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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

THERMAL FINE STRUCTURE AND MAGNETIC FIELDS IN THE SOLAR ATMOSPHERE: SPICULES AND FIBRILS

The relationship between observed structures in the solar atmosphere and the magnetic fields threading them is known only for the solar photosphere, even then imprecisely. We suggest that some of the fine structures in the more tenuous chromosphere and corona-specifically some populations of spicules and fibrils-correspond to warps in two-dimensional sheet-like structures, as an alternative to conventional interpretations in terms of tube-like structures. The sheets are perhaps related to magnetic tangential discontinuities, which Parker has argued arise naturally in low-{beta} conditions. Some consequences of this suggestion, if it can be confirmed, are discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

POLARIZATION OF THE CHARGE-EXCHANGE X-RAYS INDUCED IN THE HELIOSPHERE

We report results of a theoretical investigation of polarization of the X-ray emissions induced in charge-exchange collisions of fully stripped solar wind (SW) ions C{sup 6+} and O{sup 8+} with the heliospheric hydrogen atoms. The polarization of X-ray emissions has been computed for line-of-sight observations within the ecliptic plane as a function of SW ion velocities, including a range of velocities corresponding to the slow and fast SW, and coronal mass ejections. To determine the variability of polarization of heliospheric X-ray emissions, the polarization has been computed for solar minimum conditions with self-consistent parameters of the SW plasma and heliospheric gas and compared with the polarization calculated for an averaged solar activity. We predict the polarization of charge-exchange X-rays to be between 3% and 8%, depending on the line-of-sight geometry, SW ion velocity, and the selected emission lines.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

PLASMA IN THE HELIOSHEATH: 3.5 YEARS OF OBSERVATIONS

Voyager 2 (V2) has observed heliosheath (HSH) plasma since 2007 August. We describe how the plasma has evolved across the HSH. We show that the low solar wind dynamic pressure leads to an inward movement of the termination shock (TS) of about 10 AU to a minimum position of 73 AU in 2010. Near the TS large fluctuations are present in the HSH, but these fluctuations decrease as V2 moves further from the TS. The radial speed slowly decreases and the plasma flow slowly turns tailward. The temperature decreases across the HSH. The radial speed in 2011 remains above 100 km s{sup -1}, which implies that V2 is a substantial distance from the heliopause.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

A STANDARD-TO-BLOWOUT JET

The commonly observed jets provide critical information on the small-scale energy release in the solar atmosphere. We report a near disk-center jet on 2010 July 20, observed by the Solar Dynamics Observatory. In this event, the standard interchange magnetic reconnection between an emerging flux spanning 9 x 10{sup 3} km and ambient open fields is followed by a blowout-like eruption. In the 'standard' stage, as the emerging negative element approached the nearby positive network fields, a jet with a dome-like base in EUV grew for 30 minutes before the jet spire began to migrate laterally with enhanced flux emergence. In the 'blowout' stage, the above converging fields collided and the subsequent cancellation produced a UV microflare lasting seven minutes, in which the dome of the jet seemed to be blown out as (1) the spire swung faster and exhibited an unwinding motion before vanishing, (2) a rising loop and a blob erupted leaving behind cusped structures, with the blob spiraling outward in acceleration after the flare maximum, and (3) ejecting material with a curtain-like structure at chromospheric to transition-region temperatures also underwent a transverse motion. It is thus suggested that the flare reconnection rapidly removes the outer fields of the emerging flux to allow its twisted core field to erupt, a scenario favoring the jet-scale magnetic breakout model as recently advocated by Moore et al. in 2010.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

Low-energy charge transfer between C{sup 5+} and atomic hydrogen

Charge transfer with carbon ions has been identified as important in both magnetic fusion plasma devices and, more recently, in solar wind interactions with comets, planets, or neutrals in the heliosphere. A merged-beams technique is used to measure the absolute total charge-transfer cross section for C{sup 5+} and atomic H over four orders of magnitude in collision energy, from 12 000 to 0.64 eV/u. The present measurements are compared with previous measurements using an atomic hydrogen target and benchmark available classical trajectory Monte Carlo and molecular-orbital close-coupling calculations. An observed increasing cross section below 10 eV/u is attributed to trajectory effects due to the ion-induced dipole attraction between reactants.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

