Plage Areas Research Articles

TURBULENT DIFFUSION IN THE PHOTOSPHERE AS DERIVED FROM PHOTOSPHERIC BRIGHT POINT MOTION

On the basis of observations of solar granulation obtained with the New Solar Telescope of Big Bear Solar Observatory, we explored proper motion of bright points (BPs) in a quiet-sun area, a coronal hole, and an active region plage. We automatically detected and traced BPs and derived their mean-squared displacements as a function of time (starting from the appearance of each BP) for all available time intervals. In all three magnetic environments, we found the presence of a super-diffusion regime, which is the most pronounced inside the time interval of 10-300 s. Super-diffusion, measured via the spectral index, γ, which is the slope of the mean-squared displacement spectrum, increases from the plage area (γ = 1.48) to the quiet-sun area (γ = 1.53) to the coronal hole (γ = 1.67). We also found that the coefficient of turbulent diffusion changes in direct proportion to both temporal and spatial scales. For the minimum spatial scale (22 km) and minimum time scale (10 s), it is 22 and 19 km2 s–1 for the coronal hole and the quiet-sun area, respectively, whereas for the plage area it is about 12 km2 s–1 for the minimum time scale of 15 s. We applied our BP tracking code to three-dimensional MHD model data of solar convection and found the super-diffusion with γ = 1.45. An expression for the turbulent diffusion coefficient as a function of scales and γ is obtained.

Spicule Dynamics over a Plage Region

We studied spicular jets over a plage area and derived their dynamic characteristics using Hinode Solar Optical Telescope (SOT) high-resolution images. A target plage region was near to the west limb of the solar disk. This location permitted us to study the dynamics of spicular jets without any overlapping effect of spicular structures along the line of sight. In this work, to increase the ease with which we could identify spicules on the disk, we applied the image processing method ‘MadMax’ developed by Koutchmy et al. (1989). It enhances fine, slender structures (like jets), over a diffuse background. We identified 169 spicules over the target plage. This sample permited us to derive statistically reliable results regarding spicular dynamics. The properties of plage spicules can be summarized as follows: (1) In a plage area, we clearly identified spicular jet features. (2) They were shorter in length than the quiet region limb spicules, and followed a ballistic motion under constant deceleration. (3) The majority (80%) of the plage spicules showed a cycle of rise and retreat, while 10% of them faded out without a complete retreat phase. (4) The deceleration of the spicule was proportional to the velocity of ejection (i.e., the initial velocity).

COMPARISON AMONG Ca II K SPECTROHELIOGRAM TIME SERIES WITH AN APPLICATION TO SOLAR ACTIVITY STUDIES

Various observatories around the globe started regular full-disk imaging of the solar atmosphere in the Ca II K line in the early decades of the 20th century. The archives made by these observations have the potential of providing far more detailed information on solar magnetism than just the sunspot number and area records to which most studies of solar activity and irradiance changes are restricted. We evaluate the image quality and contents of three Ca II K spectroheliogram time series, specifically those obtained by the digitization of the Arcetri, Kodaikanal, and Mt Wilson photographic archives, in order to estimate their value for studies focusing on timescales longer than the solar cycle. We analyze the quality of these data and compare the results obtained with those achieved for similar present-day observations taken with the Meudon spectroheliograph and with the Rome-PSPT. We also investigate whether image-segmentation techniques, such as those developed for identification of plage regions on present-day Ca II K observations, can be used to process historic series. We show that historic data suffer from stronger geometrical distortions and photometric uncertainties than similar present-day observations. The latter uncertainties mostly originate from the photographic calibration of the original data and from stray-light effects. We also show that the image contents of the three analyzed series vary in time. These variations are probably due to instrument changes and aging of the spectrographs used, as well as changes of the observing programs. The segmentation technique tested in this study gives reasonably consistent results for the three analyzed series after application of a simple photographic calibration. Although the plage areas measured from the three analyzed series differ somewhat, the difference to previously published results is larger.

