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At least 1,315 records · Page 73
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DOE OSTI2012

ENTHALPY-BASED THERMAL EVOLUTION OF LOOPS. II. IMPROVEMENTS TO THE MODEL

This paper develops the zero-dimensional (0D) hydrodynamic coronal loop model 'Enthalpy-based Thermal Evolution of Loops' (EBTEL) proposed by Klimchuk et al., which studies the plasma response to evolving coronal heating, especially impulsive heating events. The basis of EBTEL is the modeling of mass exchange between the corona and transition region (TR) and chromosphere in response to heating variations, with the key parameter being the ratio of the TR to coronal radiation. We develop new models for this parameter that now include gravitational stratification and a physically motivated approach to radiative cooling. A number of examples are presented, including nanoflares in short and long loops, and a small flare. The new features in EBTEL are important for accurate tracking of, in particular, the density. The 0D results are compared to a 1D hydro code (Hydrad) with generally good agreement. EBTEL is suitable for general use as a tool for (1) quick-look results of loop evolution in response to a given heating function, (2) extensive parameter surveys, and (3) situations where the modeling of hundreds or thousands of elemental loops is needed. A single run takes a few seconds on a contemporary laptop.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

CHARGE-EXCHANGE LIMITS ON LOW-ENERGY {alpha}-PARTICLE FLUXES IN SOLAR FLARES

This paper reports on a search for flare emission via charge-exchange radiation in the wings of the Ly{alpha} line of He II at 304 A, as originally suggested for hydrogen by Orrall and Zirker. Via this mechanism a primary {alpha} particle that penetrates into the neutral chromosphere can pick up an atomic electron and emit in the He II bound-bound spectrum before it stops. The Extreme-ultraviolet Variability Experiment on board the Solar Dynamics Observatory gives us our first chance to search for this effect systematically. The Orrall-Zirker mechanism has great importance for flare physics because of the essential roles that particle acceleration plays; this mechanism is one of the few proposed that would allow remote sensing of primary accelerated particles below a few MeV nucleon{sup -1}. We study 10 events in total, including the {gamma}-ray events SOL2010-06-12 (M2.0) and SOL2011-02-24 (M3.5) (the latter a limb flare), seven X-class flares, and one prominent M-class event that produced solar energetic particles. The absence of charge-exchange line wings may point to a need for more complete theoretical work. Some of the events do have broadband signatures, which could correspond to continua from other origins, but these do not have the spectral signatures expected from the Orrall-Zirker mechanism.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

SUNSPOT ROTATION, SIGMOIDAL FILAMENT, FLARE, AND CORONAL MASS EJECTION: THE EVENT ON 2000 FEBRUARY 10

We find that a sunspot with positive polarity had an obvious counterclockwise rotation and resulted in the formation and eruption of an inverse S-shaped filament in NOAA Active Region 08858 from 2000 February 9 to 10. The sunspot had two umbrae which rotated around each other by 195 Degree-Sign within about 24 hr. The average rotation rate was nearly 8 Degree-Sign hr{sup -1}. The fastest rotation in the photosphere took place during 14:00 UT to 22:01 UT on February 9, with a rotation rate of nearly 16 Degree-Sign hr{sup -1}. The fastest rotation in the chromosphere and the corona took place during 15:28 UT to 19:00 UT on February 9, with a rotation rate of nearly 20 Degree-Sign hr{sup -1}. Interestingly, the rapid increase of the positive magnetic flux occurred only during the fastest rotation of the rotating sunspot, the bright loop-shaped structure, and the filament. During the sunspot rotation, the inverse S-shaped filament gradually formed in the EUV filament channel. The filament experienced two eruptions. In the first eruption, the filament rose quickly and then the filament loops carrying the cool and the hot material were seen to spiral counterclockwise into the sunspot. About 10 minutes later, the filament became active and finally erupted. The filament eruption was accompanied with a C-class flare and a halo coronal mass ejection. These results provide evidence that sunspot rotation plays an important role in the formation and eruption of the sigmoidal active-region filament.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

