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At least 1,405 records · Page 78
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DOE OSTI2012

THE POLARIZATION OF THE SOLAR Mg II h AND k LINES

Although the h and k lines of Mg II are expected to be of great interest for probing the upper solar chromosphere, relatively little is known about their polarization properties which encode the information on the magnetic field. Here we report the first results of an investigation whose main goal is to understand the physical mechanisms that control the scattering polarization across these resonance lines and to achieve a realistic radiative transfer modeling in the presence of arbitrary magnetic fields. We show that the joint action of partial frequency redistribution (PRD) and quantum interference between the upper J-levels of the two lines produces a complex fractional linear polarization (Q/I) pattern with large polarization amplitudes in the blue and red wings, and a negative feature in the spectral region between the two lines. Another remarkable peculiarity of the Q/I profile is a conspicuous antisymmetric signal around the center of the h line, which cannot be obtained unless both PRD and J-state interference effects are taken into account. In the core of the k line, PRD effects alone produce a triplet peak structure in the Q/I profile, the modeling of which can also be achieved via the two-level atom approximation. In addition to the Hanle effect in the core of the k line, we also emphasize the diagnostic potential of the circular polarization produced by the Zeeman effect in the h and k lines, as well as in other Mg II lines located in their wings.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

TIME-RESOLVED ULTRAVIOLET SPECTROSCOPY OF THE M-DWARF GJ 876 EXOPLANETARY SYSTEM

Extrasolar planets orbiting M-stars may represent our best chance to discover habitable worlds in the coming decade. The ultraviolet spectrum incident upon both Earth-like and Jovian planets is critically important for proper modeling of their atmospheric heating and chemistry. In order to provide more realistic inputs for atmospheric models of planets orbiting low-mass stars, we present new near- and far-ultraviolet (NUV and FUV) spectroscopy of the M-dwarf exoplanet host GJ 876 (M4V). Using the COS and STIS spectrographs on board the Hubble Space Telescope, we have measured the 1150-3140 A spectrum of GJ 876. We have reconstructed the stellar H I Ly{alpha} emission line profile, and find that the integrated Ly{alpha} flux is roughly equal to the rest of the integrated flux (1150-1210 A + 1220-3140 A) in the entire ultraviolet bandpass (F(Ly{alpha})/F(FUV+NUV) Almost-Equal-To 0.7). This ratio is {approx}2500 Multiplication-Sign greater than the solar value. We describe the ultraviolet line spectrum and report surprisingly strong fluorescent emission from hot H{sub 2} (T(H{sub 2}) > 2000 K). We show the light curve of a chromospheric + transition region flare observed in several far-UV emission lines, with flare/quiescent flux ratios {>=}10. The strong FUV radiation field of an M-star (and specifically Ly{alpha}) is important for determining the abundance of O{sub 2}-and the formation of biomarkers-in the lower atmospheres of Earth-like planets in the habitable zones of low-mass stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ON THE ORIGIN OF THE 1/f SPECTRUM IN THE SOLAR WIND MAGNETIC FIELD

We present a mechanism for the formation of the low-frequency 1/f magnetic spectrum based on numerical solutions of a shell-reduced MHD model of the turbulent dynamics inside the sub-Alfvenic solar wind. We assign reasonably realistic profiles to the wind speed and the density along the radial direction, and a radial magnetic field. Alfven waves of short periodicity (600 s) are injected at the base of the chromosphere, penetrate into the corona, and are partially reflected, thus triggering a turbulent cascade. The cascade is strong for the reflected wave while it is weak for the outward propagating waves. Reflection at the transition region recycles the strong turbulent spectrum into the outward weak spectrum, which is advected beyond the Alfvenic critical point without substantial evolution. There, the magnetic field has a perpendicular power-law spectrum with slope close to the Kolmogorov -5/3. The parallel spectrum is inherited from the frequency spectrum of large (perpendicular) eddies. The shape is a double power law with slopes of {approx_equal} - 1 and -2 at low and high frequencies, respectively, with the position of the break depending on the injected spectrum. We suggest that the double power-law spectrum measured by Helios at 0.3 AU, where the average magnetic field is not aligned with the radial (contrary to our assumptions), results from the combination of such different spectral slopes. At low frequency the parallel spectrum dominates with its characteristic 1/f shape, while at higher frequencies its steep spectral slope (-2) is masked by the more energetic perpendicular spectrum (slope -5/3).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

