PROPAGATION CHARACTERISTICS OF ULF WAVES IN THE EARTH’S
MAGNETOSPHERE AS INFERRED BY AN ANALYSIS OF POLARIZATION
PARAMETERS OF GEOMAGNETIC PULSATIONS AT A LOW LATITUDE STATION
M. Vellante(1,3), U. Villante(1,3), L. Santarelli(2,3) , M. De Lauretis(1,3)
(1)
Dip. Fisica, Università dell’Aquila, Via Vetoio, 67010 Coppito-L’Aquila, Italy,Email:massimo.vellante@aquila.infn.it
Ist. Nazionale di Geofisica e Vulcanologia, Castello Cinquecentesco, 67100 L’Aquila, Italy, Email: santarelli@ingv.it
(3)
Consorzio Area di Ricerca in Astrogeofisica, Via Vetoio, 67010 Coppito-L’Aquila, Italy
(2)
ABSTRACT
We have conducted an extensive statistical analysis of
the polarization characteristics of geomagnetic
pulsations (10-100 mHz) recorded at L’Aquila (Italy, L
= 1.6) during 15 years (1985-1999). The dependence
on frequency, local time, season, solar cycle and
interplanetary magnetic field orientation has been
investigated. The diurnal pattern of the dominant sense
of polarization suggests the presence of two distinct
sources of ULF waves, one located 1-2 hours before
noon and the other one around midnight. Significant
variations of the ellipticity occur at sunrise and sunset
with clear seasonal dependence. The results are
discussed in terms of combined effects of exogenic
wave sources, field line resonant mechanism and
ionospheric influence.
1.
pulsations. Surface waves at the magnetopause should
indeed travel westward in the morning and eastward in
the afternoon in agreement with the direction of the
solar wind propagation along the magnetopause flanks
(Fig.1).
INTRODUCTION
ULF (1 mHz - 1 Hz) waves, propagating in the
magnetosphere and observed at ground as geomagnetic
pulsations, transport to the Earth important information
about plasma dynamics in the magnetosphere. The
study of polarization characteristics of these signals
provides useful indications on the source of the
magnetospheric ULF waves as well as on their
propagation through the magnetosphere and
ionosphere. For instance, in a hydromagnetic surface
wave the plasma motion is similar to that in a surface
wave in a fluid where elements rotate with a sense
which depends on the propagation direction. Since the
magnetospheric field lines are frozen into the plasma,
elliptically polarized magnetic waves are generated.
For a wave propagating westward (eastward), the sense
of polarization in the horizontal plane would be lefthanded (right-handed).
Experimental observations at subauroral latitudes [1]
have shown that the polarization of Pc4-5 geomagnetic
pulsations (1-15 mHz) typically reverses from lefthanded (LH) to right-handed (RH) around local noon.
Surface waves generated at the magnetopause by the
Kelvin-Helmholtz (KH) instability have been then
invoked to be a possible source for geomagnetic
Fig. 1. Polarization of surface waves driven by the KH
instability on the magnetopause. (After [2]).
Such diurnal polarization pattern has been observed
also at low latitudes, both for Pc5 [3] and Pc3-4 (20100 mHz, [4, 5, 6]) pulsations. Besides, the expected
correspondence between the horizontal sense of
polarization and the longitudinal direction of wave
propagation was experimentally verified [5, 6].
However, because of the rapid spatial damping, surface
waves generated at the magnetopause by the KH
instability cannot account for low-latitude Pc3-4
pulsations [4]. Upstream waves generated in the
Earth’s foreshock region are generally considered the
most likely source for this class of pulsations [7].
Before reaching ground, ULF waves can be
significantly modified by two major effects: field line
resonance mechanism [8] and transmission through the
ionosphere [9]. Such effects can also significantly alter
polarization characteristics [10].
2.
DATA ANALYSIS
Geomagnetic pulsations have been recorded in digital
form (1 Hz sampling rate) at L’Aquila (Italy, L = 1.6)
since 1985. This has provided the opportunity to
perform an investigation of wave polarization
characteristics on an unprecedented extensive
statistical basis. In particular, using a cross-spectral
technique, we determined the horizontal ellipticity for
more than 50,000 selected events in the frequency band
10-100 mHz and investigated its dependence on
frequency, local time, season, solar cycle, and
interplanetary magnetic field (IMF) orientation.
