N. G. Kleimenova and O. V. Kozyreva
Institute of Physics of the Earth, Moscow, Russia
École et Observatoire des Sciences de la Terre, Strasbourg, France
M. Bitterly and J. Bitterly
Institute de Physique du Globe, Paris, France
Geophysical Institute, Sofia, Bulgaria
Received 10 November 1999, published online 12 February 2002
During strong geomagnetic disturbances, typical auroral zone phenomena are detected at middle and even equatorial latitudes. For instance, during strong magnetic storms long-period geomagnetic Pc5 pulsations (1.5-6.0 mHz), whose maximum amplitudes under moderately disturbed conditions are observed at F 68o-72o, occur at much lower latitudes. Bol'shakova et al.  showed that a drastic compression of the magnetosphere by the solar wind during the great magnetic storm of 13 March 1989, resulted in the excitation of Pc5 at the latitudes 53o-57o which are not typical for this class of pulsations. The maximum frequency in the pulsation spectra increased with decreasing latitude from f = 3.6 mHz at F 57o to f = 4.9 mHz at F 53o, which is consistent with the hypothesis of the resonance nature of oscillations [Samson et al., 1992; Walker et al., 1992].
Schott et al.  and Kleimenova et al.  analyzed geomagnetic Pc5 pulsations during another, also very strong, magnetic storm of 24 March 1991. In the daytime, two types of long-period geomagnetic pulsations were detected at middle latitudes. They were (1) quasimonochromatic ~1.5-2.0 mHz oscillations with a very low azimuthal wave number ( m 1 ) and synchronous wave packets on the global scale and (2) broadband oscillations in the frequency band ~2.5-3.5 mHz with m 3-5 and wave packets incoherent in space.
During the great magnetic storm of 5 August 1972, intense geomagnetic Pc5 pulsations in the magnetosphere were observed at L 4.5 [Engebretson et al., 1983]. During the magnetic storm of 10-11 January 1997, Villante et al.  observed a simultaneous occurrence of the 0.8-5.0 mHz ULF pulsations at low latitudes in Europe (observatory L'Aquila, F = 36.2 o N) and at the antipodal meridian in the southern polar cusp (observatory Terra Nova Bay, F = 80.5 o S). The authors interpreted the observed effect as a generation of global magnetospheric compressional waves due to a strong solar wind pressure pulse.
The 21 February 1994 magnetic storm (with SC at 0901 UT) was caused by the interplanetary magnetic cloud approaching the Earth [Araki et al., 1995; Petrinec et al., 1995]. The storm expansion phase began at ~1300 UT. It was characterized by strong geomagnetic disturbances with Kp up to 7+ and AL up to ~1400 nT, the ring current intensity being relatively low ( Dst (-145) nT).
The magnetic storm of 21 February 1994 was of particular interest for the analysis because during this period not only very high values of all solar wind parameters but also their unusually strong variations took place near the Earth's orbit. During the interval ~1300-1340 UT the IMF variations were observed on the background of a very high dynamic solar wind pressure ( P 100 nPa), and at ~1500-1600 UT, they were detected under a considerably lower pressure ( P 10 nPa). Yamauchi et al.  showed that during this storm the first situation (a high dynamic pressure) resulted in the expansion of the dayside polar cusp, and the second situation (a high magnetic pressure) led to its narrowing. Since the intervals mentioned above were characterized by a high (up to ~750 km s -1 ) and relatively stable solar wind velocity, variations in the dynamic pressure actually manifested variations in the solar wind density.
The goal of this work was to analyze in detail the ground-based long-period geomagnetic Pc5 pulsations (1-6 mHz) in the dayside magnetosphere during the periods of a high dynamic solar wind pressure and large variations in all IMF components (1300-1400 UT) and relatively lower dynamic pressure (1500-1600 UT) which was also accompanied by high values of the IMF parameters and their variations.
