Fine Structure of the Magnetic Chromosphere: Near-Limb Imaging,Data Processing and Analysis of Spicules and Mottles

The origin and the dynamical evolution of spicules and mottles continue to be a highly interesting research subject. Using high-resolution H observations obtained with the Dunn Solar Telescope of the Sacramento Peak Observatory and an image processing technique for the enhancement of near-limb solar images, we study the dynamics of spicules and mottles as well as their relation. Our image-processing technique is based on the correction for the limb darkening and the use of a directionally sensitive operator, the `MadMax'. The temporal evolution of characteristic cases of spicules, dark and bright mottles, indicates an association between them and supports the suggestion that the magnetic field and probably related forces play a fundamental role in their generation and dynamics. We present characteristic cases of fine bright mottles, observable in the H far wings, that appear in close juxtaposition to dark mottles. The phenomenon appears to be common, suggesting that the velocities derived from marginal resolution spectroscopic observations could be underestimated. Typical examples of individual mottles crossing the solar limb further support the association between spicules and mottles. Finally we show images of arch-shaped mottles above the limb and especially on the disk, confirming the existence of chromospheric small loops. Our image-processing method substantially enhances near-limb observations and permits an insight into the studies of the very fine chromospheric structures.     

On vortex motion in chromospheric network boundaries

A large percent of spicules shows a surge-like behavior on the solar limb, supporting a multi-component model with twisted threads. The counterpart of limb spicules foot-points is investigated on the disk, and this re-examination indicates that the interpretation of transverse motion of off-limb spicules could directly be related to rotational motion at the feet of disk spicules. Related bright elements move and vary in brightness on the timescales of chromospheric oscillations. The motions are similar to random displacements of the bright elements along the network boundaries with amplitudes of about 200 to 400 km, with evidence of “spinning” or vortex motion. We find a clear evidence and in several cases for splitting process and suggesting a formation mechanism for doublet (or multi-component) spicules. A general inter chromospheric network velocity pattern with twists existing before the emergence and eruption of spicules seems to be required. In this paper a helical- kink mode propagation consistent with the new evidence of spicule multiple structure is presented and provides an explanation for the origin of the Alfvenic wave propagation along the spicules. The evidence of spinning spicules remains unclear from disk Dopplergram observations, we use a 3D time slice “column” diagrams (2D in x and y and time in z being the 3d dimension) by consecutive partly transparent slices put in perspective to show the rotational behavior at the chromospheric rosettes, and provide a wealth of information on spinning motion, helical wave propagation and splitting.     

Photospheric and Chromospheric Oscillations in Solar Faculae

The differences between physical conditions in solar faculae and those in sunspots and quiet photosphere (increased temperature and different magnetic field topology) suggest that oscillation characteristics in facula areas may also have different properties. The analysis of 28 time series of simultaneous spectropolarimetric observations in facula photosphere (Fe?i 6569 Å, 8538 Å) and chromosphere (Hα, Ca?ii 8542 Å) yields the following results. The amplitude of five-minute oscillations of line-of-sight (LOS) velocity decreases by 20?–?40% in facula photosphere. There are only some cases revealing the inverse effect. The amplitude of four- to five-minute LOS velocity oscillations increases significantly in the chromosphere above faculae, and power spectra fairly often show pronounced peaks in a frequency range of 1.3?–?2.5 mHz. Evidence of propagating oscillations can be seen from space?–?time diagrams. We have found oscillations of the longitudinal magnetic field (1.5?–?2 mHz and 5.2 mHz) inside faculae.     

New light on solar infrared intensity oscillations

The detection of large-amplitude infrared solar intensity oscillations in the 5-min region is reported. Using a broad-band multichannel photometer, the peak-to-peak intensity variation at 2.23 m is found to be as high as 2.4% for a circular aperture of 1 arc min and 0.8% in the full disk observations, i.e., remarkably higher than at the other four observed wavelength regions.The spatially-integrated power spectrum shows the 5-min oscillation plus a strong feature near 4 mHz. This feature coincides in frequency with the fundamental p-mode resonance of the chromosphere. However, a power-spectrum autocorrelation as well as a second-order Fourier transform of the data suggest that a high-frequency tail of the 5-min power spectrum is a more likely interpretation of this feature.     