OBSERVATIONAL EVIDENCE FOR CORONAL TWISTED FLUX ROPE

Multi-instrument data sets of NOAA AR10938 on 2007 January 16 (e.g., Hinode, STEREO, GOES, MLSO, and ISOON Hα) are utilized to study the fine structure and evolution of a magnetic loop system exhibiting multiple crossing threads, whose arrangement and individual shapes are very suggestive of individual field lines in a flux rope. The footpoints of the magnetic threads are closely rooted into pores and plage areas. A C-class flare recorded by GOES at approximately 2:35 UT near one of the footpoints of the multi-thread system (along with a wisp of loop material shown by EUV data) led to the brightening of the magnetic structure revealing its fine structure with several threads that indicate a high degree of linking (suggesting a left-handed helical pattern as shown by the filament structure formed later on). EUV observations by Hinode/EIS of hot spectral lines at 2:46 UT show a complex structure of coronal loops. The same features were observed about 20 minutes later in X-ray images from Hinode/XRT and about 30 minutes further in EUV images of STEREO/SECCHI/EUVI with much better resolution. Hα and 304 A images revealed the presence of several filament fibrils in the same area. They evolved a few hours later into a denser structure seemingly showing a helical pattern, which persisted or several days forming a segment of a larger-scale filament. The present observations provide an important indication for a flux robe as a precursor of a solar filament.

On oscillations found in an active region with EIS on Hinode

Aims. Using temporal series (40 �� slot) data in the Fe xii 195 A line from the extreme-ultraviolet imaging spectrometer (EIS) on board the Hinode satellite, we seek to carry out a statistical study of oscillations in an active region. Methods. Using wavelet techniques, we measure the frequency and duration of oscillations found in the time series data. Stokes I and V data from Hinode’s SOT are used to measure photospheric magnetic fields, while an EIS raster is used to provide estimates of LOS velocity and electron density. The measured frequencies are displayed as maps in different bands to aid their analysis. Results. Oscillations over a broad range of frequencies (2–154 mHz) are found throughout the active region at the temperature of Fe xii (log T = 6.1 K). Oscillations with frequencies between 2–8 mHz are concentrated in bright plage areas, while oscillations of higher frequency preferentially group at the edges of these areas. Evidence is found for harmonics on the boundary of the active region loop, indicating the presence of standing waves. From a measurement of the lengths and electron density of loops appearing in our active region field-of-view (FOV), together with a knowledge of the periods (frequencies) of the oscillations within the area of these loops, we find a magnetic field value of between 12.2 ± 2.08−18.9 ± 3.40 G, assuming standing fast kink waves. Conclusions. We conclude that waves, which the observed oscillations are signatures of, are common in the active region studied, over a broad range of frequencies. The change in the distribution of the oscillations present at frequencies less than 8 mHz and those at all frequencies above this indicates a variation of the wave mode with frequency, e.g., possibly from slow magnetoacoustic waves in the bright plage regions to fast magnetoacoustic waves at the boundaries of these regions. The preference for higher frequency oscillations to cluster on the edges of regions of high magnetic field is considered indicative of resonant absorption. Based on our measurements of magnetic field, we conclude that the oscillations found by us in the active region loop are fast kink waves. Previous studies such as O’Shea et al. (2001 )a ndIreland et al. (1999) carried out statistical studies of the oscillatory characteristics of active regions using transition region and coronal lines observed with CDS on board the SOHO spacecraft. Here, we seek to carry out a similar statistical study but with the superior temporal resolution and imaging capabilities of the EUV imaging spectrometer (EIS) on board the Hinode spacecraft (Culhane et al. 2007). In this work we will present a global statistical analysis of all intensity oscillations, measured in a Fe xii 195 A line, present in an active region on the disk. From this statistical study we will seek to identify different wave modes present in the active region and its structures. We note that O’Shea et al. (2001) found evidence for non-compressive (transverse) waves to be more prevalent in coronal lines (at temperatures between Log T = 6.0−6.3 K) than in lower temperatures lines. We also, therefore, seek to find further evidence for these non-compressive waves. From the measured oscillation frequencies (periods), together with the observed physical quantities of loop length, radius and electron density, we will seek to provide estimates of the magnetic field in the corona. We note that Nakariakov & Ofman (2001), using a similar technique to that which will be used by us, obtained values of 13 ± 9 G for global standing kink waves in an active region.

