Helium Continuum Research Articles

Signatures of Helium Continuum in Cool Flare Loops Observed by SDO/AIA

Abstract We present an analysis of off-limb cool flare loops observed by the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) during the gradual phase of SOL2017-09-10T16:06 X8.2-class flare. In the extreme-ultraviolet (EUV) channels starting from the 335 Å one, cool loops appear as dark structures against the bright loop arcade. These dark structures were precisely coaligned (spatially and temporally) with loops observed by Swedish Solar Telescope (SST) in emission lines of hydrogen and ionized calcium. A recently published semi-empirical model of cool loops based on SST observations serves to predict the level of hydrogen and helium recombination continua. The continua were synthesized using an approximate non-LTE (i.e., departures from local thermodynamic equilibrium) approach and theoretical spectra were then transformed to AIA signals. Comparison with signals detected inside the dark loops shows that only in AIA 211 Å channel the computed level of recombination continua is consistent with observations for some models, while in all other channels that are more distant from the continua edges the synthetic continuum is far too low. In analogy with on-disk observations of flares we interpret the surplus emission as due to numerous EUV lines emitted from hot but faint loops in front of the cool ones. Finally we briefly comment on failure of the standard absorption model when used for analysis of the dark-loop brightness.

Counter-streaming flows in a giant quiet-Sun filament observed in the extreme ultraviolet

Aim. The giant solar filament was visible on the solar surface from 2011 November 8–23. Multiwavelength data from the Solar Dynamics Observatory (SDO) were used to examine counter-streaming flows within the spine of the filament.Methods. We use data from two SDO instruments, the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI), covering the whole filament, which stretched over more than half a solar diameter. Hα images from the Kanzelhöhe Solar Observatory (KSO) provide context information of where the spine of the filament is defined and the barbs are located. We apply local correlation tracking (LCT) to a two-hour time series on 2011 November 16 of the AIA images to derive horizontal flow velocities of the filament. To enhance the contrast of the AIA images, noise adaptive fuzzy equalization (NAFE) is employed, which allows us to identify and quantify counter-streaming flows in the filament. We observe the same cool filament plasma in absorption in both Hα and EUV images. Hence, the counter-streaming flows are directly related to this filament material in the spine. In addition, we use directional flow maps to highlight the counter-streaming flows.Results. We detect counter-streaming flows in the filament, which are visible in the time-lapse movies in all four examined AIA wavelength bands (λ171 Å, λ193 Å, λ304 Å, and λ211 Å). In the time-lapse movies we see that these persistent flows lasted for at least two hours, although they became less prominent towards the end of the time series. Furthermore, by applying LCT to the images we clearly determine counter-streaming flows in time series of λ171 Å and λ193 Å images. In the λ304 Å wavelength band, we only see minor indications for counter-streaming flows with LCT, while in the λ211 Å wavelength band the counter-streaming flows are not detectable with this method. The diverse morphology of the filament in Hα and EUV images is caused by different absorption processes, i.e., spectral line absorption and absorption by hydrogen and helium continua, respectively. The horizontal flows reach mean flow speeds of about 0.5 km s−1 for all wavelength bands. The highest horizontal flow speeds are identified in the λ171 Å band with flow speeds of up to 2.5 km s−1. The results are averaged over a time series of 90 minutes. Because the LCT sampling window has finite width, a spatial degradation cannot be avoided leading to lower estimates of the flow velocities as compared to feature tracking or Doppler measurements. The counter-streaming flows cover about 15–20% of the whole area of the EUV filament channel and are located in the central part of the spine.Conclusions. Compared to the ground-based observations, the absence of seeing effects in AIA observations reveal counter-streaming flows in the filament even with a moderate image scale of 0. ′′6 pixel−1. Using a contrast enhancement technique, these flows can be detected and quantified with LCT in different wavelengths. We confirm the omnipresence of counter-streaming flows also in giant quiet-Sun filaments.

Attosecond transient absorption spectroscopy of helium above the N=2 ionization threshold

Attosecond transient absorption spectroscopy (ATAS) allows for the study of electron dynamics in atoms and molecules with attosecond time resolution. Previous works reported in the literature have made use of ATAS to image and control such dynamics in the single-channel ionization continuum of helium; in particular, in the vicinity of the doubly excited autoionizing states lying between the $N=1$ and $N=2$ thresholds. In this work, we have extended these studies to autoionizing states lying above the $N=2$ threshold, where several ionization channels are open. From an accurate solution of the time-dependent Schr\odinger equation, we predict the appearance of pronounced one-photon beatings between the $3snp$ states and the adjacent ${}^{1}{S}^{e}$ and ${}^{1}{D}^{e}$ resonances, as well as, more surprisingly, two-photon beatings between the $3s3p$ doubly excited state and the ${}^{1}{P}^{o}$ nonresonant continuum. Both effects lead to a significant distortion of the $3snp$ Fano profiles and to a strong variation of these profiles with the pump-probe delay, thus demonstrating control of the corresponding multichannel two-electron correlated wave packets, in the same way as reported for resonances lying below the $N=2$ threshold.

