Solar Eruptions, Forbush Decreases, and Geomagnetic Disturbances From Outstanding Active Region 12673

Based on our tool for the early diagnostics of solar eruption geoeffectiveness (EDSEG tool; Chertok et al., 2013, https://doi.org/10.1007/s11207-012-0127-1; 2015, https://doi.org/10.1007/s11207-0140618-3; 2017, https://doi.org/10.1007/s11207-017-1081-8), we have analyzed space weather disturbances that occurred in early September 2017. Two flares, SOL2017-09-04120:33 (M5.5) and SOL2017-09-06112:02 (X9.3), accompanied by Earth-directed halo coronal mass ejections (CMEs) were found to be geoeffective. We extracted the associated extreme ultaviolet dimmings and arcades and calculated their total unsigned magnetic flux. This calculation allowed us to estimate the possible scales of the Forbush decreases (FDs) and geomagnetic storms (GMSs) in the range from moderate to strong, and they are close to the observed scales. More precisely, after the first eruption, an FD approximately equal to 2% and almost no GMS occurred because the Bz magnetic field component in front of the corresponding interplanetary CME (ICME) was northern. The stronger second eruption produced somewhat larger composite disturbances (FD approximate to 9.3% and GMS with indexes Dst approximate to -144 nT, Ap approximate to 235) than expected (FD approximate to 4.4%, Dst approximate to -135 nT, Ap approximate to 125) because the second ICME overtook the trailing part of the first ICME near Earth, and the resulting Bz component was more intense and southern. Both ICMEs arrived at Earth earlier than expected because they propagated in the high-speed solar wind emanated from an extended coronal hole adjacent to the active region AR12673 along their entire path. Overall, the presented results provide further evidence that the EDSEG tool can be used for the earliest diagnostics of actual solar eruptions to forecast the scale of the corresponding geospace disturbances. Plain Language Summary Space weather nonrecurrent disturbances such as geomagnetic storms and Forbush decreases of galactic cosmic rays are caused by coronal mass ejections that erupt from the Sun and propagate to Earth. We analyze such disturbances in connection with an outstanding outburst of solar flare activity that occurred at the beginning of September 2017 and compare their parameters with estimates from our tool for the early diagnostics of solar eruption geoeffectiveness. The estimates are based on measurements of the magnetic flux of the earliest coronal mass ejection manifestations in the extreme ultraviolet range such as bright coronal posteruptive arcades and areas with temporarily reduced brightness (so called dimmings). We demonstrate that this tool yields the earliest relatively correct estimates for the scales of geomagnetic storm and Forbush decreases despite the complexity of the near-Earth solar wind structures in September 2017.