@Article{Gonzalez-AlarconGonzTsur:1990:DuSoCy,
               author = "Gonzalez-Alarcon, Walter Demetrio and Gonzalez, Alicia Luisa Clua 
                         and Tsurutani, B. T",
                title = "Dual-peak solar cycle distribution of intense geomagnetic storms",
              journal = "Planetary and Space Science",
                 year = "1990",
               volume = "38",
               number = "2",
                pages = "181--187",
             keywords = "GEOFISICA ESPACIAL.",
             abstract = "The solar cycle distribution of intense geomagnetic storms has 
                         been studied for cycles 20 and 21 (1965-1985), using values of the 
                         Dst index <-100 nT, and for cycles 12-19 (1879-1964), using values 
                         of aa index >100 nT. It is claimed that a dominant dual-peak (DP) 
                         distribution exists in the solar cycle variability of these 
                         storms, with one peak occurring at the late ascending phase of the 
                         cycle or at solar maximum and another at the early descending 
                         phase of the cycle. From the 10 cycles studied, the average 
                         separation of the peaks from solar maximum is about 8 months ahead 
                         for the first peak and about 25 months after for the second one. 
                         For the cycles studied, the ratio of the average numbers of storms 
                         occurring at the peaks as compared with that occurring at the 
                         valley of the DP distribution was found to be statistically 
                         significant to better than a 95 confidence level. Although the 
                         present study is restricted to intense storms, their distribution 
                         including moderate events for the interval 1965-1985, in terms of 
                         Dst values <-50 nT, also shows evidence of a dual-peak 
                         distribution. Thus, in contrast with previous work on the solar 
                         cycle distribution of geomagnetic storms, which have emphasized 
                         the idea that they tend to peak at the descending phase of the 
                         cycle, the present work provides evidence for another important 
                         enhancement of this distribution at the late ascending phase of 
                         the cycle, extending sometimes to solar maximum. The origin of the 
                         dual-peak distribution of intense storms is associated to a 
                         similar dual-peak distribution obtained for large-amplitude (<-10 
                         nT) and long duration (>3 h) values of the negative Bz component 
                         of the interplanetary magnetic field (IMF), computed for the 
                         interval 1970-1981. This class of Bz fields was shown to be cause 
                         of all intense storms studied for the interval August 
                         1978-December 1979 (Gonzalez and Tsurutani, 1987, Planet. Space. 
                         Sci. 35, 1101).",
                 issn = "0032-0633",
                label = "7191",
           targetfile = "1990_gonzalez.pdf",
        urlaccessdate = "04 maio 2024"
}