Nighttime magnetic perturbation events observed in Arctic Canada: 3. Occurrence and amplitude as functions of magnetic latitude, local time, and magnetic disturbances

Rapid changes of magnetic fields associated with nighttime magnetic perturbation events (MPEs) with amplitudes |ΔB| of hundreds of nT and 5-10 min periods can induce geomagnetically-induced currents (GICs) that can harm technological systems. In this study we compare the occurrence and amplitude of nighttime MPEs with |dB/dt| [?] 6 nT/s observed during 2015 and 2017 at five stations in Arctic Canada ranging from 75.2° to 64.7° in corrected geomagnetic latitude (MLAT) as functions of magnetic local time (MLT), the SME and SYM/H magnetic indices, and time delay after substorm onsets. Although most MPEs occurred within 30 minutes after a substorm onset,  ̃10% of those observed at the four lower latitude stations occurred over two hours after the most recent onset. A broad distribution in local time appeared at all 5 stations between 1700 and 0100 MLT, and a narrower distribution appeared at the lower latitude stations between 0200 and 0700 MLT. There was little or no correlation between MPE amplitude and the SYM/H index; most MPEs at all stations occurred for SYM/H values between -40 and 0 nT. SME index values for MPEs observed more than 1 hour after the most recent substorm onset fell in the lower half of the range of SME values for events during substorms, and dipolarizations in synchronous orbit at GOES 13 during these events were weaker or more often nonexistent. These observations suggest that substorms are neither necessary nor sufficient to cause MPEs, and hence predictions of GICs cannot focus solely on substorms.


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Although early studies of nighttime magnetic perturbation events (MPEs) that induce 63 large geoelectric fields and geomagnetically-induced currents (GICs) noted the small-scale 64 character of these events (e.g., Viljanen, 1997), many efforts to predict GICs have continued to 65 focus on global processes (geomagnetic storms and substorms). Recent observational studies by  Individual events also displayed no close or consistent temporal correlation with substorm 72 onsets. 73 Here we present additional analyses of a large number of nighttime MPEs that document 74 lack of any close correlation between their occurrence and levels of the SME index, the SYM/H 75 index, or of near-tail dipolarizations, and show that a substantial fraction of these events are not 76 temporally associated with substorms. MPEs occurring in the post-midnight sector showed a 77 different dependence on both latitude and prior substorm activity than did the more numerous 78 pre-midnight MPEs.   For each of the five stations we sorted the MPE events as functions of several variables: 152 magnetic local time (MLT), the SYM/H index, the SME index (the SuperMAG version of the 153 AE index, described in Newell and Gjerloev, 2011a), and derivative amplitude. 154 Over the range of magnetic latitudes covered in this study (from 75° to 65° MLAT) all ≥ 155 6 nT/s perturbation events fell into the local time range from 17 to 07 MLT. Figure 4a shows the appears at all latitudes shown, and a distribution in the midnight to dawn sector (2 to 7 MLT) 162 that is prominent only at the lower latitude stations. This difference in latitudinal distribution, 163 which is consistent with observations of large ionospheric equivalent current perturbations by 164 Juusola et al. (2015), appears to reflect the latitudinal dependence of the auroral electrojet, which 165 is located at higher latitudes pre-midnight and lower latitudes post-midnight. As will be shown 166 in later parts of this study, the properties of these two populations also differed somewhat in their 167 association with different geomagnetic conditions. 168 Consistent with the distribution of occurrences shown in Table 2   shown with plus signs was somewhat narrower in time and shifted toward slightly later MLT, 176 and a second post-midnight peak (with similar peak occurrences) appeared between 2-3 and 6 h 177 MLT. In contrast, the distributions for events shown with squares and triangles were flat across 178 the entire MLT range shown (but with fewer occurrences).
179 Figure 4b shows that the largest-amplitude MPEs occurred at all 5 stations between 1800 180 and 2300 h MLT, but derivatives with amplitude at or above 15 nT/s also appeared after 0300 h 181 MLT at both SALU and KJPK. Table 3 shows an analysis of the distribution of these events as a 182 7 function of time delay when separated into pre-and post-midnight occurrences. In order to 183 clearly separate these categories, pre-midnight events were chosen to include those observed 184 between 1700 and 0100 MLT, and post-midnight event those between 0200 and 0700 MLT. 185 The time delay distributions were similar for pre-and post-midnight events at all 5 stations, but 186 on average over all 5 stations, post-midnight events were slightly more likely to occur within 30 187 min after substorm onsets than pre-midnight events (70% vs. 66%), and less likely to occur more 188 than 60 minutes after onset (12% vs. 17%). These differences, however, were not statistically 189 significant.  At all five stations > 6 nT/s perturbation events occurred over a wide range of SME 207 values, as shown in Figure 6a, but very few events occurred at any station for SME < 200 nT. At 208 the four highest latitude stations a large majority of events in each of the 3 time delay categories 209 occurred for SME values between 200 and 900 nT. This SME range also held at the lowest 210 latitude station (KJPK) for the Δt > 60 min category, but most of the events in the Δt ≤ 30 min 211 category were associated with SME values > 800 nT. However, fewer events occurred for high 212 SME at KJPK (64.7° MLAT) than at SALU (70.7° MLAT)note the differing vertical scales.  Figure 6b shows that there was a modest correlation between the amplitude of the largest 214 derivatives and the SME index only over the SME range between 200 and 600 nT at all 5 215 stations; the distribution of amplitudes was nearly flat for SME > 600 nT at all stations. Most 216 events at all SME values and all 3 time ranges were below 12 nT/s. Only 7 of the 842 total 217 events occurred when SME exceeded 2000 nT.   Table 4 show the number of MPE events at each station that occurred indicating that most substorms were not associated with large amplitude MPEs. The percentages 258 at CDR, IQA, and SALU were near the lower end of this range, and those at RBY and KJPK at 259 the higher end. We note the roughly inverse correlation between these percentages and the 260 number of MPE events observed at each station (Table 2). This suggests that the modest 261 differences in magnetic longitude between the five stations were a smaller factor in determining 262 the dependence of MPEs on substorm onsets than the magnetic latitude. This dependence on 263 MLAT may reflect the limited spatial extent of large MPEs, such that a station farther away from 264 the statistical auroral oval is more likely to detect an MPE with lower amplitude, and thus in 265 many cases one below our selection threshold of 6 nT/s.  We also considered the effect of multiple prior substorm onsets separately for MPEs in 269 the two populations shown in Figure 4a: the "pre-midnight" population observed between 1700 270 and 0100 MLT, and the "post-midnight" population observed between 0200 and 0700 MLT.  Table 6 shows the results of applying Pearson's Chi-squared test to the data in Table 5, 284 after reducing the number of prior substorm categories to 3: after 0, 1, and ≥ 2 onsets within 2 285 hours, respectively. The p values of << 0.05 confirm that the difference between pre-midnight 286 and post-midnight events is statistically significant at all 3 stations. Taken together, these 287 differences indicate a much stronger relation between multiple substorms and subsequent MPEs 288 in the post-midnight sector than in the pre-midnight sector.
289 Table 7 provides additional information on the relation between MPE onset and the level 290 of magnetic disturbance (as represented by the SME index) following multiple substorms. This with SME values ≥ 1000 nT occurred after two-hour intervals containing from 2 to 4 substorm 299 onsets, and 3) because of the large difference in total MPE occurrence in each bin between pre-300 midnight and post-midnight MPEs, the percentage distribution of pre-midnight MPEs 301 simultaneous with SME values ≥ 1000 nT increased greatly as the number of prior substorm 302 onsets increased from 1 to 4, but was more nearly flat for post-midnight events. The overall 303 fractions of pre-midnight MPEs associated with SME values ≥ 1000 nT were 9.2% at IQA, 8.5 304 11 % at SALU, and 19.4% at KJPK. The corresponding post-midnight fractions were much larger: 305 70%, 44%, and 52%, respectively.

