Author name code: pietarila ADS astronomy entries on 2022-09-14 author:"Pietarila, Anna" ------------------------------------------------------------------------ Title: Understanding the Chromospheric Magnetic Field Authors: Jin, C. L.; Harvey, J. W.; Pietarila, A. Bibcode: 2014ASPC..489...77J Altcode: The chromospheric magnetic field is an important and essential component for understanding solar atmospheric fields. Due to the problems of polarization radiation transfer in the chromosphere and the low detective sensitivity of chromospheric spectrum lines, observations of chromospheric magnetic fields are very difficult, so studies of chromospheric fields are infrequent. However, the understanding of chromospheric fields is evolving. In this report, we summarize our current empirical knowledge and basic physical understanding of chromospheric fields. We concentrate on the comparison of magnetic fields in the photosphere and chromosphere, and then display their difference. Title: Migration of Ca II H bright points in the internetwork Authors: Jafarzadeh, S.; Cameron, R. H.; Solanki, S. K.; Pietarila, A.; Feller, A.; Lagg, A.; Gandorfer, A. Bibcode: 2014A&A...563A.101J Altcode: 2014arXiv1401.7522J Context. The migration of magnetic bright point-like features (MBP) in the lower solar atmosphere reflects the dispersal of magnetic flux as well as the horizontal flows of the atmospheric layer they are embedded in.
Aims: We analyse trajectories of the proper motion of intrinsically magnetic, isolated internetwork Ca ii H MBPs (mean lifetime 461 ± 9 s) to obtain their diffusivity behaviour.
Methods: We use seeing-free high spatial and temporal resolution image sequences of quiet-Sun, disc-centre observations obtained in the Ca ii H 3968 Å passband of the Sunrise Filter Imager (SuFI) onboard the Sunrise balloon-borne solar observatory. Small MBPs in the internetwork are automatically tracked. The trajectory of each MBP is then calculated and described by a diffusion index (γ) and a diffusion coefficient (D). We also explore the distribution of the diffusion indices with the help of a Monte Carlo simulation.
Results: We find γ = 1.69 ± 0.08 and D = 257 ± 32 km2 s-1 averaged over all MBPs. Trajectories of most MBPs are classified as super-diffusive, i.e. γ > 1, with the determined γ being the largest obtained so far to our knowledge. A direct correlation between D and timescale (τ) determined from trajectories of all MBPs is also obtained. We discuss a simple scenario to explain the diffusivity of the observed, relatively short-lived MBPs while they migrate within a small area in a supergranule (i.e. an internetwork area). We show that the scatter in the γ values obtained for individual MBPs is due to their limited lifetimes.
Conclusions: The super-diffusive MBPs can be described as random walkers (due to granular evolution and intergranular turbulence) superposed on a large systematic (background) velocity, caused by granular, mesogranular, and supergranular flows. Title: Synoptic Mapping of Chromospheric Magnetic Flux Authors: Jin, C. L.; Harvey, J. W.; Pietarila, A. Bibcode: 2013ApJ...765...79J Altcode: We used daily full-disk Ca II 854.2 nm magnetograms from the Synoptic Optical Long Term Investigations of the Sun (SOLIS) facility to study the chromospheric magnetic field from 2006 April through 2009 November. We determined and corrected previously unidentified zero offsets in the SOLIS magnetograms. By tracking the disk passages of stable unipolar regions, the measured net flux densities were found to systematically decrease from the disk center to the limb by a factor of about two. This decrease was modeled using a thin flux tube model with a difference in signal formation height between the center and limb sides. Comparison of photospheric and chromospheric observations shows that their differences are largely due to horizontal spreading of magnetic flux with increasing height. The north polar magnetic field decreased nearly linearly with time during our study period while the south polar field was nearly constant. We used the annual change in the viewing angle of the polar regions to estimate the radial and meridional components of the polar fields and found that the south polar fields were tilted away from the pole. Synoptic maps of the chromospheric radial flux density distribution were used as boundary conditions for extrapolation of the field from the chromosphere into the corona. A comparison of modeled and observed coronal hole boundaries and coronal streamer positions showed better agreement when using the chromospheric rather than the photospheric synoptic maps. Title: Instrumental and Observational Artifacts in Quiet Sun Magnetic Flux Cancellation Functions Authors: Pietarila, A.; Pietarila Graham, J. Bibcode: 2013SoPh..282..389P Altcode: 2012arXiv1209.6388P Under the assumption that the photospheric quiet Sun magnetic field is turbulent, the cancellation function has previously been used to estimate the true, resolution-independent mean, unsigned vertical flux «|Bz|»true. We show that the presence of network elements, noise, and seeing complicate the measurement of accurate cancellation functions and their power law exponents κ. Failure to exclude network elements previously led to estimates that were too low for both the cancellation exponent κ and «|Bz|»true. However, both κ and «|Bz|»true are overestimated due to noise in magnetograms. While no conclusive value can be derived with data from current instruments, our Hinode/SP results of κ⪅0.38 and «|Bz|»true⪅270 gauss can be taken as upper bounds. Title: Ca II 854.2 nm Bisectors and Circumfacular Regions Authors: Pietarila, A.; Harvey, J. W. Bibcode: 2013ApJ...764..153P Altcode: Active regions appear bright in Ca II 854.2 nm line core intensity while the surrounding areas, referred to as circumfacular regions, are darker than the active region or the quiet Sun. We use Synoptic Optical Long-term Investigations of the Sun Vector Spectromagnetograph Ca II 854.2 nm data (photospheric and chromospheric full disk magnetograms as well as high spectral resolution Stokes I and V profiles) to study the connection between magnetic canopies, circumfacular regions, and Ca II 854.2 nm bisector amplitudes (spans). The line bisector amplitude is reduced in circumfacular regions, where the 3 minute period power in chromospheric Hα intensity oscillations is also reduced relative to the surrounding quiet Sun. The latter is consistent with magnetic canopies in circumfacular regions suppressing upward propagating steepening acoustic waves. Our results provide further strong evidence for shock waves as the cause of the inverse C-shaped bisector and explain the observed solar cycle variation of the shape and amplitude of Sun-as-a-star Ca II 854.2 nm bisectors. Title: Structure and dynamics of isolated internetwork Ca II H bright points observed by SUNRISE Authors: Jafarzadeh, S.; Solanki, S. K.; Feller, A.; Lagg, A.; Pietarila, A.; Danilovic, S.; Riethmüller, T. L.; Martínez Pillet, V. Bibcode: 2013A&A...549A.116J Altcode: 2012arXiv1211.4836J
Aims: We aim to improve our picture of the low chromosphere in the quiet-Sun internetwork by investigating the intensity, horizontal velocity, size and lifetime variations of small bright points (BPs; diameter smaller than 0.3 arcsec) observed in the Ca II H 3968 Å passband along with their magnetic field parameters, derived from photospheric magnetograms.
