North–south excess of hemispheric sunspot numbers and cosmic ray asymmetric solar modulation

Abstract

Timeline of solar activity is reviewed for 1945–2013 with data for yearly north–south excess (NSE) of hemispheric sunspot numbers (SSNs) for six cycles (18–23) and rising phase of cycle 24. There are more sunspots in north hemisphere for 1950–1970 (cycles 18–20) and excess in south hemisphere for 1980–2010 (cycles 21–23). To the best of our knowledge, the physical cause(es) for NSE and change of its sign are not known, highlighting the fact that we do not yet understand how the solar Dynamo works. Others have analyzed NSE data for shorter periods. We study the relationship between NSE and galactic cosmic ray (GCR) asymmetric solar modulation for the space age with high latitude neutron monitor data located in USA and Europe; space age began in October 1963 with in situ measurements of the solar wind parameters at 1AU. We infer an asymmetric GCR particle density gradient normal to the ecliptic plane exists for 1963–2013 and undergoes significant temporal variations unrelated to Schwabe or Hale cycle. Furthermore, it has no physical relationship with NSE for the period of our analysis, contrary to the result of a prior study for a shorter period.