  Based on our tool for the early diagnostics of solar eruption
geoeffectiveness (EDSEG tool; Chertok et al., 2013, 2015, 2017), we have
analyzed space weather disturbances that occurred in early September 2017. Two
flares, SOL2017-09-04T20:33 (M5.5) and SOL2017-09-06T12:02 (X9.3), accompanied
by Earth-directed halo coronal mass ejections (CMEs) were found to be
geoeffective. We extracted the associated EUV 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 ~
9.3% and GMS with indexes Dst ~ -144 nT, Ap ~ 235) than expected (FD ~ 4.4%,
Dst ~ -135 nT, Ap ~ 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 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.
