Introduction
Space weather impacts the operation of modern technology that relies on
global navigation satellite systems (GNSS) including GPS, GLONASS and Galileo, which have become indispensable in
precise positioning and time keeping. Ionospheric irregularities cause rapid
fluctuations of radio wave amplitude and phase that can degrade GPS
positional accuracy and affect performance of radio communication and
navigation systems. Variable ionospheric delays and scintillation cause
cycle slips, which can lead to loss of lock and affect the performance of
radio communications and navigation systems (Aarons, 1982, 1997; Aarons et
al., 2000; Basu et al., 1987, 1995, 1998). In spite of improved
understanding of ionospheric irregularities, scintillation is difficult to
predict for a given ray path because of the complexity of ionospheric
irregularity generation, coupling of the solar wind magnetic and electric
fields to the ionosphere and dynamic convection of ionospheric plasma.
Statistical (empirical) models of scintillation that have been developed for
the equatorial and high-latitude ionospheres (Secan et al., 1995, 1997) used
data from past satellite experiments (Wideband, HiLat and Polar BEAR)
covering frequencies from VHF to L-band, although the modeling was based on
VHF data only. More recently, using satellite in situ measurements of plasma
(electron) density fluctuations, the International Reference Ionosphere (IRI) model and the phase screen
propagation model, a scintillation climatology model for the high-latitude
ionosphere was developed (Wernik et al., 2007).
Canadian High Arctic Ionospheric Network (CHAIN): the GPS
Ionospheric Scintillation and TEC Monitors and Canadian Advanced Digital
Ionosondes (CADIs). The corrected geomagnetic (CGM) latitudes 70
and 80∘, in yellow, are superposed over the geographic grid.
With a growing number of GNSS Ionospheric Scintillation and TEC Monitors
(GISTMs) operating at high latitudes, it has become possible to build a
detailed scintillation climatology from accumulated scintillation data in
the auroral and polar regions (Spogli et al., 2009; Li et al., 2010; Alfonsi
et al., 2011; Prikryl et al., 2011a; Moen et al., 2013). Spogli et al. (2009) and Alfonsi et al. (2011) were the first to describe the essence of what
they called the “Ground-Based Scintillation Climatology (GBSC)” method, laying
down the basis how to obtain maps, in geographic and geomagnetic
coordinates, of scintillation occurrence and total electron content (TEC) variations observed by a
network of GISTMs, to investigate dependencies on interplanetary magnetic
field (IMF) orientation, geomagnetic activity and season and solar cycle. In a
comprehensive study of space weather impact on the cusp and polar cap
toward understanding plasma instabilities and scintillation in
association with cusp flow channels and polar cap patches, Moen et al. (2013)
applied the GBSC method to investigate the multi-scale irregularity
structures resulting from different physical processes that include flow shears and
particle precipitation. As these authors pointed out, the phase and
amplitude scintillation are biased to different irregularity scale sizes
from a few kilometers down to a few hundred meters, which are produced by
different processes.
An empirical climatology model of high-latitude GNSS scintillation appears
to be the next step to address the questions of forecasting and mitigation
of L-band scintillation at high-latitudes (Decker and Pedersen, 2001; Aquino
et al., 2007; Strangeways et al., 2011, Moen et al., 2013). One of the
ionospheric scintillation models that has been developed is WBMOD (WideBand
MODel), based on analysis of large databases of VHF scintillation
measurements (Secan et al., 1995, 1997;
http://spawx.nwra.com/ionoscint/wbmod.html). Béniguel and Hamel (2011)
formulated of a wave propagation model for equatorial regions. This global
ionospheric scintillation propagation model (GISM) aims to estimate various
radio propagation effects including phase and amplitude scintillation. The
GNSS receiver tracking performance during severe scintillation conditions
can be assessed by the analysis of receiver phase-locked-loop (PLL) jitter
(Conker et al., 2003; Sreeja et al., 2011; Aquino and Sreeja, 2013, Prikryl
et al., 2013a). Another method to mitigate the effect of ionospheric
scintillation using TEC (total electron content) at 1 Hz was described
by Tiwari and Strangeways (2015).