Methods: Several high-quality time series of disc-centre, quiet-Sun observations from the Sunrise balloon-borne solar telescope, with spatial resolution of around 100 km on the solar surface, have been analysed to study the dynamics of BPs observed in the Ca II H passband and their dependence on the photospheric vector magnetogram signal.
Results: Parameters such as horizontal velocity, diameter, intensity and lifetime histograms of the isolated internetwork and magnetic Ca II H BPs were determined. Mean values were found to be 2.2 km s-1, 0.2 arcsec (≈150 km), 1.48 ⟨ ICa ⟩ and 673 s, respectively. Interestingly, the brightness and the horizontal velocity of BPs are anti-correlated. Large excursions (pulses) in horizontal velocity, up to 15 km s-1, are present in the trajectories of most BPs. These could excite kink waves travelling into the chromosphere and possibly the corona, which we estimate to carry an energy flux of 310 W m-2, sufficient to heat the upper layers, although only marginally.
Conclusions: The stable observing conditions of Sunrise and our technique for identifying and tracking BPs have allowed us to determine reliable parameters of these features in the internetwork. Thus we find, e.g., that they are considerably longer lived than previously thought. The large velocities are also reliable, and may excite kink waves. Although these wave are (marginally) energetic enough to heat the quiet corona, we expect a large additional contribution from larger magnetic elements populating the network and partly also the internetwork. Title: Comparison of Ground-Based and Space-Based Longitudinal Magnetograms Authors: Pietarila, A.; Bertello, L.; Harvey, J. W.; Pevtsov, A. A. Bibcode: 2013SoPh..282...91P Altcode: 2012arXiv1209.6390P; 2012SoPh..tmp..274P We compare photospheric line-of-sight magnetograms from the Synoptic Optical Long-term Investigations of the Sun (SOLIS) Vector Spectro-Magnetograph (VSM) instrument with observations from the 150-foot Solar Tower at Mt. Wilson Observatory (MWO), the Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO), and the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO). We find very good agreement between VSM and the other data sources for both disk-averaged flux densities and pixel-by-pixel measurements. We show that the VSM mean flux density time series is of consistently high signal-to-noise ratio with no significant zero offsets. We discuss in detail some of the factors - spatial resolution, flux dependence, and position on the solar disk - affecting the determination of scaling between VSM and SOHO/MDI or SDO/HMI magnetograms. The VSM flux densities agree well with spatially smoothed data from MDI and HMI, although the scaling factors show a clear dependence on flux density. The factor to convert VSM to HMI increases with increasing flux density (from ≈1 to ≈1.5). The nonlinearity is smaller for the VSM vs. SOHO/MDI scaling factor (from ≈1 to ≈1.2). Title: Signature of Differential Rotation in Sun-as-a-star Ca II K Measurements Authors: Bertello, L.; Pevtsov, A. A.; Pietarila, A. Bibcode: 2012ApJ...761...11B Altcode: 2012arXiv1210.5556B The characterization of solar surface differential rotation (SDR) from disk-integrated chromospheric measurements has important implications for the study of differential rotation and dynamo processes in other stars. Some chromospheric lines, such as Ca II K, are very sensitive to the presence of activity on the disk and are an ideal choice for investigating SDR in Sun-as-a-star observations. Past studies indicate that when the activity is low, the determination of Sun's differential rotation from integrated-sunlight measurements becomes uncertain. However, our study shows that using the proper technique, SDR can be detected from these type of measurements even during periods of extended solar minima. This paper describes results from the analysis of the temporal variations of Ca II K line profiles observed by the Integrated Sunlight Spectrometer during the declining phase of Cycle 23 and the rising phase of Cycle 24, and discusses the signature of SDR in the power spectra computed from time series of parameters derived from these profiles. The methodology described is quite general, and could be applied to photometric time series of other main-sequence stars for detecting differential rotation. Title: Magnetometry with Large Solar Telescopes: Beyond the Photospheric Boundaries Authors: Pietarila, A. Bibcode: 2012ASPC..463....3P Altcode: Measuring the magnetic fields throughout the solar atmosphere, from the photosphere through the chromosphere to the corona, is one of the main goals of the upcoming large aperture solar telescopes. The observational requirements and the need for compromises will be discussed followed by a brief review of some of the available diagnostics for chromospheric and coronal field measurements. The need for a wide range of temporal and spatial scales as well as wavelength ranges in future solar observations will also be considered. Title: Nonlinear Force-free Field Modeling of a Solar Active Region Using SDO/HMI and SOLIS/VSM Data Authors: Thalmann, J. K.; Pietarila, A.; Sun, X.; Wiegelmann, T. Bibcode: 2012AJ....144...33T Altcode: 2012arXiv1206.1141T We use SDO/HMI and SOLIS/VSM photospheric magnetic field measurements to model the force-free coronal field above a solar active region, assuming magnetic forces dominate. We take measurement uncertainties caused by, e.g., noise and the particular inversion technique, into account. After searching for the optimum modeling parameters for the particular data sets, we compare the resulting nonlinear force-free model fields. We show the degree of agreement of the coronal field reconstructions from the different data sources by comparing the relative free energy content, the vertical distribution of the magnetic pressure, and the vertically integrated current density. Though the longitudinal and transverse magnetic flux measured by the VSM and HMI is clearly different, we find considerable similarities in the modeled fields. This indicates the robustness of the algorithm we use to calculate the nonlinear force-free fields against differences and deficiencies of the photospheric vector maps used as an input. We also depict how much the absolute values of the total force-free, virial, and the free magnetic energy differ and how the orientation of the longitudinal and transverse components of the HMI- and VSM-based model volumes compare to each other. Title: Circumfacular Regions in Ca II 854.2 nm Authors: Pietarila, Anna; Harvey, J. Bibcode: 2012AAS...22011005P Altcode: Active regions appear bright in Ca II 854.2 nm line core intensity while the surrounding areas, circumfacular regions, are darker than the active region or the quiet Sun. We use SOLIS VSM Ca II 854.2 nm data (high spectral resolution Stokes I and V profiles as well as photospheric and chromospheric LOS magnetograms) to study the relationship between the atmospheric dynamics, LOS magnetic field stratification and detailed spectral line properties, e.g., line bisectors and Stokes V asymmetries. The presence of circumfacular regions, magnetic canopies and flows may explain the solar cycle variation of Sun-as-star Ca II 854.2 nm bisectors. Title: Detection of Solar Differential Rotation in Disk-Integrated Ca II K Measurements Authors: Bertello, Luca; Pietarila, A.; Pevtsov, A. A. Bibcode: 2012AAS...22020311B Altcode: The characterization of solar differential rotation (SDR) from disk-integrated chromospheric measurements has important implications for the study of differential rotation and dynamo processes in other stars. Chromospheric lines, such as Ca II K, are very sensitive to the presence of activity on the disk and are an ideal choice for investigating SDR in Sun-as-a star

obervations. Here we use daily observations from the SOLIS Integrated Sunlight Spectrometer (ISS) to study the temporal variations of the Ca II K line profiles from 2006 to 2012.

