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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1997

Persistent Sub-Yearly Chromospheric Variations in Lower Main-Sequence Stars: Tau Booe and alpha Com

The recent discoveries of extrasolar planetary systems around lower main-sequence stars such as tau Booe (HD 120136) has prompted further investigation into their stellar activity. A cursory analysis of tau Booe for cyclic chromospheric activity, based on its 30-yr record of Ca 2 H and K fluxes obtained as part of the HK Project from Mount Wilson Observatory, finds an intermediate, sub-yearly period (approximately 117 d) in chromospheric activity in addition to, and separate from, both its rotation (3.3 d) and long-term variability. As a persistent subyearly period in surface magnetic activity is unprecedented, we investigate this apparent anomaly further by examining chromospheric activity levels of other stars with similar mass, searching for variability in chromospheric activity with periods of less than one year, but longer than measured or predicted rotation. An examination of the time series of 40 mid-to-late F dwarfs yielded one other star for further analysis: alpha Com (HD 114378, P approximately 132 d). The variations for these two stars were checked for persistence and coherence. Based on these determinations, we eliminate the possibilities of rotation, long-term activity cycle, and the evolution of active regions as the cause of this variation in both stars. In particular, for tau Booe we infer that the phenomenon may be chromospheric in origin; however, beyond this, it is difficult to identify anything further regarding the cause of the activity variations, or even whether the observed modulation in the two stars have the same origin.

Maulik, Davesh↗
NASA NTRS2001

Active Region Magnetic Structure Observed in the Photosphere and Chromosphere

The magnetic flux above sunspots and plage in NOAA (National Oceanic and Atmospheric Administration) Active Region 8299 has been measured in the photosphere and the chromosphere. We investigate the vertical magnetic structure above the umbrae, penumbrae and plage regions using quantitative statistical comparisons of the photospheric and chromospheric vector magnetic flux data. The results include: (1) a decrease in flux with height, (2) the direct detection of the superpenumbral canopy in the chromosphere, (3) values for dB/dz which are consistent with earlier investigations when derived from a straight difference between the two datasets but quite low when derived from the delta x B = 0 condition, (4) a monolithic structure in the umbra which extends well into the upper chromosphere with a very complex and varied structure in the penumbra and plage, as evidenced by (5) a uniform magnetic scale height in the umbrae with an abrupt jump to widely varying scale heights in the penumbral and plage regions. Further, we find (6) evidence for a very large (delta z approximately equals 3Mm) height difference between the atmospheric layers sampled in the two magnetograms, almost a factor of three larger than that implied by atmospheric models. We additionally test the apropriateness of using photospheric magnetic flux as a boundary for field-line extrapolations, and find a better agreement with observed coronal structure when the chromospheric flux is used as a boundary.

Leka, K. D.↗
NASA NTRS2015

Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP)

Chromosphere, the transition layer of the sun is a region to switch to the magnetic pressure dominated from plasma pressure dominated, simultaneous observation of the detailed magnetic field measurement and plasma of dynamic phenomenon here is what is the frontier of the next solar physics. As This is a challenge that has just mentioned, even the next solar observation satellite plan SOLAR-C, in the experiments we had used a NASA sounding rocket for the first time in the SOLAR-C plan, will address the chromosphere-transition layer magnetic field measurement there. It is, is a Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) plan, the linear polarization of Lyman 􀉲 emission lines chromosphere-transition layer shoots (121.6nm) were detected in 0.1 percent of high accuracy, a new technique called Hanre effect I get the magnetic field information of chromosphere-transition layer. In Japan, the US and Europe joint observation in November 2012 as a rocket experiment is adopted to NASA this plan that full-scale start-up, start from assembly work is 2014 spring flight observation device, currently, it is where the alignment of the optical elements have been implemented. After this, it is planned to continue with the performance evaluation towards the observation implementation of summer 2015. In addition to once again explain the contents of the plan In this presentation, we report an overview of the entire development and preparation current status.