Suspended spicules associated with the enhanced bright network in an active region

The progressive rotation from the limb onto the disk of a long-lived cluster of coaligned Hα spicules was observed at high spatial resolution on the fringe of a large complex of activity. Although individual spicules were steadily changing, the organized cluster appeared consistently suspended above the photospheric limb when viewed in the wings of Hα (|Δλ| ≈ 0.9 Å). The phenomenon is the counterpart near an active region of the dark band discovered in the quiet low chromosphere by Loughhead (1969). But in the present circumstances the effect is perceived as a weakening of emission, i.e. as a gap rather than an obscuration. The initial gap between the off-band spicules and the photospheric limb narrowed and closed in about 4 h. A day later, the cluster of spicules could be identified at the same wavelength with a cluster of elongated dark mottles, similarly coaligned; they were adjacent to, but not in contact with, a foreshortened patch of faculae. The persistence of the gap in this cluster, and its occurrence in isolated spicules reported here in quiet regions, imply that the phenomenon is an inherent property of spicules. It is proposed that the gap results from a spicule-generating process initiated above the temperature minimum.     

On the Possible Connection between Photospheric 5-Min Oscillation and Solar Flare Microwave Emission

Dynamic spectra of low-frequency modulation of microwave emission from solar flares are obtained. Data of 15 bursts observed in 1989–2000 with Metsähovi radio telescope at 37 GHz have been used. During 13 bursts a 5-min modulation of the microwave emission intensity was detected with the frequency of ν _I = 3.2± 0.24 (1σ) mHz. Five bursts revealed a 5-min wave superimposed on a ～1 Hz, linear frequency modulated signal generated, presumably, by coronal magnetic loop, this wave frequency is ν_fm = 3.38± 0.37 (1σ) mHz. Both intensity and frequency modulations detected are in good agreement with the data on 5-min global oscillations of photosphere and with the data on the umbral velocity oscillations observed in the vicinity of sunspots. Possible role of p-mode photospheric oscillations in modulation of microwave burst emission is discussed.     

Oscillations Above Sunspots

The 3-min oscillations in the sunspot atmosphere are discussed, based on joint observing with the Transition Region and Coronal Explorer – TRACE and the Solar and Heliospheric Observatory – SOHO. We find that the oscillation amplitude above the umbra increases with increasing temperature, reaches a maximum for emission lines formed close to 1–2× 10⁵ K, and decreases for higher temperatures. Oscillations observed with a high signal-to-noise ratio show deviations from pure linear oscillations. The results do not support the sunspot filter theory, based on the idea of a chromospheric resonator. Whereas the filter theory predicts several resonant peaks in the power spectra, equally spaced 1 mHz in frequency, the observed power spectra show one dominating peak, close to 6 mHz. Spectral observations show that the transition region lines contribute less than 13 percent to the TRACE 171 Å channel intensity above the umbra. The 3-min oscillations fill the sunspot umbra in the transition region. In the corona the oscillations are concentrated to smaller regions that appear to coincide with the endpoints of sunspot coronal loops, suggesting that wave propagation along the magnetic field makes it possible for the oscillations to reach the corona.     

Fine Structure of the Quiet Solar Chromosphere: Limb-Crossing Features

In this article we study chromospheric structures (spicules) crossing the solar limb in H images corrected for limb darkening. This correction enabled us to view structures both on the disk and beyond the limb in the same image. The observations were obtained at the Sacramento Peak Observatory at H±0.75 Å. The processed images reveal both bright and dark (relative to the local background) features crossing the limb. We also observed bushes (rosettes) crossing the limb, as well as structures indicating probably arch-shaped mottles beyond the limb.     

He I 1083 nm OSCILLATIONS AND DOWNFLOWS NEAR THE NORTH SOLAR POLE

Imaging spectroscopic data of the Sii 1082.7 nm (photospheric) and Hei 1083.0 nm (chromospheric) spectral lines were taken starting 22:05 UT on 23 July, 1996 with the NASA/NSO Spectromagnetograph at the NSO/Kitt Peak Vacuum Telescope. Observations were made near the north solar pole, with a field of view of 100 by 400 arc sec and with a temporal cadence of 53 s for 2 hr. Simple fitting to the line profiles measured the line position, depth, and spectral full-width at half-maximum. Power spectra of the velocity oscillations in each line were computed, and the oscillation power in the 2 to 6 mHz frequency band versus view angle was measured to search for horizontal oscillations. Horizontal waves are not detected to limiting amplitudes (1) of 22 m s^-1 in the chromosphere and 9 m s^-1 in the photosphere. These values are used to estimate limits for the energy flux into the corona. The amplitude of radial oscillations in the chromosphere is twice that of the photosphere. No statistically meaningful oscillation power is measured in the spectral parameters of the Hei line in the emission shell seen above the continuum limb. Finally, rapidly evolving red-shift events are observed in the Hei 1083 nm line on the disk; these events are some sort of coronal rain, and there are about 40 of these events on the solar disk at any moment.     

