The SWAP Filter: A Simple Azimuthally Varying Radial Filter for Wide-Field EUV Solar Images.

We present the SWAP Filter: an azimuthally varying, radial normalizing filter specifically developed for EUV images of the solar corona, named for the Sun Watcher with Active Pixels and Image Processing (SWAP) instrument on the Project for On-Board Autonomy 2 (PROBA2) spacecraft. We discuss the origins of our technique, its implementation and key user-configurable parameters, and highlight its effects on data via a series of examples. We discuss the filter's strengths in a data environment in which wide field-of-view observations that specifically target the low signal-to-noise middle corona are newly available and expected to grow in the coming years. Supplementary Information: The online version contains supplementary material available at 10.1007/s11207-023-02183-w.

Defining the Middle Corona.

The middle corona, the region roughly spanning heliocentric distances from 1.5 to 6 solar radii, encompasses almost all of the influential physical transitions and processes that govern the behavior of coronal outflow into the heliosphere. The solar wind, eruptions, and flows pass through the region, and they are shaped by it. Importantly, the region also modulates inflow from above that can drive dynamic changes at lower heights in the inner corona. Consequently, the middle corona is essential for comprehensively connecting the corona to the heliosphere and for developing corresponding global models. Nonetheless, because it is challenging to observe, the region has been poorly studied by both major solar remote-sensing and in-situ missions and instruments, extending back to the Solar and Heliospheric Observatory (SOHO) era. Thanks to recent advances in instrumentation, observational processing techniques, and a realization of the importance of the region, interest in the middle corona has increased. Although the region cannot be intrinsically separated from other regions of the solar atmosphere, there has emerged a need to define the region in terms of its location and extension in the solar atmosphere, its composition, the physical transitions that it covers, and the underlying physics believed to shape the region. This article aims to define the middle corona, its physical characteristics, and give an overview of the processes that occur there.

Mapping solar magnetic fields from the photosphere to the base of the corona.

Routine ultraviolet imaging of the Sun's upper atmosphere shows the spectacular manifestation of solar activity; yet, we remain blind to its main driver, the magnetic field. Here, we report unprecedented spectropolarimetric observations of an active region plage and its surrounding enhanced network, showing circular polarization in ultraviolet (Mg ii h & k and Mn i) and visible (Fe i) lines. We infer the longitudinal magnetic field from the photosphere to the very upper chromosphere. At the top of the plage chromosphere, the field strengths reach more than 300 G, strongly correlated with the Mg ii k line core intensity and the electron pressure. This unique mapping shows how the magnetic field couples the different atmospheric layers and reveals the magnetic origin of the heating in the plage chromosphere.

Global Non-Potential Magnetic Models of the Solar Corona During the March 2015 Eclipse.

Seven different models are applied to the same problem of simulating the Sun's coronal magnetic field during the solar eclipse on 2015 March 20. All of the models are non-potential, allowing for free magnetic energy, but the associated electric currents are developed in significantly different ways. This is not a direct comparison of the coronal modelling techniques, in that the different models also use different photospheric boundary conditions, reflecting the range of approaches currently used in the community. Despite the significant differences, the results show broad agreement in the overall magnetic topology. Among those models with significant volume currents in much of the corona, there is general agreement that the ratio of total to potential magnetic energy should be approximately 1.4. However, there are significant differences in the electric current distributions; while static extrapolations are best able to reproduce active regions, they are unable to recover sheared magnetic fields in filament channels using currently available vector magnetogram data. By contrast, time-evolving simulations can recover the filament channel fields at the expense of not matching the observed vector magnetic fields within active regions. We suggest that, at present, the best approach may be a hybrid model using static extrapolations but with additional energization informed by simplified evolution models. This is demonstrated by one of the models.

Magnetic Nulls and Super-radial Expansion in the Solar Corona.

Magnetic fields in the Sun's outer atmosphere-the corona-control both solar-wind acceleration and the dynamics of solar eruptions. We present the first clear observational evidence of coronal magnetic nulls in off-limb linearly polarized observations of pseudostreamers, taken by the Coronal Multichannel Polarimeter (CoMP) telescope. These nulls represent regions where magnetic reconnection is likely to act as a catalyst for solar activity. CoMP linear-polarization observations also provide an independent, coronal proxy for magnetic expansion into the solar wind, a quantity often used to parameterize and predict the solar wind speed at Earth. We introduce a new method for explicitly calculating expansion factors from CoMP coronal linear-polarization observations, which does not require photospheric extrapolations. We conclude that linearly polarized light is a powerful new diagnostic of critical coronal magnetic topologies and the expanding magnetic flux tubes that channel the solar wind.