Home Page of Manuel Luna

List of forthcoming and past seminars:

Schedule	Speaker	Title and Abstract
December 16, 2019 10:00-12:00 Sala Pléyades	Prof Jeff Khun IfA, Hawaii	Title: The cold Helium Corona and new ideas for sensitive coronal IR magnetometry

October 16, 2019 11:00-12:00 Sala Pléyades	Supriya Hebbur Dayananda Instituto de Astrofísica de Canarias	Title: A new method to probe the solar corona Abstract: We provide a summary of an investigation we are doing about the polarization of the Lyman-alpha lines produced by scattering by the residual hydrogen and ionized helium atoms of the solar corona.

September 23, 2019 12:00-13:00 Sala GTC	Alejandro Alvarez Laguna Laboratoire de physique des plasmas/Centre de Mathématiques Appliquées, Ecole Polytechnique, Paris (France)	Title: Mathematical and numerical modeling of solar and laboratory plasmas in chemical and thermal non-equilibrium based on multi-fluid and multi-component models Abstract: In the recent years, simulation tools solving the multi-fluid plasma equations have been proposed for studying different phenomena in astrophysical and laboratory plasmas. The multi-fluid equations consider mass, momentum, and energy conservation laws for each species within plasma, that interact among each other by means of elastic and inelastic collisions.They are able to represent non-equilibrium dynamics beyond standard single-fluid magnetohydrodynamics (MHD) models. The transport fluxes of the multi-fluid equations depend on collisional data and are derived from the kinetic theory by means of multi-scale expansion such as the Chapman-Enskog perturbative solution. In this talk, we will present the derivation of the multi-component/multi-fluid equations from kinetic theory and the consistent computation of transport properties by means of a spectral Galerkin method using the Laguerre-Sonine polynomial approximation. The numerical discretization of the non-equilibrium plasma equations is difficult to solve as it contains very dispare time scales. We discuss advanced numerical techniques for stiff systems of equations such as fully implicit, well-balanced, operator splitting or asymptotic-preserving schemes. These techniques allow for more efficient numerical algorithms, while preserving the stability and accuracy of the solution. The proposed numerical methods are implemented in massively parallel codes for GPU/CPU architectures and adaptive mesh refinement. We present results of multi-fluid models for the study of chromospheric magnetic reconnection, the propagation of magnetosonic waves in the low sun atmosphere, and laboratory plasmas.

September 19, 2019 11:00-12:00 Sala Pléyades	Vyacheslav (Slava) Lukin Program Director, Plasma Physics Division of Physics, National Science Foundation	Title: Magnetic Reconnection in the Lower Solar Atmosphere Abstract: I will describe recent efforts to self-consistently model magnetic reconnection processes in weakly ionized plasmas, with a focus on the solar chromosphere. The solar chromosphere is a complex and dynamic boundary layer of the solar atmosphere where interdependence of the magnetic field evolution, radiation transport, plasma reactivity, and dissipation mechanisms make it a particularly difficult system to model and understand. Past studies have focused on the micro-physics of multi-fluid magnetic reconnection at magnetic nulls[1]. Here, the previous work is extended by considering a range of spatial scales and plasma β values in a configuration with component magnetic reconnection[2]. We show that in all cases the non-equilibrium reactivity of a weakly ionized plasma is important for determining the properties of a reconnection region and explore current sheet stability to secondary instabilities. [1] Leake, et al, ApJ 760 (2012); Leake, et al, PoP 20 (2013); Murphy & Lukin, ApJ 805 (2015). [2] L. Ni, et al, ApJ 852 (2018); L. Ni, et al, PoP 25 (2018).