TESTING DISK-LOCKING IN NGC 2264

We test analytic predictions from different models of magnetospheric accretion, which invoke disk-locking, using stellar and accretion parameters derived from models of low-resolution optical spectra of 36 T Tauri stars (TTSs) in NGC 2264 (age {approx}3 Myr). Little evidence is found for models that assume purely dipolar field geometries; however, strong support is found in the data for a modified version of the X-wind model which allows for non-dipolar field geometries. The trapped flux concept in the X-wind model is key to making the analytic predictions which appear supported in the data. By extension, our analysis provides support for the outflows predicted by the X-wind as these also originate in the trapped flux region. In addition, we find no support in the data for accretion-powered stellar winds from young stars. By comparing the analysis presented here of NGC 2264 with a similar analysis of stars in Taurus (age {approx}1-2 Myr), we find evidence that the equilibrium interaction between the magnetic field and accretion disk in TTS systems evolves as the stars grow older, perhaps as the result of evolution of the stellar magnetic field geometry. We compare the accretion rates we derive with accretion rates based on U-band excess, finding good agreement. In addition, we use our accretion parameters to determine the relationship between accretion and H{beta} luminosity, again finding good agreement with previously published results; however, we also find that care must be taken when applying this relationship due to strong chromospheric emission in young stars, which can lead to erroneous results in some cases.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

SUPPRESSION OF ENERGETIC ELECTRON TRANSPORT IN FLARES BY DOUBLE LAYERS

During flares and coronal mass ejections, energetic electrons from coronal sources typically have very long lifetimes compared to the transit times across the systems, suggesting confinement in the source region. Particle-in-cell simulations are carried out to explore the mechanisms of energetic electron transport from the corona to the chromosphere and possible confinement. We set up an initial system of pre-accelerated hot electrons in contact with ambient cold electrons along the local magnetic field and let it evolve over time. Suppression of transport by a nonlinear, highly localized electrostatic electric field (in the form of a double layer) is observed after a short phase of free-streaming by hot electrons. The double layer (DL) emerges at the contact of the two electron populations. It is driven by an ion-electron streaming instability due to the drift of the back-streaming return current electrons interacting with the ions. The DL grows over time and supports a significant drop in temperature and hence reduces heat flux between the two regions that is sustained for the duration of the simulation. This study shows that transport suppression begins when the energetic electrons start to propagate away from a coronal acceleration site. It also implies confinement of energetic electrons with kinetic energies less than the electrostatic energy of the DL for the DL lifetime, which is much longer than the electron transit time through the source region.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THREE-DIMENSIONAL NUMERICAL SIMULATIONS OF FAST-TO-ALFVEN CONVERSION IN SUNSPOTS

The conversion of fast waves to the Alfven mode in a realistic sunspot atmosphere is studied through three-dimensional numerical simulations. An upward propagating fast acoustic wave is excited in the high-{beta} region of the model. The new wave modes generated at the conversion layer are analyzed from the projections of the velocity and magnetic field in their characteristic directions, and the computation of their wave energy and fluxes. The analysis reveals that the maximum efficiency of the conversion to the slow mode is obtained for inclinations of 25 Degree-Sign and low azimuths, while the Alfven wave conversions peak at high inclinations and azimuths between 50 Degree-Sign and 120 Degree-Sign . Downward propagating Alfven waves appear at the regions of the sunspot where the orientation of the magnetic field is in the direction opposite to the wave propagation, since at these locations the Alfven wave couples better with the downgoing fast magnetic wave which is reflected due to the gradients of the Alfven speed. The simulations show that the Alfven energy at the chromosphere is comparable to the acoustic energy of the slow mode, being even higher at high inclined magnetic fields.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

