The Variations in the Sky Brightness of Lijiang and Namco throughout the Day

Abstract

The sky brightness is an important parameter to evaluate the excellence of a coronal observatory, which directly determines whether the coronagraph can measure and monitor the coronal activity for a long time. The Sky Brightness Moniter (SBM) is a precision instrument for accurately measuring important atmospheric parameters such as the sky brightness, water vapor content, and atmospheric extinction. It is also an universal instrument for coronagraph site selection and an important equipment for solar site selection in western China. Before conducting statistical analysis on the accumulated data from Namco, this paper first performs an in-depth analysis using representative data from Namco and Lijiang. The aim is to preliminarily study and understand the characteristics of the sky brightness at the site based on the profiles of the representative data. Firstly, by considering the actual climatic characteristics and the normalized sky brightness, the paper gradually explains the different variations of the sky brightness throughout the day. Secondly, we establish an effective calculation method to measure and reveal the evolution characteristics of the sky brightness. The results show: 1) The sky brightness (green line, 530 nm) at the Lijiang station with an altitude of 3200 meters and the Namco site with an altitude of 4700 meters are both relatively good. The minimum values can reach the order of $10\times {10}^{-6}$, ensuring the feasibility of regular corona observations. 2) The sky brightness at the Namco site and the Lijiang station evolves differently over time. The Lijiang station is not suitable for all-day observations, with morning conditions being far superior to those in the afternoon. In contrast, the Namco site has a longer observation time (from 9:00 AM to 4:00 PM), and its cooperation with the observation equipment of other stations can significantly improve the efficiency of space weather ground monitoring. The analysis results not only provides a sample analysis for subsequent statistics on the sky brightness at Namco but also demonstrates the expected characteristics of the sky brightness at Namco.