Small-scale flows in SUMER and TRACE high-cadence co-observations

We report on the physical properties of small-scale transient flows observed simultaneously at high cadence with the SUMER spectrometer and the TRACE imager in the plage area of an active region. Our major objective is to provide a better understanding of the nature of transient phenomena in the solar atmosphere by using high-cadence imager and spectrometer co-observations at similar spatial and temporal resolution. A sequence of TRACE Fe IX/X 171 A and high-resolution MDI images were analysed together with simultaneously obtained SUMER observations in spectral lines covering a temperature range from 10 000 K to 1 MK. We reveal the existence of numerous transient flows in small-scale loops (up to 30 Mm) observed in the plage area of an active region. These flows have temperatures from 10 000 K (the low temperature limit of our observations) to 250 000 K. The coronal response of these features is uncertain due to a blending of the observed coronal line Mg X 624.85 A. The duration of the events ranges from 60 s to 19 min depending on the loop size. Some of the flows reach supersonic velocities. The Doppler shifts often associated with explosive events or bi-directional jets can actually be identified with flows (some of them reaching supersonic velocities) in small-scale loops. Additionally, we demonstrate how a line-of-sight effect can give misleading information on the nature of the observed phenomena if only either an imager or a spectrometer is used.

The Effect of Abnormal Granulation on Acoustic Wave Travel Times and Mode Frequencies

Observations indicate that in plage areas (i.e. in active regions outside sunspots) acoustic waves travel faster than in the quiet Sun, leading to shortened travel times and higher p-mode frequencies. Coupled with the 11-year variation of solar activity, this may also explain the solar cycle variation of oscillation frequencies. While it is clear that the ultimate cause of any difference between the quiet Sun and plage is the presence of magnetic fields of order 100 G in the latter, the mechanism by which the magnetic field exerts its influence has not yet been conclusively identified. One possible such mechanism is suggested by the observation that granular motions in plage areas tend to be slightly “abnormal”, dampened compared to the quiet Sun.

The Mn I 539.47 nm Line Variation in Solar Active Regions

It is a well-established fact that the Mn I 539.47 nm line exhibits significant cycle dependence similar to lines arising in the chromosphere which are affected by non-thermal heating in the chromospheric plages. Among these lines the case of the Mn I 539.47 nm line seems unique. It is of photospheric origin on the basis of theoretical calculations but its cyclic dependence hints at a chromospheric nature. The present work provides further evidence to both connections. The line exhibits asymmetry features and a center-to-limb variation as though influenced by the photospheric granulation. On the other hand an enhancement of the central intensity has been detected in a well identifiable Ca plage area. The line seems to be a promising candidate as an irradiance variation indicator.

The Signature of Solar Activity in the Infrared Spectral Irradiance

The effects of solar activity on the spectral irradiance have been studied using atmospheric semiempirical models developed from observations of the various surface features observed on the Sun. From these models, it has been the long-standing belief that the contributions of active regions to solar irradiance at wavelengths in the range of 1.2-3 μm is negative; that is, their net effect reduces the overall solar irradiance at these wavelengths by a small amount. For verifying the validity of the current modeling, we use the observed plage areas to compute the solar irradiance variations at two bands (centered at 0.516 and 1.553 μm wavelength). We compare in detail the predictions of the models by Fontenla et al. with measurements of the solar spectral irradiance variations obtained by the Spectral Irradiance Monitor instrument aboard the Solar Radiation and Climate Experiment spacecraft. The data comparison extends over a 6 month period in 2003 that covers several solar rotations. The comparison indicates that the variations in the short wavelength display good agreement between models and observations but also that the current models of IR spectral irradiance are inaccurate at the long wavelength. This disagreement in the IR may be due to the fact that, contrary to the current model assumptions, the presence of active regions on the disk increases the spectral irradiance at all wavelengths, even near 1.6 μm. Consequently, the modeling of solar spectral irradiance at wavelengths in the range around 1.6 μm has to be revised to match the new observations.

