1
A statistical analysis of low frequency magnetic pulsations at South Pole
P. Francia (1,2), L. J. Lanzerotti (3,4), U. Villante (1,2), S. Lepidi, (5,2) D. Di Memmo (1)
1
Dipartimento di Fisica, Università dell’Aquila, Italy
2
Consorzio Area di Ricerca Astrogeofisica, L’Aquila, Italy
3
Center for Solar Terrestrial Physics, New Jersey Institute of Technology, Newark, NJ 07102
USA
4
Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974 USA
5
Istituto Nazionale di Geofisica e Vulcanologia, L’Aquila, Italy
We report a statistical analysis of low frequency magnetic variations (magnetic pulsations,
0.8-7 mHz) at South Pole in Antarctica (74°S corrected geomagnetic latitude) during 1996.
The results show that the pulsation power exhibits two maxima during the day, one in the
local pre-midnight hours, associated with substorm occurrence, and the other in the local
morning. During quiet magnetospheric conditions (Bz > 1nT), when the cusp is expected to be
located poleward with respect to the station, the spectral and polarization characteristics of
pulsations between 1-3 mHz suggest that resonant oscillations of the outermost closed field
lines commonly occur at South Pole in the local morning. In order to investigate the spatial
extension of the observed phenomena, we extend our analysis to simultaneous measurements
made at Terra Nova Bay (80°S) and find that, in the polar cap, the magnetic variations exhibit
different spectral and polarization characteristics.
2
Introduction
In the last years the availability of long data series have allowed studies of ULF geomagnetic
variations (geomagnetic pulsations) at high geomagnetic latitudes, where several generation
mechanisms occur related to solar wind (SW) - magnetosphere interactions (Arnoldy et al.,
1988). The results have provided information on the pulsation characteristics as well as on
their spatial and/or temporal extent, all of which can be useful for distinguishing among
various possible generation mechanisms. In particular, in the frequency range ∼1-5 mHz, a
pronounced local morning/afternoon asymmetry, with higher power levels in the local
morning, was reported at auroral and cusp latitudes (Gupta, 1973; 1975; Olson, 1986;
Engebretson et al., 1998; Yagova et al., 2002). An additional power enhancement near local
midnight was explained in terms of substorm related features (Olson, 1986). Local morning
pulsations were attributed to the Kelvin-Helmholtz instability on the magnetopause boundary,
while the rare afternoon pulsations were suggested to be triggered by SW pressure pulses
(Rostoker and Sullivan, 1987).
At these high latitudes, more recent statistical studies reported pulsations at discrete
frequencies (∼1.2-1.4, 1.8-2.0, 2.4-2.6, 3.2-3.4, and 4.-4.2 mHz; Samson et al., 1992; Walker
et al., 1992; Ziesolleck and McDiarmid, 1994, 1995; Mathie et al., 1999a), which were
interpreted in terms of field line resonances excited by magnetospheric waveguide/cavity
modes (Kivelson et al., 1984; Kivelson and Southwood, 1986). Mann et al. (1999)
investigated the energization of waveguide modes by magnetosheath flows on the
magnetopause flanks and explained the observed local morning/afternoon asymmetry in terms
of a greater stability of the postnoon magnetopause to shear-flow instabilities with respect to
the dawn flank. In this sense, pulsations driven by magnetopause instabilities during intervals
of high SW speed should occur predominantly in the local morning, while impulsively driven
pulsations should extend over a wide range of local times (Mathie et al., 1999a; Mathie and
Mann, 2000).
In the polar cap, at stations located at ~78°-80°, the magnetic fluctuation power maximizes
around local magnetic noon; i.e. when the station approaches the dayside cusp (Troitskaya,
1985; Engebretson et al., 1995; Ballatore et al., 1996; 1998; Yagova et al., 2002). Evidence
for daytime power enhancements at discrete frequencies (1.2, 1.9, 2.7, 3.3, 3.9 and 4.5 mHz,
3
more clearly evident during high SW speed conditions) has been reported also at these
latitudes in a statistical study by Villante et al. (1997).
