Reconstruction and Prediction of Variations in the Open Solar Magnetic Flux and Interplanetary Conditions

Abstract

Historic geomagnetic activity observations have been used to reveal centennial variations in the open solar flux and the near-Earth heliospheric conditions (the interplanetary magnetic field and the solar wind speed). The various methods are in very good agreement for the past 135 years when there were sufficient reliable magnetic observatories in operation to eliminate problems due to site-specific errors and calibration drifts. This review underlines the physical principles that allow these reconstructions to be made, as well as the details of the various algorithms employed and the results obtained. Discussion is included of: the importance of the averaging timescale; the key differences between "range" and "interdiurnal variability" geomagnetic data; the need to distinguish source field sector structure from heliosphericallyimposed field structure; the importance of ensuring that regressions used are statistically robust; and uncertainty analysis. The reconstructions are exceedingly useful as they provide calibration between the in-situ spacecraft measurements from the past five decades and the millennial records of heliospheric behaviour deduced from measured abundances of cosmogenic radionuclides found in terrestrial reservoirs. Continuity of open solar flux, using sunspot number to quantify the emergence rate, is the basis of a number of models that have been very successful in reproducing the variation derived from geomagnetic activity. These models allow us to extend the reconstructions back to before the development of the magnetometer and to cover the Maunder minimum. Allied to the radionuclide data, the models are revealing much about how the Sun and heliosphere behaved outside of grand solar maxima and are providing a means of predicting how solar activity is likely to evolve now that the recent grand maximum (that had prevailed throughout the space age) has come to an end.

A Remarkable Recent Transition in the Solar Dynamo

“…The coronal holes are the sources of (mainly weakly magnetic) solar winds and this generates the conclusion that the solar wind must be strong and mainly with a low magnetic flux in (the) coming cycle(s). This has indeed been observed (Lockwood et al 2009, Lockwood 2013, and that finding is a fine confirmation of the Makarov-TlatovCallebaut observations. This phenomenon may, perhaps only partly, be related to the latitude drift of sunspots (Richardson and Schwarzschild 1953;Tuominen 1954): at latitudes below 26 • the spots move equatorward, the faster when closer to the equator, with an extreme value of 0.017 degrees per day at 2 • latitude.…”

“…An important feature is the coronal holes. These are non-magnetic regions or at best regions with a small magnetic field that are mainly situated in polar areas but that do sometimes extend down to fairly low latitudes as already mentioned in the preceding Section (Zwaan 1987;Makarov et al 2004;Lockwood et al 2009;Lockwood 2013). The behavior of the polar and equatorial activity during and prior to the phase transition is shown in Fig.…”

“…The late group of extreme values, those around 2010, is mainly related to the corpuscular solar emission of which the open solar flux (Lockwood et al 2009;Lockwood 2013) is an important component. The magnetized part of the corpuscular radiation is related to the cosmic ray flux (cf.…”

Overview of Solar Wind–Magnetosphere–Ionosphere–Atmosphere Coupling and the Generation of Magnetospheric Currents

“…Ganushkina et al (2015) described the observed morphology of the current systems and the measurements used to characterise them. Lockwood (2013) provided an overview of the current systems and the geomagnetic indices which are used to monitor their intensity and variability, with the aim of determining changes in interplanetary conditions over the last dozen or so solar cycles (150 years or more). Also of interest to this topic is the review of which describes our present understanding of solar wind-magnetosphere-ionosphere coupling, that is the dynamics of the magnetosphere in response to solar wind driving, which in turn are responsible for many of the current systems within the magnetosphere.…”

Transitions between states of magnetotail–ionosphere coupling and the role of solar wind dynamic pressure: the 25 July 2004 interplanetary CME case

“…We note that this is somewhat shorter than the corresponding ∼ 60 min long phases of field stretching before dipolarizations observed during the conditions of continuously high (3-4 nPa) dynamic pressure in the case of the sawtooth substorms on 18 August 2003 . Concerning the mechanism for the relation between solar wind dynamic pressure and the stretching of the magnetic field in the nearEarth PS, we refer to Karlsson et al (2000) and Lockwood (2013).…”