Photospheric Material Research Articles

Observing Solar Limb Darkening in the Classroom

The falling temperature of the photosphere with height is responsible for the effect known as limb darkening. The Sun is not equally bright all over the disc. When we observe the Sun towards the limbs, it appears to get darker. Light from the photosphere travels through an absorptive medium. Therefore, one can see only so far into the photosphere. This is the optical depth (Fig. 1). The line of sight at the center sees deeper into the Sun to an optical depth of about 2/3 into the photosphere. In other words, photons that escape from a smaller radius of the photosphere have originated in a hotter region; therefore, they will have a higher intensity. Photons that originate from a larger radius come from a cooler part of the photosphere; therefore, this results in lower intensity. This effect is known as solar limb darkening. In addition, the solar photosphere displays various phenomena that can easily be observed in images obtained with small telescopes, such as sunspots, faculae, and granulation. Limb darkening results from the fact that we are looking into hot gas when we look at the Sun and, as a consequence of this, the brightness of the Sun decreases as one looks from the center of the disc (where we see deeper inside the Sun) towards the limb (where we will not see as deeply into the solar atmosphere because we are looking at a slant through the photospheric material).

Formation of a Dense Flux Rope by a Siphon Flow

Abstract The interaction of siphon flow with an initially linear Alfvén wave within an isolated chromospheric loop is investigated. The loop is modeled using 1.5D magnetohydrodynamics (MHD). The siphon flow undergoes a hydrodynamic (HD) shock, which allows the Alfvén instability to amplify the propagating waves as they interact with the shock and loop footpoints. The amplification leads to nonlinear processes strongly altering the loop equilibrium. Azimuthal twists of are generated and the loop becomes globally twisted with an azimuthal magnetic field of . The flow is accelerated to due to the propagating shock waves that form. Near the end of the simulation, where the nonlinear processes are strongest, flow reversal is seen within the descending leg of the loop, generating upflows up to . This flow reversal leads to photospheric material being “pulled” into the loop and spreading along its entirety. Within about 2.5 hr, the density increases by a factor of about 30 its original value.

Initial formation of an “impulsive” coronal mass ejection

An “impulsive” coronal mass ejection (CME) observed on August 24, 2014 is analyzed using ultraviolet images obtained in the SDO/AIA 193, 304, 1600, and 1700 A channels and Hα (6562.8 A) data obtained with the EI Teide and Big Bear telescopes. The formation of this impulsive CME was related to a magnetic tube (rope) moving with a velocity of ≈35 km/s and containing plasma that was cooler than the photospheric material. Moving in the corona, the magnetic tube collides with a quasi-stationary coronal magnetic rope, with its two bases rooted in the photosphere. This interaction results in the formation of the CME, with the surface of the coronal magnetic rope becoming the CME frontal structure. According to SDO/HMI data, no enhancements or changes in magnetic flux were detected in the vicinity of the CME bases during its formation. This may support the hypothesis that the magnetic tube starts its motion from layers in the vicinity of the temperature minimum.

High-speed photospheric material flow observed at the polarity inversion line of a δ-type sunspot producing an X5.4 flare on 2012 March 7

Abstract Solar flares abruptly release the free energy stored as a non-potential magnetic field in the corona and may be accompanied by eruptions of the coronal plasma. Formation of a non-potential magnetic field and the mechanisms for triggering the onset of flares are still poorly understood. In particular, photospheric dynamics observed near those polarity inversion lines that are sites of major flare production have not been well observed with high spatial resolution spectro-polarimetry. This paper reports on a remarkable high-speed material flow observed along the polarity inversion line located between flare ribbons at the main energy release side of an X5.4 flare on 2012 March 7. Observations were carried out by the spectro-polarimeter of the Solar Optical Telescope on board Hinode. The high-speed material flow was observed in the horizontally oriented magnetic field formed nearly parallel to the polarity inversion line. This flow persisted from at least six hours before the onset of the flare, and continued for at least several hours after the onset of the flare. Observations suggest that the observed material flow represents neither the emergence nor convergence of the magnetic flux. Rather, it may be considered to be material flow working both to increase the magnetic shear along the polarity inversion line and to develop magnetic structures favorable for the onset of the eruptive flare.