Important information on the generation and propagation mechanisms of ULF pulsations can be
also obtained from measured polarization characteristics. For example, waves excited by the
Kelvin-Helmholtz
instability
are
expected
to
propagate
antisunward
through
the
magnetosphere, i.e. westward in the local morning and eastward in the afternoon. In such a
situation, ground-based measurements of pulsations should exhibit a polarization reversal
across local noon. However, theoretically it has been shown that the polarization pattern can be
modified by the resonant coupling between compressional waves and Alfven modes
(Southwood, 1974; Chen and Hasegawa, 1974). In particular, for a given frequency, theory
predicts a first polarization reversal at the latitude corresponding to the amplitude minimum
between the magnetopause and the resonant field line, and a second reversal at the latitude of
the resonant field line where amplitude maximizes. Due to the variable length of geomagnetic
field lines through the day (Waters et al., 1995; Mathie et al., 1999b), both these latitudes will
have a local time dependence, with highest latitude values occurring around local noon. The
theory explains well the results reported for the low frequency (1-5 mHz) pulsation
measurements reported by Samson et al. (1971) and Samson (1972) at latitudes between 60° 80°, where, depending on latitude, two or more polarization reversals were observed through
the day. In agreement with these results, a statistical analysis of low frequency (1-4 mHz)
pulsations at Terra Nova Bay (80°S; Lepidi et al., 1999) found four polarization reversals,
suggesting that the resonance effects of lower latitude field lines can be also observed in the
polar cap.
In this study we report a statistical analysis of low frequency pulsations (f = 0.8-7 mHz)
measured in data obtained at South Pole station (Antarctica, IGRF96 corrected geomagnetic
latitude 74.02°S, MLT = UT-03:35) during 1996, at the minimum phase of the solar cycle. The
analysis allows us to characterize the diurnal variation of the pulsation geomagnetic power
levels in the cusp region and to determine that, during quiet magnetospheric conditions when
the cusp is expected to be located poleward with respect to the station (Zhou et al., 2000),
resonant oscillations of the outermost closed field lines commonly occur in the local morning.
In order to compare the results found at this cusp latitude with those in the polar cap, we also
report the analyses of data acquired at Terra Nova Bay during the same year.
4
Data analysis and experimental results
Our analysis is based on 1-min values of the geomagnetic field components H and D
measured at South Pole during 1996 (data are available from January 1st to October, 12th).
We also use 1-min values of the interplanetary magnetic field (IMF) component Bz from
WIND spacecraft.
The geomagnetic data were differenced in order to eliminate the lowest frequency variations.
Dynamic power spectra were obtained by computing spectra over overlapping 3-hr intervals:
the beginning of each interval was shifted by 10 min with respect to the preceding. The
spectral analysis was conducted using the maximum entropy method at order m = 30 of the
prediction error filter. In addition, a cross-spectral analysis between the H and D components
was calculated using the technique for partially polarized waves as proposed by Fowler et al.
(1967). In particular, the polarization ratio R (i.e., the ratio between the polarized and total
intensity of the horizontal signal) and the ellipticity (i.e., the ratio between the minor and the
major axis of the polarization ellipse in the horizontal plane) were evaluated over each hour
and then averaged over the 3-hr intervals. In order to consider only polarized fluctuations,
characterized by a negligible noise component, this analysis was restricted to intervals with R
> 0.7 and | | > 0.2 (Lepidi et al., 1999). This restriction decreases the number of selected
intervals by about 70% of the data.
The local day results presented here are for the entire interval of data analyzed. A separate
investigation (not shown here) conducted for the different seasons showed that the results of
the present analysis are permanent features at South Pole, largely independent of season,
although somewhat more prominent during equinoxes (e.g., Rao and Gupta, 1978; Wright et
al., 1999).
