Signatures of photospheric convection throughout the solar atmosphere: the EVE Sun-as-a-star mHz continuum

Abstract

Abstract Convectively driven motions in the solar photosphere can generate broadband Doppler variability across the chromosphere, transition region and corona. Here we investigate this variability using “Sun-as-a-Star” observations from the Extreme Ultraviolet Variability Experiment (EVE) aboard the Solar Dynamics Observatory, constructing high signal-to-noise Doppler power spectra by incoherently averaging spectra from 3-hour sequences of the centroid wavelengths of emission lines that span wavelengths 35–104 nm. The spectra reveal a broad power-spectral continuum with two “Harvey” components, one of which extends to the Nyquist frequency at 50 mHz with a steep power-law tail. All emission lines have substantial power both above and below the nominal acoustic cutoff frequency at 5 mHz, but none of the power spectra show maxima corresponding to p-mode or chromospheric 3-minute oscillations. Lines formed in the corona, as compared with those of the chromosphere/transition region, have substantially less Doppler amplitude in the Harvey spectrum associated with granulation-scale convection. Based on the observed continuum, there is no evidence (in any of the 26 lines studied) for Kolmogorov turbulence, which predicts a flat continuum component with Doppler variance 〈v2〉 ∝ f−5/3 as a function of frequency f. The total inferred RMS velocities (>0.1 mHz) are roughly consistent with coronal “microturbulence” estimates from excess line widths. These observations provide the first clear detection of Sun-as-a-star EUV Doppler variability above ∼10 mHz and demonstrate the potential of full-disk EUV spectroscopy to probe turbulent energy transport throughout a stellar atmosphere. The predominance of the Harvey spectrum, a signature of photospheric convection, likely results from limb brightening of the Doppler variance due to horizontal flows in the upper solar atmosphere.