Daytime valley in the F1-region observed by incoherent scatter

Incoherent scatter data obtained at Saint-Santin have been analyzed between 1969 and 1972 in the height interval 120–200 km. The summer daytime electron density profiles show a maximum around 150 km followed by a minimum around 165 km, when the solar activity is low. The seasonal and solar cycle variations of this phenomena are discussed. When the day-time valley exists, the maximum density is ca. 20–30 per cent greater than the minimum value. The phenomenon cannot be fully explained on the basis of photochemical equilibrium. A wind-shear mechanism could play an important role.     

A long-lived coronal arch system observed in X-rays

A large long-lived soft X-ray emitting arch system was observed during the last Skylab mission. This arcade stayed in the same approximate position for several solar rotations. We suggest that these long-lived arches owe their stability to the stable coronal magnetic-field configuration. A global constant force-free magnetic field analysis, as developed by Nakagawaet al. (1977), is used to describe the arches, and results in a marked resemblance between the theoretical magnetic-field configuration and the observed X-ray emitting feature.     

Study of a neutral hydrogen feature previously observed by Cugnon

An anomalous velocity cloud nearl=349°,b=+3°, was investigated by Cugnon (1968). The authors made a new set of observations in order to obtain a more complete picture of the feature, including the region originally out of Cugnon's limit of observation. A comparison with optical and radio observations was made and several possibilities of interpretation as to the nature of the object were analyzed.     

MHD Seismology of a loop-like filament tube by observed kink waves

We report and analyze observational evidence of global kink oscillations in a solar filament as observed in Ha by instruments administered by National Solar Observatory(NSO)/Global Oscillation Network Group(GONG).An M 1.1-class flare in active region(AR) 11692 occurred on 2013 March 15 and induced a global kink mode in the filament lying towards the southwest of AR 11692.We find periods of about 61-67 minutes and damping times of 92-117 minutes at positions of three vertical slices chosen in and around the filament apex.We find that the waves are damped.From the observed period of the global kink mode and damping timescale using the theory of resonant absorption,we perform prominence seismology.We estimate a lower cut-off value for the inhomogeneity length scale to be around 0.34-0.44 times the radius of the filament cross-section.     

Short Gamma-Ray Bursts observed by PHEBUS

Statistical studies of Gamma-Ray Burst (GRB) properties have recently led to the discovery of a subclass within the population of classical events (Dezalayet al. 1992, Kouveliotouet al. 1993). Bursts belonging to this subclass are characterized by short durations, typically less than 2 seconds, and harder spectra on average. Using the PHEBUS GRB data set, we analyse the distributions of peak intensity, hardness ratio, and duration of the two subclasses. We also compare the sum spectra obtained with the brightest events to determine the ratio of total energies observed for each population.     

Evidence for beamed electrons in a limb X-ray flare observed by HXIS

X-ray images taken by the Hard X-Ray Imaging Spectrometer (HXIS) aboard SMM during the 1980, November 18 limb flare are analysed. The temporal and spatial evolutions of the X-radiation are described. They differ significantly for hard and soft X-rays. During the elementary flare bursts energetic photons are predominantly emitted from a region close to the solar limb. In contrast, the soft X-ray sources are situated higher in the solar atmosphere. The observed X-ray spectra, in particular those emitted from small source regions at various altitudes, were fitted to power laws. Analysis of the spatial variation of the spectral index shows that there is a systematic tendency of the spectra to get harder with decreasing source altitude, especially during the elementary flare bursts. This fact is in agreement with the existence of nonthermal electron beams precipitating from the corona towards the denser layers of the solar atmosphere.     

Energy balance in a magnetically confined coronal structure observed by OSO-7

A model of a coronal region of enhanced Fexv and Fexvi emission is developed and its energy balance is examined using extreme ultraviolet observations from OSO-7 together with calculations of possible force-free coronal magnetic field configurations. The coronal emissions overlying the photospheric boundary between regions of opposite magnetic polarity are found to be associated with generally non-potential (current-carrying) magnetic fields in the forms of arches with footpoints in regions of opposite polarity. The orientation of these arches relative to the neutral line changes with degree of ionization of the emitting ion (which we infer from our limb observations to be a function of height) and may be evidence of differing electric currents along various field lines. The appearance of a coronal arch, seen side-on, can conveniently be represented by a parabola and a detailed analysis (Appendix) shows this to be a realistic approximation that should be generally useful in analyzing two-dimensional pictures of coronal structures. Applying this analysis to the most prominent coronal region observed in the radiations of Fexv and Fexvi, we find a maximum in the electron temperature, T _e , of 2.6 × 10⁶K at the top of arches whose heights are 20000–40000 km and whose footpoints are separated by ≈ 100000 km. A temperature gradient of ▽T _e ≈5 × 10^-5K cm^-1 is found in this coronal structure. Radiative losses are typically fifteen times greater than conductive losses and the energy deposition required to maintain the coronal feature is nearly uniformly distributed along its length.     