At high latitudes, statistical characterization and climatology of
scintillation of GPS signals show prevalence of phase over amplitude
scintillation (Spogli et al., 2009; Li et al., 2010; Prikryl et al., 2011a, b, 2013b; Jiao et al., 2013; Moen et al., 2013). The general lack of
amplitude scintillation (S4 index) at high latitudes has been attributed
to a commonly used filter to detrend high-rate GPS data when amplitude and
phase scintillation indices S4 and σΦ (Beach, 2006;
Forte, 2007) are computed. Different filters and indices have been proposed
to alleviate this problem (Forte, 2005; Zhang et al., 2010; Mushini et al.,
2012). However, the phase scintillation index σΦ has been
widely used to characterize phase scintillation and is found to be correlated with
proxy phase scintillation indices that can be obtained from 1 Hz data by
geodetic-quality GPS receivers (Ghoddousi-Fard et al., 2013). In this paper
we use phase scintillation index σΦ obtained by the
Canadian High Arctic Ionospheric Network (CHAIN) to extend the initial phase
scintillation climatology study (Prikryl et al., 2011a) over a period of 6
years, from 2008 to 2013.
Instruments and data
Phase scintillation data used in this study were collected by ten
specialized GISTMs of
CHAIN (http://chain.physics.unb.ca/chain) (Jayachandran et al., 2009). CHAIN
GISTMs and ionosondes are distributed in the auroral oval, cusp and the
polar cap (Fig. 1). A table of CHAIN stations' geographic and geomagnetic
coordinates of stations can be found in Prikryl et al. (2011a).
The NovAtel OEM4 GSV 4004B dual-frequency GPS receivers (Van Dierendonck and
Arbesser-Rastburg, 2004), with special firmware specifically configured to
record the power and phase of the L1 signal at a 50 Hz sampling rate, compute
the ionospheric total electron content (TEC) using both L1 and L2 signals,
the amplitude scintillation index S4 and the phase scintillation index
σΦ. The phase scintillation index σΦ is
the standard deviation of the detrended phase using a filter in the receiver
with a 0.1 Hz cutoff frequency.
To characterize the space weather climate, additional parameters such as Kp
index and solar wind measurements are used. The Kp index of geomagnetic
activity that has been derived and distributed by the German Research Centre
for Geosciences was obtained through http://wdc.kugi.kyoto-u.ac.jp/kp/. Solar wind data are obtained from the
Goddard Space Flight Center Space Physics Data Facility OMNIWeb (http://omniweb.gsfc.nasa.gov/). This data set provides time series of
magnetic field and plasma parameters projected to the nose of the Earth's
bow shock to accommodate for propagation delays from the spacecraft. In this
paper, hourly values of the IMF BY and
BZ components are used to investigate scintillation dependence on the
polarity of the IMF components. Other solar wind parameters are used to
identify arrival times of high speed streams (HSSs). Following the criteria
of Prikryl et al. (2012), co-rotating interaction regions (CIRs) at the
leading edges of HSSs were determined. For arrival times of interplanetary
coronal mass ejections (ICMEs), a catalogue of near-Earth ICMEs (Richardson
and Cane, 2010) that is updated at http://www.srl.caltech.edu/ACE/ASC/DATA/level3 was
used.
Phase scintillation climatology
The phase scintillation occurrence is represented as a function of magnetic
local time (MLT) and the altitude-adjusted corrected geomagnetic
(AACGM) latitude (Baker and Wing, 1989; Shepherd et al., 2014). In this paper, the scintillation data are
merged on a grid with bins of 1 h MLT × 2.5∘ CGM
latitude. Although the altitude of ionospheric irregularities causing
scintillation varies and is unknown in general, we assume ionospheric pierce
point (IPP) at 350 km altitude. The scintillation occurrence is simply
defined as 100 × N(σΦ > 0.1) /
Ntot, where N is the number of cases when phase scintillation
index exceeded a given threshold and Ntot is the total number of
data points with IPPs in the bin. Bins with sparse statistics are removed and
marked as grey areas in the plots as done by other authors (Spogli et al.,
2009). Here we adopted a relatively low σΦ threshold of 0.1
radians (weak scintillation). To minimize the multipath effect, only
elevations exceeding 30∘ are used. The values of σΦ are
projected to the vertical to account for geometrical effects on the
measurements made at different elevation angles (Spogli et al., 2009; their
Eq. 1). The scope of the present paper does not allow enough space to
consider higher thresholds that would specifically characterize moderate and
strong scintillation. For typical scintillation strength in various regions
of the high-latitude ionosphere including cusp, auroral oval and polar cap, we
refer the reader to case studies (e.g., Jin et al., 2014; Prikryl et al.,
2015a, b, c). It should be noted that proxy scintillation indices such as the
rate of TEC index (ROTI) (Jacobsen and Dähnn, 2014) and disturbance
ionosphere index (DIX) (Jakowski et al., 2012) could be used instead of
σΦ to map scintillation with comparable results. Also, GPS
climatology of scintillation and TEC in the Southern Hemisphere (Spogli et
al., 2013) and interhemispheric comparison is of interest for future
studies.