Seasonal variability of Saturn’s tropospheric temperatures, winds and para-H2 from Cassini far-IR spectroscopy

Far-IR 16–1000μm spectra of Saturn’s hydrogen–helium continuum measured by Cassini’s Composite Infrared Spectrometer (CIRS) are inverted to construct a near-continuous record of upper tropospheric (70–700mbar) temperatures and para-H2 fraction as a function of latitude, pressure and time for a third of a saturnian year (2004–2014, from northern winter to northern spring). The thermal field reveals evidence of reversing summertime asymmetries superimposed onto the belt/zone structure. The temperature structure is almost symmetric about the equator by 2014, with seasonal lag times that increase with depth and are qualitatively consistent with radiative climate models. Localised heating of the tropospheric hazes (100–250mbar) create a distinct perturbation to the temperature profile that shifts in magnitude and location, declining in the autumn hemisphere and growing in the spring. Changes in the para-H2 (fp) distribution are subtle, with a 0.02–0.03 rise over the spring hemisphere (200–500mbar) perturbed by (i) low-fp air advected by both the springtime storm of 2010 and equatorial upwelling; and (ii) subsidence of high-fp air at northern high latitudes, responsible for a developing north–south asymmetry in fp. Conversely, the shifting asymmetry in the para-H2 disequilibrium primarily reflects the changing temperature structure (and hence the equilibrium distribution of fp), rather than actual changes in fp induced by chemical conversion or transport. CIRS results interpolated to the same point in the seasonal cycle as re-analysed Voyager-1 observations (early northern spring) show qualitative consistency from year to year (i.e., the same tropospheric asymmetries in temperature and fp), with the exception of the tropical tropopause near the equatorial zones and belts, where downward propagation of a cool temperature anomaly associated with Saturn’s stratospheric oscillation could potentially perturb tropopause temperatures, para-H2 and winds. Quantitative differences between the Cassini and Voyager epochs suggest that the oscillation is not in phase with the seasonal cycle at these tropospheric depths (i.e., it should be described as quasi-periodic rather than ‘semi annual’). Variability in the zonal wind field derived from latitudinal thermal gradients is small (<10m/s per scale height near the tropopause) and mostly affects the broad retrograde jets, with the notable exception of large variability on the northern flank of the equatorial jet. The meridional potential vorticity (PV) gradient, and hence the ‘staircase of PV’ associated with spatial variations in the vigour of vertical mixing, has varied over the course of the mission but maintained its overall shape. PV gradients in latitude and altitude are used to estimate the atmospheric refractive index for the propagation of stationary planetary (Rossby) waves, predicting that such wave activity would be confined to regions of real refractivity (tropical regions plus bands at 35–45° in both hemispheres). The penetration depth of these regions into the upper troposphere is temporally variable (potentially associated with stratification changes), whereas the latitudinal structure is largely unchanged over time (associated with the zonal jet system).

THE EFFICIENCY OF STELLAR REIONIZATION: EFFECTS OF ROTATION, METALLICITY, AND INITIAL MASS FUNCTION

We compute the production rate of photons in the ionizing Lyman continua (LyC) of H I (lambda < 912 A), He I (lambda < 504 A), and He II (lambda < 228 A) using recent stellar evolutionary tracks coupled to a grid of non-LTE, line-blanketed (WM-basic) model atmospheres. The median LyC production efficiency is Q_LyC = (6+/-2)x10^60 LyC photons per Msun of star formation (range [3.1-9.4]x10^60) corresponding to a revised calibration of 10^{53.3+/-0.2} photons/s per Msun/yr. Efficiencies in the helium continua are Q_HeI ~ 10^60 photons/Msun and Q_HeII ~ 10^56 photons/Msun at solar metallicity and larger at low metallicity. The critical star formation rate needed to maintain reionization against recombinations at z = 7 is rho_SFR = (0.012 Msun/yr/Mpc^3) [(1+z)/8]^3 [(C_H /3) (0.2/ f_esc) for fiducial values of IGM clumping factor C_H = 3 and LyC escape fraction f_esc = 0.2. The boost in LyC production efficiency is an important ingredient, together with metallicity, C_H, and f_esc, in assessing whether IGM reionization was complete by z ~ 7. Monte-Carlo sampled spectra of coeval starbursts during the first 5 Myr have intrinsic flux ratios of F(1500)/F(900) = 0.4-0.5 and F(912^-)/F(912^+) = 0.4-0.7 in the far-UV (1500A), the LyC (900A), and at the Lyman edge (912A). These ratios can be used to calibrate the LyC escape fractions in starbursts.

