How did astronomers first detect the high temperatures in the corona of the Sun?

Question

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Astronomers first detected high temperatures in the corona of the Sun by observing emission lines of highly ionized elements like iron. The hot corona emits light at specific wavelengths, creating spectral lines that can be detected and analyzed. These emission lines reveal the presence of extremely hot gas in the corona, with temperatures of several million degrees Celsius. Direct measurements using space probes, such as NASA's Parker Solar Probe, have since confirmed these observations.

How Did Astronomers First Detect the High Temperatures in the Corona