Table 1.
Average daily frequency of grid disturbances for three distinct selection criteria: ga within the day of high geomagnetic activity as measured by |dB/dt|(30 m); gi for a day ending a 3-day period with the lowest average |dB/dt(30 m)| within 25 days of a day with high |dB/dt(30 m)|; gr for a day selected at random between 5 and 50 days before or after high |dB/dt(30 m)|. The conditional criterion for days with high |dB/dt(30 m)| is defined in the first column for each of the three rows. The final column shows the total number of dates, Nd with high |dB/dt(30 m)| corresponding to the 2, 5, and 10 percentile levels. Uncertainties in fa and fi assume Poisson statistics; for fr the standard deviation of a sample of 100 random realizations is given. Data are shown for all grid disturbances (top), for grid disturbances attributed to weather, technical or external causes (center), and for the complementary set of grid disturbances of unclear attribution (bottom).
| Selection criterion for reference dates | ga (enhanced geomagn. act) | gi (low nearby geomagn. act) | gr (nearby random date) | Nd |
|---|---|---|---|---|
| All disturbances (1,216 cases) | ||||
| |dB/dt(30 m)| ≥ 36.1 | 0.230 ± 0.041 | 0.058 ± 0.020 | 0.136 ± 0.031 | 139 |
| |dB/dt(30 m)| ≥ 24.5 | 0.184 ± 0.023 | 0.107 ± 0.018 | 0.143 ± 0.023 | 347 |
| |dB/dt(30 m)| ≥ 18.5 | 0.167 ± 0.016 | 0.089 ± 0.012 | 0.147 ± 0.016 | 694 |
| WET: Attributed to weather/external/technical causes (743 cases) | ||||
| |dB/dt(30 m)| ≥ 36.1 | 0.137 ± 0.031 | 0.043 ± 0.018 | 0.070 ± 0.024 | |
| |dB/dt(30 m)| ≥ 24.5 | 0.115 ± 0.018 | 0.055 ± 0.013 | 0.077 ± 0.016 | |
| |dB/dt(30 m)| ≥ 18.5 | 0.099 ± 0.011 | 0.050 ± 0.009 | 0.080 ± 0.010 | |
| U: Unclear/unknown attribution (473 cases) | ||||
| |dB/dt(30 m)| ≥ 36.1 | 0.094 ± 0.026 | 0.014 ± 0.010 | 0.066 ± 0.025 | |
| |dB/dt(30 m)| ≥ 24.5 | 0.069 ± 0.010 | 0.052 ± 0.012 | 0.066 ± 0.017 | |
| |dB/dt(30 m)| ≥ 18.5 | 0.068 ± 0.010 | 0.039 ± 0.007 | 0.068 ± 0.013 | |
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