V. A. Pilipenko, N. G. Kleymenova, and O. V. Kozyreva
Institute of Physics of the Earth, Moscow, Russia
University of Nagoya, Japan
Institute of Physics of the Earth, Strasbourg, France
The geomagnetic Pc 3-4 pulsations (the frequency range 20-60 mHz) are a typical phenomenon in the daytime magnetosphere at all latitudes. The wave "signature" of the geomagnetic cusps (to be more precise, clefts, i.e., the regions including the projection of the cusp itself and the entry layers of the magnetosphere) is a complicated and insufficiently studied picture. Almost every time the highlatitude observatory is under the cusp projection, geomagnetic pulsations in a broad frequency range from 1 mHz to 100 mHz are observed [ Olson, 1986]. The frequencies of the Pc 3 pulsations observed in the cusp region correlate well with the solar wind parameters [ Engebretson et al., 1986; Wolfe et al., 1987]. The studies of the meridional distribution of averaged Pc 3 amplitudes point to the presence of maxima at the latitudes of the daytime polar cusp [ Bol'shakova and Troitskaya, 1984; Plyasova-Bakounina et al., 1986]. The body of facts listed above suggests [ Engebretson et al., 1987] that just the cusp region is the source from which Pc 3 pulsations of the extramagnetospheric origin propagate to middle and low latitudes. Note that Bol'shakova and Troitskaya  and Plyasova-Bakounina et al.  analyzed only the characteristics of the pronounced variations of the analog recording averaged over the intervals of 10 min and more. Strictly speaking, the results of these works merely indicate that the Pc 3 pulsation intensity in the polar cusps' regions is, on the average, much higher than at middle latitudes. No detailed analysis of simultaneous signals at polar and middle latitudes was carried out. In part, this can be explained by the fact that until recently the geomagnetic pulsations have been registered at ground-based observatories using analog paper recorders, which made the analysis of complicated regimes of variations difficult.
To find the answer to the question of whether the source of mid-latitude Pc 3 pulsations is indeed the daytime polar cusp/cleft, we analyze in this work simultaneous digital observations of geomagnetic pulsations at two observatories located nearly in one longitudinal sector at polar and middle latitudes.
For the detailed analysis, the data of simultaneous observations in the southern hemisphere at the Antarctic observatory Dumon d'Urville (geomagnetic coordinates F = -80.6 o, L = 234.8 o, to be referred to hereinafter as DUM) and the mid-latitude observatory Adelaide ( F = -46.7 o, L = 213.3 o, to be referred to hereinafter as ADL) were used. The ADL observatory, as a part of " 210o Magnetic Meridian" network, has a flux-gate magnetometer. The DUM observatory is equipped with an induction magnetometer with flat frequency response in the 2-100 mHz band. The cusp itself, i.e., the region of the direct injection of the low-energy plasma (30-100 eV), has small sizes: 1 hour in longitude from the geomagnetic noon and 0.5-1.0o in latitude. The DUM observatory is near the cusp projection only during very quiet periods; during the remaining time it is in the region of projection of entry layers or in the polar cap. The mean statistical location of the cusp corresponds to 75-77o of the corrected geomagnetic latitude, and therefore DUM is poleward from the cusp projection. As geomagnetic activity increases, the cusp moves toward lower latitudes, and hence the distance between the DUM and the cusp projection will grow with increasing magnetic activity. However, it should be borne in mind that Troitskaya and Bol'shakova  revealed the dependence of the cusp projection position on longitude from which it follows that, at the longitudes of the stations studied where the geomagnetic noon corresponds to 0100 UT in moderately disturbed conditions, the cusp is at the latitudes of 80-81o . In further discussion, we shall speak about DUM as about the observatory which is practically in the cusp.
