Submitted Abstracts

There are 131 abstracts


Space Oddities: The Search For Ephemeral Coronal Holes

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Presentation Type: No Preference

Session: Session 6: Atmospheric Dynamics and Sources of the Solar Wind

Abstract:

Ephemeral coronal holes are short-lived, volatile counterparts to equatorial coronal holes. Very little is known about their characteristics and behavior aside from their definition: open, unipolar magnetic field lines resulting in darkened regions of the corona. The first exemplar of this phenomenon was observed by NASA’s Solar Dynamics Observatory (SDO) on October 26, 2010, which spurred our search for other occurrences in order to understand the frequency and evolution of these phenomena. To accomplish this, we visually evaluated SDO 211 Å images on a 12-hour cadence between June 2010 and June 2016. Each compact and isolated dim region we encountered was flagged as a potential ephemeral coronal hole for further analysis. This preliminary effort resulted in 149 candidate holes. For further analysis of their characteristics, we applied a strict definition criterion of an ephemeral coronal hole. This criterion was a set of four factors that were created in order to ensure events being observed were isolated, individual events-- the candidates had to be dark relative to the surrounding material, not influenced by a nearby eruption, not obviously connected to other coronal hole structures, and their lifetime had to occur completely within the disk crossing. This criterion was designed so that events could be completely analyzed, from beginning to end, to better understand the origins. Application of this criterion eliminated all candidates but 5 of the original 149. True ephemeral coronal holes are rare occurrences, appearing only five times in six years. Future research in this area is needed to both locate additional events and study the underlying driving forces behind these rare phenomena.




Coronal and Flare Diagnostic with SDO/AIA-discovered Fast MHD Wave Trains in Active Regions

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Presentation Type: Oral

Session: Session 2: Motions Near and Above the Solar Surface

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Recently, SDO/AIA discovered quasi-periodic, fast-mode propagating MHD wave trains (QFPs) that propagate at high speeds of more than 1000 km/s. The waves provide a new diagnostic tool for coronal seismology that includes information on the flare energy release and the magnetic structure of the active regions. Many events are now available in a statistical study. However, for improved accuracy of coronal seismology, 3D MHD modeling is required and simple wave-mode analysis may be insufficient. We present new results of observationally constrained models of QFPs using our recently upgraded radiative, thermally conductive, visco-resistive 3D MHD code. The waves are excited by time-depended boundary conditions constrained by the spatial (localized) and quasi-periodic temporal evolution of a C-class flare typically associated with QFPs, and produce observable density and temperature fluctuations. We investigate parametrically the excitation, propagation, and damping of the waves for a range of key model parameters, such as the background temperature, density, magnetic field structure, and the location of the flaring site within the active region. We synthesize EUV intensities in multiple AIA channels and then obtain the model parameters that best reproduce the properties of observed QFPs, such as the recent DEM analysis. We discuss the implications of our modeling results for the seismological application of QFPs for the diagnostic of the active region field and flare pulsations.




Prediction of in-situ magnetic structure of flux ropes from coronal observations

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Presentation Type: Oral

Session: Session 5: Studies of Solar Eruptive Events (SEEs)

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Coronal Mass Ejections (CMEs) are built at the Sun as nearly force-free (J x B = 0) magnetic flux ropes. It is well-established that CMEs are the main drivers of intense magnetic storms and various space weather phenomena at Earth. The most important parameter that defines the ability of a CME to drive geomagnetic storms is the north-south magnetic field component. One of the most significant problems in current long-term space weather forecasts is that there is no method to directly measure the magnetic structure of CMEs before they are observed in situ. However, due to their influence on the coronal plasma environment, the magnetic structure of CME flux ropes can be indirectly estimated based on the properties of the source active region and characteristics of the nearby structures, such as filament details, coronal EUV arcades and X-ray sigmoids. We present here a study of two CME flux ropes, aiming at determining their magnetic properties (magnetic helicity sign, flux rope tilt, and direction of the flux rope axial field) when launched from the Sun by using a synthesis of indirect proxies based on multi-wavelength remote sensing observations. In addition, we employ a data-driven magnetofrictional method that models the CME initiation in the corona to determine the magnetic structure in the two case studies. Finally, the predictions given by the observational synthesis and coronal modeling are compared with the structure detected in situ at Earth.




The Solar Dynamics Observatory, Six Years of Science

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Presentation Type: Oral

Session: Session 3: Solar Magnetic Variability and the Solar Cycle

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The Solar Dynamics Observatory has been producing science and science data since May 2010. I will describe some highlights of SDO results, including filament eruptions, flares, and comets. The status of the observatory hardware will also be discussed. The future of SDO will be a series of extended missions, each lasting two years. Our next extended mission proposal will be due early next year and I will discuss part of that process.




Solar Wind Prediction Using HMI Synoptic Maps and Current Sheet Source Surface Model

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Presentation Type: Oral

Session: Session 7: Space Weather at the Earth and other Planets

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We present the predicted solar wind conditions near the Earth's orbit based on the Wang and Sheeley inverse relation between the magnetic flux tube expansion factors (FTEs) and the observed solar wind speed (SWS). We used the recently validated Current Sheet Source Surface (CSSS) model (developed earlier by X. P. Zhao and J. T. Hoeksema at Stanford: Zhao & Hoeksema, 1995, JGR, 100, 19) to compute the FTEs and used the quadratic functions obtained for the pair of parameters FTE and SWS, to predict the solar wind speed at a distance of 15 Rsun. This is then kinematically propagated forward to 1 AU. We also present a comparison with the observed solar wind. The HMI synoptic maps are available from CR 2096 (6 May 2010).




