@Article{MacotelaCruzRaMaCoTuMa:2017:LoIoSe,
               author = "Macotela Cruz, Edith Liliana and Raulin, Jean-Pierre and Manninen, 
                         Jyrki and Correira, Emilia and Turunen, Tauno and Magalh{\~a}es, 
                         Antonio",
          affiliation = "{University of Oulu} and {Universidade Presbiteriana Mackenzie} 
                         and {University of Oulu} and {Instituto Nacional de Pesquisas 
                         Espaciais (INPE)} and {University of Oulu} and {Universidade 
                         Presbiteriana Mackenzie}",
                title = "Lower ionosphere sensitivity to solar X-ray flares over a complete 
                         solar cycle evaluated from VLF signal measurements",
              journal = "Journal of Geophysical Research: Space Physics",
                 year = "2017",
               volume = "122",
               number = "12",
                pages = "12370--12377",
             abstract = "The daytime lower ionosphere behaves as a solar X\‐ray 
                         flare detector, which can be monitored using very low frequency 
                         (VLF) radio waves that propagate inside the 
                         Earth\‐ionosphere waveguide. In this paper, we infer the 
                         lower ionosphere sensitivity variation over a complete solar cycle 
                         by using the minimum X\‐ray fluence (FXmin) necessary to 
                         produce a disturbance of the quiescent ionospheric conductivity. 
                         FXmin is the photon energy flux integrated over the time interval 
                         from the start of a solar X\‐ray flare to the beginning of 
                         the ionospheric disturbance recorded as amplitude deviation of the 
                         VLF signal. FXmin is computed for ionospheric disturbances that 
                         occurred in the time interval of DecemberJanuary from 2007 to 2016 
                         (solar cycle 24). The computation of FXmin uses the X\‐ray 
                         flux in the wavelength band below 0.2 nm and the amplitude of VLF 
                         signals transmitted from France (HWU), Turkey (TBB), and U.S. 
                         (NAA), which were recorded in Brazil, Finland, and Peru. The main 
                         result of this study is that the long\‐term variation of 
                         FXmin is correlated with the level of solar activity, having FXmin 
                         values in the range 
                         (1\ \−\ 12)\ ×\ 10\−7 
                         J/m2. Our result suggests that FXmin is anticorrelated with the 
                         lower ionosphere sensitivity, confirming that the 
                         long\‐term variation of the ionospheric sensitivity is 
                         anticorrelated with the level of solar activity. This result is 
                         important to identify the minimum X\‐ray fluence that an 
                         external source of ionization must overcome in order to produce a 
                         measurable ionospheric disturbance during daytime.",
                  doi = "10.1002/2017JA024493",
                  url = "http://dx.doi.org/10.1002/2017JA024493",
                 issn = "2169-9402",
             language = "en",
           targetfile = "macotela_lower.pdf",
        urlaccessdate = "27 abr. 2024"
}