The preliminary analysis of the many-year observations at DUM has shown that the Pc 3 pulsations are most often detected at this observatory during a local summer, and therefore the Pc 3 pulsation intervals for November-December 1991 were chosen for a detailed analysis. Since from the data of Engebretson et al. , Olson , and Wolfe et al.  the Pc 3 pulsations in the cusp are observed almost every day, the presence of a pronounced seasonal variation in the Pc 3 occurrence at DUM is apparently the result of the seasonal shifts in the position of the daytime polar cusp which is located at higher latitudes in summer time [ Trochichev et al., 1992], i.e., nearer to DUM, than in winter time when DUM is in the polar cap.
For the analysis, the data of digital registration with the sampling of 2 s in the interval 2000-0600 UT (0700-1700 MLT) were used. The difference in longitude between the observatories is 20 o, which does not obstruct the comparison because Pc 3 pulsations are coherent in the magnetosphere in a large longitudinal region.
To eliminate the low-frequency trend and long-period pulsations, the data were preliminarily filtered in the 10-100 mHz band by the band-pass Butterworth filter. The spectral estimate was performed using the Filon technique with a subsequent averaging of the calculated values over five points. For a detailed comparison of individual wave envelopes, a narrowband digital filtering with the central frequency corresponding to the maxima in the given time interval found by the spectral analysis was used.
Let us consider a typical event of December 12-13, 1991. Intense disturbances were observed simultaneously at both observatories in the morning and prenoon hours, 2230-0030 UT (Figure 1). The onset (2230 UT) and end (2345 UT) of the Pc 4 regime (10-20 mHz) at DUM and ADL coincided, the signal shapes were also similar on the whole. The amplitude maxima also coincided in time, the largest being at 2315 UT. It amounted to 15 nT and 3 nT in the H component at DUM and ADL, respectively.
In the Pc 3 range (25-50 mHz), two pronounced bursts of activity are observed in the cusp (DUM) (Figure 1), i.e., before midday (2200-2330 UT) and after midday (0350-0430 UT). At ADL, there are two long bursts in the Pc 3 regime, i.e., the morning burst coinciding with that at DUM at 2200-2330 UT with a sharp end, and the second burst, which is also fairly pronounced, at 0015-0400 UT. The maximum amplitude of the first and second bursts at ADL was as high as 1 nT. Only a general coincidence between the onset and end of the regime was noticed at DUM and ADL in the Pc 3 range, while individual wave packets were not synchronous. This is clearly seen in Figure 2, i.e., no relation between Pc 3 wave envelopes at DUM and ADL is revealed. At the same time, something similar to wave bursts is observed in the Pc 4 range. The fluctuations here are nearly in antiphase. The correlation coefficient R for the Pc 4 event is -0.89 , while for the Pc 3 band R = 0.06 . The maximum amplitude of Pc 3 ( 12 nT) was observed at 2315 UT at DUM and somewhat later at ADL (2300 UT) and amounted to only 1.3 nT.
In the interval 2330-0130 UT, the variations in the Pc 3-4 range at
DUM were at the background level
At 0350-0430 UT, a sufficiently intense (up to 12 nT) burst in Pc 3 with a relatively narrow spectrum (25-36 mHz) occurred in the cusp. No specific features in variation of the Pc 3 regime were observed at ADL during this interval.
Thus the wideband burst of geomagnetic pulsations typical of the prenoon hours at the polar cusp latitudes [ Olson, 1986] was followed to middle latitudes. The amplitudes of Pc 4 and Pc 3 pulsations in cusp were higher by a factor of 4-5 and 8-10, respectively, than at middle latitudes.
The spectral analysis of pulsations has shown that the Pc 3-4 pulsations in the cusp and at middle latitudes differed not only in intensity but also in the spectral composition and its dynamics (Figures 3a and 3b). The maxima in the Pc 3 spectra at two observatories did not coincide and varied with time in different fashions. This can be seen in Figure 3, which shows the Pc 3-4 spectra for DUM and ADL at the prenoon (2240-2340 UT), near-noon (0100-0200 UT), and afternoon (0340-0440 UT) time.
