Prominence Cavity Regions Observed Using SWAP 174 Å Filtergrams and Simultaneous Eclipse Flash Spectra

SWAP images from PROBA2 taken at 174 Å in the Fe ix/x lines are compared with simultaneous slitless flash spectra obtained during the solar total eclipse of 11 July 2010. Myriad faint low-excitation emission lines together with the He i and He ii Paschen α chromospheric lines are recorded on eclipse spectra where regions of limb prominences are obtained with space-borne imagers. We analyzed a deep flash spectrum obtained by summing 80 individual spectra to evaluate the intensity modulations of the continuum. Intensity deficits are observed and measured at the prominences boundaries in both eclipse and SWAP images. The prominence cavities interpreted as a relative depression of plasma density, produced inside the corona surrounding the prominences, and some intense heating occurring in these regions, are discussed. Photometric measurements are shown at different scales and different, spectrally narrow, intervals for both the prominences and the coronal background.     

X-Ray Jet Dynamics in a Polar Coronal Hole Region

New X-ray observations of the north polar region taken from the X-ray Telescope (XRT) of the Hinode spacecraft are used to analyze several time sequences showing small loop brightenings with a long ray above. We focus on the formation of the jet and discuss scenarios to explain the main features of the events: the relationship with the expected surface magnetism, the rapid and sudden radial motion, and possibly the heating, based on the assumption that the jet occurs above a null point of the coronal magnetic field. We conclude that 2-D reconnection models should be complemented in order to explain the observational details of these events and suggest that alternative scenarios may exist.     

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.     

Increasing the Fine Structure Visibility of the Hinode SOT Ca ii H Filtergrams

We present an image processing technique, the improved OMC filter (also called Madmax), which selects maxima of convexities of intensity modulations of an image. The filter computes second derivatives of the image in multiple directions around each pixel. It is shown to be efficient for pattern recognition, and bright hair-like or small anisotropic features can be enhanced. The filter is tested on artificially generated images, and the effect of a different number of directions in which the second derivatives are calculated is evaluated. Ca ii H line images of spicules and prominences obtained with Hinode/SOT are also used to illustrate its efficiency on real data. It is shown how to make the choice of the sensitive parameters to be used in improving the image visibility.     

On Interpretation of the Radiation of Coronal Suprathermal Streams. II

This is the second part of the series of two articles which consider the effect of Compton scattering of the photospheric radiation on fast electrons of the coronal suprathermal streams. As compared to the previous part, a more realistic height-dependent model problem is treated. The results of numerical calculations for the mean frequency change and the proper cross-section for both the sunward and antisunward directed beams of electrons are given as a function of height and the slope angle. It is concluded that, depending on the angle between directions of the initial outburst and magnetic field, the scattering on the beams moving away the Sun may produce measurable drifts in frequency to shortwave as well as to longwave domains of the spectrum. At the same time, the sunward directed beams result only in an increase in the photon energy.     

MHD shock interactions in coronal structures

We consider the magnetohydrodynamic (MHD) interactions of solar coronal fast shock waves of flare and/or nonflare origin with the boundaries of coronal streamers and coronal holes. Boundaries are treated as MHD tangential discontinuities (TD). Different parameters of the observed corona are used in the investigation. The general case of the oblique interaction is studied.It is shown that a solar fast shock wave must be refracted usually as a fast shock wave inside the coronal streamer. For the special case of the velocity shear across TD, a slow shock wave is generated. On the contrary, the shock wave refracted inside the coronal hole is indeed a slow shock wave.The significance of different effects due to the interaction of fast and slow shock waves on the coronal magnetic field is noticed, especially at the time of a coronal mass ejection (CME). It is also shown, that an oblique fast MHD coronal shock wave may trigger an instability at the boundary of a streamer considered as a TD. It might have a relation with the observed process of abrupt disappearance of the streamer's boundary in the solar corona.On leave from the Academy of Sciences, Central Astronomical Observatory Pulkovo, 196140, St. Petersburg, Russia.     

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.     

Small-scale motions over concentrated magnetic regions of the quiet Sun

We have used a 5.5 min time-sequence of spectra in the Fe i lines 5576 (magnetically insensitive), 6301.5 and 6302.5 (magnetically sensitive) to study the association of concentrated magnetic regions and velocity in the quiet Sun. After the elimination of photospheric oscillations we found downflows of 100–300 m s ^–1, displaced by about 2 from the peaks of the magnetic field; this velocity is comparable to downflow velocity associated with the granulation and of the same order or smaller than the oscillation amplitude. Quasi-periodic time variations of the vertical component of the magnetic field up to ± 40% were also found with a period near 250 s, close to the values found for the velocity field. Finally we report a possible association of intensity maxima at the line center with peaks of the oscillation amplitude.     

Magneto-acoustic wave oscillations in solar spicules

Some observations suggest that solar spicules show small amplitude and high frequency oscillations of magneto-acoustic waves, which arise from photospheric granular forcing. We apply the method of MHD seismology to determine the period of kink waves. For this purposes, the oscillations of a magnetic cylinder embedded in a field-free environment is investigated. Finally, diagnostic diagrams displaying the oscillatory period in terms of some equilibrium parameters are provided to allow a comparison between theoretical results and those coming from observations.