We discuss the signature of SDR in the power spectra computed from time series of parameters derived from these profiles, and the implications for detecting differential rotation in other Main-Sequence stars. Title: Coronal Rain Observed On-disk with He I Spectropolarimetry from DST/FIRS Authors: Schad, Thomas A.; Penn, M. J.; Pietarila, A. Bibcode: 2012AAS...22031005S Altcode: Coronal rain refers to cool, dense blobs of plasma that presumably condense near the apex of hot coronal loops and then "rain" down upon the chromosphere traveling along curved loop-like paths. Considered to be the result of a thermal instability known as "catastrophic cooling", coronal rain places constraints on heating mechanisms for coronal loops. Nearly all observational studies of coronal rain, however, have been limited to the solar limb where cooler material within hot coronal loops is more readily identified. Here, we report observations of what we interpret to be the on-disk counterpart of coronal rain. Scanned spectropolarimetric observations in the He I triplet (1083 nm) from the Facility Infrared Spectropolarimeter (FIRS) reveal highly-redshifted material displaying an acceleration along curved trajectories terminating within a large sunspot (located at N17W21). Line-of-sight velocities in the He I triplet peak near 190 km/s, which can be consider high in relation to most coronal rain observations. This is also the largest redshift ever reported in the He I triplet. These curved loops correspond to an overarching loop structure seen in SDO/AIA anchored at its ends by a large sunspot and a group of pores. The loops observed with SDO/AIA display significant cooling as dark (EUV absorptive) blobs begin to form near the loop apex and then traverse along the same trajectories observed in the He I FIRS observations. Although the EUVI instrument of STEREO-A/SECCHI has a reduced temporal resolution compared to SDO/AIA, we are able to confidently match rain features in both spacecraft and thus stereoscopically reconstruct the three-dimensional trajectory to confirm the material is raining upon the solar surface. Title: Solar magnetism eXplorer (SolmeX). Exploring the magnetic field in the upper atmosphere of our closest star Authors: Peter, Hardi; Abbo, L.; Andretta, V.; Auchère, F.; Bemporad, A.; Berrilli, F.; Bommier, V.; Braukhane, A.; Casini, R.; Curdt, W.; Davila, J.; Dittus, H.; Fineschi, S.; Fludra, A.; Gandorfer, A.; Griffin, D.; Inhester, B.; Lagg, A.; Landi Degl'Innocenti, E.; Maiwald, V.; Sainz, R. Manso; Martínez Pillet, V; Matthews, S.; Moses, D.; Parenti, S.; Pietarila, A.; Quantius, D.; Raouafi, N. -E.; Raymond, J.; Rochus, P.; Romberg, O.; Schlotterer, M.; Schühle, U.; Solanki, S.; Spadaro, D.; Teriaca, L.; Tomczyk, S.; Trujillo Bueno, J.; Vial, J. -C. Bibcode: 2012ExA....33..271P Altcode: 2011arXiv1108.5304P; 2011ExA...tmp..134P The magnetic field plays a pivotal role in many fields of Astrophysics. This is especially true for the physics of the solar atmosphere. Measuring the magnetic field in the upper solar atmosphere is crucial to understand the nature of the underlying physical processes that drive the violent dynamics of the solar corona—that can also affect life on Earth. SolmeX, a fully equipped solar space observatory for remote-sensing observations, will provide the first comprehensive measurements of the strength and direction of the magnetic field in the upper solar atmosphere. The mission consists of two spacecraft, one carrying the instruments, and another one in formation flight at a distance of about 200 m carrying the occulter to provide an artificial total solar eclipse. This will ensure high-quality coronagraphic observations above the solar limb. SolmeX integrates two spectro-polarimetric coronagraphs for off-limb observations, one in the EUV and one in the IR, and three instruments for observations on the disk. The latter comprises one imaging polarimeter in the EUV for coronal studies, a spectro-polarimeter in the EUV to investigate the low corona, and an imaging spectro-polarimeter in the UV for chromospheric studies. SOHO and other existing missions have investigated the emission of the upper atmosphere in detail (not considering polarization), and as this will be the case also for missions planned for the near future. Therefore it is timely that SolmeX provides the final piece of the observational quest by measuring the magnetic field in the upper atmosphere through polarimetric observations. Title: Circumfacular regions and magnetic canopies as seen in Ca II 8542 Å Authors: Pietarila, Anna; Harvey, Jack Bibcode: 2012decs.confE...8P Altcode: Active regions appear bright in Ca II 8542 Å line core intensity while the surrounding areas are darker than the active region or the quiet Sun. These areas are referred to as circumfacular regions. We use SOLIS VSM Ca II 8542 Å data (photospheric and chromospheric full disk magnetograms as well as high spectral resolution Stokes I and V profiles) to study the relationship between photospheric and chromospheric LOS magnetic fields and detailed properties (e.g., line bisectors, Stokes V asymmetries) of the spectral profiles. There is a connection between magnetic canopies, circumfacular regions and Ca II 8542 Å bisector spans which may explain the observed solar cycle variation of the Sun-as-a-star Ca II 8542 Å bisectors. Title: Diffusivity of Isolated Internetwork Ca II H Bright Points Observed by SuFI/SUNRISE Authors: Jafarzadeh, S.; Solanki, S. K.; Cameron, R. H.; Feller, A.; Pietarila, A.; Lagg, A.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knoelker, M.; Martinez Pillet, V.; Schmidt, W.; Title, A. Bibcode: 2012decs.confE..99J Altcode: We analyze trajectories of the proper motion of intrinsically magnetic, isolated internetwork Ca II H BPs (with mean lifetime of 461 sec) to obtain their diffusivity behaviors. We use high spatial and temporal resolution image sequences of quiet-Sun, disc-centre observations obtained in the Ca II H 397 nm passband of the Sunrise Filter Imager (SuFI) on board the SUNRISE balloon-borne solar observatory. In order to avoid misidentification, the BPs are semi-manually selected and then automatically tracked. The trajectory of each BP is then calculated and its diffusion index is described by a power law exponent, using which we classify the BPs' trajectories into sub-, normal and super- diffusive. In addition, the corresponding diffusion coefficients (D) based on the observed displacements are consequently computed. We find a strong super-diffusivity at a height sampled by the SuFI/SUNRISE Ca II H passband (i.e. a height corresponding roughly to the temperature minimum). We find that 74% of the identified tiny BPs are super-diffusive, 18% move randomly (i.e. their motion corresponds to normal diffusion) and only 8% belong to the sub-diffusion regime. In addition, we find that 53% of the super-diffusion regime (i.e. 39% of all BPs) have the diffusivity index of 2 which are termed as "Ballistic BPs". Finally, we explore the distribution of diffusion index with the help of a simple simulation. The results suggest that the BPs are random walkers superposed by a systematic (background) velocity in which the magnitude of each component (and