CIRCULAR RIBBON FLARES AND HOMOLOGOUS JETS

Solar flare emissions in the chromosphere often appear as elongated ribbons on both sides of the magnetic polarity inversion line (PIL), which has been regarded as evidence of a typical configuration of magnetic reconnection. However, flares having a circular ribbon have rarely been reported, although it is expected in the fan-spine magnetic topology involving reconnection at a three-dimensional (3D) coronal null point. We present five circular ribbon flares with associated surges, using high-resolution and high-cadence H{alpha} blue wing observations obtained from the recently digitized films of Big Bear Solar Observatory. In all the events, a central parasitic magnetic field is encompassed by the opposite polarity, forming a circular PIL traced by filament material. Consequently, a flare kernel at the center is surrounded by a circular flare ribbon. The four homologous jet-related flares on 1991 March 17 and 18 are of particular interest, as (1) the circular ribbons brighten sequentially, with cospatial surges, rather than simultaneously, (2) the central flare kernels show an intriguing 'round-trip' motion and become elongated, and (3) remote brightenings occur at a region with the same magnetic polarity as the central parasitic field and are co-temporal with a separate phase of flare emissions. In another flare on 1991 February 25, the circular flare emission and surge activity occur successively, and the event could be associated with magnetic flux cancellation across the circular PIL. We discuss the implications of these observations combining circular flare ribbons, homologous jets, and remote brightenings for understanding the dynamics of 3D magnetic restructuring.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

CHARGE STATE EVOLUTION IN THE SOLAR WIND. II. PLASMA CHARGE STATE COMPOSITION IN THE INNER CORONA AND ACCELERATING FAST SOLAR WIND

In the present work, we calculate the evolution of the charge state distribution within the fast solar wind. We use the temperature, density, and velocity profiles predicted by Cranmer et al. to calculate the ionization history of the most important heavy elements in the solar corona and solar wind: C, N, O, Ne, Mg, Si, S, and Fe. The evolution of each charge state is calculated from the source region in the lower chromosphere to the final freeze-in point. We show that the solar wind velocity causes the plasma to experience significant departures from equilibrium at very low heights, well inside the field of view (within 0.6 R{sub sun} from the solar limb) of nearly all the available remote-sensing instrumentation, significantly affecting observed spectral line intensities. We also study the evolution of charge state ratios with distance from the source region, and the temperature they indicate if ionization equilibrium is assumed. We find that virtually every charge state from every element freezes in at a different height, so that the definition of freeze-in height is ambiguous. We also find that calculated freeze-in temperatures indicated by charge state ratios from in situ measurements have little relation to the local coronal temperature of the wind source region, and stop evolving much earlier than their correspondent charge state ratio. We discuss the implication of our results on plasma diagnostics of coronal holes from spectroscopic measurements as well as on theoretical solar wind models relying on coronal temperatures.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE HIGH-ENERGY IMPULSIVE GROUND-LEVEL ENHANCEMENT

We have studied short-lived (21 minute average duration), highly anisotropic pulses of cosmic rays that constitute the first phase of 10 large ground-level enhancements (GLEs), and which extend to rigidities in the range 5-20 GV. We provide a set of constraints that must be met by any putative acceleration mechanism for this type of solar-energetic-particle (SEP) event. The pulses usually have very short rise-times (three to five minutes) at all rigidities, and exhibit the remarkable feature that the intensity drops precipitously by 50% to 70% from the maximum within another three to five minutes. Both the rising and falling phases exhibit velocity dispersion, which indicates that there are particles with rigidities in the range 1 < P (GV) < 3 in the beam, and the evidence is that there is little scattering en route from the Sun. We name these events the high-energy impulsive ground-level enhancement (HEI GLE). We argue that the time-dependence observed at Earth at {approx}5 GV is a close approximation to that of the SEP pulse injected into the open heliospheric magnetic field in the vicinity of the Sun. We conclude that the temporal characteristics of the HEI GLE impose nine constraints on any putative acceleration process. Two of the HEI GLEs are preceded by short-lived, fast-rising neutron and >90 MeV gamma-ray bursts, indicating that freshly accelerated SEPs had impinged on higher-density matter in the chromosphere prior to the departure of the SEP pulse for Earth. This study was based on an updated archive of the 71 GLEs in the historic record, which is now available for public use.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