Figure 1 shows the yearly averaged dynamic spectra of polarized fluctuations. The Hcomponent spectra are in the two upper panels and the D-component spectra are in the two
lower panels. Column (a) are the spectra for the entire time interval (independent of Bz) and
column (b) are the spectra for northward Bz (Bz > 1 nT). From Figure 1a it can be seen that
the power levels in both components have two broad enhancements (although more
pronounced for the H component), one in the local morning (and, for frequencies lower than
5
3-4 mHz, up to ~15 MLT). The second power enhancement occurs in the pre-midnight sector
for both components.
The results in Figure 1b show that, when they are restricted to quiet magnetospheric
conditions (i.e., northward IMF Bz > 1 nT), the nighttime power intensification almost
disappears in both components, indicating that the corresponding magnetic fluctuations are
related to the occurrence of geomagnetic substorms. Under the quiet geomagnetic conditions,
the H-component spectrum clearly shows in the morning power enhancements at ~ 1.8, 2.5,
and 3.1 mHz. Under these conditions, the onset of the power intensification shifts towards
later times at the higher frequencies; further, the frequency of the power maxima increases
from ~1.8 mHz at ~ 0730 MLT to ~2.5 and ~ 3.1 mHz at ~ 0830 MLT. These daytime power
features are absent in the D-component dynamic spectrum; compared to the D-component
spectrum in Fig. 1a, the quiet time power is characterized by a strong attenuation of the low
frequency (< 4 mHz) fluctuations in the early morning.
The characteristics of the magnetic fluctuations measured during quiet magnetospheric
conditions were further examined by computing the corresponding polarization patterns.
Figure 2 shows the MLT and frequency dependence of the percentage of polarized pulsations
with clockwise (CW) polarization sense during northward interplanetary magnetic field
conditions. The sense of polarization in this study was defined by looking downward on the
Earth. The results in Figure 2 show a first polarization reversal (CW/CCW) in the early
morning hours (before 06 MLT) in all frequency bands. After ~ 0600 MLT, the polarization
has a frequency dependence; for frequencies between ~1-3 mHz, the polarization reverses
(CCW/CW) at ~10 MLT and then back to CCW after local noon; conversely, above ~3 mHz
the polarization reverses sense (CCW/CW) around local noon and after ~18 MLT returns
CCW. Finally, a CCW/CW reversal is observed at ~21 MLT, independent of frequency.
The complex polarization pattern seen in the South Pole data in Figure 2 is consistent with the
results presented by Samson et al. (1971) and Samson (1972), and explained by them in terms
of the resonant coupling between compressional modes and resonant field line modes. In
particular, the local morning CCW polarization sense indicates a station located in the
resonance region (i.e. between the latitude corresponding to the minimum amplitude and the
latitude of the resonant field line), suggesting the occurrence of field line resonances at
latitudes lower than South Pole. In this scenario, the shift of the polarization reversal toward
later times with increasing frequency, that is observed in the early morning in Figure 2, can be
6
interpreted in terms of a corresponding shift toward lower latitudes of the resonance region.
For frequencies between ~1-3 mHz, the reversal in the sense of polarization at ~10 MLT is
consistent with a crossing of the line of maximum hydromagnetic wave intensity (Fig. 13 in
Samson, 1972). This suggests that South Pole is located at the footpoint of closed resonant
field lines at this local time. This interpretation agrees with other features of the magnetic
variations that are observed in the morning during the quiet times, i.e. the spectral power
peaks at discrete frequencies, the major power content in the H component, and the time shift
in the frequency of the power maxima (Fig. 1b).
Our results suggest that, during northward IMF conditions, the cusp is located poleward with
respect to South Pole. It is expected that the cusp latitude shifts equatorward during
southward IMF (i.e. when the erosion of the dayside magnetosphere occurs, Zhou et al.,
2000). Accordingly, we conducted a polarization analysis during southward IMF conditions;
the results (percentage of CW polarization; Bz < -1 nT) are shown in Figure 3.
The
polarization reversal at ~ 10 MLT for frequencies between ~1-3 mHz disappears, indicating
that, under southward IMF conditions, South Pole is located poleward with respect to closed
resonant field lines at this local time.
