University of Oulu

Surface flux transport simulations of the photospheric magnetic field

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Author: Virtanen, Iiro1,2
Organizations: 1University of Oulu Graduate School
2University of Oulu, Faculty of Science, Physics
Format: ebook
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 4.4 MB)
Persistent link: http://urn.fi/urn:isbn:9789526223292
Language: English
Published: Oulu : University of Oulu, 2019
Publish Date: 2019-09-02
Thesis type: Doctoral Dissertation
Defence Note: Academic Dissertation to be presented with the assent of the Doctoral Training Committee of Technology and Natural Sciences of the University of Oulu for public discussion in the Auditorium L4, Linnanmaa, on 6th September, 2019, at 12 o’clock noon.
Tutor: Professor Kalevi Mursula
Doctor Alexei Pevtsov
Reviewer: Professor Paul Charbonneau
Doctor David Hathaway
Opponent: Professor Kristóf Petrovay
Kustos: Professor Kalevi Mursula
Description:

Abstract

This thesis studies the long-term evolution of the photospheric magnetic field using surface flux transport simulations. The photospheric magnetic field and magnetic activity are tightly connected to space weather, and affect the whole heliosphere including the Earth. However, due to a lack of reliable observations our understanding of the long-term evolution of the photospheric magnetic field is still poor. Surface flux transport models, which are capable of simulating the evolution of the whole surface field from observations of solar activity, can be used to study the field in times when direct observations are not available.

In this thesis we validate our surface flux transport model, optimize its parameters and test its sensitivity to uncertainties in parameter values and input data. We find a need to extend the model with a decay term to properly model the deep and long minimum between solar cycles 23 and 24, and simulate the photospheric magnetic field of cycles 21–24 using magnetographic observations as input. We also study consequences of hemispherically asymmetric activity, and show that activity in one hemisphere is enough to maintain polar fields in both hemispheres through cross-equatorial flow of magnetic flux.

We develop a new method to reconstruct active regions from calcium K line and sunspot polarity observations. We show that this reconstruction is able to accurately capture the correct axial dipole moment of active regions. We study the axial dipole moments of observed active regions and find that a significant fraction of them have a sign opposite to the sign expected from Hale’s and Joy’s laws, proving that the new reconstruction method has an advantage over existing methods that rely on Hale’s and Joy’s laws to define polarities. We show one example of a long simulation covering solar cycles 15–21, demonstrating that using the active region reconstruction and surface flux transport model presented in this thesis it is possible to simulate the large-scale evolution of the photospheric magnetic field over the past century.

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Original papers

The original publications are not included in the electronic version of the dissertation.

  1. Virtanen, I. O. I., Virtanen, I. I., Pevtsov, A. A., Yeates, A., & Mursula, K. (2017). Reconstructing solar magnetic fields from historical observations. II. Testing the surface flux transport model. Astronomy & Astrophysics, 604, A8. https://doi.org/10.1051/0004-6361/201730415

    http://jultika.oulu.fi/Record/nbnfi-fe2017103050356

  2. Virtanen, I. O. I., Virtanen, I. I., Pevtsov, A. A., & Mursula, K. (2018). Reconstructing solar magnetic fields from historical observations. III. Activity in one hemisphere is sufficient to cause polar field reversals in both hemispheres. Astronomy & Astrophysics, 616, A134. https://doi.org/10.1051/0004-6361/201732323

    http://jultika.oulu.fi/Record/nbnfi-fe201902205813

  3. Virtanen, I. O. I., Virtanen, I. I., Pevtsov, A. A., Bertello, L., Yeates, A., & Mursula, K. (2019). Reconstructing solar magnetic fields from historical observations. IV. Testing the reconstruction method. Astronomy & Astrophysics, 627, A11. https://doi.org/10.1051/0004-6361/201935606

  4. Virtanen, I. O. I., Virtanen, I. I., Pevtsov, A. A., & Mursula, K. (2019) Axial dipole moment of solar active regions in cycles 21-24. Manuscript.

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Series: Report series in physical sciences
ISSN: 1239-4327
ISSN-L: 1239-4327
ISBN: 978-952-62-2329-2
ISBN Print: 978-952-62-2328-5
Issue: 130
Type of Publication: G5 Doctoral dissertation (articles)
Field of Science: 115 Astronomy and space science
Subjects:
Copyright information: © University of Oulu, 2019. This publication is copyrighted. You may download, display and print it for your own personal use. Commercial use is prohibited.