Uniformity of the quiet solar disk: 3130 - 46700 {\AA}

TL;DR

This study measures the uniformity of the quiet solar disk across a broad wavelength range with record low noise, revealing fluctuations likely due to granulation, and provides data useful for exoplanet detection.

Contribution

The paper presents highly precise measurements of solar disk uniformity across 3130 to 46700 Å, achieving unprecedented low noise levels through specific observing procedures.

Findings

Fluctuations range from 0.3% at 3130 Å to 0.06% at 46700 Å.

Observed fluctuations are consistent with granulation effects.

No significant differences between scan directions (EW, N-S).

Abstract

Taking advantage of the absence of solar activity in the recent 2008-9 epoch (no spots, few faculae), we have made equatorial and meridian disk scans in continua from 3129 {\AA} to 46700 {\AA}. Averaging 20 scans at each wavelength to suppress granulation, which takes a total of 35 minutes, we achieve a system noise level of 0.01%. We believe this noise level is a record low, not because of instrument improvements, but simply because of observing procedures and the cooperation of the Sun and sky. The observed solar fluctuations significantly exceed the noise and range from 0.3% at 3130 {\AA}, 0.05% at 34000 {\AA}, to 0.06% at 46700 {\AA} near disk center. These fluctuations (corresponding to about 3 K) presumably arise from the incomplete averaging of granulation. Standard solar models for limb darkening fit the data for true continuum regions reasonably well. No significant differences are seen between scan directions (EW, N-S). Perhaps our results can serve as a template for exoplanet detection by the transit of quiet G2V-like stars.