CLASP↗
DOE OSTI2016

CHROMOSPHERIC HEATING BY ACOUSTIC WAVES COMPARED TO RADIATIVE COOLING

Acoustic and magnetoacoustic waves are among the possible candidate mechanisms that heat the upper layers of the solar atmosphere. A weak chromospheric plage near the large solar pore NOAA 11005 was observed on 2008 October 15, in the Fe i 617.3 nm and Ca ii 853.2 nm lines of the Interferometric Bidimemsional Spectrometer attached to the Dunn Solar Telescope. In analyzing the Ca ii observations (with spatial and temporal resolutions of 0.″4 and 52 s) the energy deposited by acoustic waves is compared to that released by radiative losses. The deposited acoustic flux is estimated from the power spectra of Doppler oscillations measured in the Ca ii line core. The radiative losses are calculated using a grid of seven one-dimensional hydrostatic semi-empirical model atmospheres. The comparison shows that the spatial correlation of the maps of radiative losses and acoustic flux is 72%. In a quiet chromosphere, the contribution of acoustic energy flux to radiative losses is small, only about 15%. In active areas with a photospheric magnetic-field strength between 300 and 1300 G and an inclination of 20°–60°, the contribution increases from 23% (chromospheric network) to 54% (a plage). However, these values have to be considered as lower limits and it might be possible that the acoustic energy flux is the main contributor to the heating of bright chromospheric network and plages.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

CHROMOSPHERIC AND CORONAL WAVE GENERATION IN A MAGNETIC FLUX SHEATH

Using radiation magnetohydrodynamic simulations of the solar atmospheric layers from the upper convection zone to the lower corona, we investigate the self-consistent excitation of slow magneto-acoustic body waves (slow modes) in a magnetic flux concentration. We find that the convective downdrafts in the close surroundings of a two-dimensional flux slab “pump” the plasma inside it in the downward direction. This action produces a downflow inside the flux slab, which encompasses ever higher layers, causing an upwardly propagating rarefaction wave. The slow mode, excited by the adiabatic compression of the downflow near the optical surface, travels along the magnetic field in the upward direction at the tube speed. It develops into a shock wave at chromospheric heights, where it dissipates, lifts the transition region, and produces an offspring in the form of a compressive wave that propagates further into the corona. In the wake of downflows and propagating shock waves, the atmosphere inside the flux slab in the chromosphere and higher tends to oscillate with a period of ν ≈ 4 mHz. We conclude that this process of “magnetic pumping” is a most plausible mechanism for the direct generation of longitudinal chromospheric and coronal compressive waves within magnetic flux concentrations, and it may provide an important heat source in the chromosphere. It may also be responsible for certain types of dynamic fibrils.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

RELATIONSHIP BETWEEN CHROMOSPHERIC EVAPORATION AND MAGNETIC FIELD TOPOLOGY IN AN M-CLASS SOLAR FLARE

Chromospheric evaporation is observed as Doppler blueshift during solar flares. It plays a key role in the dynamics and energetics of solar flares; however, its mechanism is still unknown. In this paper, we present a detailed analysis of spatially resolved multi-wavelength observations of chromospheric evaporation during an M 1.0-class solar flare (SOL2014-06-12T21:12) using data from NASA’s Interface Region Imaging Spectrograph and HMI/ SDO (the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory), and high-resolution observations from VIS/NST (the Visible Imaging Spectrometer at the New Solar Telescope). The results show that the averaged over the flare region Fe xxi blueshift of the hot (10{sup 7} K) evaporating plasma is delayed relative to the C ii redshift of the relatively cold (10{sup 4} K) chromospheric plasma by about one minute. The spatial distribution of the delays is not uniform across the region and can be as long as two minutes in several zones. Using vector magnetograms from HMI, we reconstruct the magnetic field topology and the quasi-separatrix layer, and find that the blueshift delay regions as well as the H α flare ribbons are connected to the region of the magnetic polarity inversion line (PIL) and an expanding flux rope via a system of low-lying loop arcades with a height of ≲4.5 Mm. As a result, the chromospheric evaporation may be driven by the energy release in the vicinity of PIL, and has the observed properties due to a local magnetic field topology.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