Evolutions of various solar indices around sunspot maximum and sunspot minimum years

Abstract. The smoothed monthly sunspot numbers showed that in many solar cycles, (a) during years around sunspot maxima, there was only one prominent maximum, but in some cycles there was a broad plateau. If the beginning and end of these are termed as first and second maxima (separated by several months), the first maximum was generally the higher one, and the valley in between was very shallow. Solar indices at or near the photosphere generally showed similar structures with maxima matching with sunspot maxima within a month or two. Indices originating in the chromosphere and above showed two peaks in roughly the same months as sunspots (with some exceptions, notably the Coronal green line, and the Total Solar Irradiance). Yet often, the second maximum was larger than the first maximum, and the valley between the two maxima was deeper, as compared to sunspot maxima, and (b) during years around sunspot minima, the smoothed sunspot minimum could be sharp and distinct, lasting for a month or two, or could spread over several months. Among the indices originating at or near the photosphere, the Ca K line intensity showed good matching with sunspots, but the Ca Plage area, the Sunspot Group Area, and the solar magnetic fields seemed to show minima earlier than the sunspots, indicating that these activities died out first. These also showed recoveries from the minima later than sunspots. Most of the other indices originating in the chromosphere and corona attained minima coincident with sunspot minima, but in some cases, minima earlier than sunspots were seen, while in some other cases minima occurred after the sunspot minima. Thus, the energy dissipation in the upper part of the solar atmosphere sometimes lagged or led the evolution of sunspots near sunspot minimum. In a few cases, after the minimum, the indices recovered faster than the sunspots. In general, the chromospheric indices seemed to evolve similar to sunspots, but the evolution of coronal indices was not always similar to sunspots, and may differ considerably between themselves.Key words. Solar physics, astrophysics and astronomy (Corona and transition region; Magnetic fields; Photosphere and chromosphere)

Wavelet Analysis of Hidden Periodicities in Some Indexes of Solar Activity

The wavelet transform is used to analyze periodicities in the Wolf numbers, calcium plage areas, coronal intensities, net solar magnetic field intensities, fluxes of solar radiation at a wavelength of 10.7 cm, and geomagnetic planetary indexes. The periodicities with scales varying between 5 and 100 years have been detected. A correlation between the time series of temperature in Tomsk and solar activity is analyzed. No correlation is found between the initial series; however, a significant correlation is established between the periodicities in the time series of temperature and individual solar activity indexes.

What is Moss

TRACE observations of active regions show a peculiar extreme ultraviolet (EUV) emission over certain plage areas. Termed ‘moss’ for its spongy, low-lying, appearance, observations and modeling imply that the phenomenon is caused by thermal conduction from 3–5 MK coronal loops overlying the plage: moss is the upper transition region emission of hot coronal loops. The spongy appearance is due to the presence of chromospheric jets or ‘spicules’ interspersed with the EUV emission elements. High cadence TRACE observations show that the moss EUV elements interact with the chromospheric jets on 10 s time scales. The location of EUV emission in the moss does not correlate well to the locations of underlying magnetic elements in the chromosphere and photosphere, implying a complex magnetic topology for coronal loop footpoint regions. We summarize here the key observations leading to these conclusions and discuss new implications for understanding the structuring of the outer solar atmosphere.

What Determines the Relative Areas of Spots and Faculae on Sun‐like Stars?

We analyze newly digitized Ca K plage area data extending back to 1915, and also the white-light facular area data beginning in 1874, to investigate further our earlier finding that the area ratio of faculae to spots decreases at increasing activity levels. We find that this ratio decreases in plage as well as facular data, so it cannot be an artifact of the visibility function of limb faculae. The decrease is also accentuated in daily data, compared to annual means; we explain this as a consequence of the different dependences of facular, plage, and spot lifetimes upon their emergent magnetic flux. From this we show that subphotospheric field properties are more likely to determine this ratio, rather than photospheric field diffusion rates. Systematic, cycle-to-cycle variations in its value suggest an origin in fluctuations of the field generation mechanism; specifically, a mechanism that produces a positive correlation between magnetic flux generation efficiency, and relative power in the spatial spectrum at low frequencies. Our results also suggest that main-sequence stars about 50% more magnetically active than the present Sun might exhibit ratio values an order of magnitude lower than current solar values. This evidence strengthens our earlier argument that a rapid shift toward dark photospheric structures in both active regions and network provides the most likely explanation of the recently reported sharp increase of photometric variability in late-type stars somewhat more active than the Sun.

