Hα intensity oscillations in large flares

We reinvestigate the problem of Hα intensity oscillations in large flares, particularly those classified as X-class flares. We have used high spatial and temporal resolution digital observations obtained from Udaipur Solar Observatory during the period 1998–2006 and selected several events. Normalized Lomb-Scargle periodogram method for spectral analysis was used to study the oscillatory power in quiet and active chromospheric locations, including the flare ribbons.     

The Hanle effect of the coronal Lα line of hydrogen: Theoretical investigation

This paper is devoted to a computation of the effect of a magnetic field on the linear polarization of the coronal L line of hydrogen. Recent works (Gabriel et al., 1971) have shown that the linear polarization of this line is due to resonant scattering of the incident chromospheric L line. The Hanle effect is the modification of this linear polarization, due to the magnetic field. After having briefly recalled the main features of this effect and the conditions of the coronal L line formation, we present the theoretical formalism to be used for Hanle effect computations. The effect of the hyperfine structure of the line is included. Then the results of our computations are given in terms of linear polarization as a function of the magnetic field. We get that the effect of the hyperfine structure on these results is negligible, although this is not evident a priori. When the hyperfine structure is neglected, the line structure is simplified and the Hanle effect can be expressed with analytical formulae, which we give in the last part of this paper. After integration along the line of sight, these formulae could be used for magnetic field determination in the solar corona from measurements of the linear polarization of the L line.     

Overestimating the coronal density and its effect on the velocities derived from Lα Doppler dimming

It is shown that relatively small errors of the order of 10% in the electron densities, for example derived from polarization brightness measurements, might lead to large errors in the velocities inferred from intensity measurements of the L Hi 1216 spectral line in the inner corona. It is demonstrated that in some cases this effect can result in very high velocities close to the coronal base with subsequent small acceleration with heliocentric distance. It is also pointed out that the errors in the deduced velocities can be reduced if simultaneous observations that place constraints on the mass flux are available, and by ensuring that the mass flux computed from the derived velocities and densities is constant.     

The excitation mechanism and the line-to-continuum intensity ratio of the [Fexiv] 5303 Å line in various coronal regions

The excitation mechanism of the coronal green line in various coronal regions is studied. The line-to-continuum intensity ratio is calculated using the model electron density values given by Newkirk (1961) and is compared with the observed values. Reasonably good agreement is found between the model calculations and observed values. The model calculations show that collisional excitation is more important in the innermost regions while radiative contribution progressively increases towards outer regions. The individual contributions depend strongly on the activity of the particular coronal region. An increased contribution from collisional excitation is seen in coronal active regions owing to the large electron density. A contour map of the line-to-continuum intensity ratio as derived from the observations, is plotted. At 1.10R the value is about 80 in active regions, 30–40 in coronal streamers, while it is less than 10 in polar regions.     

The clustering of Lα absorption lines in the QSO spectra

For nine published high-resolution QSO spectra a correlation analysis of their L forest lines has been performed. The two-point correlation functions show some quasi-periodic structure of magnitude ||0.3. Their characteristic separation along the line-of-sight amounts to s ₀=3×10^–3 or to s ₀=5×10^–3 for =1 and 0.2, respectively. Especially the distribution of nearest neighbouring line positions in two close QSO pairs allows for the interpretation that the absorption clouds lie in sheet-like structures as predicted by the pancake theory. The correlation data contain some hints on metal absorbers within the forest of unidentified lines.     

Associations of decimetric type Ⅲ bursts with coronal mass ejections and Hα flares

We present a statistical study of decimetric type Ⅲ radio bursts,coronal mass ejections(CMEs),and Hα flares observed in the period from July 2000 to March2005.In total,we investigated 395 decimetric type Ⅲ radio burst events,21% of which showed apparent correlation to CMEs that were associated with Hα flares.We noticed that the Hα flares which were strongly associated with CMEs were gradual events,and82% of them took place before CMEs appeared in the field of view of LASCO C2;that most of the CME-associated radio bursts started in the frequency range around750 MHz with a frequency drifting rate of several hundred MHz s-1,of which both positive and negative ones were recognized; and that the correlation of type Ⅲ radio bursts to CMEs without associated flares is fairly vague,less than 9%.     

Association of type II solar radio bursts with coronal structures above Hα filament channels

A study of type II solar radio bursts recorded at 160 MHz by the Culgoora radioheliograph during 1980 to 1982 shows that the radio emission occurs above H filaments rather than above H flares. This suggests that the type II radio emission most probably originates from within a coronal helmet streamer overlying the filament channel.     

