Robert J. Rutten

Dynamics of the solar chromosphere. I - Long-period network oscillations

We analyze differences in solar oscillations between the chromospheric network and internetwork regions from a 1 hr sequence of spectrograms of a quiet region near disk center. The spectrograms contain Ca II H, Ca I 422.7 nm, and various Fe I blends in the Ca II H wing. They permit vertical tracing of oscillations throughout the photosphere and into the low chromosphere. We find that the rms amplitude of Ca II H line center Doppler fluctuations is ∼1.5 km s -1 for both network and internetwork, but that the character of the oscillations differs markedly in these two regions. Within internetwork areas the chromospheric velocity power spectrum is dominated by oscillations with frequencies at and above the acoustic cutoff frequency

Magnetic coupling between the interior and atmosphere of the sun

Preamble.- Republication of: Radial Movement in Sunspots.- Opening Remarks.- Solar Physics at the Kodaikanal Observatory: A Historical Perspective.- Vainu Bappu Memorial Lecture: What is a Sunspot?.- Oral Presentations.- Angular-Momentum Coupling Through the Tachocline.- Outstanding Issues in Solar Dynamo Theory.- Status of 3D MHD Models of Solar Global Internal Dynamics.- Measuring the Hidden Aspects of Solar Magnetism.- Recent Advances in Chromospheric and Coronal Polarization Diagnostics.- Probability Density Functions to Represent Magnetic Fields at the Solar Surface.- Spectropolarimetry with CRISP at the Swedish 1-m Solar Telescope.- Spectropolarimetry with the NLST.- Magnetic Coupling in the Quiet Solar Atmosphere.- The Evershed Effect with SOT/Hinode.- The Evershed Flow and the Brightness of the Penumbra.- A Topology for the Penumbral Magnetic Fields.- Theoretical Models of Sunspot Structure and Dynamics.- Convection and the Origin of Evershed Flows.- The Magnetic Field of Solar Spicules.- Three-Dimensional Magnetic Reconnection.- Signatures of Coronal Heating Mechanisms.- Waves in Polar Coronal Holes.- MHD Wave Heating Diagnostics.- Coronal Mass Ejections from Sunspot and Non-Sunspot Regions.- CME Observations from STEREO.- Low-Frequency Radio Observations of Coronal Magnetic Fields.- Evolution of Near-Sun Solar Wind Turbulence.- The Solar-Stellar Connection.- Summary and Perspective.- Poster Presentations.- Evidence for Return Meridional Flows in the Convection Zone.- Long-Term Variations in Meridional Flows.- Low-Degree High-Frequency and Modes in the Solar Core.- Interior and Exterior Clues of Solar Activity.- Do Active Regions Modify Oscillation Frequencies?.- Deep-Focus Diagnostics of Sunspot Structure.- Are Polar Faculae Generated by a Local Dynamo?.- The Hanle Effect as Diagnostic Tool for Turbulent Magnetic Fields.- Phase III of the USO Solar Vector Magnetograph.- Revisit of the Classic Wilson Effect.- The Waldmeier Effect in Sunspot Cycles.- Flare-Driven Acoustic Modes in the Sun.- Dynamics of Active Regions Revealed by Tracking of Doppler Features.- The Growth of a Primitive Penumbra.- Evershed Flow Velocities During 100 Years.- The Thermal Structure of Sunspots.- Bright Points in G-Band and Ca IIH Images from Hinode.- Waves in the Transition Region.- Viscous Damping of Alfv#x00E9 n Surface Waves with Steady Flows.- Statistical Detection of Propagating Waves in a Polar Coronal Hole.- Network Loop Oscillations with EIS/Hinode.- Dynamical Evolution of X-Ray Bright Points with Hinode/XRT.- Helicity at Photospheric and Chromospheric Heights.- Evolution of Coronal Helicity in a Twisted Emerging Active Region.- Power-Law Nanoflare Heating.- Spectroscopic Diagnostics of Polar Coronal Plumes.- A Flaring Polar Filament.- Solar X-Ray Processes.- Time-Varying Thermal Emission in Solar Flares.- Multi-Wavelength View of Flare Events on 20 November 2003.- Time-Delay Between Solar Soft X-Ray and EUV Flare Emissions.- Major Surge Activity of Super-Active Region NOAA 10484.- Coronal Magnetic Field Estimation Using Type-II Radio Bursts.- Acceleration of CMEs Associated with Eruptive Prominences.- Interplanetary Consequences of a Large CME.- Solar System Resonances on Light-Travel Time Scales Set Up before Proto-Sun#x2019 s Nuclear Ignition.- Summaries of Presentations Published Elsewhere.- Cycle Prediction from Dynamo Theory.- Why Does the Torsional Oscillation Precede the Sunspot Cycle?.- The Subsurface Magnetic Structure of Solar Active Regions.- Sunspot Magnetometry from Kodaikanal.- Vector Magnetic Field in Emerging Flux Regions.- Evolution of Umbral Dots and Penumbral Grains.- Strong, Localized Downflows in a Sunspot Light Bridge.- Small-Scale Velocities in Sunspot Penumbrae.- Photospheric Temperatures from Ca IIH.- Dual-Line Spectral Imaging of the Chromosphere.- Inversions of High-Cadence SOLIS-VSM Stokes Observations.- Flows in Flaring and Dormant Active Regions.- Magnetic and Velocity Field Changes Related to the Solar Flares of 28 and 29 October 2003.- A Numerical Investigation of Unsheared Flux Cancelation.- Wave Heating of Coronal Loops with Steady Flows.- Damping of Prominence Oscillations in Steady Equilibrium.- Variation of Network Contrast with Height.- A Flaring Twisted Emerging Flux Region.- Evidence of Magnetic Reconnection Outflows in a Flare seen by Hinode/EIS.- Does Coronal Rotation Period Depend on the Sunspot Number?.- Coronal Magnetic Field from an Extreme Radio Burst.- CME Kinematics and Dynamics.- Solar Wind Monitoring with SWIM-SARA Onboard Chandrayaan-1.- Coupling of the Solar Wind and the Magnetosphere.- The FIP Effect in RV Tauri Stars.