Evolution of Chromospheric Structures Derived from CaiiK Spectroheliograms: Implications for Solar Ultraviolet Irradiance Variability

Over 1400 National Solar Observatory Ca II K spectroheliograms are analyzed for solar structure evolution for the years 1980, 1985, 1987, 1988, 1989, and 1992 through 1996 July with about a 50% time coverage. This time period includes the maximum of solar cycle 21 and almost all phases of solar cycle 22. The spectral bandpass and spatial resolution of these images are 0.5 A and about 85, respectively. The plage, enhanced-network, active-network, and quiet-Sun features are identified on each image with an algorithm that uses criteria of intensity, size, filling factor, and continuity. This decomposition leads to time series of the plage and network areas, and their spatial distribution on the solar disk, which describe the solar cycle evolution of these structures. Thus, either the resultant structure masks or structure time series can be used for modeling the solar irradiance at other wavelengths. We cannot accurately identify all of the active network on the solar disk in Ca II K because the active-network intensities are not much greater than those of the quiet Sun. Therefore, we identify only the brighter active-network structures. However, the active network we identify on the Ca II K image can be used as a proxy for active-network evolution over the solar cycle. Many results that are useful for modeling solar irradiance variability are also presented. We find that the intensity contrasts (ratio of structure intensity to quiet Sun intensity) of the plage, enhanced network, and active network, averaged over the solar disk, are 1.95, 1.51, and 1.33, respectively. These contrasts remain essentially constant over the solar cycle. As expected, we find that the plage and the enhanced-network time series show a strong 27 day rotational modulation, but we also find that the active network can have a weak rotational modulation despite its wider longitudinal dispersion. The plage and enhanced network typically cover about 13% and 10%, respectively, of the solar disk during solar-maximum time periods. During solar moderate and minimum activity levels, the total plage and enhanced-network areas can reach zero, but the active network can still cover a large portion of the solar disk.

Large-Scale Structure of the Atmosphere of the Quiet Sun, Coronal Holes, and Plages as Deduced by Tomography Study

We present here the results of emission tomography studies, based on a new differential deconvolution method (DDM) of Laplace transform inversion, which we use for reconstruction of the coronal emission measure distributions in the quiet Sun, coronal holes and plage areas. Two methods are explored. The first method is based on the deconvolution of radioemission brightness spectra in a wide wavelength range (1 mm–100 cm) for temperature profile reconstructions from the corona to the deeper chromosphere. The second method uses radio brightness measurements in the cm–dm range to give a coronal column emission measure (EM).

The Behavior of solar magnetic plages measured from Mt. Wilson observations between 1915–1984

We describe the digitization and reduction of the daily solar chromospheric spectroheliograms obtained in the K‐line of Ca II between 1915–1984 at Mt. Wilson Observatory. Our results provide the first reliable information on the behavior of solar magnetic plage area prior to 1947, thus extending by almost 70% the length of the primary data base needed to model past total, UV and EUV solar irradiances. Comparison with other solar activity indices confirms the remarkably linear relation between plage areas, and sunspot number and area, found in the post‐1947 data. Our analysis also shows a markedly non‐linear relation between the areas of chromospheric plage, and of photospheric white‐light faculae. We provide an explanation of this surprising nonlinearity between measurements of two magnetic structures that are known to be closely related in the sun's atmosphere.