KELVIN–HELMHOLTZ INSTABILITY IN SOLAR CHROMOSPHERIC JETS: THEORY AND OBSERVATION

Using data obtained by the high-resolution CRisp Imaging SpectroPolarimeter instrument on the Swedish 1 m Solar Telescope, we investigate the dynamics and stability of quiet-Sun chromospheric jets observed at the disk center. Small-scale features, such as rapid redshifted and blueshifted excursions, appearing as high-speed jets in the wings of the H α line, are characterized by short lifetimes and rapid fading without any descending behavior. To study the theoretical aspects of their stability without considering their formation mechanism, we model chromospheric jets as twisted magnetic flux tubes moving along their axis, and use the ideal linear incompressible magnetohydrodynamic approximation to derive the governing dispersion equation. Analytical solutions of the dispersion equation indicate that this type of jet is unstable to Kelvin–Helmholtz instability (KHI), with a very short (few seconds) instability growth time at high upflow speeds. The generated vortices and unresolved turbulent flows associated with the KHI could be observed as a broadening of chromospheric spectral lines. Analysis of the H α line profiles shows that the detected structures have enhanced line widths with respect to the background. We also investigate the stability of a larger-scale H α jet that was ejected along the line of sight. Vortex-like features, rapidly developing around the jet’s boundary, are considered as evidence of the KHI. The analysis of the energy equation in the partially ionized plasma shows that ion–neutral collisions may lead to fast heating of the KH vortices over timescales comparable to the lifetime of chromospheric jets.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Chromospheric and coronal activity in the 500 MYR old open cluster M37: evidence for coronal stripping?

We present the results of a spectroscopic survey to characterize chromospheric activity, as measured by Hα emission, in low-mass members of the 500 Myr old open cluster M37. Combining our new measurements of Hα luminosities (L{sub Hα}) with previously cataloged stellar properties, we identify saturated and unsaturated regimes in the dependence of the L{sub Hα}-to-bolometric luminosity ratio, L{sub Hα}/L{sub bol}, on the Rossby number R{sub o}. All rotators with R{sub o} smaller than 0.03 ± 0.01 converge to an activity level of L{sub Hα}/L{sub bol}=(1.27±0.02)×10{sup −4}. This saturation threshold (R{sub o,sat}=0.03±0.01) is statistically smaller than that found in most studies of the rotation–activity relation. In the unsaturated regime, slower rotators have lower levels of chromospheric activity, with L{sub Hα}/L{sub bol}(R{sub o}) following a power-law of index β=−0.51±0.02, slightly shallower than that found for a combined ≈650 Myr old sample of Hyades and Praesepe stars. By comparing this unsaturated behavior to that previously found for coronal activity in M37 (as measured via the X-ray luminosity, L{sub X}), we confirm that chromospheric activity decays at a much slower rate than coronal activity with increasing R{sub o}. While a comparison of L{sub Hα} and L{sub X} for M37 members with measurements of both reveals a nearly 1:1 relation, removing the mass-dependencies by comparing instead L{sub Hα}/L{sub bol} and L{sub X}/L{sub bol} does not provide clear evidence for such a relation. Finally, we find that R{sub o,sat} is smaller for our chromospheric than for our coronal indicator of activity (R{sub o,sat}=0.03±0.01 versus 0.09 ± 0.01). We interpret this as possible evidence for coronal stripping.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Multi-fluid Modeling of Magnetosonic Wave Propagation in the Solar Chromosphere: Effects of Impact Ionization and Radiative Recombination

In order to study chromospheric magnetosonic wave propagation including, for the first time, the effects of ion–neutral interactions in the partially ionized solar chromosphere, we have developed a new multi-fluid computational model accounting for ionization and recombination reactions in gravitationally stratified magnetized collisional media. The two-fluid model used in our 2D numerical simulations treats neutrals as a separate fluid and considers charged species (electrons and ions) within the resistive MHD approach with Coulomb collisions and anisotropic heat flux determined by Braginskiis transport coefficients. The electromagnetic fields are evolved according to the full Maxwell equations and the solenoidality of the magnetic field is enforced with a hyperbolic divergence-cleaning scheme. The initial density and temperature profiles are similar to VAL III chromospheric model in which dynamical, thermal, and chemical equilibrium are considered to ensure comparison to existing MHD models and avoid artificial numerical heating. In this initial setup we include simple homogeneous flux tube magnetic field configuration and an external photospheric velocity driver to simulate the propagation of MHD waves in the partially ionized reactive chromosphere. In particular, we investigate the loss of chemical equilibrium and the plasma heating related to the steepening of fast magnetosonic wave fronts in the gravitationally stratified medium.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

