Chromospheric rotation

The dependence of solar rotation on the size of the chromospheric tracers is considered. On the basis of an analysis of Ca ii K₃ daily filtergrams taken in the period 8 May–14 August, 1972, chromospheric features can be divided into two classes according to their size. Features with size falling into the range 24 000–110 000 km can be identified with network elements, while those falling into the range 120 000–300 000 km with active regions, or brightness features of comparable size present at high latitudes. The rotation rate is determined separately for the two families of chromospheric features by means of a cross-correlation technique which directly yields the average daily displacement of tracers due to rotation. Before computing the cross-correlation functions, chromospheric brightness data have been filtered with appropriate bandpass and highpass filters for separating spatial periodicities whose wavelengths fall into the two ranges of size, characteristic of the network pattern and of the activity centers. A difference less than 1% of the rotation rate of the two families of chromospheric features has been found. This is an indication for a substantial corotation at chromospheric levels of different short-lived features, both related to solar activity and controlled by the convective supergranular motions.     

Identification of the Coronal Sources of the Fast Solar Wind

Starting from the observation that kilohertz quasi-periodic oscillations (kHz QPOs) occur in a very narrow range of X-ray luminosities in neutron star low-mass X-ray binaries, we try to link the kHz QPO observability to variations of the neutron star magnetospheric radius, in response to changing mass inflow rate. At low luminosities, the drop-off of kHz QPO activity may be explained by the onset of the centrifugal barrier, when the magnetospheric radius reaches the corotation radius. At the opposite side, at higher luminosities, the magnetospheric radius may reach the neutron star and the vanishing of the magnetosphere may lead to the stopping of the kHz QPO activity. If we apply these constraints, the magnetic fields of atoll [B approximately 0.3-1x108 G for Aql X-1] and Z [B approximately 1-8x108 G for Cyg X-2] sources can be derived. These limits naturally apply in the framework of beat-frequency models but can also work in the case of general relativistic models.     

Multiwavelength analysis of a well observed flare from SMM

We describe and analyse observations of an M1.4 flare which began at 17: 00 UT on 12 November, 1980. Ground based H and magnetogram data have been combined with EUV, soft and hard X-ray observations made with instruments on-board the Solar Maximum Mission (SMM) satellite. The preflare phase was marked by a gradual brightening of the flare site in Ov and the disappearance of an H filament. Filament ejecta were seen in Ov moving southward at a speed of about 60 km s^–1, before the impulsive phase. The flare loop footpoints brightened in H and the Caxix resonance line broadened dramatically 2 min before the impulsive phase. Non-thermal hard X-ray emission was detected from the loop footpoints during the impulsive phase while during the same period blue-shifts corresponding to upflows of 200–250 km s^–1 were seen in Ca xix. Evidence was found for energy deposition in both the chromosphere and corona at a number of stages during the flare. We consider two widely studied mechanisms for the production of the high temperature soft X-ray flare plasma in the corona, i.e. chromospheric evaporation, and a model in which the heating and transfer of material occurs between flux tubes during reconnection.     

Microwave,ultraviolet, and soft X-Ray observations of hale region 16898

Hale region 16898 was observed by the Westerbork Synthesis Radio Telescope at 6 cm and by the Ultraviolet Spectrometer and Polarimeter and the X-Ray Spectrometer on the Solar Maximum Mission satellite. Optical pictures of the same active region were taken at Sacramento Peak, Big Bear, and Meudon Observatories. The radio emission mechanisms are identified by comparing radio data with ultraviolet and soft X-ray data. The height of the radio sources and the magnetic field strength at that height are deduced. A radio source above a large sunspot shows a crescent shaped depression of circular polarization and a high brightness temperature. The emission mechanism is identified as gyroresonance at the second and the third harmonic layers and it is found that the second harmonic layer, where the magnetic field strength is 900 G, must be in the corona. An extended loop-like source connecting the leading and the following part of the active region as well as the sources associated with small spots are mainly due to thermal free-free emission by hot and dense plasma which is also observed in ultraviolet and soft X-ray radiation. The calculated radio brightness temperature, using the physical parameters deduced from the ultraviolet and soft X-ray line intensities, agrees with the observed brightness temperature. The height of the low brightness temperature sources above the small spots is 6000 ± 3000 km and that above the large spot is less than 3000 km: the source above the large spot does not show any shift relative to the sunspot due to the projection effect. Very strong radio emission was found which was associated with the merging of a group of small spots into the large sunspot. In the same day, warm ( 10⁶ K) and dense matter was present above the large spot. Evidence for nonthermal emission is presented.     

