In the Trenches of the Solar-stellar Connection. IV. Solar Full-disk Scans of C ii, Si iv, and Mg ii by the Interface Region Imaging Spectrograph

Abstract

Abstract About once a month, the Interface Region Imaging Spectrograph conducts day-long raster scans of the full Sun in three ultraviolet spectral channels. These full-disk mosaics are valuable in the solar context, but provide a unique connection to the distant, unresolved stars. Here, 10 deep-exposure scans (4–8 s per slit step), collected during the peak and decline of sunspot Cycle 24, were analyzed. Spatial spectra (2″ pixels) of resonance lines of C ii (T ∼ 104 K), Si iv (8 × 104 K), and Mg ii (8 × 103 K) were fitted with a pseudo-Gaussian model to track the emission strengths, widths, and shifts in the various surface features that comprise the quiet Sun and active regions. The full-disk mosaic spectra compare well to tracings of solar-twin α Centauri A (HD 128620; G2 V). The contrast between disk-average spectra from cycles MIN and MAX is relatively modest (∼50% in Si iv), but, remarkably, the brightest solar pixels in active regions, at 2″ resolution, exceed the global-average intensities of the most active Sun-like stars, suggesting a deeper solar-stellar connection. Si iv shows a conspicuous bright ring at the limb, whereas optically thicker C ii and Mg ii are suppressed (more so for the latter). The Si iv emission favors the bright knots of the large-scale supergranulation network, while the cooler Mg ii emission is more ubiquitous and C ii intermediate. The non-Gaussian profile of full-disk C iv, similar in formation temperature to Si iv, was previously interpreted as a combination of narrow and broad dynamical components, but the prevalence of the characteristic line shape in the finest resolution spatial pixels of Si iv here provides support for alternative formation scenarios, for example, invoking κ-distributions.