ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus GmbHGöttingen, Germany10.5194/angeo-33-395-2015Are dayside long-period pulsations related to the cusp?PilipenkoV.pilipenk@augsburg.eduhttps://orcid.org/0000-0003-3056-7465BelakhovskyV.EngebretsonM. J.KozlovskyA.https://orcid.org/0000-0003-1468-7600YeomanT.Space Research Institute, Moscow, RussiaPolar Geophysical Institute, Apatity, RussiaAugsburg College, Minneapolis, MN, USASodankyla Observatory, Oulu University Branch, Oulu, FinlandUniversity of Leicester, Leicester, UKV. Pilipenko (pilipenk@augsburg.edu)24March201533339540426August20148February20156March2015This 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/33/395/2015/angeo-33-395-2015.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/33/395/2015/angeo-33-395-2015.pdf
We compare simultaneous observations of long-period ultra-low-frequency (ULF) wave activity from a
Svalbard/IMAGE fluxgate magnetometer latitudinal profile covering the
expected cusp geomagnetic latitudes. Irregular Pulsations at Cusp Latitudes
(IPCL) and narrowband Pc5 waves are found to be a ubiquitous element of ULF
activity in the dayside high-latitude region. To identify the ionospheric
projections of the cusp, we use the width of return signal of the Super Dual Auroral Radar Network
(SuperDARN)
radar covering the Svalbard archipelago, predictions of empirical cusp
models, augmented whenever possible by Defense Meteorological Satellite Program (DMSP) identification of
magnetospheric boundary domains. The meridional spatial structure of
broadband dayside Pc5–6 pulsation spectral power has been found to have a
localized latitudinal peak, not under the cusp proper as was previously
thought, but several degrees southward from the equatorward cusp boundary.
The earlier claims of the dayside monochromatic Pc5 wave association with the
open–closed boundary also seems doubtful. Transient currents producing
broadband Pc5–6 probably originate at the low-latitude boundary layer/central plasma sheet (LLBL/CPS) interface, though such
identification with available DMSP data is not very precise. The occurrence
of broadband Pc5–6 pulsations in the dayside boundary layers is a challenge
to modelers because so far their mechanism has not been firmly identified.
The most intense wave activity in the ultra-low-frequency (ULF) frequency range (from fractions of
millihertz to a few hertz) is persistently observed at high latitudes during daytime.
Irregular long-period magnetic pulsations are observed almost every day over
a wide period range (3–20 min) when a ground-based magnetic observatory
happens to be within several hours from local noon (Olson, 1986; McHarg et
al., 1995). These high-latitude disturbances were denoted as IPCL (Irregular
Pulsations at Cusp Latitudes) in Bolshakova et al. (1988) and broadband Pc5
pulsations in Rostoker et al. (1972), McHarg and Olson (1992), Engebretson et
al. (1995) and Posch et al. (1999). The issue of whether these pulsations are
characteristic of some dayside boundary domain/boundary is still unresolved,
though earlier many researchers suggested this type of ULF wave activity as a
possible cusp (or the ionospheric projection of some magnetospheric dayside
boundary layer) discriminant. So, widely used earlier terms “cusp-related
pulsation” or IPCL are likely not adequate for correctly characterizing
these events; the term “broadband dayside Pc5–6 activity” would probably
be more adequate. The driver of these broadband dayside Pc5–6 pulsations has
not been firmly established. These fluctuations were suggested to be
triggered by solar wind variations (Friis-Christensen et al., 1988), sporadic
reconnection (flux transfer events) (Bolshakova and Troitskaya, 1982; Lee and
Lanzerotti, 1986; Lanzerotti and McLennan, 1988), shear instability at the
reversal boundary of ionospheric convection associated with the LLBL (Clauer
and Ridley, 1995; Clauer et al., 1997), or they could be a manifestation of the
intermittency of turbulent solar wind flow (Kurazkovskaya and Klain, 2000).