SHORT GAS DISSIPATION TIMESCALES: DISKLESS STARS IN TAURUS AND CHAMAELEON I

We present an Advanced Camera for Surveys/Solar Blind Channel far-ultraviolet (FUV) study of H{sub 2} gas in 12 weak T Tauri stars in nearby star-forming regions. The sample consists of sources which have no evidence of inner disk dust. Our new FUV spectra show that in addition to the dust, the gas is depleted from the inner disk. This sample is combined with a larger FUV sample of accretors and non-accretors with ages between 1 and 100 Myr, showing that as early as 1-3 Myr, systems both with and without gas are found. Possible mechanisms for depleting gas quickly include viscous evolution, planet formation, and photoevaporation by stellar radiation fields. Since these mechanisms alone cannot account for the lack of gas at 1-3 Myr, it is likely that the initial conditions (e.g., initial disk mass or core angular momentum) contribute to the variety of disks observed at any age. We estimate the angular momentum of a cloud needed for most of the mass to fall very close to the central object and compare this to models of the expected distribution of angular momenta. Up to 20% of cloud cores have low enough angular momenta to form disks with the mass close to the star, which would then accrete quickly; this percentage is similar to the fraction of diskless stars in the youngest star-forming regions. With our sample, we characterize the chromospheric contribution to the FUV luminosity and find that L{sub FUV}/L{sub bol} saturates at {approx}10{sup -4.1}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

SIMULTANEOUS OBSERVATIONS OF A LARGE-SCALE WAVE EVENT IN THE SOLAR ATMOSPHERE: FROM PHOTOSPHERE TO CORONA

For the first time, we report a large-scale wave that was observed simultaneously in the photosphere, chromosphere, transition region, and low corona layers of the solar atmosphere. Using the high temporal and high spatial resolution observations taken by the Solar Magnetic Activity Research Telescope at Hida Observatory and the Atmospheric Imaging Assembly (AIA) on board Solar Dynamic Observatory, we find that the wave evolved synchronously at different heights of the solar atmosphere, and it propagated at a speed of 605 km s{sup -1} and showed a significant deceleration (-424 m s{sup -2}) in the extreme-ultraviolet (EUV) observations. During the initial stage, the wave speed in the EUV observations was 1000 km s{sup -1}, similar to those measured from the AIA 1700 A (967 km s{sup -1}) and 1600 A (893 km s{sup -1}) observations. The wave was reflected by a remote region with open fields, and a slower wave-like feature at a speed of 220 km s{sup -1} was also identified following the primary fast wave. In addition, a type-II radio burst was observed to be associated with the wave. We conclude that this wave should be a fast magnetosonic shock wave, which was first driven by the associated coronal mass ejection and then propagated freely in the corona. As the shock wave propagated, its legs swept the solar surface and thereby resulted in the wave signatures observed in the lower layers of the solar atmosphere. The slower wave-like structure following the primary wave was probably caused by the reconfiguration of the low coronal magnetic fields, as predicted in the field-line stretching model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

