  We study the solar flare index (SFI) for the solar cycles 18\,--\,24. We find
that SFI has deeper Gnevyshev gap (GG) in its first principal component than
other atmospheric parameters. The GG is extremely clear especially in the even
cycles.
  The GG of the SFI appears about a half year later as a drop in the
interplanetary magnetic field near the Earth and in the geomagnetic Ap-index.
The instantaneous response of the magnetic field to solar flares, however,
shows about two to three days after the eruption as a high, sharp peak in the
cross-correlation of the SFI and Ap-index and as a lower peak in SFI vs. IMF B
cross-correlation. We confirm these rapid responses using superposed-epoch
analysis.
  The most active flare cycles during 1944-2020 are the Cycles 19 and 21. The
Cycle 18 has very strong SFI days as much as Cycle 22, but it has least nonzero
SFI days in the whole interval. Interestingly Cycle 20 can be compared to the
Cycles 23 and 24 in its low flare activity, although it locates between the
most active SFI cycles.