Broadband Pc5–6 pulsations could be a driving factor for the simultaneous
quasi-monochromatic Pc5 pulsations observed at lower latitudes (Clauer et
al., 1997; Engebretson et al., 1989). Indeed, at high geomagnetic latitudes, Φ∼80∘,
broadband Pc5–6 pulsations are the most common dayside irregular
quasi-periodic disturbances. At the same time, at lower latitudes, Φ∼75∘, intense series of monochromatic Pc5 waves with periods of
3–5 min are observed. The dramatic difference in their appearance is
probably caused by the band-pass filtering due to field-line resonant
response at lower latitudes to a high-latitude driver.
An increase in the intensity of dayside ULF wave activity over a wide
frequency range, from Pc6/Pc5 to Pc3/Pi1 (Lepidi et al., 1996; Engebretson et
al., 2006), is commonly associated with the cusp/cleft region. Early studies
of dayside ULF activity at high latitudes gave hope that long-period
irregular variations were supposedly closely associated with the cusp/cap
interface, and thus could be used as a simple indicator of the dayside cusp
position and the polar cap boundary (Troitskaya and Bolshakova, 1977, 1988;
Troitskaya, 1985; Bolshakova et al., 1988; Dunlop et al., 1994). However,
further studies of high-latitude broadband wave activity on the dayside
showed that it cannot simply be associated with cusp proximity, but, instead,
showed a coordinated time dependence across several hours of local time
(Engebretson et al., 1995). Attempts to find a cause for these widespread
temporal variations in the solar wind, interplanetary magnetic field (IMF), or substorm injections have so
far been fruitless.
The appearance of monochromatic Pc5 pulsations during the morning hours through
local noon at stations at latitudes below 80∘ was associated with
resonant standing Alfvén waves on the last closed field lines, near the
dayside magnetopause (Ables et al., 1998; Lanzerotti et al., 1999). The band
of enhanced wave power evident in dynamic spectrograms had the form of an
“arch”, with the frequency increasing from ∼ 1 to ∼ 4 mHz
during a several-hour interval from early morning toward local noon, then
slowly decreasing until it disappeared in early afternoon. The distinction
between broadband and band-limited wave activity was suggested to
characterize the location of the open–closed field line boundary (OCB). For
synoptic monitoring of the OCB, Urban et al. (2011) used a meridional array of
Antarctic magnetometers, and retrieved a nominal Pc5 mode with periods T∼3–9 min and long-period modes with T>10 min. They assumed that
isolated Pc5 presence corresponds to closed dayside field lines, whereas high
power in both bands indicated that a close magnetotail field line was being
sampled. During quiet periods they observed at the South Pole (Φ∼74∘) U-shaped power spectra, indicating the dominance of long periods
during nighttime and Pc5 dominance during daytime.
So far, the spatial correspondence between a particular dayside boundary
domain and ULF activity peak has not been reliably established. It is not yet
known which magnetospheric boundary region is responsible for long-period
dayside ULF waves: mantle, LLBL, cusp, region I/region II current systems, or
something else. In most previous studies of long-period pulsations there had
been
no available information about the actual instantaneous location of the cusp,
or any other dayside boundary domain, so conclusions about the association of
ULF wave activity with the cusp were made on the basis of the nominal
statistical cusp location. Three data sources have emerged to provide
information on cusp location. First, particle measurements made by
low-orbiting spacecraft (e.g, Defense Meteorological Satellite Program (DMSP), NOAA) provide direct determination of the
ionospheric projections of various magnetospheric boundary domains and hence
provide the benchmark for other observations. However, their usage is
restricted as they only make snapshot observations of the particle
precipitation boundaries in each polar region every ∼ 100 min. Second,
the Doppler spectral width boundary at about 200 m s-1 has been shown
to be a good proxy of the ionospheric projection of the cusp (Baker et al.,
1995). Comparison of the cusp boundary identification with the Super Dual Auroral Radar Network
(SuperDARN) data and with DMSP particle detector showed good correspondence
between those techniques (Cai et al., 2009; Baker et al., 1990). Third, ground optical
observations of the red (630.0 nm) and green (557.7 nm) line aurorae around
noon can give very relevant information on precipitation boundaries and the
location of the cusp (e.g., Milan et al., 1999; Sandholt et al., 2002);
however, this is possible only during polar night (in December and early
January at the Svalbard location) and with clear sky conditions. For the
present study we have no suitable optical data.
