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Upcoming Seminars

TBD Dec 7, 2021

Past Seminars

Improved Measurements of the Sun’s Meridional Flow and Torsional Oscillation from Correlation Tracking on MDI and HMI Magnetograms Nov 9, 2021
Sushant Mahajan, University of Hawaii
The Sun’s axisymmetric flows, differential rotation, and meridional flow govern the dynamics of the solar magnetic cycle, and a variety of methods are used to measure these flows, each with its own strengths and weaknesses. Flow measurements based on cross-correlating images of the surface magnetic field have been made since the 1970s that require advanced numerical techniques that are capable of detecting movements of less than the pixel size in images of the Sun. We have identified several systematic errors in addition to the center-to-limb effect that influence previous measurements of these flows and propose numerical techniques that can minimize these errors by utilizing measurements of displacements at several time lags. Our analysis of line-of-sight magnetograms from the Michelson Doppler Imager on the ESA/NASA Solar and Heliospheric Observatory and the Helioseismic and Magnetic Imager on the NASA Solar Dynamics Observatory shows long-term variations in the meridional flow and differential rotation over two sunspot cycles from 1996 to 2020. These improved measurements can serve as vital inputs for solar dynamo and surface flux transport simulations.

Numerical Convergence in Implicit Large Eddy Simulations of Turbulent Convection Oct 12, 2021
Gustavo Guerrero, Universidade Federal de Minas Gerais, Brazil
Large eddy simulations (LES) and implicit LES are wise and affordable alternatives to the unfeasible direct numerical simulations (DNS) of turbulent flows at high Reynolds numbers (Re). However, for systems with few observational constraints, it is a formidable challenge to determine whether or not the physics of the system is properly captured by these strategies. Here we address this problem through an analysis of numerical convergence of ILES of turbulent convection in 2D with 2 < Re < 104. The thermodynamic atmosphere resembles the solar interior, including a fraction of the radiative zone and the convection zone. Our results indicate that the large-scale properties of convection do not change significantly with the increase of resolution and even a resolution of 1282 grid points is sufficient to capture these dynamics. This happens because the relevant scales of the system are much larger than the Kolmogorov scales. Therefore, the small-scale structures dissipate on a time scale which depends on the turbulence and not on the effective dissipation corresponding to the resolution. Special attention is needed in regions with a sharp density contrast and the resolution cannot resolve small structures. This may lead to significant changes in the integral profiles of rms quantities. Most importantly, if the system includes a convective stable layer interfacing with the convection zone, the correct resolution of interacting internal gravity waves requires low viscosities, i.e., high resolution. Our simulations with resolution ≥ 10242 mesh points show the self-consistent emergence of solar QBO as a consequence of these waves.

Giant Fields of Small Stars. New Insights into M-Dwarf Magnetism Sep 14, 2021
Oleg Kochukhov, Uppsala University, Sweden
Magnetic fields play a fundamental role for interior and atmospheric properties of M dwarfs and greatly influence terrestrial planets orbiting in the habitable zones of these low-mass stars. Determination of the strength and topology of magnetic fields, both on stellar surfaces and throughout the extended stellar magnetospheres, is a key ingredient for advancing stellar and planetary science. In this talk I will review modern methods of magnetic field measurements applied to M dwarfs, with an emphasis on direct diagnostics based on interpretation of the Zeeman effect signatures in high-resolution intensity and polarization spectra. I will summarize results of total field strength measurements derived from Zeeman broadening analyses as well as information on global magnetic geometries inferred with Zeeman Doppler imaging. The emerging understanding of complex, multi-scale nature of M-dwarf magnetic fields is discussed in the context of theoretical models of hydromagnetic dynamos.

