The solar dynamo begins near the surface
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arXiv
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| Autori principali: | , , , , , , , |
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| Natura: | Preprint |
| Pubblicazione: |
2024
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| _version_ | 1866914753081769984 |
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| author | Vasil, Geoffrey M Lecoanet, Daniel Augustson, Kyle Burns, Keaton J Oishi, Jeffrey S Brown, Benjamin P Brummell, Nicholas Julien, Keith |
| author_facet | Vasil, Geoffrey M Lecoanet, Daniel Augustson, Kyle Burns, Keaton J Oishi, Jeffrey S Brown, Benjamin P Brummell, Nicholas Julien, Keith |
| contents | The Sun's magnetic dynamo cycle features a distinct pattern: a propagating region of sunspot emergence appears around 30 degrees latitude and vanishes near the equator every 11 years. Moreover, longitudinal flows called "torsional oscillations" closely shadow sunspot migration, undoubtedly sharing a common cause. Contrary to theories suggesting deep origins for these phenomena, helioseismology pinpoints low-latitude torsional oscillations to the Sun's outer 5-10%, the "Near-Surface Shear Layer". Within this zone, inwardly increasing differential rotation coupled with a poloidal magnetic field strongly implicates the Magneto-Rotational Instability prominent in accretion-disk theory and observed in laboratory experiments. Together, these two facts prompt the general question: Is it possible that the solar dynamo is a near-surface instability? Here, we report strong affirmative evidence in stark contrast to traditional paradigms focusing on the deeper tachocline. Simple analytic estimates show that the near-surface magneto-rotational instability better explains the spatiotemporal scales of the torsional oscillations and inferred subsurface magnetic field amplitudes. State-of-the-art numerical simulations corroborate these estimates and, strikingly, reproduce hemispherical magnetic current helicity laws. The dynamo resulting from a well-understood near-surface phenomenon improves prospects for accurate predictions of full magnetic cycles and space weather, impacting Earth's electromagnetic infrastructure. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2404_07740 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | The solar dynamo begins near the surface Vasil, Geoffrey M Lecoanet, Daniel Augustson, Kyle Burns, Keaton J Oishi, Jeffrey S Brown, Benjamin P Brummell, Nicholas Julien, Keith Solar and Stellar Astrophysics Fluid Dynamics Space Physics The Sun's magnetic dynamo cycle features a distinct pattern: a propagating region of sunspot emergence appears around 30 degrees latitude and vanishes near the equator every 11 years. Moreover, longitudinal flows called "torsional oscillations" closely shadow sunspot migration, undoubtedly sharing a common cause. Contrary to theories suggesting deep origins for these phenomena, helioseismology pinpoints low-latitude torsional oscillations to the Sun's outer 5-10%, the "Near-Surface Shear Layer". Within this zone, inwardly increasing differential rotation coupled with a poloidal magnetic field strongly implicates the Magneto-Rotational Instability prominent in accretion-disk theory and observed in laboratory experiments. Together, these two facts prompt the general question: Is it possible that the solar dynamo is a near-surface instability? Here, we report strong affirmative evidence in stark contrast to traditional paradigms focusing on the deeper tachocline. Simple analytic estimates show that the near-surface magneto-rotational instability better explains the spatiotemporal scales of the torsional oscillations and inferred subsurface magnetic field amplitudes. State-of-the-art numerical simulations corroborate these estimates and, strikingly, reproduce hemispherical magnetic current helicity laws. The dynamo resulting from a well-understood near-surface phenomenon improves prospects for accurate predictions of full magnetic cycles and space weather, impacting Earth's electromagnetic infrastructure. |
| title | The solar dynamo begins near the surface |
| topic | Solar and Stellar Astrophysics Fluid Dynamics Space Physics |
| url | https://arxiv.org/abs/2404.07740 |