In the morning hours when enhancement in Pc 3 at both observatories was observed, the polarizations of Pc 3 in the horizontal plane in the cusp and at middle latitudes were different: at DUM, the pulsations were elliptically polarized with the preferential orientation along the latitude and counterclockwise rotation direction, and at middle latitudes the polarization was nearly circular with the clockwise rotation direction. In the Pc 4 range, the polarization ellipses at DUM and ADL were similar but unstationary in time. The rotation directions of the polarization vectors were similar at DUM and ADL and changed simultaneously.
For the event of December 13-14, 1991, wideband burst of geomagnetic pulsations was also observed at the premidday time (2200-2320 UT). While the major contribution in the cusp came from the low-frequency part of the spectrum (to 25 mHz), it came from the higher-frequency part at low latitudes. There were two spectral maxima (Figure 3) corresponding to Pc 3 (10-20 mHz) and Pc 3 (20-30 mHz). These maxima are also observed in the pulsation spectra at DUM. As in the previous case, individual wave envelopes at DUM and ADL coincided on the whole for in the Pc 4 range (10-15 mHz) and did not coincide for Pc 3 (20-30 mHz), though the onset and end of the regimes were simultaneous, as in the previous case. The maximum amplitudes in the Pc 4 range were 8 nT in the cusp and 0.8 nT at middle latitudes. For Pc 3, the amplitude was 5 nT in the cusp and 1 nT at ADL.
At the near midday time (0200 UT), an almost simultaneous abrupt increase in the Pc 3 intensity was observed at both observatories. As before, the onset and end of the Pc 3 regime at DUM and ADL coincided, and wave envelopes were not synchronous. The maximum amplitudes were 3 nT in the cusp and 0.7 nT at middle latitudes, i.e., they were less by nearly a factor of 4. The Pc 3 pulsations in the cusp and middle latitudes had different frequency-polarization structure. The ratios between the spectra of horizontal magnetic components of the signals shown in Figure 4 indicate that at ADL the H component is more intense than the D component in the frequency range of Pc 3 (Figure 4a). This specific feature of the spectrum does not manifest itself at DUM (Figure 4b).
At the afternoon time (0200-0500 UT), a gradual increase in the Pc 3 amplitude and extension of the frequency range to higher frequencies was observed at ADL. For instance, at 0320-0440, the maximum in the pulsation spectrum at ADL was at frequencies of 25-45 mHz, and in the cusp the intensity of this band in the pulsation spectrum was at the level of background.
In separate cases, synchronous wave Pc 3 packets could be observed at the afternoon time at DUM and ADL. Note that the signal amplitude at DUM in this case was only twice as high as that at ADL. The rotation directions of the polarization vector in the cusp and at middle latitudes were opposite, the polarization being elliptical at DUM and nearly circular at ADL.
The event of November 10-11, 1991, was preceded by a strong magnetic storm with Kp = 9 . During such periods, DUM is deep in the polar cap, far to the pole from the cusp projection. As a rule, the Pc 3 amplitude at DUM during magnetically disturbed periods is much smaller than in moderately disturbed conditions.
No characteristic wideband bursts of pulsation activity were observed at the prenoon time. Otherwise the basic regularities corresponded to magnetically quiet days. At the near-noon time, a simultaneous occurrence of wave envelopes in the Pc 4 range (10-17 mHz) was detected at DUM and ADL, while wave packets in the Pc 3 range were not synchronous. The intense bursts in Pc 3 at DUM was not associated with any response at ADL.
All three observations during the days analyzed can be fit in three scenarios: (1) a synchronous wideband noise burst of geomagnetic pulsations at the prenoon time with a simultaneous onset and end of the phenomenon in the cusp and at middle latitudes; the wave envelopes in the Pc 3 range are not synchronous in this case, and the maximum amplitude in the cusp is an order of magnitude larger; (2) the burst in Pc 3 pulsations at the near-noon time observed only at middle latitudes; and (3) the burst in Pc 3 pulsations at the afternoon time observed at polar cusp latitudes and not accompanied by an increase in the activity of corresponding pulsations at middle latitudes.
The results of the comparison of simultaneous observations at the Dumon d'Urville and Adelaide observatories in moderately disturbed conditions can be summarized as follows.