List of stations used in the study. Corrected geomagnetic
coordinates and UT of local magnetic noon (MLT) have been computed for an
altitude of 100 km using the NSSDC web facility
(http://nssdc.gsfc.nasa.gov/space/cgm/cgm.html).
110∘ combined IMAGE – Greenland east coast array. StationStation codeGeog. lat.Geog. long.Geom. lat.Geom. long.UT of MLT noonNordNRD81.60343.3380.9105.809:17Ny ÅlesundNAL78.9311.9576.3110.608:56LongyearbyenLYR78.2015.8375.3111.708:50BarentsburgBAB78.0714.2375.3110.408:57HornsundHOR76.9715.4774.2109.109:01Hopen IslandHOP76.5125.1072.9116.508:29Bear IslandBJN74.5019.2071.4108.108:56SørøyaSOR70.5422.2267.2106.708:48
In order to obtain additional clues about the mechanisms of high-latitude
Pc5–6 waves, in this paper we compare this wave activity in the dayside
high-latitude region recorded by the IMAGE and Greenland arrays of
magnetometers with simultaneous monitoring of the cusp location derived from
SuperDARN radar observations. Whenever possible, observations have been
augmented with DMSP data on the ionospheric projections of magnetospheric
boundary domains. Some implications of the observational results are
discussed.
Observational facilities and data analysis technique
Fluxgate magnetometers (10 s sampling rate) are deployed at Svalbard and
throughout Scandinavia in a closely spaced configuration. In Greenland fluxgate
magnetometers are deployed along the west and east coasts. From these arrays
of instruments it is possible to form a latitudinal profile along the
Λ∼110∘ geomagnetic meridian across Svalbard (noon
∼ 09:00 UT,
LT = UT + 3), NRD–NAL–LYR–HOR–HOP–BJN–SOR (shown in
Fig. 1 and Table 1).
Map of the combined IMAGE and Greenland magnetometer array. Beam
no. 9 (green lines) of SuperDARN high-frequency (HF) radar in Hankasalmi (indicated by the
open diamond) covers the Svalbard archipelago.
To identify the ionospheric projections of the cusp, we use data from the
SuperDARN HF radar located in Hankasalmi, Finland. This radar system provides
high time-resolution measurements (120 s in routine operations) of the
ionospheric flow vectors with a spatial resolution on the order of 50 km.
Beam no. 9 covers the Svalbard magnetometers. The SuperDARN radar technique
has been successfully used to identify the relative location of dayside
high-latitude wave activity in the Pc3 band (Yeoman et al., 2012).
Statistical models driven by the IMF/SW parameters nowcast the cusp
boundaries with correlation coefficients up to 0.8. The highest correlation
is provided by the “half-wave rectifier” model and the Newell et al. (2006)
model. The latter will be used to additionally estimate the cusp
location. According to this model, the equatorward cusp boundary is predicted
to be ∼ 78∘ when IMF Bz∼ 0, and should shift
equatorward when IMF Bz< 0.
A magnetic local time–magnetic latitude snapshot of the
line-of-sight Doppler velocity and spectral width from the Hankasalmi
SuperDARN radar from 03:00 to 15:00 UT on 4 January 2002.
To categorize the ionospheric projection of the magnetospheric boundary
layers, the low-altitude (∼ 800 km) DMSP charged-particle precipitation
characteristics have been used (Newell and Meng, 1988). The automated dayside
region identification program
(http://sd-www.jhuapl.edu/Aurora/dayside/dayside.html) distinguishes
magnetospheric regions through the characteristics of precipitating electrons
and ions (in the 30 eV to 30 keV range).
Typical events
From radar campaigns in 2001–2002, we have identified > 10 ULF events
when it was possible to determine the cusp location from the radar data. We
consider only daytime hours to avoid the contamination of ULF activity by
disturbances related to nighttime substorms. Here we present two typical
events.
4 January 2002 (day 004)
According to HAN radar data, both the ionospheric velocity pattern and the
spectral width of radio signal returns indicate that the cusp equatorward
boundary was observed on this day from ∼ 07:30 to ∼ 11:00 UT.
