@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 = "23 abr. 2024"
}