Found 21 talks width keyword solar magnetic fields
The phenomenon of magnetic reconnection in a magnetized plasma has been a subject of numerous studies over the past several decades in a variety of contexts, from high energy astrophysics, to solar and space physics, to laboratory-based experiments. However, most magnetic reconnection studies have been devoted to exploring different collisionality regimes in a fully ionized single specie plasma. Until recently, the physics of magnetic reconnection in partially, and weakly, ionized plasmas has received relatively little attention. In this talk, I will provide a brief overview of the physical effects that partial ionization of the ambient medium may introduce in the dynamics of magnetic reconnection, with a particular focus on the environment of the lower solar atmosphere.
(This seminar is organized by the IAU G5 commission on stellar and planetary atmospheres)
Task-based computing is a method where computational problems are split
into a large number of semi-independent tasks (cf.
2018MNRAS.477..624N). The method is a general one, with application not
limited to traditional grid-based simulations; it can be applied with
advantages also to particle-based and hybrid simulations, which involve
both particles and fields. The main advantages emerge when doing
simulations of very complex and / or multi-scale systems, where the
cost of updating is very unevenly distributed in space, with perhaps
large volumes with very low update cost and small but important regions
with large update costs.
Possible applications in the context of stellar atmospheres include
modelling that covers large scales, such as whole active regions on the
Sun or even the entire Sun, while at the same time allows resolving
small-scale details in the photosphere, chromosphere, and corona. In
the context of planetary atmospheres, models of pebble-accreting hot
primordial atmospheres that cover all scales, from the surfaces of
Mars- and Earth-size embryos to the scale heights of the surrounding
protoplanetary disks, have already been computed (2018MNRAS.479.5136P,
2019MNRAS.482L.107P), and one can envision a number of applications
where the task-based computing advantage is leveraged, for example to
selectively do the detailed chemistry necessary to treat atmospheres
saturated with evaporated solids, or to do complex cloud chemistry
combined with 3-D radiative transfer.
In the talk I will give a quick overview of the principles behind
task-based computing, and then use both already published and still
on-going work to illustrate how this may be used in practice. I will
finish by discussing how these methods could be applied with great
advantage to problems such as non-equilibrium ionization, non-LTE
radiative transfer, and partial redistribution diagnostics of spectral
A solar flare involves the conversion of magnetic energy stored in the coronal magnetic field into the kinetic energy of thermal and non-thermal particles, mass motion, and radiation. How this happens remains a central question in solar physics. A particular long-standing puzzle is how such a high fraction of the stored magnetic energy - up to a half - arrives in the kinetic energy of accelerated non-thermal particles. In this talk I will present an observational overview of solar flares with an emphasis on accelerated particles, discuss some ideas and constraints on particle acceleration, and present some new observations of the possible role of plasma turbulence in the acceleration process.
Solar surface convection displays highly localized sinks where cold plasma returns to the solar interior. On its way to being engulfed by a downdraft the plasma can also advect and intensify magnetic fields up to kG field strengths. Such theoretical predictions strengthen the idea that localized downdrafts are places where the concentration of magnetic fields is favored. The observational discovery of convectively driven sinks is rather recent, however, and its role in the formation and evolution of quiet-Sun magnetic features is still poorly characterized. In our work, we provide both quantitative and qualitative bases for the association between sinks and magnetic fields using high spatial resolution spectropolarimetric data acquired with the Imaging Magnetograph eXperiment on board Sunrise. We find 3.1 x 10-3 sinks Mm-2 minute-1 located at mesogranular vertices. These sinks are associated to (1) horizontal velocity flows converging to a central point and (2) long-lived downdrafts. The spatial distribution of magnetic fields in the quiet Sun is also examined. The strongest magnetic fields are preferentially located at sinks. We find that 40% of the pixels with longitudinal component of the magnetic field higher than 500 G are located in the close neighborhood of sinks. The study of individual examples reveal that sinks can play an important role in the evolution of quiet-Sun magnetic features.