A Model of the Solar Chromosphere: Structure and Internal Circulation

A model of the solar chromosphere that consists of two fundamentally different regions, a lower region and an upper region, is proposed. The lower region is covered mostly by weak locally closed magnetic field and small network areas of extremely strong, locally open field. The field in the upper region is relatively uniform and locally open, connecting to the corona. The chromosphere is heated by strong collisional damping of Alfvén waves, which are driven by turbulent motions below the photosphere. The heating rate depends on the field strength, wave power from the photosphere, and altitude in the chromosphere. The waves in the internetwork area are mostly damped in the lower region, supporting radiation in the lower chromosphere. The waves in the network area, carrying more Poynting flux, are only weakly damped in the lower region. They propagate into the upper region. As the thermal pressure decreases with height, the network field expands to form the magnetic canopy where the damping of the waves from the network area supports radiation in the whole upper region. Because of the vertical stratification and horizontally nonuniform distribution of the magnetic field and heating, one circulation cell is formed in each of the upper and lower regions. The two circulation cells distort the magnetic field and reinforce the funnel-canopy-shaped magnetic geometry. The model is based on classical processes and is semi-quantitative. The estimates are constrained according to observational knowledge. No anomalous process is invoked or needed. Overall, the heating mechanism is able to damp 50% of the total wave energy.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

RESOLVING THE FAN-SPINE RECONNECTION GEOMETRY OF A SMALL-SCALE CHROMOSPHERIC JET EVENT WITH THE NEW SOLAR TELESCOPE

Jets are ubiquitously present in both quiet and active regions on the Sun. They are widely believed to be driven by magnetic reconnection. A fan-spine structure has been frequently reported in some coronal jets and flares, and has been regarded as a signature of ongoing magnetic reconnection in a topology consisting of a magnetic null connected by a fan-like separatrix surface and a spine. However, for small-scale chromospheric jets, clear evidence of such structures is rather rare, although it has been implied in earlier works that showed an inverted-Y-shaped feature. Here we report high-resolution (0.″16) observations of a small-scale chromospheric jet obtained by the New Solar Telescope (NST) using 10830 Å filtergrams. Bi-directional flows were observed across the separatrix regions in the 10830 Å images, suggesting that the jet was produced due to magnetic reconnection. At the base of the jet, a fan-spine structure was clearly resolved by the NST, including the spine and the fan-like surface, as well as the loops before and after the reconnection. A major part of this fan-spine structure, with the exception of its bright footpoints and part of the base arc, was invisible in the extreme ultraviolet and soft X-ray images (observed by the Atmosphere Imaging Assembly and the X-Ray Telescope, respectively), indicating that the reconnection occurred in the upper chromosphere. Our observations suggest that the evolution of this chromospheric jet is consistent with a two-step reconnection scenario proposed by Török et al.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Chromospheric Activity of HAT-P-11: An Unusually Active Planet-hosting K Star

Kepler photometry of the hot Neptune host star HAT-P-11 suggests that its spot latitude distribution is comparable to the Sun’s near solar maximum. We search for evidence of an activity cycle in the Ca ii H and K chromospheric emission S-index with archival Keck/HIRES spectra and observations from the echelle spectrograph on the Astrophysical Research Consortium 3.5 m Telescope at Apache Point Observatory. The chromospheric emission of HAT-P-11 is consistent with an ≳10 year activity cycle, which plateaued near maximum during the Kepler mission. In the cycle that we observed, the star seemed to spend more time near active maximum than minimum. We compare the logR{sub HK}{sup ′} normalized chromospheric emission index of HAT-P-11 with other stars. HAT-P-11 has unusually strong chromospheric emission compared to planet-hosting stars of similar effective temperature and rotation period, perhaps due to tides raised by its planet.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1973

Summary of the Stellar Chromospheres Conference

Studies on solar-type stellar chromospheres include diagnostic techniques, observations on different kinds of apparently existing chromospheres, enhancement dynamics of chromospheric activity, and interpretation of stellar spectroscopy with theoretical explanations for chromospheric lines.