A white-light/Fe X/Hα coronal transient observation to 10 solar RadII

Multi-telescope observations of the coronal transient of 15–16 April, 1980 provide simultaneous data from the Solar Maximum Mission Coronagraph/Polarimeter, the Solwind Coronagraph, and the new Emission Line Coronagraph of the Sacramento Peak Observatory. An eruptive prominence-associated white light transient is for the first time seen as an unusual wave or brightening in Fe x gl6374 (but not in Fe xiv gl5303). Several interpretations of this fleeting enhancement are offered.The prominence shows a slowly increasing acceleration which peaks at the time of the Fe event. The white light loop transient surrounding the prominence expands at a well-documented constant speed to 10R , with an extrapolated start time at zero height coincident with the surface activity.This loop transient exemplifies those seen above 1.7R in that leading the disturbance is a bright (N _e-enhanced) loop rather than dark. This is consistent with a report of the behavior of another eruptive event observed by Fisher and Poland (1981) which began as a density depletion in the lower corona, with a bright loop forming at greater altitudes. The top of the bright loop ultimately fades in the outer corona while slow radial growth continues in the legs.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Do changes in coronal emission structure imply magnetic reconnection?

We examine three major possible interpretations of observed reconfigurations of coronal X-ray and XUV emitting structures on a scale comparable to the size of the structures themselves. One possibility is that little change in the large-scale magnetic field configuration is associated with the change in emission. The other two possibilities are processes by which the magnetic field structure can change.We demonstrate that large changes in visibility in X-rays or XUV lines can be associated with relatively minor changes in the coronal magnetic field by showing the behavior of magnetic interconnections between individual active regions in a complex of activity observed by the S-054 X-ray spectrographic telescope on Skylab. While the large-scale interconnections are continuously present for at least several days, individual loops in these structures are visible for only relatively short times (1 day).The two theoretical possibilities which we discuss are frozen-in motion of the fields, and field line reconnection. We emphasize that reconnection occurs in regions much smaller than telescopic resolution. Because there are no measurements of the magnetic field in the corona in projection against the disk, existing observations are generally not sufficient to show in detail howmuch reconnection has occurred.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

A new examination of absorption line density in the Lα forest of QSOs

The distribution of the absorption lines in the L forest of QSOs is re-examined in relation to both absorption and emission redshifts by analyzing a sample consisting of 20 QSO L forests.On the one hand, the -value in the relationN(z _abs)=N ₀(1+z _abs) is re-estimated by applying the maximum likelihood (ML) method to all the L forests in our sample as a whole as well as to the forests for individual QSOs. The global estimated is 2.20±0.28 and both the quasars with highest and lowestz _em do not individually have a significant effect on it. A non-parametric test on the estimate of from individual QSOs shows that there is no significant difference between the numbers of positive and negative -values. This does not, therefore, favour the idea that for individual QSOsN(z _abs) decreases with increasingz _abs.On the other hand, the possible correlation between the absorption line density and the emission redshift of the QSO itself, which was found by Bianet al. (1986, 1988) and re-discovered independently by Tytler (1987), is shown once again not only by the largerz _em QSOs having larger mean density, but more importantly, at a rather high confidence level, by the fact that, at the same values ofz _abs, QSOs with largerz _em possess a greater number of L absorption lines.A discussion of the inverse effect suggests that it is still hard to say whether this effect really exists.     

Chromospheric and coronal Alfvénic oscillations in non-vertical magnetic fields

We generalize previous studies of Alfvénic oscillations in the solar atmosphere to geometries in which the background magnetic field is not parallel to the gravitational acceleration. A uniform but inclined field produces only subtle changes in the mathematics, and virtually no changes to the coronal energy flux, over previous vertical field studies. We show that simple, two-layer models agree remarkably well with more sophisticated, multi-layer calculations. In addition, we derive several useful and accurate analytic results with which we highlight many features and parameter dependences. We also study a model with a spreading field geometry. For low magnetic fields (- 10 G) the corona still appears WKB to the oscillations and we do not find any significant deviations from the uniform field calculations. This is not the case for higher magnetic fields in active regions (- 3000 G) where we confirm previous results which suggest an order of magnitude increase in the coronal flux. Again, we derive useful analytic approximations.Now at: Mathematics Department, Monash University, Clayton, Victoria, Australia.     