Multiwavelength Characteristics of Microflares

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Presentation Type: Poster

Session: Session 5: Studies of Solar Eruptive Events (SEEs)

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We present the multiwavelength characteristic of microflare detected in the SDO/AIA and IRIS images using the Automated Microevent-finding Code (AMC). We have catalogued independent events with information such as location on the disk, size, lifetime and peak flux, and obtained their frequency distribution. We mapped these events to other wavelengths, using their location information, to study their associated features, and infer the temperature characteristics and evolution. Moreover, we obtained their magnetic topologies by mapping the microflare locations on to the HMI photospheric magnetic field synoptic maps. Further, we analyzed the filtered brightness profiles and light curves for each event to classify them. Finally, we carried out a differential emission measure (DEM) analysis to study their temperature characteristics.




The amplitude of the deep solar convection and the origin of the solar supergranulation

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Presentation Type: Oral

Session: Session 1: Motions Inside the Sun

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Recent observations and models have raised questions about our understanding of the dynamics of the deep solar convection. In particular, the amplitude of low wavenumber convective motions appears to be too high in both local area radiative magnetohydrodynamic and global spherical shell magnetohydrodynamic simulations. In global simulations this results in weaker than needed rotational constraints and consequent non solar-like differential rotation profiles. In deep local area simulations it yields strong horizontal flows in the photosphere on scales much larger than the observed supergranulation. We have undertaken numerical studies that suggest that solution to this problem is closely related to the long standing question of the origin of the solar supergranulation. Two possibilities have emerged. One suggests that small scale photospherically driven motions dominate convecive transport even at depth, descending through a very nearly adiabatic interior (more more nearly adiabatic than current convection models achieve). Convection of this form can meet Rossby number constraints set by global scale motions and implies that the solar supergranulation is the largest buoyantly driven scale of motion in the Sun. The other possibility is that large scale convection driven deeep in the Sun dynamically couples to the near surface shear layer, perhaps as its origin. In this case supergranulation would be the largest non-coupled convective mode, or only weakly coupled and thus potentially explaining the observed excess power in the prograde direction. Recent helioseismic results lend some support to this. We examind both of these possibilities using carefully designed numerical experiments, and weigh thier plausibilities in light of recent observations.




3D MHD simulation of a Solar Flare

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Presentation Type: Oral

Session: Session 5: Studies of Solar Eruptive Events (SEEs)

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We present results from a numerical 3D simulation of a solar flare triggered by flux emergence into a pre-existing bipolar active region. The simulation is performed with a recently developed version of the MURaM radiative MHD code and includes coronal physics in terms of optically thin radiative loss and field-aligned heat conduction. Severe time-step constraints arising from Alfven wave propagation and heat conduction are avoided through the use of the Boris correction and a hyperbolic treatment of heat conduction. In the simulation we find a flare releasing about 5x10^30 erg over a time of about 1-2 minutes. The efficient transport of energy along field lines leads to the formation of flare ribbons within seconds and at later times to chromospheric evaporation filling coronal flare loops. Since the efficiency of energy transport by electrons (classic heat conduction vs. non-thermal electrons) is one of the main uncertainties, we compare simulations with different values for the saturation of the heat flux. We present synthetic observables in the form of UV, EUV and soft and hard Xray emission.




Global Energetics of Solar Particle Events

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Presentation Type: No Preference

Session: Session 5: Studies of Solar Eruptive Events (SEEs)

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In a solar eruptive event, energy is released in many forms such flare emissions (X-rays, gamma-rays radio, thermal), kinetic and potential energy in coronal mass ejections and other mass motions, and solar energetic particles (SEPs). New insight into the global energetic of eruptive events may be obtained by incorporating multi-point observations from STEREO, and improved solar observations from SDO. We discuss ongoing efforts to estimate the global energy content of the solar energetic particles (protons, heavy ions and electrons) associated with the 398 M and X class flares that occurred between June, 2010 and January, 2014. A number of challenges will be discussed. In particular, many of these flares do not have a detectable SEP event, or they occur when another SEP event is in progress; only around 14% of these flares have an SEP event at Earth and/or the STEREO spacecraft that is a candidate for this analysis. We also discuss the various corrections that may be made, for example to account for multiple crossings of 1 AU due to scattering, and sources of uncertainty in the energy estimates. We present estimates of the energy in different particle types for a number of these events.




Measuring flows in the solar interior: current developments, results, and outstanding problems

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Presentation Type: Oral

Session: Session 1: Motions Inside the Sun

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I will present an overview of the current developments to determine flows in the solar interior and recent results from helioseismology. I will lay special focus on the inference of the deep structure of the meridional flow, which is one of the most challenging problems in helioseismology. In recent times, promising approaches have been developed for solving this problem. The time-distance analysis made large improvements in this after becoming aware of and compensating for a systematic effect in the analysis, the origin of which is not clear yet. In addition to this, a different approach is now available, which directly exploits the distortion of mode eigenfunctions by the meridional flow as well as rotation. These methods have presented us partly surprisingly complex meridional flow patterns, which, however, do not provide a consistent picture of the flow. Resolving this puzzle is part of current research since this has important consequences on our understanding of the solar dynamo. Another interesting discrepancy was found in recent studies between the amplitudes of the large- and small-scale dynamics in the convection zone estimated from helioseismology and those predicted from theoretical models. This raises fundamental questions how the Sun, and in general a star, maintains its heat transport and redistributes its angular momentum that lead, e.g., to the observed differential rotation.