Heliospheric timescale identified in surface atmospheric electricity

Spectral analysis of surface atmospheric electricity data at daily resolution is used to investigate heliospheric and cosmic ray effects on the global circuit. Using 42 years of Potential Gradient ( PG) data (1963-2004) from Nagycenk Observatory, Hungary (47 degrees 38'N, 16 degrees 43'E), periodicities are found at annual, semi-annual and 1.7 year timescales. A quasi-periodicity centred on 1.68 years in surface neutron monitor data has previously been identified as of heliospheric origin; a similar signal was apparent in Voyager spacecraft cosmic ray measurements. The heliospheric signal is strongest during the 1980s and early 1990s, but absent before 1975. The 1.7 year periodicity in the Nagycenk PG data is present 1978-1990, but absent 1963-1977, and is clearest during fair weather when the global circuit and columnar air conductivities most readily contribute to the surface PG. When poor local weather influences the PG, the 1.7 year periodicity is reduced or absent.