3. Results

[6]  1. The majority of the studies of Pg known to the authors have been performed on the basis of observations in observatories in the Northern Hemisphere. The data of some observatories of the Southern Hemisphere were attracted to analyze the conjugacy of the Pg pulsations [Annexstad and Wilson, 1968; Green, 1979; Nagata et al., 1963]. Studies of the Pg pulsations using the data of the auroral observatories Molodezhnaya (MOL) and Novolazarevskaya (NVL) (Antarctic) and midlatitude observatory Borok (BOR) have never been performed till now. So for the beginning we consider briefly the morphological characteristics of the giant pulsations detected at the MOL, NVL and BOR observatories.

[7]  During the periods analyzed 36 and 10 Pg events were detected at MOL and NVL, respectively. The average frequency of the Pg occurrence is 2.3 and 1.4 events per year at MOL and NVL, respectively. The comparison with the data (known from the publications in literature) on the occurrence frequency of Pg in the Northern Hemisphere shows that in the Southern Hemisphere the pulsations are more rare events. Actually, it follows from the Pg studies in the Northern Hemisphere that on the average 3.1 events per year are observed at the Abisko observatory ( L=5.5 ) [Rolf, 1931], 6 events per year are observed at the Sodankyla observatory ( L=5.2 ) [Sucksdorff, 1939], 8 events per year are observed at the Tromsö observatory ( L=6.4 ) [Brekke et al., 1987], and 11 events per year are observed at the chain of EISCAT magnetometers in Scandinavia ( L= 5.01-6.66 ) [Chisham and Orr, 1991].

[8]  We have found no cases of simultaneous observation of the Pg pulsations at MOL and NVL, though these observatories are distanced from each other only by two MLT belts along geomagnetic longitude and by 4^o along geomagnetic latitude. This fact manifests strong localization by longitude and latitude of the Pg pulsations also in the Southern Hemisphere.

[9]  Many of the morphological features of the Pg pulsations detected at MOL and NVL are identical. For example, at both observatories Pg are observed under quiet geomagnetic conditions mainly in the morning hours (0400-0700 MLT). The mean duration of Pg is 20-100 min, the range of the pulsation periods is nearly similar: 90-120 s. The amplitude level of Pg at MOL is slightly higher (5-30 nT) than at NVL (4-15 nT), the latter fact being explained by the dependence of the oscillation intensity on the geomagnetic latitude of the observations. It should be noted that almost in all the analyzed cases the giant pulsations at MOL and NVL were observed at the recovery phase of small geomagnetic storms.

[10]  It is widely known that the most probable region of the giant pulsation generation lies at the L shells of about 5-6. Nevertheless the Pg pulsations can be observed also at L <4 but much more seldom than at auroral latitudes [Green, 1985]. We have detected 14 cases of Pg at the midlatitude Borok observatory ( L=2.8 ) during 33 years. The mean occurrence frequency of Pg at BOR is 0.4 events per year, the latter value being much lower than the corresponding value at high latitudes of the Northern Hemisphere. The analysis of observations of midlatitude giant pulsations at BOR shows that the range of their periods (45-60 s), duration (20-60 min) and amplitude level (2-4 nT) are significantly lower than for the auroral Pg. The most often Pg at BOR were observed at 4-6 MLT. The D component polarization of Pg at BOR is less pronounced than at auroral observatories. The comparison of the D/H ratio at BOR (approximately equal to 1.0-1.9), to the D/H ratio at MOL and NVL (much higher and varies within 1.5-3.0) confirm the above statement.

