scholarly journals Increase in the Amplitude of Line-of-sight Velocities of the Small-scale Motions in a Solar Filament before Eruption

2017 ◽  
Vol 843 (2) ◽  
pp. L24 ◽  
Author(s):  
Daikichi Seki ◽  
Kenichi Otsuji ◽  
Hiroaki Isobe ◽  
Takako T. Ishii ◽  
Takahito Sakaue ◽  
...  
1970 ◽  
Vol 36 ◽  
pp. 281-301 ◽  
Author(s):  
Edward B. Jenkins

Absorption at the Lyman-α transition from interstellar neutral hydrogen has been observed in the ultraviolet spectra of 18 nearby O and B stars. Radiation damping is the dominant cause of line broadening, which makes the derived line-of-sight column densities proportional to the square of the observed equivalent widths. An average hydrogen density on the order of 0.1 atom cm−3 has been found for most of the stars observed so far. This is in contrast to the findings from surveys of 21-cm radio emission, which suggest 0.7 atom cm−3 exists in the local region of the Galaxy. Several effects which might introduce uncertainties into the Lyman-α measurements are considered, but none seems to be able to produce enough error to explain the disagreement with the 21-cm data. The possibility that small-scale irregularities in the interstellar gas could give significantly lower values at Lyman-α is explored. However, a quantitative treatment of the factor of ten discrepancy in Orion indicates the only reasonable explanation requires the 21-cm flux to come primarily from small, dense, hot clouds which are well separated from each other. The existence of such clouds, however, poses serious theoretical difficulties.


2020 ◽  
Vol 500 (1) ◽  
pp. 684-695
Author(s):  
Aabha Monga ◽  
Rahul Sharma ◽  
Jiajia Liu ◽  
Consuelo Cid ◽  
Wahab Uddin ◽  
...  

ABSTRACT The partial eruption of a filament channel with bifurcated substructures is investigated using data sets obtained from both ground-based and space-borne facilities. Small-scale flux reconnection/cancellation events in the region triggered the pile-up of ambient magnetic field, observed as bright extreme ultraviolet (EUV) loops in close proximity to the filament channel. This led to the formation of a V-shaped cusp structure at the site of interaction between the coalesced EUV loops and the filament channel, with the presence of distinct plasmoid structures and associated bidirectional flows. Analysis of imaging data from SDO/AIA further suggests vertical splitting of the filament structure into two substructures. The perturbed upper branch of the filament structure rose up and erupted with the onset of an energetic GOES M1.4 flare at 04:30 ut on 2015 January 28. The estimated twist number and squashing factor obtained from non-linear force free-field extrapolation of the magnetic field data support the vertical split in the filament structure with high twist in the upper substructure. The loss in equilibrium of the upper branch due to torus instability implies that this is a potential triggering mechanism for the observed partial eruption.


2012 ◽  
Vol 2012 ◽  
pp. 1-10 ◽  
Author(s):  
Attaphongse Taparugssanagorn ◽  
Matti Hämäläinen ◽  
Jari Iinatti

We present statistical models for wideband and ultrawideband (UWB) radio channels in a working machine cabin environment. Based on a set of measurements, it was found that such a small and confined space causes mostly diffuse multipath scattering rather than specular paths. The amplitude of the channel impulse responses in the wideband case is mostly Rayleigh distributed small-scale fading signal, with only a few paths exhibiting Ricean distributions, whereas the ones in the UWB case tend to be log-normally distributed. For the path amplitude, we suggest an exponential decay profile, which has a constant slope in dB scale, with the corresponding parameters for the UWB case. For the wideband case, a twofold exponential decay profile provides excellent fits to the measured data. It was also noted that the root-mean-square (RMS) delay spread is independent of the line-of-sight/obstructed line-of-sight situations of the channel. The multipath components contributing significant energy play a major role in such a small environment if compared to the direct path. In addition, the radio channel gains are attenuated with the presence of a driver inside the cabin.


1997 ◽  
Vol 166 ◽  
pp. 321-324
Author(s):  
R. Warwick ◽  
I. Hutchinson ◽  
R. Willingale ◽  
K. Kuntz ◽  
S. Snowden

AbstractAn overlapping set of ROSAT PSPC observations made in a region of very low Galactic foreground column density, has been used to investigate variations in the soft X-ray background on angular scales of 15′ – 5°. In the ¼ keV band there is a clear inverse correlation of the count-rate with the line-of-sight hydrogen column density. However, after correcting for this absorption effect, strong residual fluctuations remain in the data, with an amplitude which is significantly larger than that due to the counting statistics or the confusion of unresolved discrete sources. In contrast a similar analysis for the ¾ and 1.5 keV ROSAT bands shows no evidence for an excess signal. The most likely origin of the ¼ keV fluctuations would seem to be in a patchy distribution of ~ 106 K gas in the Galactic halo.


2019 ◽  
Vol 623 ◽  
pp. A68 ◽  
Author(s):  
V. Thiel ◽  
A. Belloche ◽  
K. M. Menten ◽  
A. Giannetti ◽  
H. Wiesemeyer ◽  
...  

