Long-Term Evolution of Solar Coronal Holes

Thesis event information

Date and time of the thesis defence

Place of the thesis defence

Linnanmaa, auditorium IT115. Remote connection: https://oulu.zoom.us/j/65433840465

Topic of the dissertation

Long-Term Evolution of Solar Coronal Holes

Doctoral candidate

Master of Science Amr Hamada

Faculty and unit

University of Oulu Graduate School, Faculty of Science, Space Physics and Astronomy Research Unit

Subject of study



Professor Minna Palmroth, University of Helsinki


Professor Kalevi Mursula, University of Oulu

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Long-term evolution of solar coronal holes

The studies conducted in this thesis, throughout four publications, have found new ways to identify and track the evolution of one of the most important solar features, coronal holes. The corona is the outermost layer of the solar atmosphere, and coronal holes are regions on the corona characterized by low density and temperature compared to the surrounding background. Therefore, they appear dark in solar ultraviolet images. The coronal holes have an open magnetic field extending into interplanetary space allowing solar plasma to flow out as high-speed streams of the solar wind. These streams have important consequences for the near-Earth space in form of space weather and, e.g., auroras.

To study coronal holes, a new homogeneous dataset of synoptic maps of ultraviolet emission on the solar surface based on observations of SOHO and SDO satellites were constructed. This dataset provides a consistent view of the solar corona in four different ultraviolet wavelengths and covers the time from 1996 to present.

A novel algorithm to automatically identify and track dark coronal holes from the through different satellite images was developed in the thesis. This algorithm was applied to the dataset of ultraviolet synoptic maps to extract the evolution of coronal holes for the last two solar cycles. This satellite-based coronal hole information was also extended backwards in time to 1973 with ground observations of the solar corona. This provided the longest uniform record of coronal holes and allowed to study their long-term evolution in an unprecedented manner.

Understanding the variation of solar coronal holes not only clarifies cyclic evolution of the solar magnetic field but also helps in better understanding the important consequences of solar activity in the near-Earth environment.
Last updated: 5.11.2020