The 2008–2013 phase scintillation occurrence maps for
geomagnetically (a) quiet and (b) disturbed days, and for (c) CIR/HSS and (d) ICME days.
The yearly phase scintillation occurrence maps for geomagnetically
disturbed days.
Phase scintillation occurrence maps (hIPP= 350 km) for (a) autumn, (b) winter, (c) spring and (d) summer.
Regions identified as cusp (cu), polar cap (pc), nightside auroral oval (au)
and post-midnight auroral/subauroral region (sa) are outlined in gray dashed
lines in panel (a).
Phase scintillation occurrence maps (hIPP= 110 km) for (a) autumn, (b) winter, (c) spring and (d) summer.
Monthly variation of phase scintillation occurrence in cu, au, pc and
sa sectors for (a) hIPP= 350 km and (b) hIPP= 110 km.
Scintillation occurrence dependence on geomagnetic activity
Figure 2a and b show the mean occurrence of phase scintillation with
σΦ exceeding 0.1 radians as a function of MLT and magnetic
latitude for 1858 quiet and 334 disturbed days selected over a period of 6 years. The scintillation statistics were divided into two parts,
geomagnetically quiet and disturbed days, using a 3-hourly Kp index level over
24 h with a disturbed day defined as having a Kp index greater than 2
for more than 60 % of the day. Superposed on the maps are the boundaries
of the Feldstein statistical auroral oval for a quiet and moderately disturbed
conditions (Feldstein and Starkov, 1967; Holzworth and Meng, 1975). The control parameter for the Feldstein model is the
index Q (IQ) ranging from 0 to 6 for quiet to active oval. The scintillation
occurrence during quiet conditions is confined in a smaller and narrower
area with relatively low percentages. In general, under disturbed
conditions, the scintillation occurrence is at least 10 % higher and the
scintillation area becomes larger and broader when the auroral oval expands.
In both cases, as previously observed (Spogli et al., 2009; Prikryl et al.,
2011), there are two principal regions where scintillation occurs most
frequently: the cusp with extension into dayside polar cap and the nightside
auroral oval. For the disturbed days, the area of enhanced scintillation
occurrence extends equatorward of the statistical oval particularly in the
post-midnight to noon sector. This enhancement can be attributed to the
equatorward shift of the active auroral oval with a possible addition of
sub-auroral polarization streams (SAPS) and storm-enhanced density (SED)
that are more frequent during disturbed periods (Foster and Burke, 2002;
Foster et al., 2004; Clausen et al., 2012; Prikryl et al., 2015c).
To consider the impact of specific solar wind disturbances on scintillation
occurrence, we compiled 137 days of major CIR/HSSs arrival (stream maximum
velocity VMAX exceeding 500 km s-1) and 55 start days of ICME
disturbances, for which the geomagnetic storm intensity characterized by Dst
index was less than -30 nT. Because some of these time arrivals may
occurred at later UT hours on the day of arrival (day 0), we added the
following day (day 1) to the statistics to compute the mean occurrence of
phase scintillation for CIRs and ICMEs (Fig. 2c and d). The
scintillation occurrence for ICME days is significantly higher than for CIR
days as previously shown using a superposed epoch analysis (Prikryl et al.,
2014). In particular, there is significantly higher scintillation occurrence
equatorward of the statistical auroral oval. This can be attributed to most
intense events when auroral oval expanded further equatorward and SAPS/SED
events occurred (Prikryl et al., 2013b, 2015c), which is more often the case
during ICME disturbances. On the other hand, CIR/HSSs are known to be
permeated with high-amplitude solar wind Alfvén waves coupling to the
dayside magnetopause and producing copious patches (Prikryl et al., 1999,
2015a). Polar patches are the main cause of scintillation in the polar cap
but also in the nightside auroral oval as they evolve into auroral blobs
(Jin et al., 2014).