The solar photospheric-to-coronal Fe abundance ratio from X-ray fluorescence lines

The ratio of the Fe abundance in the photosphere to that in coronal flare plasmas is determined by X-ray lines within the complex at 6.7~keV (1.9~\AA) emitted during flares. The line complex includes the He-like Fe (\fexxv) resonance line $w$ (6.70~keV) and Fe K$\alpha$ lines (6.39, 6.40~keV), the latter being primarily formed by the fluorescence of photospheric material by X-rays from the hot flare plasma. The ratio of the Fe K$\alpha$ lines to the \fexxv\ $w$ depends on the ratio of the photospheric-to-flare Fe abundance, heliocentric angle $\theta$ of the flare, and the temperature $T_e$ of the flaring plasma. Using high-resolution spectra from X-ray spectrometers on the {\em P78-1} and {\em Solar Maximum Mission} spacecraft, the Fe abundance in flares is estimated to be $1.6\pm 0.5$ and $2.0 \pm 0.3$ times the photospheric Fe abundance, the {\em P78-1} value being preferred as it is more directly determined. This enhancement is consistent with results from X-ray spectra from the {\em RHESSI} spacecraft, but is significantly less than a factor 4 as in previous work.

High ions towards white dwarfs: circumstellar line shifts and stellar temperature

Based on a compilation of OVI, CIV, SiIV and NV data from IUE, FUSE, GHRS, STIS, and COS, we derive an anti- correlation between the stellar temperature and the high ion velocity shift w.r.t. to the photosphere, with positive (resp. negative) velocity shifts for the cooler (resp. hotter) white dwarfs. This trend probably reflects more than a single process, however such a dependence on the WD's temperature again favors a CS origin for a very large fraction of those ion absorptions, previously observed with IUE, HST-STIS, HST-GHRS, FUSE, and now COS, selecting objects for which absorption line radial velocities, stellar effective temperature and photospheric velocity can be found in the literature. Interestingly, and gas in near-equilibrium in the star vicinity. It is also probably significant that the temperature that corresponds to a null radial velocity, i.e. \simeq 50,000K, also corresponds to the threshold below which there is a dichotomy between pure or heavy elements atmospheres as well as some temperature estimates for and a form of balance between radiation pressure and gravitation. This is consistent with ubiquitous evaporation of orbiting dusty material. Together with the fact that the fraction of stars with (red-or blue-) shifted lines and the fraction of stars known to possess heavy species in their atmosphere are of the same order, such a velocity-temperature relationship is consistent with quasi-continuous evaporation of orbiting CS dusty material, followed by accretion and settling down in the photosphere. In view of these results, ion measurements close to the photospheric or the IS velocity should be interpreted with caution, especially for stars at intermediate temperatures. While tracing CS gas, they may be erroneously attributed to photospheric material or to the ISM, explaining the difficulty of finding a coherent pattern of the high ions in the local IS 3D distribution.

O VI IN THE LOCAL INTERSTELLAR MEDIUM

We report the results of a search for O VI absorption in the spectra of 80 hot DA white dwarfs observed by the FUSE satellite. We have carried out a detailed analysis of the radial velocities of interstellar and (where present) stellar absorption lines for the entire sample of stars. In approximately 35% of cases (where photospheric material is detected), the velocity differences between the interstellar and photospheric components were beneath the resolution of the FUSE spectrographs. Therefore, in 65% of these stars the interstellar and photospheric contributions could be separated and the nature of the O VI component unambiguously determined. Furthermore, in other examples, where the spectra were of a high signal-to-noise, no photospheric material was found and any O VI detected was assumed to be interstellar. Building on the earlier work of Oegerle et al. (2005) and Savage & Lehner (2006), we have increased the number of detections of interstellar O VI and, for the first time, compared their locations with both the soft X-ray background emission and new detailed maps of the distribution of neutral gas within the local interstellar medium. We find no strong evidence to support a spatial correlation between O VI and SXRB emission. In all but a few cases, the interstellar O VI was located at or beyond the boundaries of the local cavity. Hence, any T ~ 300,000K gas responsible for the O VI absorption may reside at the interface between the cavity and surrounding medium or in that medium itself. Consequently, it appears that there is much less O VI-bearing gas than previously stated within the inner rarefied regions of the local interstellar cavity.