OBSERVATIONS OF PLASMA UPFLOW IN A WARM LOOP WITH HINODE/EIS

A complete understanding of Doppler shift in active region loops can help probe the basic physical mechanism involved into the heating of those loops. Here, we present observations of upflows in coronal loops detected in a range of temperatures (log T = 5.8-6.2). The loop was not discernible above these temperatures. The speed of upflow was strongest at the footpoint and decreased with height. The upflow speed at the footpoint was about 20 km s{sup -1} in Fe VIII, which decreased with temperature, being about 13 km s{sup -1} in Fe X, about 8 km s{sup -1} in Fe XII, and about 4 km s{sup -1} in Fe XIII. To the best of our knowledge, this is the first observation providing evidence of upflow of plasma in coronal loop structures at these temperatures. We interpret these observations as evidence of chromospheric evaporation in quasi-static coronal loops.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE SCATTERING POLARIZATION OF THE Ly{alpha} LINES OF H I AND He II TAKING INTO ACCOUNT PARTIAL FREQUENCY REDISTRIBUTION AND J-STATE INTERFERENCE EFFECTS

Recent theoretical investigations have pointed out that the cores of the Ly{alpha} lines of H I and He II should show measurable scattering polarization signals when observing the solar disk, and that the magnetic sensitivity, through the Hanle effect, of such linear polarization signals is suitable for exploring the magnetism of the solar transition region. Such investigations were carried out in the limit of complete frequency redistribution (CRD) and neglecting quantum interference between the two upper J-levels of each line. Here we relax both approximations and show that the joint action of partial frequency redistribution and J-state interference produces much more complex fractional linear polarization (Q/I) profiles, with large amplitudes in their wings. Such wing polarization signals turn out to be very sensitive to the temperature structure of the atmospheric model, so that they can be exploited for constraining the thermal properties of the solar chromosphere. Finally, we show that the approximation of CRD without J-state interference is however suitable for estimating the amplitude of the linear polarization signals in the core of the lines, where the Hanle effect operates.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

CHARGE STATE EVOLUTION IN THE SOLAR WIND. RADIATIVE LOSSES IN FAST SOLAR WIND PLASMAS

We study the effects of departures from equilibrium on the radiative losses of the accelerating fast, coronal hole-associated solar wind plasma. We calculate the evolution of the ionic charge states in the solar wind with the Michigan Ionization Code and use them to determine the radiative losses along the wind trajectory. We use the velocity, electron temperature, and electron density predicted by Cranmer et al. as a benchmark case even though our approach and conclusions are more broadly valid. We compare non-equilibrium radiative losses to values calculated assuming ionization equilibrium at the local temperature, and we find that differences are smaller than 20% in the corona but reach a factor of three in the upper chromosphere and transition region. Non-equilibrium radiative losses are systematically larger than the equilibrium values, so that non-equilibrium wind plasma radiates more efficiently in the transition region. Comparing the magnitude of the dominant energy terms in the Cranmer et al. model, we find that wind-induced departures from equilibrium are of the same magnitude as the differences between radiative losses and conduction in the energy equation. We investigate which ions are most responsible for such effects, finding that carbon and oxygen are the main source of departures from equilibrium. We conclude that non-equilibrium effects on the wind energy equation are significant and recommend that they are included in theoretical models of the solar wind, at least for carbon and oxygen.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE CORONAL SOURCE OF EXTREME-ULTRAVIOLET LINE PROFILE ASYMMETRIES IN SOLAR ACTIVE REGION OUTFLOWS

High-resolution spectra from the Hinode EUV Imaging Spectrometer have revealed that coronal spectral line profiles are sometimes asymmetric, with a faint enhancement in the blue wing on the order of 100 km s{sup -1}. These asymmetries could be important since they may be subtle yet diagnostically useful signatures of coronal heating or solar wind acceleration processes. It has also been suggested that they are signatures of chromospheric jets supplying mass and energy to the corona. Until now, however, there have been no studies of the physical properties of the plasma producing the asymmetries. Here we identify regions of asymmetric profiles in the outflows of AR 10978 using an asymmetric Gaussian function and extract the intensities of the faint component using multiple Gaussian fits. We then derive the temperature structure and chemical composition of the plasma producing the asymmetries. We find that the asymmetries are dependent on temperature, and are clearer and stronger in coronal lines. The temperature distribution peaks around 1.4-1.8 MK with an emission measure at least an order of magnitude larger than that at 0.6 MK. The first ionization potential bias is found to be 3-5, implying that the high-speed component of the outflows may also contribute to the slow-speed wind. Observations and models indicate that it takes time for plasma to evolve to a coronal composition, suggesting that the material is trapped on closed loops before escaping, perhaps by interchange reconnection. The results, therefore, identify the plasma producing the asymmetries as having a coronal origin.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