Wilson, O. C.↗
NASA NTRS1975

The infrared excess of cool giant stars - A chromospheric contribution

The idea that the infrared excesses of evolved M stars may contain a contribution from a chromosphere is explored using alpha Ori and W Hya as test cases. The spectrum of alpha Ori between 8 and 30 millimicrons can be interpreted satisfactorily in terms of three components: a photosphere, a silicate dust cloud, and a cool chromosphere (temperature about 5000 K), which is optically thick at 14 millimicrons. A similar modelling for W Hya suggests a hotter chromosphere (temperature about 8000 K), with unit optical depth at 30 millimicrons. Some consequences of these chromospheres are briefly discussed.

Lambert, D. L.↗
NASA NTRS1979

Chromospheric scaling laws, width-luminosity correlations, and the Wilson-Bappu effect

Simple scaling laws for the thickness and mean electron density of stellar chromospheres as functions of surface gravity and chromospheric heating are proposed. These scaling laws are shown to be a consequence of hydrostatic equilibrium, the influence of gas ionization on plasma cooling functions, and the assumption that chromospheric heating is relatively constant with height. It is argued that line width-luminosity correlations similar to those observed in the Ca II K and Mg II k resonance lines are implied by the chromospheric scaling laws if the outer edges of the K and k emission cores are formed in the Lorentzian wings of the absorption profile. The results are compared with the Wilson-Bappu effect, empirical width-luminosity correlations for Ca II K1 minimum features, and solar-plage profiles of the Ca II K and Mg II k resonance lines.

Ayres, T. R.↗
NASA NTRS1981

Heating of the solar chromosphere by ionization pumping

A new theory is proposed to explain the heating of the solar chromosphere, and possibly the corona, by the dissipation of hydrodynamic compression waves. The basis of the dissipative mechanism, here referred to as ionization pumping, is hysteresis caused by irreversible relaxation of the chromospheric medium to ionization equilibrium following pressure perturbations. In the middle chromosphere, where hydrogen is partially ionized, it is shown that ionization pumping will cause strong dissipation of waves whose periods are 200s or less. This could cause heating of the chromosphere sufficient to compensate for the radiative losses. The mechanism retains a high efficiency for waves of arbitrarily small amplitude and, thus, can be more efficient than shock dissipation for small perturbations in pressure. The formation of shocks therefore is not required for the dissipation of waves whose periods are several minutes or less.

Lindsey, C. A.↗
NASA NTRS1979

Heating of chromospheres and coronae in cool stars

Recent evidence is summarized that suggests that neither of two traditional views of chromospheric heating (that the flux of energy required to heat the solar chromosphere) is only a small fraction of the total radiative energy emerging from the solar interior and that chromospheres are heated by acoustic waves generated in the convection zone (which lies close to the surface of the star) is valid. Particular attention is given to cool stars, both dwarfs and giants. The directions in which research is currently heading in attempting to understand heating of chromospheres and coronae in these stars are indicated. Hot stars are excluded from consideration because radiation pressure plays an important role in their atmospheric heating. It is concluded that the role of magnetic fields in giants is different from the role in dwarfs.

Mullan, D. J.↗
NASA NTRS1981

Outer atmospheres of cool stars. VIII - IUE observations and chromospheric models for the supergiant stars Beta Draconis, Epsilon Geminorum, and Alpha Orionis

A semiempirical modeling of stellar chromospheres is extended to late-type supergiants, where computations match high-resolution, absolute-flux profiles of the Ca II K and Mg II h and k lines. IUE UV spectra of Epsilon Gem and Alpha Ori show no evidence of emission lines formed at temperatures greater than 10,000 K, leading to the computation of chromospheric models extending to 10 to the -6th g/sq cm at temperatures that rise to 6500 K for the former star and 7000 K for the latter. By contrast, the C II-IV, Si IV, and He II and N V strong emission lines of Beta Dra lead to a tentative chromospheric model extension to 16,000 K at 0.012 dynes/sq cm. It is pointed out that the Ca II and Mg II line analyses presented, which assume hydrostatic equilibrium with only thermal and turbulent components to the pressure, imply nearly plane-parallel chromospheres even in the case of Alpha Ori.

Basri, G. S.↗