The Nitrogen Abundance in Be Stars Determined from UV Spectra

AbstractA NLTE abundance analysis of the IUE spectra of 8 Be stars with v sin i < 150km s−1 reveals no evidence for nitrogen enrichment from mixing of core and photospheric material.

Fe Kα and Hydrodynamic Loop Model Diagnostics for a Large Flare on II Pegasi

The observation by the Swift X-ray Telescope of the Fe Kα1, α2 doublet during a large flare on the RS CVn binary system II Peg represents one of only two firm detections to date of photospheric Fe Kα from a star other than our Sun. We present models of the Fe Kα equivalent widths reported in the literature for the II Peg observations and show that they are most probably due to fluorescence following inner shell photoionization of quasi-neutral Fe by the flare X-rays. Our models constrain the maximum height of flare the to 0.15R* assuming solar abundances for the photospheric material, and 0.1R* and 0.06R* assuming depleted photospheric abundances ([M/H] = –0.2 and –0.4, respectively). Accounting for an extended loop geometry has the effect of increasing the estimated flare heights by a factor of ~3. These predictions are consistent with those derived using results of flaring-loop models, which are also used to estimate the flaring loop properties and energetics. From loop models we estimate a flare loop height of 0.13R*, plasma density of ~4 × 1012 cm−3, and emitting volume of ~6 × 1030 cm3. Our estimates for the flare dimensions and density allow us to estimate the conductive energy losses to Econd ⩽ 2 × 1036 erg, consistent with upper limits previously obtained in the literature. Finally, we estimate the average energy output of this large flare to be ~1033 erg s−1, or 1/10th of the stellar bolometric luminosity.

Argon Abundances in the Solar Neighborhood: Non‐LTE Analysis of Orion Association B‐Type Stars1

Argon abundances have been derived for a sample of B main-sequence stars in the Orion association. The abundance calculations are based on non-LTE (NLTE) metal line-blanketed model atmospheres calculated with the NLTE code TLUSTY and an updated and complete argon model atom. We derive an average argon abundance for this young population of A(Ar) = 6.66 ± 0.06. While our result is in excellent agreement with a recent analysis of the Orion Nebula, it is significantly higher than the currently recommended solar value, which is based on abundance measurements in the solar corona. Moreover, the derived argon abundances in the Orion B stars agree very well with a measurement from a solar impulsive flare during which unmodified solar photospheric material was brought to flare conditions. We therefore argue that the argon abundances obtained independently for both the Orion B stars and the Orion Nebula are representative of the disk abundance value in the solar neighborhood. The lower coronal abundance may reflect a depletion related to the first ionization potential (FIP) effect. We propose a new reference value for the abundance of argon in the solar neighborhood, A(Ar) = 6.63 ± 0.10, corresponding to Ar/O = 0.009 ± 0.002.

MASS MOTIONS IN THE PHOTOSPHERE OF BETELGEUSE

Spectroscopic observations of Betelgeuse, taken at the Elginfield Observatory, show velocity and temperature variations delineating the systematic, but generally chaotic, rise and fall of photospheric material. The characteristic timescale of the variations is 400 days, while velocities cover a range of ~9 km s?1. Macroturbulence is constant to about 13% and does not result from motions on the largest scales, but from motions within the large structures. The line bisectors have predominantly a reversed-C shape. Variations in the shapes of bisectors occur on the 1 km s?1 level and are not obviously connected to their shifts in wavelength. A likely explanation of the observations is granulation and giant convection cells accompanied by short-lived oscillations they trigger. Random convection events may account for radial-velocity jitter seen in many highly evolved stars.

Observational Features of Large-Scale Structures as Revealed by the Catastrophe Model of Solar Eruptions