The ellipticity  is defined as the ratio of the minor to
major axis of the polarization ellipse. A positive
(negative) value of  corresponds to right-hand (lefthand) sense of rotation when viewed in the direction of
the ambient geomagnetic field. The spectral analysis
was conducted over consecutive 30 min intervals with
a frequency resolution of 8 mHz. Polarization
parameters were evaluated only for high degree of
polarization (> 80%) and significant power signal.
3.
3.1
EXPERIMENTAL RESULTS
Diurnal variation of the polarization sense
Fig.2 shows the diurnal variation of the dominant sense
of polarization in three different frequency bands. Only
cases with definite sense of polarization (|| > 0.1) are
considered. The percentage of cases with RH
polarization is plotted. The main results are the
following:
In the lowest frequency band there is only a slight
evidence for a polarization reversal arond noon. A
dominant sense of polarization (RH) occurs in the time
interval 16-20 LT. As will be shown later (Fig.5) this
likely reflects an ionospheric effect occurring around
sunset.
In the highest frequency bands (40–100 mHz) four
reversals of polarization can be clearly noted at  0 LT,
 3 LT,  10 LT, 20-21 LT. It seems to suggest, for this
class of pulsations, the presence of two distinct
sources, one located  2 hours before noon and the
other one around midnight.
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Fig. 2. Diurnal variation of polarization sense at
L’Aquila during equinoxes.
3.2
IMF effect
We also investigated the possible effects of the IMF
orientation on the polarization pattern for pulsations in
the frequency band 40–100 mHz where the prenoon
polarization reversal is more clear. Separated analysis
were conducted for IMF lines along the Parker spiral
and along the radial direction ( 15° in both cases). As
can be seen in Fig.3, when the IMF is oriented along
the radial direction the polarization reversal (from LH
to RH) appears to occur about one hour closer to noon.
This might be interpreted in terms of a more symmetric
configuration of the magnetosphere around the EarthSun direction.
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Fig. 3. Diurnal variation of polarization sense under
different IMF orientations.
3.3
Field line resonance effect
When the frequency of a compressional MHD wave
propagating through the magnetosphere matches at
some point the eigenfrequency of the local field line, a
resonance occurs and the wave is transformed in a
transverse wave with linear polarization [8]. On the
other hand, the eigenfrequency of a particular field line
depends on the plasma density distribution along that
field line. In agreement with the expected variation of
the plasmasphere density, [11] showed that the
eigenfrequency of the local field line at L’Aquila
experiences a solar cycle variation ( 55 mHz and  85
mHz at maximum and minimum solar activity,
respectively). This result is confirmed by the
polarization analysis shown in Fig.4. In fact, minimum
values of the ellipticity (i.e. maximum percentage of
linear polarization), for each phase of the solar cycle,
just occur at the corresponding estimates of the local
eigenfrequency (arrows).
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Fig. 4. Frequency variation of the average ellipticity
(absolute value) during daytime hours (0700-1700 LT)
under different solar activity conditions.
3.4
Seasonal effect
Fig.5 shows the diurnal variation of the absolute value
of the ellipticity in three different frequency bands and
for different seasons. The main results are the
following:
In the daytime hours, average values of the ellipticity
are quite small (almost linear polarization).
Around sunrise and sunset times the ellipticity
increases. A seasonal effect (more clear in the lower
frequency bands) can be seen in the evening hours with
maximum ellipticity shifted in time according to the
season, i.e at  17 LT,  18 LT, and 19 LT in winter,
equinoxes, and summer, respectively.
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Fig. 5. Diurnal variation of the average ellipticity
(absolute value) in different seasons.
4.
CONCLUSIONS
The sense of polarization of geomagnetic pulsations is
usually considered to be associated with the direction
of the wave propagation in the magnetosphere: LH
(RH) polarization would imply westward (eastward)
propagation. According to this paradigm the diurnal
pattern of the dominant sense of polarization observed
at L’Aquila seems to indicate the presence of two
distinct sources of ULF waves, one located 1-2 hours
before noon and the other one around midnight.
