ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-35-139-2017A study of geomagnetic field variations along the 80∘ S
geomagnetic parallelLepidiStefaniastefania.lepidi@ingv.ithttps://orcid.org/0000-0002-5692-2560CafarellaLilihttps://orcid.org/0000-0003-2005-8923FranciaPatriziahttps://orcid.org/0000-0001-5183-9146PiancatelliAndreaPietrolungoManuelaSantarelliLuciahttps://orcid.org/0000-0002-4134-1560UrbiniStefanoIstituto Nazionale di Geofisica e Vulcanologia, Rome, 00143,
ItalyDipartimento di Scienze Fisiche e Chimiche, Università degli
Studi dell'Aquila, L'Aquila, 67100, ItalyStefania Lepidi (stefania.lepidi@ingv.it)24January20173511391467June20161December201614December2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://angeo.copernicus.org/articles/35/139/2017/angeo-35-139-2017.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/35/139/2017/angeo-35-139-2017.pdf
The availability of measurements of the geomagnetic field variations in
Antarctica at three sites along the 80∘ S geomagnetic parallel,
separated by approximately 1 h in magnetic local time, allows us to study the
longitudinal dependence of the observed variations. In particular, using
1 min data from Mario Zucchelli Station, Scott Base and Talos Dome, a
temporary installation during 2007–2008 Antarctic campaign, we investigated
the diurnal variation and the low-frequency fluctuations (approximately in
the Pc5 range, ∼ 1–7 mHz). We found that the daily variation is
clearly ordered by local time, suggesting a predominant effect of the polar
extension of midlatitude ionospheric currents. On the other hand, the
pulsation power is dependent on magnetic local time maximizing around
magnetic local noon, when the stations are closer to the polar cusp, while
the highest coherence between pairs of stations is observed in the magnetic
local nighttime sector. The wave propagation direction observed during
selected events, one around local magnetic noon and the other around local
magnetic midnight, is consistent with a solar-wind-driven source in the
daytime and with substorm-associated processes in the
nighttime.
Magnetospheric physics (polar cap phenomena)Introduction
Geomagnetic field measurements in Antarctica are particularly valuable for
the study of magnetospheric dynamics and dynamic processes controlling the
energy transfer from the solar wind (SW) to the Earth's magnetosphere in that
local field lines reach extreme magnetospheric regions where this interaction
occurs. In previous studies we used measurements from the Italian Geomagnetic
Observatory Mario Zucchelli Station (TNB, formerly Terra Nova Bay), at
80∘ S corrected geomagnetic latitude, and from the French–Italian
Observatory Concordia at Dome C (DMC) close to the geomagnetic pole, in order
to characterize the ultra low-frequency (ULF; 1 mHz–1 Hz) pulsation
activity and its relation with SW parameters in the Antarctic region (Lepidi
et al., 1996, 2003; Villante et al., 2000; Francia et al., 2005, 2009).
The global distribution of geomagnetic observatories is still quite
unbalanced in favor of the Northern Hemisphere; in this sense, the
installation of a magnetometer in a new observation site in the Antarctic
continent is useful in the study of the magnetospheric dynamics at high
latitudes. For this reason, during the 2007–2008 Antarctic campaign, we
installed a low-power magnetometer (LPM) at Talos Dome (TLD), within the
framework of the AIMNet (Antarctic International Magnetometer Network)
project, proposed and coordinated by the British Antarctic Survey and joined
by the Italian Programma Nazionale Ricerche in Antartide (PNRA). TLD is
located at ∼ 300 km from TNB, approximately at the same corrected
geomagnetic latitude.
