Solar and Stellar Magnetism

    General
    Description

    Magnetic fields are at the base of star formation and stellar structure and evolution. When stars are born, magnetic fields brake the rotation during the collapse of the mollecular cloud. In the end of the life of a star, magnetic fields can play a key role in the form of the strong winds that lead to the last stages of stellar evolution. During the whole adult life of a star, magnetic fields are the origin of stellar activity. Our Sun has magnetic fields that give rise to such spectacular activity that impacts the climate on Earth. The magnetic activity in other stars is, in some cases, of orders of magnitude more intense than the solar one, influencing – often drastically – the transport of chemical species and angular momentum, as well as affecting the possible planetary systems around them.

    The aim of this project is the study of the diverse manifestations of the magnetic field that can be observed in the solar atmosphere and in other stars. These include distinct structures as sunspots, weak quiet-sun fields or chromospheric and coronal features such as filaments and prominences. The following research topics have been gradually faced:

    Solar magnetism

    1. Structure and evolution of Sunspot magnetic fields.

    2. Structure and evolution of quiet Sun magnetic fields.

    3. Structure and evolution of the magnetism of the chromosphere and of chromospheric strcutures (promiences, spicules,...)

    4. Structure and evolution or coronal loops.

    5. Structure and evolution of the Sun's global field. Studies of the activity cycle.

    6. Empirical study of propagation of magnetohydrodynamic waves in magnetic structures.

    7. Empirical study of energy transfer mechanisms related with the heating of the external atmospheric layers.

    8. Empirical study of the influence of partial ionisation in the dynamics of the solar atmosphere.

    9. Participation in the European Solar Telescope project.

    Stellar magnetism

    1. Development of numerical tools to diagnose stellar magnetic fields, both in the surface and in the chromsphere.

    2. Study of magnetic fields in stellar prominences.

    3. Study of the role of magnetic fields in the late stages of stellar evolution.

    Principal investigator
    Project staff
    1. Spiral waves in sunspots: They have been interpreted as magnetoacoustic waves propagating from the interior to the atmosphere following the direction of the magnetic field. We have characterized the magnetic field topology, dismissing the twist of the field lines as the cause of the spiral shape (Felipe et al. 2019).
    2. Magnetic response to umbral flashes: Simultaneous spectropolarimetric observations of the chromospheric He I 10830 and Ca II 8542 lines have been used to estimate the fluctuations of the magnetic field associated to shock waves. The shocks cause expansion of the magnetic field lines (Houston et al. 2018, including A. Asensio Ramos).

    Related publications

    • Fast-to-Alfvén Mode Conversion Mediated by the Hall Current. I. Cold Plasma Model
      The photospheric temperature minimum in the Sun and solar-like stars is very weakly ionized, with an ionization fraction f as low as 10‑4. In galactic star-forming regions, f can be 10‑10 or lower. Under these circumstances, the Hall current can couple low-frequency Alfvén and magnetoacoustic waves via the dimensionless Hall parameter ε =ω /{{{Ω }}
      Cally, P. S. et al.

      Advertised on:

      12
      2015
      Citations
      29
    • Damped transverse oscillations of interacting coronal loops
      Damped transverse oscillations of magnetic loops are routinely observed in the solar corona. This phenomenon is interpreted as standing kink magnetohydrodynamic waves, which are damped by resonant absorption owing to plasma inhomogeneity across the magnetic field. The periods and damping times of these oscillations can be used to probe the physical
      Soler, R. et al.

      Advertised on:

      10
      2015
      Citations
      7
    • Model Comparison for the Density Structure across Solar Coronal Waveguides
      The spatial variation of physical quantities, such as the mass density, across solar atmospheric waveguides governs the timescales and spatial scales for wave damping and energy dissipation. The direct measurement of the spatial distribution of density, however, is difficult, and indirect seismology inversion methods have been suggested as an
      Arregui, I. et al.

