Model Fitting of Wind Magnetic Clouds for the Period 2004 – 2006

Abstract

We give the results of parameter fitting of the magnetic clouds (MCs) observed by the Wind spacecraft for the three year period – 2004 to the end of 2006 (the “Present period”) using the force-free MC model of Lepping, Jones, and Burlaga (J. Geophys. Res.95, 11957, 1990). There were 19 MCs identified in the Present period, which was mainly in the declining phase of the solar cycle. The long-term occurrence rate of MCs is 10.3/year (1995-2015), whereas the occurrence rate for the Present period is only 6.3/year, similar to that for the period 2007-2009. Hence, the MC occurrence rate has had an appreciable decrease for the six years 2004-2009. The MC modeling gives such basic MC quantities as size, axial orientation, field handedness, axial magnetic field strength, center time, and closest approach vector. A statistically based modification of the modeled field intensity is tested. Also calculated are derived quantities, such as axial magnetic flux, axial current density, and axial current. Quality ( $Q_{0}$ ) estimates are assigned representing excellent (1), good/fair (2), and poor (3). We provide error estimates on the specific fit parameters for the individual MCs for the $Q_{0} =1,2$ cases, and give a distribution of MC types (i.e. N ⇒ S, S ⇒ N, All N, All S, etc., ten categories in all). There is an inordinately large percentage of the N ⇒ S type in the Present period (32%). The Present period basic model fitting results are compared to the results of the full Wind mission and other 3-year periods. First, we notice that during the Present period the MCs are, on average, significantly faster (by 21%), distinctly stronger in axial magnetic field (by 37%), and smaller in diameter (by 5.5%), than those in the Long-term period. The quality of the MCs in the Present period is significantly better than that of the Long-term period, where the ratio $N(Q_{0} =1,2)/N(Q_{0} =1,2,3)$ for each is 0.79 and 0.58, respectively. The Present period is quite different from the Long-term period (1995-2015), it is from the other three 3-year periods between 2006 and the end of 2015. In the Present period upstream shocks occur for the first 12 MCs of the 19 cases (63%); for comparison the Long-term rate is 56%.