As discussed in the Introduction, data from Terra Nova Bay (IGRF96 corrected geomagnetic
latitude 80.01°S, MLT = UT-08:10) were analyzed for the same year as the South Pole data in
order to examine the differences between cusp latitude magnetic variations (South Pole) and
those in the polar cap. Figure 4 shows the results in the same format as Figure 1. The yearly
averaged dynamic spectra appear to be essentially independent of magnetosphere conditions.
Importantly, the spectral characteristics are quite different at the pole and at the cusp station.
The spectral power levels in both magnetic components at Terra Nova Bay are much lower
than at South Pole; the power shows a dominant maximum around local magnetic noon at
Terra Nova when the station approaches closer to the polar cusp. Geomagnetic substorm
signatures are almost absent in the polar cap on an average basis as well. There also do not
appear to be any clear evidence for selected frequencies at Terra Nova, even during northward
IMF conditions.
A separate analysis for the different seasons found similar characteristics independent of
season; it was found (not shown here), however, that the power maximum around noon was
more enhanced during local summer, consistent with a cusp located at higher latitudes during
7
the local summer (Zhou et al., 1999), and then closer to Terra Nova Bay, as well as with a
possible sunlight effect on the ionosphere.
The polarization pattern at Terra Nova Bay (Figure 5; same polarization formats as Figure 2)
is very similar for northward (Figure 5a) and southward (Figure 5b) IMF conditions. In
particular, for frequencies between ~1-4 mHz, the polarization is characterized by four
reversals during the day: in the early morning, around noon, in the local afternoon and just
before local midnight. This pattern is very similar to the one observed at South Pole for
frequencies higher than ~3 mHz; the pattern is consistent with the results reported by Samson
et al. (1971) and Samson (1972) as well as with previous results obtained at Terra Nova Bay
by Lepidi et al. (1999) and indicates the occurrence of field line resonances at geomagnetic
latitudes lower than Terra Nova Bay. At frequencies higher than ~4 mHz the polarization
pattern is less well defined; the four reversals tend to disappear and the polarization sense is
generally CW in the local morning and CCW in the local afternoon, suggesting that Terra
Nova Bay is poleward of the resonance region.
Summary and discussion
In this study we have reported a statistical analysis of low frequency geomagnetic pulsations
observed at the cusp latitude station South Pole in Antarctica during 1996, at the minimum
phase of the solar cycle. A comparison with pulsations occurring deep in the polar cap was also
performed using data acquired simultaneously at Terra Nova Bay.
At South Pole, for both horizontal geomagnetic field components, the pulsation power shows
a diurnal variation with two maxima, one in the local morning and the second in the
premidnight hours. When the analysis is restricted to northward IMF conditions, the
premidnight power maximum disappears, suggesting that this enhancement is associated with
substorm occurrence (Olson, 1986). In the local morning, power peaks at discrete frequencies
below ~4 mHz appear in the H component spectrum, while the D component spectrum shows
a much lower energy content, especially at frequencies lower than 4 mHz.
8
The polarization analysis for South Pole data during quiet magnetospheric conditions shows
that the yearly average diurnal pattern is consistent with results obtained at similar northern
hemisphere geomagnetic latitudes by Samson (1972) and, more recently, by Lepidi and
Francia (2003) for frequencies < 4 mHz, and by Samson et al. (1971) for higher frequencies.
In particular, the morning CCW polarization indicates the occurrence of field line resonances
at latitudes lower than South Pole; in this sense, the time shift of the polarization reversal with
increasing frequency observed before 06 MLT might reflect the corresponding shift of the
resonance region toward lower latitudes.
Interestingly, for frequencies between ~1-3 mHz, the polarization sense becomes CW
between ~10-12 MLT, indicating that South Pole is located equatorward of the resonance
region. This result suggests that field line resonances in the frequency range 1-3 mHz
commonly occur at the South Pole latitude before 10 MLT during quiet magnetospheric
conditions when the cusp is at higher latitudes (78°-80°; Zhou et al., 2000); therefore, South
Pole can be located at the footpoint of the outermost closed magnetospheric field lines that
have eigenfrequencies ~ 1-3 mHz.