      Advertised on:

      10
      2015
      Citations
      24
    • On the Source of Propagating Slow Magnetoacoustic Waves in Sunspots
      Recent high-resolution observations of sunspot oscillations using simultaneously operated ground- and space-based telescopes reveal the intrinsic connection between different layers of the solar atmosphere. However, it is not clear whether these oscillations are externally driven or generated in situ. We address this question by using observations
      Krishna Prasad, S. et al.

      Advertised on:

      10
      2015
      Citations
      47
    • The WEBT campaign on the BL Lac object PG 1553+113 in 2013. An analysis of the enigmatic synchrotron emission
      A multifrequency campaign on the BL Lac object PG 1553+113 was organized by the Whole Earth Blazar Telescope (WEBT) in 2013 April-August, involving 19 optical, two near-IR, and three radio telescopes. The aim was to study the source behaviour at low energies during and around the high-energy observations by the Major Atmospheric Gamma-ray Imaging
      Raiteri, C. M. et al.

      Advertised on:

      11
      2015
      Citations
      37
    • On the nature of transverse coronal waves revealed by wavefront dislocations
      Context. Coronal waves are an important aspect of the dynamics of the plasma in the corona. Wavefront dislocations are topological features of most waves in nature and also of magnetohydrodynamic waves. Are there dislocations in coronal waves? Aims: The finding and explanation of dislocations may shed light on the nature and characteristics of the
      López Ariste, A. et al.

      Advertised on:

      7
      2015
      Citations
      5
    • Are Tornado-like Magnetic Structures Able to Support Solar Prominence Plasma?
      Recent high-resolution and high-cadence observations have surprisingly suggested that prominence barbs exhibit apparent rotating motions suggestive of a tornado-like structure. Additional evidence has been provided by Doppler measurements. The observations reveal opposite velocities for both hot and cool plasma on the two sides of a prominence barb
      Luna, M. et al.

      Advertised on:

      7
      2015
      Citations
      22
    • Where are the solar magnetic poles?
      Regardless of the physical origin of stellar magnetic fields - fossil or dynamo induced - an inclination angle between the magnetic and rotation axes is very often observed. Absence of observational evidence in this direction in the solar case has led to generally assume that its global magnetic field and rotation axes are well aligned. We present
      Collados, M. et al.

      Advertised on:

      10
      2015
      Citations
      11
    • Equation of state for solar near-surface convection
      Numerical 3-D radiative hydrodynamical simulations are the main tool for the analysis of the interface between the solar convection zone and the photosphere. The equation of state is one of the necessary ingredients of these simulations. We compare two equations of state that are commonly used, one ideal and one nonideal, and quantify their
      Khomenko, E. et al.

      Advertised on:

      6
      2015
      Citations
      4
    • Spatial deconvolution of spectropolarimetric data: an application to quiet Sun magnetic elements
      Context. One of the difficulties in extracting reliable information about the thermodynamical and magnetic properties of solar plasmas from spectropolarimetric observations is the presence of light dispersed inside the instruments, known as stray light. Aims: We aim to analyze quiet Sun observations after the spatial deconvolution of the data. We
      Quintero Noda, C. et al.

      Advertised on:

      7
      2015
      Citations
      30
    • Model comparison for the density structure along solar prominence threads
      Context. Quiescent solar prominence fine structures are typically modelled as density enhancements, called threads, which occupy a fraction of a longer magnetic flux tube. This is justified from the spatial distribution of the imaged plasma emission or absorption of prominences at small spatial scales. The profile of the mass density along the
      Arregui, I. et al.

      Advertised on:

      6
      2015
      Citations
      11
    • Beyond MHD: modeling and observation of partially ionized solar plasma processes
      The temperature and density conditions in the magnetized photosphere and chromosphere of the Sun lead to a very small degree of atomic ionization. At particular heights, the magnetic field may be strong enough to give rise to a cyclotron frequency larger than the collisional frequency for some species. These circumstances influence the collective
      Khomenko, E.

