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In this work, we present a metastable helium lidar system for the measurements of metastable helium He(23S) density in the thermosphere and lower exosphere. This lidar system consists of a high power 1,083 nm pulsed laser, a 1 m aperture laser beam expander, six 1 m aperture receiving telescopes and a superconducting nanowire single‐photon detector (SNSPD). This system realizes metastable helium density detection up to 1,000 km. Daily rapid variation of metastable helium density within several hours was measured with a height resolution of 50 km for the first time. It demonstrates the capability of ground‐based lidar for continuous height‐resolved detection of the atmospheric metastable helium in the height range of 200–1,000 km. This is a promising tool to help study the coupling of neutral and ionized atmosphere in this height range and further providing observation basis for space weather prediction. Plain Language Summary: Many different kinds of lidar have been well developed for tens of years and can detect the atmosphere up to 180 km by different principles (such as aerosol scattering, Rayleigh scattering, resonance fluorescence scattering and so on). However, in the higher height range of 200–1,000 km, there are almost no suitable methods to monitoring the atmosphere continuously from the ground, except for one choice: metastable helium lidar. In this work, we built a powerful 1,083 nm pulsed laser and emit it vertically in to the atmosphere, the metastable helium atoms resonance with the laser and emits fluorescence photons. The scattered photons from the very high atmosphere are collected and focused by the large aperture telescope on the ground and detected by the photoelectric detector. Building such a lidar has many difficulties that have never been encountered by other conventional lidars, such as the large signal attenuation by long distance. This powerful lidar is built successfully in 2023 and make observations for several months. The first high quality initial data of the observed metastable helium density profile in 200–1,000 km is shown in this paper. This observation will continuously provide useful observation for the study of space physics and space weather forecasting. Key Points: A metastable helium lidar system for the measurements of metastable helium He(23S) density in 200–1,000 km has been developedDaily rapid variation of metastable helium density within several hours was measured with a height resolution of 50 km for the first timeThis is a promising tool to help providing observation basis for space weather prediction [ABSTRACT FROM AUTHOR]

The influence of the non‐ideality (NI) of the classical plasmas on the doubly excited singlet S states of the helium atom (He) embedded in the plasma has been investigated theoretically. A pseudopotential containing the Debye length and the non‐ideality parameter (NIP) as its characteristics is used to represent the screened interaction potentials in the plasma. Using a large wavefunction within the framework of the stabilization method, it has been possible to recognize six doubly excited singlet S states (five lying below the He+(2S) excitation threshold and one lying below the He+(3S) excitation threshold) for the plasma‐free case. The energies and the autoionization life‐times of those states are computed by fitting the density of states to the Lorentzian form. Convergence of the computed results is corroborated by increasing the number of terms in the employed wavefunction. For the plasma‐free case, these results are in excellent agreement with the established results in the literature. A comprehensive analysis has been made on changes induced on those doubly excited states by varying NI over a wide range. It has been observed that the energies of the states gradually approach the corresponding threshold energies with the increasing NI of the plasma, whereas the change in the life‐times (alternatively the widths of the states) of the states shows distinctive features depending on the angular momentum of an individual electron. [ABSTRACT FROM AUTHOR]

In this article, we study the helium atom confined in a spherical impenetrable cavity by using informational measures. We use the Ritz variational method to obtain the energies and wave functions of the confined helium atom as a function of the cavity radius r0$$ {r}_0 $$. As trial wave functions we use one uncorrelated function and five explicitly correlated basis sets in Hylleraas coordinates with different degrees of electronic correlation. We computed the Shannon entropy, Fisher information, Kullback–Leibler entropy, Tsallis entropy, disequilibrium and Fisher–Shannon complexity, as a function of r0$$ {r}_0 $$. We found that these entropic measures are sensitive to electronic correlation and can be used to measure it. As expected these entropic measures are less sensitive to electron correlation in the strong confinement regime (r0<1$$ {r}_0<1 $$ a.u.). [ABSTRACT FROM AUTHOR]