The solar abundance of chemical elements play an important role in addressing such important issues as the formation, structure, and evolution of the Sun and the solar system, the origin of the chemical elements, the evolution of stars and galaxies. Despite the large number of papers published on this issue, debates about the solar composition of the Sun continue. In this talk we start summarizing the current understanding of the solar abundances of iron and CNO elements, which play a crucial role on the determination of the solar metallicity. We then pay especial attention to the impact of the quiet Sun magnetism on the determination of the abundances of these elements. The solar photosphere is significantly magnetized, due to the ubiquitous presence of a small-scale magnetic field whose mean strength is thought to be of the order of 100 gauss. Here we address the problem of the determination of the abundances of chemical elements taking into account the significant magnetization of the quiet Sun photosphere. To this end, we use 3D models of the quiet solar photosphere resulting from a state-of-the-art magneto-convection simulation with small-scale dynamo action where the net magnetic flux is zero. We conclude that if the magnetism of the quiet solar photosphere is mainly produced by a small-scale dynamo,then its impact on the determination of the solar abundance of iron and CNO elements is negligible.
The Chromosphere and Prominence Magnetometer (ChroMag) is a synoptic instrument with the goal of quantifying the intertwined dynamics and magnetism of the solar chromosphere and in prominences through imaging spectro-polarimetry of the full solar disk in a synoptic fashion. The picture of chromospheric magnetism and dynamics is rapidly developing, and a pressing need exists for breakthrough observations of chromospheric vector magnetic field measurements at the true lower boundary of the heliospheric system. ChroMag will provide measurements that will enable scientists to study and better understand the energetics of the solar atmosphere, how prominences are formed, how energy is stored in the magnetic field structure of the atmosphere and how it is released during space weather events like flares and coronal mass ejections. An essential part of the ChroMag program is a commitment to develop and provide community access to the `inversion' tools necessary to interpret the measurements and derive the magneto-hydrodynamic parameters of the plasma. Measurements of an instrument like ChroMag provide critical physical context for the Solar Dynamics Observatory (SDO) and Interface Region Imaging Spectrograph (IRIS) as well as ground-based observatories such as the future Daniel K. Inouye Solar Telescope (DKIST). A prototype is currently deployed in Boulder, CO, USA. We will present an overview of instrument design and capabilities, show some recent observations, and discuss the future of the project.
Flares are among the most energetic magnetic solar phenomena. They are often accompanied by ejections of charged particles, which have a direct influence on the Earth in terms of Aurora or radio and satellite outages. The sudden nature of flares - some of them only last minutes - makes them an elusive feature when observed from ground-based telescopes. These measurements are especially challenging when we focus on magnetic fields and velocities in the different solar layers where flares develop and occur. I will present flare observations taken with different instruments, each targeting different observables, and I will show what we can learn from ground-based polarization measurements.
This talk will give an overview of our understanding of the Sun in the 1960's, the major discoveries since then, and the main questions that need to be answered in future. It will focus on the role of the magnetic field in the solar interior, the photosphere, prominences, coronal heating and eruptive flares.
The formation of active regions and its most visible outcome-sunspots-are still a matter of research. Magnetic flux tubes theory tends to explain the formation of sunspots, but it still faces some unresolved questions: How are they generated? Why can they survive all along the convective zone? How do they rise? I will review this theory and introduce a new way to understand sunspot formation: the negative effective magnetic pressure instability (NEMPI). NEMPI was predicted long ago (Kleeorin et al., 1989, 1990; Kleeorin \& Rogackevskii, 1994; Kleeorin et al., 1996) but has only been seen recently (Branderburg et. al., 2011). It arised as a effect of strong stratication and the presence of turbulence with a weak mean magnetic field. Under suitable conditions, a large-scale instability resulting in the formation of non-uniform magnetic structures, can be excited over the scale of many turbulent eddies or convection cells. This instability is caused by a negative contribution of turbulence to the effective (mean-field) magnetic pressure and has previously been discussed in connection with the formation of active regions and perhaps sunspots. Now, we want to understand the effects of rotation on this instability in both two and three dimensions. We use mean-field magnetohydrodynamics in a parameter regime in which the properties of the negative effective magnetic pressure instability have previously been found to be in agreement with those of direct numerical simulations. We find that the instability is suppressed already for relatively slow rotation with Coriolis numbers (i.e. inverse Rossby numbers) around 0.2. The suppression is strongest at the equator. In the nonlinear regime, we find traveling wave solutions with propagation in the prograde direction at the equator with additional poleward migration away from the equator. The prograde rotation of the magnetic pattern near the equator is argued to be a possible explanation for the faster rotation speed of magnetic tracers found on the Sun. In the bulk of the domain, kinetic and current helicities are negative in the northern hemisphere and positive in the southern.
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