DETERMINATION OF PROMINENCE PLASMA {beta} FROM THE DYNAMICS OF RISING PLUMES

Observations by the Hinode satellite show in great detail the dynamics of rising plumes, dark in chromospheric lines, in quiescent prominences that propagate from large ({approx}10 Mm) bubbles that form at the base of the prominences. These plumes present a very interesting opportunity to study magnetohydrodynamic (MHD) phenomena in quiescent prominences, but obstacles still remain. One of the biggest issues is that of the magnetic field strength, which is not easily measurable in prominences. In this paper we present a method that may be used to determine a prominence's plasma {beta} when rising plumes are observed. Using the classic fluid dynamic solution for flow around a circular cylinder with an MHD correction, the compression of the prominence material can be estimated. This has been successfully confirmed through simulations; application to a prominence gave an estimate of the plasma {beta} as {beta} = 0.47 {+-} 0.079 to 1.13 {+-} 0.080 for the range {gamma} = 1.4-1.7. Using this method it may be possible to estimate the plasma {beta} of observed prominences, therefore helping our understanding of a prominence's dynamics in terms of MHD phenomena.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ESTIMATING THE ''DARK'' ENERGY CONTENT OF THE SOLAR CORONA

The discovery of ubiquitous low-frequency (3-5 mHz) Alfvenic waves in the solar chromosphere (with Hinode/Solar Optical Telescope) and corona (with CoMP and SDO) has provided some insight into the non-thermal energy content of the outer solar atmosphere. However, many questions remain about the true magnitude of the energy flux carried by these waves. Here we explore the apparent discrepancy in the resolved coronal Alfvenic wave amplitude ({approx}0.5 km s{sup -1}) measured by the Coronal Multi-channel Polarimeter (CoMP) compared to those of the Hinode and the Solar Dynamics Observatory (SDO) near the limb ({approx}20 km s{sup -1}). We use a blend of observational data and a simple forward model of Alfvenic wave propagation to resolve this discrepancy and determine the Alfvenic wave energy content of the corona. Our results indicate that enormous line-of-sight superposition within the coarse spatio-temporal sampling of CoMP hides the strong wave flux observed by Hinode and SDO and leads to the large non-thermal line broadening observed. While this scenario has been assumed in the past, our observations with CoMP of a strong correlation between the non-thermal line broadening with the low-amplitude, low-frequency Alfvenic waves observed in the corona provide the first direct evidence of a wave-related non-thermal line broadening. By reconciling the diverse measurements of Alfvenic waves, we establish large coronal non-thermal line widths as direct signatures of the hidden, or ''dark'', energy content in the corona and provide preliminary constraints on the energy content of the wave motions observed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

SIMULTANEOUS OBSERVATION OF SOLAR OSCILLATIONS ASSOCIATED WITH CORONAL LOOPS FROM THE PHOTOSPHERE TO THE CORONA

The solar oscillations along one coronal loop in AR 11504 are observed simultaneously in white light emission and Doppler velocity by SDO/HMI, and in UV and EUV emissions by SDO/AIA. The technique of the time-distance diagram is used to detect the propagating oscillations of the emission intensities along the loop. We find that although all the oscillation signals were intercorrelated, the low chromospheric oscillation correlated more closely to the oscillations of the transition region and corona than to those of the photosphere. Situated above the sunspot, the oscillation periods were {approx}3 minutes in the UV/EUV emissions; however, moving away from the sunspot and into the quiet Sun, the periods became longer, e.g., up to {approx}5 minutes or more. In addition, along another loop we observe both the high-speed outflows and oscillations, which roughly had a one-to-one corresponding relationship. This indicates that the solar periodic oscillations may modulate the magnetic reconnections between the loops of the high and low altitudes that drive the high-speed outflows along the loop.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