The solar Lα flux near solar minimum

After being turned off in 1972 the OSO-5 satellite was reactivated during the summer of 1974 for one year. The University of Paris experiment designed to monitor the solar L flux operated almost perfectly during that period which occurred near a minimum in solar activity. This new set of data is presented here, showing that neither the total L flux nor the flux emitted at the center of the line correlate with the solar activity indices in the same manner as previously found at higher levels of activity.These new observations confirm that the solar L flux varies approximately by a factor of two from solar maximum to solar minimum.Furthermore, the lower boundary of the transition region seems to be strongly perturbated near solar minimum, since the flux variations observed at the center of the L line are drastically different from all those previously reported. This seems to be related to the presence of large coronal holes over the Sun.     

Rapid changes in the fine structure of a coronal ‘bright point’ and a small coronal ‘active region’

A coronal bright point is resolved into a pattern of emission which, at any given time, consists of 2 or 3 miniature loops (each 2500 km in diameter and 12 000 km long). During the half-day lifetime of the bright point individual loops evolved on a time scale 6 min. A small ctive region seemed to evolve in this way, but the occasional blurring together of several loops made it difficult to follow individual changes.     

Periodic variations of the Hα profile width in the chromosphere of coronal holes as a possible indicator of Alfvén waves

We analyze the oscillations of the Hα profile width based on our observations of the chromosphere at the base of solar coronal holes. The maximum oscillation amplitude averaged over ten time series is 64 m 0 A. Direct calculations show that this value cannot be reached through temperature oscillations, because the periodic intensity fluctuations observed during our experiment did not exceed 5%, corresponding to Hα profile broadening only by 1.5–2 m Å. We hypothesize that the observed variations can result from the propagation of torsional Alfvén waves in the chromosphere of coronal holes.     

Moment method for multiple scattering of solar Lα radiation in the nearby interstellar medium

The problem of solar L (1216 Å) photons scattered coherently by interplanetary medium is solved for a realistic density distribution using a simple three-stream division of the radiation field.     

Non-inductive current driven by Alfvén waves in solar coronal loops

It has been shown that Alfvén waves can drive non-inductive current in solar coronal loops via collisional or collisionless damping. Assuming that all the coronal-loop density of dissipated wave power (W= 10–3 erg cm^–3 s^–1), which is necessary to keep the plasma hot, is due to Alfvén wave electron heating, we have estimated the axial current density driven by Alfvén waves to be jz 10³–10⁵ statA cm^–2. This current can indeed support the quasi-stationary equilibrium and stability of coronal loops and create the poloidal magnetic field up to B 1–5 G.     

Photoelectric measurement of the coronal emission line λ 5303 by an automatic coronal photometer

A 5303 Å-photometer was constructed for the 20 cm coronagraph of the Wendelstein solar observatory. Details of the photometer, the method of measurement and an example for measured isophotes are given. The Wendelstein photometer is compared with the 5303 Å-Sac Peak photometer.Mitteilungen der Sternwarte München Bd. 2 Nr. 5.     

A low β coronal loop model

A localized force-free current is proposed as a model for the observed coronal loops.An upper limit for the growth rate of kink instabilities in this model is found by solving numerically, in cylinder symmetry, the MHD equation of motion, with the boundary condition = 0 outside the loop.For various current densities a spectrum of kinks is found. These instabilities will disrupt the loops that are long or strongly twisted, on a time scale of a few seconds.The kinks in short or barely twisted loops are too long to fit.     

Cerenkov line emission: The Lα/Hβ, Hα/Hβ, Pα/Hα intensity ratios of quasars

We use the Cerenkov line emission mechanism to give a new explanation of the observed intensity ratios, particularly the L/H ratio, of the emission lines of quasars. We give equations that restrict the choice of the parameter values. The parameters are the characteristic energy of the relativistic electrons, the number density of neutral hydrogen and its relative level populations. With reasonable choice of the parmaeters, we can obtain calculated L/H, H/H, P/H ratios in agreement with observed values. Our estimate for the gas density in the broad line region of quasars is 10¹⁵ cm^–3, very different from previous estimates. Unlike previous theories, such a high density causes no difficulties with the Cerenkov line emission.     

A low-β coronal loop model

The low- coronal loop model of Sillen and Kattenberg (1980) is extended to include a surrounding current-free plasma. We calculated the dispersion curves of kink modes by solving the linearized MHD-equations of motion.We found a strong stabilizing influence on the growth rates of kink instabilities due to the surrounding plasma.In loops that are thick, have small current densities and that have a high density and a low magnetic field strength the growth times for kinks become of the order of days.Presently at Caltech, Pasadena, U.S.A.Presently at the FOM-Institute for Plasma Physics, Rijnhuizen, Nieuwegein, The Netherlands.