A loop prominence system observed on May 24, 1972

A flare-loop prominence appeared over a complex sunspot group in McMath region No. 11895, on May 24,1972. The evolution of the loop prominence and its relation to a flare were observed using H and Ca K filters. Soft X-ray emission between 0.5–20 Å was found to be emitted from the flare-loop prominence system.     

Solar spicules observed through a K-filter

Some properties of the solar spicules as observed outside the limb through a Halle K-filter at the centre of the line are investigated. The observations have been made at the Pic-du-Midi Observatory. The results are as follows: (a) The chromosphere at the limb appears more diffuse in the K line than in the H line, the spicules being not well resolved. (b) The ratio of spicule intensity to the background intensity increases with height, as in the case of H spicules, but it is, in every height, smaller than that in H. (c) The spicule mean width is measured photometrically and found equal to 1900 km. (d) The lifetime is found to be 4.5 min. (e) The mean chromospheric height is estimated to be 6.000 km.     

Mountains on Titan observed by Cassini Radar

The Cassini Titan Radar mapper has observed elevated blocks and ridge-forming block chains on Saturn's moon Titan demonstrating high topography we term “mountains.” Summit flanks measured from the T3 (February 2005) and T8 (October 2005) flybys have a mean maximum slope of 37° and total elevations up to 1930 m as derived from a shape-from-shading model corrected for the probable effects of image resolution. Mountain peak morphologies and surrounding, diffuse blankets give evidence that erosion has acted upon these features, perhaps in the form of fluvial runoff. Possible formation mechanisms for these mountains include crustal compressional tectonism and upthrusting of blocks, extensional tectonism and formation of horst-and-graben, deposition as blocks of impact ejecta, or dissection and erosion of a preexisting layer of material. All above processes may be at work, given the diversity of geology evident across Titan's surface. Comparisons of mountain and blanket volumes and erosion rate estimates for Titan provide a typical mountain age as young as 20-100 million years.     

Dunes on Titan observed by Cassini Radar

Thousands of longitudinal dunes have recently been discovered by the Titan Radar Mapper on the surface of Titan. These are found mainly within ±30° of the equator in optically-, near-infrared-, and radar-dark regions, indicating a strong proportion of organics, and cover well over 5% of Titan's surface. Their longitudinal duneform, interactions with topography, and correlation with other aeolian forms indicate a single, dominant wind direction aligned with the dune axis plus lesser, off-axis or seasonally alternating winds. Global compilations of dune orientations reveal the mean wind direction is dominantly eastwards, with regional and local variations where winds are diverted around topographically high features, such as mountain blocks or broad landforms. Global winds may carry sediments from high latitude regions to equatorial regions, where relatively drier conditions prevail, and the particles are reworked into dunes, perhaps on timescales of thousands to tens of thousands of years. On Titan, adequate sediment supply, sufficient wind, and the absence of sediment carriage and trapping by fluids are the dominant factors in the presence of dunes.     

Study of white-light flares observed by Hinode

White-light flares are considered to be the most energetic flaring events that are observable in the optical broad-band continuum of the solar spectrum. They have not been commonly observed. Observations of white-light flares with sub-arcsecond resolution have been very rare. The continuous high resolution observations of Hinode provide a unique opportunity to systematically study the white-light flares with a spatial resolution around 0.2 arcsec. We surveyed all the flares above GOES magnitude C5.0 since the launch of Hinode in 2006 October. 13 of these kinds of flares were covered by the Hinode G-band observations. We analyzed the peak contrasts and equivalent areas (calculated via integrated excess emission contrast) of these flares as a function of the GOES X-ray flux, and found that the cut-off visibility is likely around M1 flares under the observing limit of Hinode. Many other observational and physical factors should affect the visibility of white-light flares; as the observing conditions are improved, smaller flares are likely to have detectable white-light emissions. We are cautious that this limiting visibility is an overestimate, because G-band observations contain emissions from the upper atmosphere.Among the 13 events analyzed, only the M8.7 flare of 2007 June 4 had near-simultaneous observations in both the G-band and the blue continuum. The blue continuum had a peak contrast of 94% vs. 175% in G-band for this event. The equivalent area in the blue continuum is an order of magnitude lower than that in the G-band. Very recently, Jess et al.studied a C2.0 flare with a peak contrast of 300% in the blue continuum. Compared to the events presented in this letter, that event is probably an unusual white-light flare: a very small kernel with a large contrast that can be detected in high resolution observations.     