DETECTION OF RADIO EMISSION FROM THE HYPERACTIVE L DWARF 2MASS J13153094-2649513AB

We report the detection of radio emission from the unusually active L5e + T7 binary 2MASS J13153094-2649513AB made with the Australian Telescope Compact Array. Observations at 5.5 GHz reveal an unresolved source with a continuum flux of 370 {+-} 50 {mu}Jy, corresponding to a radio luminosity of L{sub rad} = {nu}L{sub {nu}} = (9 {+-} 3) Multiplication-Sign 10{sup 23} erg s{sup -1} and log{sub 10} L{sub rad}/L{sub bol} = -5.44 {+-} 0.22. No detection is made at 9.0 GHz to a 5{sigma} limit of 290 {mu}Jy, consistent with a power-law spectrum S{sub {nu}}{proportional_to}{nu}{sup -{alpha}} with {alpha} {approx}> 0.5. The emission is quiescent, with no evidence of variability or bursts over three hours of observation, and no measurable polarization (V/I < 34%). 2MASS J1315-2649AB is one of the most radio-luminous ultracool dwarfs detected in quiescent emission to date, comparable in strength to other cool sources detected in outburst. Its detection indicates no decline in radio flux through the mid-L dwarfs. It is unique among L dwarfs in having strong and persistent H{alpha} and radio emission, indicating the coexistence of a cool, neutral photosphere (low electron density) and a highly active chromosphere (high electron density and active heating). These traits, coupled with the system's mature age and substellar secondary, make 2MASS J1315-2649AB an important test for proposed radio emission mechanisms in ultracool dwarfs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

OBSERVATION OF MAGNETIC RECONNECTION DRIVEN BY GRANULAR SCALE ADVECTION

We report the first evidence of magnetic reconnection driven by advection in a rapidly developing large granule using high spatial resolution observations of a small surge event (base size {approx} 4'' Multiplication-Sign 4'') with the 1.6 m aperture New Solar Telescope at the Big Bear Solar Observatory. The observations were carried out in narrowband (0.5 A) He I 10830 A and broadband (10 A) TiO 7057 A. Since He I 10830 A triplet has a very high excitation level and is optically thin, its filtergrams enable us to investigate the surge from the photosphere through the chromosphere into the lower corona. Simultaneous space data from the Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory were used in the analysis. It is shown that the surge is spatio-temporally associated with magnetic flux emergence in the rapidly developing large granule. During the development of the granule, its advecting flow ({approx}2 km s{sup -1}) squeezed the magnetic flux into an intergranular lane area, where a magnetic flux concentration was formed and the neighboring flux with opposite magnetic polarity was canceled. During the cancellation, the surge was produced as absorption in He I 10830 A filtergrams while simultaneous EUV brightening occurred at its base. The observations clearly indicate evidence of a finest-scale reconnection process driven by the granule's motion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