According to the OMNI database the mean IMF and solar wind parameters for the
period 07:00–10:00 UT were: V∼330 km s-1, N∼6 cm-3, By∼-5 nT, and
Bz was varying between +1 and -4 nT. During this period, the boundary
was on average around 76∘ (Fig. 2); however, it was moving north and
south within this interval following the variability (fluctuations) of the
IMF parameters (not shown here). For an averaged Bz=-3 nT, the
statistical model predicts the cusp boundary location to have been at
∼ 76∘.
Along the latitudinal magnetometer array, quasi-periodic variations with timescales from ∼ 7 to ∼ 15 min were observed with peak-to-peak
amplitudes larger than 10 nT (Fig. 3). These variations are most evident in
the X component from ∼ 08:00 to ∼ 10:00 UT at HOP and BJN.
Fourier spectra for the time interval 07:00–10:00 UT show the dominance of
frequencies ∼ 0.6 and ∼ 1.5 mHz (not shown). Visual inspection
of waveforms demonstrates that they were time shifted between different
latitudes, indicating an apparent poleward propagation. This result has been
validated with cross-correlation analysis for the period 07:00–10:00 UT
using NAL as a reference station. Indeed, the time lag Δt of the
cross-correlation function R(Δt) varied from ∼ 10 s for LYR to
∼ 60 s for BJN (not shown).
Magnetograms of X component magnetic variations along the
IMAGE + Greenland 100∘ magnetic longitude profile from 06:00 to
12:00 UT on 4 January 2002.
The latitudinal distribution of X component spectral power for the time
period 07:00–10:00 UT integrated in the 0.4–3.0 mHz band shows a maximum
around 73–74∘ (HOP-HOR) (Fig. 4). Thus, the ULF maximum is shifted
∼ 2∘ equatorward from the equatorward cusp boundary (shown by
the vertical dashed line). The vertical Z component, on the other hand, has
a local minimum in the region of maximal X component power, and increases
towards higher latitudes.
The latitudinal distribution of band-integrated (0.4–1.7 mHz)
hourly spectral power in the IPCL band for the time interval 06:00 to
12:00 UT 4 January 2002. The vertical dotted line (red) indicates the
equatorward cusp boundary location.
The superposition of all DMSP-identified boundary domain projections from
available satellite tracks and station locations during the period of cusp
observations by radar, i.e., 07:00–10:00 UT, is shown in Fig. 5. This
comparison shows that the latitudinal maximum of the broadband Pc5–6 power
(highlighted by the solid triangle) corresponds to the projection of the
LLBL.
The superposition of the DMSP-identified boundary domain ionospheric
projections in the dayside hemisphere and station locations during the period
of cusp observations by radar, from 07:00 to 11:00 UT (in geomagnetic
coordinates). The station with maximal ULF power is marked by a solid
triangle.
9 December 2001 (day 343)
SuperDARN radar identified a slow equatorward drift of the equatorward cusp
boundary near ∼ 78∘ for the period from ∼ 07:30 to
∼ 11:00 UT (Fig. 6). According to the OMNI database, from
07:00 to 10:00 UT the IMF Bz component was slightly negative
(∼ 0/-2 nT), and the By component gradually changed from -4 to
0 nT. The solar wind velocity was ∼ 370 km s-1 and the plasma density ∼ 2–3 m-3.
The statistical position of the equatorward cusp boundary estimated from the
model (Newell et al., 2006) corresponds to the SuperDARN observations.
A magnetic local time–magnetic latitude snapshot of line-of-sight
Doppler velocity and spectral width from the Hankasalmi SuperDARN radar from
03:00 to 15:00 UT 9 December 2001.
Intense irregular pulsations were observed along the Svalbard array from
∼ 06:30 to ∼ 09:30 UT (Fig. 7). In their broadband spectra, the
frequencies ∼ 0.4 mHz and ∼ 1.2 mHz are highlighted (not shown).
Visual inspection of X component magnetograms indicates an apparent
poleward propagation. This conclusion is supported by a cross-correlation
analysis for the period 07:00–10:00 UT using NAL as a reference station,
which shows that the time lag Δt of the cross-correlation function
R(Δt) varied from ∼ 50 s for LYR to ∼ 330 s for BJN.
