ALBERT

Notes: [Auszug] Nearly a decade of high-precision monitoring of solar luminosity by the ACRIM I experiment ended with re-entry of the Solar Maximum Mission (SMM) spacecraft in December 1989. Significant solar variability was found on all timescales, ranging from individual samples (1.024 seconds) to the full ...

Notes: Abstract Since its launch on March 8, 1967, the OSO-III has continuously observed solar and cosmic X-rays over the 7.7–210 keV range. The sun emits many impulsive X-ray bursts having fluxes several orders of magnitude above the background level of 8 × 10−9 ergs(cm2-sec)−1 at 7.7 keV and characteristic times on the order of 5 min. Ninety-five such events having fluxes 〉3 × 10−5 ergs(cm2-sec)−1 were detected in the period from March 8 to June 15, 1967. The cosmic X-ray source Lupus XR-1 has been observed to have a power law spectral form and no significant time variations over a 40-day period. Upper limits have been obtained on the hard X-ray flux of the peculiar galaxy M 87.

Notes: [Auszug] Ten months of solar total irradiance data from the Solar Maximum Mission satellite have generated accurate frequencies, amplitudes and linewidths for individual ∼5-min solar p-mode ocillations of low degree. The modes can be described as independent and chaotically excited oscillators, and ...

Notes: Abstract We demonstrate several events where an eruptive flare close to the limb gave rise to a transient coronal streamer visible in X-rays in Yohkoh SXT images, and analyze one of these events, on 28–29 October 1992, in detail. A coronal helmet streamer began to appear 2 hours after the flare, high above rising post-flare loops; the streamer became progressively narrower, reaching its minimum width 7–12 hours after the flare, and widened again thereafter, until it eventually disappeared. Several other events behaved in a similar way. We suggest that the minimum width indicates the time when the streamer became fully developed. All the time the temperature in the helmet streamer structure was decreasing, which can explain the subsequent fictitious widening of the X-ray streamer. It is suggested that we may see here two systems of reconnection on widely different altitudes, one giving rise to the post-flare loops while the other creates (or re-forms) the coronal helmet streamer. A similar interpretation was suggested in 1990 by Kopp and Polettofor post-flare giant arches observed on board the SMM; indeed, there are some similarities between these post-flare helmet streamers and giant arches and, with the low spatial resolution of SMM instruments, it is possible that some helmet streamers could have been considered to be a kind of a giant arch.

Notes: Abstract Subphotospheric current systems inferred from recent vector magnetograph observations (e.g. Gary et al., 1987) imply the existence of electric currents penetrating the photosphere and thus flowing deep in the solar convection zone. These currents presumably originate in an internal dynamo that supplies the observed photospheric magnetic fields through the buoyant motions of the initially deeply-buried flux tubes. The coronal fields resulting from this process therefore must carry slowly-varying currents driven by emf's remote from the surface. These currents may then drive solar-flare energy release. This paper discusses the consequences of such a deep origin of the coronal parallel currents. Simple estimates for a large active region suggest a mean current-closure depth ≥ 10,000 km, with a subphotospheric inductance ≥ 100 H and a subphotospheric stored energy ≥ 1033 ergs.

Notes: Abstract Solar flares emit line and continuum γ-radiation as well as neutrons and charged particles. These high-energy emissions require the presence of energetic ions within the magnetic structures of the flare proper. We have already learned a great deal about the location and mode of particle acceleration. The observations have now become extensive enough so that we can begin to study the dynamics of the energetic ions within the flare structures themselves. This paper reviews the γ-ray and neutron observations and the theory of their emission, and discusses on this basis the presence of energetic ions deep within the flaring atmosphere.

Notes: Abstract This paper considers the discoveries that have appreciably changed our understanding of the physics of solar flares. I identify a total of 42 discoveries from all disciplines, ranging from Galileo's initial observation of faculae to the recent discovery of strong limb brightening in 10-MeV γ-radiation. The rate of discovery increased dramatically over the past four decades as new observational tools became available. My assessment of significance suggests that recent discoveries - though more numerous - are individually less significant; perhaps this is because the minor early discoveries tend to be taken for granted. In spite of the many facets of flare physics that have been explained or at least well-described, many fundamental questions remain unresolved.

