Phase scintillation data used in this study were collected from 2008 and 2012 by ten specialized GPS ionospheric scintillation and total electron content (TEC) monitors (GISTMs) of the Canadian High Arctic Ionospheric Network (CHAIN) (http://chain.physics.unb.ca/chain) (Jayachandran et al. 2009). CHAIN instruments, including six ionosondes, are distributed in the auroral oval, cusp, and the polar cap (Figure 1). A brief description of CHAIN, the scintillation parameters obtained, and the geographic and corrected 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 50-Hz sampling rate, compute the ionospheric TEC using both L1 and L2 signals, the amplitude scintillation index S4, and the phase scintillation index σ
Φ
. The phase scintillation index σ
Φ
is the standard deviation of the detrended phase using a filter in the receiver with 0.1-Hz lower cutoff. Intense scintillation is usually accompanied by cycle slips. A cycle slip (Horvath and Crozier 2007) is defined here as a jump in differential phase TEC of more than or equal to 1.5 TECU in 1 s (1 TEC unit corresponds to 1016 electrons/m2).
Solar wind data, projected to the Earth's bow shock, are obtained from the Goddard Space Flight Center Space Physics Data Facility OMNIWeb (http://omniweb.gsfc.nasa.gov/). To characterize the solar wind by one index, we use the quasi-invariant (QI) index (Osherovich et al. 1999) that is defined as the ratio of the solar-wind magnetic to ram pressures.
High-speed streams generate CIRs, regions between the fast and slow solar wind where the solar wind plasma is compressed. Following the criteria of (Prikryl et al. 2012), arrival times of high-speed stream interfaces CIR/HSSs are determined. A total of 108 HSS/CIRs with a maximum velocity VMAX exceeding 500 km/s are used in the SPE analysis of scintillation data from 2008 to 2012. For ICMEs, we use a catalogue of near-Earth ICMEs (Richardson and Cane 2010) that is updated at http://www.srl.caltech.edu/ACE/ASC/DATA/level3. In this table, ICMEs are classified into three categories based on the properties of magnetic clouds in column l: a value of 2 in this column indicating that a magnetic cloud (MC) has been reported in association with the ICME; a value of 1 indicating that the ICME shows evidence of a rotation in field direction but lacks some other characteristics of an MC, for example, an enhanced magnetic field; and a value of 0 indicating that the ICME has not been a reported magnetic cloud and lacks most of the typical features of a magnetic cloud, such as a smoothly rotating, enhanced magnetic field.
The time of the leading edge (front) of an HSS or of the ICME upstream shock/wave/disturbance provides key times of a geoeffective solar wind disturbance observed upstream from the Earth. Projected to the Earth's bow shock (OMNI), these key arrival times can be used in an SPE analysis to obtain an average response of the ionosphere, e.g., geomagnetic disturbance (McPherron and Siscoe 2004; McPherron and Weygand 2006), riometer absorption (Kavanagh et al. 2012), and scintillation and cycle slip occurrence (Prikryl et al. 2012). Such results can then be used in probabilistic forecasting of a given ionospheric variable due to specific solar wind conditions. This can be thought of as an analogy to traditional meteorology approach in weather forecasting based on atmospheric air mass climatologies separated by boundaries (weather fronts) as discussed by McPherron and Siscoe (2004).
SPE analysis of solar wind and scintillation data
To obtain the mean variations of solar wind parameters relative to HSS interface (CIR) SPE analysis of solar wind plasma time series is performed. Figure 2a shows the mean solar wind velocity, V, mean flow angle Φ
V
, density, n
p
, magnetic field magnitude, |B|, and standard deviation of IMF B
Z
component, σ
Bz
, over 12 days centered at the key time (day 0). The mean solar wind velocity decreases to a minimum just before the stream interface and then steeply rises to a maximum 2 days later. All other mean solar wind parameters peak at, or very close to, the key time. The mean flow angle Φ
V
shows a bipolar east-west deflection. Figure 2b shows the results of the SPE analysis for 40 ICMEs for which the geomagnetic disturbance, Dst, was less than -30 nT. Because the ICMEs are often preceded by shocks, the mean V, n
p
, |B|, and σ
Bz
increase near the key time. The mean flow angle Φ
V
does not show a strong bipolar east-west deflection that is typical for CIRs. These results are similar to those obtained previously (Prikryl et al. 2012) for 2008 to 2010.
The GPS phase scintillation predominantly occurs in the ionospheric footprint of the cusp, auroral oval, and polar cap (Spogli et al. 2009; Prikryl et al. 2011a, [b], 2012). In response to varying geomagnetic activity, the scintillation regions shift in latitude. To examine a response of scintillation and to propose a method of scintillation forecasting relative to arrival time of CIRs and ICMEs, Prikryl et al. (2012) focused on the cusp stations in Cambridge Bay and Taloyoak that showed steep increases in scintillation occurrence on the zero epoch day tapering off a few days later. We now include ten CHAIN stations in the SPE analysis. The scintillation and cycle slip occurrence is mapped as a function of CGM latitude and magnetic local time (MLT).