Conversely, during southward IMF conditions the cusp shifts to lower latitudes (73°; Zhou et
al., 2000) and South Pole can be located on open field lines; indeed, the polarization
characteristics (CCW polarization until noon, independent of frequency) indicate that the
station is located poleward with respect to the resonant field lines. It is interesting to note that
the discrete power peak frequencies (1.8, 2.5, and 3.1 mHz) correspond approximately to the
lowest expected frequencies of the magnetospheric waveguide modes (Samson et al., 1992);
in our interpretation, these modes might be responsible for the energization of the 1.8, 2.5 and
possibly 3.1 mHz field line resonances, while the higher frequency waveguide modes might
couple to such resonances at lower latitudes. These waveguide modes can be driven by
magnetosheath flows on the magnetopause flanks and will occur mainly in the local morning,
due to the higher duskside magnetopause stability (Mann et al., 1999).
In this scenario, the local time variation of the maximum power frequency at South Pole can
be explained in terms of the diurnal variation of the local field line eigenfrequency, mainly
due to field line stretching near dawn with respect to local noon. It is interesting to note that
Waters et al. (1995) and Mathie et al. (1999b), using data from the CANOPUS and IMAGE
arrays respectively, found evidence for a diurnal, U-shaped variation in the frequency of the
9
field line resonance associated with near magnetopause field lines. Their results are very
similar to those presented here; for example, Mathie et al. (1999b) found that at 73.5° in the
northern hemisphere the local field line eigenfrequency increases from 1.7 mHz at 08 MLT to
2.3 mHz at 11 MLT. Moreover, in a previous statistical study of Pc5 pulsations at a cusp
latitude station in Svalbard (Norway, approximately at the same geomagnetic latitude in the
northern hemisphere as South Pole), McHarg et al. (1995) found a similar local time variation
of the maximum power frequency, from less than 2 mHz at 08 MLT to 5 mHz around local
noon; they called this feature an ‘arch’ and related it to the passage of the cusp in the field of
view of the station. Also Lanzerotti et al. (1999), analysing low frequency geomagnetic
pulsations at Antarctic stations located between 71° - 80°, observed that, at lower latitudes,
geomagnetic power enhances during the morning through local noon with the frequency
tending to increase from ∼1 to ∼4 mHz; the ‘arch’ disappears around local noon. They
interpreted this feature as possible evidence of Alfven waves on closed field lines, with the
dominant frequency related to the outermost closed field line of the magnetosphere.
The corresponding analysis at Terra Nova Bay shows that low frequency pulsations in the
polar cap have quite different characteristics with respect to the cusp latitudes. The spectral
power is much lower; it maximizes for both components around local noon, when Terra Nova
Bay approaches the cusp, and the power does not show clear evidence for enhancements at
discrete frequencies. Moreover, the polarization pattern indicates that Terra Nova Bay,
independent of IMF conditions, is always located poleward with respect to resonant field
lines. In particular, for frequencies below 4 mHz, in the daytime hours, Terra Nova Bay is
located within the resonance region, while for higher frequencies no effects of resonance
phenomena can be detected.
Acknowledgements.
The research at South Pole was supported in part by the Office of Polar Program of the United
States National Science Foundation. This work was supported by Italian PNRA (Programma
Nazionale di Ricerche in Antartide) and MIUR.
10
References
Arnoldy, R. L., L. J. Cahill Jr., M. J. Engebretson, L. J. Lanzerotti, and A. Wolfe, Review of
hydromagnetic wave studies in the Antarctic, Rev. Geophys., 26, 181-207, 1988.
Ballatore, P., S. Lepidi, L. Cafarella, U. Villante, A. Meloni, M. Candidi, and P. Palangio,
Low-frequency (1.7-6.7 mHz) geomagnetic field fluctuations at high southern latitudes, Il
Nuovo Cimento, 19, 517-525, 1996.
Ballatore, P., L.J. Lanzerotti, and C.G. Maclennan, Multistation measurements of Pc5
geomagnetic power amplitudes at high latitudes, J. Geophys. Res., 103, 29455-29465, 1998.