The latitudinal distribution of band-integrated (0.4–3 mHz) X component
spectral power (Fig. 8, top panel) had its maximum around 74∘ (HOR). This maximum is
∼ 4∘ equatorward from the cusp boundary. At the same time, the
latitudinal distribution of the Z component power (Fig. 8, bottom panel)
had a local minimum nearly in the same region, ∼ 73∘, and two
maxima away from it. Visual inspection of waveforms shows that the
Z component variations in the maxima (NAL–LYR and BJN) were roughly in
anti-phase to each other (not shown).
The superposition of the DMSP-identified boundary domain projections and
station locations during the period of Pc5–6 pulsation occurrence, i.e.,
06:00–10:00 UT (Fig. 9), shows that the latitudinal maximum of the
broadband dayside Pc5–6 power was inside the CPS.
Magnetograms of the X component along the IMAGE profile from 06:00
to 12:00 UT on 9 December 2001. Data from NRD are missing.
The CGM latitudinal distribution of band-integrated (0.7–6.0 mHz)
hourly spectral power for the time interval 05:00 to 10:00 UT
9 December 2001. The vertical dotted line (red) indicates the equatorward
cusp boundary location, identified from SuperDARN data.
Interplanetary parameters and cusp location during selected events.
DateUTVsw,N,Bz,By,SuperDARNULF-maxkm s-1cm-3nTnTcusp (Lat)(Lat)2 Feb 200106.30–09.3042040; +227673.17 Dec 200107.00–10.0046035; 0-57874.125 Jan 201108.00–11.003206-30; -27573.119 Dec 200909.00–12.0042040; -2-37773.119 Feb 201007.00-10.004601–22; -467873.126 Jan 201107.30–10.3031082; -1-2; 07774.18 Feb 201107.00–10.004303–4-2; 0-27674.123 Jan 201007.00–10.003605-22; 07674.1Statistical latitudinal distribution
The normalized statistical distribution of X component broadband Pc5–6
spectral power in the 0.4–3.0 mHz band for the eight most intense ULF events
observed along the Λ∼110∘ latitudinal profile with
respect to the distance from the cusp boundary, identified for each event
from SuperDARN observations, is shown in Fig. 10. Table 2 summarized the
interplanetary parameters and the cusp location information during each
event. It shows that all events were observed during low-to-moderate solar
wind velocities of 310–460 km s-1. Though the individual distributions
are widely dispersed, the summary plot evidently shows that broadband dayside
Pc5–6 X component power is shifted by 1∘–5∘ from the cusp
equatorward boundary.
The superposition of the DMSP-identified boundary domain projections
in the dayside hemisphere and station locations during the period of cusp
observations by radar, from 07:00 to 10:00 UT 9 December 2001 (in polar
geomagnetic coordinates). The station with maximal ULF power is highlighted
by a solid triangle.
Discussion
Long-period broadband pulsations in the nominal Pc5–6 band have been known
to be a persistent feature in the ULF activity at dayside high latitudes. At
high latitudes during daytime, a mixture of broadband Pc5–6 and narrowband
Pc5 is observed, though there is no well-established criterion to separate
these phenomena. In the events under consideration a regular transition from
irregular broadband pulsations at high latitudes to more intense and
monochromatic Pc5 pulsations at lower latitudes can be seen, similar to
Engebretson et al. (2002). These events indicate that quasi-monochromatic Pc5
and broadband pulsations are not separate wave phenomena, but the
manifestations of the same wave process, whereas the difference in their
appearance is related to resonant amplification deeper into the
magnetosphere, probably on closed dipole-like field lines. It is natural to
suppose that broadband Pc5–6 corresponds to a source, and Pc5 pulsations are
its resonant response (Clauer et al., 1997). However, there is no one-to-one
correspondence between their occurrence: sometimes one can see broadband
Pc5–6 without a Pc5 response at lower latitudes, and sometimes one can see
monochromatic Pc5 without broadband Pc5–6 at higher latitudes (Kleimenova et
al., 1985, 1998). Thus, it is still uncertain whether the excitation
mechanisms of these pulsations are firmly coupled or not. It is likely that
there are a variety of source mechanisms for these pulsations and until the
driving conditions are sorted properly, the situation will seem confusing.
