Surface flux transport simulations of the photospheric magnetic field

Virtanen, Iiro (2019-09-02)
 
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Virtanen, Iiro
University of Oulu
02.09.2019
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Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:9789526223292

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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.
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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.

 

Original papers

Original papers 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

    Self-archived version

  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

    Self-archived version

  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

    Self-archived version

  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 submitted for publication. https://doi.org/10.1051/0004-6361/201936134

    Self-archived version

 

Osajulkaisut

Osajulkaisut eivät sisälly väitöskirjan elektroniseen versioon.

  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

    Rinnakkaistallennettu versio

  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

    Rinnakkaistallennettu versio

  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

    Rinnakkaistallennettu versio

  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 submitted for publication. https://doi.org/10.1051/0004-6361/201936134

    Rinnakkaistallennettu versio

 
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