Z. Ts. Rapoport and V. M. Sinel'nikov
Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation, Troitsk, Moscow Region, Russia
Experimental electron concentration profiles ( Ne(h) ) are undoubtedly of interest to both radio wave propagation problems and the development of reliable models. The attempts to derive the Ne vertical profiles experimentally have been made by both ground-based and rocket techniques. Among the ground-based methods, the cross-modulation [Fejer, 1970] and partial reflection [Belrose, 1970] experiments and the techniques based on measurements of radio wave absorption at a number of frequencies of the HF and MF ranges [Belikovich et al., 1964; Bremer and Singer, 1974; Parthasarathy et al., 1963] and also of the LF and VLF wave field parameters [Baybulatov and Krasnushkin, 1966; Deeks, 1966] have been discussed in the literature. The ground-based techniques make it possible to monitor the state of the lower ionosphere, which is an advantage over rocket techniques which can be used only episodically and at a limited number of locales. However, the latter in-situ measurements are no doubt more precise.
To obtain Ne(h) profiles by rockets, different probing techniques [Holt and Lerfald, 1967; Smith, 1969], the exploitation of ionospheric radio wave propagation (differential absorption, Faraday rotation [Jacobsen and Friedrich, 1979; Knoebel and Skaperdas, 1966] and coherent frequency technique [Seddon, 1953]) have been applied. All of these methods (both rocket-borne and ground-based) involve some degree of measurement error. The most precise measurements of the absolute value of electron concentration have been carried out using radio physics techniques (differential absorption, Faraday rotation, and coherent frequency) (see, e.g., Mechtly ).
Various attempts to compile catalogs of the Ne(h) profiles for the lower ionosphere from previous work have been made [Danilov and Ledomskaya, 1983; Nesterova, 1987]. Unfortunately, it is very difficult to use these because the profiles were obtained by different techniques at different latitudes and longitudes and under varying solar and geophysical conditions.
To study the lower ionosphere, the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation and the Central Aerological Observatory of Roskomgidromet carried out joint measurements ionospheric electron concentration by the coherent frequency technique, using a M100B meteorological rocket at the Volgograd range ( j = 48o 41' N, l = 44o 21' E) from 1979 till 1990. A significant fraction of the rocket launches were performed within the framework of International Campaigns [Knyazev et al., 1994; Rapoport, 1984; Williams et al., 1987]. Over the 11 year interval, 41 profiles were obtained.
The goal of the work presented here is a generalization of these experimental data and their possible interpretation.
A brief description of the coherent frequency technique follows. A transmitter emitting two coherent radio frequencies f1 and f2 = f1/p is installed on a rocket. For these experiments, frequencies of 10 and 40 MHz were used. It can be shown that in the case of this choice of working frequencies,
Here, Ne1,2 is the mean electron concentration in the altitude range Dh = h1-h2; DF1,2 = F1 - pF2 is the reduced phase difference between two coherent frequencies; j is the angle between the wave trajectory and vertical; and cos j1,2 is the mean cosine of angle j for the same altitude range ( Ne is in cm -3, DF is in degrees, and Dh is in meters).
Thus to find Ne, it is necessary only to measure the increment DF for successive time intervals and to record information about the rocket trajectory (h(t) and j(t) ), which is acquired from standard trajectory measurements. According to the work of Sinel'nikov and Tolstova , the total experimental error in determining electron concentration in the 80-100 km range is 10%.
Figure 1 shows examples of the registrograms of the amplitude (A) and the reduced phase difference (DF) of the coherent signals from a transmitter on board of the M100B meteorological rocket for daytime and nighttime conditions. Figure 1 shows also the electron concentration vertical profiles at the ascent (1) and descent (2) parts of the trajectory of the rockets launched on May 18, 1979 under c =83o (a) and May 23, 1979 under c = 100o (b).
Table 1 shows a total list of the rocket flights. The third column shows the measurement height interval, however in this paper we consider only the Ne values up to the height of 100 km, that is in the most interesting altitude range. The last column of Table 1 contains the smoothed values of the relative sunspot numbers Rz [Solar-Geophysical Data, 1980-1990] to indicate solar activity level.