Context. The diffuse and translucent molecular clouds traced in absorption along the line of sight to strong background sources have so far been investigated mainly in the spectral domain because of limited angular resolution or small sizes of the background sources. Aims. We aim to resolve and investigate the spatial structure of molecular clouds traced by several molecules detected in absorption along the line of sight to Sgr B2(N). Methods. We have used spectral line data from the EMoCA survey performed with the Atacama Large Millimeter/submillimeter Array (ALMA), taking advantage of its high sensitivity and angular resolution. The velocity structure across the field of view is investigated by automatically fitting synthetic spectra to the detected absorption features, which allows us to decompose them into individual clouds located in the Galactic centre (GC) region and in spiral arms along the line of sight. We compute opacity maps for all detected molecules. We investigated the spatial and kinematical structure of the individual clouds with statistical methods and perform a principal component analysis to search for correlations between the detected molecules. To investigate the nature of the molecular clouds along the line of sight to Sgr B2, we also used archival Mopra data. Results. We identify, on the basis of c-C3H2, 15 main velocity components along the line of sight to Sgr B2(N) and several components associated with the envelope of Sgr B2 itself. The c-C3H2 column densities reveal two categories of clouds. Clouds in Category I (3 kpc arm, 4 kpc arm, and some GC clouds) have smaller c-C3H2 column densities, smaller linewidths, and smaller widths of their column density PDFs than clouds in Category II (Scutum arm, Sgr arm, and other GC clouds). We derive opacity maps for the following molecules: c-C3H2, H13CO+, 13CO, HNC and its isotopologue HN13C, HC15N, CS and its isotopologues C34S and 13CS, SiO, SO, and CH3OH. These maps reveal that most molecules trace relatively homogeneous structures that are more extended than the field of view defined by the background continuum emission (about 15′′, that is 0.08–0.6 pc depending on the distance). SO and SiO show more complex structures with smaller clumps of size ~5–8′′. Our analysis suggests that the driving of the turbulence is mainly solenoidal in the investigated clouds. Conclusions. On the basis of HCO+, we conclude that most line-of-sight clouds towards Sgr B2 are translucent, including all clouds where complex organic molecules were recently detected. We also conclude that CCH and CH are good probes of H2 in both diffuse and translucent clouds, while HCO+ and c-C3H2 in translucent clouds depart from the correlations with H2 found in diffuse clouds.


2017 ◽  
Vol 852 (1) ◽  
pp. 22 ◽  
Author(s):  
Michael Walther ◽  
Joseph F. Hennawi ◽  
Hector Hiss ◽  
Jose Oñorbe ◽  
Khee-Gan Lee ◽  
...  

1990 ◽  
Vol 139 ◽  
pp. 307-316
Author(s):  
Francesco Paresce

Due mainly to the minimal contaminating effects of zodiacal light and direct stellar emission, the far UV wavelength band from 912 to à 2000 å is ideally suited, in principle at least, for an accurate measurement of a diffuse background component due to sources outside our own galaxy. The cosmological significance of such radiation is of great current interest as it certainly includes the cumulative line-of-sight effect of galaxies and quasars and may include emission from both a lukewarm intergalactic medium and decaying massive particles such as neutrinos, photinos, etc. The radiation required to maintain the IGM at its known high ionization level should also, in any case, appear clearly in this band due to redshift and lookback time effects, thereby providing a crucial clue as to its presently obscure origins. Just how accurately this component can be measured in practice, however, clearly depends on how well we understand the probably dominant galactic component from which it must be disentangled in one way or another except, possibly, at the galactic poles. The residual emission there in this band is on the order of a few ×102 photons cm−2 s−1 sr−1 å−1, of which perhaps as many as 50 units are almost certainly due to galaxies since the small-scale spatial fluctuations corresponding to this flux almost exactly mimic those expected from the known spatial distribution of galaxies. The rest must come from a presently uncertain source, most likely a residual tenuous dust layer at the galactic poles. This latter possibility is at least consistent with recent IRAS results on the diffuse IR background at 100 μm and very sensitive HI, 21 cm measurements in these regions, but an extragalactic origin cannot be presently ruled out. Higher spatial and spectral resolution observations throughout the entire far UV range planned for the near future from orbiting platforms are expected to resolve this last but critically important issue.


2013 ◽  
Vol 9 (S297) ◽  
pp. 321-329
Author(s):  
S. R. Federman

AbstractGround-based and space-borne observations of diffuse molecular clouds suggest a number of areas where further improvements to modeling efforts is warranted. I will highlight those that have the widest applicability. The range in CO fractionation caused by selective isotope photodissociation, in particular the large 12C16O/13C16O ratios observed toward stars in Ophiuchus, is not reproduced well by current models. Our ongoing laboratory measurements of oscillator strengths and predissociation rates for Rydberg transitions in CO isotopologues may help clarify the situtation. The CH+ abundance continues to draw attention. Small scale structure seen toward ζ Per may provide additional constraints on the possible synthesis routes. The connection between results from optical transitions and those from radio and sub-millimeter wave transitions requires further effort. A study of OH+ and OH toward background stars reveals that these species favor different environments. This brings to focus the need to model each cloud along the line of sight separately, and to allow the physical conditions to vary within an individual cloud, in order to gain further insight into the chemistry. Now that an extensive set of data on molecular excitation is available, the models should seek to reproduce these data to place further constraints on the modeling results.


1990 ◽  
Vol 138 ◽  
pp. 129-146 ◽  
Author(s):  
Sara F. Martin

Small-scale solar features identifiable on the quiet sun in magnetograms of the line-of-sight component consist of network, intranetwork, ephemeral region magnetic fields, and the elementary bipoles of ephemeral active regions. Network fields are frequently observed to split into smaller fragments and equally often, small fragments are observed to merge or coalesce into larger clumps; this splitting and merging is generally confined to the borders and vertices of the convection cells known as supergranules. Intranetwork magnetic fields originate near the centers of the supergranule convection cells and appear to increase in magnetic flux as they flow in approximate radial patterns towards the boundaries of the cells.


Sign in / Sign up

Export Citation Format

Share Document