Owing to the longitudinal arrangement of the selected observatories, characteristics of the geomagnetic pulsations could be studied simultaneously in the morning, afternoon, and evening sectors of the Earth. For instance, during the interval 1300-1400 UT observatories NAQ ( F = 68.1o ) and STJ ( F = 55.5o ) were near the geomagnetic noon; observatories PBQ ( F = 66.2o ) and OTT (F =59.3o) were in the morning sector, 0800-0900 LT; observatories FCC (F= 69.8o) and GLN (F = 60.4o) were also in the morning sector, but in the earlier one (0600-0700 LT); and the European and Scandinavian stations were in the evening sector. The geomagnetic latitudes of these observatories allowed us to study the geomagnetic pulsations in both the closed magnetosphere and the dayside polar cusp.
To analyze morphological characteristics of the long-period geomagnetic pulsations, the initial data were preliminary, digitally filtered in the 0.6-10 mHz band (Pc5), and then amplitude and dynamic spectra of the oscillations were calculated for the chosen time intervals. After this the waves were analyzed in a narrower frequency band corresponding to the maxima in the pulsation spectra.
Let us consider the morphological features of the dayside geomagnetic pulsations detected at the Earth's surface during two chosen time intervals: (a) 1300-1400 UT under a very high dynamic solar wind pressure and (b) 1500-1600 UT under strong IMF variations on the background of a lower dynamic solar wind pressure.
The geomagnetic pulsation amplitudes at similar latitudes near noon were much smaller than in the morning and evening sectors, the amplitude in the evening sector being larger than in the morning (Figure 4). For instance, at 1300-1400 UT the 3-5 mHz subauroral pulsation amplitude at observatory NUR (1700 LT) was nearly 4 times as large as that at observatory STJ (1200 LT) located at the same latitude. At the European middle-latitude observatories the Pc5 pulsation amplitudes increased from the western to the eastern observatories. For instance, in the longitudinal interval ~80o-115o, at a latitude of ~50o, the pulsation amplitude at MOS was nearly twice as high as at ESK, though ESK is located at a somewhat higher latitude.
Wave packets on the morning and evening sides were similar, with opposite directions of the wave polarization vector rotation. At ~1330 UT the polarization vector of the 3-5 mHz pulsations rotated counterclockwise in the morning (OTT) and clockwise in the afternoon sector (for instance, NUR, ESK, CLF, and others). These facts indicate that waves propagated westward and eastward from the noon meridian. The phase (j1) of the X component oscillations at STJ (l =31 o) was ahead of the wave phase (j2) at NUR (l =103 o) by ~2 min, which corresponds to very low azimuthal wave numbers [m=(j2-j1)/Dl ] m 1-2. The same values of m were obtained from comparison of phase differences at observatories HAD ( 1300 LT) and AMS (~1800 LT) at F 50o. Note that 1-min sampling data were used in the analysis, which means that the time of the phase lag could be determined with an accuracy not better than 1 min.
The reversal of the polarization vector rotation sense took place not only in the longitudinal but also in the latitudinal direction. At the Scandinavian chain of stations the maximum amplitudes of the 3-5-mHz pulsations at 1330-1400 UT were observed at F 62o (OUJ). On both sides of the amplitude maximum the polarization vector rotation directions were opposite (counterclockwise poleward and clockwise equatorward), which agrees with the theory of field line resonances (FLR).
Comparison of observations at the quasi-conjugate observatories CLF and HER at F 43o has shown that oscillations in the X component occur in phase and oscillations in the Y component are in antiphase. The same picture was obtained for observatories MOS-CZT at F 51o and for NUR-PAF at F 57o. This means that the motion of field lines at their north and south ends was symmetric with respect to the equatorial plane, which corresponds to the first (odd) harmonic of the standing wave. The polarization vector rotated in the Northern and Southern Hemispheres (CLF and HER) in opposite directions, as at MOS-CZT and NUR-PAF.
