Abstract An interplanetary shock and a magnetic cloud (MC) reached the Earth on 2012 July 14 and 15 one after another. The shock sheath and the MC triggered an intense geomagnetic storm. We find that only small part of the MC from 06:45 UT to 10:05 UT on 2012 July 15 made contribution to the intense geomagnetic storm, while the rest part of the MC made no contribution to the intense geomagnetic storm. The averaged southward component of interplanetary magnetic field (Bs) and duskward-electric fields (Ey) within the MC from 10:05 UT, 2012 July 15 to 09:08 UT, 2012 July 16 (hereafter MC_2), are 15.11 nT and 8.01 mV m−1, respectively. According to the empirical formula established by Burton et al. (hereafter Burton equation), the geoeffectiveness of MC_2 should be −655.42 nT, while the geoeffectiveness of MC_2 is −324.68 nT according to the empirical formula established by O’Brien & McPherron (hereafter OM equation). However, the real geoeffectiveness of MC_2 is 39.74 nT. The results indicate that the Burton equation and the OM equation cannot work effectively. The geoeffectiveness of MC_2 shows that large and long duration of Bs or Ey cannot guarantee the occurrence of an intense geomagnetic storm if the solar wind dynamic pressure is very low. If we use 0.52 as γ, the geoeffectiveness of MC_2 is 40.36 nT according to the empirical formula established by Wang et al., which is very close to the observed value, indicating that the empirical formula established by Wang et al. is much better than the Burton equation and the OM equation.
Keywords Solar coronal mass ejections – Solar-terrestrial interactions – Solar wind
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