The magnetic field in the solar atmosphere     

Thomas?WiegelmannEmail author  Julia?K.?Thalmann  Sami?K.?Solanki 《Astronomy and Astrophysics Review》2014,22(1):78

This publication provides an overview of magnetic fields in the solar atmosphere with the focus lying on the corona. The solar magnetic field couples the solar interior with the visible surface of the Sun and with its atmosphere. It is also responsible for all solar activity in its numerous manifestations. Thus, dynamic phenomena such as coronal mass ejections and flares are magnetically driven. In addition, the field also plays a crucial role in heating the solar chromosphere and corona as well as in accelerating the solar wind. Our main emphasis is the magnetic field in the upper solar atmosphere so that photospheric and chromospheric magnetic structures are mainly discussed where relevant for higher solar layers. Also, the discussion of the solar atmosphere and activity is limited to those topics of direct relevance to the magnetic field. After giving a brief overview about the solar magnetic field in general and its global structure, we discuss in more detail the magnetic field in active regions, the quiet Sun and coronal holes.  相似文献   

A ‘charge bunching’ approach to the gravitohydrodynamic instability     

Sami Cuperman 《Astrophysics and Space Science》1969,3(2):229-233

The criterion for the gravitational hydrodynamic instability of an infinite uniform selfgravitating system in uniform translational motion is established by making use of the conservation laws written in the system of coordinates in which the pressure wave produced by an initial density fluctuation is at rest.An analogous method, based on the hydrodynamic formulation of charge bunching has been used in the past to establish a criterion for the instabilities of plasma oscillations (Jackson, 1960).  相似文献   

Using landforms of Saudi Arabia for scenic and recreational purposes (With special reference to the Riyadh area)     

Sami S. A. Al-Amiri Ph.D. 《GeoJournal》1986,13(2):143-151

The landform of Saudi Arabia in general and the Riyadh area in particular have their own potentiality for scenic and recreational purposes. The higher lands or features which have well cemented material in some areas of Riyadh have been recommended for scenic use, while lower lands like wide natural lakes, sand dunes and others can be mostly used for recreational purposes. Such features and purposes found here may be extrapolated into other similar areas within Saudi Arabia and may be compared with other areas elsewhere in the world.  相似文献   

Unusual diatoms linked to climatic events in the northeastern English Channel   总被引：1，自引：0，他引：1  

Fernando Gmez  Sami Souissi 《Journal of Sea Research》2007,58(4):283-290

The composition of the micro-phytoplankton was investigated at two fixed stations in the northeastern English Channel from November 1997 to December 2005. In late summer-autumn 2005, proliferations of the centric diatoms Proboscia indica and Rhizosolenia hebetata f. semispina were recorded for the first time in this area. The weather in 2005 was abnormal: a cold winter and early arrival of summer conditions that extended to late autumn. This resulted in an unusually long, sustained calm and warm period. We suggest that the reduction in mixing may have allowed diatoms of more-stratified waters to be competitive in the normally highly mixed northeastern English Channel in summer-autumn 2005. In 2003 and in 2005, two warm-water diatom species were recorded for the first time. A colony of Eucampia cornuta was observed after an exceptional heat wave in September of 2003, and again in September 2005. Simultaneously at both fixed stations, Chaetoceros peruvianus was recorded in the warm October of 2005. The climatic events are associated with the northward spreading of thermophilic diatoms such as E. cornuta and Ch. peruvianus in the European Atlantic waters, and the proliferation of diatoms of stratified environments such as P. indica and R. hebetata f. semispina in the usually highly turbulent waters of the northeastern English Channel.  相似文献   

Impacts of an uncontrolled phosphogypsum dumpsite on summer distribution of phytoplankton, copepods and ciliates in relation to abiotic variables along the near-shore of the southwestern Mediterranean coast     

Rekik A  Drira Z  Guermazi W  Elloumi J  Maalej S  Aleya L  Ayadi H 《Marine pollution bulletin》2012,64(2):336-346

In connection with the Taparura Project, studies of spatial distribution of the crustacean zooplankton community, nutrients, phytoplankton and ciliates were conducted in July 2007 at 45 stations spread over fifteen transects along the coast north of Sfax. The results showed that the N/P ratio was lower than the Redfield ratio, suggesting potential N limitation. Phytoplankton was characterised by the proliferation of several diatoms, while ciliates were largely dominated by spirotrichs. Copepods were the most abundant zooplankton present during the entire study period, comprising 61% of the total zooplankton community. Twelve copepod families were identified at every station, with a high percentage of Oithonidae (77% of copepods) dominated by Oithona nana. The abundance of this species was correlated with that of diatoms, Cocoolithophorideae and ciliated Colpodea, suggesting that O. nana may feed on a wide range of prey. Despite human pressure and industrial activities, the coastal waters north of Sfax showed a wide diversity of phytoplankton, ciliates and zooplankton.  相似文献   

Inactivation of the gene katA or sodA affects the transient entry into the viable but non-culturable response of Staphylococcus aureus in natural seawater at low temperature     

Salma Masmoudi  Michel Denis  Sami Maalej 《Marine pollution bulletin》2010,60(12):2209-2214

