A Review of the Extended EUV Corona Observed by the Sun Watcher with Active Pixels and Image Processing (SWAP) Instrument

West, Matthew J.; Seaton, Daniel B.; D’Huys, Elke; Mierla, M.; Laurenza, M.; Meyer, Karen; Berghmans, D.; Rachmeler, Laurel; Rodriguez, L.; Stegen, K.

doi:10.1007/s11207-022-02063-9

Cited by 6 publications

(5 citation statements)

References 152 publications

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“…In which case, the tangential speed of the solar wind may be more strongly influenced (rigid-rotation up to 1.5R results in a ∼ 7% increase in the Sun's wind-braking torque). Measurements of the middle-corona are needed in order to evaluate impact of strong active regions on coronal rotation (investigations into this area have become more frequent West et al 2022;Chitta et al 2022).…”

Section: Equatorial Connectivity and Rotation Ratementioning

confidence: 99%

Accounting for differential rotation in calculations of the Sun’s angular momentum-loss rate

Finley¹,

Brun²

2023

A&A

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Context. Sun-like stars shed angular momentum due to the presence of magnetised stellar winds. Magnetohydrodynamic models have been successful in exploring the dependence of this "wind-braking torque" on various stellar properties, however the influence of surface differential rotation is largely unexplored. As the wind-braking torque depends on the rotation rate of the escaping wind, the inclusion of differential rotation should effectively modulate the angular momentum-loss rate based on the latitudinal variation of wind source regions.Aims. Here we aim to quantify the influence of surface differential rotation on the angular momentum-loss rate of the Sun, in comparison to the typical assumption of solid-body rotation. Methods. To do this, we exploit the dependence of the wind-braking torque on the effective rotation rate of the coronal magnetic field, which is known to be vitally important in magnetohydrodynamic models. This quantity is evaluated by tracing field lines through a Potential Field Source Surface (PFSS) model, driven by ADAPT-GONG magnetograms. The surface rotation rates of the open magnetic field lines are then used to construct an open-flux weighted rotation rate, from which the influence on the wind-braking torque can be estimated. Results. During solar minima, the rotation rate of the corona decreases with respect to the typical solid-body rate (the Carrington rotation period is 25.4 days), as the sources of the solar wind are confined towards the slowly-rotating poles. With increasing activity, more solar wind emerges from the Sun's active latitudes which enforces a Carrington-like rotation. Coronal rotation often displays a north-south asymmetry driven by differences in active region emergence rates (and consequently latitudinal connectivity) in each hemisphere.Conclusions. The effect of differential rotation on the Sun's current wind-braking torque is limited. The solar wind-braking torque is ∼ 10 − 15% lower during solar minimum, (compared with the typical solid body rate), and a few percent larger during solar maximum (as some field lines connect to more rapidly rotating equatorial latitudes). For more rapidly-rotating Sun-like stars, differential rotation may play a more significant role, depending on the configuration of the large-scale magnetic field.

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Section: Equatorial Connectivity and Rotation Ratementioning

confidence: 99%

Accounting for differential rotation in calculations of the Sun’s angular momentum-loss rate

Finley¹,

Brun²

2023

A&A

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“…Contemporary imaging techniques include the stacking of multiple short-exposure observations to approximate a long exposure (e.g. West et al., 2022 ), and the use of detectors with locally variable exposure times (Mason et al., 2022 ). Post-processing techniques, which improve the display of these high-dynamic-range images, include computational radial-graded filters (e.g.…”

Section: How We Observe the Middle Coronamentioning

confidence: 99%

“…The three images show the same SWAP (17.4 nm) observation from 10 November 2014, processed nominally ( top left ), using a stacking technique ( top right , see West et al. ( 2022 ) for further details), and using the MGN technique ( bottom : Morgan and Druckmüller, 2014 ). …”

Section: How We Observe the Middle Coronamentioning

confidence: 99%

Defining the Middle Corona

et al. 2023

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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.

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“…Early in the SWAP mission, we realized that SWAP’s large FOV might reveal features and evolution in the solar corona that had not been well observed previously in these wavelengths. We began to produce movies composed of deep-exposure images spanning full Carrington rotations, using median stacking – that is, computing the median of each image pixel in the temporal direction – of many individual SWAP observations to suppress noise and enhance the visibility of faint features in the middle corona (see Section 2.2 in West et al., 2022 ). We quickly realized, however, that without a technique to equalize the dynamic range in the corona, there was no path to take full advantage of these new observations, since we could not clearly visualize the large-scale features in the data.…”

Section: The Origins Of Our Filter Techniquementioning

confidence: 99%

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

et al. 2023

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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.

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A Review of the Extended EUV Corona Observed by the Sun Watcher with Active Pixels and Image Processing (SWAP) Instrument

Cited by 6 publications

References 152 publications

Accounting for differential rotation in calculations of the Sun’s angular momentum-loss rate

Accounting for differential rotation in calculations of the Sun’s angular momentum-loss rate

Defining the Middle Corona

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

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