Inferring the Structure of the Solar Corona and Inner Heliosphere During the Maunder Minimum Using Global Thermodynamic Magnetohydrodynamic Simulations

Abstract

Observations of the Sun's corona during the space era have led to a picture of relatively constant, but cyclically varying solar output and structure. Longer-term, more indirect measurements, such as from 10 Be, coupled by other albeit less reliable contemporaneous reports, however, suggest periods of significant departure from this standard. The Maunder Minimum was one such epoch where:(1) sunspots effectively disappeared for long intervals during a 70 yr period; (2) eclipse observations suggested the distinct lack of a visible K-corona but possible appearance of the F-corona; (3) reports of aurora were notably reduced; and (4) cosmic ray intensities at Earth were inferred to be substantially higher. Using a global thermodynamic MHD model, we have constructed a range of possible coronal configurations for the Maunder Minimum period and compared their predictions with these limited observational constraints. We conclude that the most likely state of the corona during-at least-the later portion of the Maunder Minimum was not merely that of the 2008/2009 solar minimum, as has been suggested recently, but rather a state devoid of any large-scale structure, driven by a photospheric field composed of only ephemeral regions, and likely substantially reduced in strength. Moreover, we suggest that the Sun evolved from a 2008/2009-like configuration at the start of the Maunder Minimum toward an ephemeral-only configuration by the end of it, supporting a prediction that we may be on the cusp of a new grand solar minimum.

Modeling the Sun’s Small-Scale Global Photospheric Magnetic Field

“…From the table, it is interesting to note that a factor of 6 decrease in the source surface height (2.5 R e to 1.25 R e ) produces a factor of 20 increase in the IMF at 1 au. The lower source surface extrapolations do produce IMF values closer to those of Riley et al (2015) (around 100 times smaller than presently measured), but it should be noted that a PFSS is an very simplified and limited approximation to the coronal magnetic field. On further development of the global photospheric evolution model, we will consider a more realistic nonpotential, time-evolving coronal magnetic field, for more detailed future studies.…”

“…2 ). This is around 1000 times smaller than the typical values observed this century (≈3 nT=3×10 −5 G, e.g., Yeates et al 2010), but it is also an order of magnitude lower than the ephemeral-only Sun estimates of Riley et al (2015) (2.9×10 −6 G for ±10 G ERs, 8×10 −7 G for ±3 G ERs). Lee et al (2011) suggest that the source surface for a PFSS should be lower during low solar activity (although interestingly, Réville et al (2015) find the opposite).…”

“…Here we consider potential field extrapolations purely for visual comparison and to speculate on the coronal structure of a grand minimum state Sun, as potential field source surface (PFSS) extrapolations are widely used by the scientific community. For a more in-depth study of grand minimum coronae, Riley et al (2015) compare the magnetic structure and emission of coronae for various hypothetical grand minimum photospheric states using a thermodynamic MHD model.…”

“…One possibility is that only smallscale magnetic features were present during such a state. Indeed, Riley et al (2015) suggest that in the later stages of the Maunder Minimum, the solar photosphere may have consisted entirely of small-scale ephemeral regions (ERs), with no significant large-scale magnetic dipole structure. Low solar magnetic activity results in less magnetic flux leaving the Sun (open flux) and hence a weaker interplanetary magnetic field (IMF), which in turn leads to a higher galactic cosmic ray intensity at Earth (see, e.g., Usoskin 2013 and references therein).…”

Sunspot number recalibration: The ~1840–1920 anomaly in the observer normalization factors of the group sunspot number

“…In addition, have specified a constant non-linear relationship between the sunspot number and group counts. Finally, the reexamination of the sunspot number and related long-term series has led to detailed investigation of the Maunder Minimum (e.g., Riley et al 2015;Usoskin et al 2015;Zolotova & Ponyavin 2015) which will lead to a firmer view of solar activity during that intriguing low-activity period.…”