Probing the Solar Atmosphere Using Oscillations of Infrared CO Spectral Lines

Oscillations were observed across the whole solar disk using the Doppler shift and line depth of spectral lines from the CO molecule near 4666~nm with the National Solar Observatory's McMath/Pierce solar telescope. Power, coherence, and phase spectra were examined, and diagnostic diagrams reveal power ridges at the solar global mode frequencies to show that these oscillations are solar p-modes. The phase was used to determine the height of formation of the CO lines by comparison with the IR continuum intensity phase shifts as measured in Kopp et al., 1992; we find the CO line formation height varies from 425 < z < 560 km as we move from disk center towards the solar limb 1.0 > mu > 0.5. The velocity power spectra show that while the sum of the background and p-mode power increases with height in the solar atmosphere as seen in previous work, the power in the p-modes only (background subtracted) decreases with height, consistent with evanescent waves. The CO line depth weakens in regions of stronger magnetic fields, as does the p-mode oscillation power. Across most of the solar surface the phase shift is larger than the expected value of 90 degrees for an adiabatic atmosphere. We fit the phase spectra at different disk positions with a simple atmospheric model to determine that the acoustic cutoff frequency is about 4.5 mHz with only small variations, but that the thermal relaxation frequency drops significantly from 2.7 to 0 mHz at these heights in the solar atmosphere

Proper orthogonal decomposition of solar photospheric motions

The spatio-temporal dynamics of the solar photosphere is studied by performing a Proper Orthogonal Decomposition (POD) of line of sight velocity fields computed from high resolution data coming from the MDI/SOHO instrument. Using this technique, we are able to identify and characterize the different dynamical regimes acting in the system. Low frequency oscillations, with frequencies in the range 20-130 microHz, dominate the most energetic POD modes (excluding solar rotation), and are characterized by spatial patterns with typical scales of about 3 Mm. Patterns with larger typical scales of 10 Mm, are associated to p-modes oscillations at frequencies of about 3000 microHz.Comment: 8 figures in jpg in press on PR

Time series of high resolution photospheric spectra in a quiet region of the Sun. I. Analysis of global and spatial variations of line parameters

A 50 min time series of one-dimensional slit-spectrograms, taken in quiet sun at disk center, observed at the German Vacuum Tower Telescope (Observatorio del Teide), was used to study the global and spatial variations of different line parameters. In order to determine the vertical structure of the photosphere two lines with well separated formation heights have been considered. The data have been filtered of p-modes to isolate the pure convective phenomenon. From our studies of global correlation coefficients and coherence and phase shift analyzes between the several line parameters, the following results can be reported. The convective velocity pattern preserves structures larger than 1.0" up to the highest layers of the photosphere (~ 435 km). However, at these layers, in the intensity pattern only structures larger than 2.0" are still connected with those at the continuum level although showing inverted brightness contrast. This confirms an inversion of temperature that we have found at a height of ~140 km. A possible evidence of gravity waves superimposed to the convective motions is derived from the phase shift analysis. We interpret the behavior of the full width at half maximum and the equivalent width as a function of the distance to the granular borders, as a consequence of enhanced turbulence and/or strong velocity gradients in the intergranular lanes.Comment: 16 pages, 15 figures, 5 tables; Astronomy & Astrophysics, Volume 408, p.363-378, 200

Dynamics of the solar chromosphere. V. High-frequency modulation in ultraviolet image sequences from TRACE

We search for signatures of high-frequency oscillations in the upper solar photosphere and low chromosphere in the context of acoustic heating of outer stellar atmospheres. We use ultraviolet image sequences of a quiet center-disk area from the Transition Region and Coronal Explorer (TRACE) mission which were taken with strict cadence regularity. The latter permits more reliable high-frequency diagnosis than in earlier work. Spatial Fourier power maps, spatially averaged coherence and phase-difference spectra, and spatio-temporal k-f decompositions all contain high-frequency features that at first sight seem of considerable intrinsic interest but actually are more likely to represent artifacts of different nature. Spatially averaged phase difference measurement provides the most sensitive diagnostic and indicates the presence of acoustic modulation up to f=20 mHz (periods down to 50 seconds) in internetwork areas.Comment: 9 pages, 8 figure

Quiet-Sun imaging asymmetries in NaI D1 compared with other strong Fraunhofer lines