Multi-wavelength study of Network Bright Points near the limb*

We analyze the morphology of network bright points (NBPs), their relation to fine dark mottles and their temporal variations, using observations of a quiet region near the polar solar limb, obtained during the solar minimum. For our analysis we used an image-processing method for the selection and identification of NBPs. Further we constructed 'maximum power frequency' maps computing the power spectrum for each pixel of a field and selecting the frequency of maximum spectral power as the most representative oscillating frequency for this pixel. The morphological analysis of our data indicates that the enhancement of NBPs is part of a complicated process that is responsible for the formation of mottles. The analysis of the lifetimes of NBPs indicates that although the general patterns of NBPs remain constant for time intervals larger than an hour, the lifetime of individual bright points is of the order of 16 min. Furthermore, our results indicate an association of the lifetime of NBPs with that of mottles. From the analysis of temporal variations we confirm that the power of NBPs is smaller than the corresponding power for intra-network points at both the 3-min and the 5-min oscillating modes and in both the Mg b₁–0.4 Å and the continuum. Our analysis revealed enhanced power in the 7-min range, which is theoretically considered a candidate frequency for transverse waves related to NBPs at the base of the chromosphere (Kalkofen, 1997). However, we consider more likely that this period is related to the evolution of individual NBPs.     

Quasi-harmonic faraday-rotation fluctuations of radio waves when sounding the outer solar corona

A statistical analysis of the Faraday-rotation fluctuations (FRFs) of linearly polarized radio signals from the Helios 1 and Helios 2 spacecraft shows that the FRF time power spectra can be of three types. Spectra of the first type are well fitted by a single power law in the range of fluctuation frequencies 1–10 mHz. Spectra of the second type are a superposition of a power law and two quasi-harmonic components with fluctuation frequencies of about v₁=4 mHz (fundamental frequency) and v₂=8 mHz (second harmonic). Spectra of the third type exhibit only one of the two quasi-harmonic components against the background of a power law. The spectral density of the quasi-harmonic components can be represented by a resonance curve with a fairly broad [Δυ ≈ (0.5–1.3)υ_1,2] distribution relative to the v=v_{1, 2} peak. The intensity of the quasi-harmonic FRF has a radial dependence that roughly matches the radial dependence for the background FRF, while their period at the fundamental frequency is approximately equal to the period of the wellknown 5-min oscillations observed in the lower solar atmosphere. The fluctuations with 5-min periods in FRF records can be explained by the presence in the outer corona of isolated trains of Alfvén waves generated at the base of the chromosphere-corona transition layer and by acoustic waves coming from deeper layers.     

A study of the fine structure of the solar chromosphere at the limb

We have measured the dimensions, distances from the inner limb and the lifetime of bright mottles at the limb. Spicule lifetimes have been measured too. The problem of the dark band, lying just above the inner limb, as well as the relations between bright mottles and spicules are discussed.     

A search for periodicities from a ULX in the LINER galaxy NGC 4736

We report our findings on a new quasi-periodic oscillation (QPO) and a long period from the ultraluminous X-ray source (ULX) X-2 in nearby galaxy NGC 4736 based on the Chandra and XMM-Newton archival data. To examine the timing properties, power density spectra of the source have been obtained using Fast Fourier Transform. Also the spectral parameters of the source have been calculated by obtaining and fitting the energy spectra. Power density spectrum of this source reveals a QPO peak at \(0.73_{-0.14}^{+0.16}~\mathrm{mHz}\) with an fractional rms variability of 16 % using the Chandra data (in the year 2000-lower state of the source). The XMM-Newton data analysis indicates a peak at \(0.53_{-0.35}^{+0.09}~\mathrm{mHz}\) with a fractional rms variation of 5 % (in the year 2006-higher state of the source). These recovered QPOs overlap within errors and may be the same oscillation. In addition, we detect a long periodicity or a QPO in the Chandra data of about (5.2±2.0)×10^?5 Hz (～5.4 h) over 3 σ confidence level. If this is a QPO, it is the lowest QPO detected from a ULX. The mass of the compact object in ULX X-2 is estimated using the Eddington luminosity and a disk blackbody model in the range (10–80) M _⊙.     

