Decomposing the AIA 304 Å Channel into Its Cool and Hot Components

Abstract

The AIA 304 Å channel on board the Solar Dynamics Observatory (SDO) offers a unique view of $\approx 10^{5}\text{ K}$ ≈ 10 5 K plasma emitting in the He ii 304 Å line. However, when observing off-limb, the emission of the (small) cool structures in the solar atmosphere (such as spicules, coronal rain and prominence material) can be of the same order as the surrounding hot coronal emission from other spectral lines included in the 304 Å passband, particularly over active regions. In this paper, we investigate three methods based on temperature and morphology that are able to distinguish the cool and hot emission within the 304 Å passband. The methods are based on the Differential Emission Measure (DEM), a linear decomposition of the AIA response functions (RFit) and the Blind Source Separation (BSS) technique. All three methods are found to produce satisfactory results in both quiescent and flaring conditions, largely removing the diffuse corona and leading to images with cool material off-limb in sharp contrast with the background. We compare our results with co-aligned data from the Interface Region Imaging Spectrograph (IRIS) in the SJI 1400 Å and 2796 Å channels, and find the RFit method to best match the quantity and evolution of the cool material detected with IRIS. Some differences can appear due to plasma emitting in the $\log T=5.1\,\text{--}\,5.5$ log T = 5.1 – 5.5 temperature range, particularly during the catastrophic cooling stage prior to rain appearance during flares. These methods are, in principle, applicable to any passband from any instrument suffering from similar cool and hot emission ambiguity, as long as there is good coverage of the high-temperature range.

From Chromospheric Evaporation to Coronal Rain: An Investigation of the Mass and Energy Cycle of a Flare

“…This similarity is due to the surrounding diffuse hot corona being more extended along the line of sight (LOS) than the He II emission, which comes from the smaller, clumpy CR (Antolin et al 2015;Froment et al 2020). To remove the hot emission from the AIA 304 Å passband, we follow the procedure by Antolin et al (2024). We first fit the response function of AIA 304 Å over the hot temperature range with the other EUV passbands of AIA (which offer good coverage over the temperature range of 10 5.5 -10 7.2 K), as shown in Equation (1).…”

“…Here we take the response function using the "aia_get_response" command in Interactive Data Language (IDL) with the chiantifix and eve keywords and specifying the time of the observation with the help of the CHIANTI (ver. 10) atomic database (Landi et al 2012;Del Zanna et al 2021; for further details, see Antolin et al 2024):…”

“…The cool AIA 304 Å (middle panel) emission is then obtained by subtracting this hot component from the original AIA 304 Å (left panel) intensity. Further details on this procedure can be found in Antolin et al (2024).…”