The sub-auroral scintillation was less likely during the past solar minimum
2008–2009 (Prikryl et al., 2011), even when extended by 1 more year to
2008–2010 (Prikryl et al., 2012). In the present data analysis we included 3
more years spanning the rising solar activity up to the current, though
relatively low, solar maximum. Figure 3a–d show yearly maps of the mean
occurrence of phase scintillation for disturbed days from 2010 to 2013.
Approaching the solar maximum, the scintillation occurrence progressively
increased in all regions, and the total area of enhanced scintillation
occurrence extended further equatorward and poleward into the polar cap. As
already stated for ICME days, this is a result of more frequent expansion of
the
auroral oval during intense auroral events that can include SAPS events and
increased activity in the polar cap including the tongue of ionization (TOI)
that is drawn from dense SED plasma producing patches of enhanced plasma
density. In case studies, weak-to-moderate scintillation was found at
sub-auroral latitudes, collocated with SAPS, and events of moderate-to-strong scintillation associated with TOI and polar patches were observed in
the polar cap (Prikryl et al., 2013b, 2015a, b, c).
Seasonal dependence of scintillation occurrence
The scintillation data are divided into four 3-month intervals that are
approximately centered on equinoxes and solstices to examine the seasonal
variation of the mean scintillation occurrence maps (Fig. 4a–d). Regions
identified as cusp (cu), polar cap (pc), nightside auroral oval (au) and
post-midnight auroral/subauroral region (sa) with possible contribution from
SAPS are outlined in gray dashed lines in Fig. 4a. The cu sector stretches
between 9 and 15 MLT, and between 72.5 and 80∘ CGM
latitude, the pc region is above 75∘ CGM latitude with exclusion
of the cusp, the au sector lies between 19 and 2 MLT, and between
65 and 75∘ CGM latitude, while the sa belt lies
between 0 and 7 MLT and is defined here by two pairs of latitude ranges
between 57.5 and 65.0∘ CGM and between 60.0
and 67.5∘ CGM latitude. The highest scintillation occurrence is
observed in the cusp and the dayside polar cap in the winter months (Fig. 4b). The scintillation occurrence in the nightside auroral oval, at the
poleward edge in particular, is highest in autumn (Fig. 4a) and spring (Fig. 4c). The scintillation region in the nightside auroral oval shows a clear
dawn–dusk asymmetry (predominance of scintillation in pre-midnight hours) in
all seasons. The sa belt of scintillation is most likely associated with fast convection in
the expanded dawn convection cell with possible contribution from SAPS is
noticeable in all seasons but is weakest in the winter (Fig. 4b).
So far we assumed an IPP altitude of 350 km for scintillation mapping. This
is plausible in the polar cap and cusp when considering polar cap
patches and convection of irregularities. However, the energetic particles
that cause aurora, discrete aurora in particular, produce maximum ionization
at much lower heights. For mapping the scintillation caused by bright aurora,
an IPP height of 110 km has been used (e.g., Prikryl et al., 2013b, 2015a).
This results in IPPs that are closer to GPS receivers and shrinks the latitudinal and
longitudinal range of IPPs. The grid cell latitude width of 2.5∘
is fairly wide but the mapping at 110 km will significantly redistribute the
IPPs in magnetic latitude and MLT when compared with mapping at 350 km. With
a small number of receivers, the mapping at 110 km will result in some gaps
in latitude coverage or reduced statistical samples due to small number of
data points in grid cells between widely spaced receivers. This is the case
particularly in the auroral oval that was, until 2013, covered only by two
CHAIN receivers in Edmonton and Sanikiluaq (Fig. 1).
