International Journal of Geomagnetism and Aeronomy
Published by the American Geophysical Union
Vol. 1, No. 1, April 1998

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

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



Data on peculiar features of the ionosphere at solar zenith angles zodot , close to 60o  are analyzed. The increased values of ne  at zodot approx 60o  are explained by reduced values of the [NO + ]/[O 2+ ] ratio. Owing to the increase of ne  the values of the power index p  in the relation between ne ; (E)  and cosine of zodot  are reduced under low zodot  and increased under high zodot . The low values of [NO + ]/[O 2+ ] are due to their dependence on the governing ratios [NO]/ ne  and [N]/ ne , in which numerator and denominator increase with a decrease of zodot , but ne  changes more rapidly under the transition of zodot  from 90o  to 60o , and NO or N are changing more rapidly under further change of zodot .


Some peculiarities are observed in the ionosphere under the solar zenith angles close to 60o. 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 ne  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 ne  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 zodot simeq 60o and to provide an explanation for these effects.

Experimental Facts

First of all, we would like to note that indication of the value of zodot = 60o  is slightly conventional. One should speak of some interval about 60 pm 10o . 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 zodot . Also for certainty we limit our consideration to low and middle latitudes.

fig01 The rocket measurements of ne  allowed the detailed picture of the ne(h)  profile behavior with zodot  to be created still in 1965 [ Kazachevskaya and Ivanov-Kholodny, 1965]. The empirical model of ne (h)  at altitudes of 100-300 km at various solar zenith angles zodot  has been developed on the basis of these measurements. As an example, Figure 1 shows the ne (h)  profiles in summer under high solar activity from Ivanov-Kholodny and Nicolsky [1972]. The numbers at the curves indicate zodot . It can be seen that there are profiles of an intermediate type between two series of similar profiles obtained at zodot = 0-50o and during sunset and sunrise at zodot = 70-80o . At altitudes of 170-190 km an unusual grow of ne  with zodot  increase is observed at the intermediate profiles instead of a decrease, and in the E  region only under zodot > 60o  the region with a well pronounced maximum of ne  (the E  region) and a minimum of ne  above it (the "valley") is formed. It can be easily seen in Figure 1 that ne  under low zodot = 50-60o  at altitudes 110-130 km is lower than the one extrapolated from high values of zodot . That means that the p = d (ne)/d ( coszodot)  gradient decreases under zodot < 50-60o .

fig02 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 zodot  change from 90o  to 60o. 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 zodot = 90o  and also under low values of zodot . A minimum of C  is observed at zodot = 60o . Until recently this peculiarities of the ion composition variations with zodot  as well as the above mentioned features of the ne(h)  profiles under zodot simeq 60o has been given no physical interpretation.

Dependence of ne  on zodot  in the E  layer may be presented in the form

  ne = ne0 (cos zodot)2p  

where ne0  is the value of ne  under zodot = 0 . The value of p  in the simple layer theory is equal to 0.25, but according to observations this value is higher: p simeq 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 zodot  are obtained.

fig03 Figure 3 shows a dependence of the mean power index p , p(zodot ), for the Dzodot = 16o  intervals. The dependence was recently obtained by the authors, using near-noon observations of foE  at the network of about 30 ionospheric stations in the 40-60o N latitude band during minimum solar activity (June-July of 1976 and 1986). The total number of the foE  estimates used was about 4 times 104 . It can be seen in Figure 3 that the p(zodot ) function has a maximum equal to about 0.32 at zodot = 60 - 70o .

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 :


Generally speaking, ne  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 zodot were calculated earlier by Ivanov-Kholodny and Nicolsky [1972]. It has been found that the theoretical gradient


has the vertical structure similar to that of the experimental gradient


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 ne  dependence on zodot  is governed by the q  dependence on zodot . The minimum at the m(h)  theoretical profile appears due to the fact that a more narrow qy(h)  profile produced by the solar UV radiation is superimposed on the wide qx(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 qx/qy approx 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 ne  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 (F10.7 = 150-160)  have been analyzed and it was found that on the average qx/qy = 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 zodot  are very small and can not explain the considerable variations of the ionosphere under zodot = 60o .

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 ne  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 ne  by sim 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 zodot simeq 40o  to zodot = 60o  takes place in the entire altitude region below about 200 km, the additional decrease of ne  under zodot simeq 40o  and the increase of ne  near zodot simeq 60o  should also happen at almost all these altitudes. That can be seen at the initial ne (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 ne(h)  profiles: a depletion of ne  at zodot < 60o and an increase of ne  at zodot simeq 60-75o .

The effect of a  decrease due to the reduction of C  under zodot approx 60o in the E  region should be significantly lower, because first, the ne  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 ne  and corresponding increase of the p  parameter under values of zodot  slightly exceeding 60o  and by its depletion under zodot  values slightly lower than 60o. This very picture of p  variations with zodot  is observed (see Figure 3). In this case on the basis of the measured variations of p(zodot ) 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 zodot = 60o , variation of the gradients of ne (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 zodot simeq 60o  may contribute to the ne  increase at these altitudes.

As far as the initial reason of ne  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 ne , which is really observed under zodot  changes from 70o to 90o . The reverse effect of C  increase with a decrease of zodot  is observed under zodot < 60o . 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 ne increase. The minimum of C  is observed at zodot = 60o  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].


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 zodot  in the vicinity of 60o  should be considered together. Increased values of ne  under zodot simeq 60o  are explained by the depleted values of the [NO + ]/[O 2+ ] ratio. Owing to this increase of ne  the p  values are depleted under lower values of zodot  and enhanced under the higher ones. The low value of C  is related to its dependence on the governing parameters [NO]/ ne  and [N]/ ne , in which both numerator and denominator increase with a decrease of zodot , but ne  changes more rapidly under transition from 90o  to 60o , and NO and/or N change more rapidly under further decrease of zodot .

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


This paper was supported by the Russian Fund for Basic Research (project 93-05-9323).


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