May 20th, 2019 11:00-12:00 Aula	Momchil E. Molnar George Ellery Hale Graduate Fellow CU Boulder/National Solar Observatory	Title: PROBING THE TURBULENT QUIET CHROMOSPHERE WITH ALMA AND IBIS Abstract: We present observations of the solar chromosphere observed simultaneously with the Atacama Large Millimeter Array (ALMA) and the Interferometric BIdimensional Spectropolarimeter (IBIS) at the Dunn Solar Telescope (DST). This dataset combines spectrally resolved observations in the optical and near infrared with the high cadence (∼2 seconds) images in the millimeter continuum. We compare the different diagnostics available in multiple lines. A key result from our study is the demonstration that certain NLTE chromospheric diagnostics are well correlated to the LTE temperature measurements from ALMA. Spectral synthesis with RH and 1D model atmospheres provides insight into the source of these correlations, but also highlights the need for caution when interpreting the ALMA observations.

March 20th, 2019 11:00-12:00 Sala Pléyades	Dr Pere L. Palle Dra Marian Martínez González IAC	Title: Solar-SONG, present and future Abstract: Solar-SONG is a solar infrastructure that allow Sun-as-a-star observations using the spectroscopic capabilities of the Hertzsprung-SONG telescope. It was conceived for helioseismology with the aim to increase our knowledge on the solar interior. Nowadays, this initiative is fully operational and is providing interesting scientific results. In this talk, we will summarise the concept of Solar-SONG, we will present the latest results on global helioseismology, and we will present new future ideas to upgrade this instrument.

March 6th, 2019 11:00-12:00 Sala Pléyades	Dr Valentín Martinez-Pillet NSO	DKIST status and SPRING-GBSON network

November 7th, 2018 11:00-12:00 Sala Pléyades	Dr Hendrik C. Spruit Max-Planck Institut für Astrophysik, Garching, Germany	Title: Dynamo surprises Abstract: Numerical simulations of dynamos over the past 2 decades, as well as lab experiments, have turned up several most curious and unexpected phenomena. They are sufficiently severe to put our common intuition of astrophysical dynamos in disarray. The field is still in the process of making sense of these problems. I'll try to summarize them from a broader perspective, with examples from accretion disks, the solar "small-scale field" and lab experiments.

October 23, 2018 11:00-12:00 Sala Pléyades	Piyush Agrawal Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder	Title: Transport of Internetwork Magnetic Flux Elements in the Solar Photosphere Abstract: The motions of small-scale magnetic flux elements in the solar photosphere can provide some measure of the Lagrangian properties of the convective flow. Measurements of these motions have been critical in estimating the turbulent diffusion coefficient in flux-transport dynamo models and in determining the Alfvén wave excitation spectrum for coronal heating models. We examine the motions of internetwork flux elements in Hinode/ Narrowband Filter Imager magnetograms and study the scaling of their mean squared displacement and the shape of their displacement probability distribution as a function of time. We find that the mean squared displacement scales super-diffusively with a slope of about 1.48. Super-diffusive scaling has been observed in other studies for temporal increments as small as 5 s, increments over which ballistic scaling would be expected. Using high cadence MURaM simulations, we show that the observed super-diffusive scaling at short increments is a consequence of random changes in barycenter positions due to flux evolution. We also find that for long temporal increments, beyond granular lifetimes, the observed displacement distribution deviates from that expected for a diffusive process, evolving from Rayleigh to Gaussian. This change in distribution can be modeled analytically by accounting for supergranular advection along with granular motions. These results complicate the interpretation of magnetic element motions as strictly advective or diffusive on short and long timescales and suggest that measurements of magnetic element motions must be used with caution in turbulent diffusion or wave excitation models. We propose that passive tracer motions in measured photospheric flows may yield more robust transport statistics. NASA ADS link


