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There are 146 abstracts


Can Alfven wave turbulence explain inverted temperature loops in the quiet corona?

Author(s): Zhenguang Huang, Bart van der Holst, Richard A. Frazin, Igor V. Sokolov, Ward B. Manchester IV, Rona Oran, Meng Jin, Alberto M. Vasquez, Tamas I. Gombosi

Institution(s): University of Michigan

Abstract:

We recently studied the quiet Sun (QS) corona during solar minimum with the Michigan Loop Diagnostic Technique (MLDT), which combines Differential Emission Measure Tomography (DEMT) with magnetic models to determine the electron density and electron temperature along individual magnetic field lines (ApJ v. 755 p. 86, 2012). In that work, we found two types of QS coronal loops: "up" loops in which the temperature increases with height, and "down" (inverted temperature) loops in which the temperature decreases with height. Furthermore, we found the "up" loops are dominant in high latitude regions while "down" loops are ubiquitous in low latitude regions. In the present work, we use one of the most sophisticated solar corona models, the Michigan Two-Temperature Alfven Wave Driven Model, to explore the corona heating problem. The model is based on the the BATSRUS code, and is developed as part of the Space Weather Modeling Framework (SWMF). It is a 3D global model, driven by synoptic magnetograms and with an upper chromospheric inner boundary. The solar corona is heated by Alfven waves: open field lines with reflected wave dissipation and closed field lines with counter-propagating waves. We find that enhanced Alfven wave reflection in the low corona may explain the observed "up" and "down" loops feature in the solar corona.




High resolution He I 1083 nm imaging with NST at BBSO

Author(s): Haisheng Ji

Institution(s): Purple Mountain Observatory

Abstract:

I will report some interesting results of 1083 nm narrow-band imaging with NST at BBSO. Simultaneous observations made by AIA and HMI on board SDO are used for coordinated analysis. Following results will be reported: 1) Continuous small scale, impulsive events have been tracked from their origin in the photosphere's intergranular lanes on through to their brightening of the local corona. 2) Observation of magnetic reconnection initiated and driven by granule scale advection. 3) Eruption of a flux rope triggered by shear motion around a sunspot light bridge. 4) Preliminary results of an M-class flare.




Properties of Reversed Helicity Injection in Active Region 11158

Author(s): Ju Jing, Xin Chen, Jeongwoo Lee, Shuo Wang, Peter W. Schuck, Chang Liu, Yan Xu, Haimin Wang

Institution(s): NJIT

Abstract:

The MHD model by Kusano et al. (2004) shows that flares are triggered by the injection of reversed helicity into the magnetic polarity inversion line (PIL). Here we present observations of reversed helicity injection in the active region NOAA 11158 from 2011 February 12 to 16 during which two major flares with GOES class M6.6 and X2.2, respectively, occurred. We have calculated magnetic helicity by counting the emerging motion as well as shearing motion of the magnetic fields by applying the Differential Affine Velocity Estimator for Vector Magnetograms (DAVE4VM, Schuck 2008) to a sequence of photospheric vector magnetograms of the Helioseismic and Magnetic Imager (HMI) on the Solar Dynamic Observatory (SDO). Amount of the accumulated helicity injected in this active region over the five-day period is estimated to be $\sim2\times10^{43}$ Mx$^2$, mainly contributed by the shearing motion of magnetic fields. By examining the series of helicity density maps, we find the intermittent injection of negative helicity as well as the persistent accumulation of positive helicity in the region located around the magnetic PIL. We further use the Southwest Automatic Magnetic Identification Suite (SWAMIS) to detect and track each feature with reversed helicity (i.e., negative helicity in this case). A total of 19 reversed-helicity features are detected, and their location, lifetime, maximum area and flux variation are presented in this paper. The two most noticeable injections of reverse helicity appear prior to each of two major flares, M6.6 and X2.2, respectively. The presented observation may have implications on the tearing mode instability growing on the shear inversion layer and magnetic reconnections leading to flare/CME initiation.




Energetic characterisation and statistics of solar coronal bright points with SDO/AIA

Author(s): Joulin, Buchlin, Guennou, Solomon

Institution(s): Institut d'Astrophysique Spatiale

Abstract:

To explain the high temperature of the corona, much attention has been paid to the distribution of energy in dissipation events. Indeed, if the power-law slope of the dissipated energy distribution is less than -2, the smallest, unobservable events could be the largest contributors to the total energy dissipation in the corona. Observations in EUV and X-rays have actually shown a distribution of event energies extending over 8 decades, with a slope close to -2, but they remain inconclusive about the precise slope. Furthermore, these results rely on a very crude estimate of the (thermal) energy. On the other hand, more detailed spectroscopic studies of events such as coronal bright points do not provide enough statistical information to derive their total contribution to heating. In this work we aim at getting a better estimate of the distributions of the energy dissipated in coronal heating events, by detecting EUV brightenings at small spatial and temporal scales (minutes to few hours) in high-cadence multi-chanel SDO/AIA data (94Å, 131Å, 171Å, 193Å, 211Å, 335Å). And using temperature and emission mesure maps derived from the same data to compute the thermal energy assiociated to these events. We compare these distributions of event energies by this method with previous results.




