Peculiarities of ionospheric behavior under the solar zenith angles close to 60^o

L. A. Antonova

Institute of Applied Geophysics, Moscow, Russia

G. S. Ivanov-Kholodny and V. E. Chertoprud

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

Contents

Abstract

Introduction

Some peculiarities are observed in the ionosphere under the solar zenith angles close to 60^o. They were detected while constructing an electron concentration empirical model [ Ivanov-Kholodny and Nicolsky, 1972; Kazachevskaya and Ivanov-Kholodny, 1965] and in the ion structure [ Antonova and Ivanov-Kholodny, 1990].

In the E  layer the peculiarities are known as "the noon depression" in electron density [ Ivanov-Kholodny and Nusinov, 1979]; in the F  layer they are manifested by the effect of n_e  enhancement in the morning and evening hours [ Kelley, 1989; Mitra, 1952]. Not all these peculiarities are explained, and they have never been considered jointly. Some cases, for example, well-pronounced E  layer and the "valley" in the n_e  vertical profiles, present a special interest for some problems of radio wave propagation.

The aim of this paper is to demonstrate a mutual relation of the revealed effects in ionospheric behavior under z 60^o and to provide an explanation for these effects.

Experimental Facts

First of all, we would like to note that indication of the value of z = 60^o  is slightly conventional. One should speak of some interval about 60 10^o . At the same time one has to keep in mind that some of the experimental facts discussed here do not have a solid relation to fixed values of z . Also for certainty we limit our consideration to low and middle latitudes.

The rocket measurements of n_e  allowed the detailed picture of the n_e(h)  profile behavior with z  to be created still in 1965 [ Kazachevskaya and Ivanov-Kholodny, 1965]. The empirical model of n_e (h)  at altitudes of 100-300 km at various solar zenith angles z  has been developed on the basis of these measurements. As an example, Figure 1 shows the n_e (h)  profiles in summer under high solar activity from Ivanov-Kholodny and Nicolsky [1972]. The numbers at the curves indicate z . It can be seen that there are profiles of an intermediate type between two series of similar profiles obtained at z = 0-50^o and during sunset and sunrise at z = 70-80^o . At altitudes of 170-190 km an unusual grow of n_e  with z  increase is observed at the intermediate profiles instead of a decrease, and in the E  region only under z > 60^o  the region with a well pronounced maximum of n_e  (the E  region) and a minimum of n_e  above it (the "valley") is formed. It can be easily seen in Figure 1 that n_e  under low z = 50-60^o  at altitudes 110-130 km is lower than the one extrapolated from high values of z . That means that the p = d (n_e)/d ( cosz)  gradient decreases under z < 50-60^o .

On the basis of the first rocket mass-spectrometer measurements was it was found by Ivanov-Kholodny and Nicolsky [1972] that the molecular ion concentration decreases under z  change from 90^o  to 60^o. Nowadays this conclusion may be broadened. Figure 2 from Antonova and Ivanov-Kholodny [1990] presents all the midlatitude measurements of the C = [ NO⁺]/[ O₂⁺]  ratio under low and moderate solar activity. One can see that the C  value grows to z = 90^o  and also under low values of z . A minimum of C  is observed at z = 60^o . Until recently this peculiarities of the ion composition variations with z  as well as the above mentioned features of the n_e(h)  profiles under z 60^o has been given no physical interpretation.

Dependence of n_e  on z  in the E  layer may be presented in the form

  n_e = n_e0 (cos z)^2p  

where n_e0  is the value of n_e  under z = 0 . The value of p  in the simple layer theory is equal to 0.25, but according to observations this value is higher: p 0.3-0.35 . Moreover, the p  variations related to local time (depletion of p  near noon and also to the morning and evening hours) were detected earlier by Robinson [1960]. Now more definite conclusions on p dependence on z  are obtained.