Figure 2 shows the generalized experimental results. The nighttime profiles were obtained over a whole year irrespective of the season, while winter daytime profiles were obtained in winter months (December-January-February), and nonwinter profiles were obtained in the daytime during the remaining months of the year; n is the number of electron concentration profiles Ne used to obtain the average. In the majority of cases, the maximum number was used, although fewer values were available for the lowest heights in the profile. Horizontal error bars show confidence intervals for a confidence probability of 0.95. So as not to overload the figure, the confidence intervals at some points are shown on only one side, from the mean value of Ne. The derived profiles shown here are representative mid-latitude values of Ne for both the winter night and day, as well as other seasons.
The monthly smoothed mean sunspot number Rz during the observations varied from 163 (February, 1980) to 12 (September, 1986), while the corresponding solar radio emission flux F10.7 varied from 200 to 73 [Solar-Geophysical Data, 1980-1990]. These variations correspond to changes from maximum to the minimum solar activity. The solar zenith angle also changed appreciably during rocket launches (see Table 1); however, the winter day launches were performed at a solar zenith angle of about 78o. Unfortunately, it is very difficult to get a statistically reliable estimate of the effects of solar activity and solar zenith angle during the daytime hours because of limited experimental data. Note that one of the most recent attempts to find these dependencies was made by Belikovich et al. .
A difference between the Ne(h) profiles for day and night hours is clearly seen. The major feature of the winter profiles is a large daytime value of Ne compared with summer in the height range of 75-95 km, reflecting the presence of the so-called winter anomaly of the lower ionosphere. (Note that it has been shown on the basis of several examples that the winter anomaly is observed not only in the daytime but also at night [Knyazev et al., 1994; Pakhomov et al., 1985]). First of all, the rocket measurements allow us to refine the height range where anomalies are observed ( 75-95 km). The lower boundary of this range is much lower (by 5-10 km) than assumed earlier, from various ground-based observations (see, e.g., Lastovicka ). The largest increase in Ne is observed in the lower part of the profile; at a height of 80 km, the mean values of Ne is increased during winter day hours relative to other seasons by nearly a factor of ~8 and attains a density of 3.5 103 cm-3, which exceeds the nighttime values by more than one order of magnitude.
The day-to-day variability of the electron concentration (and, as a consequence, of the radio wave absorption) is a characteristic feature of the winter anomaly. The variability may be observed under a continuous measurements of radio wave absorption (see e.g. Schwentek, ). It is practically impossible to carry out such a monitoring by rocket flights, because rockets are flown episodically. However, the electron concentration variability is clearly seen in the results of rocket measurements of the Ne(h) profiles. For example, Ne = 1.75 104 cm -3 in the daytime on December 20, 1983 at the 85 km height and Ne = 2.9 103 cm -3 at the same height on February 23, 1984 [Williams et al., 1987]. The variability may also be seen in the standard deviation s values. For example, at 80 km s = 1.9 102 cm -3 in nonwinter months in the daytime and s = 3.8 103 cm -3 in winter.
A discussion of the mechanisms and specific features of the winter anomaly is beyond the scope of this paper. We note here only one aspect which is important from our point of view. It is commonly accepted that a winter increase in the lower ionosphere electron concentration at middle latitudes is caused by an increase in the concentration of nitric oxide ionized by the solar Lyman- a-radiation ( l = 121.57 nm). Nitric oxide is a relatively long-lived minor constituent of the atmosphere [Taubenheim, 1984], and since increased concentrations of NO cannot be formed in the mesosphere or lower thermosphere, i.e, the height region corresponding to the lower ionosphere, the increase is attributed to dynamic processes, such as the transport of NO from regions where the NO concentration is greater.
In general, three possibilities can be considered: 1) Movement of air due to both the mean mass transport and eddy diffusion from higher regions of the thermosphere; 2) Transport from the stratosphere; and 3) Transport of air masses enriched by NO from the auroral zone, where nitric oxide is formed by injection of energetic particles into the atmosphere.
The winter anomaly could be explained by the transport from above. Unfortunately, this concept does not agree with the experimental data. Permanent monitoring of the vertical motions may be provided by radars directed to the zenith. There are few publications on this kind of measurements [Balsley and Riddle, 1984; Hocking, 1988]. The continuous measurements during 15 months at Alaska show that at 75-110 km there is an upwelling in winter and downwelling in summer [Balsley and Riddle, 1984]. The two-year (1985-1986) continuous measurements of the turbulence parameters in the upper mesosphere and lower thermosphere by the 2-MHz radar in Adelaide (Australia) did not detect any turbulence increase in winter [Hocking, 1988]. If we assume that the radar measurements provide information on the vertical mean motions (we see no other possibilities for such monitoring), we come to the conclusion that there is yet not enough experimental basis to confirm the role of the NO vertical transport. Nonetheless, the question is still open and important and so a continuation of such studies is very desirable.