Coronal Cavities from SDO Observations

Author(s): N. Karna, J.Zhang, W. D. Pesnell, S. A. Hess Webber

Institution(s): GMU, GMU, GSFC-NASA, GMU

Abstract:

Coronal cavities are circular darkened regions observed above the solar limb in white light and EUV coronal images. It is believed a region of low density relative to the surrounding corona. In this study, we are using synoptic maps made from EUV images from the Atmospheric Imager Assembly (AIA) instrument on the SDO to determine the structure and evolution of cavities. The synoptic maps, constructed from circular rings above the limb, are found to best show cavities in 211Å (Fe XIV, ~2.0 MK) and 193 Å (Fe XII, ~1.6 MK) and171 Å (Fe IX, ~0.6 MK) pass bands. Moreover, 304Å (He II, ~0.05 MK) synoptic map best shows the evolution of prominence associated with cavity. The high spatial and time resolution combined with the broad temperature coverage provides a consistent picture of the cavity material and the dynamics of the structure. Our goal in this work is to study the cavity’s sizes (height, diameter and length), density and temperature properties.




Using realistic MHD simulations for modeling HMI observables

Author(s): I.Kitiashvili^1, S.Couvidat^1, N.Mansour^2, A.Wray^2, A.Kosovichev^1

Institution(s): 1) Stanford Unievrsity; 2) NAA Ames Research Center

Abstract:

The solar atmosphere is extremely dynamic, and many important phenomena which develop on small scales are unresolved in the SDO/HMI observations. For correct calibration and interpretation of HMI observations it is very important to investigate the effects of small-scale structures and dynamics on the HMI observables. We use radiative MHD simulations of the upper turbulent convective layer and atmosphere of the Sun and spectro-polarimetric radiative transfer codes to study Stokes profiles of the FeI 6173 line for different conditions in the solar atmosphere, including quiet-Sun regions with various background magnetic field strengths and sunspot umbrae and penumbrae, and discuss effects on HMI observables and interpretation of the HMI data.




Where is Coronal Plasma Heated?

Author(s): James A. Klimchuk

Institution(s): NASA-GSFC

Abstract:

The coupling between the chromosphere and corona is a question of great interest at the moment. It has long been understood that coronal mass originates in the chromosphere and that the energy which powers the corona flows up through the chromosphere. However, the details of how this happens are now being questioned. In the traditional view, “mechanical” energy flows into the corona in the form of waves or gradually increasing magnetic stresses. The waves or stresses dissipate and heat the plasma. The resulting downward thermal conduction flux causes material to evaporate from the chromosphere and fill the corona. If the heating is steady, an equilibrium is established whereby radiation and thermal conduction balance the energy input. If the heating is impulsive (a nanoflare), the evaporated plasma cools and drains, only to reappear during the next event. In either case, the heating occurs in the corona. A new idea is that the heating occurs instead in the chromosphere. Cold plasma is directly heated to coronal temperatures and then flows upward due to expansion and perhaps also an ejection process. The hot tips of type II spicules are one example, though spicules need not be involved. I will discuss these two fundamentally different scenarios and the observational predictions that they make. A comparison with actual observations leads to the conclusion that only a small fraction of the hot plasma in the corona comes from chromospheric heating. Most coronal plasma is a consequence of heating that occurs in the corona itself.




Sunquake Observations from HMI: Time-Distance and Holography Analyses

Author(s): Kosovichev, Alexander (1), Zhao, Junwei (1)

Institution(s): (1) Stanford University, Stanford, CA

Abstract:

HMI observations of solar flares revealed new sunquake events. We analyze and compare the results obtained using two different approaches: time-distance analysis and holography. The time-distance analysis provides detailed information about physical properties of the flare-excited helioseismic waves, including their interaction with strong magnetic field regions. On the other hand, the holography approach, which measures the integrated signal, allows us to detect weak events and investigate their relative power and its frequency dependence. A remarkable features of the observed events are their strong anisotropy, associated with the rapid motion of the flare impacts in the low atmosphere, which are the source of the energy and momentum of sunquakes. We identify the sunquake source spatial-temporal characteristics and show that source locations determined from the holographic method systematically differ from the HMI observed Doppler impact locations. We discuss the potential mechanisms of sunquakes, and their relationship to the energy release and transport in solar flares.




Solar EUV irradiance variability as observed by SDO/EVE: from flares to long-term variations

Author(s): M. Kretzschmar

Institution(s): LPC2E - CNRS & University of Orléans, France

Abstract:

The observations of the solar EUV irradiance by SDO/EVE at both high temporal and spectral resolution allow us for the first time to investigate irradiance variations both on the long term and on the time scale of flares. I will first present the variations of the EUV irradiance spectrum during several flares, with an emphasis on the spectral distribution of the flare energy and how it may change with the soft X-ray amplitude of the flare. The second part of the presentation will be devoted to the comparison of the long term variations of the EUV irradiance observed by PROBA2/LYRA and SDO/EVE.




Mass Flows in a Prominence Spine as Observed in EUV

Author(s): T.A. Kucera and H.R. Gilbert

Institution(s): NASA/GSFC

Abstract:

We analyze flows in the spine of a quiescent prominence observed by SDO/AIA. We discuss the appearance of the prominence in absorption and emission and estimate quantities including the mass of material flowing in the spine, measured by analyzing Lyman absorption, velocities and sizes of moving features, and lifetimes of flows. These quantities provide constraints on models of prominence formation and dynamics such as the thermal non-equilibrium model of prominence condensation. This work is funded by NASA's LWS program





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Last Updated on Tuesday, 29 March 2011 09:36