Short-term cosmic-ray increases and magnetic cloud-like structures during Forbush decreases

The analysis of the nucleonic intensity recorded during Forbush decreases (amplitude ≥ 1.5 % at high-latitude stations), occurring in the period 1966-1979, allowed us to identify 49 short-term cosmic ray increases (STIs) of non-solar and non-geomagnetic origin, with time duration from 5 to 30 hours. They show often a large North-South anisotropy and begin 4 to 40 hours after the interplanetary shock arrival; moreover they take place during the passage at the Earth of peculiar solar wind structures characterized by cold material (∼ 6 x 104 K), high speed solar wind (≥ 470 km/s), intense magnetic field strength (B ≥ 10 nT) and large changes in the B-direction which appear mainly as a quasi-continuous rotation of the B-component on a plane. The characteristics of these interplanetary regions are consistent with those of magnetic cloud structures strongly related with the driver gas ejected by the sun. The high number of cosmic ray short-term increases seems to suggest that these increases are common features of flare-related interplanetary perturbations producing Forbush decreases