I. P. Gabis
Arctic and Antarctic Research Institute, Saint-Petersburg, Russia
It is known that the basic type of sporadic layers in the E region of the polar cap ionosphere is flat Esf [Troshichev, 1986]. The mechanism of formation of this anomalous electron concentration is still unclear. The goal of this paper is to study the conditions for formation of the anomalous ionization in the E region of the polar cap ionosphere, its relationship with auroras, and the dependence of the probability of its occurrence on Bz of IMF.
For the analysis, ascaplots [Apatity, 1981, 1982] and ionograms of the ionosphere vertical sounding (IVS) obtained at the Antarctic observatory Vostok (corrected geomagnetic latitude F' = 83.29 o S) for May-August, 1978 and 1979, were used. The anomalous electron concentration at ionospheric E region altitudes was studied by the parameter fbEs MHz, i.e., the screening frequency of the sporadic layer which characterizes the electron concentration in the E region [ Ne=1.24104(fbEs)2 cm-3]. Ascaplots contain information about auroras observed by the all-sky camera at the Vostok observatory. The field of view of the camera allows determination of the spatial distribution of auroras in the 9-degree latitudinal range with the center at the zenith, i.e., at F' 78.8 - 87.8 o. For the Vostok observatory, MLT is nearly equal to UT.
Figure 1a shows the diurnal variations in the probability P(fb Es) of occurrence of fbEs ( fb Es being a blanketing frequency of the sporadic layer), obtained for eight winter months (May-August) of 1978 and 1979. The curves demonstrate a pronounced premidnight maximum in P(fbEs ). At 1900-0200 UT, the probability from the curve for fbEs >1 MHz exceeds 0.2, which is nearly twice as high as its minimum value at 0600-1100 UT, and at 2100-0000 UT it equals 0.30-0.35.
Earlier studies [Besprozvannaya and Shirochkov, 1976] of the probability of appearance of sporadic layers P(Es) in the polar cap showed that the diurnal variations in P(Es ) at the Vostok observatory has two maxima, i.e., the premidnight and near-midday maxima. The latter is more pronounced in summer. Two analogous maxima occur in the probability P(fbEs ). In this study, a significant seasonal dependence is observed, i.e., the noon probability in summer is 5 times as high as in winter [Gabis and Pudovkin, 1991]. It is likely that this fact explains the absence of the noon peak on the curves in Figure 1a plotted for a winter season. However, it can be seen that P(fbEs ) grows beginning from 1000 UT, and the curves are not symmetric with respect to the middle of the premidnight peak (2200-2300 UT) because of the enhanced value of P(fbEs ) near the geomagnetic noon.
The origin of the noon maximum was studied by Gabis et al.  and Gabis and Pudovkin . They showed that the anomalous electron concentration at the geomagnetic noon at F' = 83o can result from the neutral atmosphere ionization by the particles precipitated at the poleward cusp boundary during the pulsed reconnection of the interplanetary and geomagnetic fields at the high-latitude magnetopause in the case of the northward directed IMF. The premidnight maximum in P(fbEs) corresponds to the location of the Vostok observatory in the central part of the polar cap, and the factors responsible for formation of anomalous ionization in the premidnight MLT sector are therefore likely to be associated with the processes in the magnetotail. It is interesting to study the dependence of appearance of near-midnight fbEs on the IMF direction and their relationship with auroras.
Figure 1b shows the diurnal variations in the probability of occurrence of auroras Pa from observations at the Vostok observatory. To determine Pa, the same time interval as for Figure 1a was used, i.e., May-August, 1978-1979. Note that the probability Pa determined from ascaplots can be overestimated because a half-hour interval in ascaplots is assumed to have an aurora if the aurora is observed at least at one ascafilm for this interval. Comparison of the curves in Figure 1 first of all leads to the conclusion that the premidnight maximum in P(fbEs) is observed during the period of the local minimum in the probability of aurora occurrence, and the morning maximum in Pa falls on the minimum in the probability of appearance of anomalous ionization. The morning maximum in Pa is mainly associated with auroras observed within the zenith angles 0 60o (which corresponds to F' 82 - 85o ). The minimum in P(fbEs) during these hours indicates that the particles responsible for auroras at 0600-0900 UT do not have sufficient flux energies and/or intensities to give rise to changes in the ionospheric E region ionization. The local peak in auroras in the noon hours (1000-1400 UT) is formed mainly by auroras at the northern horizon, i.e., by auroras located equatorward of the Vostok observatory at F' 79- 82o. These are auroras of the daytime sector of the oval or auroras in the region of the poleward cusp boundary [Gabis and Pudovkin, 1992]. The number of auroras at the zenith is also larger during these hours. After 1500 UT, the probability of occurrence of auroras at the zenith is higher than at the northern horizon. At the southern horizon, i.e., to the pole of the Vostok observatory at F' 85 -88o, Pa is low during the whole day and night.
The comparison of simultaneous observations of fbEs and auroras allowed us to answer the question whether auroras are connected with anomalous ionization in every particular case. Then we analyzed 93 days for April-September, 1979, during which either fbEs or auroras, or both of them were observed. The periods (days) during which auroras were not photographed or there were no IVS data and also the days when observations were carried out but there were neither auroras nor ionization fbEs were excluded from the analysis. It was found that the relation between fbEs and auroras depends on the MLT sector. The probability of coincidence of the two phenomena (i.e., the ratio between the number of simultaneous observations of auroras and fbEs and the number of occurrences of fbEs ) for two time intervals was determined: for the daytime (0600-1800 UT) anomalous ionization is accompanied by auroras in 70 10% of the events, and in the nighttime sector (1800-0600 UT) it is accompanied by auroras in only 20 10% of the events. No relation between the occurrence of high fb Es (more than 2 MHz) and auroras was found for 1800-0600 UT. It is important to note that the major contribution to a high correlation between fbEs and auroras in the daytime sector comes from the local noon values of fbEs because P(fbEs) at 0600-1000 UT is at its minimum and at 1500-1800 UT the decay phase of the near-midday maximum in P(fbEs) is superimposed on the growth phase of the premidnight one. If the near-midday and premidnight values of fbEs have a high and low correlation with auroras, respectively, then inclusion of the interval 1500-1800 UT into the data volume decreases the correlation between the obtained values of fbEs and auroras.