In the morning and prenoon hours, a sharp enhancement of geomagnetic variations in a wide frequency range is observed both near the cusp and at middle latitudes. This enhancement lasts for 2-5 hours and is detected in the frequency bands 10-20 mHz (Pc 4) and 30-50 mHz (Pc 3). Though the regimes of the pulsation activity enhancement near the cusp and at middle latitudes are similar, there is no one-to-one correspondence between individual wave Pc 3 envelopes, and the Pc 3 spectra and dynamics of their variation are different.
In the afternoon hours, intense Pc 3 pulsations which do not manifest themselves in any way at middle latitudes can be observed near the cusp projection. At the same time, the situations are possible when the occurrence of Pc 3 at middle latitudes is not accompanied by an analogous pulsation activity at the cusp latitudes. Only in the near-noon hours, we sometimes succeeded in detecting synchronous Pc 3 wave envelopes in the cusp and at middle latitudes. The observed diversity in the spatial and temporal specific features of pulsations indicate that Pc 3 near the cusp and at middle latitudes originate from different independent sources. In spite of the fact that the pulsation intensity near the cusp typically exceeds by several times (sometimes by an order of magnitude) the intensity of pulsations of the same range at middle latitudes, we cannot claim that the cusp is the main source of the midlatitude pulsations.
In the low-frequency (Pc 4) range, the variations in the H components are correlated much more closely; the tendency to the antiphase behavior is seen. We can suppose that the midlatitude pulsations of this range are the consequence of disturbances at the cusp latitudes.
Using the regularities described above, we try to draw a possible qualitative physical picture of the origin of the pulsations discussed. One of the main sources of daytime pulsations is the MHD turbulence of the solar wind plasma. Turbulent MHD noise can penetrate the inner magnetosphere in two ways. The first is associated with the propagation of waves of the compressional (magnetosonic) type in the near-equatorial magnetospheric plane. Formation of narrowband wave envelopes of pulsations occurs during propagation under the action of the filtering properties of both the magnetospheric cavity on the whole and local resonance features of the field lines. This mechanism does not exclude a possibility of formation of a maximum in the spatial distribution of pulsation amplitudes at the near-cusp latitudes, near the projection of closed boundary field lines.
The other channel for transfer of energy from the solar wind into the ionosphere is the cleft region where a nearly direct penetration of turbulent MHD noise from the transitional region occurs. The dominance of the Pc 3 spectral range is due to either the preferential generation of noise in this frequency range in the solar wind by fluxes of reflected protons or formation of the spectrum when pulsations leak through the geomagnetic funnel (cusp). In principle, the pulsations which leaked through the cusp into the magnetosphere can then propagate to lower latitudes through the magnetosphere or along the ionosphere. However, the efficiency of these propagation channels has not so far been estimated. It cannot be excluded that the pulsations observed at middle latitudes are the superposition of signals resulting from the action of different generation mechanisms and propagation channels.
The similar regimes of Pc 3 pulsations at DUM and ADL in the morning hours is apparently caused by a common primary source of this noise, with appreciably differing channels of penetration of these waves into the ionosphere. The difference in the propagation mechanisms is also confirmed by different polarization properties of pulsations. At middle latitudes the preferential enhancement of the H component at the local resonance frequency typical for the mechanism of transformation of a compressional wave to resonant Alfv\'en oscillations is observed. This effect is absent at the near-cusp station. The directions of the polarization vector rotation are also opposite. Thus though the magnetospheric cusp is the source of intense pulsations in the Pc 3 range, the hypothesis that these pulsations are also a source of midlatitude Pc 3 pulsations is not confirmed by our investigation.