      Advertised on:

      5
      2015
      Citations
      1
    • Variation in sunspot properties between 1999 and 2014
      Aims: We study the variation in the magnetic field strength, area, and continuum intensity of umbrae in solar cycles 23 and 24. Methods: We analyzed a sample of 374 sunspots observed from 1999 until 2014 with the Tenerife Infrared Polarimeter at the German Vacuum Tower Telescope and the Facility InfRared Spectropolarimeter at the Dunn Solar
      Rezaei, R. et al.

      Advertised on:

      6
      2015
      Citations
      17
    • Magnetohydrodynamic wave propagation from the subphotosphere to the corona in an arcade-shaped magnetic field with a null point
      Aims: The aim of this work is to study the energy transport by means of Magnetohydrodynamic (MHD) waves propagating in quiet-Sun magnetic topology from layers below the surface to the corona. Upwardly propagating waves find obstacles, such as the equipartition layer with plasma β = 1, the transition region, and null points, and they get transmitted
      Santamaria, I. C. et al.

      Advertised on:

      5
      2015
      Citations
      36
    • Estimating the magnetic field strength from magnetograms
      A properly calibrated longitudinal magnetograph is an instrument that measures circular polarization and gives an estimation of the magnetic flux density in each observed resolution element. This usually constitutes a lower bound of the field strength in the resolution element, given that it can be made arbitrarily large as long as it occupies a
      Asensio Ramos, A. et al.

      Advertised on:

      5
      2015
      Citations
      2
    • Multiwavelength behaviour of the blazar OJ 248 from radio to γ-rays
      We present an analysis of the multiwavelength behaviour of the blazar OJ 248 at z = 0.939 in the period 2006-2013. We use low-energy data (optical, near-infrared, and radio) obtained by 21 observatories participating in the Gamma-Ray Large Area Space Telescope (GLAST)-AGILE Support Program of the Whole Earth Blazar Telescope, as well as data from
      Carnerero, M. I. et al.

      Advertised on:

      7
      2015
      Citations
      34
    • Alfvén wave coupled with flow-driven fluid instability in interpenetrating plasmas
      The Alfvén wave is analyzed in case of one quasineutral plasma propagating with some constant speed v0 through another static quasineutral plasma. A dispersion equation is derived describing the Alfvén wave coupled with the flow driven mode ω = k v 0 and solutions are discussed analytically and numerically. The usual solutions for two oppositely
      Vranjes, J.

      Advertised on:

      5
      2015
      Citations
      4
    • Drift wave stabilized by an additional streaming ion or plasma population
      It is shown that the universally unstable kinetic drift wave in an electron-ion plasma can very effectively be suppressed by adding an extra flowing ion (or plasma) population. The effect of the flow of the added ions is essential, their response is of the type (vp h-vf 0) exp[-(vph-vf 0) 2] , where vf 0 is the flow speed and vp h is the phase
      Bashir, M. F. et al.

      Advertised on:

      3
      2015
      Citations
      7
    • An open-source, massively parallel code for non-LTE synthesis and inversion of spectral lines and Zeeman-induced Stokes profiles
      With the advent of a new generation of solar telescopes and instrumentation, interpreting chromospheric observations (in particular, spectropolarimetry) requires new, suitable diagnostic tools. This paper describes a new code, NICOLE, that has been designed for Stokes non-LTE radiative transfer, for synthesis and inversion of spectral lines and
      Socas-Navarro, H. et al.

      Advertised on:

      5
      2015
      Citations
      151
    • Spectro-Polarimetric Imaging Reveals Helical Magnetic Fields in Solar Prominence Feet
      Solar prominences are clouds of cool plasma levitating above the solar surface and insulated from the million-degree corona by magnetic fields. They form in regions of complex magnetic topology, characterized by non-potential fields, which can evolve abruptly, disintegrating the prominence and ejecting magnetized material into the heliosphere
      Martínez González, M. J. et al.

      Advertised on:

      3
      2015
      Citations
      32

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