Enhanced diurnal variation and Forbush decreases recorded with Lead-Free Gulmarg Neutron Monitor during the solar active period of late October 1989

Abstract The cosmic ray variations consequent upon the interplanetary disturbances during the solar active period of late October 1989 have been investigated by using the data recorded with the Lead-Free Gulmarg Neutron Monitor (LFGNM). Apart from recording total neutron count, it is also set to record neutron bunches with occurrence rates of 1–9 in short time intervals of 50 ms duration. Whereas the Forbush decrease (FD) profile observed in the total hourly count rate of this monitor compares well with the conventional neutron monitors, the neutron bunch-1 data reveals clear and large amplitude diurnal variation that persisted for about 10 consecutive days with average phase close to ∼ 1700 h direction. The power spectrum analysis of the neutron bunch-1 data by maximum entropy method (MEM) further revealed presence of semi- and tri-diurnal components also. The most intriguing result on anisotropic variation observed with LFGNM in a distinct mode of recording bunch-1 neutrons is unique thus far, though consistent with the strong unidirectional diurnal anisotropy observations made by conventional neutron monitors during the said period. We report the observations and the results in this work.