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Register a new userOn Doppler shift and its Center-to-Limb Variation in Active Regions in the Transition Region
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A comprehensive understanding of the structure of Doppler motions in transition region including the center-to-limb variation and its relationship with the magnetic field structure is vital for the understanding of mass and energy transfer in the solar atmosphere. In this paper, we have performed such a study in an active region using the Si IV 1394~{AA} emission line recorded by the Interface Region Imaging Spectrograph (IRIS) and the line-of-sight photospheric magnetic field obtained by the Helioseismic and Magnetic Imager (HMI) on-board the Solar Dynamics Observatory (SDO). The active region has two opposite polarity strong field regions separated by a weak field corridor, which widened as the active region evolved. On average the strong field regions (corridor) show(s) redshifts of 5{--}10 (3{--}9)~km~s$^{-1}$ (depending on the date of observation). There is, however, a narrow lane in the middle of the corridor with near-zero Doppler shifts at all disk positions, suggesting that any flows there are very slow. The Doppler velocity distributions in the corridor seem to have two components---a low velocity component centered near 0 km/s and a high velocity component centered near 10~km~s$^{-1}$. The high velocity component is similar to the velocity distributions in the strong field regions, which have just one component. Both exhibit a small center-to limb variation and seem to come from the same population of flows. To explain these results, we suggest that the emission from the lower transition region comes primarily from warm type II spicules, and we introduce the idea of a `chromospheric wall---associated with classical cold spicules---to account for a diminished center-to-limb variation.
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We derive the non-thermal velocities (NTVs) in the transition region of an active region using the ion{Si}{4}~1393.78~{AA} line observed by the Interface Region Imaging Spectrograph (IRIS) and compare them with the line-of-sight photospheric magnetic fields obtained by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). The active region consists of two strong field regions with opposite polarity, separated by a weak field corridor, that widened as the active region evolved. The means of the NTV distributions in strong-field regions (weak field corridors) range between $sim$18{--}20 (16{--}18)~km~s$^{-1}$, albeit the NTV maps show much larger range. In addition, we identify a narrow lane in the middle of the corridor with significantly reduced NTV. The NTVs do not show a strong center-to-limb variation, albeit somewhat larger values near the disk center. The NTVs are well correlated with redshifts as well as line intensities. The results obtained here and those presented in our companion paper on Doppler shifts suggest two populations of plasma in the active region emitting in ion{Si}{4}. The first population exists in the strong field regions and extends partway into the weak field corridor between them. We attribute this plasma to spicules heated to $sim$0.1 MK (often called type II spicules). They have a range of inclinations relative to vertical. The second population exists in the center of the corridor, is relatively faint, and has smaller velocities, likely horizontal. These results provide further insights into the heating of the transition region.
We present the properties of the inverse Evershed flow (IEF) based on the center-to-limb variation of the plasma speed and loop geometry of chromospheric superpenumbral fibrils in eleven sunspots that were located at a wide range of heliocentric angles from 12 to 79 deg. The observations were acquired at the Dunn Solar Telescope in the spectral lines of Halpha at 656nm, CaII IR at 854 nm and HeI at 1083 nm. All sunspots display opposite line-of-sight (LOS) velocities on the limb and center side with a distinct shock signature near the outer penumbral edge. We developed a simplified flexible sunspot model assuming axisymmetry and prescribing the radial flow speed profile at a known loop geometry to replicate the observed two-dimensional IEF patterns under different viewing angles. The simulated flow maps match the observations for chromospheric loops with 10-20 Mm length starting at 0.8-1.1 sunspot radii, an apex height of 2-3Mm and a true constant flow speed of 2-9km/s. We find on average a good agreement of the simulated velocities and the observations on elliptical annuli around the sunspot. Individual IEF channels show a significant range of variation in their properties and reach maximal LOS speeds of up to 12km/s. Upwards or downwards directed flows do not show a change of sign in the LOS velocities for heliocentric angles above 30 deg. Our results are consistent with the IEF being caused by a siphon flow mechanism driving a flow at a constant sonic speed along elevated loops with a flattened top in the chromosphere.