hence their ratio) depends on the time and spatial scales. We further discuss a simple sketch to explain the diffusivity of observed BPs while they migrate within a supergranule (i.e. internetwork areas) or close to the network regions. Title: Chromospheric Observations of a Kink Wave in an On-disk Active Region Fibril Authors: Pietarila, A. M.; Aznar Cuadrado, R.; Hirzberger, J.; Solanki, S. Bibcode: 2011AGUFMSH13B1951P Altcode: Most observations of kink and Alfven waves in the chromosphere are made in off-limb spicules. Here we present observations of a kink wave in high spatial and temporal resolution Ca II 8542 data of an active region fibril on the solar disk. The properties of the observed wave are similar to kink waves in spicules. From the inferred wave phase and period we estimate the lower limit for the field strength in the chromospheric fibril to be a few hundred Gauss. The observations indicate that the event may have been triggered by a small-scale reconnection event higher up in the atmosphere. Title: The Longitudinal Solar Magnetic Field Measured by SOLIS Authors: Bertello, L.; Pevtsov, A. A.; Harvey, J. W.; Pietarila, A. Bibcode: 2011AGUFMSH13B1934B Altcode: The SOLIS (Synoptic Optical Long-term Investigations of the Sun) Vector SpectroMagnetograph (VSM) instrument operating at the National Solar Observatory at Kitt Peak (Arizona) measures the full-disk longitudinal solar magnetic field in the neutral iron spectral lines at 630.15-630.25 nm (photosphere) and ionized calcium spectral line at 854.2 nm (lower chromosphere). Over the past two years a considerable effort has been made to improve the quality of the computed magnetograms. Major modifications to the original data reduction pipeline include a new determination of the instrumental magnetic bias and an improved calibration of the computed solar magnetic flux density. As a result of these changes SOLIS-VSM is now able to provide, among other products, more reliable values of the full-disk mean magnetic flux measured in the photosphere and low chromosphere. In addition, these improvements open the possibility to extend the current SOLIS-VSM catalog with new products of interest to the solar and heliospheric communities. We describe some of these changes and their impact on the quality of the derived SOLIS-VSM data. Title: Erratum:"Convective Nature of Sunspot Penumbral Filaments: Discovery of Downflows in the Deep Photosphere" (2011, ApJ, 734, L18) Authors: Joshi, Jayant; Pietarila, A.; Hirzberger, J.; Solanki, S. K.; Aznar Cuadrado, R.; Merenda, L. Bibcode: 2011ApJ...740L..55J Altcode: No abstract at ADS Title: Kink Waves in an Active Region Dynamic Fibril Authors: Pietarila, A.; Aznar Cuadrado, R.; Hirzberger, J.; Solanki, S. K. Bibcode: 2011ApJ...739...92P Altcode: 2011arXiv1107.3113P We present high spatial and temporal resolution Ca II 8542 Å observations of a kink wave in an on-disk chromospheric active region fibril. The properties of the wave are similar to those observed in off-limb spicules. From the observed phase and period of the wave we determine a lower limit for the field strength in the chromospheric active region fibril located at the edge of a sunspot to be a few hundred gauss. We find indications that the event was triggered by a small-scale reconnection event higher up in the atmosphere. Title: Solar Cycle Variation in Sun-as-a-star Ca II 854.2 nm Bisectors Authors: Pietarila, A.; Livingston, W. Bibcode: 2011ApJ...736..114P Altcode: 2011arXiv1105.3437P The bisector of the strong chromospheric Ca II 854.2 nm line has an inverse-C shape, the cause of which is not yet fully understood. We show that the amplitude of the bisector in Sun-as-a-star observations exhibits a solar cycle variation with smaller amplitudes during highest activity. The line core intensity is lower during solar minima while the part of the bisector most sensitive to the line core shows no systematic change with activity. Our results support the use of Ca II 854.2 nm bisectors in studying the relationship between convection and magnetic fields, not only in the Sun but in other stars as well. Title: Convective Nature of Sunspot Penumbral Filaments: Discovery of Downflows in the Deep Photosphere Authors: Joshi, Jayant; Pietarila, A.; Hirzberger, J.; Solanki, S. K.; Aznar Cuadrado, R.; Merenda, L. Bibcode: 2011ApJ...734L..18J Altcode: 2011arXiv1105.1877J We study the velocity structure of penumbral filaments in the deep photosphere to obtain direct evidence for the convective nature of sunspot penumbrae. A sunspot was observed at high spatial resolution with the 1 m Swedish Solar Telescope in the deep photospheric C I 5380 Å absorption line. The Multi-Object Multi-Frame Blind Deconvolution method is used for image restoration and straylight is filtered out. We report here the discovery of clear redshifts in the C I 5380 Å line at multiple locations in sunspot penumbral filaments. For example, bright head of filaments show larger concentrated blueshift and are surrounded by darker, redshifted regions, suggestive of overturning convection. Elongated downflow lanes are also located beside bright penumbral fibrils. Our results provide the strongest evidence yet for the presence of overturning convection in penumbral filaments and highlight the need to observe the deepest layers of the penumbra in order to uncover the energy transport processes taking place there. Title: Ca Ii K and H Measurements from the SOLIS ISS Instrument Authors: Bertello, Luca; Pevtsov, A. A.; Pietarila, A.; Harvey, J. W.; Toussaint, R. M.; SOLIS Team Bibcode: 2011SPD....42.1744B Altcode: 2011BAAS..43S.1744B Precise measurements of the disk integrated solar Ca II K and H lines play a critical role in many investigations of solar activity and studies related to solar irradiance variability. The Integrated Sunlight Spectrometer(ISS) operating at the National Solar Observatory at Kitt Peak (Arizona) since December 2006 is designed to obtain high spectral resolution (R = 300,000) observations of the Sun as a star in a broad range of wavelengths (350 nm -1100 nm). The ISS is one of three instruments, with the Vector Spectro-Magnetograph (VSM) and the Full Disk Patrol (FDP), comprising the Synoptic Optical Long-term Investigations of the Sun (SOLIS) - a synoptic facility for solar observations operating at NSO/Kitt Peak. The ISS takes daily observations of solar spectra in nine spectral bands, including the Ca II K and H lines. We describe recent improvements in data reduction of Ca II K and H observations, and present time variations of parameters derived from the profiles of these spectral lines. Some properties of these time series are also discussed. Title: Solar Cycle Variation of the Ca 854.2 nm Line Bisectors Authors: Pietarila, Anna; Livingston, W. Bibcode: 2011SPD....42.1703P Altcode: 2011BAAS..43S.1703P The bisector of the strong chromospheric Ca II 854.2 nm line has an inverse-C shape the cause of which is not yet fully understood. We show that the amplitude of the bisector in Sun-as-a-star observations exhibits a solar cycle variation with smaller amplitudes during highest activity. The line core intensity is lower during solar minima while the part of the bisector most sensitive to the line core