. V.,
G. I. Sukhomazova,
and L. A. Shchepkin,
Relation of the parameters of the
*F*2 and
*F*1 ionospheric layers
in the day-to-day variations,
* Geomagn. Aeron. (in Russian), 22*, 862, 1982.

Zhang, M.-L.,
and S. M. Radicella,
Variability of the
electron density at fixed heights near
*F*1 region,
in * Proceedings of the IRI Task Force Activity 1995*,
edited by S. M. Radicella,
IC/IR/96, 14, p. 55, Trieste, Italy, 1996.