The statistical distribution of IPCL spectral power (X component)
for 8 events along the NAL–LYR–HOR–HOP–BJN–SOR profile with respect to the
distance from the equatorward cusp boundary. All the spectra have been
normalized by their maximal value.
While narrowband Pc5 pulsations are evidently produced by the resonant response
of closed field lines inside the magnetosphere, it is tempting to associate
broadband dayside Pc5–6 pulsations with disturbances at the magnetosphere
boundary. In this case, on the ground the “epicenter” of their activity is
to be observed near the ionospheric projection of the equatorward boundary.
However, our examination of the local latitudinal distribution of
band-integrated ULF power in the Pc5–6 range with respect to the equatorward
cusp boundary has shown that in fact “cusp pulsations” are not related to
the cusp proper. The peak of their power distribution is shifted several
degrees southward from the equatorward cusp boundary, identified from
simultaneous SuperDARN observations. Moreover, the longitudinal extent of
pulsations observed on the ground is wider than the cusp. Commonly,
pulsations along the Svalbard array that begin earlier than the time that the cusp is detected by radar. It is worth mentioning the parallel story about high-latitude
dayside Pc3 waves. They were also at first thought to be closely associated
with the cusp, but as Yeoman et al. (2012) showed, they instead maximize
∼ 2∘ equatorward of it. This justifies the usage of more
adequate term “broadband dayside Pc5–6 activity” instead of “cusp-related
pulsation” or IPCL. Broadband dayside Pc5–6 activity is probably comprised of
multiple generation mechanisms, so our conclusions may be invalid for
other classes of high-latitude ULF pulsations. In general, the physics of
these pulsations will remain confusing until the conditions under which they
are produced are sorted properly.
A related question is the location of the observed maximum of pulsation
activity with respect to the OCB. Indeed, Lockwood (1997) pointed out that
the OCB can be located equatorward of the cusp. Such a precipitation regime
is identified as the open LLBL adjacent the cusp (Newell and Meng, 1998).
According to the Lockwood (1997) model newly open field lines are
characterized by the disappearance of CPS electrons giving rise to a void
regime between the CPS and the LLBL (open field lines), and this model
obtained experimental support for cases of southward IMF (Sandholt and
Farrugia, 2002; Sandholt et al., 2002, 2004). Statistically, the open LLBL
has sizes on the order of 1 h in MLT and 1∘ in MLat (Newell et
al., 2004). The largest separation in MLat between the spectral width
boundary and the OCB is expected in intervals of Bz-dominated (|Bz/By|>1; Bz<0) IMF orientations (Lockwood, 1997).
However, in the present study all the cases are characterized by |Bz/By|≤1, so we may hardly expect a substantial (>1∘) width of
open LLBL for 3 h. Because the maximum of pulsation power was well more than
2∘, poleward of the spectral width boundary, we believe that the
maximum was predominantly on closed magnetic field lines.
Here it is necessary to mention that identification of the cusp from ground
radar data is not a straightforward problem. The broadening of spectral lines
in SuperDARN radar backscatter is frequently considered as a signature of the
cusp (e.g., Chisham et al., 2005), though in some cases, especially during
northward or near-zero IMF Bz (Safargaleev et al., 2008), a large spectral
width was observed on both open and closed field lines. The region of
increased spectral width may also include closed LLBL or other turbulent
precipitation regions, which has been shown by Villain et al. (2002). Moreover,
Kozlovsky et al. (2011) have shown that spectral width increase up to
200 m s-1 can be due to the gradient drift instability cascade in the
regions of enhanced electric field (up to 50 mV m-1). Such electric
fields are indeed observed equatorward of the cusp on closed field lines (see
e.g., Figs. 4 and 8 in Sandholt and Farrugia (2007). Thus, the latitude of
equatorial cusp boundary derived from the spectral width data may be
underestimated. This is one more argument for the location of pulsation
maxima well equatorward of the cusp, on closed magnetic field lines.
Additionally, we tried to use data from the SuperDARN radar in Goose Bay to
check the cusp location above the Greenland west coast magnetometer profile.
