Investigating the Response of Loop Plasma to Nanoflare Heating Using RADYN Simulations

Abstract

We present the results of 1D hydrodynamic simulations of coronal loops which are subject to nanoflares, caused by either in-situ thermal heating, or non-thermal electrons (NTE) beams. The synthesized intensity and Doppler shifts can be directly compared with IRIS and AIA observations of rapid variability in the transition region (TR) of coronal loops, associated with transient coronal heating. We find that NTE with high enough low-energy cutoff (E C ) deposit energy in the lower TR and chromosphere causing blueshifts (up to ∼ 20 km/s) in the IRIS Si IV lines, which thermal conduction cannot reproduce. The E C threshold value for the blueshifts depends on the total energy of the events (≈ 5 keV for 10 24 ergs, up to 15 keV for 10 25 ergs). The observed footpoint emission intensity and flows, combined with the simulations, can provide constraints on both the energy of the heating event and E C . The response of the loop plasma to nanoflares depends crucially on the electron density: significant Si IV intensity enhancements and flows are observed only for initially low-density loops (< 10 9 cm −3 ). This provides a possible explanation of the relative scarcity of observations of significant moss variability. While the TR response to single heating episodes can be clearly observed, the predicted coronal emission (AIA 94Å) for single strands is below current detectability, and can only be observed when several strands are heated closely in time. Finally, we show that the analysis of the IRIS Mg II chromospheric lines can help further constrain the properties of the heating mechanisms.

IRIS Observations of Short-term Variability in Moss Associated with Transient Hot Coronal Loops

“…Each symbol corresponds to one RADYN simulation, and we offset the values in the y direction in order to make them distinguishable even when different simulations have similar values of the shown parameters. used in Testa et al (2014); Polito et al (2018) and here, and explore different heating properties. Additional effects, such as e.g., non-equilibrium ionization, are also expected to increase the predicted Si iv emission and likely bring the C ii/Si iv and Mg ii/Si iv intensity ratios expected from our simulations in better agreement with the observations.…”

“…• nanoflare energies: 10 24 to 10 26 ergs Some of these simulations have already been presented in detail in Polito et al (2018). In the following we discuss in detail nine of these (dozens of) simulations summarized above, for which the synthetic spectra have characteristics similar to the observed spectra in terms of e.g., duration of the brightenings and line intensities (see discussions in Sec.…”

“…In the previous section we have presented an analysis of the IRIS chromospheric and TR spectral observations, which can provide much tighter constraints on the properties of the heating. Therefore we expanded on our previous investigation of RADYN nanoflare heated loops, by calculating the synthetic chromospheric emission in C ii and Mg ii triplet for the simulations previously discussed in Polito et al (2018). The very broad variety of properties of the spectral observations analyzed in the previous section (section 2) motivated us to also explore a few additional simulations to explore different properties of the heating including: (a) combining heating by thermal conduction and non-thermal particles; (b) longer heating duration of 20-30 s; (c) repeated heating in the same loop (in particular, two 20s heating episodes separated by 60 s intervals when the heating is switched off).…”

“…The details of the RADYN numerical code (Carlsson & Stein 1997;Allred et al 2005Allred et al , 2015 and the general description of how these simulations of nanoflare heated loops are run are discussed in detail in Polito et al (2018), so here we only provide a brief description. The RADYN code solves the 1D equation of charge conservation and the level population rate equations for the magnetically confined plasma; the loop's atmosphere encompasses the photosphere, chromosphere, TR, and corona.…”

“…4 and 5). In particular, as summarized in Table 2, we selected four of the simulations presented in Polito et al (2018) -with an initially empty cool loop (T ∼ 1 MK), of semi-length of 15 Mm, heated for 10 s by TC or NTE with 5/10/15 keV energy cutoff -, and five new simulations, all run on the same cool initial condition: (1) 10 s heating with TC and NTE with 10 keV cutoff (and total energy of 6 × 10 24 ergs, as in the previous runs), (2) intermittent heating (two 20 s heating episodes with 60 s pause in between) by NTE with 10 keV cutoff, (3) 30 s heating by NTE with 10 keV cutoff, (4) 30 s heating with TC and NTE with 10 keV cutoff, and total energy of 1.8×10 25 ergs, (5) 30 s heating by NTE with 15 keV cutoff, and total energy of 1.8 × 10 25 ergs. Figure 5 shows the predicted IRIS lightcurves (left column) for the nine selected RADYN simulations, synthesized by assuming an integration time (and cadence) of 4 s. A qualitative comparison of these synthetic lightcurves with the observed ones (shown in Fig.…”