Unfortunately, even this extended interval does not contain a “grand solar minimum” of activity, the most recent of which was the Maunder minimum (MM), circa 1650–1710, when there were a dearth of sunspots and a greatly reduced occurrence of reported aurora 14, 15, 16. The local heliosphere in near-Earth space has been directly sampled for the last 60 years 11, while geomagnetic proxies can be used to reliably infer annual means of HMF and solar wind speed back to 1845 12, 13. Finally, coronagraph and eclipse observations reveal coronal density structures and hence can give indirect insight into the solar wind structure. The accuracy of such estimates depends on both the underlying observations and the model assumptions and at present they are more reliable at solar minimum than at solar maximum 10. Observationally constrained coronal modelling, particularly photospheric magnetogram extrapolation (routinely possible since 1975) 7, 8, 9, provides global estimates of HMF and, indirectly, solar wind structure. Interplanetary scintillation (routinely performed since circa 1989) 6 can infer the solar wind density and speed integrated along the line-of-sight to suitable astrophysical radio sources and, when combined with tomographic techniques, can provide greater spatial sampling than in- situ observations, though with greater uncertainty. At sunspot maximum regions of fast and slow wind are found at all latitudes 3, 4 and the corona is much more dynamic as a result of increased coronal mass ejection activity 5.ĭirect observations of the solar wind and heliospheric magnetic field (HMF) outside the ecliptic plane are primarily limited to single-point in- situ measurements taken by the solar polar-orbiting Ulysses mission 3, 1991–2008. Fast, tenuous solar wind originates from polar coronal holes associated with “open” magnetic flux 1 and slower, denser solar wind arises from equatorial streamer belts associated with closed magnetic loops 2. At sunspot minimum, the solar wind is highly structured by solar latitude. The structure of the solar wind, and of the magnetic field it drags from the Sun to form the heliosphere, varies in a fundamental way with the phase of the sunspot cycle. The global heliosphere was both smaller and more symmetric under MM conditions, which has implications for the interpretation of cosmogenic radionuclide data and resulting total solar irradiance estimates during grand minima. ![]() Thus solar wind energy input into the Earth’s magnetosphere was reduced, resulting in a more Jupiter-like system, in agreement with the dearth of auroral reports from the time. Relative to the modern era, the MM shows a factor 2 reduction in near-Earth heliospheric magnetic field strength and solar wind speed, and up to a factor 4 increase in solar wind Mach number. Using these empirical relations, we produce the first quantitative estimate of global solar wind variations over the last 400 years. Here, we use nearly 30 years of output from a data-constrained magnetohydrodynamic model of the solar corona to calibrate heliospheric reconstructions based solely on sunspot observations. The most recent “grand minimum” of solar activity, the Maunder minimum (MM, 1650–1710), is of great interest both for understanding the solar dynamo and providing insight into possible future heliospheric conditions.
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