A KEY PHYSICAL MECHANISM FOR UNDERSTANDING THE ENIGMATIC LINEAR POLARIZATION OF THE SOLAR Ba II AND Na I D{sub 1} LINES

The linearly polarized spectrum of the solar limb radiation produced by scattering processes is of great diagnostic potential for exploring the magnetism of the solar atmosphere. This spectrum shows an impressive richness of spectral details and enigmatic Q/I signals, whose physical origin must be clearly understood before they can be exploited for diagnostic purposes. The most enduring enigma is represented by the polarization signals observed in the D{sub 1} resonance lines of Na I (5896 A) and Ba II (4934 A), which were expected to be intrinsically unpolarizable. The totality of sodium and 18% of barium have hyperfine structure (HFS), and it has been argued that the only way to produce a scattering polarization signal in such lines is through the presence of a substantial amount of atomic polarization in their lower HFS levels. The strong sensitivity of these long-lived levels to depolarizing mechanisms led to the paradoxical conclusion that the observed D{sub 1}-line polarization is incompatible with the presence in the lower solar chromosphere of inclined magnetic fields sensibly stronger than 0.01 G. Here we show that by properly taking into account the fact that the solar D{sub 1}-line radiation has a non-negligible spectral structure over the short frequency interval spanned by the HFS transitions, it is possible to produce scattering polarization signals in the D{sub 1} lines of Na I and Ba II without the need of ground-level polarization. The resulting linear polarization is not so easily destroyed by elastic collisions and/or magnetic fields.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

SOLAR TRANSITION REGION LINES OBSERVED BY THE INTERFACE REGION IMAGING SPECTROGRAPH: DIAGNOSTICS FOR THE O IV AND Si IV LINES

The formation of the transition region O IV and Si IV lines observable by the Interface Region Imaging Spectrograph (IRIS) is investigated for both Maxwellian and non-Maxwellian conditions characterized by a κ-distribution exhibiting a high-energy tail. The Si IV lines are formed at lower temperatures than the O IV lines for all κ. In non-Maxwellian situations with lower κ, the contribution functions are shifted to lower temperatures. Combined with the slope of the differential emission measure, it is possible for the Si IV lines to be formed at very different regions of the solar transition region than the O IV lines; possibly close to the solar chromosphere. Such situations might be discernible by IRIS. It is found that photoexcitation can be important for the Si IV lines, but is negligible for the O IV lines. The usefulness of the O IV ratios for density diagnostics independently of κ is investigated and it is found that the O IV 1404.78 Å/1399.77 Å ratio provides a good density diagnostics except for very low T combined with extreme non-Maxwellian situations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

THE EFFECT OF ISOTOPIC SPLITTING ON THE BISECTOR AND INVERSIONS OF THE SOLAR Ca II 854.2 nm LINE

The Ca II 854.2 nm spectral line is a common diagnostic of the solar chromosphere. The average line profile shows an asymmetric core, and its bisector shows a characteristic inverse-C shape. The line actually consists of six components with slightly different wavelengths depending on the isotope of calcium. This isotopic splitting of the line has been taken into account in studies of non-solar stars, but never for the Sun. We performed non-LTE radiative transfer computations from three models of the solar atmosphere and show that the line-core asymmetry and inverse C-shape of the bisector of the 854.2 nm line can be explained by isotopic splitting. We confirm this finding by analyzing observations and showing that the line asymmetry is present irrespective of conditions in the solar atmosphere. Finally, we show that inversions based on the Ca II 854.2 nm line should take the isotopic splitting into account, otherwise the inferred atmospheres will contain erroneous velocity gradients and temperatures.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

MEASUREMENTS OF ABSOLUTE ABUNDANCES IN SOLAR FLARES

We present measurements of elemental abundances in solar flares with the EUV Variability Experiment (EVE) on the Solar Dynamics Observatory. EVE observes both high temperature Fe emission lines (Fe XV-Fe XXIV) and continuum emission from thermal bremsstrahlung that is proportional to the abundance of H. By comparing the relative intensities of line and continuum emission it is possible to determine the enrichment of the flare plasma relative to the composition of the photosphere. This is the first ionization potential or FIP bias (f). Since thermal bremsstrahlung at EUV wavelengths is relatively insensitive to the electron temperature, it is important to account for the distribution of electron temperatures in the emitting plasma. We accomplish this by using the observed spectra to infer the differential emission measure distribution and FIP bias simultaneously. In each of the 21 flares that we analyze we find that the observed composition is close to photospheric. The mean FIP bias in our sample is f = 1.17 ± 0.22. This analysis suggests that the bulk of the plasma evaporated during a flare comes from deep in the chromosphere, below the region where elemental fractionation occurs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