Solar and stellar granulation

Workshop Introduction.- 1: Observational Techniques.- Observations of the Solar Granulation at the Pic Du Midi Observatory and with THEMIS.- The Gregory-Coude-Telescope at the Observatorio Del Teide, Tenerife.- The Status of the Latest German Solar Facility on Tenerife.- Ground-Based Tunable Filter Observations.- An Overview of the Orbiting Solar Laboratory.- Solar Image Restoration by Adaptive Optics.- Improving Solar Image Quality by Image Selection.- Reconstruction from Focal Volume Information.- Granulation in the Photospheres of Stars.- Solar Granulation Speckle Interferometry Using Cross-Spectrum Techniques.- Active Optics, Anisoplanatism, and the Correction of Astronomical Images.- On Pressure Shifts of Fel Lines in Stellar Atmospheres.- Pressure Broadening and Solar Spectral Line Bisectors.- 2: Observations.- Solar Granulation: Overview.- Line Shifts in the Infrared Spectra of Late-Type Stars.- Asymmetry of Absorption Lines in the Solar and Procyon Spectra.- Line Asymmetries in Late-Type Dwarf Photospheres.- Challenges and Opportunities in Stellar Granulation Observations.- High Resolution Granulation Observations from La Palma: Techniques and First Results.- Spatially Resolved Spectra of Solar Granules.- Granulation Line Asymmetries.- Granulation and Waves?.- High Resolution Diagnostic of the Mesocells in the Solar Temperature Minimum Region.- Pole-Equator Difference of the Size of the Chromospheric Ca II-K Network in Quiet and Active Solar Regions.- Temperature Waves and Solar Granulation.- Flows, Random Motions and Oscillations in Solar Granulation Derived 225 the SOUP Instrument on Spacelab 2.- Granulation In and Out of Magnetic Regions.- Observation and Interpretation of Photospheric Line Asymmetry Changes near Active Regions.- Line Bisectors In and Out Magnetic Regions.- RMS Velocities in Solar Active Regions.- Report on the Session: Position and Discussion - Solar Data.- The Limb Effect of the KI 769.9 nm Line in Quiet Regions.- Vortex Motion of the Solar Granulation.- The Granulation Sensitivity of Neutral Metal Lines.- Granule Lifetimes and Dimensions as Function of Distance from a Solar Pore Region.- Spatio-Temporal Analysis of Waves in the Solar Atmosphere.- On the Granule Lifetime Near and Far Away from Sunspots.- Observations of High Frequency Waves in the Solar Atmosphere.- Photoelectric Analysis of the Solar Granulation in the Infrared.- Constraints Imposed by Very High Resolution Spectra and Images on Theoretical Simulations of Granular Convection.- The Variation of the Mean Size of the Photospheric Granules Near and Far from a Sunspot.- Abnormal Granulation.- Supergranular Pattern Formed by the Large Granules.- Details of Large Scale Solar Motions Revealed by Granulation Test Particles.- 3: Modelling.- Convection and Waves.- How Much Can Theoretical Models of Compressible Convection Tell Us about Solar Granulation?.- Two and Three-Dimensional Simulations of Compressible Convection.- Spectroscopic Properties of Solar Granulation obtained from 2-D Numerical Simulations.- On the Influence of Opacity Fluctuations on the Energy Transfer by Radiation.- Simulating Magnetoconvection.- Time Dependent Compressible Magnetoconvection.- Models of Magnetic Flux Sheets.- Stellar Granulation: Modeling of Stellar Surfaces and Photospheric Line Asymmetries.- Three-Dimensional Simulation of Convective Motions in the Procyon Envelope.- Numerical Simulations of Gas Flows in an A-Type Stellar Atmosphere.- Inhomogeneous Chromospheric Structures in the Atmosphere of Arcturus Generated by Acoustic Wave Propagation.- Interaction of Solar Granulation with Weak and Strong Magnetic Fields.- Granulation and the NLTE Formation of K I 769.9.- The Effects of Inhomogeneities in 3-D Models on Abundance Determinations.- 2-D Flux Tube in Radiative Equilibrium.- Isolated Magnetohydrodynamic Thermal Eddies in a Thermally Stratified Fluid.- What Do the Mg II Lines Tell Us About Waves and Magnetic Fields?.- A Simple Model of Mesogranular and Supergranular Flows.- Probing of Sunspot Umbral Structure by Oscillations.- Workshop Impressions.- Granulation Bibliography (compiled by K.L. Harvey).

