Abstract
Abstract. We present a new reconstruction of the interplanetary magnetic field (IMF, B) for 1846–2012 with a full analysis of errors, based on the homogeneously constructed IDV(1d) composite of geomagnetic activity presented in Part 1 (Lockwood et al., 2013a). Analysis of the dependence of the commonly used geomagnetic indices on solar wind parameters is presented which helps explain why annual means of interdiurnal range data, such as the new composite, depend only on the IMF with only a very weak influence of the solar wind flow speed. The best results are obtained using a polynomial (rather than a linear) fit of the form B = χ · (IDV(1d) − β)α with best-fit coefficients χ = 3.469, β = 1.393 nT, and α = 0.420. The results are contrasted with the reconstruction of the IMF since 1835 by Svalgaard and Cliver (2010).
Highlights
This paper is the second of a series of three
We present a new reconstruction of the interplanetary magnetic field (IMF, B) for 1846–2012 with a full analysis of errors, based on the homogeneously constructed interdiurnal variation (IDV)(1d) composite of geomagnetic activity presented in Part 1 (Lockwood et al, 2013a)
Because the aa index derived during the space age correlates with both the southward component, Bz of the near-Earth interplanetary magnetic field (IMF), B, and the solar wind speed, V, observed by interplanetary spacecraft, Feynman and Crooker (1978) noted that the upward drift in the aa index over the first half of the 20th century reveals that averages of either Bz or V had probably changed, or both
Summary
This paper is the second of a series of three. As discussed in Part 1 (Lockwood et al, 2013a), for many years, the only available long-term record of geomagnetic activity was the aa index, compiled for 1868–1968 by Mayaud (1971, 1972, 1980) and subsequently continued to the present day. As discussed in Part 1, several new geomagnetic indices have been produced from hourly means of data or from hourly samples (“spot values”) which were often recorded in observatory yearbooks Three examples of this are the median index m, as implemented by Lockwood et al (2006b) and used by Rouillard et al (2007); the inter-hour variability (IHV) index designed by Svalgaard and Cliver (2007); and the interdiurnal variation (IDV) index introduced by Svalgaard and Cliver (2005) (hereafter SC05), and developed by Svalgaard and Cliver (2010) (hereafter SC10).
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