Solar modulation parameter for cosmic rays since 1936 reconstructed from ground-based neutron monitors and ionization chambers

The differential energy spectrum of galactic cosmic rays near Earth is often parameterized by the force field model with the only time-dependent parameter, the modulation potential phi. Here we present a series of reconstructed monthly values of the modulation potential for the period from July 1936 through December 2009. This work extends our earlier study by employing new data and improving the reconstruction method. The presented series is a composite of three parts. The most reliable part is based on data from the world network of sea level neutron monitors and covers the period since April 1964. The part between February 1951 and March 1964 is based on data from one to two mountain neutron monitors of IGY type and is characterized by larger uncertainties and possible systematic error. The part related to the period before 1951 is based on data from Forbush ground-based ionization chambers and is characterized by large uncertainties and should be taken with caveats. The reconstructed series has been tested against long-term data of balloon-borne measurements of flux of cosmic ray ionizing radiation in the stratosphere performed by the Lebedev Institute since 1957. The comparison shows good agreement since 1964 but suggests that the result before 1964 may contain larger errors in that the NM-based reconstruction method may underestimate the low energy part of GCR spectrum.