Chen, L., and A. Hasegawa, A theory of long period magnetic pulsations, 1, steady state
excitation of field line resonance, J. Geophys. Res., 79, 1024-1037, 1974.
Engebretson, M. J., W. J. Hughes, J. L. Alford, E. Zesta, L. J. Cahill Jr., R. L. Arnoldy, G. D.
Reeves, Magnetometer array for cusp and cleft studies observations of the spatial extent of
broadband ULF magnetic pulsations at cusp/cleft latitudes, J. Geophys. Res., 100, 1937119386, 1995.
Engebretson, M., K.H. Glasmeier, M. Stellmacher, W.J. Hughes, and H. Luhr, The
dependence of high latitude Pc5 wave power on solar wind velocity and on the phase of high
speed solar wind streams, J. Geophys. Res., 103, 26271-26283, 1998.
Fowler, R.A., B.J. Kotick, and R.D. Elliott, Polarization analysis of natural and artificially
induced geomagnetic micropulsation, J. Geophys. Res., 72, 2871-2883, 1967.
Gupta, J.C., Occurrence studies of geomagnetic micropulsations Pc5 at high latitudes, J.
Atmos. Terr. Phys., 35, 2217, 1973.
Gupta, J.C., Long period Pc5 pulsations, Planet. Space Sci., 23, 733-750, 1975.
Kivelson, M. and D. Southwood, Resonant ULF waves:a new interpretation, Geophys. Res.
Lett., 12, 49-52, 1985.
Kivelson, M. and D. Southwood, Coupling of global magnetospheric MHD eigenmodes to
field line resonances, J. Geophys. Res., 91, 4345-4351, 1986.
Lanzerotti, L.J., A. Shono, H. Fukunishi, and C.G. Maclennan, Long period hydromagnetic
waves at very high geomagnetic latitudes, J. Geophys. Res., 104, 28423-28435, 1999.
Lepidi, S., P. Francia, U. Villante, L.J. Lanzerotti, and A. Meloni, Polarization pattern of low
frequency geomagnetic field fluctuations (0.8-3.6 mHz) at high and low latitude, J. Geophys.
Res., 104, 305-310, 1999.
11
Lepidi, S., and P. Francia, Diurnal polarization pattern of ULF geomagnetic pulsations in the
Pc5 band from low to polar latitudes, J. Atm. Solar Terr. Phys., 65, 1179-1185, 2003.
Mann, I.R., A.N. Wright, K.J. Mills, and V.M. Nakariakov, Excitation of magnetospheric
waveguide modes by magnetosheath flows, J. Geophys. Res., 104, 333-353, 1999.
Mathie, R.A. and I.R. Mann, Observations of Pc5 field line resonance azimuthal phase speeds:
a diagnosis of their excitation mechanism, J. Geophys. Res., 105, 10713-10728, 2000.
Mathie, R.A., I.R. Mann, F.W. Menk, and D. Orr, Pc5 ULF pulsations associated with
waveguide modes observed with the IMAGE magnetometer array, J. Geophys. Res., 104,
7025-7036, 1999a.
Mathie, R.A., F.W. Menk, I.R. Mann, and D. Orr, Discrete field line resonances and the
Alfven continuum in the outer magnetosphere, Geophys. Res. Lett., 26, 659-662, 1999b.
McHarg, M.G., J.V. Olson, and P.T. Newell, ULF cusp pulsations: diurnal variations and
interplanetary magnetic field correlations with ground-based observations, J. Geophys. Res.,
100, 19729-19742, 1995.
Olson, J.V., ULF signatures of polar cusp, J. Geophys. Res., 91, 10055, 1986.
Rao, D.R.K., and J.C. Gupta, Some features of Pc5 pulsations during a solar cycle, Planet.
Space Sci., 26, 1-20, 1978.
Rostoker, G., and B.T. Sullivan, Polarization characteristics of Pc5 magnetic pulsations in the
dusk hemisphere, Planet. Space Sci., 35, 429-438, 1987.
