Effect Of Time-dependent Ionization Research Articles

Nonequilibrium Ionization Plasma during a Large Solar Limb Flare Observed by Hinode/EIS

Abstract This study on plasma heating considers the time-dependent ionization process during a large solar flare on 2017 September 10, observed by Hinode/EUV Imaging Spectrometer (EIS). The observed Fe xxiv/Fe xxiii ratios increase downstream of the reconnection outflow, and they are consistent with the time-dependent ionization effect at a constant electron temperature T e = 25 MK. Moreover, this study also shows that the nonthermal velocity, which can be related to the turbulent velocity, reduces significantly along the downstream of the reconnection outflow, even when considering the time-dependent ionization process.

Theoretical basal Ca II fluxes for late-type stars: results from magnetic wave models with time-dependent ionization and multi-level radiation treatments

In the current study we present ab initio numerical computations of the generation and propagation of longitudinal waves in magnetic flux tubes embedded in the atmospheres of late-type stars. The interaction between convective turbulence and the magnetic structure is computed and the obtained longitudinal wave energy flux is used in a self-consistent manner to excite the small-scale magnetic flux tubes. In the current study we reduce the number of assumptions made in our previous studies by considering the full magnetic wave energy fluxes and spectra as well as time-dependent ionization (TDI) of hydrogen, employing multi-level Ca II atomic models, and taking into account departures from local thermodynamic equilibrium. Our models employ the recently confirmed value of the mixing-length parameter $\alpha=1.8$ . Regions with strong magnetic fields (magnetic filling factors of up to 50%) are also considered in the current study. The computed Ca II emission fluxes show a strong dependence on the magnetic filling factors, and the effect of time-dependent ionization (TDI) turns out to be very important in the atmospheres of late-type stars heated by acoustic and magnetic waves. The emitted Ca II fluxes with TDI included into the model are decreased by factors that range from 1.4 to 5.5 for G0V and M0V stars, respectively, compared to models that do not consider TDI. The results of our computations are compared with observations. Excellent agreement between the observed and predicted basal flux is obtained. The predicted trend of Ca II emission flux with magnetic filling factor and stellar surface temperature also agrees well with the observations but the calculated maximum fluxes for stars of different spectral types are about two times lower than observations. Though the longitudinal MHD waves considered here are important for chromosphere heating in high activity stars, additional heating mechanism(s) are apparently present.

Hydrodynamics of Exploding Foil X-Ray Lasers with Time-Dependent Ionization Effect**supported by National Natural Science Foundation of China (Nos. 11574390, 11374360, 41472130) and the National Basic Research Program of China (No. 2013CBA01504)

A simple modified model is presented based on R. A. London's self-similarity model on time-independent ionization hydrodynamics of exploding foil X-ray lasers. In our model, the time-dependent ionization effect is under consideration and the average ion charge depends on the temperature. Then we obtain the new scaling laws for temperature, scale length and electron density, which have better agreement with experimental results.

Observation and numerical modeling of chromospheric evaporation during the impulsive phase of a solar flare

We have studied the chromospheric evaporation flow during the impulsive phase of the flare by using the Hinode/EUV Imaging Spectrometer observation and 1D hydrodynamic numerical simulation coupled to the time-dependent ionization. The observation clearly shows that the strong redshift can be observed at the base of the flaring loop only during the impulsive phase. We performed two different numerical simulations to reproduce the strong downflows in FeXII and FeXV during the impulsive phase. By changing the thermal conduction coefficient, we carried out the numerical calculation of chromospheric evaporation in the thermal conduction dominant regime (conductivity coefficient κ0 = classical value) and the enthalpy flux dominant regime (κ0 = 0.1 × classical value). The chromospheric evaporation calculation in the enthalpy flux dominant regime could reproduce the strong redshift at the base of the flare during the impulsive phase. This result might indicate that the thermal conduction can be strongly suppressed in some cases of flare. We also find that time-dependent ionization effect is important to reproduce the strong downflows in Fe XII and Fe XV.