MAGNETOHYDRODYNAMIC WAVES AND CORONAL HEATING: UNIFYING EMPIRICAL AND MHD TURBULENCE MODELS

We present a new global model of the solar corona, including the low corona, the transition region, and the top of the chromosphere. The realistic three-dimensional magnetic field is simulated using the data from the photospheric magnetic field measurements. The distinctive feature of the new model is incorporating MHD Alfven wave turbulence. We assume this turbulence and its nonlinear dissipation to be the only momentum and energy source for heating the coronal plasma and driving the solar wind. The difference between the turbulence dissipation efficiency in coronal holes and that in closed field regions is because the nonlinear cascade rate degrades in strongly anisotropic (imbalanced) turbulence in coronal holes (no inward propagating wave), thus resulting in colder coronal holes, from which the fast solar wind originates. The detailed presentation of the theoretical model is illustrated with the synthetic images for multi-wavelength EUV emission compared with the observations from SDO AIA and STEREO EUVI instruments for the Carrington rotation 2107.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

FAST EXTREME-ULTRAVIOLET DIMMING ASSOCIATED WITH A CORONAL JET SEEN IN MULTI-WAVELENGTH AND STEREOSCOPIC OBSERVATIONS

We have investigated a coronal jet observed near the limb on 2010 June 27 by the Hinode/X-Ray Telescope (XRT), EUV Imaging Spectrograph (EIS), and Solar Optical Telescope (SOT), and by the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA), and on the disk by STEREO-A/EUVI. From EUV (AIA and EIS) and soft X-ray (XRT) images we have identified both cool and hot jets. There was a small loop eruption seen in Ca II images of the SOT before the jet eruption. We found that the hot jet preceded its associated cool jet by about 2 minutes. The cool jet showed helical-like structures during the rising period which was supported by the spectroscopic analysis of the jet's emission. The STEREO observation, which enabled us to observe the jet projected against the disk, showed dimming at 195 A along a large loop connected to the jet. We measured a propagation speed of {approx}800 km s{sup -1} for the dimming front. This is comparable to the Alfven speed in the loop computed from a magnetic field extrapolation of the photospheric field measured five days earlier by the SDO/Helioseismic and Magnetic Imager, and the loop densities obtained from EIS Fe XIV {lambda}264.79/274.20 line ratios. We interpret the dimming as indicating the presence of Alfvenic waves initiated by reconnection in the upper chromosphere.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

COMPUTING INTRINSIC LY{alpha} FLUXES OF F5 V TO M5 V STARS

The Ly{alpha} emission line dominates the far-ultraviolet spectra of late-type stars and is a major source for photodissociation of important molecules including H{sub 2}O, CH{sub 4}, and CO{sub 2} in exoplanet atmospheres. The incident flux in this line illuminating an exoplanet's atmosphere cannot be measured directly as neutral hydrogen in the interstellar medium (ISM) attenuates most of the flux reaching the Earth. Reconstruction of the intrinsic Ly{alpha} line has been accomplished for a limited number of nearby stars, but is not feasible for distant or faint host stars. We identify correlations connecting the intrinsic Ly{alpha} flux with the flux in other emission lines formed in the stellar chromosphere, and find that these correlations depend only gradually on the flux in the other lines. These correlations, which are based on Hubble Space Telescope spectra, reconstructed Ly{alpha} line fluxes, and irradiance spectra of the quiet and active Sun, are required for photochemical models of exoplanet atmospheres when intrinsic Ly{alpha} fluxes are not available. We find a tight correlation of the intrinsic Ly{alpha} flux with stellar X-ray flux for F5 V to K5 V stars, but much larger dispersion for M stars. We also show that knowledge of the stellar effective temperature and rotation rate can provide reasonably accurate estimates of the Ly{alpha} flux for G and K stars, and less accurate estimates for cooler stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

