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

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.