@unpublished{LiperovskayaParrotBogdanovetal.2007,
  author  = {E. V. Liperovskaya and M. Parrot and V. V. Bogdanov and Claudia-Veronika Meister and M. V. Rodkin and V. A. Liperovsky},
  title   = {On long-term variations of foF2 in the mid-latitude ionosphere before strong earthquakes},
  institution = {Interdisziplin{\"a}res Zentrum f{\"u}r Dynamik komplexer Systeme},
  series    = {NLD Preprints},
  number    = {66},
  year    = {2007},
}