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).

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.

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.

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.