Flare-associated shock waves observed by interplanetary scintillation

Daily observations of a grid of scintillating sources during the period January–August 1971 indicate that enhancements in scintillation index which cannot be related to corotating structure, are related to interplanetary shock waves associated with solar flares. Only 3 enhancements in scintillation index associated with shock waves were observed during the eight months period of observations.     

A giant coronal arch observed by Skylab

The limb event of 13 August 1973, observed by Skylab in soft X-rays, has all the typical characteristics of the giant post-flare arches observed by HXIS and FCS on board SMM in the 1980s. Skylab images provide us with 4–5 times better angular resolution than the SMM experiments and thus, for the first time, make it possible to distinguish the real fine structure of a giant post-flare arch. The image processing of the 13/14 August 1973 event is now in progress.     

Evidence for a new MeV source observed by the COMPTEL experiment aboard CGRO

We report first evidence for a new unidentified and variable MeV source, located near the galactic plane at (l,b)∼(284.5°, 2.5°). The source, GRO J1036-55, is found at a significance level of ∼5.6σ by COMPTEL in its 3–10 MeV band. The energy spectrum indicates a spectral maximum at 3–4.3 MeV with a steep slope at higher energies. Since the COMPTEL 3–4.3 MeV data contain contamination by an instrumental background line, we performed several consistency checks, which all are consistent with an astrophysical nature of this emission feature.     

On the relations between theoretical and observed colours in theBVRI system

Transformation relations from theoretical apparent magnitude and (b-v), (v-r), (v-i) colurs to the observedm _v and (B-V), (V-R), (V-I) quantities are derived.Black body colours in theBVRI system are also computed.     

In situ acceleration and gradients of charged particles in the outer solar system observed by the voyager spacecraft

The probable connection between cosmic rays and the electromagnetic state of the interplanetary medium was recognized by Hannes Alfvén as early as 1949 (Alfvén, 1949, 1950); he pointed out that the properties of cosmic rays necessitate a mechanism, external to Earth but within the solar system, capable of accelerating particles to extremely high energies. In advocating the view of local origin for part of the cosmic-ray spectrum, Alfvén and his colleagues developed a very general type of acceleration mechanism called magnetic pumping. The unique data set of the two Voyagers extends over an entire decade (1977–1987) and is most suitable to explore the problem of acceleration of charged particles in the heliosphere. The energy coverage of the Low Energy Charged Particle (LECP) experiment covers the range 30 keV to several hundred MeV for ions and 22 keV to several MeV for electrons. Selected observations of interplanetary acceleration events from 1 to 25 AU are presented and reviewed. These show frequent acceleration of ions to several tens of MeV in association with shocks; highest energies (220 MeV oxygen) were measured in the near-perpendicular ( _Bn 87.5°) shock of January 5, 1978 at 1.9 AU, where electron acceleration was also observed. Examples of ion acceleration in association with corotating interaction regions are presented and discussed. It is shown that shock structures have profound effects on high-energy (70 MeV) cosmic rays, especially during solar minimum, when a negative latitudinal gradient was observed after early 1985 at all energies from 70 MeV down to 30 keV. By early 1987, most shock acceleration activity in the outer heliosphere (25 to 30 AU) had ceased both in the ecliptic (Voyager-2) and at higher (30°) ecliptic latitudes (Voyager-1). The totality of observations demonstrate that local acceleration to a few hundred MeV, and as high as a few GeV is continually present throughout the heliosphere. It should be noted that in 1954 when Alfvén suggested local acceleration and containment of cosmic rays within the solar system, no one treated his suggestion seriously, at any energy. The observations reviewed in this paper illustrate once more Alfvén's remarkable prescience and demonstrate how unwise it is to dismiss his ideas.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.