Abstract. Movies of quiet Sun regions at disc centre obtained with the Coronal Diagnostic Spectrometer (CDS) onboard the SOHO spacecraft are used to study the properties of transient brightenings seen in the extreme ultraviolet (EUV), so-called blinkers, at three different temperatures sampled simultaneously in the chromospheric He I 584.3Å (2 × 10 4 K), the transition region O V 629.7Å (2.5 × 10 5 K) and coronal Mg IX 368.1Å (10 6 K) lines. Blinkers, here defined somewhat differently than in previous studies, were clearly detected in the O V and He I lines. Brightenings of the Mg IX line were also seen. A thorough analysis of blinker properties is carried out and their detailed properties are determined. Blinkers are found to be present in both bright (network) and dark (intranetwork) regions, but their number density is larger in the brighter areas (in O V) although the rest of their properties appear to be unaffected. The average sizes of brightenings range from 2.8 Mm 2 in Mg IX, 12.4 Mm 2 in He I to 23.5 Mm 2 in O V. The durations of blinkers are in the range 3-110 min, with the average durations being 23 min in He I, about 16 min in O V and 12 min in Mg IX. The frequency distributions of ratio of peak to background intensity, excess energy and size follow power laws with exponents <−5 for the intensity ratio, and between −1 and −3 for the other two parameters. The correlation coefficients between pairs of ratio, energy and size are at least 0.5, while other pairs of parameters describing the blinkers appear to be uncorrelated. The best correlation is between size and energy. The blinker durations exhibit a distribution whose form is compatible with a log-normal function. Finally, blinkers in the 3 lines (i.e. 3 temperature regimes) are poorly correlated; with the correlation coefficient being always less than 0.4. This suggests that to a large extent the transition region reacts independently of the corona and chromosphere to energy deposition, so that these parts of the atmosphere are at least partly decoupled from each other. This agrees with the expectations from models having separate transition-region loops, but contradicts the classical picture of the transition region, as being heated dominantly by energy conduction from the corona.
We present 90 cm VLA imaging of the COSMOS field, comprising a circular area of 3.14 square degrees at 8.0 ′′ × 6.0 ′′ angular resolution with an average rms of 0.5 mJy/beam. The extracted catalog contains 182 sources (down to 5.5σ), 30 of which are multi-component sources. Using Monte Carlo artificial source simulations we derive the completeness of the catalog, and we show that our 90 cm source counts agree very well with those from previous studies. Using X-ray, NUV-NIR and radio COSMOS data to investigate the population mix of our 90 cm radio sample, we find that our sample is dominated by active galactic nuclei (AGN). The average 90-20 cm spectral index (S ν ∝ ν α , where S ν is the flux density at frequency ν, and α the spectral index) of our 90 cm selected sources is -0.70, with an interquartile range of -0.90 to -0.53. Only a few ultra-steep-spectrum sources are present in our sample, consistent with results in the literature for similar fields. Our data do not show clear steepening of the spectral index with redshift. Nevertheless, our sample suggests that sources with spectral indices steeper than -1 all lie at z 1, in agreement with the idea that ultra-steep-spectrum radio sources may trace intermediate-redshift galaxies (z 1).
Abstract.We present an analysis of 14 ultraviolet emission lines belonging to different atoms and ions observed inside polar coronal holes and in the normal quiet Sun. The observations were made with the Coronal Diagnostic Spectrometer (CDS) onboard the Solar and Heliospheric Observatory (SOHO). This study extends previous investigations made with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer to higher temperatures. We compare line intensities, shifts and widths in coronal holes with the corresponding values obtained in the quiet Sun. While all lines formed at temperatures above 7 × 10 5 K show clearly the presence of the hole in their intensities, differences in line width are more subtle, with cooler lines being broader in coronal holes, while hotter lines tend to be narrower. According to the present data all lines are blueshifted inside the coronal hole compared to the normal quiet Sun. Almost all the lines formed between 80 000 K and 600 000 K (i.e. transition-region lines) show a correlation between blueshifts and brightness within coronal holes. This is in agreement with the conclusion reached by Hassler et al. (1999) that the fast solar wind emanates from the network and supports our previous study (Stucki et al. 2000b). For coronal lines, this trend seems to be reversed.
Abstract. Explosive events and blinkers are two observational classes of transients seen on the quiet Sun and an investigation of the significance of and relationship between such events may be critical for understanding basic processes at work in the solar atmosphere. We analysed five time-series spectra of the quiet Sun of transition region O V 629 Å, O VI 1032 Å and O VI 1038 Å lines. We investigated how often explosive events occurred during the course of a blinker at the same location and found that slightly more than a half of all explosive events happened during about one third of all blinkers. In some cases during a blinker more than one explosive event was registered. The largest average maximum relative intensity enhancement was for blinkers with explosive events, followed by the blinkers without explosive events, with the least being for the explosive events which did not happen during the course of a blinker. Due to these differences among the maximum enhancements between these events we suppose that blinkers and explosive events are two independent phenomena. Intensity light curves of blinkers show that events can be separated into two classes, specifically: 1) simple blinkers with smooth increase in intensity having only one significant peak, and 2) complex blinkers characterised by multiple (2−4) significant peaks. These two classes were equally represented when frequencies of their occurrences were averaged over five analysed data-sets. The analysis of the line profile parameters and their correlations for these two classes did not give any result which could further distinguish between them. During blinkers the intensity peaks mostly at the middle of their durations, while the line width peaks somewhat earlier than the intensity. This was a general character for both simple and complex blinkers, as well as for all explosive events.
Abstract.A number of small-scale, globally distributed solar transient event-types have been reported in the literature. Their potential role in fundamental processes in the solar atmosphere, such as coronal heating and wind acceleration, is under active investigation. However, the event-types, such as those known as blinkers, explosive events, EUV (extreme-UV) network and cell brightenings, network flares, heating events, nanoflares and EUV brightenings are basically classifications which are driven to a large extent by different observational techniques and different instruments rather than the identification of a clear differing physical phenomenon. We investigate the different instrumental and technique limitations and attempt to identify any unification of the reported quiet-Sun transient, small-scale phenomena. We find that once observational techniques have been considered, a number of the different classifications appear to be the same. This suggests that events known as blinkers, network and cell brightenings and EUV brightenings are the same event-type. We suggest that the term blinker be used as a generic term to describe these events. However, there appears to be little evidence that blinkers and explosive events are directly related. Furthermore, although a small percentage of blinkers and nanoflares/heating events appear to be related to one another, these events pose a number of important questions suggesting that either (i) blinkers and nanoflare/heating events are all created by the same mechanism, i.e. for some blinker events, the conditions are such that higher temperatures are found, or (ii) there are two types of event, including the "traditional" blinker which is effectively a transition region brightening driven by a density or filling factor enhancement, and a mini-flare-like event which reaches higher temperatures, presumably driven by reconnection.
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