However, the cusp signatures at this radar were not very clear. The
difference in the cusp detection by SuperDARN radars in Hankasalmi and Goose
Bay is probably related to their different beam orientation to the cusp. Additionally,
we tried to use particle precipitation data from DMSP satellites, which are a
reliable means to obtain the cusp location (Safargaleev et al., 2008). Though
for all the analyzed events the DMSP tracks were rather far from the meridian
under examination, a tendency was observed that maximal ULF activity was
situated around the CPS/LLBL interface, though sometimes shifted into the
region of the ionospheric projection of the CPS or LLBL. This is consistent
with the Clauer and Ridley (1995) case study of Pc6 (∼ 20 min) pulsation
observed with maximum amplitude at the convection reversal boundary
determined by incoherent scatter radar. It was reliably shown that the
convection reversal boundary is in the region of LLBL particle precipitation
measured by DMSP satellites. Certainly not all dayside ULF waves result from
this source, but this is certainly one definite source of the broadband
Pc5–6 pulsations linked to a specific boundary.
The poleward Pc5–6 propagation observed near cusp latitudes, earlier noticed
by Olson (1989), indicates that ground ULF variations cannot simply be
attributed to a quasi-stationary oscillatory current system. The latitudinal
phase delay can be caused by several mechanisms:
peculiar frequency-dependent amplitude-phase spatial structure formed
in the region of Alfvén field line resonance. Such structure is typical for
narrowband Pc5 pulsations and gives evidence that their excitation mechanism
is related to the resonance of standing Alfvén oscillations. Another feature
of this resonant structure is the maximum in latitudinal distribution on the
ground of the vertical Z component amplitude coinciding with the maximum of
the horizontal N–S component (Southwood and Hughes, 1978; Pilipenko et al.,
2000a). This feature is not seen in the observed latitudinal structure of the
broadband Pc5–6 power; the opposite, in fact, was observed: in the region of
maximal H component power, a local minimum of Z component power was
observed. Thus, broadband dayside Pc5–6 cannot be associated with field line
Alfvén resonance;
latitude-dependent time-of-flight propagation from the equatorial plane
towards the polar ionosphere. In this case, the Z component amplitude has a
minimum and phase reversal in the region of the N–S component amplitude
maximum (Pilipenko et al., 2000b). The observed broadband Pc5–6 latitudinal
structure with a localized maximum of coherent N–S components and
out-of-phase vertical components beyond this maximum probably corresponds to
the variations produced by fluctuations of E–W extended currents transported
by latitude-dependent field-aligned currents from the magnetosphere.
It may be supposed that broadband Pc5–6 activity is the ground manifestation
of turbulent flow around the magnetosphere. Moreover, the clear dependence of
the intensity of long-period dayside pulsations at high latitudes on the
solar wind velocity (Engebretson et al., 1995) favors this suggestion.
However, one aspect of the scenario must be understood. A turbulent magnetosheath
plasma flow should provide a maximal ground response around the ionospheric
projection of the magnetopause, whereas the periodicity of Pc5–6
oscillations may be imposed by the very extended non-dipole field line
length. However, our study has found that the epicenter of Pc5–6 power is
located inside the magnetosphere, probably around the CPS/LLBL interface or
inside the auroral oval. Therefore, it still has to be comprehended why the
response to the magnetosheath turbulence driving is displaced from the
magnetosheath/magnetosphere interface. Moreover, dayside narrowband Pc5
pulsations, located even deeper in the magnetosphere than broadband Pc5–6,
can hardly be associated with oscillations of last closed field lines, as had
been suggested by Lanzerotti et al. (1999) and Urban et al. (2011). It is
worth noticing that in the same period scale as Pc5–6 pulsations,
there is a class of impulsive/transient disturbances, traveling convection
vortices (TCVs), which are commonly observed in the prenoon sector. Comparison
of TCV events with the particle precipitation boundaries obtained from low-altitude DMSP satellite measurements indicated that TCVs mapped to the
CPS/boundary plasma sheet (BPS) boundary well within the magnetosphere (Yahnin et al., 1997),
contrary to the belief that TCV-related currents originated at the
magnetopause. However, they did not suggest a mechanism for TCV location.
