A significant decrease of the fundamental Schumann resonance frequency during the solar cycle minimum of 2008-9 as observed at Modra Observatory

Abstract

The  Schumann resonances (SR)  are  electromagnetic eigenmodes  of the resonator  bounded by  the Earth’s surface  and  the lower  ionosphere.   The  SR  frequency variability has  been  studied for more  than 4 decades.   Using  data from  the period  1988 to 2002, S´atori  et al.  (2005)  showed  that the  SR fundamental mode frequency  decreased on  the 11-year  time  scale  by  0.07 – 0.2  Hz,  depending on  which  component  of the field was  used  for estimation  and  likely  also  on  the location  of the observer. A decrease  by 0.30 Hz from  the latest solar  cycle maximum to the minimum of 2009 is found  in data from  Modra  Observatory.  This  extraordinary fall  of the  fundamental  mode  frequency can be attributed to the  unprecedented drop  in the ionizing  radiation in X-ray  frequency band.  Although the patterns of the daily  and  seasonal  variations remain  the same in the solar cycle minimum as in the solar cycle maximum, they are significantly shifted  to lower frequencies  during  the minimum. Analysis  of the  daily  frequency  range  suggests that the main  thunderstorm regions  during  the north hemisphere summer are smaller  in the solar cycle minimum than in the maximum.