We have investigated the fate of Staphylococcus aureus by starving the cells and maintaining them in natural seawater at 22 and 4 °C. At 22 °C, cells developed a long-term survival state where about 0.037% of the initial population remained culturable over more than 7 months, whereas at 4 °C, bacteria lost culturability and transiently entered into the viable but non-culturable state (VBNC). However, after 22 days of entry into the VBNC state, the number of viable cells detected via the direct viable count method decreased significantly. We show here that mutational inactivation of catalase (KatA) or superoxide dismutase (SodA) rendered strains hypersensitive to seawater stress at 4 °C and consequently, part of the seawater lethality on S. aureus at low temperature is mediated by reactive oxygen species (ROS) during microcosm-survival process. Shifting the temperature from 4 to 22 °C of totally non-culturable wild-type cells induced a partial recovery of the population. However, deficiencies in catalase or superoxide dismutase prevent resuscitation ability.  相似文献   

Considering groundwater use to improve the assessment of groundwater pumping for irrigation in North Africa   总被引：1，自引：0，他引：1  

Sylvain Massuel  Farida Amichi  Fatah Ameur  Roger Calvez  Zakia Jenhaoui  Sami Bouarfa  Marcel Kuper  Hamadi Habaieb  Tarik Hartani  Ali Hammani 《Hydrogeology Journal》2017,25(6):1565-1577

Groundwater resources in semi-arid areas and especially in the Mediterranean face a growing demand for irrigated agriculture and, to a lesser extent, for domestic uses. Consequently, groundwater reserves are affected and water-table drops are widely observed. This leads to strong constraints on groundwater access for farmers, while managers worry about the future evolution of the water resources. A common problem for building proper groundwater management plans is the difficulty in assessing individual groundwater withdrawals at regional scale. Predicting future trends of these groundwater withdrawals is even more challenging. The basic question is how to assess the water budget variables and their evolution when they are deeply linked to human activities, themselves driven by countless factors (access to natural resources, public policies, market, etc.). This study provides some possible answers by focusing on the assessment of groundwater withdrawals for irrigated agriculture at three sites in North Africa (Morocco, Tunisia and Algeria). Efforts were made to understand the different features that influence irrigation practices, and an adaptive user-oriented methodology was used to monitor groundwater withdrawals. For each site, different key factors affecting the regional groundwater abstraction and its past evolution were identified by involving farmers’ knowledge. Factors such as farmer access to land and groundwater or development of public infrastructures (electrical distribution network) are crucial to decode the results of well inventories and assess the regional groundwater abstraction and its future trend. This leads one to look with caution at the number of wells cited in the literature, which could be oversimplified.  相似文献   

Erratum à l'article Mise en évidence et analyse d'une structure atlasique ennoyée au front de la Chaîne alpine tunisienne [C. R. Geoscience 336 (2004) 1293–1300]     

Sami Khomsi  Mourad Bédir  M. Ghazi Ben Jemia 《Comptes Rendus Geoscience》2005,337(5):547

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Dynamical friction in an isentropic gas     

Fazeleh Khajenabi  Sami Dib 《Astrophysics and Space Science》2012,340(1):117-125

When a gravitating object moves across a given mass distribution, it creates an overdense wake behind it. Here, we performed an analytical study of the structure of the flow far from object when the flow is isentropic and the object moves subsonically within it. We show that the dynamical friction force is the main drag force on the object and by using a perturbation theory, we obtain the density, velocity and pressure of the perturbed flow far from the mass. We derive the expression of the dynamical friction force in an isentropic flow and show its dependence on the Mach number of the flow and on the adiabatic index. We find that the dynamical friction force becomes lower as the adiabatic index increases. We show analytically that the wakes are less dense in our isentropic case in comparison to the isothermal ones.  相似文献   

Coronal Density Structures and CMEs: Superior Solar Conjunctions of Mars Express, Venus Express, and Rosetta: 2004, 2006, and 2008     

Martin Pätzold  Matthias Hahn  Silvia Tellmann  Bernd Häusler  Michael K. Bird  G. Leonard Tyler  Sami W. Asmar  Bruce T. Tsurutani 《Solar physics》2012,279(1):127-152

Coronal radio-sounding experiments were carried out using the S-band (2.3 GHz) and X-band (8.4 GHz) signals of the ESA Mars Express, Venus Express, and Rosetta spacecraft during five superior conjunctions occurring in 2004, 2006 (3×), and 2008/2009. Differential frequency and propagation delay (ranging) observations were recorded during these opportunities over the better part of a solar cycle, yielding information on the large-scale structure of the coronal electron-density distribution and its variations, including fluctuations on time scales from seconds to hours. These results concern primarily regions of slow solar wind because the radio propagation path is generally confined to the low heliolatitude regions by the conjunction. The mean frequency fluctuation and total electron content are determined as a function of heliocentric distance, and, with a few exceptions caused by streamers and CMEs, are found to be consistent with previous results from experiments on Ulysses. Dense coronal streamers and several coronal mass ejection (CME) events were identified in the radio-frequency data, some of which were observed in white light by the LASCO coronagraphs onboard SOHO. For those events with sufficient mutual coverage, good correlations are found between the electron-content fluctuations and structure imaged by the LASCO instrument.  相似文献