Abstract:
This study presents the global and mid-latitude ionospheric response to three geomagnetic storms occurred in January  2005: the first  one  on  7-8  January,  the  second  one  on 17-19  January,  and  the  last one  on  21-22 January. This period has been selected, because no major sudden stratospheric warming (SSW) occurred during this month and according to many scientists this winter is represented as an example of a background reference case corresponding to a ‘normal’ year.  Therefore,  the  observed  ionospheric  response  to  the  considered geomagnetic  storms  could  be  attributed  mainly  to  the  external  forcing.  The  reaction  is  explored  by considering N(h) profiles registered by manually scaled ionosonde measurements at station Sofia (42.5°N, 25°E),  which  are  used  for  calculating  the  total  electron  content  (TEC)  up  to  the  F2-layer  maximum (bottom-TEC).  The  full-TEC  data  are  provided  by  the  Center  for  Orbit  Determination  of  Europe (CODE)-Bern,  for  the  nearest  point  to  Sofia.  The  main  aim  of  this  work  is  to  compare  in  details  the temporal  variability  of  the  full-TEC  with  bought that  below (bottom-TEC) and  up  (top-TEC)  the  F2-layer maximum for each of the considered geomagnetic storms. It is found that for all investigated geomagnetic storms  in  January  2005  the  bottom-TEC  is  considerably  different  from  bought  top-TEC and  full-TEC. An explanation of the main mechanisms responsible for the observed difference has been proposed.