Large-scale magnetic structures are the main carrier of major eruptions in the solar atmosphere. These structures are rooted in the photosphere and are driven by the unceasing motion of the photospheric material through a series of equilibrium configurations. The motion brings energy into the coronal magnetic field until the system ceases to be in equilibrium. The catastrophe theory for solar eruptions indicates that loss of mechanical equilibrium constitutes the main trigger mechanism of major eruptions, usually shown up as solar flares, eruptive prominences, and coronal mass ejections (CMEs). Magnetic reconnection which takes place at the very beginning of the eruption as a result of plasma instabilities/turbulence inside the current sheet, converts magnetic energy into heating and kinetic energy that are responsible for solar flares, and for accelerating both plasma ejecta (flows and CMEs) and energetic particles. Various manifestations are thus related to one another, and the physics behind these relationships is catastrophe and magnetic reconnection. This work reports on recent progress in both theoretical research and observations on eruptive phenomena showing the above manifestations. We start by displaying the properties of large-scale structures in the corona and the related magnetic fields prior to an eruption, and show various morphological features of the disrupting magnetic fields. Then, in the framework of the catastrophe theory, we look into the physics behind those features investigated in a succession of previous works, and discuss the approaches they used.

Photospheric physical conditions before a two-ribbon solar flare

We present preliminary results of the study of the photospheric physical conditions about 30 minutes before the 2N/M2 two-ribbon solar flare. The soft X-ray enhancement, brightening and microwave bursts occured prior to the flare. We demonstrate the semiempirical models of the preflaring photosphere fot two kernels of the future flare. These models derived from the inversion reproduce the spectral observations in seven Fraunhofer lines. The models consist of two atmospheres: the magnetic flux tube and its nonmagnetic surroundings. The inferred models show strong changes of the photospheric physical conditions in the preflare phase. The preheating was found in the flux tube. Upward motions of the surrounding photospheric material were revealed. Significant discrepancies of the flux-tube temperature and density are found for two sites of the future flare. The inner kernel located near the filament was cooler in the photosphere than the outer kernel.

Preflare chromospheric and photospheric line-of-sight velocities

The preflare plage spectra were obtained with the ATsU-26 solar telescope mounted at the Peak Terskol. The spectra were acquired from 1 h to 20 min before the flare onset. The most prominent peculiarity of all the observed events are the tilted loops were observed to interact before 1n flare. As a result of their interaction, the redistribution of velocities occurs in the both chromosphere and photosphere. This can be a consequence of the preflare reconnection at the lower atmospheric layers. As the flare onset approaches the ascend of photospheric material start to dominate. The preflare line-of-sight velocities exceed considerably those in the neighbouring unperturbed regions.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The structure of the photospheric velocity field near Hα filaments

The velocity field of the plasma in the solar photosphere beneath chromospheric Hα filaments is studied. Observations were conducted in 1999–2000 using the magnetograph and tachometer of the tower telescope of the Institute of Terrestrial Magnetism, Ionosphere, and Radio Propagation, recently upgraded to improve both its sensitivity and spatial resolution. The results confirm that, as noted earlier, filaments are frequently found near velocity-inversion lines between regions of upward and downward motion of solar material, and lie predominantly above regions with upward motion of photospheric material. This tendency is characteristic of both the stable filaments of active regions and quiescent filaments far from active regions, though it is more distinct for the former case. The upward motion of photospheric material beneath filaments may play an important role in supporting the filaments against gravity.

Photospheric and Chromospheric Gas Motions around a Dark Filament

In this paper, we investigate how photospheric material moves below a dark filament, and we study chromospheric gas motions inside the filament. In the photosphere we trace granular motions by means of a local correlation tracking (LCT) technique to derive horizontal velocity field, while inside the filament we obtain the line-of-sight velocity field by subtracting a blueshifted Hα image from a redshifted Hα image. We find that a typical value of horizontal photospheric velocity is 1 km s-1, and its divergence map maintains a large-scale pattern during several hours. We also find that photospheric motions around a filament channel are random in space and changeable in time. As for the motion inside the filament, our results show that there is an area of upward motions at one side of the filament axis and an area of downward motions at the other side, which means that filament material has a helical motion inside the filament. We think that these results provide new important information on theories of filament formation.

Solar Energetic Particles: Sampling Coronal Abundances

In the large solar energetic particle (SEP) events, coronal mass ejections (CMEs) drive shock waves out through the corona that accelerate elements of the ambient material to MeV energies in a fairly democratic, temperature-independent manner. These events provide the most complete source of information on element abundances in the corona. Relative abundances of 22 elements from H through Zn display the well-known dependence on the first ionization potential (FIP) that distinguishes coronal and photospheric material. For most elements, the main abundance variations depend upon the gyrofrequency, and hence on the charge-to-mass ratio, Q/A, of the ion. Abundance variations in the dominant species, H and He, are not Q/A dependent, presumably because of non-linear wave-particle interactions of H and He during acceleration. Impulsive flares provide a different sample of material that confirms the Ne:Mg:Si and He/C abundances in the corona.