PENUMBRAL-LIKE FILAMENTS IN THE SOLAR PHOTOSPHERE AS A MANIFESTATION OF FLUX EMERGENCE

Rare observations of the solar photosphere show the appearance of orphan penumbrae, filamentary structures very similar to a bundle of sunspot penumbral filaments not connected to any umbra. Lim et al. found an orphan penumbra in active region NOAA 11391 near a mature sunspot. We analyze a different data set to study the same structure using the Solar Optical Telescope on board the Hinode satellite. Spectropolarimetric measurements along the Fe I 630.2 nm pair, complemented by G-band and Ca II H filtergrams, show the evolution of this penumbral-like structure and reveal that an emerging flux region is its ancestor. We find new evidence for the interaction between the emerging flux and the pre-existing field that leads to a brightening observed near the base of the chromosphere. Our analysis suggests that as a result of the combination of photospheric flux emergence and magneto-convection in inclined fields the horizontal component of the emerging field can be trapped in the photosphere by the overlying fields and form a structure resembling penumbral filaments.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

HYDROGEN BALMER CONTINUUM IN SOLAR FLARES DETECTED BY THE INTERFACE REGION IMAGING SPECTROGRAPH (IRIS)

We present a novel observation of the white light flare (WLF) continuum, which was significantly enhanced during the X1 flare on 2014 March 29 (SOL2014-03-29T17:48). Data from the Interface Region Imaging Spectrograph (IRIS) in its near-UV channel show that at the peak of the continuum enhancement, the contrast at the quasi-continuum window above 2813 Å reached 100%-200% and can be even larger closer to Mg II lines. This is fully consistent with the hydrogen recombination Balmer-continuum emission, which follows an impulsive thermal and non-thermal ionization caused by the precipitation of electron beams through the chromosphere. However, a less probable photospheric continuum enhancement cannot be excluded. The light curves of the Balmer continuum have an impulsive character with a gradual fading, similar to those detected recently in the optical region on the Solar Optical Telescope on board Hinode. This observation represents a first Balmer-continuum detection from space far beyond the Balmer limit (3646 Å), eliminating seeing effects known to complicate the WLF detection. Moreover, we use a spectral window so far unexplored for flare studies, which provides the potential to study the Balmer continuum, as well as many metallic lines appearing in emission during flares. Combined with future ground-based observations of the continuum near the Balmer limit, we will be able to disentangle various scenarios of the WLF origin. IRIS observations also provide a critical quantitative measure of the energy radiated in the Balmer continuum, which constrains various models of the energy transport and deposit during flares.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

MAGNETOHYDRODYNAMIC MODELING OF THE ACCRETION SHOCKS IN CLASSICAL T TAURI STARS: THE ROLE OF LOCAL ABSORPTION IN THE X-RAY EMISSION

We investigate the properties of X-ray emission from accretion shocks in classical T Tauri stars (CTTSs), generated where the infalling material impacts the stellar surface. Both observations and models of the accretion process reveal several aspects that are still unclear: the observed X-ray luminosity in accretion shocks is below the predicted value, and the density versus temperature structure of the shocked plasma, with increasing densities at higher temperature, deduced from the observations, is at odds with that proposed in the current picture of accretion shocks. To address these open issues, we investigate whether a correct treatment of the local absorption by the surrounding medium is crucial to explain the observations. To this end, we describe the impact of an accretion stream on a CTTS by considering a magnetohydrodynamic model. From the model results, we synthesize the X-ray emission from the accretion shock by producing maps and spectra. We perform density and temperature diagnostics on the synthetic spectra, and we directly compare the results with observations. Our model shows that the X-ray fluxes inferred from the emerging spectra are lower than expected because of the complex local absorption by the optically thick material of the chromosphere and of the unperturbed stream. Moreover, our model, including the effects of local absorption, explains in a natural way the apparently puzzling pattern of density versus temperature observed in the X-ray emission from accretion shocks.

79 ASTRONOMY AND ASTROPHYSICS↗