Figure 5a–d show the scintillation occurrence maps for 3-month intervals
assuming hIPP= 110 km. When compared with corresponding maps for
hIPP= 350 km (Fig. 4a–d), some of the occurrence shifted poleward
from the cusp area (defined for hIPP= 350 km; Fig. 4a) and
filled
a larger proportion of the polar cap area. In the nightside auroral oval,
similar poleward shift of scintillation occurrence is discernible although
the latitude peak in scintillation occurrence remains within the au area
previously selected (Fig. 4a). The scintillation occurrence drops
significantly in the latitude band between 67.5 and 75∘ CGM that is approximately centered between the latitudes of
receivers in Iqaluit and Sanikiluaq. When mapping IPPs at hIPP= 110 km, there is a reduction of data points at that latitude. Because of low
statistical significance of the computed scintillation occurrence in some
grid cell the values are rejected and the grid cells are shown in grey
shading. However, the change of the mapping height from 350 to 110 km does not significantly affect the results of seasonal variations when
comparing Figs. 4 and 5 as discussed below.
To better resolve differences in seasonal dependence of scintillation in cu, pc, au and sa regions the data are grouped by month and averaged
over the four regions defined by AACGM latitude and MLT as indicated by grey
dashed lines in Figs. 4a and 5a. Figure 6a and b show monthly variations
of the mean phase scintillation occurrence in these regions for mapping IPPs
at 350 and 110 km, respectively. The au scintillation (shown in
dark blue) shows a semiannual oscillation with equinoctial maxima known to
be associated with the occurrence of aurora, while the scintillation
occurrence in the cu (shown in red) is the highest in late autumn and
winter and low in summer, with the maximum in November and the minimum in
July. The monthly variation of scintillation occurrence in the polar cap
(shown in yellow) is very similar to that observed in the cusp, although it
is much reduced in amplitude due to averaging over a large area (pc) with
the strongest contribution from the dayside polar cap and the cusp proper,
considering quite variable boundary between the cu and pc
sectors. The mean scintillation occurrence is very low in the sa sector. Similarly to the au sector, it has
a minimum in winter and poorly defined maxima around equinoxes, with an
anomalously high value in July.
From comparing Fig. 6a and b, we can conclude that the choice of the IPP
height in mapping does not significantly affect the results for seasonal
variations obtained for selected magnetic latitude and MLT except for small
differences in amplitude. In the cu region, the amplitude of annual
variation of scintillation occurrence is larger for hIPP= 350 km. In
the pc, au and sa
regions the seasonal variations are about the same for both values of
hIPP.
Scintillation occurrence dependence on the IMF orientation
The magnetosphere responds to a number of solar wind plasma parameters and a
variety of coupling functions (Newell et al., 2007) have been introduced to
understand and predict the state of the magnetosphere and, ultimately, the
magnetosphere–ionosphere–thermosphere (MIT) system. While a single parameter
does not suffice to describe the complex MIT coupling process, the IMF
orientation, and the IMF BZ component in particular, has been frequently
singled out as the most geo-effective parameter controlling the merging
process (Dungey, 1961). The IMF BZ component has been known to control
the occurrence of aurora, the dayside merging and its ionospheric signatures
in the cusp, the production of polar patches and the occurrence of
sun-aligned arcs in the polar cap and the sub-auroral ionospheric dynamics
including the SAPS and SED phenomena. These are the regions (Fig. 4a) that
are identified with enhanced phase scintillation.
The phase scintillation occurrence dependence on the IMF (a)
BZ and (b) BY. The percentages of negative BZ or BY
values for σΦ exceeding given thresholds are shown on the left.
The centers of Gaussian fits to distributions of occurrence number as a
function of BZ or BY for each σΦ interval are shown
by the yellow line.
The 2008–2013 phase scintillation occurrence maps for (a) BZ > +3 nT, (b) BZ < -3 nT, (c) BY > +3 nT and (d) BY < -3 nT.
The 2008–2013 phase scintillation occurrence maps for strongly southward
BZ < -3 nT for the cases of (a) BY
> +3 nT and (b) BY < -3 nT, and for
strongly northward BZ > +3 nT for the cases of (c) BY > +3 nT and (d) BY < -3 nT.
Maps of phase scintillation occurrence as a function of off-meridian and
off-shell angles assuming hIPP= 350 km for (a) all
latitudes and MLTs combined, (b) au, (c) cu and (d) sa.