CONTEXT: The quiet Sun magnetic fields produce ubiquitous bright points (BPs) that cover a significant fraction of the solar surface. Their contribution to the total solar irradiance (TSI) is so-far unknown. AIMS: To measure the center-to-limb variation (CLV) of the fraction of solar surface covered by quiet Sun magnetic bright points. The fraction is referred to as fraction of covered surface, or FCS. METHODS: Counting of the area covered by BPs in G-band images obtained at various heliocentric angles with the 1-m Swedish Solar Telescope on La Palma. Through restoration, the images are close to the diffraction limit of the instrument (~0.1 arcsec). RESULTS: The FCS is largest at disk center (~1 %), and then drops down to become 0.2 % at mu= 0.3 (with mu the cosine of the heliocentric angle. The relationship has large scatter, which we evaluate comparing different subfields within our FOVs. We work out a toy-model to describe the observed CLV, which considers the BPs to be depressions in the mean solar photosphere characterized by a depth, a width, and a spread of inclinations. Although the model is poorly constrained by observations, it shows the BPs to be shallow structures (depth < width) with a large range of inclinations. We also estimate how different parts of the solar disk may contribute to TSI variations, finding that 90 % is contributed by BPs having mu > 0.5, and half of it is due to BPs with mu > 0.8.
One of the necessary parameters needed for the interpretation of the light curves of transiting exoplanets or eclipsing binaries, as well as interferometric measurements of a star or microlensing events is how the intensity and polarization of a light change from the center to the limb. Scattering and absorption processes in stellar atmosphere affect both the center-to limb variation of intensity (CLVI) and polarization (CLVP). In this paper, we present a study of the CLVI and CLVP in continuum spectra considering different contributions of scattering and absorption opacity for different spectral type stars with spherical atmospheres. We solve the polarized radiative transfer equation in the presence of continuum scattering, considering spherical stellar model atmospheres. We developed two independent codes based on Feautrier and short characteristics methods to cross-check our results. We calculate the CLVI and CLVP in continuum for the Phoenix grid of spherical stellar model atmospheres for a range of $T_{eff} = 4000 - 7000 rm K$, $log g = 1.0 - 5.5$ and $lambda = 4000 - 7000 rm AA$, which are tabulated and available at the CDS. For sub-giant and dwarf stars ($log g = 3.0 - 4.5$), lower $log g$ and lower $T_{eff}$ of a star lead to higher limb polarization of the star. For giant and supergiant stars ($log g = 1.0 - 2.5$), the highest effective temperature yields the largest polarization. By decreasing of the $T_{eff}$ of a star down to $4500 - 5500 rm K$ (depending on $log g$) the limb polarization decreases and reaches a local minimum. It increases again down to $T_{eff}$ of $4000 rm K$. For the most compact dwarf stars ($log g = 5.0 - 5.5$) the limb polarization degree shows a maximum for models with $T_{eff}$ in the range $4200 - 4600 rm K$ (depending on $log g$) and decreases toward higher and lower temperatures.
The relationships among coronal loop structures at different temperatures is not settled. Previous studies have suggested that coronal loops in the core of an active region are not seen cooling through lower temperatures and therefore are steadily heated. If loops were cooling, the transition region would be an ideal temperature regime to look for a signature of their evolution. The Extreme-ultraviolet Imaging Spectrometer (EIS) on Hinode provides monochromatic images of the solar transition region and corona at an unprecedented cadence and spatial resolution, making it an ideal instrument to shed light on this issue. Analysis of observations of active region 10978 taken in 2007 December 8 -- 19 indicates that there are two dominant loop populations in the active region: core multi-temperature loops that undergo a continuous process of heating and cooling in the full observed temperature range 0.4-2.5 MK and even higher as shown by the X-Ray Telescope (XRT); and peripheral loops which evolve mostly in the temperature range 0.4-1.3 MK. Loops at transition region temperatures can reach heights of 150 Mm in the corona above the limb and develop downflows with velocities in the range of 39-105 km/s.
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