Bulletin of the American Physical Society
2017 Annual Meeting of the APS Mid-Atlantic Section
Volume 62, Number 19
Friday–Sunday, November 3–5, 2017; Newark, New Jersey
Session N5: Astro V: Solar Physics |
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Chair: Gregory Fleishman, New Jersey Institute of Technology Room: Atrium, Campus Center, NJIT |
Sunday, November 5, 2017 12:15PM - 12:27PM |
N5.00001: Global Solar MHD Modeling Andrey Stejko, Alexander Kosovichev In our work, we use a 3D global MHD model to accurately reproduce the structure and long-term pattern of the solar magnetic dynamo. We use the EULAG-MHD (Smolarkiewicz {\&} Charbonneau, 2013) code to simulate a full spherical shell (0.61 R - 0.96 R) over which we map an ambient profile for a polytropic ideal gas in a superadiabatic state to induce the kind of convection we observe in top 30{\%} of the Sun; we use this to solve for the resulting mass flows with an anelastic approximation of the Navier-Stokes equations. What we have found is that the storage and evolution of the solar magnetic fields can be heavily dependent on the level of turbulent shear that is seen on the surface of the Sun and exhibited by the tachocline layer (Guerrero et al. 2016, Stejko et al. 2017). The structure of these two layers can have enormous impacts on the time-scale of the dynamo evolution as well as the non-axisymmetric emergence of the magnetic field onto the stellar surface. We show that different ambient profiles with different levels of super- or subadiabticity can generate stark differences in the overall pattern of the magnetic field. [Preview Abstract] |
Sunday, November 5, 2017 12:27PM - 12:39PM |
N5.00002: Sunspot oscillations in the umbral region of AR 12470 observed with ALMA and BBSO Yi Chai We present a study of waves and oscillations in the umbral region of AR 12470 using the data from both the Atacama Large Millimeter/submillimeter Array (ALMA) and NJIT’s Big Bear Solar Observatory (BBSO), along with spacecraft data from the Interface Region Imaging Spectrometer (IRIS) and the Solar Dynamics Observatory (SDO), for an approximately 1-h period on December 17, 2015. The high cadence of ALMA images (2 s) allows us to well-resolve the three-minute oscillations of temperature in the sunspot chromosphere. We performed Fourier analysis of the mm and H-alpha data sets to achieve power spectra as well as phase information. A comparison of phase information between different wavelengths of the same region provides the phase relationship between wavelengths, which are sensitive to different physical parameters and come from different heights in the chromosphere. We discuss possible interpretations of the observed phase relationships, and discuss future work to be done. [Preview Abstract] |
Sunday, November 5, 2017 12:39PM - 1:15PM |
N5.00003: Electric Fields on the Sun: How Can We Determine Them and Why Should We Care? Invited Speaker: Maria Kazachenko The most violent space weather events, eruptive solar flares and coronal mass ejections, are driven by the release of free magnetic energy stored in the solar corona. Energy can build up on timescales of hours to days, and then may be suddenly released in the form of a magnetic eruption. Can we use the observed evolution of the magnetic fields in the solar photosphere to model the evolution of the overlying coronal field, including the storage and release of magnetic energy in such eruptions? The objective of CGEM, the Coronal Global Evolutionary Model, is to develop and evaluate such a model. The primary innovation of the CGEM, as compared to the majority of current generation of coronal field models that are constrained to match the surface magnetic field, is the ability of CGEM's corona to support electric currents generated inductively by electric fields, thereby allowing for a consistent energy buildup process in the corona. In this talk I will discuss the new electric field inversion technique we use in CGEM and present its recent applications to find energy fluxes in an evolving active region and to drive the time-dependent coronal magnetic field. [Preview Abstract] |
Sunday, November 5, 2017 1:15PM - 1:27PM |
N5.00004: The Correlation Between Speed of Flare Ribbon and the Normal Component of Magnetic Field Yuqian Wei, Yan Xu In a simplified two-ribbon flare model, the expansion speed of flare ribbons V$_{\mathrm{r}}$ and the normal component of magnetic field B$_{\mathrm{n}}$ swept by the flare ribbons are two contributing factors in deriving the local magnetic reconnection rate, i.e., E$_{\mathrm{rec}} \quad =$ V$_{\mathrm{r}}$B$_{\mathrm{n}}$. In this project, I investigate the correlation between V$_{\mathrm{r}}$ and B$_{\mathrm{n}}$ in a case study of a two-ribbon M6.5 flare (SOL2015-06-22T18:23). The morphology and evolution of one ribbon of this flare were well captured by exceptionally high resolution H-alpha images from the Visible Imaging Spectrometer (VIS) at the 1.6 m New Solar Telescope (NST), with which I am able to track the ribbon motion and calculate V$_{\mathrm{r}}$. The photospheric line-of-sight (LOS) magnetograms, obtained with the Goode Solar Telescope(GST), are used as an approximation of B$_{\mathrm{n}}$. Based on a sample of four sections of the ribbon, a moderate negative correlation is found between V$_{\mathrm{r}}$ and B$_{\mathrm{n}}$This result suggests a tendency for flare ribbons to slow down in strong magnetic field regions. [Preview Abstract] |
Sunday, November 5, 2017 1:27PM - 1:39PM |