The introduction of Sargassum muticum modifies epifaunal patterns in a Moroccan seagrass meadow

Marine macrophytes sustain valuable epiphytic biodiversity. Nonindigenous macroalgae may induce changes in composition and structure of epifaunal assemblages and therefore support different assemblages from those associated with native species. In this study, differences in faunal community structure between the introduced fucoid Sargassum muticum and the native seagrass Cymodocea nodosa were tested over a year on an intertidal shallow sandy bottom at the southern introduction front, the El Jadida coastline (NW Morocco). Epifaunal community structure consistently differed between macrophytes through seasons, with more species‐rich assemblages associated with S. muticum than C. nodosa despite comparable abundances. The significantly greater epifauna diversity on S. muticum may be related to its structural complexity. However, the species contributing most to differences in assemblages between both macrophytes, such as Steromphala umbilicalis and S. pennanti, were found on both habitats with temporally varying abundances. Some species‐specific affinities were detected (Stenosoma cf. acuminatum, Elasmopus vachoni, Chauvetia brunnea). Nitrogen, dissolved oxygen, suspended matter and temperature were identified as the best explanatory variables contributing to the observed macroepifaunal patterns. This study provides evidence that S. muticum acts as a favourable and additional habitat for epifaunal species and supports a more diverse epifaunal assemblage in this Moroccan seagrass meadow.     

Vegetative reproduction by multicellular propagules in Rhodophyta: an overview

This paper deals with vegetative reproduction by multicellular propagules in Rhodophyta. An extensive examination of the relevant literature shows that this phenomenon in Rhodophyta is not well known. A propagule is here defined as a vegetative multicellular structure which spontaneously detaches from the parent thallus and gives rise to a new individual. The origin and morphological features of multicellular propagules are examined in the various known propagule‐forming species. The importance of multicellular propagules as both overwintering and resting organs and as a taxonomic feature is also shown. Their role in increasing local populations and/or for long‐distance dispersal is discussed. The relative abundance of vegetative reproduction by multicellular propagules versus sexual reproduction, as well as the advantages of this additional mode of reproduction, are shown and discussed. The production of multicellular propagules may contribute to the capacity of species to increase populations, to weather unpredictable environmental changes, to survive in conditions that would be lethal for entire thalli, and to reach new habitats. As resting organs, they may also be responsible for long‐distance dispersal and may account for the introduction of some alien species. Accordingly, propagule‐forming species are probably more competitive than taxa that do not produce such propagules. These considerations should be taken into account in future studies of the biology, ecology and demography of Rhodophyta.     

Comparative Analysis of Stability—Genetic Diversity in Seagrass (<Emphasis Type="Italic">Posidonia oceanica</Emphasis>) Meadows Yields Unexpected Results

The diversity–stability relationship is the subject of a long-standing debate in ecology, but the genetic component of diversity has seldom been explored. In this study, we analyzed the interplay between genetic diversity and demographic responses to environmental pressures. This analysis included 30 meadows formed by the Mediterranean endemic seagrass, Posidonia oceanica, showing a wide range of population dynamics ranging from a near equilibrium state to steep decline due to strong environmental pressures close to aquaculture installations. Our results show that sedimentation rates are much better predictors of mortality than clonal or genetic components. An unexpected positive trend was observed between genotypic diversity and mortality, along with a negative relationship between allelic richness and net population growth. Yet such trends disappeared when excluding the most extreme cases of disturbance and mortality, suggesting the occurrence of a threshold below which no relationship exists. These results contrast with the positive relationship between genotypic diversity and resistance or resilience observed in previous manipulative experiments on seagrass. We discuss the reasons for this discrepancy, including the difficulties in designing experiments reflecting the complexity of natural meadows.     

Rotation and lifetime of coronal features

A study on the differences of rotation properties, based on the lifetime of coronal features, has been performed for the period 1972–1974. The short-lived component of the green corona associated with solar activity is differentially rotating, while long-lived coronal features persisting more than one synodic rotation period, show little or no differential rotation. These two components coexist at a same latitude within a wide latitude range at least in one of the solar hemispheres.