Vol. 2, No. 3, December 2001

*A. D. Danilov and L. B. Vanina*

**Institute of Applied Geophysics, Moscow, Russia**

The problem of
*D* -region modeling is a rather complicated one. One
of the difficulties in the modeling is the presence of many
ionization sources. It is especially true for the high-latitude
*D* region, where energetic particles (precipitating
electrons, solar protons) may create an increased electron
concentration during solar-geophysical events.

The principal idea of this study is to carry out comparative studies and obtain information on the electron concentration [e] variations depending on conditions (solar zenith angle, season, and geomagnetic activity). We also compare the corresponding dependencies for the Arctic and Antarctic to consider possible differences between the polar ionospheres of both hemispheres.

The first results of this study were published by
* Danilov and Vanina* [1999],
who considered some dependencies of [e] for
the fixed height of 80 km. Below we continue analysis of the same
data, analyzing two more altitudes (75 and 84 km)
and trying to
obtain some information on the vertical profile of various effects.
The altitude of 84 km was taken because the peak altitude of the
M100B rocket flights was around 85 km so at this altitude there are
much less measurements than at 84 km.

We use for the analysis the data bank of rocket measurements of the
electron concentration in the
*D* region
[*Knyazev et al.,* 1993].
The MR100B meteorological rockets have been flown by
the Central Aerological Observatory in 1979-1992 from two sites:
Heiss Island
( *j* = 80.6 ^{o} N,
*l* = 58.0^{o} E,
*F* = 72^{o} N)
and Molodezhnaya
( *j* = 67.7^{o} S,
*l* =45.9^{o} E,
*F* = 69^{o} S).
The device of an
electrostatic probe type has been
used. The instrumentation and data processing were described by
* Borisov et al.* [1981]
and
* Sinel'nikov et al.* [1980].

The databank used contains the results of 326 flights at Heiss Island and 250 flights at the Molodezhnaya station. The vast majority of the flights at Molodezhnaya were performed at 1400 UT which corresponds to about 1700 LT (1500 MLT). The station almost the entire day is situated within the auroral oval, so the data presented below may be considered typical for the dusk sector of the auroral zone.

The Heiss Island flights were conducted at various LT moments.
During different periods of the day Heiss Island may be located within
the auroral oval, polar cap, and subauroral zone. Since the physics
of the
*D* region may be different in the auroral zone and polar cap,
the analysis of the data on [e] should be performed separately for
the first two regions. The separation of the Heiss Island data to
auroral oval (ao) and polar cap (pc) was done according to the
local time of the observations. There are only a few flights
corresponding to the Heiss Island location within the subauroral
zone, so below we do not consider this region. The Heiss Island
(ao) flights were conducted at evening hours, so comparing them
with the Molodezhnaya data we compare the dusk sectors of the
auroral oval in two hemispheres.

Figures 1,
2
and 3
show typical dependence of the electron concentration
on solar zenith angle
*c* for the Molodezhnaya station at
altitudes of 75, 80, and 84 km. The strong scatter of the points is
evident, which is quite natural, because in the polar
ionosphere there are additional corpuscular sources of ionization
which are very changeable. Similar to Figures 1, 2 and 3 pictures were
drawn for Heiss Island (separately for the auroral oval and polar
cap).

The lines in Figures 1, 2 and 3 show some sort of a lower envelope for
each set of data and it seems reasonable to interpret them as a
quiet background, that is the level of [e] provided by quiet-time
sources of ionization. Certainly, there is
an arbitrary element in
drawing the lines. Nevertheless, it was shown by
* Danilov and Vanina* [1999]
that some conclusions may be obtained comparing
these lines for various conditions (following that paper we denote
the [e] values corresponding to the envelope lines as
[e]^{*} ).

For example, it was shown that at 80 km the
[e]^{*} values in the
nonsunlit period ( *c* > 100^{o} )
at the Molodezhnaya station are
higher than at Heiss Island. Now we consider this problem in
detail.