The Factory and the Beehive. IV. A Comprehensive Study of the Rotation X-Ray Activity Relation in Praesepe and the Hyades

X-ray observations of low-mass stars in open clusters are critical to understanding the dependence of magnetic activity on stellar properties and their evolution. Praesepe and the Hyades, two of the nearest, most-studied open clusters, are among the best available laboratories for examining the dependence of magnetic activity on rotation for stars with masses ≲1 M ⊙ . We present an updated study of the rotation–X-ray activity relation in the two clusters. We updated membership catalogs that combine pre-Gaia catalogs with new catalogs based on Gaia Data Release 2. The resulting catalogs are the most inclusive ones for both clusters: 1739 Praesepe and 1315 Hyades stars. We collected X-ray detections for cluster members, for which we analyzed, re-analyzed, or collated data from ROSAT, the Chandra X-ray Observatory, the Neil Gehrels Swift Observatory, and XMM-Newton. We have detections for 326 Praesepe and 462 Hyades members, of which 273 and 164, respectively, have rotation periods—an increase of 6× relative to what was previously available. We find that at ≈700 Myr, only M dwarfs remain saturated in X-rays, with only tentative evidence for supersaturation. We also find a tight relation between the Rossby number and fractional X-ray luminosity L X /L bol in unsaturated single members, suggesting a power-law index between –3.2 and –3.9. Lastly, we find no difference in the coronal parameters between binary and single members. These results provide essential insight into the relative efficiency of magnetic heating of the stars' atmospheres, thereby informing the development of robust age-rotation-activity relations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

Interim Report on the Power Law Index of Interplanetary Suprathermal Ion Spectra

There is a continuing debate about the applicability of the theory presented by Fisk and Gloeckler (FG) regarding the formation of suprathermal ion tails in phase space density vs. velocity spectra; in the solar wind frame the FG theory predicts a power law index of-5 (which is equivalent to a differential intensity vs. energy index of-1.5). There has also been uncertainty and perhaps misunderstanding regarding the extent to which such spectra are actually observed; i.e., is there really a significant preference for the -5 index? Here we report the results of an interim technique we use to analyze {approx}1-100 keV/nucleon interplanetary suprathermal H{sup +}, He{sup +}, and He{sup ++}, spectra measured at the Cassini spacecraft by the Charge Energy Mass Spectrometer (CHEMS) instrument of the Magnetospheric Imaging Instrument (MIMI) suite during the cruise to Saturn. We analyzed 18 active periods and report a mean index in the solar wind frame of 4.9{+-}0.4 for protons, 5.2{+-}0.5 for He{sup +}, and 4.7{+-}0.2 for alpha particles. MIMI/CHEMS offers much needed independent observations of heliospheric ions in the suprathermal energy range.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

MICRO-SIGMOIDS AS PROGENITORS OF CORONAL JETS: IS ERUPTIVE ACTIVITY SELF-SIMILARLY MULTI-SCALED?

Observations from the X-ray telescope (XRT) on Hinode are used to study the nature of X-ray-bright points, sources of coronal jets. Several jet events in the coronal holes are found to erupt from small-scale, S-shaped bright regions. This finding suggests that coronal micro-sigmoids may well be progenitors of coronal jets. Moreover, the presence of these structures may explain numerous observed characteristics of jets such as helical structures, apparent transverse motions, and shapes. Analogous to large-scale sigmoids giving rise to coronal mass ejections (CMEs), a promising future task would perhaps be to investigate whether solar eruptive activity, from coronal jets to CMEs, is self-similar in terms of properties and instability mechanisms.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

IS THE POLAR REGION DIFFERENT FROM THE QUIET REGION OF THE SUN?