Surface waves generated at the magnetopause by a
solar wind driven KH instability mechanism and/or
upstream waves generated in the Earth’s foreshock
region are possible candidates for the daytime source.
In particular, the earlier occurrence with increasing
frequency of the prenoon reversal might indicate an
increasing contribution of upstream waves which are
preferentially generated in the morning sector [7].
However, possible different ionospheric modifications
of the ellipticity at different frequencies should be also
taken into account [12]. The night-time source likely
consists of transient waves generated in the
magnetotail during substorm onset [7].
The LT position of the prenoon polarization reversal
for Pc3 pulsations has been found to depend on the
IMF orientation (at  10 LT and  11 LT for IMF lines
oriented along the Parker spiral and the Sun-Earth
direction, respectively). This pattern is consistent with
the expected IMF dependence of the longitudinal
location of the upstream wave source [7]. The IMF
dependence of the position of the stagnation point of
the solar wind flow around the magnetosphere [13]
might also be a concurrent cause.
We also found significant variations of the ellipticity at
sunrise and sunset with clear seasonal dependence. The
high east-west inhomogeneity of the ionospheric
conductivity at sunrise/sunset has been invoked in the
past [14] as a possible cause for abrupt changes
observed in the azimuth angle of the polarization
ellipse. Our analysis suggests, for the first time, that
ionospheric inhomogeneities likely affect also the wave
ellipticity. So, further theoretical work is necessary to
understand the role of the ionosphere in the
transmission of magnetospheric ULF signals to the
ground.
5.
REFERENCES
1. Samson J. C., et al., Latitude-dependent
characteristics of long-period geomagnetic pulsations,
J. Geophys. Res., Vol. 76, 3675 - 3683, 1971.
2. Hughes W. J., Magnetospheric ULF waves: a
tutorial with a historical perspective, in Solar Wind
Sources of Magnetospheric Ultra-Low-Frequency
Waves, Geophys. Monogr. Ser., Vol. 81, AGU,
Washington D. C., 1 – 11, 1994.
3. Ziesolleck C. W. S. and Chamalaun F. H., A twodimensional array study of low-latitude Pc 5
geomagnetic pulsations, J. Geophys. Res., Vol. 98,
13703 - 13713, 1993.
4. Lanzerotti L. J., et al., Polarization characteristics of
hydromagnetic waves at low geomagnetic latitudes, J.
Geophys. Res., Vol. 86, 5500 - 5506, 1981.
5. Saka O. and Kim J. S., Spatial phase structure of
low-latitude Pc3-4 pulsations, Planet. Space Sci., Vol.
33, 1073 - 1079, 1985.
6. Ansari I. A. and Fraser B. J., A multistation study of
low latitude Pc3 geomagnetic pulsations, Planet. Space
Sci., Vol. 34, 519 - 536, 1986.
7. Yumoto K., Generation and propagation
mechanisms of low-latitude magnetic pulsations – A
review, J. Geophys., Vol. 60, 79 - 105, 1986.
8. Southwood D. J., Some features of field line
resonances in the magnetosphere, Planet. Space Sci.,
Vol. 22, 483 - 491, 1974.
9. Hughes W. J., The effect of the atmosphere and
ionosphere
on
long
period
magnetospheric
micropulsations, Planet. Space Sci., Vol. 22, 1157 1172, 1974.
10. Hughes W. J. and Southwood D. J., An illustration
of modification of geomagnetic pulsation structure by
the ionosphere, J. Geophys. Res., Vol. 81, 3241 - 3247,
1976.
11. Vellante M., et al., Solar cycle variation of the
dominant frequencies of Pc3 geomagnetic pulsations at
L = 1.6, Geophys. Res. Lett., Vol. 23, 1505 - 1508,
1996.
12. Nenovski P., Polarization effects of the finite-size
low-altitude ionosphere, Space Sci. Rev., Vol. 95, 581
– 598, 2001.
13. Walters G. K., Effect of oblique interplanetary
magnetic field on shape and behavior of the
magnetosphere, J. Geophys. Res., Vol. 69, 1769 - 1783,
1964.
14. Itonaga M. and Kitamura T.-I., Numerical
simulations on ionospheric control of polarization of
low-latitude geomagnetic pulsations, Ann. Geophys.,
Vol. 11, 1018 – 1025, 1993.