Solar nanoflares in different spectral ranges

Abstract

Aims. The rates and other characteristics of solar nanoflares were measured for the same area of the Sun in different extreme-ultravioilet (EUV) channels to find how the main properties of nanoflares depend on the spectral range. Methods. We used images of the quiet Sun obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) in seven spectral channels, 94 Å, 131 Å, 171 Å, 193 Å, 211 Å, 304 Å, and 335 Å. We analyzed 300 images for each AIA/SDO channel covering one hour from 12:00 UT to 13:00 UT on 20 May 2019 with a 12 s cadence. We searched for nanoflares in two 360″×720″ fields of view above (N) and below (S) the Sun’s equator to measure nanoflare latitudinal distributions and their N–S asymmetry. To detect nanoflares, we used a threshold-based algorithm with 5σ threshold. Results. The integral nanoflare rate measured in seven spectral ranges is 3.53 × 10−21 cm−1 s−1; the corresponding frequency is 215 events s−1 for the entire surface of the Sun. A search for nanoflares in any single AIA-channel leads to significant underestimation of their frequency and rate: 171 Å −34% of the total value; 193 Å −33%; 211 Å −24%; other channels – less than 16%. Most EUV nanoflares are single-pixel (∼78%) and mono-channel (∼86%) events. In channel 304 Å, multipixel events dominate over single-pixel events (68% vs. 32%). The average duration of nanoflares is in the range of (89 − 141)±(40 − 61) s depending on the spectral region with the mean value being 129 ± 59 s. The latitudinal distribution of nanoflares is approximately uniform in the range from 0° to 45° for all channels. We find a slight difference between the N and S hemispheres (up to 20% depending on channel), but we do not find it to be statistically significant. Conclusions. We demonstrate that solar nanoflares can be found in all AIA EUV channels. The detection probability strongly depends on the spectral range and the channels can be approximately ranked as follows (from high to low probability): 171 Å, 193 Å, 211 Å, 131 Å, 304 Å, 335 Å, and 94 Å. The first three channels, 171, 193, and 211 Å, allow the detection of ∼78% of all the nanoflares. The remaining four add only 22%. Other characteristics of nanoflares, including duration and spatial distribution, weakly depend on spectral range.