MESSENGER Observations of Magnetohydrodynamic Waves in the Solar Corona from Faraday Rotation

During the declining phase of the longest solar minimum in a century, the arrival of the MESSENGER spacecraft at superior conjunction allowed the measurement of magnetohydrodynamic (MHD) waves in the solar corona with its 8 GHz radio frequency signal. MHD waves crossing the line of sight were measured via Faraday rotation fluctuations (FRFs) in the plane of polarization (PP) of MESSENGER’s signal. FRFs in previous observations of the solar corona (at greater offset distances) consisted of a turbulent spectrum that decreased in power with increasing frequency and distance from the Sun. Occasionally a spectral line, a distinct peak in the power spectral density spectrum around 4 to 8 mHz, was also observed in these early data sets at offset distances of about 5 to 10 solar radii. The MESSENGER FRF data set shows a spectral line at an offset distance between 1.55 to 1.85 solar radii with a frequency of 0.6±0.2 mHz. Other possible spectral lines may be at 1.2, 1.7, and 4.5 mHz; MHD waves with these same frequencies have been observed in X-ray data traveling along closed coronal loops at lower offset distances. An initial analysis of the MESSENGER spectral line(s) shows behavior similar to turbulent spectra: decreasing power with increasing frequency and distance from the Sun. Here we detail the steps taken to process the MESSENGER change in PP data set for the MHD wave investigation.     

Dynamics of Hα spicules according to spectral observations at various heights of the solar chromosphere

Almost simultaneous height sequences of 69 spicules in the Hα line have been studied. The spectra are obtained at six heights during 6 s on the east side of the solar disk with the 53-cm Lyot coronagraph of Abastumani Astrophysical Observatory. Radial velocities V _r, total intensities or equivalent widths W, full widths at half maximum of intensity (FWHM) at all heights are determined (about 300 profiles of the Hα line). It is found that:

1. Absolute values of radial velocities increase linearly with the height (see Equation (1));
2. variation of the sign of the radial velocity along single spicules was never observed.

These results combined with the findings on the spicules radial velocities and shifts obtained earlier (Kulidzanishvili and Nikolsky, 1978; Nikolsky and Platova, 1970) led us to the conclusion that the 5-min tangential oscillations of spicules involve the entire spicule at once. The intensity height scales for single spicules and for the chromosphere ‘in toto’ are determined; they turned out to be 2.5 × 10³ km and 1.9 × 10³ km respectively (see Equations (2) and (3)). The dependence curve of the Hα line half-widths Δλ on the height h is drawn. The Hα line half-width for those spicule groups which are traced at all heights (10 spicules) decreases with the height (Figure 4); for the majority (～60 spicules) it remains essentially constant. Non-thermal ‘turbulent’ velocities V _t, in Hα spicules are defined. A mean value of the ‘turbulent’ velocity V _t at T = 6000° appeared to be 20–30km s^?1. The hydrogen concentration in the spicules at 5000 km is 6 × 10¹¹ cm^?3.     

A statistical analysis of the SOT-Hinode observations of solar spicules and their wave-like behavior

Spicules are an important very dynamical and rather cool structure extending between the solar surface and the corona. They are partly filling the space inside the chromosphere and they are surrounded by a transition thin layer. New space observations taken with the SOT of the Hinode mission shed some light on their still mysterious formation and dynamics. Here we restrict the analysis to the most radial and the most interesting polar spicules situated at the base of the fast solar wind of coronal holes.We consider a first important parameter of spicules as observed above the solar visible limb: their apparent diameter as a function of the height above the limb which determines their aspect ratio and leads to the discussion of their magnetic origin using the flux tube approximation. We found that indeed spicules show a whole range of diameters, including unresolved “interacting spicules” (I-S), depending of the definition chosen to characterize this ubiquitous dynamical phenomenon occurring into a low coronal surrounding. Superposition effects along the line of sight have to be taken into account in order to correctly measure individual spicules and look at I-S. We take advantage of the so-called mad-max operator to reduce these effects and improve the visibility of these hair-like features. An excellent time sequence of images obtained above a polar region with the Hinode SOT through the HCaII filter with a cadence of 8 s was selected for analysis. 1-D Fourier amplitude spectra (AS) made at different heights above the limb are shown for the first time. A definite signature in the 0.18–0.25 Mm range exists, corresponding to the occurrence of the newly discovered type II spicules and, even more impressively, large Fourier amplitudes are observed in the 0.3–1.2 Mm range of diameters and spacing, in rough agreement with what historical works were reporting. Additionally, some statistically significant behavior, based on AS computed for different heights above the limb, is discussed.“Time slice or x–t diagrams” revealing the dynamical behavior of spicules are also analyzed. They show that most of spicules have multiple structures (similarly to the doublet spicules) and they show impressive transverse periodic fluctuations which were interpreted as upward kink or Alfven waves. Evidence of the helical motion in spicules is now well evidenced, the typical periods of the apparent oscillation being around 120 s. A fine analysis of the time-slice diagram as a function of the effective heights shows an interesting new feature near the 2 Mm height. We speculate on the interpretation of this feature as being a result of the dynamical specificities of the spicule helical motion as seen in these unprecedented high resolution HCaII line emission time series.     

