Abstract 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 to produce weak fields during the poleward drift of the surge. / Original papers Original papers are not included in the electronic version of the dissertation. Getachew, T., Virtanen, I., & Mursula, K. (2017). Structure of the Photospheric Magnetic Field During Sector Crossings of the Heliospheric Magnetic Field. Solar Physics, 292(11). https://doi.org/10.1007/s11207-017-1198-9 http://jultika.oulu.fi/Record/nbnfi-fe201802083259 Getachew, T., Virtanen, I., & Mursula, K. (2019). Asymmetric Distribution of Weak Photospheric Magnetic Field Values. The Astrophysical Journal, 874(2), 116. https://doi.org/10.3847/1538-4357/ab0749 http://jultika.oulu.fi/Record/nbnfi-fe2019061320447 Getachew, T., Virtanen, I., & Mursula, K. (2019). A New Signal of the Solar Magnetic Cycle: Opposite Shifts of Weak Magnetic Field Distributions in the Two Hemispheres. Geophysical Research Letters, 46(16), 9327–9333. https://doi.org/10.1029/2019gl083339 Mursula, K., Getachew, T., & Virtanen, I. (2019). Spatial-temporal evolution of photospheric weak-field shifts in solar cycles 21-24. Astron. Astrophys., submitted.

Abstract This thesis studies the structure and evolution of the large scale heliospheric magnetic field. The work covers the space age, the period when satellite measurements revolutionized our knowledge about space. Now, this period is known to be the declining phase of the grand modern maximum of solar activity. The thesis addresses how the hemispherical asymmetry of solar activity is seen in the photospheric magnetic field and how it appears in the corona and in the heliosphere until the termination shock. According to geomagnetic and heliospheric observations, the heliospheric current sheet has been southward shifted around the solar minima since 1930s. Using Ulysses probe observations, we derive an accurate estimate of 2° for the southward shift of the heliospheric current sheet during two very different solar minimum in the mid 1990s and 2000s. The overall structure of the heliospheric magnetic field has changed significantly now when the grand modern maximum has come to an end. During the present low solar activity the polar fields are weaker and the heliospheric current sheet covered a wide latitudinal range during the previous minimum. When the heliospheric current sheet is wide the asymmetry is less visible at the Earth’s orbit. We extend our study to the outer heliosphere using measurements made by Voyager and Pioneer probes and show that the hemispherical asymmetry in the coronal hole evolution, and the related southward shift of the heliospheric current sheet, are seen until the termination shock. In order to understand the origin of the hemispherical asymmetry, we complete a multipole analysis of the solar magnetic field since 1976. We find that the minimum time southward shift of the heliospheric current sheet is due to the quadrupole component of the coronal magnetic field. The quadrupole term exists because the generation and transport of the magnetic flux in the Sun tends to proceed differently in the northern and southern hemispheres. During this and the following decade the Sun is most likely going to be less active than it has been since 1920s. Therefore it is probable that the hemispherical asymmetry of the heliospheric magnetic field will be less visible in the ecliptic plane in the near future. Now, when the Sun seems to be at the maximum of cycle 24, we are looking forward to see how the polar fields and the heliospheric magnetic field are formed when approaching the following solar minimum. It is possible that, as the activity rises again after the present and future low cycles, the hemispherical asymmetry will be opposite to that of the 20th century and the minimum time heliospheric current sheet would be northward shifted.

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