Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Theoretical models are presented to show that expansion of plasma in the radial direction from a denser solar surface to a rarefied upper atmosphere with short-scale inhomogeneous field-aligned flows and currents in the form of thin threads itself is an important source of electrostatic instabilities. Multifluid theory shows that the shear flow–driven purely growing electric fields appear in the transition region. On the other hand, plasma kinetic theory predicts that the short-scale current sheets (or filaments) produce current-driven electrostatic ion acoustic (CDEIA) waves in the hydrogen plasma of the transition region that damp out in the system through wave–particle interactions and increase the temperature. Similar processes take place in the solar corona and act positively for increasing the temperature further and maintaining it. The shear flow–driven instabilities and CDEIA waves have short perpendicular wavelengths of the order of 1 m and low frequencies of the order of 1 or several Hz when the ions’ shear flow scale length is considered to be of the order of 1 km. It is pointed out that the purely growing fluid instabilities turn into oscillatory instabilities and the growth rates of kinetic CDEIA wave instabilities are reduced when the dynamics of 10% helium ions is taken into account along with 90% hydrogen ions. Therefore, the role of helium ions should not be ignored in the study of wave dynamics in solar plasma.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Galactic disk is expected to be spatially and kinematically clustered on many scales due to both star formation and the Galactic potential. In this work we calculate the spatial and kinematic two-point correlation functions (TPCF) using a sample of 1.7 × 10<jats:sup>6</jats:sup> stars with radial velocities from Gaia DR2. Clustering is detected on spatial scales of 1–300 pc and a velocity scale of 15 km s<jats:sup>−1</jats:sup>. After removing bound structures, the data have a power-law index of <jats:italic>γ</jats:italic> ≈ −1 for 1 pc &lt; Δ<jats:italic>r</jats:italic> &lt; 100 pc and <jats:italic>γ</jats:italic> ≲ −1.5 for Δ<jats:italic>r </jats:italic>&gt; 100 pc. We interpret these results with the aid of a star-by-star simulation of the Galaxy, in which stars are born in clusters orbiting in a realistic potential that includes spiral arms, a bar, and giant molecular clouds. We find that the simulation largely agrees with the observations at most spatial and kinematic scales. In detail, the TPCF in the simulation is shallower than the data at ≲20 pc scales, and steeper than the data at ≳30 pc. We also find a persistent clustering signal in the kinematic TPCF for the data at large Δ<jats:italic>v</jats:italic> (&gt;5 km s<jats:sup>−1</jats:sup>) that is not present in the simulations. We speculate that this mismatch between observations and simulations may be due to two processes: hierarchical star formation and transient spiral arms. We also predict that the addition of ages and metallicities measured with a precision of 50% and 0.05 dex, respectively, will enhance the clustering signal beyond current measurements.</jats:p>

<jats:title>Abstract</jats:title> <jats:p> <jats:sup>3</jats:sup>He enrichment is one distinctive feature of impulsive solar energetic particle events. This study is designed to investigate the process of plasma wave–particle resonance, which plays a key role in selectively accelerating heavy ions. We apply a 1.5 dimensional particle-in-cell simulation to model the electron-beam–plasma interaction that generates electron and ion cyclotron waves, namely proton and <jats:sup>4</jats:sup>He cyclotron waves, whose dispersions are dependent on the magnetization parameter <jats:italic>α</jats:italic> = <jats:italic>ω</jats:italic> <jats:sub>pe</jats:sub>/Ω<jats:sub>ce</jats:sub> and the temperature ratio <jats:italic>τ</jats:italic> = <jats:italic>T</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub>/<jats:italic>T</jats:italic> <jats:sub> <jats:italic>p</jats:italic> </jats:sub>. The background particles, e.g., <jats:sup>3</jats:sup>He and <jats:sup>4</jats:sup>He, resonate with the excited cyclotron waves and experience selective heating or acceleration. Specifically, the resonant modes of <jats:sup>3</jats:sup>He ions lead to a more effective acceleration rate compared to those of the <jats:sup>4</jats:sup>He ions. The simulation results provide a potential solution for understanding the abundance of heavy ions in the solar wind.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>It is generally accepted that collisionless magnetic reconnection is initiated on electron scales, which is mediated by electron kinetics. In this paper, by performing a two-dimensional particle-in-cell simulation, we investigate the transition of collisionless magnetic reconnection from electron scales to ion scales in a Harris current sheet with and without a guide field. The results show that after magnetic reconnection is triggered on electron scales, the electrons are first accelerated by the reconnection electric field around the X line, and then leave away along the outflow direction. In the Harris current sheet without a guide field, the electron outflow is symmetric and directed away from the X line along the center of the current sheet, while the existence of a guide field will distort the symmetry of the electron outflow. In both cases, the high-speed electron outflow is decelerated due to the existence of the magnetic field <jats:italic>B</jats:italic> <jats:sub> <jats:italic>z</jats:italic> </jats:sub>, then leading to the pileup of <jats:italic>B</jats:italic> <jats:sub> <jats:italic>z</jats:italic> </jats:sub>. With the increase of <jats:italic>B</jats:italic> <jats:sub> <jats:italic>z</jats:italic> </jats:sub>, the ions are accelerated by the Lorentz force in the outflow direction, and an ion outflow at about one Alfvén speed is at last formed. In this way, collisionless magnetic reconnection is transferred from the electron scales to the ion scales.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the composite optical spectrum for the largest sample of giant radio quasars (GRQs). They represent a rare subclass of radio quasars due to their large projected linear sizes of radio structures, which exceed 0.7 Mpc. To construct the composite spectrum, we combined the optical spectra of 216 GRQs from the Sloan Digital Sky Survey (SDSS). As a result, we obtained the composite spectrum covering the wavelength range from 1400 Å to 7000 Å. We calculated the power-law spectral slope for the GRQ’s composite, obtaining <jats:italic>α</jats:italic> <jats:sub> <jats:italic>λ</jats:italic> </jats:sub> = −1.25, and compared it with that of the smaller-sized radio quasars, as well as with the quasar composite spectrum obtained for a large sample of SDSS quasars. We obtained that the GRQ’s continuum is flatter (redder) than the continuum of comparison quasar samples. We also show that the continuum slope depends on core and total radio luminosity at 1.4 GHz, being steeper for higher radio luminosity bins. Moreover, we found that there is a flattening of the continuum with the increase in the projected linear size of the radio quasar. We show that <jats:italic>α</jats:italic> <jats:sub> <jats:italic>λ</jats:italic> </jats:sub> is orientation-dependent, being steeper for a higher radio core-to-lobe flux density ratio, which is consistent with AGN unified model predictions. For two GRQs, we fit the spectral energy distribution using the X-CIGALE code to compare the consistency of results obtained in the optical part of the electromagnetic spectrum with broadband emission. The parameters obtained from the SED fitting confirmed the larger dust luminosity for the redder optical continuum.</jats:p>