In addition to the extramagnetospheric sources of Pc 3 pulsations, a specific mechanism of pulsations excitation can exist directly in the cusp region. In this case the cusp not only passively channels waves from the magnetosheath, but also plays an active role in pulsation generation. Among these mechanisms are generation of PilC emission by irregular fluxes of precipitated electrons, excitation of surface waves at the cusp boundaries, and different types of gradient and current plasma instabilities. A special class of specific cusp emission is the pulsed disturbances caused by flux transfer events (FTEs) and low-frequency irregular pulsations ipcl, vlp, np, etc. [ Lanzerotti and MacLennan, 1988]. Probably some of the sources listed above contributed to the observed Pc 4 pulsations; however, this problem has not been specially studied.
Engebretson et al.  noted that one should distinguish between the wideband noise bursts in Pc 3 frequency range typical of the polar cusp latitudes and accompanied by injection of electrons and the narrowband Pc 3 discussed above. The excitation of noise bursts is likely to occur in the region of the entry layers of the magnetosphere rather than in the cusp because the cusp [ Marklund et al., 1990] is characterized by uniform fluxes of injected low-energy electrons and the region of entry layers is characterized by irregular variations in fluxes with the energy of more than 10 keV. According to Engebretson et al. , the wave activity in the Pc 3 range is a superposition of the waves having the extramagnetospheric origin and the waves arising in the region of high-latitude field-aligned electric currents accompanied by modulated precipitation of electrons. In the suggested model [ Engebretson et al., 1987], hydromagnetic waves give rise to the ionospheric conductivity modulation, which leads to the intensity modulation of field-aligned currents of region 2. The modulated currents can become a source of supply of wave energy into the daytime magnetosphere.
In spite of a relation between intensifications of Pc 3 and Pc 4 pulsations, these types of disturbances apparently have different physical natures. This can be evidenced by differing characters of attenuation of these signals between the cusp region and middle latitudes. The low-frequency disturbances also manifest themselves at middle latitudes, their amplitude being 4-5 times smaller in comparison with the cusp region. At the same time, the Pc 3 pulsations of the cusp origin practically do not manifest themselves at middle latitudes at the background of the midlatitude pulsations, i.e., their amplitude decreases by more than an order of magnitude. Different degrees of signal attenuation can be the evidence of different characteristic sizes of the source, i.e., Pc 3 pulsations in the cusp region are much more local in comparison with the low-frequency disturbances.
The existence of an interrelation between low-frequency disturbances and Pc 3-4 pulsations in high-latitude regions was also noted by Yagodkina and Vorob'yev . It was found that the occurrence of magnetic pulses arising owing to moving ionospheric vortices is accompanied by a short-term increase in the power of geomagnetic pulsations, preferentially of the Pc 3 range. These observations and also the time coincidence between the Pc 3 activity and the enhancement of low-frequency disturbances noted above can be the evidence of a local increase in the efficiency of penetration of the wave hydromagnetic energy into the magnetosphere during a sporadic breakdown of the magnetopause when the pulsed reconnection develops.
Comparison of direct simultaneous observations of geomagnetic Pc 3 pulsations at two observatories located at the polar ( F = 80o ) and middle ( F = 46o ) latitudes along approximately one geomagnetic longitude has shown that the general character of the regime of the Pc 3-4 pulsation intensification in the morning hours coincides at the two stations, the pulsation intensity near the cusp being several times higher than those at middle latitudes. However, individual wave envelopes of Pc 3 (25-40 mHz) turn out to be uncorrelated in the overwhelming majority of cases. In the afternoon hours the wave activity in the Pc 3 range at one observatory is not typically accompanied by an analogous activity at the other station.
Lower-frequency geomagnetic pulsations of the Pc 4 range (18-20 mHz) are usually better correlated at the two stations. It can be supposed that the Pc 3 pulsations in the morning hours at the near-cusp and middle latitudes have a common source, apparently the turbulent solar wind. However, the basic channels of penetration of the wave energy to middle and high latitudes are different. This is a direct penetration to the cusp and entry layers regions for DUM and propagation and resonant transformation in the near-equatorial magnetospheric plane for ADL. To study experimentally the\linebreak
\noindent Pc 3 pulsations of different origins and to obtain the picture of their spatial distribution along the meridian, further experiments with a dense network of stations at high latitudes are needed.
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