The availability of simultaneous measurements from TNB and Scott Base (SBA;
data provided by INTERMAGNET database) allows us to make an interesting
comparison in that the three stations are located approximately at the same
geomagnetic latitude (∼ 80∘ S); stations at such latitudes are
located generally in the polar cap but approach the dayside cusp around local
magnetic noon. Moreover, SBA, TNB and TLD are at approximately 2 h total
displacement in magnetic local time (MLT; see Table 1 and Fig. 1). This
location is particularly useful for investigating the azimuthal signal
distribution and propagation. A previous analysis has shown that Pc5
pulsations at SBA and TNB (separated by 1 h in MLT) are highly coherent in
the magnetic noon and midnight sectors and that they propagate preferably from
midnight for the southward interplanetary magnetic field (IMF) and from noon
for the northward IMF (Santarelli et al., 2007). More recently, Lepidi et al. (2011a)
made a comparative analysis of Pc5 pulsations at TNB and Dumont d'Urville, both located at 80∘ S and separated by 5 h in MLT; they observed
coherent fluctuations when the stations are on the same side with respect to
the cusp; also, in this case, the propagation direction was found to be away from
midnight, as expected for substorm-related phenomena, and from noon, as
expected for the Kelvin–Helmholtz instability mechanism or SW pressure
fluctuation transmission into the magnetosphere.
Geographic coordinates, IGRF08 corrected geomagnetic coordinates and
time in UT of the geomagnetic local noon (NN) for the three stations.
Polar areas are important also to study the daily variation, which, at high
latitudes, is due to two different contributions: the polar extension of the
midlatitude ionospheric current systems and an additional electric current
system, related to field-aligned currents, characteristic of the polar cap
(Matsushita and Xu, 1982; Akasofu et al., 1983). In previous papers we also
investigated the 24 h periodicity at TNB and DMC, as well as in other polar
observatories, to ascertain its dependence on solar cycle season, IMF
conditions and geomagnetic activity level (Cafarella et al., 2007, 2009;
Pietrolungo et al., 2008; Lepidi et al., 2011b). One result, evident for
different latitudes within the polar cap and in both hemispheres, is that the
geographic reference system, with X and Y geomagnetic field
components against local time (LT) as a sorting parameter, is more suitable
than the geomagnetic one (with H, D and MLT) to describe and compare the
diurnal variation at such high latitudes.
Location of the three Antarctic sites TNB, SBA and TLD. The
geographic and the corrected geomagnetic coordinate systems are indicated
as dashed and solid lines, respectively.
In this study we show a comparative analysis of geomagnetic field horizontal
components recorded at TLD, TNB and SBA from 18 January to 14 March 2008,
focusing attention on the daily variation and low-frequency pulsations at
the three sites.
Upper panels: hourly average values of the horizontal
component H and D variations at TLD for the whole analyzed time period
(from 18 January to 14 March, 2008); lower panel: Kp index.
Data analysis
Variations in the Earth's magnetic field were measured at the three sites by
means of three-axis fluxgate magnetometers. The field variations are measured
along three directions oriented with reference to the local magnetic
meridian: the horizontal magnetic field intensity H component (south–north),
the orthogonal component D in the horizontal plane (west–east) and the
vertical intensity Z component (consequently increasing inward). For this
analysis we used 1 min values of the H and D horizontal geomagnetic field
components recorded in the period 18 January–14 March 2008.
Spectral and coherence analysis was performed with MATLAB processing tools,
based on the fast Fourier transform (FFT) method. The use of the Fourier
transform in comparison to other spectral analysis methods has been
extensively discussed in Balasis et al. (2012, 2015).
We first focused on the analysis of the daily variation; in Fig. 2 the hourly
average values of the horizontal component H and D variations are shown,
together with the Kp index. The presence of a quite regular daily variation
is evident; its amplitude strongly varies from day to day and closely follows
the level of magnetospheric activity, as can be seen from the Kp index.
We also performed a dynamical spectral analysis of these hourly data,
computing the spectra for each 3-day interval with a 1-day step size. The
dynamic spectra (Fig. 3; H and D components) show a sharp, persistent
power peak corresponding to the 24 h period; sometimes, also the 12 h
harmonic emerges.
Dynamic spectra from hourly data at TLD.
A comparison of the daily variation at SBA, TNB and TLD is shown in Fig. 4
(thick lines), which reports the daily distribution of the average 10 min
values of the two horizontal geomagnetic field components at the three
stations; in addition to the H–D horizontal components ordered
according to MLT, for this analysis we also show data rotated into the geographically oriented
reference system, i.e., the X and Y geomagnetic field components (along
the geographic meridian and parallel, respectively) ordered according to LT
(Pietrolungo et al., 2008). The results of Fig. 4 show that the daily
variation at the three stations is very similar, both regarding the shape and
the amplitude. It is interesting to note that when ordered according to LT,
the daily variations are perfectly in phase, while, according to MLT, there is
a slight time shift, especially between TLD and the other two stations. This
feature is more evident from the comparison of the fits of the experimental
curves (Fig. 4, thin lines), where the short period fluctuations due to the
quite short data series are eliminated.
