arXiv subjects

Mijie Shi

Publications and source records attributed to Mijie Shi.

26 records · Page 2Linked to original sources

arXiv2021

The First 3D Coronal Loop Model Heated by MHD Waves against Radiative Losses

In the quest to solve the long-standing coronal heating problem, it has been suggested half a century ago that coronal loops could be heated by waves. Despite the accumulating observational evidence of the possible importance of coronal waves, still no 3D MHD simulations exist that show significant heating by MHD waves. Here we report on the first 3D coronal loop model heating the plasma against radiative cooling. The coronal loop is driven at the footpoint by transverse oscillations and subsequently the induced Kelvin-Helmholtz instability deforms the loop cross-section and generates small-scale structures. Wave energy is transfered to smaller scales where it is dissipated, overcoming the internal energy losses by radiation. These results open up a new avenue to address the coronal heating problem.

astro-ph.SR↗

arXiv2019

Transition region loops in the very late phase of flux-emergence in IRIS sit-and-stare observations

Loops are one of the fundamental structures that trace the geometry of the magnetic field in the solar atmosphere. Their evolution and dynamics provide a crucial proxy for studying how the magnetized structures are formed and heated in the solar atmosphere. Here, we report on spectroscopic observations of a set of transition region loops taken by the Interface Region Imaging Spectrograph (IRIS) at Si IV 1394 Å with a sit-and-stare mode. The loops are corresponding to the flux emergence at its very late phase when the emerged magentic features in the photosphere have fully developed. We find the transition region loops are still expanding and moving upward with a velocity of a few kilometers per second ($\lesssim$10 km/s) at this stage. The expansion of the loops leads to interactions between themselves and the ambient field, which can drive magnetic reconnection evidenced by multiple intense brightenings, including transition region explosive events and IRIS bombs in the footpoint region associated with the moving polarity. A set of quasi-periodic brightenings with a period of about 130 s is found at the loop apex, from which the Si IV 1394 Å profiles are significantly non-Gaussian with enhancements at both blue and red wings at Doppler velocities of about 50 km/s. We suggest that the transition region loops in the very late phase of flux emergence can be powered by heating events generated by the interactions between the expanding loops and the ambient fields and also by (quasi-)periodic processes, such as oscillation-modulated braiding reconnection.

astro-ph.SR↗

arXiv2019

Synthetic Extreme-ultraviolet Emissions Modulated by Leaky Fast Sausage Modes in Solar Active Region Loops

We study the extreme-ultraviolet (EUV) emissions modulated by leaky fast sausage modes (FSMs) in solar active region loops and examine their observational signatures via spectrometers like EIS. After computing fluid variables of leaky FSMs with MHD simulations, we forward-model the intensity and spectral properties of the Fe X 185~Å~and Fe XII 195~Å~lines by incorporating non-equilibrium ionization (NEI) in the computations of the relevant ionic fractions. The damping times derived from the intensity variations are then compared with the wave values, namely the damping times directly found from our MHD simulations. Our results show that in the equilibrium ionization cases, the density variations and the intensity variations can be either in phase or in anti-phase, depending on the loop temperature. NEI considerably impacts the intensity variations but has only marginal effects on the derived Doppler velocity or Doppler width. We find that the damping time derived from the intensity can largely reflect the wave damping time if the loop temperature is not drastically different from the nominal formation temperature of the corresponding emission line. These results are helpful for understanding the modulations to the EUV emissions by leaky FSMs and hence helpful for identifying FSMs in solar active region loops.

astro-ph.SR↗

arXiv2019

Synthetic Emissions of the Fe XXI 1354 Å Line from Flare Loops Experiencing Fundamental Fast Sausage Oscillations

Inspired by recent IRIS observations, we forward model the response of the Fe XXI 1354 Å line to fundamental, standing, linear fast sausage modes (FSMs) in flare loops. Starting with the fluid parameters for an FSM in a straight tube with equilibrium parameters largely compatible with the IRIS measurements, we synthesize the line profiles by incorporating the non-Equilibrium Ionization (NEI) effect in the computation of the contribution function. We find that both the intensity and Doppler shift oscillate at the wave period ($P$). The phase difference between the two differs from the expected value ($90^\circ$) only slightly because NEI plays only a marginal role in determining the ionic fraction of Fe XXI in the examined dense loop. The Doppler width modulations, however, posses an asymmetry in the first and second halves of a wave period, leading to a secondary periodicity at $P/2$ in addition to the primary one at $P$. This behavior results from the competition between the broadening due to bulk flow and that due to temperature variations, with the latter being stronger but not overwhelmingly so. These expected signatures, with the exception of the Doppler width, are largely consistent with the IRIS measurements, thereby corroborating the reported detection of a fundamental FSM. The forward modeled signatures are useful for identifying fundamental FSMs in flare loops from measurements of the Fe \MyRoman{21} 1354 Å line with instruments similar to IRIS, even though a much higher cadence is required for the expected behavior in the Doppler widths to be detected.