MAGNETIC RECONNECTION IN NON-EQUILIBRIUM IONIZATION PLASMA

We have studied the effect of time-dependent ionization and recombination processes on magnetic reconnection in the solar corona. Petschek-type steady reconnection, in which model the magnetic energy is mainly converted at the slow-mode shocks, was assumed. We carried out the time-dependent ionization calculation in the magnetic reconnection structure. We only calculated the transient ionization of iron; the other species were assumed to be in ionization equilibrium. The intensity of line emissions at specific wavelengths were also calculated for comparison with {\it Hinode} or other observations in future. What we found is as follows: (1) iron is mostly in non-equilibrium ionization in the reconnection region, (2) the intensity of line emission estimated by the time-dependent ionization calculation is significantly different from that with the ionization equilibrium assumption, (3) the effect of time-dependent ionization is sensitive to the electron density in the case that the electron density is less than $10^{10}$ cm$^{-3}$, (4) the effect of thermal conduction lessens the time-dependent ionization effect, (5) the effect of radiative cooling is negligibly small even if we take into account time-dependent ionization.

Effect of time-dependent ionization on properties of the ultrashort pulse propagation and optical power limiting in a two-photon absorption molecular medium

By taking the strong two-photon absorption 4,4′-bis （dimethylamino） stilbene molecules as medium, the propagation of femosecond laser pulses in this medium is simulated by numerically solving the Maxwell-Bloch equations using an iterative predictor-corrector and finite-difference time-domain method. The influences of time-dependent ionization on two-photon absorption and optical power limiting behavior are emphatically investigated. The numerical results show that the nonlinear interactions between the pulses and the medium and the corresponding spontaneous emission become weaker when the time-dependent photoionization is considered. The effects of photonionization on the main pulse become more evident as the amplitude of input electric field is increased. When the photonionization cross section is larger, the dynamic optical limiting range becomes broader, which demonstrates that the limited photonionization ability is favorable for the optical power limiting. Furthermore, the propagating-distance dependence of the dynamical behavior of the ultrashort laser pulse in the medium is observed.

Effect of Time-Dependent Ionization on Propagation of a Few-Cycle Circularly Polarized Laser Pulse in Two-Level Medium

Influence of multiphoton ionization on the propagation and spectrum of few-cycle circularly (elliptically) polarized laser pulses in an open two-level medium (two-level plus continuum model) is investigated based on the conventional two-level model proposed by Slavcheva and Hess (Phys. Rev. A 72 (2005) 053804), and the propagation dynamics of an arbitrary elliptically polarized laser pulse is reduced into that of right and left circularly polarized laser pulses. When the laser intensity is high enough to cause ionization, there are significant impacts of ionization on the pulse reshaping and on the higher order spectral components, and the impacts for the open two-level model are different from those for the closed two-level model.

Publisher's Note: Effect of time-dependent ionization on the propagation of a few-cycle laser pulse in a two-level medium [Phys. Rev. A72, 043820 (2005)

Publisher's Note: Effect of time-dependent ionization on the propagation of a few-cycle laser pulse in a two-level medium [Phys. Rev. A<b>72</b>, 043820 (2005)

Effect of time-dependent ionization on the propagation of a few-cycle laser pulse in a two-level medium

We investigate the influence of ionization on the propagation and spectral effects of a few-cycle ultrashort laser pulse in a two-level medium. It is found that when the fractional ionization is weak, the production of higher spectral components makes no difference. However, when the two states are essentially depleted before the peak of the laser pulse, the impact of ionization on the higher spectral components is very significant.

Effect of time-dependent ionization on the harmonics generated by bound–bound transitions

We present a study of the effect of time-dependent ionization on harmonic generation by bound–bound transitions. The introduction of tunneling and above-barrier ionization rates in a simple bound system, a two-level atom, induces an unexpected richness in the harmonic spectra, leading to the production of radiation at high frequencies. Such frequencies are not reached when no ionization or steady ionization is considered. Time-dependent ionization is, therefore, a key process in the understanding of generation of middle plateau frequency harmonics from atoms in the tunneling ionization regime and shows that bound–free transitions are not uniquely responsible for the higher-harmonic frequencies in realistic atoms. Following previous authors’ studies, we use an adiabatic approach to understand the presence of these higher frequencies.