October 19, 2018 11:00-12:00 Sala Pléyades	Travis Metcalfe Space Science Institute, Boulder	Title: The Limitations of Gyrochronology for Old Field Stars Abstract: Nearly half a century has passed since the initial indications that stellar rotation slows while chromospheric activity weakens with a power-law dependence on age, the so-called Skumanich relations. Subsequent characterization of the mass-dependence of this behavior up to the age of the Sun led to the advent of gyrochronology, which uses the rotation rate of a star to infer its age from an empirical calibration. The efficacy of the method relies on predictable angular momentum loss from a stellar wind entrained in the large-scale magnetic field produced by global dynamo action. Recent observational evidence suggests that the global dynamo begins to shut down near the middle of a star's main-sequence lifetime, leading to a disruption in the production of large-scale magnetic field, a dramatic reduction in angular momentum loss, and a breakdown of gyrochronology relations. For solar-type stars this transition appears to occur near the age of the Sun, when rotation becomes too slow to imprint Coriolis forces on the global convective patterns, reducing the shear induced by differential rotation, and disrupting the large-scale dynamo. After summarizing the evidence for this mid-life magnetic transition, I will reveal its signature in the observations that were originally used to validate gyrochronology. Chromospheric activity may ultimately provide a more reliable age indicator for older stars, and asteroseismology can be used to help calibrate activity-age relations for field stars beyond the middle of their main-sequence lifetimes.


October 17, 2018 11:00-12:00 Sala Pléyades	Piyush Agrawal Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder	Title: A preliminary report on inverting for gas pressure using SIR inversion code Abstract: Here i will talk about our preliminary results on inverting for gas pressure in the solar photosphere using SIR inversion code and the problems we have faced so far in trying to do so. We are trying to synthesize and invert the temperature (T) and electronic pressure (Pe) of a smooth Muram simulation, as SIR computes gas pressure (Pg) from these thermodynamic parameters. To idealize the problem, all other atmospheric parameters (B, Vmic, Vz etc) are set to zero (not inverted) for now. Here, we try to answer the following questions : 1. How much error in T, Pe translates into Pg. 2. Is there any correlation between best fit stokes profile and error in T and Pe. 3. Does more spectral lines lead to a better T, Pe inversion or not. 4. What spectral lines are sensitive to Pe. 5. What amplitude of perturbations in T or Pe results in a considerable change in synthesized stokes profiles, such that SIR can atleast sense that perturbation and try to invert for it.
October 10, 2018 11:30-12:00 Sala Ómega	Dr. Sarah A. Jaeggli Daniel K. Inouye Solar Telescope Scientist, Assistant Astronomer National Solar Observatory, Maui, HI	Title: Status update on DL-NIRSP Abstract: The Diffraction Limited Near-Infrared Spectropolarimeter is an innovative new integral field spectrograph that is part of the first generation of instruments for the National Solar Observatory’s Daniel K. Inouye Solar Telescope on Haleakala, Maui. DL-NIRSP is currently being integrated and tested at the University of Hawaii’s Institute for Astronomy and will be delivered to the telescope in 2019. I will give an overview of the instrument and discuss some the the challenges we’re facing.
October 10, 2018 11:00-11:30 Sala Ómega	Lucas Tarr George Mason University & U.Hawaii IfA, USA	Title: MHD simulations of stratified atmospheres containing magnetic null points Abstract: In this talk I will discuss my recent simulations using the resistive MHD code LARE (Arber et al 2001), in which I inject compressive MHD wavepackets into a stratified atmosphere that contains a magnetic null point. The 2.5D simulation represents a slice through a small ephemeral region or area of strong plage. The strong gradients in field strength and connectivity related to the presence of the null produce substantially different dynamics compared to the more slowly varying fields typically used in simple sunspot models. The wave-null interaction collapses the null into a current sheet and generates a set of outward propagating (from the null) slow mode shocks confined to field lines near each separatrix. A combination of oscillatory reconnection and shock dissipation ultimately raise the plasma's internal energy at the null and along each separatrix by 25-50% above the background. The resulting pressure gradients must be balanced by Lorentz forces, so that the final state has contact discontinuities along each separatrix and a persistent current at the null.

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