Figure 3 shows a dependence of the mean power index p , p(z ), for the Dz = 16^o  intervals. The dependence was recently obtained by the authors, using near-noon observations of f_oE  at the network of about 30 ionospheric stations in the 40-60^o N latitude band during minimum solar activity (June-July of 1976 and 1986). The total number of the f_oE  estimates used was about 4 10⁴ . It can be seen in Figure 3 that the p(z ) function has a maximum equal to about 0.32 at z = 60 - 70^o .

Possible Explanation

Theoretically the electron concentration in the E  region is determined by the ratio of the electron production rate by the solar UV radiation and the effective recombination coefficient a :

(1)

Generally speaking, n_e  variations may be due to variations of both q  and a . Let us consider the both possibilities.

The vertical profiles of the ion production rate at various z were calculated earlier by Ivanov-Kholodny and Nicolsky [1972]. It has been found that the theoretical gradient

(2)

has the vertical structure similar to that of the experimental gradient

(3)

In particular, the m  and p  gradients are equal to each other at altitudes of 150-200 km. Similar minima at the m(h)  and p(h) profiles are seen in the E  region. This fact has been considered as an argument that the n_e  dependence on z  is governed by the q  dependence on z . The minimum at the m(h)  theoretical profile appears due to the fact that a more narrow q_y(h)  profile produced by the solar UV radiation is superimposed on the wide q_x(h) profile produced by the solar X ray radiation [ Ivanov-Kholodny and Nicolsky, 1972]. Currently this explanation is reconsidered, because Ivanov-Kholodny and Nicolsky [1972] used overestimated values of the X ray fluxes. For example, even under low solar activity the ratio q_x/q_y 0.5  has been obtained in the E  layer maximum, which appeared to be by several times higher than the observed value [ Antonova and Ivanov-Kholodny, 1990; Ivanov-Kholodny and Nusinov, 1979]. The E  region observations during solar flares, when the solar X ray radiation is enhanced by several times, do not show an n_e  increase by several times, but only by 20-50%. The 15-minute observations of the E  layer at the midlatitude ionospheric stations during more than 50 flares in stable conditions under moderate solar activity (F_10.7 = 150-160)  have been analyzed and it was found that on the average q_x/q_y = 0.22 [ Ivanov-Kholodny and Nusinov, 1979]. That means that earlier the role of the X ray radiation in the E  layer ionization has been significantly overestimated, whereas the input of this radiation is by 5 times lower than the input of the UV radiation. Therefore, the q  changes with z  are very small and can not explain the considerable variations of the ionosphere under z = 60^o .

We are going to explain the above mentioned peculiarities of the ionospheric behavior by variations of the effective recombination coefficient a . The value of a  in the considered altitude range may be presented in the form:

where a₁  and a₂  are the dissociative recombination coefficients of NO ⁺  and O ₂⁺  ions, respectively. Let us first evaluate the effect of a  variation in the scope of the old scheme (without any allowance for excited ions). As far as the value of a₁  is about twice that of a₂ , the effect of a reduction of C  leads to a depletion of a . A change of C  from 2 to 0.5 leads to a depletion of a  by about 1.25 times and to a corresponding decrease of n_e  by about 10%.

It was suggested earlier by Antonova and Ivanov-Kholodny [1990] that significant changes of a  in comparison with the old scheme are related to a presence in the ionosphere of vibrationally excited NO ⁺  ions with low recombination coefficient. However, it follows from the new laboratory measurements of the corresponding processes [ Gritsay et al., 1993a] that the amount of such excited ions at the considered altitudes should be much lower than has been suggested. So Gritsay et al. [1993b] suggested a different mechanism of appearance in the ionosphere of the region with a reduced effective recombination coefficient: because of the existence of vibrationally excited O ₂⁺  ions, which are presented in the ionosphere in a sufficient amount. With allowance for that, the condition a₁ > a₂  is also fulfilled in the new scheme. The magnitude of the effect in the new scheme increases. Under the same changes of C , as in the first case, the depletion of a may be as high as by about 1.8 times (an increase of n_e  by  30% ) in the case when all the O ₂⁺  ions are in the exited state, i.e., during the illuminated part of a day above about 120 km [ Gritsay et al., 1993b].