shows no systematic change with activity. We also show preliminary results of a connection between magnetic shadows and bisector amplitudes. Our results support the use of Ca II 854.2 nm bisectors in studying the relationship between convection and magnetic fields, not only in the Sun but in other stars as well. Title: Solar Magnetic Fields As Observed By Solis Authors: Pietarila, Anna; Bertello, L.; Callahan, L.; Harker, B.; Harvey, J.; Marble, A.; Pevtsov, A.; Toussaint, R. Bibcode: 2011SPD....42.1746P Altcode: 2011BAAS..43S.1746P The Vector Spectromagnetograph (VSM), part of the Synoptic Optical Long-term Investigations of the Sun (SOLIS), makes spectropolarimetric observations of the full-disk of the Sun in the photospheric Fe I lines around 630 nm (Stokes I, Q, U and V) and the chromospheric Ca II 854.2 nm (Stokes I and V) line. We present some of the updated SOLIS VSM data products and show how they compare with data from other instruments, e.g., SDO/HMI and Hinode/SP. We also illustrate some of the differences between the photospheric and chromospheric magnetograms, and how they can be used to study the height variation of the magnetic field. Title: Status of Synoptic Optical Long-term Investigation of the Sun (SOLIS) Project. Authors: Pevtsov, Alexei A.; Streander, K.; Harvey, J.; Bertello, L.; Branston, D.; Britanik, J.; Callahan, L.; Cole, L.; Galayda, E.; Harker, B.; Hauth, D.; Jaksha, D.; Leiker, C.; Marble, A.; Mills, N.; Pietarila, A.; Schramm, K.; Stover, E.; Suarez-Sola, I.; Toussaint, R. Bibcode: 2011SPD....42.1747P Altcode: 2011BAAS..43S.1747P Synoptic Optical Long-term Investigation of the Sun (SOLIS) is a suite of three instruments to study various aspects of solar activity. Vector Stokes Magnetograph (VSM) takes full disk longitudinal magnetograms in the photosphere (Fe I 6301 A-6302 A) and the chromosphere (Ca II 8542A), the photospheric vector magnetograms, and the full disk images of equivalent width for He I 10830A. The VSM data set goes back to August 2003. Integrated Sunlight Spectrometer (ISS) provides high-resolution spectra of sun-as-a-star for nine selected spectral bands (starting from December 2006). Full-Disk Patrol (FDP) observes full disk images of the Sun at high cadence in several selected wavelengths (starting from June 2011). In the last two years, the SOLIS instruments and data reduction went through a series of upgrades resulting in higher data quality. These upgrades include new CCD cameras, photo guider, and improved magbias calculations and data reduction of ISS line profiles. We will present the current status of SOLIS, and show the comparison between SOLIS observations with other instruments. Title: Transport of Magnetic Flux from the Canopy to the Internetwork Authors: Pietarila, A.; Cameron, R. H.; Danilovic, S.; Solanki, S. K. Bibcode: 2011ApJ...729..136P Altcode: 2011arXiv1102.1397P Recent observations have revealed that 8% of linear polarization patches in the internetwork (INW) quiet Sun are fully embedded in downflows. These are not easily explained with the typical scenarios for the source of INW fields which rely on flux emergence from below. Using radiative MHD simulations, we explore a scenario where magnetic flux is transported from the magnetic canopy overlying the INW into the photosphere by means of downward plumes associated with convective overshoot. We find that if a canopy-like magnetic field is present in the simulation, the transport of flux from the canopy is an important process for seeding the photospheric layers of the INW with magnetic field. We propose that this mechanism is relevant for the Sun as well, and it could naturally explain the observed INW linear polarization patches entirely embedded in downflows. Title: Constructing Semi-Empirical Sunspot Models for Helioseismology Authors: Cameron, R. H.; Gizon, L.; Schunker, H.; Pietarila, A. Bibcode: 2011SoPh..268..293C Altcode: 2010arXiv1003.0528C; 2010SoPh..tmp..167C One goal of helioseismology is to determine the subsurface structure of sunspots. In order to do so, it is important to understand first the near-surface effects of sunspots on solar waves, which are dominant. Here we construct simplified, cylindrically-symmetric sunspot models that are designed to capture the magnetic and thermodynamics effects coming from about 500 km below the quiet-Sun τ5000=1 level to the lower chromosphere. We use a combination of existing semi-empirical models of sunspot thermodynamic structure (density, temperature, pressure): the umbral model of Maltby et al. (1986, Astrophys. J. 306, 284) and the penumbral model of Ding and Fang (1989, Astron. Astrophys. 225, 204). The OPAL equation-of-state tables are used to derive the sound-speed profile. We smoothly merge the near-surface properties to the quiet-Sun values about 1 Mm below the surface. The umbral and penumbral radii are free parameters. The magnetic field is added to the thermodynamic structure, without requiring magnetostatic equilibrium. The vertical component of the magnetic field is assumed to have a Gaussian horizontal profile, with a maximum surface field strength fixed by surface observations. The full magnetic-field vector is solenoidal and determined by the on-axis vertical field, which, at the surface, is chosen such that the field inclination is 45° at the umbral - penumbral boundary. We construct a particular sunspot model based on SOHO/MDI observations of the sunspot in active region NOAA 9787. The helioseismic signature of the model sunspot is studied using numerical simulations of the propagation of f, p1, and p2 wave packets. These simulations are compared against cross-covariances of the observed wave field. We find that the sunspot model gives a helioseismic signature that is similar to the observations. Title: Transverse Component of the Magnetic Field in the Solar Photosphere Observed by SUNRISE Authors: Danilovic, S.; Beeck, B.; Pietarila, A.; Schüssler, M.; Solanki, S. K.; Martínez Pillet, V.; Bonet, J. A.; del Toro Iniesta, J. C.; Domingo, V.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knölker, M.; Schmidt, W.; Title, A. M. Bibcode: 2010ApJ...723L.149D Altcode: 2010arXiv1008.1535D We present the first observations of the transverse component of a photospheric magnetic field acquired by the imaging magnetograph SUNRISE/IMaX. Using an automated detection method, we obtain statistical properties of 4536 features with significant linear polarization signal. We obtain a rate of occurrence of 7 × 10-4 s-1 arcsec-2, which is 1-2 orders of magnitude larger than the values reported by previous studies. We show that these features have no characteristic size or lifetime. They appear preferentially at granule boundaries with most of them being caught in downflow lanes at some point. Only a small percentage are entirely and constantly embedded in upflows (16%) or downflows (8%). Title: Expansion of magnetic flux concentrations: a comparison of Hinode SOT data and models Authors: Pietarila, A.; Cameron, R.; Solanki, S. K. Bibcode: 2010A&A...518A..50P Altcode: 2010arXiv1005.3405P Context. The expansion of network magnetic fields with height is a fundamental property of flux tube models. A rapid expansion is required to form a magnetic canopy.