Spectral investigation of chromospheric fine structure

H spectra and effectively simultaneous filtergrams were taken at the Fraunhofer Observatory on Capri with the 35 cm domeless Coudé. The spatial resolution of the 19 best spectra selected for analysis was estimated to be 1–2 arc sec. The comparison of several hundred H line profiles emitted by typical chromospheric structure elements with theoretical prediction yielded strong evidence to suggest that the chromosphere consists of two parts: A lower, rather uniform layer at rest superposed by clouds (condensations of great spatial variability) which constitute the well-known structure pattern of H filtergrams. For most image points the line-of-sight velocity, optical thickness, source function and Doppler broadening of these clouds could be determined. While the values of the latter three quantities were found to be similar to what Beckers (1968) has found for limb spicules the velocity of the bright and of the dark mottles is considerably smaller than to be expected if these features were the spicules as seen on the disk. However, our results do not rule out the possibility that the spicules rise at the centers of rosettes where they are difficult to detect.Mitteilung aus dem Fraunhofer Institut No. 105.     

Center-to-limb variation of Ca II line brightness oscillations in the solar chromosphere

We have investigated 15 time series of Ca II line spectrograms in quiet-Sun regions located at various distances from the disk center. Our goal is to reveal the center-to-limb variation of the brightness oscillations. The residual intensities at the centers of the Ca II K and 849.806-nm lines and the K index have been analyzed. We have considered separately two components of the chromospheric network. Our main result is that the power of the brightness oscillations in the chromosphere of the average quiet Sun decreases to the limb. This change for the boundaries of supergranulation cells (networks) is considerably larger than that for their inner parts (cells). It is mainly determined by the 5-min oscillations; the 3-min oscillations show virtually no center-to-limb variation. In addition to studying the dependence of the oscillation power on the heliocentric angle, we also consider other characteristics of the oscillatory regime of the chromosphere. For example, the low-frequency oscillations with periods longer than 700 s, which are inherent predominantly in the K line core in networks, have been separated into an isolated mode. No center-to-limb variation has been revealed for them. As a result of our discussion of the patterns found based on present-day publications on the chromosphere dynamics, we conclude that different mechanisms and sources of its heating can simultaneously make their contributions.     

The bright streaks in the Hα disk chromosphere

Evidence is presented demonstrating the existence of a type of chromospheric structure in the form of bright streaks. These are extensions across the solar disk of elongated bright mottles which originate in the central regions of clusters of mottles. They are best observed on good filtergrams at H ± 0.5 Å through comparison with filtergrams at other positions on the line profile. Their length can be as much as 200 sec of arc. The bright streaks appear to be predominantly horizontal loop structures, while the well-known spicules are mainly vertical structures. A bright streak may be well defined or rather diffuse along its length, and many of them are accompanied by darker boundaries or envelopes. It is usual to find a loop of bright streak bridging the central areas of two mottle clusters. It seems that the observed pattern in the space between the chromospheric network at H ± 0.5 Å results partly from the interactions of the bright streaks of different stages of evolution traversing the area in different directions.     

Spatial Distribution of Oscillations in Faculae

We find that oscillations of the LOS velocity in Hα vary within facula regions. The power spectra show that the contributions of low-frequency modes (1.2 – 2 mHz) increase at the network boundaries. Three- and five-minute periods dominate inside cells. The spectra of photospheric and chromospheric LOS-velocity oscillations differ for most faculae. We detected several cases where oscillations in faculae seem to propagate horizontally with phase velocities of 50 – 70 km s⁻¹. Their location in space and time coincided with the local maximum of the longitudinal magnetic field.