Figure 1

[11]  2. Now we come to the results obtained at the comparative analysis of the simultaneous observations of geomagnetic pulsations in two frequency ranges: (7.0-22.0) mHz and (0.5-2.0) Hz. For each Pg pulsation event at the MOL, NVL and BOR observatories (using the analog recordings to magnetic tape of geomagnetic pulsations by sonograph) dynamical spectra of high-frequency geomagnetic pulsations in the frequency range (0.5-2.0) Hz were created. The study of the structure of the dynamical spectra corresponding simultaneous observations of the Pg pulsations shows that in 70% cases the auroral and midlatitude Pg are observed against a background of globally excited structured pulsations of the Pc1 type (the “pearl” series). The latter fact makes it possible to assume that there is a relation between these two phenomena. For example, out of 36 cases of Pg pulsations detected at MOL 25 Pg events were observed against a background of the “pearl” series registered both at the same observatory and at other observatories distanced by longitude and latitude (BOR, VOS, and CMO). According to Guglielmi and Troitskaya [1973], Pc1 observed simultaneously on a huge territory from high to lower latitudes and enveloped by a large longitudinal interval are called global Pc1. The frequency of the globally excited pulsations Pc1 are usually F > 0.5 Hz [Guglielmi and Troitskaya, 1973]. The frequency of "pearls" accompanying Pg as a rule exceeded 0.5 Hz. It is an additional confirmation that the excitation of the “pearl” pulsations has a global character. Figure 1 shows examples of observations of the Pg pulsations in the D component and fragments of the dynamical spectra of the “pearl” series at middle and high latitudes: a typical event of simultaneous Pg pulsations at MOL and BOR on 29 September 1985 (Figure 1a); a typical event of simultaneous Pg pulsations at NVL and “pearl” series at MOL and BOR on 10 October 1985 (Figure 1b), and a typical event of simultaneous Pg pulsations at NVL and pulsations and “pearl” series at BOR on 24 February 1985 (Figure 1c). The presented examples visually illustrate the fact of the giant pulsation occurrence against a background excitation of the “pearl” series.

Figure 2

[12]  3. It is known that the Pg pulsations appear rather seldom, whereas "pearls" are observed relatively often. In order to be convinced that occurrence of the Pg pulsations against a background of the “pearl” series is not casual we tried to reveal the relation between the main characteristics of these pulsations. To do this one can use the data of any pair of the observatories where the Pg cases are accompanied by “pearl” series. The largest statistical material we have available, was selected from the observations of the giant pulsations in MOL and “pearl” series at BOR. Therefore we studied the relation between the characteristics of Pg and "pearls" using the data of these observatories. We considered the oscillation period ( T ) and the repetition period of "pearls" ( t ) as the main characteristics of the Pg and Pc1 pulsations, respectively. The repetition period of "pearls" was determined from the dynamical spectra. Figure 2 shows the dependence of the giant pulsations period on the "pearls" repetition period. Every point in Figure 2 corresponds to a particular event of Pg observation against a background of “pearl” series and line shows the linear approximation by the least squares method. Dotted line indicates events for which the Pg pulsation period T is the same as the pearl repetition period t. The correlation coefficient between these parameters is 0.70 and indicates to the significance of the found relation. The regression equation for T and t has the form

It is important that the tangent (  tan a = T/t ) of the angle of the slope of the line approximating the relation between T and t depends significantly of the Dst

Figure 3

variation. Figure 3 shows the dependence of the ratio of the giant pulsations period to the repetition period of "pearls" ( T/t or tan a ) on Dst variations. One can see that the value of tan a decreases with a decrease of the negative values of the Dst variations. The line shows an approximation of this dependence. Horizontal dotted line at T/t = 1 indicate where the pulsation period T and the pearl repetition period t are equal. The regression relation between tan a and Dst may be presented in the form

The existence of the correlation between tan a and Dst is manifested by the relatively high value of the correlation coefficient equal to -0.79.

[13]  Thus the comparison of magnetograms of individual events of observations of the Pg pulsations and dynamical spectra of the pulsations in the Pc1 range, as well as the results of the obtained experimentally relations between the characteristics of these pulsations ( T and t ) indicate to the relation between the two phenomena and so to the relation between the processes developing on the plasmapause during generation of Pg pulsation and "pearls" series.

[14]  4. Now we consider the interplanetary situation during Pg observations against a background of “pearl” series. Using the King digital catalog (the data from WDC B) containing mean hourly values of the parameters of the solar wind plasma and interplanetary magnetic field (IMF) the dependencies of the occurrence frequency of Pg against a background of “pearl” series on the velocity ( V ) and density ( n ) of the solar wind, IMF magnitude ( B ), and the IMF component B_z in the solar ecliptic plane were analyzed. It was found that Pg are observed against a background of “pearl” series in the dominating number of cases at V 350-400 km s^-1, n 6-8 cm^-3, B 2-4 nT, and -2 nT < B_z < 2 nT. The fact draws attention that usually the pulsations of the Pc1 type are excited at much higher density of the solar wind (30-35 cm^-3 ) and typical values of the IMF component B_z: -3 nT > B_z > -5 nT and 3 nT < B_z< 5 nT [Matveeva et al., 1972]. At the same time Pc1 are more often observed at extremely low values of the solar wind velocity (200-250) km s^-1 [Matveeva et al., 1972], the latter values being considerably lower than at Pg occurrence against a background of "pearls". So the above mentioned events of Pg observations against a background of "pearls" are typical for lower values of the density and relatively higher values of the velocity of the solar wind.