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Register a new userA Laboratory Experiment of Magnetic Reconnection: Outflows, Heating and Waves in Chromospheric Jets
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Hinode observations have revealed intermittent recurrent plasma ejections/jets in the chromosphere. These are interpreted as a result of non-perfectly anti-parallel magnetic reconnection, i.e. component reconnection, between a twisted magnetic flux tube and the pre-existing coronal/chromospheric magnetic field, though the fundamental physics of component reconnection is unrevealed. In this paper, we experimentally reproduced the magnetic configuration and investigated the dynamics of plasma ejections, heating and wave generation triggered by component reconnection in the chromosphere. We set plasma parameters as in the chromosphere (density 10^14 cm^-3, temperature 5-10 eV, i.e. (5-10)x10^4 K, and reconnection magnetic field 200 G) using argon plasma. Our experiment shows bi-directional outflows with the speed of 5 km/s at maximum, ion heating in the downstream area over 30 eV and magnetic fluctuations mainly at 5-10 us period. We succeeded in qualitatively reproducing chromospheric jets, but quantitatively we still have some differences between observations and experiments such as jet velocity, total energy and wave frequency. Some of them can be explained by the scale gap between solar and laboratory plasma, while the others probably by the difference of microscopy and macroscopy, collisionality and the degree of ionization, which have not been achieved in our experiment.
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Magnetic reconnection, a fundamentally important process in many aspects of astrophysics, is believed to be initiated by the tearing instability of an electric current sheet, a region where magnetic field abruptly changes direction and electric currents build up. Recent studies have suggested that the amount of magnetic shear in these structures is a critical parameter for the switch-on nature of magnetic reconnection in the solar atmosphere, at fluid spatial scales much larger than kinetic scales. We present results of simulations of reconnection in 3D current sheets with conditions appropriate to the solar corona. Using high-fidelity simulations, we follow the evolution of the linear and non-linear 3D tearing instability, leading to reconnection. We find that, depending on the parameter space, magnetic shear can play a vital role in the onset of significant energy release and heating via non-linear tearing. Two regimes in our study exist, dependent on whether the current sheet is longer or shorter than the wavelength of the fastest growing parallel mode (in the corresponding infinite system), thus determining whether sub-harmonics are present in the actual system. In one regime, where the fastest growing parallel mode has sub-harmonics, the non-linear interaction of these sub-harmonics and the coalescence of 3D plasmoids dominates the non-linear evolution, with magnetic shear playing only a weak role in the amount of energy released. In the second regime, where the fastest growing parallel mode has no-sub-harmonics, then only strongly sheared current sheets, where oblique mode are strong enough to compete with the dominant parallel mode, show any significant energy release. We expect both regimes to exist on the Sun, and so our results have important consequences for the the question of reconnection onset in different solar physics applications.
Hot collisionless accretion flows, such as the one in Sgr A$^{*}$ at our Galactic center, provide a unique setting for the investigation of magnetic reconnection. Here, protons are non-relativistic while electrons can be ultra-relativistic. By means of two-dimensional particle-in-cell simulations, we investigate electron and proton heating in the outflows of trans-relativistic reconnection (i.e., $sigma_wsim 0.1-1$, where the magnetization $sigma_w$ is the ratio of magnetic energy density to enthalpy density). For both electrons and protons, we find that heating at high $beta_{rm i}$ (here, $beta_{rm i}$ is the ratio of proton thermal pressure to magnetic pressure) is dominated by adiabatic compression (adiabatic heating), while at low $beta_{rm i}$ it is accompanied by a genuine increase in entropy (irreversible heating). For our fiducial $sigma_w=0.1$, the irreversible heating efficiency at $beta_{rm i}lesssim 1$ is nearly independent of the electron-to-proton temperature ratio $T_{rm e}/T_{rm i}$ (which we vary from $0.1$ up to $1$), and it asymptotes to $sim 2%$ of the inflowing magnetic energy in the low-$beta_{rm i}$ limit. Protons are heated more efficiently than electrons at low and moderate $beta_{rm i}$ (by a factor of $sim7$), whereas the electron and proton heating efficiencies become comparable at $beta_{rm i}sim 2$ if $T_{rm e}/T_{rm i}=1$, when both species start already relativistically hot. We find comparable heating efficiencies between the two species also in the limit of relativistic reconnection ($sigma_wgtrsim 1$). Our results have important implications for the two-temperature nature of collisionless accretion flows, and may provide the sub-grid physics needed in general relativistic MHD simulations.