Spectroscopic Measurement of Shock Waves in an Arcjet Plasma Expanding Through a Conical Nozzle

A diamond-shaped shock wave was created in a helium arcjet plasma. Visible/ultraviolet emission spectroscopy was used to investigate the condition for the formation of stable shocks and to determine characteristics of the plasma. Dependence of the position of the shock front on the gas pressure in the expansion region was investigated. It was found that the shock wave arises from the collision of plasma particles and residual neutral atoms in that region. Continuum and line spectra of neutral helium were measured, from which the electron temperatures were derived. The electron density was deduced from the Inglis-Teller limit of the He I 2p3P-3d3D series. The temperature and density were found to have almost constant values of 0.2 eV and 8.5×1013 cm−3, respectively, across the shock front.

Correlation effects in two-photon single and double ionization of helium

We study the effects of two-photon absorption in the region of the double continuum of helium, with extreme ultraviolet (xuv) laser fields. The nonrelativistic time-dependent Schroedinger equation is solved for the two active electron systems, interacting with strong laser pulses. The dynamic calculations include electronic correlations. The final double continuum states are calculated by treating the electronic term 1/r{sub 12} within the zero- and first-order perturbation theory. We calculate the total single and double ionization probabilities and cross sections for photon energies of 45 and 57 eV, as well as the electron energy distributions in the double continuum. The effects of electronic correlations are discussed in the context of ultrashort pulses.

Multi-dimensional cosmological radiative transfer with a Variable Eddington Tensor formalism

We present a new approach to numerically model continuum radiative transfer based on the Optically Thin Variable Eddington Tensor (OTVET) approximation. Our method insures the exact conservation of the photon number and flux (in the explicit formulation) and automatically switches from the optically thick to the optically thin regime. It scales as N log N with the number of hydrodynamic resolution elements and is independent of the number of sources of ionizing radiation (i.e. works equally fast for an arbitrary source function). We also describe an implementation of the algorithm in a Soften Lagrangian Hydrodynamic code (SLH) and a multi-frequency approach appropriate for hydrogen and helium continuum opacities. We present extensive tests of our method for single and multiple sources in homogeneous and inhomogeneous density distributions, as well as a realistic simulation of cosmological reionization.

A Study of Hydrogen Density in Emerging Flux Loops from a CoordinatedTransition Region and Coronal Explorerand Canary Islands Observation Campaign

During an international ground-based campaign in the Canary Islands coordinated with space instruments (i.e., Transition Region and Coronal Explorer [TRACE]), we observed an active region on 1998 September 10 with high spatial and temporal resolution. New emerging flux in the central part of the active region was observed in magnetograms of the Swedish Vacuum Solar Telescope, La Palma. Emerging loops (arch-filament systems [AFSs]) are well developed in Hα and Ca II according to the observations made at the Vacuum Tower Telescope (VTT) and THEMIS telescope in Tenerife with the Multichannel Subtractive Double Pass (MSDP) spectrographs. The TRACE images obtained at 171 and 195 A show low-emission regions that are easily identified as the individual AFS. They are due to absorption by hydrogen and helium continua in the cool filament plasma. We compare two techniques of measuring the hydrogen density in the cool dense fibrils of AFSs. The first method based on TRACE observations derived the neutral hydrogen column density of the plasma absorbing coronal lines. The second one using Hα line profiles provided by the MSDP spectrographs is based on the cloud model. The results are consistent. We derive also electron density values using Hα lines that are in good agreement with those derived from the 8542 A Ca II line observed with THEMIS (Mein et al.). The three types of observations (TRACE, VTT, THEMIS) are well complementary: absorption of coronal lines giving a good approximation for the maximum value of the neutral hydrogen column density, the Hα line giving a good determination of ne, and the 8542 A Ca II line a good determination of the electronic temperature.