Emerging Active Regions on the Sun and the Photospheric Abundance of Neon

Early work on Skylab spectroheliograms showed that deep-lying impulsive flares and newly emerged active regions showed a high relative abundance of neon, thought to be representative of the unknown neon abundance in the underlying photosphere. In pursuit of this idea a survey of emerging magnetic flux sequences on Skylab spectroheliograms (300-600 A) has been made, and from these the Ne/Mg abundance ratio has been derived by combining plots of the ion differential emission measures. From the initial phase of five emerging flux sequences a stable value of the Ne/Mg abundance ratio equal to 3.16 ± 0.07 has been determined. Normalizing this to the well-determined abundance of magnesium, the photospheric abundance of neon is This result is compared with previous estimates which used local galactic values as a proxy for the unknown neon abundance. The present results are also compared with a slightly larger Ne/Mg ratio of 3.6 ± 0.4 (log Neon = 8.14) observed in a deep-lying impulsive flare, also thought to expose a sample of photospheric material.

A Self‐consistent Optical, Ultraviolet, and Extreme‐Ultraviolet Model for the Spectrum of the Hot White Dwarf G191‐B2B

The star G191-B2B is one of a number of hot DA white dwarfs whose atmospheres have been found to contain significant quantities of heavy elements, including C, N, O, Si, Fe, and Ni. Several earlier studies have measured their abundances using IUE echelle data in conjunction with synthetic spectra derived from theoretical model atmosphere calculations of varying degrees of sophistication. However, predictions of the EUV spectrum based on these observations failed completely to match either its shape or absolute flux level. We present here the results of new non-LTE model calculations including the effects of line blanketing from more than 9 million (mainly Fe and Ni) transitions. For the first time, we are able to find an effective temperature and composition that can consistently match the optical, FUV and EUV data. However, to maintain this agreement below the He ii Λ228 Lyman limit, it is necessary to incorporate additional He ii opacity in the form of photospheric, circumstellar or interstellar material.

Sun center observations of the Evershed effect

view Abstract Citations (77) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Sun Center Observations of the Evershed Effect Rimmele, Thomas R. Abstract Results of observations of the Evershed effect for a round sunspot at disk center are presented. Using the 20 mA UBF/FP filter at the VTT of NSO/Sacramento Peak we recorded a 2 hr time sequence of Fe I 5576.099 A velocity maps, Mn I 5394.675 A core intensity maps, and white-light images. By computing the 2 hr time average we were able to filter out the vertical Evershed component of a few hundred m/s from the background of oscillatory and granular velocities, which dominate individual images. The averaged velocity fields show distinct filaments which extend beyond the white-light boundary of the sunspot by as far as 10,000 km. The velocity profile along these filaments is consistent with the picture of an arched magnetic loop carrying the Evershed flow. These loops reach their maximum elevation at less than 300 km above continuum height. The portions of the loops seen in velocity maps have a length of up to 20,000 km. Within the penumbra the velocity filaments are correlated with dark filaments observed in the core intensity map of the temperature-sensitive Mn I line. However, beyond the penumbral boundary the same velocity filaments coincide with enhanced brightness, relative to the photospheric intensity, suggesting that the gas in the downstream legs of the loop is at a higher temperature than the surrounding photospheric material. The temperature excess in the downstream legs is of the order of 200 K. A possible explanation is a standing tube shock that occurs in the downstream legs and near the penumbral boundary as modeled by Montesinos & Thomas (1993). Some velocity filaments end in pore-like features which are 5%-10% darker than the average photosphere and reveal a pronounced redshift. Publication: The Astrophysical Journal Pub Date: May 1995 DOI: 10.1086/175716 Bibcode: 1995ApJ...445..511R Keywords: Coronal Loops; Solar Atmosphere; Solar Granulation; Solar Magnetic Field; Sunspots; Velocity Distribution; Data Reduction; Flux Density; Interferometry; Solar Spectra; Solar Physics; SUN: ATMOSPHERE; SUN: SUNSPOTS full text sources ADS |