It is evident from comparison of the spectral distributions of pulsations at observatory BJN ( F 72o ) and observatory SOR (F 67.5o) that the oscillation spectra extended to higher frequencies with increasing latitude. Below F 62o the pulsation amplitudes at f < 2 mHz became comparable with the wave amplitudes in the 3-5 mHz range (Figure 5b).
Two well-defined temporal wave packets, at 1320-1345 UT and 1355-1410 UT (Figures 4-6), were observed at all observatories located inside the closed magnetosphere. The first wave packet contained only low-frequency pulsations with f < 2 mHz, and the second packet contained high-frequency pulsations with f > 3 mHz. This picture was especially pronounced in the morning sector at observatories OTT and FRD (Figure 5a), and also in the afternoon time at observatories HAN and NUR (Figure 6b) where pulsation amplitudes were nearly an order of magnitude larger (see the scale of the graphs). In the noon time (observatory STJ in Figure 5a) the low-frequency oscillations had a much greater intensity than the high-frequency oscillations.
At the Scandinavian meridian (Figure 6b), at 1320-1410 UT, two wave packets are also clearly seen. The first was characterized by a strong burst of low-frequency pulsations at polar latitudes (Figure 6a, NAL-BJN). Their amplitudes decreased rapidly with decreasing latitude. Note that the bursts of the 3-5-mHz oscillations were observed in these two wave packets both at the latitudes of the dayside polar cusp (NAL, LYR, HOR) and in the closed magnetosphere (Figure 6b). At lower latitudes (HAN, NUR) the amplitudes of the 3-5 mHz oscillations were larger in the second burst than in the first burst. A similar picture was observed in the morning sector (FRD, OTT). The SOR and MAS stations were probably located near the boundary between the closed and the open magnetosphere.
At observatories NAQ (noon) and PBQ (morning), which is located at a close latitude but 40o to the West, the low-frequency 0.6-2.0-mHz wave packets did not coincide, though the spectra exhibited a broad maximum in this frequency range. The pulsation amplitude at NAQ was as high as 100 nT, and at STJ (the same meridian but 12o lower in the latitude), it was only ~20 nT. This means that the low-frequency oscillation amplitude decreased sharply with latitude, as at the Scandinavian meridian. The dynamic spectra shown in Figure 5a suggest that PBQ and NAQ were located near the projection of the equatorial boundary of the dayside polar cusp where intense pulsations at f < 2 mHz, typical of these latitudes, were excited. At lower latitudes (OTT and STJ), in the closed magnetosphere, the amplitudes of these pulsations were 4-5 times smaller.
The differences between the morphological characteristics and the dynamics of the pulsations at the frequencies above and below 3 mHz suggest that these pulsations have different origins.
The beginning of this interval was characterized by a decrease in the dynamic solar wind pressure to P 10 nPa on the background of a very high magnetic pressure. The IMF B was greater than 60 nT under large positive Bz (to +20 nT) and very large negative By (to -60 nT); the solar wind velocity was about 600 km s -1 (Figure 1).
At ~1500 UT a small jump in the dynamic pressure of the solar wind from ~4.5 nPa to ~11 nPa due to a sharp increase in its velocity from ~550 to ~700 km s -1 was observed. It was accompanied by a new burst of long-period geomagnetic pulsations with the highest intensity in the afternoon sector, as during the interval discussed earlier. Compared to 1300-1400 UT, only a slight enhancement of the f < 2 mHz pulsations and weak higher-frequency oscillations were observed at the polar cusp latitudes (Figures 5a and 6a).
At the Scandinavian meridian, i.e., at ~1700 LT (Figure 6a), two bands in the wave spectrum, at f < 2 mHz and f > 2 mHz, were well pronounced at the latitudes lower than ~65o, similar to 1300-1400 UT. The low-frequency maximum had a much higher intensity than the high-frequency one. The Pc5 pulsations had the largest amplitudes at f 3-4 mHz. The wave intensity maximum was observed at F 60o-62o (OUJ, HAN) at 1520-1530 UT, i.e., somewhat later than the low-frequency maximum. At the midlatitude European observatories (Figure 5b) the main maximum in the oscillation spectrum was observed at low frequencies, f < 2 mHz.