Figure 4 shows the
[e]^{*} values for three heights and three
situations (Molodezhnaya station, Heiss Island (ao), and Heiss
Island (pc)). One can see that there are some visual features in
the
[e]^{*}(*n*) behavior in the nonsunlit period
(*c* > 100^{o} ). At
75 km the
[e]^{*}(*n*) values are close to each other for all three
situations: the difference between the highest values corresponding
to the Molodezhnaya station and the lowest values corresponding to
Heiss Island (pc) is by a factor of 1.6 (0.2 in the logarithmic
scale). At 84 km this difference is very high: 1.3 or a factor of
20. At 80 km an intermediate situation is observed: the
[e]^{*}(*n*) values at the Molodezhnaya station exceed the values
at Heiss
Island (pc) by a factor of 4 ( *D* lg [e]^{*}(*n*) = 0.6).

Similar picture is seen if we compare the
[e]^{*}(*n*) values for the
two situations at Heiss Island. At 80 km
the values of
[e]^{*}(*n*) for
the auroral oval exceed that for the polar cap by a factor of 1.6.
At 84 km the difference reaches a factor of 2.5.

We interpret such changes of the relation between the
[e]^{*}(*n*) values for different situations in the following
way. At the 75 km
height in quiet geomagnetic conditions there is no additional
sources of ionization in the form of precipitating corpuscles.
Therefore the equilibrium electron concentration is determined by
"normal" (the same as at middle latitudes) ionization sources. It
is mainly the ultraviolet radiation (first of all in the Lyman-
*a* line)
scattered at the geocorona. The small difference between the
[e]^{*}(*n*) values for the Molodezhnaya station and Heiss Island
(pc)
may be of an occasional nature (for example it may be a result of
the arbitrary drawing of the lower envelope) or may manifest some
interhemisphere changes of aeronomical parameters (the nitric oxide
concentration, temperature, etc.) which determine the electron
equilibrium concentration, the ionization rate being given.

The
[e]^{*}(*n*) increase both for the Molodezhnaya station and
for
Heiss Island (ao) as compared with Heiss Island (pc) shows
evidently that even in quiet conditions in the auroral oval there
are particle precipitations which are absent (or weak) in the polar
cap. The particle energy should be such that they were able to
penetrate down to the height of 84 km,
partly absorbed above 80 km
and were not able to penetrate (completely absorbed) down to 75 km.

Since the flights both at the Molodezhnaya station and Heiss Island
(ao) correspond to the dusk sector of the auroral oval, one would
expect similar values of
[e]^{*}(*n*) in both situations. The real
difference at 84 km
is by an order of magnitude. Such a strong
difference manifests a strong difference either in the aeronomical
parameters governing
[e]^{*}(*n*) or in corpuscular fluxes. The former
seems very improbable. Actually, to have the difference in the
equilibrium electron concentration by a factor of 10 one needs a
change in the effective recombination coefficient (due to changes
of the aeronomical parameters which determine
*a*_{eff} ) by a
factor of 100. There is hardly any ground to assume such a strong
difference in aeronomical parameters between hemispheres. At the
same time, variations of the ionization rate by a factor of 100 due
to changes in the fluxes (and/or rigidity) of corpuscles seems
quite real. In this case the results obtained most probably
indicate to the hemisphere asymmetry (which have been numerously
discussed before, see, for example,
* Bythrow et al.* [1982],
* Newell and Meng* [1988],
* Suzuki and Sato* [1987], and
* Zanetti et al.* [1982])
in auroral precipitation intensity
and spectrum.

The best parameter to compare the [e]
data with would be the
geomagnetic
*AE* index, but there are gaps in the data on
*AE* for some
period when considerable portion of the flights has been conducted.
Neither is it possible to find a bank of precipitating particle
measurements adequate to the bank of [e] measurements. Therefore,
following the previous paper we compared the electron concentration
with the daily sum of the
*Kp* indices
*S**Kp* available in the
databank used.

Since any rocket flight lasts only a few minutes, it was
possible
to prescribe to any flight a three-hour
*Kp* index and to make the
comparison with
*Kp* instead of
*S**Kp*. We have tried this way for
several types of
[e] dependence on magnetic activity (analyzed in this paper
as well as in other publications) and have found that the statistical
characteristics of the results are much worse if
*Kp* is used. Our choice of
the
*S**Kp* as a geomagnetic index was also initiated
by the fact that
the characteristic time of the large-scale magnetospheric processes is about
one day. Therefore, one should not expect the particle
precipitation
to be active only in a particular 3-hour interval with high
*Kp*, but rather
depend on the disturbance degree of the total
day. It seems that our results
mentioned confirm this point of view.