The curious case of the greenwich faculae

We analyze the record of facular areas compiled by the Royal Greenwich Observatory (RGO) from daily white-light observations between 1874 and 1976. Curiously, the relative amplitudes of the three largest sunspot cycles 17, 18, and 19 in this record are reversed when they are ranked by facular area. We show that this negative correlation arises from a general decrease of the ratioA F/A S, of facular to sunspot area, with increasingA S. Within a given cycle,A F/A Sdecreases in active regions of largeA S, butA F/A Sis also lower at allA S, in cycles of higher peak amplitude inA S. This decrease ofA F/A Sin large spot groups is consistent with its decrease in younger, more active solar-mass stars, and it may explain why stars only slightly more magnetically active than the Sun tend to exhibit much greater variability in broad-band photometry. We suggest that the physical explanation is an increased spatial filling factor of magnetic flux, favoring formation of sunspots over faculae. We also explain why the decrease inA F/ASis not seen in the disc-integrated Ca K plage areas, nor in theF10.7 microwave index, both of which exhibit remarkable linearity when plotted against smoothed sunspot area. This explanation suggests how complementary data on faculae and plages from RGO and Mt. Wilson could be used to improve empirical models of total irradiance variation, extending back to 1874.

LAMBDA PERSEI: AN UNUSUAL B9 IV STAR

ABSTRACT Spectrograms of Lambda Per obtained at 30 A/mm in the red region show weak emission peaks in the core of the H-alpha absorption line. The V/R for the emission components changed from about 1.0 on October 28 1990 UT to less than 1.0 on October 29, 1990 UT. The midpoints of absorption lines from He I, Si II, Fe II, and the midpoints of the H-alpha profile measured at residual intensities between 0.80 and 0.96 are shifted slightly shortward of the wavelengths suggested by the catalog radial velocities of Lambda Per and by the wavelength of the sharp central absorption core of H-alpha. It is plausible that the emission features originate in plage areas on the surface of Lambda Per, and that the aspect of the plages presented to the observer changes as the star rotates. Plages imply the presence of surface magnetic fields.

Reconstructing the emission at 10.7 cm from individual active regions in terms of optical data

A detailed statistical analysis of F10.7 emission and its relation to manifestations of optical activity as evidenced by plages and sunspots has been carried out. The data were selected from days with complete radio and optical coverage during the years 1976 to 1982. Starting with individual peaks as shown by east–west scans at 10.7 cm, we find an excellent correlation (0.93) with the reported spot area for the individual active region (AR) from which the radio flux emanated. The correlation with the reported plage area is 0.84; the combination of plage and spot areas (0.96) only slightly improves the correlation with spot area alone. An equally close correlation of F10.7 exists with the 3/4 power of spot areas. These results suggest that the F10.7 emission is tightly bound to the magnetic flux of ARs. Center‐to‐limb variations indicate that the emitting region at 10.7 cm can be described as an optically thin volume with, in first order, spherical symmetry. Next, calculated on the basis of the sum of individual areas, one finds the expected total emission from the Sun for a carefully selected sample to be of comparable quality of correlation with spot and plage areas (0.88 and 0.80, respectively). Several tests were carried out to determine time variations of the reconstruction parameters, in particular, concerning a possible dependence of the optical data on the choice of observing station. We were unable to find in the data any evidence for the existence of a significant third component in the F10.7 emission, that is, a component that is not localized yet still follows roughly the solar cycle variation (network contribution). Finally, we compared for our sample days these findings with published daily indices for solar activity, that is, plage index and relative sunspot number, and conclude that the latter, with a correlation of 0.94, is overall the most trustworthy stand‐in for F10.7 data.

Stokes profile analysis and vector magnetic fields. V - The magnetic field structure of large sunspots observed with Stokes II

Results are presented on the analysis of Stokes II polarimeter data on four large sunspots during 1980, yielding the vector magnetic field structure and thermodynamic parameters of these sunspots. It is found that the magnetic field occupies significant fractions of the area in both light and dark penumbral filaments, and that the intrinsic field does not fall below 600 G, either in the sunspot or in the surrounding plage areas, and is typically 700-800 G at the outer edge of the penumbra. The radial variation of the vector field in these sunspots does not agree with the return-flux models of Fla et al. (1982). The variation of the poloidal field strength and inclination with distance from the center of the symmetric sunspots exhibits little nonpotential character and is well represented by the potential field of a buried dipole. 37 refs.