Speaker
Description
Solar active regions, accumulations of strong magnetic field, play a crucial role in driving space weather. Their evolution can influence the solar wind, and they can trigger eruptive phenomena. Active regions have long been studied from Earth’s vantage point using instruments such as the Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO). While HMI allows for uninterrupted monitoring of the photospheric magnetic field as seen from Earth, the full evolution of active regions can usually not be captured due to their extended lifetimes. Helioseismic far-side imaging offers indirect detection of active regions beyond Earth’s view. However, this method remains limited by the lack of polarimetric information, which prevents a complete study of the magnetic flux evolution of the regions from emergence to their eventual decay.
The ESA/NASA Solar Orbiter mission addresses these limitations by observing the Sun from different vantage points. Among its suite of instruments, the Polarimetric and Helioseismic Imager (SO/PHI) is the first instrument to acquire polarimetric data of the solar photosphere from outside the Sun–Earth line. The unique heliocentric orbit of the spacecraft allows observing the Sun for extended periods each year at a longitudinal separation angle of more than 130º from the Sun-Earth line. During these periods, SO/PHI performs regular synoptic observations with its Full Disk Telescope.
The combination of SO/PHI’s direct observations of the solar far side with direct observations of the near-Earth side gives rise to new opportunities for the study of the evolution of the photospheric magnetic field, especially of active regions. Thus, the regions can be observed well beyond the very limited window available from a single viewpoint. An almost uninterrupted tracking enables studying the evolution of active regions over multiple solar rotations, which in some cases means over their full lifetime. In combination with the extreme ultraviolet instruments on board Solar Orbiter and SDO this offers a deeper insight into the interplay between the surface magnetic fields of the active regions and their structures and dynamics in the overlying corona. Furthermore, co-temporal observations of the photospheric magnetic field from opposite sides of the Sun allow a more frequent production of synoptic maps, which enhances coronal and space weather models. Ultimately, the dual-perspective has permitted, for the first time, the direct calibration of helioseismic far-side measurements. Such calibrations are crucial, especially when direct far-side observations are unavailable, as they provide a more complete view of the Sun’s full 360º magnetic field which significantly influences solar wind modelling.
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