The onset of the burst of the 3-4-mHz oscillations coincided with the sign reversal of the Bz component, from +25 nT to -24 nT (Figure 1). At that time the change of the By from -44 nT to +30 nT and Bx from -17 nT to +23 nT also occurred. In the dayside sector of the magnetosphere (Figure 5a), observatories NAQ and STJ detected only low-frequency ( f < 2 mHz) oscillations with much smaller amplitudes than at 1300-1400 UT. In before noon hours (1000 LT), the pulsation spectra at FRD ( F 50o ) had two pronounced frequency bands (Figure 5a), similar to the pre-evening post-noon sector (the Scandinavian meridian), but the oscillation amplitude was much smaller than in the evening. Figure 3b shows the amplitude pulsation spectra at observatories GLN (0800 LT), OTT (1000 LT), and PAF (2000 LT). It is obvious that oscillations are considerably enhanced in the evening sector.
Thus a sudden southward reversal of the Bz direction led to the excitation of the 3-5 mHz geomagnetic Pc5 pulsations with the maximum amplitude in the afternoon hours, both at 1300-1400 UT, under the conditions of a high dynamic solar wind pressure (P > 70 nPa), and at 1500-1600 UT, under a lower pressure (P 5-10 nPa). A sharp large decrease in the solar wind density resulted in an abrupt suppression of the Pc5 generation. In the low-frequency range ( f < 2 mHz), bursts of oscillations occurred during both intervals. The highest pulsation amplitudes were observed at the dayside polar cusp latitudes during the first interval and in the evening sector inside the closed magnetosphere during the second interval.
Thus on 21 February 1994, at 1300-1600 UT, a passage of a large-scale solar wind irregularity with very strong variations in the interplanetary magnetic field and solar wind (Figure 1) happened in the near-Earth space. It was accompanied by bursts of long-period geomagnetic Pc5 pulsations with maximum amplitudes in the afternoon sector.
Many authors attribute the generation of Pc5 pulsations to development of the Kelvin-Helmholtz instability at the magnetopause or flanks of the entry layers of the magnetosphere flowed by the solar wind. Generally, symmetric flowing around the Earth must give rise to surface waves symmetric in space with respect to noon. However, the majority of researchers [e.g., Chisham and Orr, 1997; Kokubun et al., 1989; Nosé et al., 1995; Ol', 1963; Pilipenko et al., 1997; Yumoto et al., 1983] convincingly showed the asymmetry of pulsations on the morning and evening sides of the Earth. It is probably the result of a spiral structure of the solar wind when the IMF direction is closer to the normal to the magnetopause boundary in the morning hours than in the afternoon. Hence the criteria for excitation of the Kelvin-Helmholtz instability on the morning side are satisfied more easily than on the afternoon side [Lee and Olson, 1980].
Comparison of the ground-based and satellite observations has shown [Kokubun et al., 1989; Yumoto et al., 1983] that in the morning magnetosphere, transverse azimuthally polarized waves well correlating with the oscillations at the Earth's surface are mostly detected, while in the afternoon sector, radially polarized compressional waves poorly correlating with the ground-based observations dominate.
In the case we discuss here the largest amplitudes of the Pc5 pulsations (3-5 mHz) were observed in the afternoon sector rather than in the morning sector. Hence it is unlikely that their source is the Kelvin-Helmholtz instability.
The morphological characteristics of the 3-5-mHz geomagnetic pulsations observed on 21 February 1994, such as opposite polarizations in the morning and afternoon sectors, polarization reversal along latitude near the wave amplitude maximum, a discrete spectrum, the phase relations between oscillations, opposite directions of the polarization vector rotation in conjugate regions, low azimuthal wave numbers ( m 1-2 ), and the temporal structure of the pulsations in the form of individual wave packets. These characteristics suggest that the pulsations have a resonance nature.
