@Article{EcherGGPVDGS:2004:LoTeCo,
               author = "Echer, Ezequiel and Gonzalez, Walter Demetrio and Gonzalez, Alicia 
                         Luisa Clua de and Prestes, A. and Vieira, Luis Eduardo A. and Dal 
                         Lago, Alisson and Guarnieri, Fernando Luis and Schuch, Nelson 
                         Jorge",
                title = "Long term correlation between solar and geomagnetic activity",
              journal = "Journal of Atmospheric and Solar-Terrestrial Physics",
                 year = "2004",
               volume = "66",
               number = "12",
                pages = "1019--1025",
                month = "Mar.",
             keywords = "Geomagnetic activity, Solar activity, Solar wind, Sunspot 
                         number.",
             abstract = "A long-term correlation study between solar and geomagnetic 
                         activity is reported in this work using annual averages of the aa 
                         index and of the sunspot number Rz in the period of 1868?2000. Dst 
                         and AE geomagnetic indices and solar wind speed data are used for 
                         more recent periods. It is shown that the geomagnetic and solar 
                         activity correlation has decreased since the end of the 19th 
                         century, and the lag between them has increased. The variations of 
                         Rz and aa were in phase in the early period (solar cycles 11?14, 
                         around 1868?1910), and became out of phase in later periods (with 
                         a lag of 2 years in solar cycle 22, with aa maximum after Rz). 
                         Nevertheless, this trend is not monotonic and superposed 
                         <uctuations are seen, which does not permit determine if this 
                         correlation decrease is part or not of a long period solar 
                         activity cycle. The probable cause of the correlation decrease 
                         seems to be related to the aa index dual peak structure. The 
                         second aa peak seems to have increased relative to the =rst one. 
                         This second peak is more related to the high-speed streams 
                         originated from co-rotating structures whereas the =rst one is 
                         related to sunspot (coronal mass ejections) activity. In recent 
                         periods, since 1964, it has been observed that aa annual values 
                         have higher correlation with the fraction of days per year with 
                         daily solar wind speed peaks larger than 500 km=s (Fpk) than with 
                         Rz . The aa index also shows larger correlation with AE index than 
                         with Dst. Thus, it seems that average aa is strongly in<uenced by 
                         AE activity, which is in<uenced mainly by high speed streams from 
                         coronal holes. One can conclude that the decrease in correlation 
                         between aa and Rz occurs because the second aa peak has becoming 
                         stronger relative to the =rst one. The cause seems to be that open 
                         solar magnetic =eld structures have increased their activity 
                         relative to the closed (sunspot-related) solar magnetic =eld 
                         structures. This implies that the global solar magnetic =eld could 
                         have experienced a di?erential (between closed and open 
                         structures) large-scale variation in the last 130 years. c 2004 
                         Elsevier Ltd. All rights reserved.",
           copyholder = "SID/SCD",
                 issn = "1364-6826",
             language = "en",
           targetfile = "sdarticle.pdf",
        urlaccessdate = "20 maio 2024"
}