* Danilov and Vanina* [1999]
found some statistically
significant correlation of [e] at 80 km
with
*S**Kp* for the
Molodezhnaya station and for Heiss Island in the auroral oval. It
was found that the best pronounced dependence of the electron
concentration on
*S**Kp* is seen for the nonsunlit conditions
(*c* > 84^{o} )
at the Molodezhnaya station.
It was detected also
that there is some sort of a "saturation"
effect in the dependence
of [e] on
*S**Kp* : a direct relation between
[e] and
*S**Kp* is
evident and statistically significant (the correlation coefficient
*r*= 0.49 )
for
*S**Kp* < 30,
whereas it is absent for
*S**Kp* > 30 (see Figure 5 in
* Danilov and Vanina* [1999]).

Similar comparison for the heights of 75 and 84 km
is presented in
Figures 5
and 6,
respectively. It is seen that at 75 km
there is an evident increase of [e] with
*S**Kp* up to
*S**Kp* about
25 and then the increase stops. Statistical analysis shows that the
highest correlation coefficient
(*r* = 0.52 ) between [e] and
*S**Kp* is obtained if we cut the analysis
at
*S**Kp* = 27.

At 84 km all the data shown in Figure 6 give
*r* = 0.53, but there is
only a few points for
*S**Kp* > 30.
Therefore it is difficult to
evaluate the "boundary"
value similar to that at 75 and 80 km, we
can only believe that the above boundary lies within the
*S**Kp*= 35-40 interval.

The scatter of individual points makes it difficult to evaluate
exactly the slope of the [e]
( *S**Kp* ) dependence at
*S**Kp* < 27,
but visually the dependence at 75 km is much steeper than at 84 km,
the data at 80 km
showing an intermediate situation. This
difference in the slope evidently indicates stronger dependence
on magnetic activity level of the electrons (with higher energy)
providing ionization at 75 km
than of the electrons producing
ionization at 84 km.

Thus, the consideration of the altitude profile of the [e]
dependence on
*S**Kp* in the nonsunlit conditions at the
Molodezhnaya station shows that an increase of the daily mean
magnetic activity (the
*S**Kp* index) leads to a significant
increase of [e] in the upper
*D* region only under
*S**Kp* below
some boundary value, the value increasing with altitude from 75 to
84 km.

The presence of such boundary value may be interpreted in two ways.
Either under
*S**Kp* increase above this value there is
a
"saturation" of the intensity of the precipitating particle
fluxes, the boundary value being different for different particle
energies, or under high enough magnetic activity there occur some
changes of aeronomical parameters which lead to an increase of the
effective recombination coefficient and thus compensate the
increase of the corpuscular flux intensity and stop further
systematic increase of [e] with
*S**Kp*.
To decide which of the
mechanisms mentioned really determines the observed effect in [e] a
special study of dependence of
*D* region aeronomical parameters
(first of all, minor constituents such as NO, O, O_{3} ) on
geomagnetic activity is required. Currently very little is known about
such dependence.

the electron concentration should depend on two principal
parameters, that is on the ionization rate
*q* and effective
recombination coefficient
*a*_{eff}.
In the majority of cases
(except the envelope values
[e]^{*} considered above) the values of
*q* are determined by precipitating particle fluxes.
Having no data on
such fluxes for every day of rocket launch, we considered above the
daily magnetic index
*S**Kp* as an indicator of the intensity of
these fluxes. The presence of statistically significant correlation
between [e] and
*S**Kp* (see Figures 5 and 6 and also Figure 5 in
*Danilov and Vanina* [1999])
demonstrates that the
proportionality between [e] and
*S**Kp* does exist. However in
all the above figures there is a significant scatter of the points
at any fixed value of
*S**Kp*.
Part of this scatter may be due to
the fact that the
*S**Kp* index is not related unambiguously
to
precipitating particle fluxes and is not the best index for their
description. However, one should not forget that not only
*q* but
*a*_{eff} as well may change in equation (1).
Variations of the
latter may not be related to changes of geomagnetic activity (and
precipitation) but occur due to changes of meteorological
parameters (first of all the atmospheric temperature), that is due
to the well known meteorological control of the
*D* region
[*Danilov,* 1986].

