X-ray and ultraviolet (UV) observations of the outer solar atmosphere have been used for many decades to measure the fundamental parameters of the solar plasma. This review focuses on the optically thin emission from the solar atmosphere, mostly found at UV and X-ray (XUV) wavelengths, and discusses some of the diagnostic methods that have been used to measure electron densities, electron temperatures, differential emission measure (DEM), and relative chemical abundances. We mainly focus on methods and results obtained from high-resolution spectroscopy, rather than broad-band imaging. However, we note that the best results are often obtained by combining imaging and spectroscopic observations. We also mainly focus the review on measurements of electron densities and temperatures obtained from single ion diagnostics, to avoid issues related to the ionisation state of the plasma. We start the review with a short historical introduction on the main XUV high-resolution spectrometers, then review the basics of optically thin emission and the main processes that affect the formation of a spectral line. We mainly discuss plasma in equilibrium, but briefly mention non-equilibrium ionisation and non-thermal electron distributions. We also summarise the status of atomic data, which are an essential part of the diagnostic process. We then review the methods used to measure electron densities, electron temperatures, the DEM, and relative chemical abundances, and the results obtained for the lower solar atmosphere (within a fraction of the solar radii), for coronal holes, the quiet Sun, active regions and flares.

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Solar UV and X-ray spectral diagnostics

The broad bandwidth and long storage lifetimes of Nd3+: glass and ruby lasers have made possible the generation of picosecond laser pulses having peak powers it excess of one gigawatt and repetition rates in the microwave range. The numerous application areas of these pulses include research in nonlinear optics, transient response of atomic and molecular systems, optically generated plasmas, spectroscopy, ranging, optical information processing, and high-speed photography. This paper reviews several experimental techniques for generating, measuring, and utilizing these ultrashort laser pulses.

Picosecond laser pulses

This paper provides a review of papers published on laser induced gas breakdown. The material is divided into four parts: 1) breakdown mechanism, 2) plasma expansion under the influence of the laser pulse, 3) plasma decay after the end of the laser pulse, and 4) bibliography of papers published on the subject up to the end of May, 1968.

Laser induced gas breakdown: A bibliographical review

Picosecond Laser Pulses

This paper describes a spectroscopic study of the plasma produced when a giant pulse ruby laser of 5 J energy and 17 ns duration is focused into a 0.3 mm diameter spot on a polyethylene target in vacuum. Quantitative measurements of spectral intensities in the range from 20 Angstrom to 6000 Angstrom have yielded estimates of the electron and ion densities, the electron temperature and the streaming velocities of the ions. From these results it is estimated that the electron temperature, the ionization energy, the energy lost by radiation and the energy of recoil of the target account for only about 10% of the incident laser energy. The ion kinetic energy accounts for 70% of the incident energy with an experimental uncertainty of ± 70%. It is calculated that there are about 1016 ions produced by the laser beam striking the target. It is shown that the electrons contained in the plasma expanding away from the target cool adiabatically. In the course of the work there have been identified several new lines hitherto unclassified.

https://iopscience.iop.org/article/10.1088/0022-3700/1/6/324/pdf

A spectroscopic study of the plasma generated by a laser from polyethylene

Plasmas produced by focusing a 400 MW ruby laser beam onto surfaces of C, Ti, Mn, Fe, Ni, Cu and Zn, and into argon gas, have been observed in the extreme ultra-violet region. Line and continuum radiation have been recorded down to 25 A, including new spectra of Fe XV and Fe XVI and Ni XVII and Ni XVIII. The value of such plasmas for line classification work is demonstrated.

https://iopscience.iop.org/article/10.1088/0370-1328/88/4/129/pdf

Extreme ultra-violet spectra from laser-produced plasmas

The use of vacuum sparks and laser-produced plasmas has enabled many of the stronger transitions from these ions to be classified. New lines of Fe XII and XIII complete the classification of the intense members of the solar emission line group at 167 A to 220 A.

P A H Saunders

Papers

Extreme ultra-violet spectra from laser-produced plasmas

New spectra of Fe XII to Fe XVIII and their isoelectronic sequences