Observations of the polar region of the Sun are critically important for understanding the solar dynamo and the acceleration of solar wind. We carried out precise magnetic observations on both the north polar region and the quiet Sun at the east limb with the spectropolarimeter of the Solar Optical Telescope aboard Hinode to characterize the polar region with respect to the quiet Sun. The average area and the total magnetic flux of the kilo-Gauss magnetic concentrations in the polar region appear to be larger than those of the quiet Sun. The magnetic field vectors classified as vertical in the quiet Sun have symmetric histograms around zero in the strengths, showing balanced positive and negative fluxes, while the histogram in the north polar region is clearly asymmetric, showing a predominance of the negative polarity. The total magnetic flux of the polar region is larger than that of the quiet Sun. In contrast, the histogram of the horizontal magnetic fields is exactly the same for both the polar region and the quiet Sun. This is consistent with the idea that a local dynamo process is responsible for the horizontal magnetic fields. A high-resolution potential field extrapolation shows that the majority of magnetic field lines from the kG-patches in the polar region are open with a fanning-out structure very low in the atmosphere, while in the quiet Sun, almost all the field lines are closed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

LARGE-SCALE SOFT X-RAY LOOPS AND THEIR MAGNETIC CHIRALITY IN BOTH HEMISPHERES

The magnetic chirality in the solar atmosphere has been studied based on soft X-ray and magnetic field observations. It is found that some of the large-scale twisted soft X-ray loop systems occur for several months in the solar atmosphere, before the disappearance of the corresponding background large-scale magnetic field. This paper provides observational evidence of the helicity of the large-scale magnetic field in the solar atmosphere and the reverse one relative to the helicity rule in both hemispheres with solar cycles. The transfer of the magnetic helicity from the subatmosphere is consistent with the formation of large-scale twisted soft X-ray loops in both the solar hemispheres.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

CAN IBEX IDENTIFY VARIATIONS IN THE GALACTIC ENVIRONMENT OF THE SUN USING ENERGETIC NEUTRAL ATOMS?

The Interstellar Boundary Explorer (IBEX) spacecraft is providing the first all-sky maps of the energetic neutral atoms (ENAs) produced by charge exchange between interstellar neutral H{sup o} atoms and heliospheric solar wind and pickup ions in the heliosphere boundary regions. The 'edge' of the interstellar cloud presently surrounding the heliosphere extends less than 0.1 pc in the upwind direction, terminating at an unknown distance, indicating that the outer boundary conditions of the heliosphere could change during the lifetime of the IBEX satellite. Using reasonable values for future outer heliosphere boundary conditions, ENA fluxes are predicted for one possible source of ENAs coming from outside of the heliopause. The ENA-production simulations use three-dimensional MHD plasma models of the heliosphere that include a kinetic description of neutrals and a Lorentzian distribution for ions. Based on this ENA-production model, it is then shown that the sensitivities of the IBEX 1.1 keV skymaps are sufficient to detect the variations in ENA fluxes that are expected to accompany the solar transition into the next upwind cloud. Approximately 20% of the IBEX 1.1 keV pixels appear capable of detecting the predicted model differences at the 3{sigma} level, with these pixels concentrated in the Ribbon region. Regardless of the detailed ENA production model, the success of the modeled B {center_dot} R {approx} 0 directions in reproducing the Ribbon locus, together with our results, indicates that the Ribbon phenomenon traces the variations in the heliosphere distortion caused by the relative pressures of the interstellar magnetic and gaseous components.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2010