Similarly to solar wind parameters in Figure 2, Figure 3a,b shows the SPE analysis results for QI and Kp indices, shown in green and black lines, respectively. The mean QI index increases more gradually and peaks later but reaches a higher value for ICMEs than for HSS/CIRs. This is because many ICMEs are actually magnetic clouds, and the ratio of the magnetic to ram pressures is higher inside the magnetic cloud. Also, the average background levels of QI and Kp before the arrival of a solar wind disturbance are higher for ICMEs than for HSS/CIRs because most of the ICMEs occurred in the years of rising solar activity (2010 to 2012), while the HSS/CIRs events are distributed more evenly over the whole period. However, the focus of this study is phase scintillation occurrence and number of cycle slips Ncys that are shown in Figure 3a,b in blue and red lines, respectively. They are averaged over the cusp region defined here by latitude range from 70.0°N to 82.5°N and MLT from 0600 to 1800 hours. Both the scintillation and cycle slip occurrence peaked about three to four times higher for ICMEs than for HSS/CIRs. As we already noted for QI and Kp indices, the background average levels of occurrences are higher for the case of ICMEs than for HSS/CIRs.
Figures 4 and 5 show the SPE analysis results for CIRs and ICMEs, respectively, for all magnetic latitudes and MLTs. Figure 4a,b shows the SPE analysis results based on 108 CIRs, namely, the maps of percentage occurrence of phase scintillation σ
Φ
exceeding 0.1 radians and average number of cycle slips, respectively. The positions of the statistical auroral oval (Feldstein and Starkov 1967; Holzworth and Meng 1975) are superposed in white line for conditions from very quiet (IQ = 0) to disturbed (IQ = 6), proportionally to the daily mean value of the Kp index. For each epoch day, the phase scintillation occurrence and mean number of cycle slips maximized in the cusp and extended into the polar cap with the grand maximum occurring on the epoch day 0. However, significant scintillation also occurred at auroral and even at subauroral latitudes. The occurrence of scintillation and cycle slips tapered off, reaching the pre-event background level a few days later.
Figure 5a,b shows the results of the SPE analysis based on 40 ICMEs (Dst < -30 nT). Similarly to CIRs, the mean phase scintillation occurrence and mean number of cycle slips maximized in the cusp and extended into the polar cap with the grand maximum occurring on the epoch day 0, but the peak scintillation occurrence and cycle slip number were higher and tapered off faster, reaching the pre-event background level on epoch day 2. Significant scintillation also occurred at auroral and even subauroral latitudes on epoch days 0 and 1.
Cumulative probability distribution functions
Figures 6 and 7 show cumulative probability distribution functions (PDFs) for the phase scintillation occurrence in the ‘cusp’ region defined here by latitude range from 70.0°N to 82.5°N and MLT from 0600 to 1800 hours. The PDFs are based on the SPE analysis results shown in Figures 4a and 5a for CIRs and ICMEs, respectively. The curves represent the probabilities that phase scintillation will exceed a given value as plotted on the abscissa.
In Figure 6, the solid lines show overall PDFs for epoch days -1 and 0 (for slow solar wind just before and fast solar wind after the key time) including all solar wind conditions. The overall probability curves for both the slow and fast streams (shown by solid lines) are bracketed by probability curves (broken lines) for scintillation data subdivided by corresponding solar wind low and high (below and above median) values of V, |B|, and σ
Bz
. In general, the probabilities are higher for ICMEs than for CIRs. As discussed further in the ‘Results and discussion’ section, the period of slow solar wind before the arrival of CIR/HSS is characterized by very low geomagnetic activity and, expectedly, the lowest level of scintillation. It gives the lowest background levels of scintillation occurrence and thus the lowest probabilities, particularly for the case of low (below median) values of |B| and σ
Bz
. Because ICMEs can arrive when a HSS is active, the mean ‘background’ level of probabilities before the arrival of ICMEs is expectedly higher. By the same token, the scintillation occurrence (probability) level after an ICME arrival is a superposition of that ‘background’ level and scintillation caused by the ICME interaction with the magnetosphere-ionosphere system. The PDFs shown for low and high V, |B|, and σ
Bz
indicate that variability about the average of the solar wind speed affects the computed probability values the least. The scintillation occurrence (probability) is strongly affected by the variability of the magnetic field.In Figure 7a,b, the overall PDFs are shown from -6 to +6 epoch days stepped by 1-day intervals for HSS/CIRs and ICMEs, respectively. In both cases, the highest probabilities are observed for day 0 (i.e., over 24 h after the key time) but are significantly higher for ICMEs. Although the background levels of occurrences, and thus the PDF values, are higher for the case of ICMEs than for HSS/CIRs; the range of probabilities is larger for the latter. It is noted that the lowest occurrences (Figures 3a and 4) and the lowest probabilities (Figures 7a and 8b) are observed 2 days before HSS/CIR arrivals, i.e., on epoch day -2.
To differentiate between the response in the cusp and auroral zone, Figures 8 and 9 show the cumulative PDFs for the phase scintillation occurrence in the nighttime auroral zone defined here by a latitude range from 60.0°N to 75.0°N and MLT from 1800 to 0600 hours. Clearly, the probabilities are significantly lower as already indicated by lower occurrence rate of phase scintillation in the auroral oval as compared with the cusp (Figures 4a and 5a). However, in all instances, both in the cusp and in the auroral zone, the highest probability of phase scintillation occurrence is on epoch day 0, i.e., just after the arrival of CIRs or ICMEs. Also, the probabilities are higher for larger values of |B|, σ
Bz
, and V. The variability of the magnetic field affects the probabilities the most.