Aims: We characterize the observed expansion properties of magnetic network elements and compare them with the thin flux tube and sheet approximations, as well as with magnetoconvection simulations.
Methods: We used data from the Hinode SOT NFI NaD1 channel and spectropolarimeter to study the appearance of magnetic flux concentrations seen in circular polarization as a function of position on the solar disk. We compared the observations with synthetic observables from models based on the thin flux tube approximation and magnetoconvection simulations with two different upper boundary conditions for the magnetic field (potential and vertical).
Results: The observed circular polarization signal of magnetic flux concentrations changes from unipolar at disk center to bipolar near the limb, which implies an expanding magnetic field. The observed expansion agrees with expansion properties derived from the thin flux sheet and tube approximations. Magnetoconvection simulations with a potential field as the upper boundary condition for the magnetic field also produce bipolar features near the limb while a simulation with a vertical field boundary condition does not.
Conclusions: The near-limb apparent bipolar magnetic features seen in high-resolution Hinode observations can be interpreted using a simple flux sheet or tube model. This lends further support to the idea that magnetic features with vastly varying sizes have similar relative expansion rates. The numerical simulations presented here are less useful in interpreting the expansion since the diagnostics we are interested in are strongly influenced by the choice of the upper boundary condition for the magnetic field in the purely photospheric simulations. Title: STEREO quadrature observations of coronal dimming at the onset of mini-CMEs Authors: Innes, D. E.; McIntosh, S. W.; Pietarila, A. Bibcode: 2010A&A...517L...7I Altcode: 2010arXiv1005.2097I Context. Using unique quadrature observations with the two STEREO spacecraft, we investigate coronal dimmings at the onset of small-scale eruptions. In CMEs they are believed to indicate the opening up of the coronal magnetic fields at the start of the eruption.
Aims: It is to determine whether coronal dimming seen in small-scale eruptions starts before or after chromospheric plasma ejection.
Methods: One STEREO spacecraft obtained high cadence, 75 s, images in the He II 304 Å channel, and the other simultaneous images in the Fe IX/Fe X 171 Å channel. We concentrate on two well-positioned chromospheric eruptions that occurred at disk center in the 171 Å images, and on the limb in 304 Å. One was in the quiet Sun and the other was in an equatorial coronal hole. We compare the timing of chromospheric eruption seen in the 304 Å limb images with the brightenings and dimmings seen on disk in the 171 Å images. Further we use off-limb images of the low frequency 171 Å power to infer the coronal structure near the eruptions.
Results: In both the quiet Sun and the coronal hole eruption, on disk 171 Å dimming was seen before the chromospheric eruption, and in both cases it extends beyond the site of the chromospheric eruption. The quiet Sun eruption occurred on the outer edge of the enclosing magnetic field of a prominence and may be related to a small disruption of the prominence just before the 171 Å dimming.
Conclusions: These small-scale chromospheric eruptions started with a dimming in coronal emission just like their larger counterparts. We therefore suggest that a fundamental step in triggering them was the removal of overlying coronal field.

Movies are only available in electronic form at http://www.aanda.org Title: STEREO quadrature observations of mass flows in prominences Authors: Innes, Davina; McIntosh, Scott; Pietarila, Anna Bibcode: 2010cosp...38.2917I Altcode: 2010cosp.meet.2917I Understanding the structure and dynamics of prominences is much easier when both the promi-nence on the limb and the filament on the disk are seen together. In February 2009, we obtained STEREO quadrature observations with a cadence of 75 s and simultaneous images of promi-nences in 304 A at the limb, and 171 A at disk center. We show how the observed flows in the prominence are associated with microflares seen in 171 at disk center for a couple of representative cases. Title: Relation between the Sunrise photospheric magnetic field and the Ca II H bright features Authors: Jafarzadeh, Shahin; Hirzberger, J.; Feller, A.; Lagg, A.; Solanki, S. K.; Pietarila, A.; Danilovic, S.; Riethmueller, T.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knülker, M.; Martínez Pillet, V.; Schmidt, W.; Schüssler, M.; Title, A. Bibcode: 2010cosp...38.2856J Altcode: 2010cosp.meet.2856J Recent observations from the Sunrise balloon-borne solar telescope have enabled us to reach an unprecedented high spatial resolution on the solar surface with the near-ultraviolet photo-spheric and chromospheric images as well as the magnetograms. We use these high resolution observations to investigate the structure of the solar upper photosphere and lower chromosphere as well as their temporal evolutions. We study the relation between the inter-granular Ca II 397 nm bright structures in images obtained by the Sunrise Filter Imager (SuFI) and their corresponding photospheric vector magnetic field computed from the Imaging Magnetogram eXperiment (IMaX) observations. The targets under study are in a quiet Sun region and close to disc-centre. Title: Expansion of Magnetic Flux Concentrations with Height: A Comparison of Hinode SOT Data and MHD Simulations Authors: Pietarila, A.; Cameron, R.; Solanki, S. Bibcode: 2009ASPC..415...91P Altcode: The Hinode SOT (Tsuneta et al. 2008) NFI Na I D1 and SP Fe I data sampled at different positions on the solar disk provide a unique diagnostic for studying the expansion of magnetic flux concentrations with height. We make a comparative study of SOT observations and 2-dimensional (2D) radiative MHD-simulations to see how well the simulations capture the expansion properties. The expansion of flux concentrations is clearly seen in the SOT Na I D1 data, where most of the magnetic features appear unipolar at disk center while close to the limb bipolar appearance strongly dominates. This trend, albeit not as strong, is seen in the SP data as well. Some aspects of the observations are qualitatively reproduced by simulations with a potential (as opposed to vertical) upper boundary condition for the magnetic field. Title: Bright fibrils in Ca II K Authors: Pietarila, A.; Hirzberger, J.; Zakharov, V.; Solanki, S. K. Bibcode: 2009A&A...502..647P Altcode: 2009arXiv0905.3124P Context: Except for the Ca II resonance lines, fibrils are ubiquitously present in most high-resolution observations of chromospheric lines.
Aims: We show that fibrils are also a prevailing feature in Ca II K, provided the spatial-resolution is sufficiently high.
Methods: We present high spatial resolution observations of an active region in the Ca II K line from the Swedish Solar Telescope. Through a comparison between photospheric intensity and magnetic field data, we study the connection between bright chromospheric fibrils and photospheric structures. Additionally, using Fourier analysis we study how the fibrils are linked to the observed dynamics.
Results: We find that very narrow, bright fibrils are a prevailing feature over large portions of the observed field. We also find a clear connection between the fibril footpoints and photospheric magnetic features. We show that the fibrils play two distinct roles in the observed dynamics: depending on their location they can act as a canopy suppressing oscillations or they can channel low-frequency oscillations into the chromosphere.