FLUCTUATING ENERGY STORAGE AND NONLINEAR CASCADE IN AN INHOMOGENEOUS CORONAL LOOP

The dynamics and the energy balance of large-scale fluctuations in a coronal loop are studied. The loop is represented by a simplified structure where the curvature is neglected and the background magnetic field is uniform. In a previous paper, we studied a similar model where a uniform background density was assumed. The present paper represents a generalization of the previous one and it has the purpose of investigating possible modifications to the large-scale energy balance and dynamics due to a more realistic longitudinally nonuniform density. Large-scale fluctuations are dominated by coherent eigenmodes that nonlinearly couple to produce an energy cascade to smaller scales. Eigenmodes properties are calculated by a simplified linear dissipative model, deriving an expression for the input energy flux that is not substantially modified by the presence of the density inhomogeneity and is independent of dissipation. For typical values of the parameters, the derived input energy flux is comparable with that required to heat the active region corona. Nonlinear couplings are dominated by coherence effects due to the symmetry properties of eigenmodes; the consequences are that the system is in a weakly nonlinear regime that produces fluctuating energy storage in the loop, and that the kinetic and magnetic nonlinear energy fluxes are of the same order, despite the dominance of magnetic energy at large scales. From the energy balance, an expression for the velocity fluctuation is derived, which is valid in the more general case of a nonuniform background density; this estimate is in agreement both with measures of nonthermal velocities in the solar corona and with previous numerical results.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

STRUCTURE AND DYNAMICS OF QUIESCENT FILAMENT CHANNELS OBSERVED BY HINODE/XRT AND STEREO/EUVI

We present a study of the structure and dynamics of quiescent filament channels observed by Hinode/XRT and STEREO/EUVI at the solar minimum 23/24 from 2006 November to 2008 December. For 12 channels identified on the solar disk (Group I channels), we find that the morphology of the structure on the two sides of the channel is asymmetric in both X-rays and EUV: the eastern side has curved features while the western side has straight features. We interpret the results in terms of a magnetic flux rope model. The asymmetry in the morphology is due to the variation in axial flux of the flux rope along the channel, which causes the field lines from one polarity to turn into the flux rope (curved feature), while the field lines from the other polarity are connected to very distant sources (straight). For most of the 68 channels identified by cavities at the east and west limbs (Group II channels), the asymmetry cannot be clearly identified, which is likely due to the fact that the axial flux may be relatively constant along such channels. Corresponding cavities are identified only for 5 of the 12 Group I channels, while Group II channels are identified for all of the 68 cavity pairs. The studied filament channels are often observed as dark channels in X-rays and EUV. Sheared loops within Group I channels are often seen in X-rays, but are rarely seen in Group II channels as shown in the X-ray Telescope daily synoptic observations. A survey of the dynamics of studied filament channels shows that filament eruptions occur at an average rate of 1.4 filament eruptions per channel per solar rotation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

FAST GYROSYNCHROTRON CODES

Radiation produced by charged particles gyrating in a magnetic field is highly significant in the astrophysics context. Persistently increasing resolution of astrophysical observations calls for corresponding three-dimensional modeling of the radiation. However, available exact equations are prohibitively slow in computing a comprehensive table of high-resolution models required for many practical applications. To remedy this situation, we develop approximate gyrosynchrotron (GS) codes capable of quickly calculating the GS emission (in non-quantum regime) from both isotropic and anisotropic electron distributions in non-relativistic, mildly relativistic, and ultrarelativistic energy domains applicable throughout a broad range of source parameters including dense or tenuous plasmas and weak or strong magnetic fields. The computation time is reduced by several orders of magnitude compared with the exact GS algorithm. The new algorithm performance can gradually be adjusted to the user's needs depending on whether precision or computation speed is to be optimized for a given model. The codes are made available for users as a supplement to this paper.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

INTERMITTENCY AND MULTIFRACTALITY SPECTRA OF THE MAGNETIC FIELD IN SOLAR ACTIVE REGIONS

We present the results of a study of intermittency and multifractality of magnetic structures in solar active regions (ARs). Line-of-sight magnetograms for 214 ARs of different flare productivity observed at the center of the solar disk from 1997 January until 2006 December are utilized. Data from the Michelson Doppler Imager (MDI) instrument on board the Solar and Heliospheric Observatory operating in the high resolution mode, the Big Bear Solar Observatory digital magnetograph, and the Hinode SOT/SP instrument were used. Intermittency spectra were derived from high-order structure functions and flatness functions. The flatness function exponent is a measure of the degree of intermittency. We found that the flatness function exponent at scales below approximately 10 Mm is correlated with flare productivity (the correlation coefficient is -0.63). The Hinode data show that the intermittency regime is extended toward small scales (below 2 Mm) as compared to the MDI data. The spectra of multifractality, derived from the structure functions and flatness functions, are found to be broader for ARs of higher flare productivity as compared to those of low flare productivity. The magnetic structure of high-flaring ARs consists of a voluminous set of monofractals, and this set is much richer than that for low-flaring ARs. The results indicate the relevance of the multifractal organization of the photospheric magnetic fields to the flaring activity. The strong intermittency observed in complex and high-flaring ARs is a hint that we observe a photospheric imprint of enhanced sub-photospheric dynamics.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

