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@InProceedings{AlvesEchTsuCosLak:2015:HiLaHe,
               author = "Alves, Maria Virg{\'{\i}}nia and Echer, Ezequeil and Tsurutani, 
                         B. T. and Costa J{\'u}nior, E. da and Lakhina, G. S.",
          affiliation = "{Instituto Nacional de Pesquisas Espaciais (INPE)} and {Instituto 
                         Nacional de Pesquisas Espaciais (INPE)} and {Jet Propulsion 
                         Laboratory} and {Instituto Federal de Minas Gerais} and {Indian 
                         Institute for Geomagnetism}",
                title = "High latitude heliospheric magnetic decrease properties observed 
                         by Ulysses",
                 year = "2015",
         organization = "IUGG General Assembly, 26.",
             abstract = "Magnetic Decreases (MDs) are regions with abrupt decreases in the 
                         interplanetary magnetic field (IMF) magnitude. Ulysses spacecraft 
                         magnetic field and plasma data provide a unique opportunity to 
                         study MDs at high heliographic latitudes (HHL). We use a computer 
                         code on Ulysses high-resolution magnetic field data to obtain the 
                         MD occurrence rates and features under a variety of solar cycle 
                         and solar wind conditions. Ulysses first orbit had polar passes in 
                         1994 (South Pole, S1) and 1995 (North Pole, N1), near the 
                         declining and solar minimum phases. The second orbit, with polar 
                         passes in 2000 (S2) and 2001 (N2), occurred during a solar maximum 
                         phase. The third orbit, with polar passes in 2007 (S3) and 2008 
                         (N3), occurred near solar minimum, with an opposite solar magnetic 
                         field polarity of first pass. Among the six polar passes we found 
                         S1 and N1 are representatives for MDs occurring at HHLs during a 
                         typical solar minimum, when the solar plasma is mainly composed of 
                         high speed streams coming from coronal holes. For S1 we found a MD 
                         occurrence rate of ~12 MDs/day, an angular variation of IMF (Da) 
                         of ~27 and a duration time for MDs (Dt) of ~17s. For N1 we found 
                         a MD occurrence rate of ~15 MDs/day, Da~26 and Dt~20s. These 
                         results are quite different from the ones found for MDs at low 
                         heliographic latitudes: ~4 MDs/day, Dt~42s, and Da~6. MDs that 
                         present Da<10 are usually attributed to mirror instability. The 
                         origin of MDs observed at HHLs is still an open question, and 
                         depends on a careful analysis of high resolution plasma parameters 
                         where MDs occur. The S2 pass shows several days where the MD 
                         occurrence rate is zero. Preliminary results indicate the absence 
                         of MDs are associated with low solar wind velocities, <500km/s, 
                         low density Np <1cm-3, and low beta, <1.0.",
  conference-location = "Prague, Czech Republic",
      conference-year = "22 June - 02 July",
        urlaccessdate = "01 dez. 2020"
}


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