Engineering topics

Kerr, Graham S.

Publications and source records attributed to Kerr, Graham S..

4 records

NASA NTRS2019

Modeling Mg II During Solar Flares. II. Nonequilibrium Effects

To extract the information that the Mg II NUV spectra (observed by the Interface Region Imaging Spectrograph) carry about the chromosphere during solar ﬂares, and to validate models of energy transport via model–data comparison, forward modeling is required. The assumption of statistical equilibrium (SE) is typically used to obtain the atomic level populations from snapshots of ﬂare atmospheres, due to computational necessity. However, it is possible that relying on SE could lead to spurious results. We compare solving the atomic level populations via SE versus a non-equilibrium (NEQ) time-dependent approach. This was achieved using ﬂare simulations from RADYN alongside the minority species version MS_RADYN from which the time-dependent Mg II atomic level populations and radiation transfer were computed in complete frequency redistribution. The impacts on the emergent proﬁles, light curves, line ratios, and formation heights are discussed. In summary we note that NEQ effects during ﬂares are typically important only in the initial stages and for a short period following the cessation of the energy injection. An analysis of the timescales of ionization equilibrium reveals that for most of the duration of the ﬂare, when the temperatures and densities are sufﬁciently enhanced, the relaxation timescales are short (τrelax < 0.1 s), so that the equilibrium solution is an adequate approximation. These effects vary with the size of the ﬂare, however. In weaker ﬂares, effects can be more pronounced. We recommend that NEQ effects be considered when possible but that SE is sufﬁcient at most stages of the ﬂare.

Kerr, Graham S.↗

NASA NTRS2019

Modeling Mg II During Solar Flares. I. Partial Frequency Redistribution, Opacity, and Coronal Irradiation

The Interface Region Imaging Spectrograph has routinely observed the ﬂaring Mg II near-ultraviolet (NUV) spectrum, offering excellent diagnostic potential and a window into the location of energy deposition. A number of studies have forward-modeled both the general properties of these lines and speciﬁc ﬂare observations. Generally these have forward-modeled radiation via post-processing of snapshots from hydrodynamic ﬂare simulations through radiation transfer codes. There has, however, not been a study of how the physics included in these radiation transport codes affects the solution. A baseline setup for forward-modeling Mg II in ﬂares is presented and contrasted with approaches that add or remove complexity. It is shown for Mg II that (1) partial frequency distribution (PRD) is still required during ﬂare simulations despite the increased densities; (2) using full angle-dependent PRD affects the solution but takes signiﬁcantly longer to process a snapshot; (3) including Mg I in non-LTE (NLTE) results in negligible differences to the Mg II lines but does affect the NUV quasi-continuum; (4) only hydrogen and Mg II need to be included in NLTE; (5) ideally the nonequilibrium hydrogen populations, with nonthermal collisional rates, should be used rather than the statistical equilibrium populations; (6) an atom consisting of only the ground state, h and k upper levels, and continuum level is insufﬁcient to model the resonance lines; and (7) irradiation from a hot, dense ﬂaring transition region can affect the formation of Mg II. We discuss modiﬁcations to the RH code allowing straightforward inclusion of the transition region and coronal irradiation in ﬂares.

Kerr, Graham S.↗

NASA NTRS2019

SI IV Resonance Line Emission during Solar Flares: Non-LTE, Non-equilibrium, Radiation Transfer Simulations

The Interface Region Imaging Spectrograph routinely observes the Si IV resonance lines. When analyzing quiescent observations of these lines, it has typically been assumed that they form under optically thin conditions. This is likely valid for the quiescent Sun, but this assumption has also been applied to the more extreme flaring scenario. We used 36 electron-beam-driven radiation hydrodynamic solar flare simulations, computed using the RADYN code, to probe the validity of this assumption. Using these simulated atmospheres, we solved the radiation transfer equations to obtain the non-LTE, nonequilibrium populations, line profiles, and opacities for a model silicon atom, including charge exchange processes. This was achieved using the "minority species" version of RADYN. The inclusion of charge exchange resulted in a substantial fraction of Si IV at cooler temperatures than those predicted by ionization equilibrium. All simulations with an injected energy flux F> 5 x 10(exp 10) erg cm(exp -2) s(exp -1) resulted in optical depth effects on the Si IV emission, with differences in both intensity and line shape compared to the optically thin calculation. Weaker flares (down to F ~ 10(exp 9) erg cm(exp -2) s(exp -1)) also resulted in Si IV emission forming under optically thick conditions, depending on the other beam parameters. When opacity was significant, the atmospheres generally had column masses in excess of 5 x 10-6 g cm-2 over the temperature range 40-100 kK, and the Si IV formation temperatures were between 30 and 60 kK. We urge caution when analyzing Si IV flare observations, or when computing synthetic emission without performing a full radiation transfer calculation.

Kerr, Graham S.↗

NASA NTRS2016

Simulations of the Mg II K and Ca II 8542 Lines From an Alfvén Wave-Heated Flare Chromosphere

We use radiation hydrodynamic simulations to examine two models of solar flare chromospheric heating: Alfven wave dissipation and electron beam collisional losses. Both mechanisms are capable of strong chromospheric heating, and we show that the distinctive atmospheric evolution in the mid-to-upper chromosphere results in Mg II k-line emission that should be observably different between wave-heated and beam-heated simulations. We also present Ca II 8542 A profiles that are formed slightly deeper in the chromosphere. The Mg II k-line profiles from our wave-heated simulation are quite different from those from a beam-heated model and are more consistent with Interface Region Imaging Spectrograph observations. The predicted differences between the Ca II 8542 A in the two models are small. We conclude that careful observational and theoretical study of lines formed in the mid-to-upper chromosphere holds genuine promise for distinguishing between competing models for chromospheric heating inflares.

Kerr, Graham S.↗