Abstract

Solar coronal magnetic field is the source of the heliospheric magnetic field, and it drives many of the heliospheric processes affecting the Earth. In this thesis we study the long-term evolution of the solar coronal magnetic field by using two different models, the potential field source surface (PFSS) model and the current sheet source surface (CSSS) model. We cover the past four solar activity cycles from 1976 until 2017.

We compare the prediction capability of the PFSS model on the topology and intensity of the heliospheric magnetic field. We find that the PFSS model can predict the overall topology of the coronal field fairly well, with an accuracy of about 80 %, over the whole period of study. We find that on average the optimum value for the source surface parameter of the PFSS model is 3.5, which is slightly larger than the traditionally used value of 2.5 solar radii. We also find a temporal variation for the optimum source surface distance, it being on average highest during solar minima. However, during the peculiar solar cycle 23 minimum in 2008 the optimum source surface distance remained small, showing a shrinking of the solar corona during the latest solar cycles. The CSSS model is not able to much improve the topology of the PFSS model, despite its more complex current system. However, the CSSS model can produce a larger total coronal flux than the PFSS model, and it corrects the latitudinal variation of the PFSS model field.

The heliospheric current sheet has been observed in several earlier studies to be systematically southward shifted in the late declining and minimum phase of the solar cycle, both in the corona and in the heliosphere. We compare the simultaneous coronal and heliospheric shifts, and find that the shift is consistently larger in the corona than at the Earth’s distance during the 20th century. In 2008 such a clear difference between the shift in the two locations is seen no longer.

Osajulkaisut / Original papers

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

1. Koskela, J. S., Virtanen, I. I., & Mursula, K. (2017). Comparing coronal and heliospheric magnetic fields over several solar cycles. The Astrophysical Journal, 835(1), 63. https://doi.org/10.3847/1538-4357/835/1/63

   Rinnakkaistallennettu versio / Self-archived version

2. Koskela, J. S., Virtanen, I. I., & Mursula, K. (2018). Southward shift of the coronal neutral line and the heliospheric current sheet: Evidence for radial evolution of hemispheric asymmetry. Astronomy & Astrophysics, 618, A105. https://doi.org/10.1051/0004-6361/201832609

   Rinnakkaistallennettu versio / Self-archived version

3. Koskela, J., Virtanen, I., & Mursula, K. (in press). Revisiting the coronal current sheet model: Parameter range analysis and comparison with the potential field model. Astronomy & Astrophysics. https://doi.org/10.1051/0004-6361/201935967

   Rinnakkaistallennettu versio / Self-archived version

4. Virtanen, I. I., Koskela, J. S., & Mursula, K. (2019). Abrupt Shrinking of Solar Corona in the Late 1990s. Manuscript submitted for publication. https://doi.org/10.3847/2041-8213/ab644b

   Rinnakkaistallennettu versio / Self-archived version