Figure 2 shows the diurnal variations in the probability of appearance of anomalous ionization fbEs ( N is number of events) for different signs of Bz of IMF (from the hourly averages of IMF [Couzens and King, 1986]). It can be seen that the curve for Bz > 0 has two maxima, i.e., the near-midday and premidnight maxima. The curve for Bz < 0 has only one maximum, i.e., the premidnight one. Thus the anomalous ionization at the geomagnetic noon appears mainly for the northward directed IMF (see also Figure 2 of [Gabis et al., 1992], which shows that in summer fbEs is observed at 1200 5 UT at positive Bz of IMF in 86% of the events). The analysis of the events of the near-midday ionization which contribute to the curve for Bz < 0 in Figure 2 shows that when IMF reverses its direction from the northern to the southern, frequencies fbEs disappear, but the moment of sign inversion, as evidenced by the hourly averages of IMF, does not necessarily coincide with the moment of disappearance of fbEs which are detected every 15 min. This explains a small number of the events when midday fbEs are observed for negative Bz of IMF. Analysis of 5-min values of IMF leads to the conclusion that near the geomagnetic noon, at 1200 3 MLT, anomalous ionization is observed only for northward directed IMF, short-term fbEs can appear in the case of bursts of Bz > 0 on the background of negative values of Bz, and fbEs is never observed in this MLT sector in the case of a stable negative direction of IMF.
The premidnight maximum in Figure 2 for Bz < 0 is nearly twice as intense as that for Bz > 0 Hence, the probability of occurrence of anomalous ionization at 1700-0300 UT at F' 83o (i.e., in the evening-nighttime sector of the polar cap) turns out to be higher for the southward directed IMF. In this case, fbEs which appear at Bz < 0 are not associated with auroras. This result is consistent with the well known fact that auroras in the polar cap (Sun-aligned arcs) are observed at Bz > 0 [Troshichev, 1991]. Long-term (during several hours) negative values of Bz of IMF do not prevent occurrence of anomalous ionization with fbEs up to 2-5 MHz. The magnitudes fbEs 2 MHz were observed for any direction of IMF. While frequencies fbEs disappear at the IMF reversal from the northern to the southern direction at 0900-1700 UT, at 1800-0300 UT fbEs was observed during long periods (up to several hours). The Bz direction changed several times during the occurrence of fbEs
Specific features of the anomalous ionization morphology at the geomagnetic noon [Gabis and Pudovkin, 1991]; its relationship with a definite type of aurora [Gabis and Pudovkin, 1992]; a coincidence with the geomagnetic disturbances which point to the presence of the sunward convection in the anomalous ionization region [Gabis and Pudovkin, 1994]; and the dependence on the IMF direction and the angle of inclination of the Earth dipole are satisfactorily explained in terms of the theory of reconnection of the interplanetary and geomagnetic fields at the high-latitude magnetopause beyond the cusps at Bz > 0 [Pudovkin and Semyenov, 1985 and references therein]. During the magnetic reconnection, a field-aligned acceleration of plasma particles precipitated along the reconnecting field lines into the ionosphere occurs, which is manifested in a set of phenomena which can be detected by ground-based observations (in the enhancement of the electron concentration at the ionospheric altitude corresponding to the particle energy, in the occurrence of auroras, corresponding magnetic disturbances indicating currents/convection in the area of ionospheric projection of the reconnection region, etc.). Gosling et al.  gave direct proofs (obtained from satellite measurements) for the existence of reconnection of IMF and field lines of the magnetotail part near the terminator plane at which accelerated plasma flows toward the Sun were detected when the satellite crossed the magnetopause. Thus the appearance of noon particles fbEs at the Vostok observatory is associated with the observatory coming to the area of the ionospheric projection of the reconnection region at the high-latitude magnetopause in case of the northward Bz of IMF.
In the night hours, some of the observed fbEs, i.e., those observed at Bz > 0 and accompanied by auroras, are likely to be also associated with precipitation. Because of the nearly vertical direction of geomagnetic field lines, the wind shear theory is inapplicable for the polar cap. Other mechanisms of formation of anomalous ionization in the premidnight sector associated with electric fields and field-aligned currents probably depend on the processes in the magnetospheric tail for negative Bz of IMF.
1. The probability of occurrence of the anomalous ionization P(fbEs) at F' 83o has a maximum in the premidnight hours (2100-0000 MLT) equal to 0.30-0.35. The near-midday increase in the probability in winter months manifests itself in asymmetry of the growth and decay parts of the premidnight maximum.
2. In the premidnight hours, the probability of appearance of auroras is minimum. Simultaneous observations of sporadic E layers and auroras also show that anomalous ionization in the near-midnight sector coincides with occurrence of auroras in only 20 10% of the events, while in the daytime 70 10% of the events of anomalous ionization are associated with auroras.
3. The probability of observation of anomalous ionization in the premidnight hours at Bz < 0 is nearly twice as high as that at Bz > 0, while the midday ionization is observed only at Bz > 0.
4. The obtained results lead to the conclusion that the occurrence of anomalous ionization in the ionospheric E region at F' 83o has different origins in different MLT sectors.
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