Zdenek F. Svestka

Flare-like dynamic phenomena

Abstract Using data from Skylab , SMM, and Yohkoh spacecraft, as well as Hα observations on the ground, we summarize the present knowledge about dynamic phenomena on the Sun which usually are not classified as flares, although they resemble processes observed in flares, often are associated with flares, and sometimes can be considered for special kinds of flares. This involves field openings out-side active regions (eruptive flares without chromospheric response), loop eruptions, with emphasis on brightenings (and possibly eruptions) of loops which interconnect active regions, sprays, flaring arches (injections of hot and cool plasma into closed field configurations), Hα and X-ray surges (injections of hot and cool plasma into ‘open’ fields), X-ray jets, recently discovered by Yohkoh , as well as Hα jets observed at Wroclaw and Ottawa. We will try to find relationships between all the latter phenomena so that one gets a more homogeneous picture of plasma injections and jets observed on the Sun.

A comparison of solar activity during the decline of several solar cycles

Abstract This review summarizes the general characteristics of solar activity during the declining part of solar cycles and compares the behavior of the Sun during several last cycles. Different characteristics of solar activity are considered and tentative reasons for their different behavior in different cycles are suggested. The activity decline is usually very irregular, characterized by quiet periods interrupted by temporary bursts of activity. These bursts can be quite impressive: actually, as a rule, the most outstanding events of solar activity do not occur at the cycle maximum, but shortly before it, or a few years later. The duration of quiet periods between activity enhancements increases as the cycle declines, but even close to the solar minimum one may still encounter quite interesting flare events. From the point of view of solar-terrestrial relations, these late events are particularly favorable, because their effects in interplanetary space and in planetary magnetospheres can be followed much easier than during high solar activity: one can uniquely ascribe the observed consequencies in space to their isolated sources on the Sun.

Pre- and Post-Flare X-ray Variations in Active Regions

Extremely low background noise of the HXIS experiment aboard the SMM made it possible to detect > 3.5 keV X-ray emissions from non-flaring active regions which are 103–104 times weaker than the X-ray flux from flares. Short-lived X-ray bursts and long-lived X-ray enhancements of various intensities seem to characterize active regions in different phases of their development. After major two-ribbon flares, giant X-ray arches are seen in the corona, slowly decaying for many hours after the flare end. Associated with these arches appear to be quasi-periodic flare-like variations of purely coronal nature.

On the behaviour of the hydrogen Lyman series in flares

The Lyman spectrum of hydrogen has been computed for a number of flare models, characterized by the column density of hydrogen atoms in the ground state (N1), the electron density (ne) and the electron temperature (Te). Broadening by the thermal Doppler effect and by Stark effect has been accounted for. The source functions for the individual lines of the series have been derived from non-LTE calculations of the excitation in hydrogen flares. The aim of the investigation is to evaluate the use of the intensities in between two successive Lyman lines for a determination of the value for N1, which is a physical parameter of the flare for which only indirect determinations are available. Whilst in principle this method could give reliable results, its practical application meets with difficulties which hardly can be overcome. Therefore, one probably has to base the N1 determination on the highest line resolvable in the spectrum.

Relationships among flare images at different wavelengths

The talk reviews briefly historical development of the imaging of the Sun, and flare imaging in particular, at different wavelengths. The present state is then critically analyzed, emphasizing and demonstrating problems related to simultaneity of observed data, differences in time resolution, limitations of the imaged field of view, unequal spatial resolution, and alignment of the images. Prospects for the future are briefly sketched, including imaging in very hard X-rays and γ-rays.

X-ray observations of limb flare loops and post-flare coronal arch

We present observations of another post-flare arch following an eruptive flare, detected in X-ray lines above the western solar limb on 2 May 1985.

Needs and constraints for solar flares space-borne cooperative programs

Abstract There are three basic reasons for cooperation in space observations of flares: (1) observations in different spectral regions, (2) stereoscopic observations, and (3) in situ data. Using experience from the past and considering the planned experiments, problems encountered by flare-related cooperative programs on spacecraft are briefly summarized.