Daily distribution of the average 10 min values of the
two horizontal geomagnetic field component variation in the geographic
reference system vs. LT (upper panels, thick lines) and in the geomagnetic
reference system vs. MLT (lower panels, thick lines). Each point represents
the variation at a fixed 10 min interval, averaged over the whole analyzed
time period. In all panels thin lines represent the fits of the experimental
curves, upward shifted of 25 nT.
Besides the daily variation, we also investigated the ULF activity at the
three stations. In Fig. 5 two examples of daily magnetograms of the
horizontal components H and D at SBA, TNB and TLD are shown; it is
evident that the observations at the three stations are very similar. The 23 February (top panels) is a quiet day, with the sum of the eight 3 h Kp
values sum(Kp) = 10; the plots show that the geomagnetic fluctuations
increase in the last hours of the day, when the stations are closer to their
magnetic local noon (indicated by the arrows). The magnetograms for 11 March
(lower panels) show a more intense activity; indeed, it is a more disturbed
day, with sum(Kp) = 23; also, in this case, there is evident
geomagnetic activity in the dayside MLT sector, around local magnetic noon
with simultaneous signals of comparable amplitude at the three stations. We
may note that the signal observed at all stations around 07:00 UT has a
different amplitude, maximizing at SBA, which is at MLT midnight; it can be
related to substorm activity, as confirmed by the high AE (auroral electrojet index) values (∼ 500 nT) observed in the same time interval (http://wdc.kugi.kyoto-u.ac.jp/).
Daily magnetograms of the horizontal components H and D at SBA,
TNB and TLD. The arrows indicate MLT noon at the three stations. An
arbitrary value has been added to each time series to show the variations in
the same plot.
Upper panels: average power spectra of the H component at the
three stations. Lower panels: average ratio between power spectra at pairs
of stations.
We analyzed the low-frequency geomagnetic field fluctuations in the H
component at the three stations, computing the power spectra and coherence
between pairs of stations, with TNB as reference station. The spectral and
coherence analysis was performed computing the spectra for each 2 h interval
(averaging four 30 min subintervals) with a 1 h step size.
Average coherence between H component fluctuations at pairs of
stations.
The average power spectra of the H component as a function of UT at the three
stations are shown in Fig. 6 (upper panels). It is evident that the power at
each station maximizes at all frequencies around MLT noon (indicated by the
arrows), while the minimum power occurs in the postmidnight/early morning
sector (around 03:00 MLT). The shift in the maximum, due to the different
MLT noon at the stations, is made more evident by computing the average ratio
between the power spectra at the pairs of stations SBA–TNB and TNB–TLD
(Fig. 6, lower panels). The ratios show a bipolar variation, varying sharply
from maximum to minimum values around the MLT noon at each pair of
stations.
Figure 7 shows the daily distribution of the average coherence between
H component fluctuations at the pairs of stations TNB–SBA and TNB–TLD. In
the nighttime (02:00–12:00 UT), when the stations are well within the polar
cap, far from the cusp, the coherence maximizes at all frequencies;
conversely, in the daytime, when the stations are close to the cusp, the
coherence is high only for the lowest frequencies (up to 1.5–2 mHz).
Moreover, the signal correspondence is more evident between TNB and TLD than
between TNB and SBA, probably due to the smaller separation both in MLT and
in geographic latitude (Table 1).
We lastly analyzed the fluctuations during the 2 days shown in Fig. 5 for
a comparison between the geomagnetic signals at the three stations. Figures 8
and 9 show, from the top, the variations in the geomagnetic field H
component, filtered in the 1–5 mHz frequency range, and the dynamical power
spectra (computed for 1 h overlapping intervals with a 30 min step size).