VLA High-Sensitivity [TSUP]4[/TSUP]H[CLC]e[/CLC] Imaging of Galactic H [CSC]ii[/CSC] Regions

We have carried out 8.3 GHz hydrogen and helium radio recombination line and continuum observations of three Galactic H II regions with the NRAO 140 Foot Telescope and the Very Large Array. These observations measure the spatial variations in the singly ionized helium-to-hydrogen abundance ratio, 4He+/H+ (y+), and probe for any localized enhancements in y+. The three observed regions (W43, NGC 6334A, and K3-50) are morphologically complex, and high-resolution, wide field observations are crucial to measure the helium abundances on scales that would be associated with individual massive stars or clusters of stars. The nearly constant y+ value across the W43 source (W43 = 7.7% ± 1%) indicates that any ionization correction over the spatial scales investigated is small. Such ionization corrections are important for calculating total abundances of the light element 3He, which is measured in the Galaxy via a singly ionized hyperfine transition. Localized enhancements in the y+ value have been detected in both NGC 6334A (y+ = 20% ± 4%) and K3-50A (y+ = 24% ± 5%). The 4He abundance results are discussed along with other measurements of the physical properties of these H II regions, such as the kinematics and excitation.

Photoionization suppression by continuum coherence: Experiment and theory

We present experimental and theoretical results of a detailed study of laser-induced continuum structures (LICS) in the photoionization continuum of helium out of the metastable state 2s $^1S_0$. The continuum dressing with a 1064 nm laser, couples the same region of the continuum to the {4s $^1S_0$} state. The experimental data, presented for a range of intensities, show pronounced ionization suppression (by as much as 70% with respect to the far-from-resonance value) as well as enhancement, in a Beutler-Fano resonance profile. This ionization suppression is a clear indication of population trapping mediated by coupling to a contiuum. We present experimental results demonstrating the effect of pulse delay upon the LICS, and for the behavior of LICS for both weak and strong probe pulses. Simulations based upon numerical solution of the Schr\"{o}dinger equation model the experimental results. The atomic parameters (Rabi frequencies and Stark shifts) are calculated using a simple model-potential method for the computation of the needed wavefunctions. The simulations of the LICS profiles are in excellent agreement with experiment. We also present an analytic formulation of pulsed LICS. We show that in the case of a probe pulse shorter than the dressing one the LICS profile is the convolution of the power spectra of the probe pulse with the usual Fano profile of stationary LICS. We discuss some consequences of deviation from steady-state theory.

Population trapping and laser-induced continuum structure in helium: Experiment and theory

We report the observation of laser-induced structure (LICS) in the flat photoionization continuum of helium. The structure is a strong and spectrally sharp resonance, showing both enhanced and diminished ionization (a Beutler-Fano profile). Our observations show, in an otherwise unstructured continuum, pronounced ionization suppression (as much as 70{percent}), which is due to population trapping associated with LICS. We also show the effect of dynamic Stark shifts upon the LICS profile. A discussed theoretical model, incorporating significant dynamic Stark shifts, gives a quantitatively accurate description of the line profile for a wide range of pulse intensities. {copyright} {ital 1998} {ital The American Physical Society}

Optical constants of aluminum films in the extreme ultraviolet interval of 82–77 nm

Measurements of the extreme ultraviolet (EUV) reflectance of unoxidized aluminum films versus the angle of incidence in the interval of 82-77 nm, just below the aluminum plasma wavelength (83 nm), are presented. The continuum of helium was used as a radiation source for the first time in EUV reflectometry, to our knowledge. The surface roughness of substrates and samples was characterized by atomic force microscopy. The complex refractive index of unoxidized aluminum was obtained from reflectance measurements at each wavelength for the first time in this spectral range. Data on the refractive index, the dielectric constant, and the energy loss function in the above interval are shown together with our previous data obtained in an interval of 82.6-113.5 nm. Current results on the refractive index show a good match with the data in the literature calculated through the Kramers-Kronig analysis, the largest differences being in the imaginary part of the refractive index at the shortest wavelengths.

Far-ultraviolet absolute reflectometer for optical constant determination of ultrahigh vacuum prepared thin films