Imaging spectroscopy of the solar atmosphere using the NaI D1 line yields marked asymmetry between the blue and red line wings: sampling a quiet-Sun area in the blue wing displays reversed granulation, whereas sampling in the red wing displays normal granulation. The MgI b2 line of comparable strength does not show this asymmetry, nor does the stronger CaII 8542 line. We demonstrate the phenomenon with near-simultaneous spectral images in NaI D1, MgI b2, and CaII 8542 from the Swedish 1-m Solar Telescope. We then explain it with line-formation insights from classical 1D modeling and with a 3D magnetohydrodynamical simulation combined with NLTE spectral line synthesis that permits detailed comparison with the observations in a common format. The cause of the imaging asymmetry is the combination of correlations between intensity and Dopplershift modulation in granular overshoot and the sensitivity to these of the steep profile flanks of the NaI D1 line. The MgI b2 line has similar core formation but much wider wings due to larger opacity buildup and damping in the photosphere. Both lines obtain marked core asymmetry from photospheric shocks in or near strong magnetic concentrations, less from higher-up internetwork shocks that produce similar asymmetry in the spatially averaged CaII 8542 profile.Comment: Accepted by Astron & Astrophys. In each in-text citation the year links to the corresponding ADS abstract pag

Analysis of the solar cycle and core rotation using 15 years of Mark-I observations:1984-1999. I. The solar cycle

High quality observations of the low-degree acoustic modes (p-modes) exist for almost two complete solar cycles using the solar spectrophotometer Mark-I, located at the Observatorio del Teide (Tenerife, Spain) and operating now as part of the Birmingham Solar Oscillations Network (BiSON). We have performed a Fourier analysis of 30 calibrated time-series of one year duration covering a total period of 15 years between 1984 and 1999. Applying different techniques to the resulting power spectra, we study the signature of the solar activity changes on the low-degree p-modes. We show that the variation of the central frequencies and the total velocity power (TVP) changes. A new method of simultaneous fit is developed and a special effort has been made to study the frequency-dependence of the frequency shift. The results confirm a variation of the central frequencies of acoustic modes of about 450 nHz, peak-to-peak, on average for low degree modes between 2.5 and 3.7 mHz. The TVP is anti-correlated with the common activity indices with a decrease of about 20% between the minimum and the maximum of solar cycle 22. The results are compared with those obtained for intermediate degrees, using the LOWL data. The frequency shift is found to increase with the degree with a weak l-dependence similar to that of the inverse mode mass. This verifies earlier suggestions that near surface effects are predominant.Comment: Accepted by A&A October 3 200

The Quiet-Sun Photosphere and Chromosphere

The overall structure and the fine structure of the solar photosphere outside active regions are largely understood, except possibly important roles of a turbulent near-surface dynamo at its bottom, internal gravity waves at its top, and small-scale vorticity. Classical 1D static radiation-escape modelling has been replaced by 3D time-dependent MHD simulations that come closer to reality. The solar chromosphere, in contrast, remains ill-understood although its pivotal role in coronal mass and energy loading makes it a principal research area. Its fine structure defines its overall structure, so that hard-to-observe and hard-to-model small-scale dynamical processes are the key to understanding. However, both chromospheric observation and chromospheric simulation presently mature towards the required sophistication. The open-field features seem of greater interest than the easier-to-see closed-field features.Comment: Accepted for special issue "Astrophysical Processes on the Sun" of Phil. Trans. Royal Soc. A, ed. C. Parnell. Note: clicking on the year in a citation opens the corresponding ADS abstract page in the browse

Theory of Stellar Oscillations

In recent years, astronomers have witnessed major progresses in the field of stellar physics. This was made possible thanks to the combination of a solid theoretical understanding of the phenomena of stellar pulsations and the availability of a tremendous amount of exquisite space-based asteroseismic data. In this context, this chapter reviews the basic theory of stellar pulsations, considering small, adiabatic perturbations to a static, spherically symmetric equilibrium. It starts with a brief discussion of the solar oscillation spectrum, followed by the setting of the theoretical problem, including the presentation of the equations of hydrodynamics, their perturbation, and a discussion of the functional form of the solutions. Emphasis is put on the physical properties of the different types of modes, in particular acoustic (p-) and gravity (g-) modes and their propagation cavities. The surface (f-) mode solutions are also discussed. While not attempting to be comprehensive, it is hoped that the summary presented in this chapter addresses the most important theoretical aspects that are required for a solid start in stellar pulsations research.Comment: Lecture presented at the IVth Azores International Advanced School in Space Sciences on "Asteroseismology and Exoplanets: Listening to the Stars and Searching for New Worlds" (arXiv:1709.00645), which took place in Horta, Azores Islands, Portugal in July 201

The Position of High Frequency Waves with Respect to the Granulation Pattern

High frequency velocity oscillations were observed in the spectral lines Fe I 543.45nm and 543.29nm, using 2D spectroscopy with a Fabry- Perot and speckle reconstruction, at the VTT in Tenerife. We investigate the radial component of waves with frequencies in the range 8 - 22mHz in the internetwork, network and a pore. We find that the occurrence of waves do not show any preference on location and are equally distributed over down-flows and up-flows, regardless of the activity of the observed area in the line of Fe I 543.45nm. The waves observed in the lower formed line of Fe I 543.29nm seem to appear preferentially over down-flows.Comment: Article has 12 pages and 7 images. It is accepted in Solar Physics Journa