Abstract
I have made a detailed study of the fundamental properties of the solar photospheric magnetic field, which helps in better understanding the Sun’s radiative and particle outputs that affect the Earth’s near-space environment, as well as the entire heliosphere. Photospheric magnetic field is an essential parameter for space weather and space climate. The photospheric magnetic field includes a wide range of large-scale and small-scale structures, but the contribution of weak, small-scale fields to the total flux on the solar surface is dominant.
This thesis discusses the spatial-temporal structure and long-term evolution of the solar photospheric magnetic field. Particularly, the thesis presents, for the first time, the spatial distribution of the asymmetry of weak field values and its evolution in solar cycles 21–24. I found that the asymmetry (also called shift) of the distribution of positive and negative weak-field values is a real physical phenomenon. I also found that the shifts are most effectively produced at the supergranulation scale.
I studied the asymmetry of the distribution of weak field values separately in the two solar hemispheres. My results show that the shifts of weak-field field distributions in the two solar hemispheres have always the same sign as the new polarity of the polar field in the respective hemisphere and solar cycle. I also found that the hemispheric shifts change their sign in the late ascending to maximum phase of the solar cycle and attain their maximum in the early to mid-declining phase. This evolution of the hemispheric weak-field gives a new signal of the solar magnetic cycle.
We also studied the long-term spatial-temporal evolution of the weak-field shift and skewness of the distribution of photospheric magnetic field values during solar cycles 21–24 in order to clarify the role and relation of the weak field values to the overall magnetic field evolution. Our results give evidence for the preference of even the weakest field elements toward the prevailing magnetic polarity since the emergence of an active region, and for a systematic coalescence of stronger magnetic fields of opposite to produce weak fields during the poleward drift of the surge. / Original papers
Original papers are not included in the electronic version of the dissertation.
Getachew, T., Virtanen, I., & Mursula, K. (2017). Structure of the Photospheric Magnetic Field During Sector Crossings of the Heliospheric Magnetic Field. Solar Physics, 292(11). https://doi.org/10.1007/s11207-017-1198-9
http://jultika.oulu.fi/Record/nbnfi-fe201802083259
Getachew, T., Virtanen, I., & Mursula, K. (2019). Asymmetric Distribution of Weak Photospheric Magnetic Field Values. The Astrophysical Journal, 874(2), 116. https://doi.org/10.3847/1538-4357/ab0749
http://jultika.oulu.fi/Record/nbnfi-fe2019061320447
Getachew, T., Virtanen, I., & Mursula, K. (2019). A New Signal of the Solar Magnetic Cycle: Opposite Shifts of Weak Magnetic Field Distributions in the Two Hemispheres. Geophysical Research Letters, 46(16), 9327–9333. https://doi.org/10.1029/2019gl083339
Mursula, K., Getachew, T., & Virtanen, I. (2019). Spatial-temporal evolution of photospheric weak-field shifts in solar cycles 21-24. Astron. Astrophys., submitted.
| Identifer | oai:union.ndltd.org:oulo.fi/oai:oulu.fi:isbn978-952-62-2436-7 |
| Date | 29 October 2019 |
| Creators | Getachew, T. (Tibebu) |
| Contributors | Mursula, K. (Kalevi), Virtanen, I. (Ilpo) |
| Publisher | Oulun yliopisto |
| Source Sets | University of Oulu |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/doctoralThesis, info:eu-repo/semantics/publishedVersion |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess, © University of Oulu, 2019 |
| Relation | info:eu-repo/semantics/altIdentifier/pissn/1239-4327 |
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