As far as the decrease of the C  values under a transition from z 40^o  to z = 60^o  takes place in the entire altitude region below about 200 km, the additional decrease of n_e  under z 40^o  and the increase of n_e  near z 60^o  should also happen at almost all these altitudes. That can be seen at the initial n_e (h)  profiles [ Ivanov-Kholodny and Nicolsky, 1972; Kazachevskaya and Ivanov-Kholodny, 1965]. The effects at altitudes above 170-190 km (see Figure 42 of Ivanov-Kholodny and Nicolsky [1972]) is seen especially clearly at the model n_e(h)  profiles: a depletion of n_e  at z < 60^o and an increase of n_e  at z 60-75^o .

The effect of a  decrease due to the reduction of C  under z 60^o in the E  region should be significantly lower, because first, the n_e  variations are not detected owing to a small number of measurements and a strong scatter of the data, and second, the portion of excited O ₂⁺  ions at 110 km should be lower. It is worth noting, however, that the decrease of a  should be followed by an increase of n_e  and corresponding increase of the p  parameter under values of z  slightly exceeding 60^o  and by its depletion under z  values slightly lower than 60^o. This very picture of p  variations with z  is observed (see Figure 3). In this case on the basis of the measured variations of p(z ) one can determine a limit of possible a  variations. Estimates show that even if all the variations of p  from its maximum value of 0.33 to its minimum value of 0.26 would be due to a variations, the latter should not exceed about 10%.

Above 200 km the atomic (O ⁺ ), but not molecular, ions start to dominate, and the character of the principal processes changes. We merely note that near z = 60^o , variation of the gradients of n_e (h)  profiles occurs, which indicates their relation to changes of some parameters of these processes, in particular, recombination processes, which are controlled by the concentration of molecular ions NO ⁺  and O ₂⁺ .

Earlier, Kelley [1989] suggested taking into account variations of the ionization vertical drift induced by the horizontal wind in order to explain the morning and evening maxima in the F  layer. Nevertheless, one can suggest that the effect of a  depletion under z 60^o  may contribute to the n_e  increase at these altitudes.

As far as the initial reason of n_e  variations in a wide altitude range (and also p  in the E  layer) is the observed variation of C , it would be desirable to understand the reason of the effect itself. We indicate some ideas staying in the scope of qualitative considerations. The general expression for C  was presented by Antonova and Ivanov-Kholodny [1990]. For the altitudes of h < 140  km the expression is simplified and takes the form

where g₁  and g₂  are the rate constants of the interchange reactions O ₂⁺ +  NO   NO ⁺ +  O ₂  and O ₂⁺ +  N   NO ⁺ +  O. At higher altitudes this expression evidently is also true. The formula shows that C  increases with a decrease of n_e , which is really observed under z  changes from 70^o to 90^o . The reverse effect of C  increase with a decrease of z  is observed under z < 60^o . This is due to the fact that according to the satellite measurements there occurs an enhancement of NO near noon [ Krasnopol'skiy, 1974], and it influences the C  increase more significantly than the n_e increase. The minimum of C  is observed at z = 60^o  when both considered effects become equal. At altitudes of h > 140-150  km the role of NO passes to N [ Antonova and Ivanov-Kholodny, 1990].

Conclusions

The well-known features of distribution of the electron concentration and ion composition in the ionosphere at altitudes of 100-200 km (and of the p  parameter in the E  region) under the values of z  in the vicinity of 60^o  should be considered together. Increased values of n_e  under z 60^o  are explained by the depleted values of the [NO ⁺ ]/[O ₂⁺ ] ratio. Owing to this increase of n_e  the p  values are depleted under lower values of z  and enhanced under the higher ones. The low value of C  is related to its dependence on the governing parameters [NO]/ n_e  and [N]/ n_e , in which both numerator and denominator increase with a decrease of z , but n_e  changes more rapidly under transition from 90^o  to 60^o , and NO and/or N change more rapidly under further decrease of z .

Thus the different ionospheric effects, observed in the vicinity of z 60^o  are compiled into a united picture.

Acknowledgments

References

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