THE OPTICAL DEPTH OF WHITE-LIGHT FLARE CONTINUUM

The white-light continuum emission of a solar flare remains a puzzle as regards its height of formation and its emission mechanism(s). This continuum and its extension into the near-UV contain the bulk of the energy radiated by a flare, and so its explanation is a high priority. We describe a method to determine the optical depth of the emitting layer and apply it to the well-studied flare of 2002 July 15, making use of MDI pseudo-continuum intensity images. We find the optical depth of the visible continuum in all flare images, including an impulsive ribbon to be small, consistent with the observation of Balmer and Paschen edges in other events.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

TESTING AUTOMATED SOLAR FLARE FORECASTING WITH 13 YEARS OF MICHELSON DOPPLER IMAGER MAGNETOGRAMS

Flare occurrence is statistically associated with changes in several characteristics of the line-of-sight magnetic field in solar active regions (ARs). We calculated magnetic measures throughout the disk passage of 1075 ARs spanning solar cycle 23 to find a statistical relationship between the solar magnetic field and flares. This expansive study of over 71,000 magnetograms and 6000 flares uses superposed epoch (SPE) analysis to investigate changes in several magnetic measures surrounding flares and ARs completely lacking associated flares. The results were used to seek any flare associated signatures with the capability to recover weak systematic signals with SPE analysis. SPE analysis is a method of combining large sets of data series in a manner that yields concise information. This is achieved by aligning the temporal location of a specified flare in each time series, then calculating the statistical moments of the 'overlapping' data. The best-calculated parameter, the gradient-weighted inversion-line length (GWILL), combines the primary polarity inversion line (PIL) length and the gradient across it. Therefore, GWILL is sensitive to complex field structures via the length of the PIL and shearing via the gradient. GWILL shows an average 35% increase during the 40 hr prior to X-class flares, a 16% increase before M-class flares, and 17% increase prior to B-C-class flares. ARs not associated with flares tend to decrease in GWILL during their disk passage. Gilbert and Heidke skill scores are also calculated and show that even GWILL is not a reliable parameter for predicting solar flares in real time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

DYNAMICS OF THE FLARING LOOP SYSTEM OF 2005 AUGUST 22 OBSERVED IN MICROWAVES AND HARD X-RAYS

We studied the spatial dynamics of the flaring loop in the 2005 August 22 event using microwave (NoRH) and hard X-ray (RHESSI) observations together with complementary data from SOHO/MDI, SMART at Hida, SOHO/EIT, and TRACE. We have found that (1) the pre-flare morphology of the active region exhibits a strongly sheared arcade seen in H{alpha} and the J-shape filament seen in EUV; (2) energy release and high-energy electron acceleration occur in a sequence along the extensive arcade; (3) the shear angle and the parallel (to the magnetic neutral line) component of the footpoint (FP) distance steadily decrease during the flare process; (4) the radio loop shrinks in length and height during the first emission peak, and later it grows; after the fourth peak the simultaneous descending of the brightest loop and formation of a new microwave loop at a higher altitude occur; (5) the hard X-ray coronal source is located higher than the microwave loop apex and shows faster upward motion; (6) the first peak on microwave time profiles is present in both the loop top and FP regions. However, the emission peaks that follow are present only in the FP regions. We conclude that after the first emission peak the acceleration site is located over the flaring arcade and particles are accelerated along magnetic field lines. We make use of the collapsing magnetic trap model to understand some observational effects.

79 ASTRONOMY AND ASTROPHYSICS↗