Fast prograde coronal flows in solar active regions

Abstract

We report the discovery and characterization of high-speed (>100 km/s) horizontal flows in solar active regions, making use of the Sun-as-a-star spectroscopy in the range 5-105 nm provided by the EVE(Extreme Ultraviolet Variability Experiment) spectrometers on the Solar Dynamics Observatory. These apparent flows are persistent on time scales of days, and are well observed in lines of Mg x, Si xii and Fe xvi for example. They are prograde, as evidenced directly by blueshifts/redshifts peaking at the east/west limb passages of isolated active regions. The high-speed flow behaviour does not depend upon active-region latitude or solar cycle, with similar behaviour in Cycles 24 and 25.

The Temperature Dependence of Hot Prograde Flows in Solar Active Regions

“…This paper extends the analysis of EVE Doppler signatures on active-region time scales of days to weeks, as reported earlier for the MEGS-B data by Hudson et al (2022). Our new analysis focuses on MEGS-A, where astigmatism in the optics competes with the true Doppler signatures.…”

“…We validate our methods by independently quantifying this effect and comparing to Chamberlin's results, and explore the wavelength dependence as an extension of his formula as derived for He ii 304 Å. MEGS-B is unaffected by this instrumental effect in any case, and this has allowed us to find evidence of hot prograde flows in active regions. Using our image-based models for astigmatism and flows, we independently confirm the MEGS-B results (Hudson et al, 2022). We now extend our knowledge of the temperature dependence of these flows via the additional Fe emission lines available in MEGS-A.…”

“…In this case, the wavelength shifts exhibit a positive correlation with expected variations from astigmatism due to the rotation of active regions across the solar surface. Surprisingly, as shown graphically in Hudson et al (2022), similar oscillatory structures are present in the wavelength shifts of hot lines observed by MEGS-B, where the effects of the astigmatism are negligible (Woods et al, 2012). Hudson et al (2022) provide strong evidence to suggest that these time structures result from hot prograde flows in active regions.…”

“…Contributions from the astigmatism, and also the prograde flow component, will be largest for activity at or above the limbs. This can be seen in the figures in Hudson et al (2022): In the purely astigmatic case Equation (1) shows that maxima correspond symmetrically to activity at either limb (see Figure 8); the prograde flows instead have an asymmetric character: positive at the E and negative at the W. Furthermore, any emission from coronal loops extending above the solar surface at the limbs would be misrepresented by the use of heliographic coordinates. It is exactly in these regions of strong contributions where the pixel-clipping occurs.…”

“…In this paper, we build on the work of Chamberlin (2016), and similar work by Cheng, Wang, and Liu (2021), by using AIA images to develop a method to quantify this instrumental astigmatism and compare to the previous estimates. We extend the work of Hudson et al (2022) by modelling the hot flows using emission lines observed by MEGS-A and MEGS-B over a broad range of wavelengths and temperatures. This analysis makes it possible to use of many additional EVE/MEGS-A spectral lines for Doppler measurements.…”