As a public facility, the Lijiang 2.4-m telescope of Yunnan Observatories, is required to change one auxiliary instrument with another in as short a time as possible at night, in order to quickly respond to scientific programs (e.g. transient observation, time domain studies) and changes of observation conditions (e.g. seeing and weather condition). In this paper, we describe the design, construction and test of a rapid instrument exchanging system (RIES), both the hardware and software, for the Cassegrain focal station of this telescope, which enables the telescope to quickly change instruments at night without much loss of observing time. The tests in the laboratory and at the telescope show that the image quality and pointing accuracy of the RIES are satisfied. With the RIES, we observed the same Landolt standard stars almost at the same time with the Princeton Instrument VersArray 1300B Camera and the Yunnan Faint Object Spectrograph and Camera (YFOSC), both mounted at Cassegrain focus. The Quasi-simultaneous comparison shows that the image quality of the optical system inside the YFOSC is comparable with PICCD.

We use the observations from the incoherent scatter radar and the magnetometers over Jicamarca (11.95° S, 76.87° W) sector to investigate the equatorial ionospheric electrodynamics on 6–11 September 2017. In this period, there are three X‐class solar flares peaked at 12:02, 14:36, and 16:06 UT on 6, 7, and 10 September, respectively, in the local daytime of Jicamarca and a storm with the minimum SYM‐H of −146 nT at 01:08 UT on 8 September. The results reveal that the three X‐class flares drive the westward disturbance electric field in equatorial region. The flares intensify the westward and eastward equatorial electrojets on 6 and 10 September, while the eastward equatorial electrojet enhances first and then declines on the 7 September flare. The depletion in equatorial electrojet during flare is suggested to be the effects of the weakened electric field. In the storm recovery, the disturbance electric field is westward at ~10–16 LT and eastward at postmidnight and early morning sectors on 9–11 September, which is mainly driven by disturbance dynamo electric field (DDEF). Further, it is found that the westward DDEF first occurs at high altitudes and then at low altitudes around 10 LT, and the westward DDEF at 200–500 km has magnitudes increasing with altitudes. Meanwhile, the Jicamarca incoherent scatter radar observations show distinct altitudinal differences in the responses of the F region electron density to the disturbance electric field.

The Lijiang 2.4-meter Telescope (LJT), the largest common-purpose optical telescope in China, has been available to the worldwide astronomical community since 2008. It is located at the Gaomeigu site, Lijiang Observatory (LJO), in the southwest of China. The site has very good observational conditions. During its 10-year operation, several instruments have been equipped on the LJT. Astronomers can perform both photometric and spectral observations. The main scientific goals of LJT include recording photometric and spectral evolution of supernovae, reverberation mapping of active galactic nuclei, investigating the physical properties of binary stars and near-earth objects (comets and asteroids), and identification of exoplanets and all kinds of transients. Until now, the masses of 41 high accretion rate black holes have been measured, and more than 168 supernovae have been identified by the LJT. More than 190 papers related to the LJT have been published. In this paper, the general observation conditions of the Gaomeigu site is introduced at first. Then, the structure of the LJT is described in detail, including the optical, mechanical, motion and control system. The specification of all the instruments and some detailed parameters of the YFOSC is also presented. Finally, some important scientific results and future expectations are summarized.