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The SME index is well correlated with auroral power (Newell and Gjerloev, 2011a). In 307 general, the relationship among discrete precipitation, ionospheric conductance, and upward 308 FAC density is instantaneous. In contrast, diffuse precipitation has a certain time lag; particles 309 are injected and then later forced to precipitate into the ionosphere. The associated enhancement 310 of ionospheric conductance lasts longer, which is favorable for more tail current to short-circuit 311 through the ionosphere at subsequent substorms. As a result, SME may increase following   Some of the smaller GOES 13 Bz perturbations, and especially those in the Δt ≥ 60 min 332 category, were associated with brief (few min) transient pulses rather than step functions 333 (dipolarizations). It is difficult to discern whether such pulses arise from spatial or temporal 12 effects. If spatial, GOES 13 may have been rather distant in MLT from the center of a more 335 large-scale dipolarization. If temporal, the perturbation may have been associated with a bursty 336 bulk flow, dipolarization front, and/or pseudobreakup (e.g., Palin et al., 2015). Further analysis 337 of the features of the GOES 13 dataset during these MPE events is certainly warranted, but is 338 beyond the scope of this paper.      onset. There was a modest correlation between the amplitude of the largest MPEs and the SME 379 index over the SME range from ~200 to ~600 nT at all 5 stations, but the distribution of  restricted to times when SYM/H is large and negative; it simply means that they occur at higher 419 latitudes at these times. 420 We have also found that only 60 -67% of the ≥ 6 nT/s MPEs we observed occurred

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The main implications of this study are 1) that neither a magnetic storm nor a fully 479 developed substorm is a necessary or sufficient condition for the occurrence of the extreme 480 nighttime magnetic perturbation events that can cause GICs, and 2) that the pre-midnight and                                      30 min < Δt < 60 min Δt ≥ 60 min Δt ≤ 30 min            Table 3 shows an analysis of the distribution of these events as a 182 7 function of time delay when separated into pre-and post-midnight occurrences. In order to 183 clearly separate these categories, pre-midnight events were chosen to include those observed 184 between 1700 and 0100 MLT, and post-midnight event those between 0200 and 0700 MLT.