First attempts to relate and correlate the high-latitude scintillation
occurrence with the IMF orientation, particularly the polarity of the IMF
BZ, clearly indicated its importance in controlling the scintillation
occurrence level (Li et al., 2010; Alfonsi et al., 2011; Aquino and Sreeja,
2013). Li et al. (2010) also investigated the influence of the IMF BY
highlighting the dawn–dusk asymmetry of the scintillation response. To
further investigate scintillation dependence on the IMF BY and BZ, we
use hourly values of the magnetic field components in geocentric solar
magnetospheric (GSM) coordinate system obtained from the OMNIWeb database
over the period 2008–2013. Instead of simply plotting all values of
σΦ versus corresponding values of IMF BZ (Aquino and
Sreeja, 2013), which would result in a very dense plot, Fig. 7a shows the
occurrence number of σΦ values in bins of 0.05 radians × 1 nT combining the data from all CHAIN stations, i.e., all data
points/IPPs at all latitudes and longitudes covered by CHAIN. The
distributions of occurrence number as a function of BZ are fitted with a
Gaussian curves for each σΦ interval and the centers of the
Gaussians are shown by the yellow line. The distributions of the occurrence
number across negative and positive values of IMF BZ are skewed towards
negative BZ values except for σΦ < 0.1 radians. The
percentage of negative BZ values for σΦ exceeding given
thresholds are shown on the left. This shows that phase scintillation
preferentially occurs when IMF BZ < 0. In contrast, the
distributions of phase scintillation occurrence number with IMF BY (Fig. 7b) are relatively symmetric or only slightly skewed toward negative values
of BY.
Figure 8a and b show maps of the mean occurrence of phase scintillation
σΦ > 0.1 for IMF BZ > +3 nT
and BZ < -3nT, respectively. Here the scintillation
occurrence for IMF BZ > +3 nT is defined as
100 × N(σΦ > 0.1 and
BZ > +3 nT) / Ntot(BZ > +3 nT) and similarly for IMF
BZ < -3 nT, i.e., the scintillation data sets are selected
by the signed mean hourly value of the IMF BZ to obtain Ntot
of IPPs (for the same UT hour) that fall in a given bin
(1 h MLT × 2.5∘ CGM latitude). In other words, when
computing the scintillation occurrence for the case of
BZ < -3 nT, Ntot is the number of σΦ measurements associated with the latter condition, i.e., excluding
IPPs in the same grid cell for which BZ≥-3 nT. We chose arbitrary
thresholds of -3 and +3 nT to characterize strongly southward or
northward conditions as opposed to a threshold of 0 nT. The latter does not
sufficiently discriminate between the two IMF conditions of interest.
For IMF BZ > +3 nT (Fig. 8a), scintillation occurrence in
the auroral oval is low but it persists at a relatively high level in the cusp
and the dayside polar cap, extending more than 12 MLT h from dawn to
dusk. The dusk-side and dawn-side scintillation is likely caused by
sun-aligned arcs that tend to occur either in the dusk-side or dawn-side of
the polar cap (Valladares et al., 1994). For IMF BZ < -3 nT
(Fig. 8b), the scintillation is enhanced in the expanded auroral oval and
even at sub-auroral latitudes, which is similar to the case of ICMEs (Fig. 2d) and geomagnetically disturbed days particularly in 2012 and 2013 (Fig. 3c and d). However, scintillation occurrence is highest in the cusp from
where scintillation associated with TOIs and/or patches extends into the
central polar cap.
The dawn–dusk asymmetry in the polar cap is controlled by the polarity of
IMF BY. Figure 8c and d show maps of the mean occurrence of phase
scintillation σΦ > 0.1 for IMF BY > +3 nT and BY < -3 nT, respectively. The regions
of phase scintillation occurrence in the cusp and polar cap are
preferentially shifted toward dawn for IMF BY > +3 nT and
toward dusk for IMF BY < -3 nT. This is caused by the
dawn–dusk asymmetry in the location of sun-aligned arcs and TOIs that are
observed preferentially during conditions of IMF BZ > 0 and
BZ < 0, respectively (Prikryl et al., 2015a). To separate these
two cases and to show that the relevant scintillation occurrence is caused
by one or the other phenomenon, the scintillation data are more strictly
limited by using IMFBZ thresholds of +3 or -3 nT in addition to
splitting the data by the polarity of the IMF BY (Fig. 9).