Far-side Activity in Surface Flux Transport Simulations Aug 10, 2021
Alexei A. Pevtsov, National Solar Observatory
Strength of the polar field during the minimum of sunspot activity could serve as a good predictor of the amplitude of the next solar cycle. However, the polar fields are difficult to measure: the magnetic fields near the poles are relatively weak and nearly radial, and thus, the line-of-sight component is small and noisy. Each pole is not observable from Earth orbit for about a half a year. The evolution of the photospheric magnetic field is simulated reasonably well by the surface flux transport (SFT) models, which can be used to reconstruct the polar fields. However, active regions that emerge and decay on the far-side are not included in the simulations. How large is the impact of such regions on the strength of polar field? Using the near-side observations, we identified the statistical properties of small, short lived active regions, and we used these parameters to simulate the flux emergence on far-side of the Sun and their evolution using the SFT model. We find that adding active regions with short lifetimes to the far-side of the Sun results in (1) significantly stronger polar fields during the period of sunspot minima and (2) slightly delayed polarity reversals. The small, short lived active regions, which emerge and decay on far-side of the Sun, do not significantly affect the poleward flux surges, which are mostly caused by larger long-living active regions. Instead, the far-side emergence leads to a weak continuous flow of magnetic flux, which affects polar fields over long periods of time.
Solar Polar Vortices Jul 13, 2021
David H. Hathaway, Stanford University
Measurements of the large cellular flows on the Sun were made by correlation tracking of supergranules seen in Dopplergrams obtained by HMI. Measurements were averaged over 34 days to produce daily maps of the vector velocities. Flows at all latitudes are in the form of vortices with helicity left-handed in the north and right-handed in the south. There are key distinctions between the low latitude and high latitudes cells. The high latitude cells (the polar vortices) have long extensions that spiral inward toward the poles, they have lifetimes of several months, rotate differentially, drift poleward at speeds approaching 2 m s−1, and have a strong equatorward momentum flux needed to maintain the Sun’s differential rotation. The low latitude cells (Rossby waves) have roughly circular shapes, lifetimes of about one month, rotate rigidly, do not drift in latitude, and do not exhibit any momentum flux. The polar vortices have a constant temporal frequency for all ℓ indicating that they are advected by differential rotation at rates representative of the base of the convection zone. The poleward motions of these vortices suggest that the meridional flow at the base of the convection zone is poleward. The polar vortices facilitate surface flux transport by transporting flux elements along their boundaries. Their association with polar faculae and polar plumes needs further investigation.
Sunny-Side Up: The Promise of Solaris May 11, 2021
Sarah Gibson, High Altitude Observatory
Current observations from ground and space present a tantalizing glimpse of the Sun’s polar evolution when the data are pieced together, despite limitations arising from the fact that such observations are currently from vantages close to the ecliptic plane. Solar Orbiter (SO) will provide the first mid-latitude imaging of the Sun and in the process further reshape our expectations and understanding; however, as an encounter mission it will not provide extended, synoptic measurements. The sustained solar polar views of the Solaris mission will directly reveal the Sun’s global-scale dynamics, greatly improve our understanding of the upper boundary on the dynamo and the lower boundary on the heliosphere, and provide a novel, sunny-side-up view of the origins of coronal dynamic structures.
Solar Activity and Dynamics in a Stellar Context April 13, 2021
Travis Metcalfe, Space Science Institute
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. This coupled evolution is ultimately driven by 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 decoupling of further evolution in rotation and activity. 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. I will review the latest evidence for this new understanding of magnetic stellar evolution, including recent constraints on the rate of solar angular momentum loss and measurements of sun-like stars that place solar variability in context.
Machine-learning Framework for Solar Data Processing March 9, 2021
Egor Illarionov, Lomonosov Moscow State University
Machine learning assumes working with large datasets. It is a common situation where the datasets do not fit into memory. To iterate and process datasets of arbitrary size, one needs a specific framework. In the seminar we discuss the key concepts of such frameworks that include data indexing, batch processing and pipelines construction. As a particular implementation we consider the framework and show how it helps to implement unified pipelines for synoptic maps construction and coronal holes segmentation using SDO and SOHO data. We also discuss the questions related to Python code optimization and acceleration.
Lessons Learned from Stellar Simulation Feb 9, 2021
Alan Wray, NASA Ames Research Center
The lessons are of course many. This talk will emphasize numerical and modeling techniques, both successful and failed ones, and the reasons for the choices made. Topics include: equations solved, space and time discretization, boundary conditions, conservation, radiation, subgrid turbulence modeling, shock capturing, MHD methodology, scaling and other speed issues, and results both technical and graphical. Enhancements in-progress will also be mentioned.
How Good Is the Bipolar Approximation of ARs for Surface Flux Transport? Jan 13, 2021
Anthony Yeates, Durham University
We investigate how representing active regions with bipolar magnetic regions (BMRs) affects the end-of-cycle polar field predicted by the surface flux transport model. Our study is based on a new database of BMRs derived from the SDO/HMI active region patch data between 2010 and 2020. An automated code is developed for fitting each active region patch with a BMR, matching both the magnetic flux and axial dipole moment of the region and removing repeat observations of the same region. By comparing the predicted evolution of each of the 1090 BMRs with the predicted evolution of their original active region patches, we show that the bipolar approximation leads to a 24% overestimate of the net axial dipole moment, given the same flow parameters. This is caused by neglecting the more complex multipolar and/or asymmetric magnetic structures of many of the real active regions, and may explain why previous flux transport models had to reduce BMR tilt angles to obtain realistic polar fields. Our BMR database and the Python code to extract it are freely available.
The Emergence of Magnetic Structures from the Deep Solar Interior Sep 1, 2020
Nicholas Brummell, University of California Santa Cruz
In this informal talk, I will address potential explanations for the origin of the magnetic structures that we observe as active regions and sunspots. I will try to point to some of the deeper questions that COFFIES could address on this topic, using (as an excuse) some work that we (here at UCSC) have been doing recently on a theory for the origin of the solar hemispheric helicity rules. This particular subject combines many aspects of COFFIES and I think is one avenue of study that has substantial potential for collaboration leading to a significant enhancement of understanding.
A Biased View on 20+ Years of Global Helioseismology Aug 4, 2020
Sylvain G. Korzennik, Center for Astrophysics | Harvard & Smithsonian
We have by now over two decades of nearly uninterrupted high quality and high cadence global helioseismogical data. GONG has been producing science quality data since 1995, MDI started in 1996 and HMI took over in 2010. Fundamental new constraints have been imposed by helioseismic inferences, yet global helioseismology seems frozen in time in some of its methodologies and has, at times, claimed to have detected what ended up being spurious features. I will present a biased - i.e. partial and personal - review of  the current state of global helioseismology, with some emphasis on issues and challenges presented by mode fitting, inversion techniques and the hunt for the residual systematic errors.

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