Besides the Kelvin-Helmholtz instability, the Alfvén resonance of field lines (FLR) can also be caused, for instance, by a magnetic impulse in the IMF. As a rule, FLRs are observed mainly in the morning sector of the magnetosphere. However, some authors reported on the cases of simultaneous occurrence of the geomagnetic Pc5 pulsations on the morning and afternoon sides of the Earth. For instance, in the initial phase of the great magnetic storm of 24 March 1991, quasi-sinusoidal oscillations with a period of 8-10 min, synchronous wave packets, and maximum amplitude in the afternoon sector were observed on the dayside of the magnetosphere [Araki et al., 1995; Kleimenova et al., 1998; Schott et al., 1998].
Shimazu et al.  also described the events of simultaneous occurrence of the geomagnetic Pc5 pulsations with similar waveforms in the morning (0400 LT, observatory College) and evening (1800 LT, observatory Kiruna) sectors. The H -component variations at observatories Kiruna and College were in antiphase and corresponded to opposite directions of the polarization vector rotation in the morning and evening times, as in our case. The intensification of pulsations was observed under a high (~620 km s -1 ) solar wind velocity and increased dynamic solar wind pressure. Shimazu et al.  concluded that the source of global Pc5 is associated with the magnetosphere cavity resonance excited by the passage of a high-pressure front of the solar wind irregularity in the interplanetary space.
The global geomagnetic 3-5-mHz pulsations that we observed are likely to have the same origin, namely, a global compressional wave in the magnetosphere. This frequency range agrees with the numerical calculations of Kivelson et al. . The oscillations are enhanced at the frequencies at which the wave cavity mode matches local FLRs [Kivelson and Southwood, 1986]. Yeoman et al.  showed that the closest interaction between the compressional mode and Alfvén FLR is observed for the waves with low wave numbers ( m 1-3 ). We observed the same values of m for the 3-5 mHz range on 21 February 1994.
Comparison of Figure 8 with Figures 5 and 6 reveals that the burst of geomagnetic pulsations at 1300-1400 UT coincides with a similar burst of the interplanetary magnetic field and solar wind density variations. As at the ground-based observatories, the main maximum in the spectrum of variations in the solar wind parameters is observed at f < 2 mHz. Hence it allows us to propose hypotheses that the geomagnetic pulsations can be caused by corresponding oscillations in the IMF which penetrate immediately into the dayside polar cusp region.
The Pc5 generation was abruptly suppressed when a sudden impulse Si occurred (Figure 7). Such a sudden disappearance of Pc5 pulsation was also observed during the magnetic storm of 24 March 1991 [Kleimenova et al., 1998]. The effect of an abrupt suppression of the Pc2-4 pulsations on the global scale (observatory Petropavlovsk, Kamchatka, observatory Borok, Europe, and observatory Soroa, Cuba) after Si was also noted by Troitskaya et al.  who attributed it to a sudden expansion of the magnetosphere. However, in our case a sharp decay of Pc5 after ~1405 UT can be caused by changes in the conditions in the IMF, or namely, a sudden sharp decrease of the cone angle, because the Bx/B ratio suddenly decreased from ~0.45 to ~0.003 under very high values of the IMF B -component. This confirms our hypothesis that the source of Pc5 at 1300-1400 UT is the IMF variations.
Note that during the interval 1500-1600 UT, contrary to 1300-1400 UT, the geomagnetic pulsation spectra on the ground did not coincide with the spectra of variations in the IMF parameters. At ~1530 UT the intensity of low-frequency pulsations in the solar wind was much lower than at ~1330 UT. However, the afternoon and evening f < 2 mHz pulsations on the ground were much stronger at ~1530 UT than at ~1330 UT (Figures 5 and 6). They were not observed at the polar cusp latitudes (Figure 6a). Therefore it can be supposed that under a high dynamic solar wind pressure the magnetic pulsations at the Earth's surface at 1300-1400 UT had an external origin, while under the conditions of a high magnetic pressure at 1500-1600 UT, the source of oscillations was located inside the magnetosphere.