For the event occurring on 23 February 2008, the wave activity starts from
∼ 17:30 UT, when the stations approach the cusp. Satellite
measurements at the Lagrangian point show, corresponding to this, SW speed higher
than 430 km s-1 and dynamic pressure fluctuations (OMNI data from http://omniweb.gsfc.nasa.gov; not
shown here). As can be seen both from the filtered data and the power
spectra, the time interval of higher activity shifts from 17:00–21:00 UT at
SBA to 18:00–22:00 UT at TNB until 19:00–23:00 UT at TLD, corresponding to the different magnetic noon sectors (in Fig. 8 the magnetic
noon at each station is indicated by an arrow). In order to make more evident
the effect of cusp activation, we show in Fig. 9 the MLT dependence of the
hourly pulsation power-integrated in the 1–5 mHz frequency band; as
expected, the power is strongly enhanced around magnetic local noon at each
station. From Fig. 8 it can also be seen that when the activity is high at
all stations, approximately between 19:30 and 20:00 UT, it is characterized
by simultaneous power enhancements at discrete frequencies, in particular at
∼ 1.1, 1.7 and 2.5 mHz, corresponding to clear, regular fluctuations
at the three stations; we may note that SBA observes them in advance; then
they occur at TNB and lastly at TLD. From a visual inspection of the filtered
data, we also estimated the time delay (note that using 1 min data, it can
be determined with an accuracy not better than 1 min), which is
∼ 3 min between SBA and TNB and ∼ 2 min between TNB and TLD and
indicates waves propagating from SBA (the station closest to the noon) to TNB
to TLD in the antisunward direction, with an azimuthal number m∼ 4
(computed by the formula m=f⋅Δt⋅360∘/Δλ, where f is the wave frequency, Δt is the time shift and
Δλ is the longitudinal distance in degrees between stations).
From top: variations in the geomagnetic field H
component filtered in the 1–5 mHz frequency range and dynamical power
spectra for the event on 28 February 2008. The arrows indicate MLT noon at
the three stations.
The MLT dependence of the pulsation power for the event
on 28 February 2008, integrated in the 1–5 mHz frequency band at the three
stations.
For the event occurring on 11 March 2008, a more disturbed day, we focused on
the nighttime sector (Fig. 10). We found a burst of activity around
07:00 UT, with a definitely greater amplitude at SBA, i.e., the station that
at 07:03 UT is at magnetic midnight. The power spectra at TNB and TLD show
several similar enhancements in the whole frequency range; at SBA a broad
power enhancement between 1 and 2.5 mHz dominates the spectrum. Also, in this
case SBA observes the signal in advance; it indicates a sunward propagation from a source located around midnight. In this case, the time delay is
definitely shorter, of the order of 1 min, from which an azimuthal number m∼1–2 can be estimated. An inspection of IMF data at the Lagrangian
point shows, corresponding to this, a high SW speed and a definitely southward
IMF, suggesting a substorm-related generation mechanism as confirmed also by
high AE values (http://wdc.kugi.kyoto-u.ac.jp/).
From top: variations in the geomagnetic field H
component filtered in the 1–5 mHz frequency range and dynamical power
spectra for the event on 11 March 2008.
Summary and discussion
During the 2007–2008 Antarctic campaign, we installed a magnetometer at
Talos Dome (TLD), a new observation site in Antarctica to extend the
observation facilities in the southern polar cap. The availability of
simultaneous measurements from TNB and SBA allows us to investigate the
azimuthal signal distribution and propagation in that the three stations are
located approximately at the same geomagnetic latitude
(∼ 80∘ S), with approximately 2 h total displacement in
magnetic local time. In this work we present a comparative analysis of
geomagnetic field variations observed at the temporary station TLD and at the
two observatories TNB and SBA; the three Antarctic sites are situated along
the 80∘ S parallel, with ∼ 1 h separation in MLT (actually
70 min for the pair SBA–TNB and 53 min for the pair TNB–TLD). The
analysis is based on measurements recorded during a 2-month campaign at TLD
in the local summer (January–March 2008).