The importance of the chromosphere in the mass and energy transport within the solar atmosphere is now widely recognised. This review discusses the physics of magnetohydrodynamic (MHD) waves and instabilities in large-scale chromospheric structures as well as in magnetic flux tubes. We highlight a number of key observational aspects that have helped our understanding of the role of the solar chromosphere in various dynamic processes and wave phenomena, and the heating scenario of the solar chromosphere is also discussed. The review focuses on the physics of waves and invokes the basics of plasma instabilities in the context of this important layer of the solar atmosphere. Potential implications, future trends and outstanding questions are also delineated.
Aims. To investigate the role of acoustic and magneto-acoustic waves in heating the solar chromosphere, observations in strong chromospheric lines are analyzed by comparing the deposited acoustic-energy flux with the total integrated radiative losses. Methods. Quiet-Sun and weak-plage regions were observed in the Ca II 854.2 nm and H-alpha lines with the Fast Imaging Solar Spectrograph (FISS) at the 1.6-m Goode Solar Telescope (GST) on 2019 October 3 and in the H-alpha and H-beta lines with the echelle spectrograph attached to the Vacuum Tower Telescope (VTT) on 2018 December 11 and 2019 June 6. The deposited acoustic energy flux at frequencies up to 20 mHz was derived from Doppler velocities observed in line centers and wings. Radiative losses were computed by means of a set of scaled non-LTE 1D hydrostatic semi-empirical models obtained by fitting synthetic to observed line profiles. Results. In the middle chromosphere (h = 1000-1400 km), the radiative losses can be fully balanced by the deposited acoustic energy flux in a quiet-Sun region. In the upper chromosphere (h > 1400 km), the deposited acoustic flux is small compared to the radiative losses in quiet as well as in plage regions. The crucial parameter determining the amount of deposited acoustic flux is the gas density at a given height. Conclusions. The acoustic energy flux is efficiently deposited in the middle chromosphere, where the density of gas is sufficiently high. About 90% of the available acoustic energy flux in the quiet-Sun region is deposited in these layers, and thus it is a major contributor to the radiative losses of the middle chromosphere. In the upper chromosphere, the deposited acoustic flux is too low, so that other heating mechanisms have to act to balance the radiative cooling.
Solid evidence of magnetic reconnection is rarely reported within sunspots, the darkest regions with the strongest magnetic fields and lowest temperatures in the solar atmosphere. Using the worlds largest solar telescope, the 1.6-meter Goode Solar Telescope, we detect prevalent reconnection through frequently occurring fine-scale jets in the H${alpha}$ line wings at light bridges, the bright lanes that may divide the dark sunspot core into multiple parts. Many jets have an inverted Y-shape, shown by models to be typical of reconnection in a unipolar field environment. Simultaneous spectral imaging data from the Interface Region Imaging Spectrograph show that the reconnection drives bidirectional flows up to 200~km~s$^{-1}$, and that the weakly ionized plasma is heated by at least an order of magnitude up to $sim$80,000 K. Such highly dynamic reconnection jets and efficient heating should be properly accounted for in future modeling efforts of sunspots. Our observations also reveal that the surge-like activity previously reported above light bridges in some chromospheric passbands such as the H${alpha}$ core has two components: the ever-present short surges likely to be related to the upward leakage of magnetoacoustic waves from the photosphere, and the occasionally occurring long and fast surges that are obviously caused by the intermittent reconnection jets.
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