An absolute reflectometer is described, which can determine the optical constants of thin films prepared and maintained in ultrahigh vacuum (UHV) in the 30–200 nm far-ultraviolet (FUV) region. The optical constants are calculated with Fresnel equations from reflectance measurements as a function of the incidence angle, which can be continuously varied from near-normal to grazing incidence. Corrections for the surface roughness effects of both the deposited film and the substrate on the reflectance in the calculation of optical constants were incorporated. The surface roughness was determined by atomic force microscopy. An innovative feature of the instrument is an oxidation chamber containing an atomic oxygen source attached to the reflectometer. Inside this chamber the FUV reflectance degradation of materials, caused by oxidation processes in low earth orbits, can be simulated on freshly prepared UHV samples. Another improvement of the reflectometer is the capability of performing the reflectance measurements in two perpendicular incidence planes. The average of these measurements obviates the unknown of the polarization state of the light beam emerging from the monochromator. However, this averaging may become inexact in the presence of a lateral inversion error in the reflected beam caused by the lack of uniformity of the beam and the detector. A modification in the calculation algorithm was introduced to reduce the influence of this error. The implementation of a differential pressure system in four stages and a specifically designed device for modulation and monitoring of the light beam has allowed the use of weak spectral lines and the continuum of helium from the emission of a windowless capillary discharge lamp. Some results of reflectance measurements on freshly prepared aluminum films and after exposure to controlled doses of atomic oxygen are presented.

Helium photoionization between the N=2 and N=3 thresholds including angular distribution and resonance properties: A K-matrix L2 basis-set calculation.

A K-matrix method employing ${\mathit{L}}^{2}$ basis sets has been utilized to investigate the $^{1}$P\ifmmode^\circ\else\textdegree\fi{} continuum of helium between the N=2 and N=3 thresholds (65.4 --73.0 eV). The results include the partial and total photoionization cross sections, the asymmetry parameters \ensuremath{\beta}, and the positions and the widths of the autoionizing states. Good agreement is found with the available experimental data for the N=2 cross section. In agreement with other recent theoretical calculations, at 68.88 eV the total cross section is predicted to be slightly greater than the recommended value that is generally employed to normalize the experimental data.

Total and partial generalized oscillator strengths for transitions to the continuum of helium

The generalized oscillator strengths (GOS) for transitions to the continuum of helium between the second and third thresholds of ionization of helium by fast electrons are calculated. The total and partial GOS, the GOS ratios in the ionization of helium to the n=1 and n=2 states of the He+ residual ion, the parameters of the autoionized states converging to the threshold n=3 in the total and partial GOS and the dependence of the spectroscopic characteristics of the resonances on momentum transfer are determined. The ground state of helium is described by a many-parameter wavefunction. The continuum of helium between the second and third thresholds is described with allowance for the coupling among 20 configurations, which correspond to the two-electron excitations and four configurations with the free electron above the n=1 and n=2 states of the He+ ion.

Theoretical study of double-resonance processes in the helium continuum.

The ionization spectrum of the He atom under double-resonance irradiation is studied theoretically. A previously developed theory is extended and adapted to treat this process. The first source (synchrotron) is tuned around the transition from the ground state to the 23sp-- $^{1}P^{o}$ autoionizing state, and the second source (laser) is put in resonance with the transition from 23sp-- $^{1}P^{o}$ to the higher-lying quasibound state 2p3p-- $^{1}P^{e}$ (or 2p4p, 2p5p). Accurate ab initio calculations have been performed to obtain all the necessary parameters. It is found that the single-field ionization spectrum is strongly modified, exhibiting coherent trapping and ac Stark splitting. These results are in consonance with previous model calculations as well as with other ab initio calculations on a different zone of the He continuum. The results show that for the resonances considered these strong effects can be observed at megawatt laser intensities.

Photon and electron ionisation of helium to the n=2 state of He+

Partial photoionisation cross sections, the ratios of partial cross sections and anisotropy parameters in the angular distributions of photoelectrons are calculated in the region between the second and third ionisation thresholds of helium with the inclusion of all configuration interactions for a limited number of states of the subspaces of open and closed channels. The generalised partial oscillator strengths for transitions to the continuum of helium are calculated in the same approximation. The results are compared with experiment and calculations of different authors.

Spectral intensity of the hopfield helium continuum: An analysis of the ground state potential for He 2

The intensity distribution of the Hopfield helium continuum has been analyzed using accurate numerical wavefunctions. The best comparison of our calculated intensity to the experimental continuum of Huffman et al. yields a repulsive energy of 3.4 ± 0.1 eV for the ground X 1Σ g + state of He 2 in the vicinity of 1.04 Å internuclear separation.

High-resolution laser and classical spectroscopy of xenon autoionization

Xenon autoionized lines between 942.4 and 923.2 A have been measured using a tunable coherent extreme-ultraviolet source in conjunction with a Fizeau wavemeter, and resulting line profiles are reported for the first time. The same resonance lines of xenon have also been obtained with a 6.65-m vacuum spectrograph with a helium continuum as a background. A comparison is made of the line positions measured by the two independent techniques, and agreement of the measurements is remarkably good—well within the experimental error predicted.