For IMF BZ < -3 nT, enhanced scintillation in the cusp that is
extended into the dawn-side polar cap for BY > +3 nT (Fig. 9a) and into the dusk-side polar cap for BY < -3 nT (Fig. 9b)
is caused by TOI irregularities, namely polar cap patches. In both cases,
there is much enhanced scintillation occurrence in the expanded auroral oval
and subauroral regions because of strongly southward IMF. For IMF BZ > +3 nT, scintillation occurrence remains highest in the cusp
but is very low in the auroral oval and subauroral regions. In the polar
cap, the scintillation extends dawnward for BY > +3 nT
(Fig. 9c) and duskward for BY < -3 nT (Fig. 9d), as expected for the dependence of occurrence of sun-aligned arcs on the IMF BY.
The shape of scintillation-causing ionospheric irregularities
Ionospheric scintillation is thought to be caused by irregularities of scale
sizes from a few hundred meters to a kilometer, which are less elongated
(Fremouw et al., 1985; Wernik et al., 1990) than small-scale irregularities,
showing anisotropy characterized by a range of axial ratios (Rino and Owen,
1980; Livingston et al., 1982; Gola et al., 1992) and various shapes from
cylindrical to sheet-like, as discussed by these authors and briefly
summarized previously (see, e.g., Prikryl et al., 2011). Assuming an IPP
height of 350 km, Gola et al. (1992) generated maps of amplitude
scintillation as a function of angles between the ray path and the
geomagnetic L-shell and the plane of magnetic meridian indicating a
predominance of field-aligned irregularities. Prikryl et al. (2011) applied
this approach to phase scintillation occurrence based on the first 2 years
of CHAIN data with the results that suggested a mixture of field-aligned and
L-shell-aligned irregularity shapes. Here we extend this analysis by 4
more years of data.
Figure 10 shows phase scintillation occurrence as a function of off-meridian
and off-shell angles that were computed for the receiver–satellite ray at the
IPP height 350 km. The data are binned into 5 × 5∘
grid points with the off-shell angle positive for IPPs northward of the receiver
and off-meridian angle positive for IPPs eastward of the receiver. High
scintillation occurrence at small off-meridian and small off-shell angles,
i.e., near magnetic zenith, suggest field-aligned irregularities. On the
other hand, large off-meridian angles along with relatively small off-shell
angles indicate L-shell-aligned irregularities. The angle distribution map
for all MLTs and latitudes combined (Fig. 10a) shows a composite of a peak
in scintillation occurrence at small angles near magnetic zenith and a broad
band of scintillation occurrence at low off-shell angles less than
20∘ but a range of off-meridian angles ±60∘,
suggesting that phase scintillation is caused by a mixture of field-aligned
and L-shell oriented irregularities. In comparison, the irregularities
causing scintillation in the au region show a large degree
of field alignment (Fig. 10b), while the irregularities causing the
scintillation in the cu region are L-shell aligned (Fig. 10c). For the
dayside (dawn and dusk) auroral oval excluding the cusp, the distributions of
off-meridian and off-shell angles (not shown) also indicate strongly
field-aligned irregularities. However, for the sa region
that includes the post-midnight equatorward edge of the auroral oval and SAPS
(Fig. 10d), there is rather low scintillation occurrence with a broad peak
approximately centered near magnetic zenith. For the dayside subauroral
region representing SEDs, we obtained similar results (not shown). The same
data analysis was repeated for the IPP height at 110 km, giving very similar
results for all regions.
These results are in agreement with the initial results (Prikryl et al.,
2011), indicating that scintillation-causing irregularities are more
shell aligned in the cusp than in the nightside auroral oval where there is
a higher percentage of scintillation near the magnetic zenith suggesting
field-aligned irregularities. However, it should be noted that, at high
latitudes in the cusp, the observations (IPPs) are mostly confined to the
south of the receiver. They are scarce close to magnetic zenith and
completely absent poleward of it.