Adelaide is located at rather low latitude (35o S) and so a doubt may appear, whether the winter anomaly is at all observed in this latitude zone, because there are data, which indicate that the equator boundary of the winter anomaly zone in the northern hemisphere is situated between 37o N and 40o N [Lauter and Schaning, 1970]. However, other data demonstrate that the equator boundary of the winter anomaly zone may be as low as 30o N [Elling et al., 1974]. There are also indications that the winter anomaly equator boundary in the southern hemisphere may be situated at lower latitude than in the northern hemisphere [Schwentek et al., 1980]. There are also data for the Southern Australia. Ferguson  showed that the winter anomaly is observed in Canberra (35o S) and Camden (34o S).
Transport from lower heights should lead to an increase in the concentration of water vapor, which will contribute to the formation of cluster ions and an increase in the effective recombination coefficient. This will have the effect of weakening the enhancement of electron concentration due to an increase in the nitric oxide concentration. Therefore, this mechanism cannot be regarded as efficient [Danilov and Ledomskaya, 1979].
Numerous rocket and satellite measurements have shown that nitric oxide concentrations at auroral latitudes are much higher than at middle latitudes (see, e.g., Rush and Barth ). It is natural to assume that NO can be transported from the auroral region in the meridional direction to middle latitudes [Lauter et al., 1976]. However, Lauter et al.  noted that transport of auroral air parcels from north to south in the northern hemisphere is opposite to the direction of the prevailing wind in winter, and attempts to correlate the meridional wind with electron concentration and radio wave absorption in the D region have yielded ambiguous results [Meek and Manson, 1978]. In addition, the meridional wind component is typically unstable in direction and small in absolute magnitude. Nevertheless, we believe that it is quite probable that the meridional wind component can provide a contribution to the formation of increased winter concentration of NO in the mesosphere and lower thermosphere at middle latitudes. However, this mechanism must not be considered as the major factor.
Rapoport  first suggested the mechanism of the transport of NO-enriched air from the auroral region by the stable winter circumpolar cyclonic vortex. A displacement of the vortex position relative to the auroral region ensures transport of auroral air to middle latitudes. The possibility of this mechanism can be verified from maps of multi-year mean charts of the atmosphere circulation in the northern hemisphere obtained for the height of 95 km on the basis of a generalization and synoptic analysis of the set of wind measurements by different techniques in different months [Portnyagin et al., 1978]. Figure 3 shows the charts for four months typical for four seasons (December, March, June, and September) from Portnyagin et al. . The oval at the Fc = 67o corrected geomagnetic latitude is shown in the charts by the dashed line and provides information on the auroral oval position. One can see that in winter (December) a transportation of the auroral air to lower latitudes is possible due to the cyclonic vortex. The transportation is impossible in other seasons. Zonal wind profiles at h = 25 - 80 km obtained by rockets launched from the Volgograd site and averaged over 20 years (1960-1980), also indicate that in this height range eastward zonal wind prevails in winter and westward zonal wind dominates in summer [Entzian and Lauter, 1982]. In our opinion, the latter transport mechanism is dominant in winter. The known variability in energetic particle fluxes in the auroral zone and their spatial nonuniformity explain the corresponding variability in the concentration of nitric oxide and electrons and also in the winter absorption in the D region at middle latitudes.
To explain specific features of the winter anomaly at middle latitudes, the mechanism of planetary waves has also been invoked [Labitzke et al., 1979]. In their comprehensive work, Garcia et al.  described model calculations involving photochemistry and auroral air transport by mean meridional circulation and planetary waves. This mechanism should not be regarded as an alternative to the mechanism described above. Presumably, planetary waves also contribute to NO transport, which then leads to the periodic variations in radio wave absorption in the lower ionosphere that are sometimes observed.
Experimental electron concentration profiles in the lower ionosphere ( h = 70 - 100 km) were obtained using the coherent radio frequency technique carried on board meteorological rockets launched between 1979 and 1990 from Volgograd. We obtained average profiles for both nighttime and daytime, in winter and other seasons, from the ensemble of in-situ profiles.
We conclude that one of the reasons for the ionospheric winter anomaly at mid-latitude is the presence of auroral air parcels that have been enriched by nitric oxide and transported from high latitudes by the circumpolar cyclonic vortex.
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