Dynamics of the solar chromosphere - III. Ultraviolet brightness oscillations from TRACE

We analyze oscillations in the solar atmosphere using image sequences from the Transition Region and Coronal Explorer (TRACE) in three ultraviolet passbands which sample the upper solar photosphere and low chromosphere. We exploit the absence of atmospheric seeing in TRACE data to furnish comprehensive Fourier diagnostics (amplitude maps, phase-dierence spectra, spatio-temporal decomposition) for quiet-Sun network and internetwork areas with excellent sampling statistics. Comparison displays from the ground-based Ca ii Hs pec- trometry that was numerically reproduced by Carlsson & Stein are added to link our results to the acoustic shock dynamics in this simulation. The TRACE image sequences conrm the dichotomy in oscillatory behaviour between network and internetwork and show upward propagation above the cuto frequency, the onset of acoustic shock formation in the upper photosphere, phase-dierence contrast between pseudo-mode ridges and the inter- ridge background, enhanced three-minute modulation aureoles around network patches, a persistent low-intensity background pattern largely made up of internal gravity waves, ubiquitous magnetic flashers, and low-lying mag- netic canopies with much low-frequency modulation. The spatio-temporal occurrence pattern of internetwork grains is found to be dominated by acoustic and gravity wave interference. We nd no sign of the high-frequency sound waves that have been proposed to heat the quiet chromosphere, but such measurement is hampered by non-simultaneous imaging in dierent passbands. We also nd no signature of particular low-frequency fluxtube waves that have been proposed to heat the network. However, internal gravity waves may play a role in their excitation.

Ca ii H2v and K2v cell grains

The bright Ca ii H2v and K2v grains, which are intermittently present in the interiors of network cells in quiet-Sun areas, should provide important diagnostics of the dynamical interaction between the quiet photosphere and the chromosphere above it, but their nature has so far eluded identification. We review the extensive observational literature on these grains and on related phenomena. We resolve various contradictions, connect hitherto unconnected observations, distill new constraints and relate signatures in the measurement domain to signatures in the Fourier domain. We then review interpretations and simulation efforts, adding computations of our own to illustrate modeling options. We conclude that the grains are a hydrodynamical phenomenon in which magnetic fields do not play a major role. The grains are due to interference between a pervasive standing oscillation with about a 180 s periodicity and an 8 Mm horizontal wavelength in the chromosphere and the wave trains of the evanescent p-mode interference pattern in the upper photosphere. The roles of short-period waves, shock formation and granular piston excitation and the issue of long-lived patterning remain open; we suggest avenues for further research.

