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Register a new userOn the spectroscopic detection of periodic plasma flows in loops undergoing thermal non-equilibrium
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Context: Long-period intensity pulsations were recently detected in the EUV emission of coronal loops, and have been attributed to cycles of plasma evaporation and condensation driven by thermal non-equilibrium (TNE). Numerical simulations that reproduce this phenomenon also predict the formation of periodic flows of plasma at coronal temperatures along some of the pulsating loops. Aims: In this paper, we aim at detecting these predicted flows of coronal-temperature plasma in pulsating loops. Methods: To this end, we use time series of spatially resolved spectra from the EUV imaging spectrometer (EIS) onboard Hinode, and track the evolution of the Doppler velocity in loops in which intensity pulsations have previously been detected in images of SDO/AIA. Results: We measure signatures of flows that are compatible with the simulations, but only in a fraction of the observed events. We demonstrate that this low detection rate can be explained by line of sight ambiguities, combined with instrumental limitations such as low signal to noise ratio or insufficient cadence.
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Long-period EUV pulsations, recently discovered to be common in active regions, are understood to be the coronal manifestation of thermal non-equilibrium (TNE). The active regions previously studied with EIT/SOHO and AIA/SDO indicated that long-period intensity pulsations are localized in only one or two loop bundles. The basic idea of this study is to understand why. For this purpose, we tested the response of different loop systems, using different magnetic configurations, to different stratifications and strengths of the heating. We present an extensive parameter-space study using 1D hydrodynamic simulations (1,020 in total) and conclude that the occurrence of TNE requires specific combinations of parameters. Our study shows that the TNE cycles are confined to specific ranges in parameter space. This naturally explains why only some loops undergo constant periodic pulsations over several days: since the loop geometry and the heating properties generally vary from one loop to another in an active region, only the ones in which these parameters are compatible exhibits TNE cycles. Furthermore, these parameters (heating and geometry) are likely to vary significantly over the duration of a cycle, which potentially limits the possibilities of periodic behavior. This study also confirms that long-period intensity pulsations and coronal rain are two aspects of the same phenomenon: both phenomena can occur for similar heating conditions and can appear simultaneously in the simulations.
Thermal non-equilibrium (TNE) is a phenomenon that can occur in solar coronal loops when the heating is quasi-constant and highly-stratified. Under such heating conditions, coronal loops undergo cycles of evaporation and condensation. The recent observations of ubiquitous long-period intensity pulsations in coronal loops and their relationship with coronal rain have demonstrated that understanding the characteristics of TNE cycles is an essential step in constraining the circulation of mass and energy in the corona. We report unique observations with the Solar Dynamics Observatory (SDO) and the Swedish 1-m Solar Telescope (SST) that link the captured thermal properties across the extreme spatiotemporal scales covered by TNE processes. Within the same coronal loop bundle, we captured 6 hr period coronal intensity pulsations in SDO/AIA and coronal rain observed off-limb in the chromospheric Halpha and Ca II K spectral lines with SST/CRISP and SST/CHROMIS. We combined a multi-thermal analysis of the cycles with AIA and an extensive spectral characterisation of the rain clumps with the SST. We find clear evidence of evaporation-condensation cycles in the corona which are linked with periodic coronal rain showers. The high-resolution spectroscopic instruments at the SST reveal the fine-structured rain strands and allow us to probe the cooling phase of one of the cycles down to chromospheric temperatures. These observations reinforce the link between long-period intensity pulsations and coronal rain. They also demonstrate the capability of TNE to shape the dynamics of active regions on the large scales as well as on the smallest scales currently resolvable.
In solar coronal loops, thermal non-equilibrium (TNE) is a phenomenon that can occur when the heating is both highly-stratified and quasi-constant. Unambiguous observational identification of TNE would thus permit to strongly constrain heating scenarios. Up to now, while TNE is the standard interpretation of coronal rain, the long-term periodic evolution predicted by simulations has never been observed yet. However, the detection of long-period intensity pulsations (periods of several hours) has been recently reported with SoHO/EIT, and this phenomenon appears to be very common in loops. Moreover, the three intensity-pulsation events that we recently studied with SDO/AIA show strong evidence for TNE in warm loops. In the present paper, a realistic loop geometry from LFFF extrapolations is used as input to 1D hydrodynamic simulations. Our simulations show that for the present loop geometry, the heating has to be asymmetrical to produce TNE. We analyse in detail one particular simulation that reproduces the average thermal behavior of one of the pulsating loop bundle observed with AIA. We compare the properties of this simulation with the properties deduced from the observations. The magnetic topology of the LFFF extrapolations points to the presence of sites of preferred reconnection at one footpoint, supporting the presence of asymmetric heating. In addition, we can reproduce the temporal large-scale intensity properties of the pulsating loops. This simulation further strengthens the interpretation of the observed pulsations as signatures of TNE. This thus gives important information on the heating localization and time scale for these loops.
Thermal non-equilibrium (TNE) is a fascinating situation that occurs in coronal magnetic flux tubes (loops) for which no solution to the steady-state fluid equations exists. The plasma is constantly evolving even though the heating that produces the hot temperatures does not. This is a promising explanation for isolated phenomena such as prominences, coronal rain, and long-period pulsating loops, but it may also have much broader relevance. As known for some time, TNE requires that the heating be both (quasi) steady and concentrated at low coronal altitudes. Recent studies indicate that asymmetries are also important, with large enough asymmetries in the heating and/or cross-sectional area resulting in steady flow rather than TNE. Using reasonable approximations, we have derived two formulae for quantifying the conditions necessary for TNE. As a rough rule of thumb, the ratio of apex to footpoint heating rates must be less than about 0.1, and asymmetries must be less than about a factor of 3. The precise values are case dependent. We have tested our formulae with 1D hydrodynamic loop simulations and find a very acceptable agreement. These results are important for developing physical insight about TNE and assessing how widespread it may be on the Sun.
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.
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