UV Spectra, Bombs, and the Solar Atmosphere

Abstract

A recent analysis of UV data from the Interface Region Imaging Spectrograph (IRIS) reports plasma “bombs” with temperatures near 8 × 10⁴ K within the solar photosphere. This is a curious result, first because most bomb plasma pressures p (the largest reported case exceeds 10³ dyn cm^-2) fall well below photospheric pressures (\gt 7× {10}³), and second, UV radiation cannot easily escape from the photosphere. In the present paper the IRIS data is independently analyzed. I find that the bombs arise from plasma originally at pressures between ≤ 80 and 800 dyne cm^-2 before explosion, i.e., between ≥ 850 and 550 km above {τ }₅₀₀=1. This places the phenomenon’s origin in the low-mid chromosphere or above. I suggest that bomb spectra are more compatible with Alfvénic turbulence than with bi-directional reconnection jets.