Long-Term Evolution of the Solar Corona Using PROBA2 Data

Abstract

We use The Sun Watcher with Active Pixel System detector and Image Processing (SWAP) imager onboard the Project for Onboard Autonomy 2 (PROBA2) mission to study the evolution of large-scale EUV structures in the solar corona observed throughout Solar Cycle 24 (from 2010 to 2019). We discuss the evolution of the on-disk coronal features and at different heights above the solar surface based on EUV intensity changes. We also look at the evolution of the corona in equatorial and polar regions and compare them at different phases of the solar cycle, as well as with sunspot-number evolution and with the PROBA2/Large Yield RAdiometer (LYRA) signal. The main results are as follows: The three time series (SWAP on-disk average brightness, sunspot number, and LYRA irradiance) are very well correlated, with correlation coefficients around 0.9. The average rotation rate of bright features at latitudes of +15^∘, 0^∘, and −15^∘ was around 15 degree day⁻¹ throughout the period studied. A secondary peak in EUV averaged intensity at the poles was observed on the descending phase of SC24. These peaks (at North and South Poles, respectively) seem to be associated with the start of the development of the (polar) coronal holes. Large-scale off-limb structures were visible from around March 2010 to around March 2016, meaning that they were absent at the minimum phase of solar activity. A fan at the North Pole persisted for more than 11 Carrington rotations (February 2014 to March 2015), and it could be seen up to altitudes of 1.6 R_⊙.

Appendices

Appendix A: Synoptic Maps

1.1 A.1 On-Disk Synoptic Maps

The SWAP on-disk synoptic maps (see, e.g., Figure 2) were constructed using averaged 3^∘-wide longitudinal stripes (see left panel of Figure 14), centered on the central meridian for each image in a CR stack. The synoptic map shows an overview of the Sun for each Carrington rotation as observed by the PROBA2/SWAP instrument. The horizontal axis represents the time (in days) and the vertical axis represents the Stonyhurst latitude in degrees. For a description of this coordinate system see Thompson (2006).

Figure 14

SWAP image on 11 November 2014. Left panel: The vertical white stripe shows the region at central meridian that is used to build the synoptic map. Middle panel: The horizontal white stripe shows the region at 1.3 solar radii from the Sun center that is used to build the off-limb synoptic map at South Pole. Right panel: Example of a sector where the average value is calculated (white pixels). The sector is centered at South Pole, it has a width of 90^∘ and the height from 1.3 to 1.6 R_⊙.

We also build the East–West (EW) SWAP synoptic maps (or time–longitude maps), similar to the classical synoptic maps described above, but this time we used averaged 3^∘-wide latitudinal stripes centered on the solar equator (see Figure 3). The vertical axis represents the time [days] and the horizontal axis represents the Stonyhurst longitude [degrees]. Similar to this, we built EW synoptic maps at different latitudes (±20^∘, ±40^∘) – see, e.g., Figure 4.

1.2 A.2 Off-Limb Synoptic Maps

We build off-limb synoptic maps at the Equator and poles by taking horizontal three-pixel stripes, respectively, at 1.1, 1.3, and 1.6 solar radii from the Sun center, average them over the three-pixel width, and stack them in time – see Figure 5 for examples of off-limb synoptic maps. In the case of equatorial off-limb maps the time [days] is on the horizontal axis and the helioprojective latitude [arcseconds] is on the vertical axis. The cuts are done at different distances from the Sun center. For the polar off-limb images, the $y$-axis represents the time [days] and the $x$-axis represents the helioprojective longitude [arcseconds]. The central panel of Figure 14 shows an example of a horizontal stripe at a distance of 1.3 R_⊙ south from the Sun center from which the polar off-limb synoptic maps were created.

Appendix B: Sectors

In order to study the evolution of the solar corona at different altitudes, and over different regions, we divided each image to sectors where we measured the mean brightness. The right panel of Figure 14 shows an example of a sector centered at the South Pole, with a width of 90^∘, and the height from 1.3 to 1.6 R_⊙.