Conclusions: The Ca II K fibrils share many characteristics with fibrils observed in other chromospheric lines, but some features, such as the very small widths, are unique to these observations. Title: Fibrils in Ca II K Authors: Pietarila, A.; Solanki, S.; Hirzberger, J.; Zakharov, V. Bibcode: 2008ESPM...12.2.51P Altcode: High spatial resolution observations have revealed that fibril-like structures are a ubiquitous feature of the solar chromosphere. They are observed in most chromospheric lines, e.g., H-? and Ca II IRT. Until recently, there have been no reports of these structures in the Ca II H and K lines. Instead, these lines have revealed a hazy, uniform chromosphere and only in the highest resolution images have there been any indications of fibril structures.

We present high spatial resolution Ca II K observations from the Swedish Solar Telescope which show that fibrils are a prevailing feature in regions where the bulk of the signal is chromospheric. Based on the cotemporal continuum and nearly cotemporal magnetic field observations it is clear that the fibril foot points originate from photospheric magnetic concentrations. The fibrils share many characteristics, e.g. lifetime and dynamics, with fibrils observed in other spectral lines. They are also found to play an important role in the dynamics: in the plage they channel low frequency waves into the chromosphere while in the more quiet regions the highly inclined fibrils form a multi-layer canopy that suppresses oscillations from below. Title: Spectropolarimetric Inversions of the Ca II 8498 and 8542 Å Lines in the Quiet Sun Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T. Bibcode: 2007ApJ...670..885P Altcode: We study non-LTE inversions of the Ca II infrared triplet lines as a tool for inferring physical properties of the quiet Sun. The inversion code is successful in recovering the temperature, velocity, and longitudinal magnetic flux density in the photosphere and chromosphere, but the height range where the inversions are sensitive is limited, especially in the chromosphere. We present results of inverting spectropolarimetric observations of the lines in a quiet-Sun region. We find three distinct ranges in chromospheric temperature: low temperatures in the internetwork, high temperatures in the enhanced magnetic network, and intermediate temperatures associated with low magnetic flux regions in the network. The differences between these regions become more pronounced with height as the plasma-β decreases. These inversions support the picture of the chromosphere, especially close to the magnetic network, being highly inhomogeneous in both the vertical and horizontal directions. Title: Spectropolarimetric Observations of the Ca II λ8498 and λ8542 in the Quiet Sun Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T. Bibcode: 2007ApJ...663.1386P Altcode: 2007arXiv0707.1310P The Ca II infrared triplet is one of the few magnetically sensitive chromospheric lines available for ground-based observations. We present spectropolarimetric observations of the 8498 and 8542 Å lines in a quiet Sun region near a decaying active region and compare the results with a simulation of the lines in a high plasma-β regime. Cluster analysis of Stokes V profile pairs shows that the two lines, despite arguably being formed fairly close, often do not have similar shapes. In the network, the local magnetic topology is more important in determining the shapes of the Stokes V profiles than the phase of the wave, contrary to what our simulations show. We also find that Stokes V asymmetries are very common in the network, and the histograms of the observed amplitude and area asymmetries differ significantly from the simulation. Both the network and internetwork show oscillatory behavior in the Ca II lines. It is stronger in the network, where shocking waves, similar to those in the high-β simulation, are seen and large self-reversals in the intensity profiles are common. Title: The Ca II Infrared Triplet Lines as Diagnostics of Chromospheric Magnetism Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T. Bibcode: 2007ASPC..368..139P Altcode: The Ca II infrared (IR) triplet lines are a promising candidate for studying chromospheric magnetism and dynamics. To study how good of a diagnostic the lines are for chromospheric magnetism in the quiet Sun we have constructed a MHD simulation in the high plasma-β regime, analyzed quiet Sun spectropolarimetric data of the lines and used a non-local thermodynamic equilibrium (nLTE) inversion code on the observations. In the simulation, where shocking acoustic waves dominate the dynamics, the Ca lines show a time-varying pattern of disappearing and reappearing Stokes V lobes. Waves are seen in the observations as well, but the dynamics are more complex. Unlike in the simulation, the observed Ca lines do not have similar shapes and the Stokes V asymmetries are determined by the local magnetic topology, not the phase of the wave. The fundamental differences between the observations and the simulation lead one to conclude that a 1D plane parallel atmosphere is not a valid approximation for the chromosphere. Nor can the effects of magnetic fields on the dynamics be neglected. This is further supported by the inversions failure to reproduce line profile asymmetries caused by gradients in the velocity and/or magnetic field. To explain the asymmetries, 3D structures and strongly localized gradients need to be included. The work presented here will be published in more detail elsewhere. Title: Spectropolarimetric observations of the Ca II 8498 A and 8542 A lines in the quiet Sun Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T. Bibcode: 2007arXiv0704.0617P Altcode: The Ca II infrared triplet is one of the few magnetically sensitive chromospheric lines available for ground-based observations. We present spectropolarimetric observations of the 8498 A and 8542 A lines in a quiet Sun region near a decaying active region and compare the results with a simulation of the lines in a high plasma-beta regime. Cluster analysis of Stokes V profile pairs shows that the two lines, despite arguably being formed fairly close, often do not have similar shapes. In the network, the local magnetic topology is more important in determining the shapes of the Stokes V profiles than the phase of the wave, contrary to what our simulations show. We also find that Stokes V asymmetries are very common in the network, and the histograms of the observed amplitude and area asymmetries differ significantly from the simulation. Both the network and internetwork show oscillatory behavior in the Ca II lines. It is stronger in the network, where shocking waves, similar to those in the high-beta simulation, are seen and large self-reversals in the intensity profiles are common. Title: The Ca Ir Triplet As A Diagnostic For Chromospheric Magnetism Authors: Pietarila, Anna M.; Socas-Navarro, H.; Bogdan, T. J. Bibcode: 2006SPD....37.1902P Altcode: 2006BAAS...38..247P The Ca IR triplet lines are a good candidate for observing chromospheric magnetic fields. Simulations of the lines in the high-beta regime, spectropolarimetric observations and inversions of observations give insight to their usability\suitability in observing QS magnetic fields and dynamics. Propagating waves are clearly visible in the simulations, both in Stokes V and I. Observed line profiles are found to be more complicated, though they do indicate the presence of waves. We present both simulated and observed profiles and analyze the ability of the NLTE inversion algorithm to retrieve the underlying physics in the solar atmosphere. Title: Spectro-Polarimetric Observations and Non-Lte Modeling of Ellerman Bombs Authors: Socas-Navarro, Hector; Pillet, Valentín Martínez; Elmore, David; Pietarila, Anna; Lites, Bruce W.; Sainz, Rafael Manso Bibcode: 2006SoPh..235...75S Altcode: 2005astro.ph..8667S Ellerman bombs are bright emission features observed in the wings of Hα, usually in the