We are not aware of detailed in situ observations of a magnetospheric
counterpart of persistent dayside high-latitude ULF activity in the Pc5–6
band. Only in one event, when the magnetosphere was strongly compressed and a
number of geosynchronous satellites happened to be in the LLBL, Takahashi et
al. (1991) observed 5–10 min irregular compressional oscillations.
Moreover, we suppose that prospects to find such a counterpart in the
equatorial magnetosphere are not very promising because broadband dayside
ULF wave power over a range of longitudes across local noon, near the
footpoint of the cusp/cleft/LLBL are predominantly nonconjugate (Engebretson
et al., 2002).
As another possibility, it may be suggested that broadband Pc5–6 are related
to quasi-periodic fluctuations of field-aligned currents or to irregular
particle precipitation. Indeed, at dayside high latitudes there are several
regions of field-aligned currents, strongly dependent on the IMF components
Bz and By. Region I corresponds to the equatorward boundary of the
polar cusp, where currents flow into the ionosphere in the morning side and
from the ionosphere in the afternoon side. Region III (or NBZ) is
concentrated at the poleward border of the cusp, with current direction
opposite to that in region I. The upward field-aligned currents are
transported by precipitating electrons, while downward currents are due to
upward-moving thermal ionospheric electrons. It is interesting that in nearly
half of the events considered here, the onset of the cusp-related ULF
pulsations is marked by an isolated bay-like disturbance, in accordance with
earlier observations by Olson (1986), which may be a signature of a
large-scale current system. However, we are unable to verify
a possible correspondence between ULF wave power and global
magnetosphere–ionosphere current systems because of a lack of reliable
monitors of the field-aligned current system above the magnetometer array
(e.g., low-orbiting satellites with magnetometers onboard). Nonetheless, we
do not believe that high-latitude broadband geomagnetic pulsations can be
attributed to the fluctuating energetic electron precipitation. Such
precipitation would barely produce a coherent latitudinally phase-shifted
magnetic response on the ground, which observations have clearly revealed.
Conclusions
Dayside high-latitude ULF pulsations will still continue to be difficult to
understand until they are sorted into proper classes. Examination of the
local latitudinal structure of high-latitude pulsations by magnetometers at
Svalbard and Greenland, covering near-cusp latitudes, has shown the
occurrence of a localized peak in the distribution of broadband dayside
Pc5–6 power about 3∘± 2∘ southward from the
equatorward cusp boundary, but not under the cusp proper as was suggested in
early studies. For the dayside high-latitude ULF events examined in this
paper, broadband Pc5–6 pulsations, and moreover narrowband dayside Pc5
waves, are unlikely to be associated with oscillations of last closed field
lines. The broadband dayside Pc5–6 pulsations are supposed to be the ground
response to transient currents, probably originating at the LLBL/CPS
interface, though such identification with available DMSP data is not very
precise. The obtained result imposes important limitations on possible
mechanisms of high-latitude ULF variations. In general, dayside high-latitude
ULF waves still remains a zone of confusion.
Acknowledgements
This work was supported by grant MK-4210.2015.5 of the President of the
Russian Federation (V. Belakhovsky), grant 14-05-00588 from the Russian Fund for Fundamental Research
(V. Pilipenko, T. Yeoman), and US NSF grant ATM-0827903 (M. J. Engebretson).
The facility in Ny Ålesund is maintained by the Norwegian Polar Institute
(www.npolar.no); Longyearbyen is supported by the Kjell Henriksen
Observatory (http://kho.unis.no); Hornsund is maintained by the Polish
Institute of Geophysics, the IMAGE magnetometer array is maintained by
Finnish Meteorological Institute (www.geo.fmi.fi/image), and the
Greenland magnetometer array is operated by the Danish Space Research
Institute (www.space.dtu.dk). The DMSP particle detectors were designed
by D. Hardy of AFRL, and data obtained from JHU/APL
(http://sd-www.jhuapl.edu/Aurora). We thank D. Hardy, F. Rich, and
P. Newell for its use. We thank all participants of the SuperDARN project
(www.superdarn.ac.uk), who collected data used in this study. The
constructive criticism of the reviewers is appreciated. Topical Editor L. Blomberg thanks two anonymous referees
for their help in evaluating this paper.
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