Evaluation of the day-time ground-level turbulence at Mt Wumingshan with a microthermal sensor

Abstract

ABSTRACT Surface-layer turbulence, especially the turbulence close to the ground (ground-level, GL), critically determines the height at which to locate a ground-based telescope. It is also vital to assess the contribution of the GL to the total turbulence. To characterize the day-time turbulence at the GL at the Mt Wumingshan (Mt WMS) site (a candidate site for the Chinese next-generation large Solar Telescope), a microthermal sensor (MS) was installed to detect fluctuations of the temperature at the GL and estimate the contribution of the GL to the total turbulence. The MSs were placed at six levels on a 22-m-high tower to measure the refractive index $C_{n}^{2}$ configuration constant. This study presents the day-time optical turbulences due to the GL at the Mt WMS site and compares them with the integrated day-time seeing that was obtained by the Wumingshan Mountain Automated Solar Differential Image Motion Monitor (WMA-SDIMM). MS and WMA-SDIMM data were obtained simultaneously for a total of 16 d during June to October 2017. The average r0 of the GL was found to be $r_{0}^{\rm gl}(\rm {1\, m,16\, m})=11.04\, {\rm cm}$ and $r_{0}^{\rm gl}({\rm 16\,m,22\,m})=61.64\,{\rm cm}$. The contribution of the GL (8 to 22 m) to the total turbulence was $13.05{{\ \rm per\ cent}}$ on average, complying with a $8.0{{\ \rm per\ cent}}$ reduction of the total seeing. At the height of 22 m, most of the GL turbulence was produced below 16 m.