Abstract

Solar coronal holes are regions of open magnetic field lines in the solar corona. They are the source region of high-speed solar wind streams that play an important role for geomagnetic activity and are the most important driver of energetic particle precipitation into the Earth’s atmosphere. Understanding the evolution of coronal holes is critical for solar magnetism as well as for accurate space weather forecasts. Historically, coronal holes have been identified visually and hand-traced by experienced observers. In this thesis, we develop a novel automated method to identify coronal holes from the extreme ultraviolet synoptic maps by determining the intensity threshold of coronal hole regions separately for each synoptic map. We quantify the spatio-temporal evolution of coronal hole during solar cycles 23 and 24. We studied geomagnetic storms triggered by two sequences of high-speed streams from persistent low-latitude coronal hole sources, observed in the declining phase of solar cycle 23. We found a systematic change in the Bz(GSM)–Bz(GSE) difference which offers the first detailed analysis of the onset of the Rusell-McPherron mechanism, increasing geomagnetic activity of the negative polarity sequence as we approach the March equinox. We also present a novel homogenized solar extreme ultraviolet synoptic dataset based on full-disk images from Solar and Heliospheric Observatory/Extreme ultraviolet Imaging Telescope and Solar Dynamics Observatory/Atmospheric Imaging Assembly. These maps provide a systematic and homogenous view of the entire solar surface in the extreme ultraviolet wavelengths from 1996 until 2019. Using this long-running homogenized dataset of solar extreme ultraviolet observations together with the coronal hole automated identification algorithm, we prepared a 23-year (1996–2019) coronal hole synoptic dataset. This coronal hole dataset is combined with the MacIntosh Archive (1973–2009) to produce the longest dataset of equatorial coronal hols covering more than four solar cycles.

Osajulkaisut / Original papers

Osajulkaisut eivät sisälly väitöskirjan elektroniseen versioon. / Original papers are not included in the electronic version of the dissertation.

1. Hamada, A., Asikainen, T., Virtanen, I., & Mursula, K. (2018). Automated identification of coronal holes from synoptic EUV maps. Solar Physics, 293(4), 71. https://doi.org/10.1007/s11207-018-1289-2

   Rinnakkaistallennettu versio / Self-archived version

2. Munteanu, C., Hamada, A., & Mursula, K. (2019). High‐speed solar wind streams in 2007–2008: Turning on the Russell‐McPherron effect. Journal of Geophysical Research: Space Physics, 124(11), 8913–8927. https://doi.org/10.1029/2019JA026846

   Rinnakkaistallennettu versio / Self-archived version

3. Hamada, A., Asikainen, T., & Mursula, K. (2020). New homogeneous dataset of solar EUV synoptic maps from SOHO/EIT and SDO/AIA. Solar Physics, 295(1), 2. https://doi.org/10.1007/s11207-019-1563-y

   Rinnakkaistallennettu versio / Self-archived version

4. Hamada, A., Asikainen, T., & Mursula, K. (2020). A uniform series of coronal holes in 1973–2018. Manuscript submitted for publication. https://doi.org/10.1007/s11207-021-01781-w

   Rinnakkaistallennettu versio / Self-archived version