vicinity of magnetic concentrations. Here we show that they can also be detected in the Ca II infrared triplet lines, which are easier to interpret and therefore allow for more detailed diagnostics. We present full Stokes observations of the 849.8 and 854.2 nm lines acquired with the new spectro-polarimeter SPINOR. The data show no significant linear polarization at the level of 3 × 10−4. The circular polarization profiles exhibit measureable signals with a very intricate pattern of peaks. A non-LTE analysis of the spectral profiles emerging from these features reveals the presence of strong downflows (∼10 {km s−1}) in a hot layer between the upper photosphere and the lower chromosphere. Title: Spinor: Visible and Infrared Spectro-Polarimetry at the National Solar Observatory Authors: Socas-Navarro, Hector; Elmore, David; Pietarila, Anna; Darnell, Anthony; Lites, Bruce W.; Tomczyk, Steven; Hegwer, Steven Bibcode: 2006SoPh..235...55S Altcode: 2005astro.ph..8685S The Spectro-Polarimeter for Infrared and Optical Regions (SPINOR) is a new spectro-polarimeter that will serve as a facility instrument for the Dunn Solar Telescope at the National Solar Observatory. This instrument is capable of achromatic polarimetry over a very broad range of wavelengths, from 430 to 1600 nm, allowing for the simultaneous observation of several visible and infrared spectral regions with full Stokes polarimetry. Another key feature of the design is its flexibility to observe virtually any combination of spectral lines, limited only by practical considerations (e.g., the number of detectors available, space on the optical bench, etc.). Title: Simulation of Quiet-Sun Waves in the Ca II Infrared Triplet Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T.; Carlsson, M.; Stein, R. F. Bibcode: 2006ApJ...640.1142P Altcode: 2005astro.ph.10744P The Ca II infrared triplet lines around 8540 Å are good candidates for observing chromospheric magnetism. Model spectra of these lines are obtained by combining a radiation hydrodynamic simulation with a Stokes synthesis code. The simulation shows interesting time-varying behavior of the Stokes V profiles as waves propagate through the formation region of the lines. Disappearing and reappearing lobes in the Stokes V profiles as well as profile asymmetries are closely related to the atmospheric velocity gradients. Title: On the Formation of Extreme-Ultraviolet Helium Lines in the Sun: Analysis of SOHO Data Authors: Judge, Philip G.; Pietarila, Anna Bibcode: 2004ApJ...606.1258J Altcode: The resonance lines of helium in the Sun are several times stronger than expected, relative to lines of other ions. To explore the origins of this ``helium enhancement,'' we have studied data from the SUMER, CDS, MDI, and EIT instruments on the Solar and Heliospheric Observatory (SOHO). Time series data obtained in a quiet region and a coronal hole indicate that the spatio-temporal properties of the He I 584 Å, and He II 304 and 1084 Å lines are qualitatively unlike other chromospheric and transition region lines. Helium line intensities vary slowly compared to chromospheric oscillations and to transient transition region brightenings seen in other lines, yet they vary rapidly (both spatially and temporally) compared to the coronal radiation. This suggests that photoionization/recombination plays a minor excitation role in these lines. The Doppler shift of the 584 Å line shows a remarkably clear 4-8 mHz oscillation, with no accompanying line intensity oscillation. Transient brightenings are used to show that nonradial photon scattering might explain the previously found diffuse appearance of the network but cannot account for the reduced network/internetwork intensity contrast in the 584 Å line. We propose a new enhancement mechanism, based on arguments in a companion paper, in which predominantly neutral species such as helium diffuse across magnetic field lines into regions of hot coronal plasma, but charged ions do not. The enhanced internetwork helium emission may arise from the diffusion of helium atoms across the chromospheric canopy; the enhanced network emission may have contributions from diffusion from spicules. This mechanism may naturally explain why the 584 Å line shows Doppler shift, but not intensity, oscillations. Finally, if a mechanism such as ``velocity redistribution'' dominates helium emission, spectroheliograms of helium lines will be bright in regions of large temperature gradients parallel to the magnetic field. Differences between helium and other spectroheliograms may then reveal the relative contributions of ``classical'' and ``cool loop'' models to the observed emission. Title: On the Formation of the Resonance Lines of Helium in the Sun Authors: Pietarila, Anna; Judge, Philip G. Bibcode: 2004ApJ...606.1239P Altcode: To investigate the cause of anomalously bright resonance lines of helium in the Sun, we have studied the magnitude of the enhancements and some proposals made to explain them. Calculations in new semiempirical solar models indicate that the resonance lines of helium are enhanced by factors of 2-5 for He I and between 2 and 9 for He II, depending on the elemental abundances assumed. These enhancements are substantially less than earlier work has suggested, with the differences arising from radiative transfer not only in helium lines but also in lines of less abundant elements. Photon scattering, even with small line center optical depths (of order unity or less) throughout the transition region, is shown to significantly modify line intensities and their center-to-limb variation. This effect has important consequences both for our analysis and also for the analysis of solar EUV lines in general, including emission measure analyses. We have re-examined some proposals to explain the enhancements based on the ``ionizing plasma'' picture. The proposals include explicit heating (the ``burst'' picture) and heating via advection (diffusive and/or flowing models and the ``velocity redistribution'' [VR] proposal). We argue that the original VR mechanism must be modified to include kinetic effects for helium atoms, which reduce the effects of VR. The VR mechanism also naturally predicts helium lines that are blue-shifted relative to lines less sensitive to VR, which contradicts observations for the He I 584 Å line. Ionizing plasma models also potentially lead to enhancement of Δn>=1 transitions (n is the principal quantum number) in other atoms and ions. Existing observational evidence for enhanced Δn>=1 transitions in other ions is weak. We conclude that the ionizing plasma scenarios are not the sole cause of the helium enhancement. Instead, in a companion paper, we propose that the thermodynamic properties of noble gases in the upper chromosphere will lead to enhancement of their spectral lines if cross-field diffusion into coronal plasma is important. Finally, we show that, surprisingly, multifluid flows of the kind computed by Fontenla and colleagues can reproduce observed intensities of several lines of C and Si and may contain a resolution to the helium enhancement problem. Title: He I 584 A Line in the Solar Spectrum Authors: Pietarila, A. Bibcode: 2002PhDT.........2P Altcode: This thesis tries to look into the formation of the neutral helium resonance line in the solar EUV spectrum. A time dependent approach is taken when dealing with the helium emission. In the beginning, the idea was to analyze some observations and put helium into context with other lines. In the end, it proved to be a difficult task, and the scope of this thesis broadened. First a brief review of the Sun and its atmosphere is given in chapter 1. The existing observational facts and models for the helium line formation will are discussed in chapters 2 and 3. Chapter 4 focuses on time series observations done with the SUMER instrument. In chapter 5 two of the models for helium line formation reviewed earlier are explored. Finally, chapter 6 draws conclusions from the observations and calculations.