Non-equilibrium hydrogen ionization in 2D simulations of the solar atmosphere

Context: The ionization of hydrogen in the solar chromosphere and transition region does not obey LTE or instantaneous statistical equilibrium because the timescale is long compared with important hydrodynamical timescales, especially of magneto-acoustic shocks. Since the pressure, temperature, and electron density depend sensitively on hydrogen ionization, numerical simulation of the solar atmosphere requires non-equilibrium treatment of all pertinent hydrogen transitions. The same holds for any diagnostic application employing hydrogen lines. Aims: To demonstrate the importance and to quantify the effects of non-equilibrium hydrogen ionization, both on the dynamical structure of the solar atmosphere and on hydrogen line formation, in particular Hα. Methods: We implement an algorithm to compute non-equilibrium hydrogen ionization and its coupling into the MHD equations within an existing radiation MHD code, and perform a two-dimensional simulation of the solar atmosphere from the convection zone to the corona. Results: Analysis of the simulation results and comparison to a companion simulation assuming LTE shows that: a) non-equilibrium computation delivers much smaller variations of the chromospheric hydrogen ionization than for LTE. The ionization is smaller within shocks but subsequently remains high in the cool intershock phases. As a result, the chromospheric temperature variations are much larger than for LTE because in non-equilibrium, hydrogen ionization is a less effective internal energy buffer. The actual shock temperatures are therefore higher and the intershock temperatures lower. b) The chromospheric populations of the hydrogen n = 2 level, which governs the opacity of Hα, are coupled to the ion populations. They are set by the high temperature in shocks and subsequently remain high in the cool intershock phases. c) The temperature structure and the hydrogen level populations differ much between the chromosphere above photospheric magnetic elements and above quiet internetwork. d) The hydrogen n = 2 population and column density are persistently high in dynamic fibrils, suggesting that these obtain their visibility from being optically thick in Hα also at low temperature.

DOT tomography of the solar atmosphere. IV. Magnetic patches in internetwork areas

We use G-band and Call H image sequences from the Dutch Open Telescope (DOT) to study magnetic elements that appear as bright points in internetwork parts of the quiet solar photosphere and chromosphere. We find that many of these bright points appear recurrently with varying intensity and horizontal motion within longer-lived magnetic patches. We develop an algorithm for detection of the patches and find that all patches identified last much longer than the granulation. The patches outline cell patterns on mesogranular scales, indicating that magnetic flux tubes are advected by granular flows to mesogranular boundaries. Statistical analysis of the emergence and disappearance of the patches points to an average patch lifetime as long as 530 ± 50 min (about nine hours), which suggests that the magnetic elements constituting strong internetwork fields are not generated by a local turbulent dynamo.

Empirical NLTE analyses of solar spectral lines

The center-to-limb behaviour of the Ba ii λ4554 resonance line is analyzed together with data from the extreme limb, flash intensities and profiles of other Ba ii lines. An empirical NLTE method is employed in which the observed profiles are compared with synthesized profiles based on a standard one-dimensional model atmosphere, with the line source function, the barium abundance, the collisional damping and the atmospheric turbulence as free parameters.The line profiles from the extreme limb furnish considerable constraints on the formation of Ba ii λ4554. Its wings reverse into emission well inside the solar limb, a phenomenon which cannot be explained by any frequency-independent line source function. Accounting for effects of partially coherent scattering in the line source function is a necessary and adequate step to reproduce the observations both over the disk and near the limb. The form of the empirically derived frequency-dependent line source function is discussed.Results are given for various parameters (gf-values, solar barium abundance and isotope ratios, collisional damping, microturbulence and macroturbulence).The λ4554 profile of disk center shows a depression in its blue wing resembling asymetries found in various stellar spectral lines.

ELLERMAN BOMBS AT HIGH RESOLUTION. III. SIMULTANEOUS OBSERVATIONS WITH IRIS AND SST

Ellerman bombs are transient brightenings of the extended wings of the solar Balmer lines in emerging active regions. We describe their properties in the ultraviolet lines sampled by the Interface Region Imaging Spectrograph (IRIS), using simultaneous imaging spectroscopy in H

The solar chromosphere at high resolution with IBIS. IV. Dual-line evidence of heating in chromospheric network

\alpha