N5.00005: Solar Flare Observations by the Karl G. Jansky Very Large Array: A Survey Yingjie Luo Solar flares are the most powerful energy release events in solar system and flare observations at multiple wavelengths provide us important information on not only physical properties of their source region in the solar atmosphere, but also the accompanying high-energy charged particles accelerated to nearly the speed of light. As high-energy electrons emit radio waves through a variety of emission mechanisms, radio observations serve as an excellent tool to study the origin and transport of the high-energy electrons at the flare site. The Karl G. Jansky Very Large Array (VLA) is one of the world’s most advanced general-purpose radio observatories. It can observe the Sun with relatively large instantaneous bandwidths up to 2 GHz, more than 1000 frequency channels for simultaneous imaging and ultra-high time resolution of up to 50 milliseconds. Since 2011, we have recorded dozens of solar flares using the VLA. Here I report a survey of these VLA flare observations and put them into context with observations at other wavelengths including extreme ultraviolet (EUV) and X-rays. Examples of selected events will be shown to demonstrate some potential investigations on important science questions in solar flare studies made possible by the radio and complementary data. [Preview Abstract] |
Sunday, November 5, 2017 1:39PM - 1:51PM |
N5.00006: Radio Imaging Spectroscopy of a Reflective MHD Wave in a Solar Flare Sijie Yu, Bin Chen We report a new type of coherent radio bursts observed by the Very Large Array (VLA) during a two-ribbon flare, which we interpret as MHD waves reflected near a footpoint of flaring loops. In the dynamic spectrum, this burst starts with a positive frequency drift toward higher frequencies before it turns over and drifts toward lower frequencies. The frequency drift rate is around 100 MHz/s, which is much slower than type III radio bursts associated with fast electron beams but close to the intermediate drift bursts (fiber bursts) which are usually attributed to propagating whistler or Alfvenic waves. VLA's unique capability of imaging with temporal and spectral resolution (50 ms and 2 MHz) enables us to trace the spatial evolution of the bursts. We find that the radio source firstly moves downward toward one of the flaring ribbons before it ``bounces off'' at the lowest height corresponding to the turnover frequency in the dynamic spectrum, and moves upward again. The measured speed in projection is the same order of the typical velocity of Alfvenic or fast-mode MHD waves in the low corona. We conclude that the radio burst is emitted by trapped nonthermal electrons in the flaring loop carried along by a large-scale MHD wave launched during the eruption of a magnetic flux rope. [Preview Abstract] |
Sunday, November 5, 2017 1:51PM - 2:03PM |
N5.00007: Flat Optically Thick Microwave Spectra Observed by EOVSA Shaheda Begum Shaik, Dale E. Gary The aim of this research is to examine the spectral dynamics of the low frequency gyrosynchrotron emission in association to the burst source of a solar flare. Studies focusing on the low frequency characteristics of bursts are rare, mainly due to the previous lack of the combination of spectral and spatial resolution in observations. High-resolution spectra observed by newly upgraded Expanded Owens Valley Solar Array(EOVSA) in the frequency range of 2.5 to 18 GHz are presented. Out of 14 events analysed in this study, 6 bursts display 'flat' optically thick spectrum (spectral index αl < 1.0), in contrary to the expected slopes and predictions of a homogeneous source model. This flat spectrum in a few events is observed especially in the decay phase of the burst and moreover with a constant reduction of spectral slope over the duration of the burst. This feature of flat spectrum can be explained as the emission from a spatially inhomogeneous gyrosynchrotron source of the flare, which evolves with the burst time. Additionally, the decrease in the flux with decreasing frequency leading to low index value in the optically thick side can also be often due to the gyrosynchrotron absorption. The physical parameters with probable dependence supporting the flat spectrum are presented. [Preview Abstract] |
Sunday, November 5, 2017 2:03PM - 2:15PM |
N5.00008: Imaging Spectroscopy of CME-Associated Solar Radio Bursts using OVRO-LWA Sherry Chhabra, Dale Gary, Bin Chen, Gregg Hallinan, Marin Anderson Energetic phenomenon on the Sun, such as solar flares and CMEs are a dynamic laboratory to study radio emission. We use Owens Valley Radio Observatory - Long Wavelength Array (OVRO-LWA) for the study. The new array with its 251 crossed broadband dipoles spread over a 200 m diameter core and 37 long baseline antennas extending to 1600 m baselines allows spatially resolving the Sun in the frequency range 24-82 MHz, with high spectral resolution. We examine coherent Type III and Type IV burst emission associated with a CME from 2015 Sep 20, as well as quiet Sun images before and after the bursts. Images of 9s cadence are used to study the event over a 100 minute period, out to a distance of about 2 solar radii, over the frequency range of 40-70 MHz available at that time. In order to understand better the behavior and structural evolution of the bursts, we image the event at hundreds of frequencies and use the source centroids to obtain the velocity of outward motion. A co-alignment with LASCO(C2) and SWAP data allows spatial and temporal comparison with observations of the CME in white light and EUV. We also place the bursts in context of AIA-EUV, Fermi hard X-ray and EOVSA Microwave emission associated with the event. [Preview Abstract] |
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