Multi-decadal evolution of the solar coronal magnetic field

In this thesis, we have studied the evolution of the solar coronal magnetic field over several decades. The corona is the outermost layer of the solar atmosphere. It consists of very hot, ionized plasma. Solar wind, which has an effect on for example the Earth’s magnetic field and the occurrence of aurora, is formed when the coronal plasma expands to the outer space. Also the heliospheric magnetic field originates from the corona, and extends all the way out to the outer reaches of the solar system.

The plasma in the corona is very sparse, which is why the coronal magnetic field is very difficult to measure directly. Currently, only a few direct measurements exist. However, the coronal magnetic field can be modelled using even very simple mathematical models. The models can use, for example, measurements of the solar surface, i.e., the photospheric magnetic field. These photospheric measurements exist over several decades.

In my thesis I have modelled the corona using two different models with several different parameters. I have compared these results to the measurements of the heliospheric magnetic field at the Earth’s orbit. The coronal field calculated with both models corresponds very well, in large scales, to the heliospheric measurements. We also found that the size of the corona has been shrinking over four centuries. This shrinking has been occurring simultaneously with the general weakening of the solar magnetic field.