The diurnal variation in the geomagnetic field at TLD shows an amplitude
dependence on the geomagnetic activity level, as previously found at TNB
(Pietrolungo et al., 2008). Its shape is the same at the three stations,
perfectly in phase when considering the X and Y components ordered in LT:
the X component shows a minimum around 13:00 LT and a maximum in the
postmidnight hours; the Y component shows a negative–positive bipolar
behavior around 13:00 LT. When considering the H and D components
ordered in MLT, a slight time shift between the stations emerges; this shift
is much smaller than the one found by Pietrolungo et al. (2008), who
considered stations with a much wider spatial separation. The observed LT
dependence demonstrates that the effects of midlatitude ionospheric currents
extend to such high latitudes, being dominant with respect to the field-aligned currents in determining the diurnal variation in the geomagnetic
field.
The Pc5 fluctuation power at all stations presents the well-known maximum
around local magnetic noon, when the stations approach the polar cusp and the
local field lines are closer to the magnetopause (Lepidi et al., 1996;
Villante et al., 2000; Francia et al., 2005), and a minimum in the magnetic
postmidnight sector. It is worth noting that the daytime Pc5 power maximum
at high-latitude stations could be considered as a marker of the auroral oval
position (Lepidi and Francia, 2003). The maximum around noon extends to the
whole analyzed frequency range (0.6–5 mHz), but the coherence between
pairs of stations is high only at the lowest frequencies, up to
1–1.5 mHz; this result can be interpreted taking into account that these
frequencies are related to the Kelvin–Helmholtz instability on the
magnetopause, which is a large-scale process. On the other hand, fluctuations
at higher frequencies are, more likely, signatures of field line resonances
(FLRs) occurring on lower-latitude closed field lines which each station
approaches at its own local noon (Lepidi et al., 1999; De Lauretis et al.,
2009). Conversely, during the nighttime, when the power is lower, the
fluctuations are coherent independently of frequency; this result can be
explained in terms of substorm-related phenomena (Menk, 2011), which extend
to a large portion of the nightside magnetosphere, as observed in the
11 March 2008 event (Fig. 10), as well as in terms of specific cap activity
which is characterized by low-amplitude, coherent fluctuations (Yagova et
al., 2004).
The analysis of a daytime event shows that for simultaneous long-duration
fluctuations, the amplitude maximizes at each station around local magnetic
noon (i.e., not simultaneously), the propagation direction is antisunward, as
for SW-driven waves (Kepko et al., 2002; Kim et al., 2002), and the estimated
azimuthal wave number m is ∼ 4, in agreement with values found in
previous studies for daytime fluctuations (Lepidi et al., 2011a) and
consistently with the classification of dayside Pc5 resonances with small m
as waves excited by an external mechanism (Glassmeier, 1995; Baker et al.,
2003; Samson, 1991). We note that the 80∘ S stations used in the
present analysis are usually located at the footprint of open field lines, so
they cannot directly observe FLRs; however, the FLR effects occurring at
somewhat lower latitudes can be detected also at such high latitudes around
local noon, when the stations approach the cusp (Lepidi et al., 1999; De
Lauretis et al., 2009). The observational evidence of FLR effects at open
field lines is also discussed by Yagova et al. (2010); of course, the
contribution of the Alfvén FLRs to broadband ULF disturbances in the
dayside polar cap does not imply that all the spectral content is due to
lower-latitude FLRs.
The analysis of a nightside event shows just a few oscillation cycles, with
maximum amplitude at the station which, at the moment, is located at local
magnetic midnight; in this case, the propagation direction is sunward (with an m value ∼ 1–2), consistently with waves originating in the tail and
associated with substorm instabilities (Yagova et al., 2002).
Data availability
TNB data can be downloaded from the INGV web site: http://geomag.rm.ingv.it/index.php.
SBA data can be downloaded from the INTERMAGNET web site: http://www.intermagnet.org.
TLD data can be requested from Stefania Lepidi:
stefania.lepidi@ingv.it.
Acknowledgements
The research activity at Mario Zucchelli station and Talos Dome has been
supported by the Italian PNRA (Programma Nazionale Ricerche in Antartide). The
results presented in this paper rely on the data collected at Scott Base; we
thank the Institute of Geological & Nuclear Sciences Limited (New
Zealand), for supporting its operation and INTERMAGNET for promoting high
standards of magnetic observatory practice
(http://www.intermagnet.org). The
topical editor, G. Balasis, thanks N. Yagova and one anonymous referee for
help in evaluating this paper.
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