Samson, J.C., J.A. Jacobs, and G. Rostoker, Latitude-dependent characteristics of long period
geomagnetic micropulsations, J. Geophys. Res., 76, 3675-3683, 1971.
Samson, J.C., Three-dimensional polarization characteristics of high latitude Pc5 geomagnetic
micropulsations, J. Geophys. Res., 77, 6145-6160, 1972.
Samson J. C., B. G. Harrold, J. M. Ruohoniemi, R. A. Greenwald, and A. D. M. Walker, Field
line resonances associated with MHD waveguides in the magnetosphere, Geophys. Res. Lett.,
19, 441-448, 1992.
Southwood, D.J., Some features of field line resonances in the magnetosphere, Planet. Space
Sci., 22, 482-491, 1974.
Troitskaya, V.A., ULF investigations in the dayside cusp, Adv. Space Res., 5, 219, 1985.
Villante, U., S. Lepidi, P. Francia, A. Meloni, and P. Palangio, Long period geomagnetic field
fluctuations at Terra Nova Bay (Antartica), Geophys. Res. Lett., 24, 1443-1446, 1997.
Walker, A. D. M., J. M. Ruohoniemi, K. B. Baker, R. A. Greenwald, and J. C. Samson,
Spatial and temporal behaviour of ULF pulsations observed by the Goose Bay HF radar, J.
Geophys. Res, 97, 12187-12202, 1992.
12
Waters, C.L., J.C. Samson, and E.F. Donovan, The temporal variation of the frequency of
high latitude field line resonances, J. Geophys. Res., 100, 7987-7996, 1995.
Wright, D.M., T.K. Yeoman, and T.B. Jones, ULF wave occurrence statistics in a highlatitude HF Doppler sounder, Ann. Geophysicae, 17, 749-758, 1999.
Yagova, N.V., L.J. Lanzerotti, U. Villante, V.A. Pilipenko, S. Lepidi, P. Francia, V.O.
Papitashvili, and A.S. Rodger, ULF Pc5-6 magnetic activity in the polar cap as observed
along a geomagnetic meridian in Antarctica, J. Geophys. Res., 107, SMP 22-1, 2002.
Ziesolleck C. W. S., and D. R. McDiarmid, Auroral latitude Pc5 field line resonances:
quantized frequencies, spatial characteristics and diurnal variations, J. Geophys. Res., 99,
5817-5830, 1994.
Ziesolleck C. W. S., and D. R. McDiarmid, Statistical survey of auroral latitude Pc5 spectral
and polarization characteristics, J. Geophys. Res., 100, 19299-19312, 1995.
Zhou, X.W., C.T. Russell, G. Le, S.A. Fuselier, and J.D. Scudder, The polar cusp location and
its dependence on dipole tilt, Geophys. Res. Lett., 26, 429-432, 1999.
Zhou, X.W., C.T. Russell, G. Le, S.A. Fuselier, and J.D. Scudder, Solar wind control of the
polar cusp at high altitude, J. Geophys. Res., 105, 245-251, 2000.
13
Figure captions
Figure 1. Magnetic local time and frequency distribution of the pulsation power of the H
(upper panels) and D (lower panels) geomagnetic field components at South Pole for the
whole examined period (a) and during northward IMF conditions (b).
Figure 2. Magnetic local time and frequency distribution of the percentage of CW events at
South Pole during northward IMF conditions.
Figure 3. Magnetic local time and frequency distribution of the percentage of CW events at
South Pole during southward IMF conditions.
Figure 4. Magnetic local time and frequency distribution of the pulsation power of the H
(upper panels) and D (lower panels) geomagnetic field components at Terra Nova Bay for the
whole examined period (a) and during northward IMF conditions (b).
Figure 5. Magnetic local time and frequency distribution of the percentage of CW events at
Terra Nova Bay during southward (a) and northward (b) IMF conditions.
14
(a)
(b)
Fig. 1
15
Fig. 2
Fig. 3
16
(a)
(b)
Fig. 4
17
Fig. 5