Evolution of Solar and Stellar Dynamo Theory

Charbonneau, Paul; Sokoloff, Dmitry

doi:10.1007/s11214-023-00980-0

Cited by 17 publications

(9 citation statements)

References 177 publications

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“…Larmor (1919) proposed that the solar magnetic fields observed by Hale and his colleagues were generated through the process of electromagnetic induction in the electrically conducting solar plasma. This idea became known as dynamo theory and its evolution as applied to the Sun and stars is discussed extensively in Charbonneau and Sokoloff (2023).…”

mentioning

confidence: 99%

Solar Cycle Observations

Norton,

Howe,

Upton

et al. 2023

Space Sci Rev

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We describe the defining observations of the solar cycle that provide constraints for the dynamo processes operating within the Sun. Specifically, we report on the following topics: historical sunspot numbers and revisions; active region (AR) flux ranges and lifetimes; bipolar magnetic region tilt angles; Hale and Joy’s law; the impact of rogue ARs on cycle progression and the amplitude of the following cycle; the spatio-temporal emergence of ARs that creates the butterfly diagram; polar fields; large-scale flows including zonal, meridional, and AR in-flows; short-term cycle variability; and helioseismic results including mode parameter changes.

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confidence: 99%

Solar Cycle Observations

Norton,

Howe,

Upton

et al. 2023

Space Sci Rev

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“…Whilst the severity of the technological and societal impact of a geomagnetic storm depends on many factors (3), from the details of the structure emitted from the corona, its solar wind propagation, to how it interacts with earth's magnetosphere (4; 5; 6), the most extreme events are directly driven by, and directly correlate with, large-scale solar eruptions (7; 8; 9). Quantifying extreme space weather risk is thus intimately related to understanding the solar cycle of activity, which in turn is a longstanding unsolved problem in physics (4), informing our understanding of the solar dynamo which is as yet not complete (10).…”

Section: Mainmentioning

confidence: 99%

A solar cycle clock for extreme space weather

Chapman,

de Wit

2024

Preprint

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The variable solar cycle of activity is a long-standing problem in physics. It modulates the overall level of space weather activity at earth, which in turn can have significant societal impact. The Hilbert transform of the sunspot number is used to map the variable length, approximately 11 year Schwabe cycle onto a uniform clock. The clock is used to correlate extreme space weather seen in the aa index, the longest continuous geomagnetic record at earth, with the record of solar active region areas and latitudes since 1874. This shows that a clear switch-off of the mostextreme space weather events occurs when the solar active regions move to within 15 degrees of the solar equator, from regions of high gradient in solar differential rotation which can power coronal mass ejections, to a region where solar differential rotation is almost constant with latitude. This overlaps with the onset of more moderate space weather events which coincide with 27 day solar rotation recurrences in the aa index, consistent with stable, persistent source regions.This offers a physical explanation for the longstanding identification of a two component cycle of activity in the aa index.

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“…In Section 2 we present evidence that F 10.7 is a proxy for the Sunʼs surface magnetism and compare how the magnetic field strength at the Sunʼs surface and in the SW approach their respective floor values at 11 yr cycle minima. In Section 3, we make conjectures based on these floors regarding (1) the existence of a turbulent small-scale dynamo (Durney et al 1993;Petrovay & Szakaly 1993;Cattaneo 1999;Rempel et al 2023) that produces a constant unsigned photospheric flux independently of the large-scale dynamo (Charbonneau 2020;Charbonneau & Sokoloff 2023) that produces the 11 yr cycle; and (2) the role of this small-scale dynamo in the generation of the floor-like slow SW (SSW). Our results are summarized and discussed in Section 4.…”

Section: Introductionmentioning

confidence: 99%

A Floor in the Sun's Photospheric Magnetic Field: Implications for an Independent Small-scale Dynamo

Cliver,

White,

Richardson

2024

ApJL

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Clette recently showed that F 10.7 systematically approaches a quiet Sun daily value of 67 solar flux units (sfu) at solar minima as the number of spotless days on the Sun increases. Previously, a floor of ∼2.8 nT had been proposed for the solar wind (SW) magnetic field strength (B). F 10.7, which closely tracks the Sun's unsigned photospheric magnetic flux, and SW B exhibit different relationships to their floors at 11 yr solar minima during the last ∼50 yr. While F 10.7 approaches 67 sfu at each minimum, the corresponding SW B is offset above ∼2.8 nT by an amount approximately proportional to the solar polar field strength—which varied by a factor of ∼2.5 during this interval. This difference is substantiated by ∼130 yr of reconstructed F 10.7 (via the range of the diurnal variation of the East-component (rY) of the geomagnetic field) and SW B (based on the interdiurnal variability geomagnetic activity index). For the last ∼60 yr, the contribution of the slow SW to SW B has exhibited a floor-like behavior at ∼2 nT, in contrast to the contributions of coronal mass ejections and high-speed streams that vary with the solar cycle. These observations, as well as recent SW studies based on Parker Solar Probe and Solar Dynamics Observatory data, suggest that (1) the Sun has a small-scale turbulent dynamo that is independent of the 11 yr sunspot cycle; and (2) the small-scale magnetic fields generated by this nonvarying turbulent dynamo maintain a constant open flux carried to the heliosphere by the Sun's floor-like slow SW.

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Evolution of Solar and Stellar Dynamo Theory

Cited by 17 publications

References 177 publications

Solar Cycle Observations

Solar Cycle Observations

A solar cycle clock for extreme space weather

A Floor in the Sun's Photospheric Magnetic Field: Implications for an Independent Small-scale Dynamo

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