TEMPORAL VARIATIONS OF X-RAY SOLAR FLARE LOOPS: LENGTH, CORPULENCE, POSITION, TEMPERATURE, PLASMA PRESSURE, AND SPECTRA

The spatial and spectral properties of three solar flare coronal X-ray loops are studied before, during, and after the peak X-ray emission. Using observations from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), we deduce the temporal changes in emitting X-ray length, corpulence, volume, position, number density, and thermal pressure. We observe a decrease in the loop length, width, and volume before the X-ray peak, and an increasing number density and thermal pressure. After the X-ray peak, volume increases and loop corpulence grows due to increasing width. The volume variations are more pronounced than the position variations, often known as magnetic field line contraction. We believe this is the first dedicated study examining the temporal evolution of X-ray loop lengths and widths. Collectively, the observations also show for the first time three temporal phases given by peaks in temperature, X-ray emission, and thermal pressure, with the minimum volume coinciding with the X-ray peak. Although the volume of the flaring plasma decreases before the peak in X-ray emission, the relationship between temperature and volume does not support simple compressive heating in a collapsing magnetic trap model. Within a low {beta} plasma, shrinking loop widths perpendicular to the guiding field can be explained by squeezing the magnetic field threading the region. Plasma heating leads to chromospheric evaporation and growing number density. This produces increasing thermal pressure and decreasing loop lengths as electrons interact at shorter distances and we believe after the X-ray peak, the increasing loop corpulence.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

THE EMISSION MEASURE DISTRIBUTION OF IMPULSIVE PHASE FLARE FOOTPOINTS

The temperature distribution of the emitting plasma is a crucial constraint when studying the heating of solar flare footpoints. However, determining this for impulsive phase footpoints has been difficult in the past due to insufficient spatial resolution to resolve the footpoints from the loop structures, and a lack of spectral and temporal coverage. We use the capabilities of Hinode/Extreme Ultraviolet Imaging Spectrometer to obtain the first emission measure distributions (EMDs) from impulsive phase footpoints in six flares. Observations with good spectral coverage were analyzed using a regularized inversion method to recover the EMDs. We find that the EMDs all share a peak temperature of around 8 MK, with lines formed around this temperature having emission measures (EMs) peaking between 10{sup 28} and 10{sup 29} cm{sup -5}, indicating a substantial presence of plasma at very high temperatures within the footpoints. An EMD gradient of EM(T) {approx} T is found in all events. Previous theoretical work on EM gradients shows this to be consistent with a scenario in which the deposited flare energy directly heats only the top layer of the flare chromosphere, while deeper layers are heated by conduction.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

He I VECTOR MAGNETOMETRY OF FIELD-ALIGNED SUPERPENUMBRAL FIBRILS

Atomic-level polarization and Zeeman effect diagnostics in the neutral helium triplet at 10830 A in principle allow full vector magnetometry of fine-scaled chromospheric fibrils. We present high-resolution spectropolarimetric observations of superpenumbral fibrils in the He I triplet with sufficient polarimetric sensitivity to infer their full magnetic field geometry. He I observations from the Facility Infrared Spectropolarimeter are paired with high-resolution observations of the H{alpha} 6563 A and Ca II 8542 A spectral lines from the Interferometric Bidimensional Spectrometer from the Dunn Solar Telescope in New Mexico. Linear and circular polarization signatures in the He I triplet are measured and described, as well as analyzed with the advanced inversion capability of the ''Hanle and Zeeman Light'' modeling code. Our analysis provides direct evidence for the often assumed field alignment of fibril structures. The projected angle of the fibrils and the inferred magnetic field geometry align within an error of {+-}10 Degree-Sign . We describe changes in the inclination angle of these features that reflect their connectivity with the photospheric magnetic field. Evidence for an accelerated flow ({approx}40 m s{sup -2}) along an individual fibril anchored at its endpoints in the strong sunspot and weaker plage in part supports the magnetic siphon flow mechanism's role in the inverse Evershed effect. However, the connectivity of the outer endpoint of many of the fibrils cannot be established.

79 ASTRONOMY AND ASTROPHYSICS↗