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We extend an existing Born approximation model for calculating the linear sensitivity of helioseismic travel-times to flows from plane-parallel to spherical geometry. This extension is necessary especially for dealing with deep flows. Furthermore, we present first results for the sensitivity kernels and validate our model with the help of artificial helioseismic data for a standard meridional flow pattern from Hartlep et al. (2013, ApJ).
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  • Video
The magnetic field in the Sun undergoes a cyclic modulation with a reversal typically every 11 years due to a dynamo operating under the surface. We simulate slowly to rapidly rotating solar-type stars, where the interplay between convection and rotation self-consistently drives large-scale magnetic field. We apply the test-field method to characterise the dynamo mechanisms acting in this simulations by determining turbulent transport coefficients of the electromotive force. We find that the alphaeffect has a complex nature and does not follow the profile expected from kinetic helicity. However, the alpha effects in these simulations show strong rotational dependency resulting in highly anisotropic tensors and vanish alpha_zz components for rapid rotation. Furthermore, I will present the determination of magnetic helicity fluxes across the equator and through the surface, which are important quantities for the alleviation of catastrophically alpha quenching. Unlike in simulation of forced turbulence of Warnecke et al. (2011), the helicity fluxes across the equator are found to be much weaker in convection simulations. Moreover, I discuss the the relevants of magnetic and current helicity production in the dynamo region for the coronal heating process as well as to understand the activity-rotation-relation of main-sequence stars.
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  • Video
The meridional flow inside the Sun is a large-scale flow observed on both hemispheres of the Sun. Its structure and variation inside the Sun is of major interest as it transports mass, angular momentum and magnetic flux. Therefore a detailed knowledge is required to understand the magneto-hydrodynamics of the Sun's convection zone and consequently the Sun's magnetic variability. In the past local helioseismology has drawn a detailed picture on this flow in the near-surface layers 15 Mm beneath the solar surface, i.e. in the outer 2% of the Sun. Recently, the helioseismic measurement of this flow throughout the whole convection zone of the Sun has made significant progress by the development of improved helioseismic methods. These reveal that the meridional flow has a possible multi-cellular structure in latitude and depth. Furthermore there are indications that this flow varies during the solar activity cycle. However a final consensus on the structure and temporal evolution of the flow has not been achieved, especially as the signal is weak and affected by noise and systematic effects. In this talk I will review the current status and efforts of helioseismology of the Sun’s meridional flow.
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  • Video
I will consider some new developments in absolute measures of helicity (as opposed to measures relative to a vacuum field). These measures are defined for arbitrary foliations of space by simply connected surfaces (e.g. start with a set of nested spheres with spherical coordinates, then deform the spheres). They are based on generalizations of Poloidal-Toroidal decompositions. One application lies in measuring the helicity contained within the Northern hemisphere interior of the sun (or Southern hemisphere).
Data Types:
  • Video
Damping of transverse waves in different solar coronal structures is a commonly observed property and a source of information about coronal conditions. Although resonant damping seems to be the most accepted mechanism for damping of transverse waves, there are other possible mechanisms. We have carried out a Bayesian analysis comparing three different models which could explain the damping in coronal loops. Our results indicate that resonant absorption is the most probable mechanism for low ratios between damping time and wave period, while the wave leakage mechanism is the best candidate for high ratios. Nonetheless, the evidence for one model against another shows a strong dependence on the data errors.
Data Types:
  • Video
In this study, we present first inversion results for deep meridional flow using spherical Born approximation kernels and time-distance helioseismology. The computation of Born approximation kernels for flows has only recently become available in spherical geometry. Compared to the ray approximation, the Born approximation is considered to provide a more realistic model of the advection and scattering processes in the solar interior, which are captured in travel time measurements. We first validate this method using artificial data from a linear 3D simulation of solar interior wave propagation. We find that the prediction of the Born approximation model coincides well with the simulated data. We then perform standard SOLA inversions of the solar meridional flow. First, inversion results of the simulated data are discussed and compared to the original flow profile included in the simulation. Finally, we apply the validated method to GONG data spanning periods of low, medium and high solar activity (2001-2003, 2004-2006, and 2007-2009). The results are discussed and compared to literature.
Data Types:
  • Video
The 1980s and 1990s saw seminal works on the role of magnetic helicity in the magnetized solar atmosphere. Thanks to helicity, our understanding of the quiescent and eruptive solar magnetism has progressed substantially since then. From the plausible necessity of coronal mass ejections (CMEs) as sinks of excess solar magnetic helicity in the heliosphere to the solar cycle-independent hemispheric helicity preference, likely imposed by the steady solar differential rotation, to the actual estimates of active-region, quiet Sun, and CME helicities, we have come to place magnetic helicity on equal footing with the electric-current-induced (i.e., non-potential) magnetic energy that fundamentally fuels solar instabilities and eruptions. In spite of this tremendous progress, however, there is still is a lot to learn: first, we need to optimize the way magnetic helicity is practically calculated in local and global solar scales. Then, we need to determine the interplay between different helicity terms that seem to hold distinct aspects of the physics of the system. Furthermore, the role of spectral helicity characteristics, as well as the competition between opposite senses of helicity in a single magnetic structure and its role to eruptions, need to be further clarified. We attempt a resume of what we know, what we are hinted about, and what we should aim to achieve in hopes of spurring a discussion between involved researchers that could further advance the state of the art in the field. This account will be attempted in a plain, physical language to hopefully enable cross-fertilization between different physical domains where helicity is deemed to play a role.
Data Types:
  • Video
Damping of transverse waves in different solar coronal structures is a commonly observed property and a source of information about coronal conditions. Although resonant damping seems to be the most accepted mechanism for damping of transverse waves, there are other possible mechanisms. We have carried out a Bayesian analysis comparing three different models which could explain the damping in coronal loops. Our results indicate that resonant absorption is the most probable mechanism for low ratios between damping time and wave period, while the wave leakage mechanism is the best candidate for high ratios. Nonetheless, the evidence for one model against another shows a strong dependence on the data errors.
Data Types:
  • Video
Having satellites positioned in-orbit at both Lagrangian L5 and L4 points offers several major advantages. For example, the L5 vantage point provides an early view of the solar surface, which Earth will be facing 4-5 days later. In turn, the L4 viewing point enables a better view of the source regions of eruptions responsible for SEPs affecting the near-Earth environment. Taken together, observations from L4 and L5 cover about 83% of solar surface, which will significantly improve both short- and long-term forecasts. However, in the most likely scenario that funding will support only a single L5 mission, not both, one alternative that the space weather community may want to explore is to encourage other spacefaring nations such as Russia, China, and India, to launch their own spacecraft to L4 in close coordination with the L5 mission. Launching two separate spacecraft to L4 and L5 will allow us to reap the benefits of having two new vantage points for space weather in addition to the L1 vantage point, to more-fully share the costs of such combined missions, and avoid the restrictions related to the transfer of technology (predominantly affected the L5 and L1 concepts to date).
Data Types:
  • Video
What if there were a way to identify **where** the magnetic helicity is concentrated within a three- dimensional magnetic field? At first sight this question appears meaningless, since magnetic helicity is an integral over the whole volume of the magnetic field. But, in fact, it is possible to decompose this total helicity as an integral over individual "field line helicities" for each magnetic field line in the domain. All of these are ideal-invariant, topological quantities, and they allow us to quantify in a meaningful way how magnetic helicity is distributed within the domain. In this talk, I will show how this idea can be practically applied to typical extrapolations of the Sun's coronal magnetic field that are used in solar physics.
Data Types:
  • Video
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