Thesis defence in the University of Oulu

Doctoral Candidate

Master of Science Tibebu Getachew

Faculty and research unit

University of Oulu Graduate School, Faculty of Science, Space Climate Research Unit

Field of study

Physics

Date and time of the thesis defence

8.11.2019 12:00

Place of the thesis defence

Linnanmaa, hall L10

Topic of the dissertation

Spatial-temporal structure and distribution of the solar photospheric magnetic field

Opponent

Professor Lidia van Driel-Gesztelyi, Mullard Space Science Laboratory, University College London, UK

Custos

Professor Kalevi Mursula, University of Oulu

Spatial-temporal structure and distribution of the solar photospheric magnetic field

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 polarity to produce weak fields during the poleward drift of the surge.

Last updated: 4.11.2019