The temperature of the ambient atmosphere
*T* was measured up to a
height of 75 km almost in all rocket flights where [e] was
measured. Thus it is worth trying to look for [e] dependence on
*T* on the basis of the same databank. However such attempt meets
serious difficulties since one has to analyze a multi-dimensional
picture of electron concentration variations (with solar zenith
angle, magnetic disturbance degree, season, temperature).

* Danilov and Vanina* [1999]
tried to reveal the relation
between [e] ] and
*T* in the following way. The dependence of [e] on
*S**Kp* for the nonsunlit conditions at the
Molodezhnaya station (Figure 5 in
* Danilov and Vanina* [1999])
was used. Since
there was found no systematic seasonal effect, all points were
considered with equal significance. A mean dependence of [e] on
*S**Kp* for
*S**Kp* below the boundary value (see above)
was
calculated, and using this dependence, points for different
*S**Kp* were reduced to a fixed value of
*S**Kp*
(*S**Kp* = 9). Then
the dependence of the reduced values [e] (9) on the temperature
(measured in the same rocket flight at 75 km) was derived. This
dependence (see Figure 6 in
* Danilov and Vanina* [1999])
was found well pronounced and statistically significant with a
correlation coefficient
*r* = 0.63.

Since in the procedure described the effect obtained depends
strongly on the correct relation between [e] and
*S**Kp* used for
the reduction of measured [e] values to a fixed value of
*S**Kp* and since it is difficult to derive
an exact dependence
[e] = *f*(*S**Kp*) because of the
data scatter (see, for example, Figure 5),
here we made an attempt to reveal the dependence of [e] on
*T* in
a different way.

Fixed values of the
*S**Kp* index for which there are several
(not less than three) measurements of [e] on different days were
considered and a dependence of [e] on
*T* was derived for each value
of
*S**Kp*. The number of points for each fixed
value of
*S**Kp* in this case is lower than in the method
described above, but
possible uncertainty in the dependence of [e] on
*S**Kp* used in
the previous method to reduce the data to
*S**Kp* = 9 is avoided.
In fact we thus get rid of one more dependence of the electron
concentration and reduce the picture to a two-dimensional one.

The results of the approach described are shown in Figures 7 and 8
for altitudes of 75 and 80 km, respectively. One can see that,
though the scatter of the individual points for some values of
*S**Kp* is rather strong, almost in all cases
the approximating
lines
lg [e] = *A* + *B* *T* (formally calculated by computer)
give an
increase of [e] with
*T*.

The averaged values of
*B* for all the values of
*S**Kp* considered
give
*B*_{aver} = 0.023 and 0.024 for the heights of 75 and 80 km,
respectively. Similar analysis for
*h* = 84 km was not performed
because both there is not enough points in Figure 5 and the real
atmospheric temperature at 84 km may differ significantly from that
at 75 km.

Thus the results obtained confirm existence of a positive relation
between [e] and
*T* found earlier by
* Danilov and Vanina* [1999]
and make it possible to obtain mean values of the
*B* coefficient in this relation.

* Danilov and Vanina* [1999]
showed that the electron
concentration at 80 km
over Heiss Island depends on particular
polar zone the station is in at the moment of rocket launch.

We have already emphasized above that comparison of each particular
rocket flight with geomagnetic and meteorological data is limited
by the absence of an adequate databanks. Therefore it is worth
considering the [e] values averaged over all the flights in this or
that situation. In such procedure it is assumed that (due to large
number of the flights averaged) the mean values
[e]_{mean} obtained
correspond to some mean disturbance level which does not differ
significantly for the flight groups in comparison. In other words,
we assume that the percents of strong, moderate and weak
disturbances is nearly the same among the days when the
measurements were conducted in the auroral oval and polar cap. Such
approach was used by
* Danilov and Vanina* [1999] and
* Vanina and Danilov* [1998]
to reveal the seasonal behavior of
[e]_{mean} at the Molodezhnaya station and Heiss Island and to
compare
[e]_{mean} at 80 km at Heiss Island in the auroral oval and
polar cap.

