IRIS Movie of the Day
At least once a week a movie of the Sun taken by NASA's Interface Region Imaging Spectrograph (IRIS) is posted by one of the scientists operating the instrument.
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10 Apr 2019
Catching the rain
Credit: IRIS, LMSAL/NASA, Paola Testa (SAO)
"Coronal rain" is often observed above solar active regions: when the plasma in coronal loops is rapidly cooling from temperatures of millions of degrees to temperatures orders of magnitude lower, it condenses and falls back down into the solar surface appearing as "rain". The slit of the IRIS spectrograph is observed as a vertical dark line in the left movie, which shows the time series of slit-jaw images in the 1400A passband (dominated by Si IV emission, formed around 80,000 degrees K). The slit moves, rastering a large region, so that spectroscopic observations can be obtained for a large field of view. In this observation, IRIS perfectly "catches" the rain: the speed of the rastering slit almost perfectly matches the apparent speed of the coronal rain in the plane of the sky (~50 km/s). The movie on the right shows a spectral scan of the active region at different wavelengths in the near-ultraviolet, including the strong Mg II chromospheric lines. The coronal rain is evident as a bright emission streak in the red wings of the Mg II (the downward speed of the "rain" causes the emission to be redshifted). The long black features evident in the core of the Mg II lines are "filaments", which are structures of cool and dense chromospheric-temperature plasma suspended in the million-degree hot corona.