astro-ph.SR↗

arXiv2018

Non-equilibrium Ionization Effects on Extreme-Ultraviolet Emissions Modulated by Standing Sausage Modes in Coronal Loops

Forward-modeling the emission properties in various passbands is important for confidently identifying magnetohydrodynamic waves in the structured solar corona. We examine how Non-equilibrium Ionization (NEI) affects the Extreme Ultraviolet (EUV) emissions modulated by standing fast sausage modes (FSMs) in coronal loops, taking the Fe IX 171 Å and Fe XII 193 Å emission lines as examples. Starting with the expressions for linear FSMs in straight cylinders, we synthesize the specific intensities and spectral profiles for the two spectral lines by incorporating the self-consistently derived ionic fractions in the relevant contribution functions. We find that relative to the case where Equilibrium Ionization (EI) is assumed, NEI considerably impacts the intensity modulations, but shows essentially no effect on the Doppler velocities or widths. Furthermore, NEI may affect the phase difference between intensity variations and those in Doppler widths for Fe XII 193 Å when the line-of-sight is oblique to the loop axis. While this difference is $180^\circ$ when EI is assumed, it is $\sim 90^\circ$ when NEI is incorporated for the parameters we choose. We conclude that in addition to viewing angles and instrumental resolutions, NEI further complicates the detection of FSMs in spectroscopic measurements of coronal loops in the EUV passband.

astro-ph.SR↗

arXiv2018

Damping of slow surface sausage modes in photospheric waveguides

There has been considerable interest in sausage modes in photospheric waveguides like pores and sunspots, and slow surface sausage modes (SSSMs) have been suggested to damp ufficiently rapidly to account for chromospheric heating. Working in the framework of linear resistive magnetohydrodynamics, we examine how efficient electric resistivity and resonant absorption in the cusp continuum can be for damping SSSMs in a photospheric waveguide with equilibrium parameters compatible with recent measurements of a photospheric pore. For SSSMs with the measured wavelength, we find that the damping rate due to the cusp resonance is substantially less strong than theoretically expected with the thin-boundary approximation. The damping-time-to-period ratio ($τ/P$) we derive for standing modes, equivalent to the damping-length-to-wavelength ratio for propagating modes given the extremely weak dispersion, can reach only $\sim 180$. However, the accepted values for electric resistivity ($η$) correspond to a regime where both the cusp resonance and resistivity play a role. The values for $τ/P$ attained at the largest allowed $η$ may reach $\sim 30$. We conclude that electric resistivity can be considerably more efficient than the cusp resonance for damping SSSMs in the pore in question, and it needs to be incorporated into future studies on the damping of SSSMs in photospheric waveguides in general.

astro-ph.SR↗

arXiv2018

Fast Standing Modes in Transversely Nonuniform Solar Coronal Slabs: Effects of a Finite Plasma Beta

We examine the dispersive properties of linear fast standing modes in transversely nonuniform solar coronal slabs with finite gas pressure, or, equivalently, finite plasma beta. We derive a generic dispersion relation governing fast waves in coronal slabs for which the continuous transverse distributions of the physical parameters comprise a uniform core, a uniform external medium, and a transition layer (TL) in between. The profiles in the TL are allowed to be essentially arbitrary. Restricting ourselves to the first several branches of fast modes, which are of most interest from the observational standpoint, we find that a finite plasma beta plays an at most marginal role in influencing the periods ($P$), damping times ($τ$), and critical longitudinal wavenumbers ($k_{\rm c}$), when both $P$ and $τ$ are measured in units of the transverse fast time. However, these parameters are in general significantly affected by how the TL profiles are described. We conclude that, for typical coronal structures, the dispersive properties of the first several branches of fast standing modes can be evaluated with the much simpler theory for cold slabs provided that the transverse profiles are properly addressed and the transverse Alfvén time in cold MHD is replaced with the transverse fast time.

astro-ph.SR↗

arXiv2017

The Parametric Decay Instability of Alfven waves in Turbulent Plasmas and the Applications in the Solar Wind

We perform three dimensional (3D) ideal magnetohydrodynamic (MHD) simulations to study the parametric decay instability of Alfven waves in turbulent plasmas and explore its possible applications in the solar wind. We find that, over a broad range of parameters in background turbulence amplitudes, the parametric decay instability of an Alfven wave with various amplitudes can still occur, though its growth rate in turbulent plasmas tends to be lower than both the theoretical linear theory prediction and that in the non-turbulent situations. Spatial - temporal FFT analyses of density fluctuations produced by the parametric decay instability match well with the dispersion relation of the slow MHD waves. This result may provide an explanation of the generation mechanism of slow waves in the solar wind observed at 1 AU. It further highlights the need to explore the effects of density variations in modifying the turbulence properties as well as in heating the solar wind plasmas.

physics.space-ph↗