Conditions for Coronal Observation at the Lijiang Observatory in 2011

TL;DR

This study assesses the sky brightness at Lijiang Observatory in 2011 to determine its suitability for coronal observations, highlighting optimal observation times and the influence of aerosols on sky conditions.

Contribution

It provides the first continuous sky brightness measurements at Lijiang Observatory, establishing conditions favorable for solar coronal observations and identifying key factors affecting sky clarity.

Findings

Sky brightness remains below 20 millionths per airmass before noon most of the year.

Optimal observation time is from 9:00 to 13:00 Beijing time.

Aerosol scattering significantly influences diurnal sky brightness variation.

Abstract

The sky brightness is a critical parameter for estimating the coronal observation conditions for solar observatory. As part of a site-survey project in Western China, we measured the sky brightness continuously at the Lijiang Observatory in Yunnan province in 2011. A sky brightness monitor (SBM) was adopted to measure the sky brightness in a region extending from 4.5 to 7.0 apparent solar radii based on the experience of the Daniel K. Inouye Solar Tele- scope (DKIST) site survey. Every month, the data were collected manually for at least one week. We collected statistics of the sky brightness at four bandpasses located at 450, 530, 890, and 940 nm. The results indicate that aerosol scattering is of great importance for the diurnal variation of the sky brightness. For most of the year, the sky brightness remains under 20 millionths per airmass before local Noon. On average, the sky brightness is less than 20 millionths accounts for 40.41 % of the total observing time in a clear day. The best observation time is from 9:00 to 13:00 (Beijing time). Lijiang Observatory is therefore suitable for coronagraphs investigating the structures and dynamics of the corona.