Notes: Abstract Calculations which predict that a phenomenon analogous to stellar negative pre-flares could also exist on the Sun were published by Hénouxet al. (1990), and Aboudarhamet al., (1990), who showed that at the beginning of a solar white-light flare (WLF) event an electron beam can cause a transient darkening before the WLF emission starts, under certain conditions. They named this event a “black light flare” (BLF). Such a BLF event should appear as diffuse dark patches lasting for about 20 seconds preceding the WLF emission, which would coincide with intense and impulsive hard X-ray bursts. The BLF location would be at (or in the vicinity of) the forthcoming bright patches. Their predicted contrast depends on the position of the flare on the solar disc and on the wavelength band of the observation. TheYohkoh satellite provided white-light data from the aspect camera of the SXT instrument (Tsunetaet al., 1991), at 431 nm and with a typical image interval of 10–12 s. We have studied nine white-light flares observed with this instrument, with X-ray class larger than M6. We have found a few interesting episodes, but no unambiguous example of the predicted BLF event. This study, although the best survey to date, was not ideal from the observational point of view. We therefore encourage further searches. Successful observations of this phenomenon on the Sun would greatly strengthen our knowledge of the lower solar atmosphere and its effects on solar luminosity variations.

Notes: Abstract The photometric sunspot index (PSI) was developed to study the effects of sunspots on solar irradiance. It is calculated from the sunspot data published in theSolar-Geophysical Data catalogue. It has been shown that the formerPSI models overestimate the effect of dark sunspots on solar irradiance; furthermore results of direct sunspot photometry indicate that the contrast of spots depends on their area. An improvedPSI calculation is presented; it takes into account the area dependence of the contrast and calculates ’true’ daily means for each observation using the differential rotation of the spots. Moreover, the observations are screened for outliers which improves the homogeneity of the data set substantially, at least for the period after December 1981 when NOAA started to report data from a few instead of one to two stations. A detailed description of the method is provided. The correlation between the newly calculatedPSI and total solar irradiance is studied for different phases of the solar cycles 21 and 22 using bi-variate spectral analysis. The results can be used as a ‘calibration’ ofPSI in terms of gain, the factor by whichPSI has to be multiplied to yield the observed irradiance change. This factor changes with time from about 0.6 in 1980 to 1.1 in 1990. This unexpected result cannot be interpreted by a change of the contrast relative to the quiet Sun (as it is normally defined and determined by direct photometry) but rather as a change of the contrast between the spots and their surrounding as seen in total irradiance (integrated over the solar disk). This may partly be explained by a change in the ratio between the areas of the spots and the surrounding faculae.

Notes: Abstract We have found several occurrences of slowly rising giant arches inYohkoh images. These are similar to the giant post-flare arches previously discovered by SMM instruments in the 80s. However, we see them now with 3–5 times better spatial resolution and can recognize well their loop-like structure. As a rule, these arches followeruptive flares with gradual soft X-ray bursts, and rise with speeds of 1.1–2.4 km s−1 which keep constant for 〉5 to 24 hours, reaching altitudes up to 250 000 km above the solar limb. These arches differ from post-flare loop systems by their (much higher) altitudes, (much longer) lifetimes, and (constant) speed of growth. One event appears to be a rise of a transequatorial interconnecting loop. In the event of 21–22 February 1992 one can see both the loop system, rising with a gradually decreasing speed to an altitude of 120 000 km, and the arch, emerging from behind the loops and continuing to rise with a constant speed for many more hours up to 240 000 km above the solar limb. In the event of 2–3 November 1991 three subsequent rising large-scale coronal systems can be recognized: first a fast one with speed increasing with altitude and ceasing to be visible at about 300 000 km. This most probably shows the X-ray signature of a coronal mass ejection (CME). A second one, with gradually decreasing speed, might represent very high rising flare loops. A third one continues to rise slowly with a constant speed up to 230 000 km (and up to 285 000 km after the speed begins to decay), and this is the giant arch. This event, including an arch revival on November 4–5, is very similar to rising giant arches observed by the SMM on 6–7 November 1980. Other events of this kind were observed on 27–28 April 1992, 15 March 1993, and 4–6 November 1993, all seen above the solar limb, where it is much easier to identify them. The temperature in the brightest part of the arch of 2–3 November 1991 was increasing with its altitude, from 2 to 4 × 106 K, which seems to be an effect of slower cooling at lower densities. Under an assumption of line-of-sight thickness of 50 000 km, the emission measure indicates densities from 1.1 × 1010 cm−3 at an altitude of 150 000 km to 1.0 × 109 cm−3 at 245 000 km 11.5 hours later. It appears that the arch is composed of plasma of widely different temperatures, and that hot plasma rises faster than the cool component. Thus the whole arch expands upward, and its density gradient increases with time, which explains whyYohkoh images show only the lowest and coolest parts of the expanding structure. The whole arch may represent an energy in excess of 1031 erg, and more if conduction contributes to the arch cooling. We suggest that the rise of the arch is initiated by a CME which removes the magnetic field and plasma in the upper corona, and the coronal structures remaining below this cavity begin to expand into the “vacuum” left behind the CME. However, we are unable to explain why the speed of rise stays constant for so many hours.