For the interval 1330-1340 UT the Geotail measurements were not available, and it was impossible to define whether the burst of the ground (3-5 mHz) Pc5 at that time coincided with the onset of pulsations in the solar wind density in the same frequency range. A sudden suppression of the middle-latitude Pc5 coincided with a sharp decrease in the solar wind density and disappearance of pulsations of the same periods in the density as well as in IMF Bz and By. It is interesting to note that during the period when the first wave packet of Pc5 (1325-1350 UT) was detected, the strongest variations occurred in the IMF Bx (the data on density variations were not available), which means that the waves had a compressional structure. During the second Pc5 wave packet (1355-1406 UT) the strongest variations were detected in the IMF By, though pulsations of the IMF Bz were also observed. Therefore the wave in the solar wind had both the transverse and the compressional field components, and source of ground Pc5 could be associated with compressional waves of the same periods in the solar wind. A similar excitation of FLR due to quasi-periodical oscillations in the solar wind parameters was reported by Prikryl et al. .
The regression analysis of the relationship between the Pc5 amplitude on the ground and the IMF parameters has shown that during the first interval, under the strong solar wind dynamic pressure and Bz>0, Bx<0, and By >0, the dayside pulsation amplitude was mostly controlled by the IMF By (with the regression coefficients up to 0.93), as well as it is typical for field-aligned currents. During the second interval, under the strong IMF magnetic pressure and Bz <0, Bx >0, and By >0, the most effective IMF component was Bx (with the regression coefficients up to 0.89). During both intervals, the strongest bursts of pulsation were observed under By>0.
Thus under the extreme conditions of the magnetic storm of 21 February 1994, characterized by very high values of the interplanetary magnetic field and solar wind parameters and their strong variations (Figure 1), the dayside geomagnetic pulsation spectra showed two well-defined bands of enhanced oscillations (low frequency, f < 2 mHz, and high frequency, 3-5 mHz).
During the first interval (1300-1400 UT), variations in the IMF occurred on the background of a very high solar wind dynamic pressure (to ~100 nPa). According to the Geotail data, oscillations of the interplanetary magnetic field and density were in antiphase, which can be the evidence of the approach to the Earth of an interplanetary compressional wave causing generation of geomagnetic pulsations at f < 2 mHz with the largest amplitudes near the dayside polar cusp (Figures 5a, 6a, and 8).
During the second interval (1500-1600 UT) the solar wind dynamic pressure decreased to ~10 nPa, and no intense pulsations at f <2 mHz were observed either in the solar wind or in the polar cusp. However, a more intense burst of oscillations at f < 1.5 mHz than at 1300-1400 UT was observed in the afternoon closed magnetosphere. The onset of these oscillations coincided with the approach of a sharp gradient of the dynamic solar wind pressure to the Earth. It can be supposed that in the first case the low-frequency oscillations had the external origin, and in the second case, the source of waves was located inside the magnetosphere.
The high-frequency Pc5 (3-5 mHz) geomagnetic pulsations in the dayside magnetosphere were observed during both intervals. Their morphological characteristics corresponded to FLR (polarization reversal near the noon longitude and in the latitude region of the wave amplitude maximum, a discrete spectrum with coinciding maxima in the morning and evening hours, the antiphase H components in conjugate regions, very low azimuthal wave numbers, the temporal structure of the pulsations in the form of individual wave packets). However, contrary to typical Pc5, these pulsations were more intense on the evening side than on the morning side (Figure 4); the onset of their generation coincided with a sudden change of the IMF Bz from positive to negative; the abrupt suppression of pulsations coincided with a sharp drop in the solar wind density. The observation of simultaneous similar pulsations in the IMF (Figure 9) suggests that a possible source of FLR are IMF oscillations in the solar wind.
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