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The time delay distributions were similar for pre-and post-midnight events at all 5 stations, but 186 on average over all 5 stations, post-midnight events were slightly more likely to occur within 30 187 min after substorm onsets than pre-midnight events (70% vs. 66%), and less likely to occur more 188 than 60 minutes after onset (12% vs. 17%). These differences, however, were not statistically 189 significant.  indicating that most substorms were not associated with large amplitude MPEs. The percentages 258 at CDR, IQA, and SALU were near the lower end of this range, and those at RBY and KJPK at 259 the higher end. We note the roughly inverse correlation between these percentages and the 260 number of MPE events observed at each station (Table 2). This suggests that the modest 261 differences in magnetic longitude between the five stations were a smaller factor in determining 262 the dependence of MPEs on substorm onsets than the magnetic latitude. This dependence on 263 MLAT may reflect the limited spatial extent of large MPEs, such that a station farther away from 264 the statistical auroral oval is more likely to detect an MPE with lower amplitude, and thus in 265 many cases one below our selection threshold of 6 nT/s.  We also considered the effect of multiple prior substorm onsets separately for MPEs in 269 the two populations shown in Figure 4a: the "pre-midnight" population observed between 1700 270 and 0100 MLT, and the "post-midnight" population observed between 0200 and 0700 MLT.  Table 6 shows the results of applying Pearson's Chi-squared test to the data in Table 5, 284 after reducing the number of prior substorm categories to 3: after 0, 1, and ≥ 2 onsets within 2 285 hours, respectively. The p values of << 0.05 confirm that the difference between pre-midnight 286 and post-midnight events is statistically significant at all 3 stations. Taken together, these 287 differences indicate a much stronger relation between multiple substorms and subsequent MPEs 288 in the post-midnight sector than in the pre-midnight sector.
289 Table 7 provides additional information on the relation between MPE onset and the level 290 of magnetic disturbance (as represented by the SME index) following multiple substorms. This with SME values ≥ 1000 nT occurred after two-hour intervals containing from 2 to 4 substorm 299 onsets, and 3) because of the large difference in total MPE occurrence in each bin between pre-300 midnight and post-midnight MPEs, the percentage distribution of pre-midnight MPEs 301 simultaneous with SME values ≥ 1000 nT increased greatly as the number of prior substorm 302 onsets increased from 1 to 4, but was more nearly flat for post-midnight events. The overall 303 fractions of pre-midnight MPEs associated with SME values ≥ 1000 nT were 9.2% at IQA, 8.5 304 11 % at SALU, and 19.4% at KJPK. The corresponding post-midnight fractions were much larger: 305 70%, 44%, and 52%, respectively.

306
The SME index is well correlated with auroral power (Newell and Gjerloev, 2011a). In 307 general, the relationship among discrete precipitation, ionospheric conductance, and upward 308 FAC density is instantaneous. In contrast, diffuse precipitation has a certain time lag; particles 309 are injected and then later forced to precipitate into the ionosphere. The associated enhancement 310 of ionospheric conductance lasts longer, which is favorable for more tail current to short-circuit 311 through the ionosphere at subsequent substorms. As a result, SME may increase following  Some of the smaller GOES 13 Bz perturbations, and especially those in the Δt ≥ 60 min 332 category, were associated with brief (few min) transient pulses rather than step functions 333 (dipolarizations). It is difficult to discern whether such pulses arise from spatial or temporal 12 effects. If spatial, GOES 13 may have been rather distant in MLT from the center of a more 335 large-scale dipolarization. If temporal, the perturbation may have been associated with a bursty 336 bulk flow, dipolarization front, and/or pseudobreakup (e.g., Palin et al., 2015). Further analysis 337 of the features of the GOES 13 dataset during these MPE events is certainly warranted, but is 338 beyond the scope of this paper.    onset. There was a modest correlation between the amplitude of the largest MPEs and the SME 379 index over the SME range from ~200 to ~600 nT at all 5 stations, but the distribution of 380 amplitudes was nearly flat for SME > 600 nT. The amplitude of most MPEs at all SME values 381 and in all 3 time categories was below 12 nT/s. restricted to times when SYM/H is large and negative; it simply means that they occur at higher 419 latitudes at these times. 420 We have also found that only 60 -67% of the ≥ 6 nT/s MPEs we observed occurred  The separation of nighttime MPEs into two populations in MLT, a pre-midnight one that 426 appeared at all 5 stations and a post-midnight one that was prominent only at the two lowest 427