Yearly Comparison of Magnetic Cloud Parameters, Sunspot Number, and Interplanetary Quantities for the First 18 Years of the Wind Mission

Abstract

In the scalar part of this study, we determine various statistical relationships between estimated magnetic cloud (MC) model fit-parameters and sunspot number (SSN) for the interval defined by the Wind mission, i.e., early 1995 until the end of 2012, all in terms of yearly averages. The MC-fitting model used is that of Lepping, Jones, and Burlaga (J. Geophys. Res. 95, 11957 – 11965, 1990). We also statistically compare the MC fit-parameters and other derived MC quantities [e.g., axial magnetic flux (ΦO) and total axial current density (J O)] with some associated ambient interplanetary quantities (including the interplanetary magnetic field (B IMF), proton number density (N P), and others). Some of the main findings are that the minimum SSN is nearly simultaneous with the minimum in the number of MCs per year (N MC), which occurs in 2008. There are various fluctuations in N MC and the MC model-fit quality (Q′) throughout the mission, but the last four years (2009 – 2012) are markedly different from the others; Q′ is low and N MC is large over these four years. N MC is especially large for 2012. The linear correlation coefficient (c.c.≈0.75) between the SSN and each of the three quantities J O, MC diameter (2R O), and B IMF, is moderately high, but none of the MC parameters track the SSN well in the sense defined in this article. However, there is good statistical tracking among the following: MC axial field, B IMF, 2R O, average MC speed (V MC), and yearly average solar wind speed (V SW) with relatively high c.c.s among most of these. From the start of the mission until late 2005, J O gradually increases, with a slight violation in 2003, but then a dramatic decrease (by more than a factor of five) occurs to an almost steady and low value of ≈ 3 μA km−2 until the end of the interval of interest, i.e., lasting for at least seven years. This tends to split the overall 18-year interval into two phases with a separator at the end of 2005. There is good tracking between 2R O and the total axial current density, as expected. The MC duration is also correlated well with these two quantities. ΦO shows marked variations throughout the mission, but has no obvious trend. N P, B IMF, V MC, Q′, and V SW are all quite steady over the full 18 years and have markedly low relative variation. Concerning vector quantities, we examine the distribution of MC type for the 18 years, where type refers to the field directional change through a given MC starting at first encounter (i.e., North-to-South, or South-to-North, All South, All North, etc.). Combining all 18 years of MC types shows that the occurrence rate varies strongly across the various MC types, with N-to-S being most prevalent, with a 27 % occurrence rate (of all MCs), and S-to-N being second, with a 23 % occurrence. Then All N and All S come next at 16 % and 10 % occurrence rate, respectively. All others are at 7 % or lower. For the variation of MC types with time, the southern types (i.e., those that start with a southern magnetic field, a negative B Z in geocentric-solar-ecliptic coordinates) decrease, as the northern types (i.e., those that start with a northern field) increase, apparently consistent with the specific timing of the polarity change of the solar magnetic field, as predicted by Bothmer and Rust (in Crooker, N., Joselyn, J., Feynman J. (eds), Geophys. Monogr., 139 – 146, 1997).