Table 1 shows the
[e]_{mean} values for three altitudes and two
situations at Heiss Island. One can see that there is only
relatively small difference in the averaged electron concentration
in the sunlit period ( *D* lg [e] < 0.13 ). This difference
may be
attributed to occasional errors of averaging. However, in the
nonsunlit period the difference is higher ( *D* lg [e] = 0.24-0.25) and agrees with
the result found earlier at 80 km.

That means that the
[e]_{mean} values over Heiss Island in the
nonsunlit conditions are higher by about a factor of 1.8 when the
station is in the auroral oval than when it is within the polar
cap. That means that the intensity of the precipitating particle
fluxes producing ionization in the upper
*D* region are
systematically higher by about a factor of 3.5 in the auroral oval
than in the polar cap. Stating that, we naturally assume (since we
deal with the measurements at the same geographic point) that there
is no systematic difference between the auroral oval and polar cap
in the values of meteorological and aeronomical parameters. It is
worth noting that the effect found (contrary to other effects
discussed above) does not show any pronounced altitude behavior.

The analysis of the rocket measurements of the electron
concentration in the upper
*D* region performed for three heights
(75, 80, and 84 km) confirmed the conclusions drawn earlier
[*Danilov and Vanina*, 1999]
for a height of 80 km. In
particular, the [e] dependence on the daily index of magnetic
activity
*S**Kp* and existence of some sort of a "saturation
effect"
under high
*S**Kp* were confirmed and it was found that
the boundary value of
*S**Kp* tends to increase with height. The
electron concentration dependence on the atmospheric temperature
was also confirmed. This fact demonstrates that the meteorological
control of the
*D* region known for midlatitude ionosphere does exist
and may be revealed also at high latitudes.

The comparison of the minimum values of [e] in three situations (Molodezhnaya station, Heiss Island (ao), and Heiss Island (pc)) at three altitudes (75, 80, and 84 km) showed that even in quiet geomagnetic conditions there do exist corpuscular precipitations in the auroral oval, the intensity of the precipitations being significantly higher in the southern hemisphere (Molodezhnaya) than in the northern hemisphere (Heiss Island). The characteristics of the corpuscular spectrum should be such that their flux is able to penetrate down to 84 km, is absorbed significantly on its way to 80 km, and does not reach 75 km.

Borisov, A. I., V. N. Kikhtenko, and S. V. Pakhomov, The
preliminary results of the measurements of the upper atmosphere
charged component onboard meteorological rockets,
* Proc. Central Aerological Observatory (in Russian), 144*, 3,
Dolgoprudny, Moscow Region,
1981.

Bythrow, P. F., T. A. Potemra, and R. A. Hoffman, Observations of
field-aligned currents, particles, and plasma drift in the polar
cusps near solstice,
* J. Geophys. Res., 87*, 5131, 1982.

Danilov, A. D., Meteorological control of the
*D* region,
* Ionosfern. Issled. (in Russian), 39*, 33,
1986.

Danilov, A. D., and L. B. Vanina, Comparison of the polar
*D* region
behavior in the Arctic and Antarctic,
* Adv. Space Res., 24* (12), 1655, 1999.

Knyazev, A. K., L. B. Vanina, L. V. Korneeva, and V. N. Avdeev,
Specification of the empirical model of the dependence of the
*D* region electron concentration on the solar zenith angle from the
rocket measurements,
* Geomagn. Aeron. (in Russian), 33* (5), 145, 1993.

Newell, P. T., and C. I. Meng, Hemispheric asymmetry in cusp
precipitation near solstices,
* J. Geophys. Res., 93*, 2643, 1988.

Sinel'nikov, V. M., et al., A rocket radiobeacon experiment on the
electron density profile measurement in the bottomside ionosphere,
in
* Proceedings of Satellite Beacon Symposium*, p. 453, Warsaw,
Poland, 1980.

Suzuki, H., and N. Sato, Seasonal and diurnal variations of ELF
emission occurrences at 750-Hz band observed at geomagnetically
conjugate stations,
* J. Geophys. Res., 92*, 6153, 1987.

Vanina, L. B., and A. D. Danilov, High-latitude
*D* region and the
asymmetry of the hemispheres,
* Geomagn. Aeron. (in Russian), 38* (4), 173, 1998.

Zanetti, L. J., et al., Interplanetary magnetic field control of
high-latitude activity on July 29, 1977,
* J. Geophys. Res., 87,*
5963, 1982.