MHD ANALYSIS OF PETSCHEK-TYPE RECONNECTION IN NON-UNIFORM FIELD AND FLOW GEOMETRIES

Analytical studies of reconnection have, for the most part, been confined to steady and uniform current sheet geometries. In contrast to these implifications, natural phenomena associated with the presence of current sheets indicate highly non-uniform structure and time-varying behaviour. Examples include the violent outbursts of energy on the Sun known as solar flares, and magnetospheric phenomena such as flux transfer events, plasmoids, and auroral activity. Unlike the theoretical models, reconnection therefore occurs in a highly dynamic and structured plasma environment. In this article we review the mathematical tools and techniques which are available to formulate models capable of describing the effects of reconnection in such situations. We confine attention to variants of the reconnection model first discussed by Petschek in the 1960s, in view of its successful application in predicting and interpreting phenomena in the terrestrial magnetosphere. The analysis of Petschek-type reconnection is based on the equations of ideal magnetohydrodynamics (MHD), which describe the large-scale behaviour of the magnetic field and plasma flow outside the diffusion region, which we determine as a localised part of the current sheet in which reconnection is initiated. The approach we adopt here is to transform the MHD equations into a Lagrangian or so-called 'frozen-in' coordinate system. In this coordinate system, the equation of motion transforms into a set of coupled nonlinear equations, in which the presence of inhomogeneous magnetic fields and/or plasma flows gives rise to a term similar to that which appears in the study of the ordinary string equation in a non-homogeneous medium. As demonstrated here, this approach not only clarifies and highlights the effects of such non-uniformities, it also simplifies the solution of the original set of MHD equations. In particular, this is true for those types of problem in which the total pressure can be considered as a known quantity from the outset. To illustrate the method, we solve several 2D problems involving magnetic field and flow non-uniformities: reconnection in a stagnation-point flow geometry with antiparallel magnetic fields; reconnection in a Y-type magnetic field geometry with and without velocity shear across the current sheet; and reconnection in a force-free magnetic field geometry with field lines of the form xy = const. These case examples, chosen for their tractability, each incorporate some aspects of the field and flow geomtries encountered in solar-terrestrial applications, and they provide a starting point for further analytical as well as numerical studies of reconnection.     

Extreme deviations of landing parameters for heavy aircraft

We estimate the statistic characteristics and extreme deviations of the state vector components for the Ilyushin Il 96–300 systems at automatic landing.     

Conductance of the channels and the knudsen paradox

In this paper, a physical model of gas flows through the channels is refined. The influence of gas absorbed by the walls of a channel on its conductance is shown. The experimentally obtained minimum on the channel conductance-pressure dependence (as the Knudsen paradox) is interpreted.     

On the hypothesis of adhesion

In this paper, the hypothesis of adhesion in fluid dynamics is analyzed and the field of its application is discussed.     

A synthetic aviation hydraulic fluid of new generation

The results of investigating a domestic synthetic hydraulic fluid of new generation are presented. By comparing with the existing domestic and foreign analogues, we specify the main advantages and disadvantages of this hydraulic fluid.     

Monthly trends in temporal and spatial distribution of Ionospheric Irregularities across the African region during the descending phase of solar cycle 24

Ionospheric irregularities are well-known phenomena associated with ionospheric scintillation. These irregularities comprise steep electron density gradients in the equatorial F region some 1 to 2 h after sunset in regions close to the geomagnetic equator. Using the IGS network of GNSS receivers spread across the low-latitude region over the African sector, we present the monthly trends in ionospheric irregularity activity levels based on Rate of TEC Index (ROTI) during the declining phase of solar 24. The monthly trends are statistically represented by counts of the night time ROTI values exceeding a threshold of 0.4 TECU/min. A clear trend emerges on the irregularity occurrence across the African sector: during the first four months of the year, the irregularity occurrence is highly pronounced on the western side of the region. The irregularity occurrence then shifts to the eastern side during the months of May, June, July, and August. During the last four months of the year, the irregularity occurrence is again more intense on the western side of Africa than on the eastern side. The occurrence of irregularity structures on only one side of the region during a given night is an unusual feature reported here for the first time.     

A Study of Vibrations of Space Station Structure Elements on the Basis of Video Information

Cosmic Research - The results of the Sreda–MKS space experiment showed that visual data on the vibrations of the ISS structural elements allow one to acquire quantitative characteristics of...     

Reconnection and fast particle production in tokamak and solar plasmas

Detailed in situ studies of magnetic reconnection and particle acceleration, which play a crucial role in the release and redistribution of energy in solar flares, can be performed in tokamak plasmas under conditions resembling those of the flaring solar corona. Recent measurements and modelling of fast particle production during reconnection events in the Mega-Amp Spherical Tokamak (MAST) are described. Specifically, observations in this device of electron acceleration during edge localised modes, and of both ion and electron acceleration during merging-compression plasma start-up, are presented, and possible implications of these studies for particle acceleration in flares are discussed. The results from MAST lend weight to the conjecture that large numbers of ions are accelerated to sub-MeV energies in flares.     

A comprehensive statistical study of cloud base height using ceilometer over western India

A comprehensive statistical analysis of the cloud base height (CBH) measured by ground-based Vaisala ceilometer (CL31) has been performed to study different layers of the cloud in the lower troposphere up to 7.5?km height over Ahmedabad, western India during 2014 and 2015. The total observations (～69%) of cloud by using ceilometer show annual cloud occurrence frequency of around 64%. Seasonal variation of CBH and cloud occurrence frequency reveal that the maximum/minimum cloud cover is found during southwest (SW) Indian summer monsoon/pre-monsoon season. Three CBHs (CBH1, CBH2, and CBH3) are presented in monsoon period due to high cloud occurrence, and two CBHs (CBH1 and CBH2) are observed in other seasons due to low cloud occurrence by ceilometer over the observational site. The CBH1 (～100–2000?m) and CBH2 (500–3000?m) are observed during SW monsoon and summer season, respectively. The CBH3 is occurred usually in SW monsoon season. Moreover, the cloud cover during the day and night time shows that the occurrence of cloud is more frequent in daytime than nighttime during pre-monsoon and post-monsoon season. The statistical analysis of cloud with ground-based observations is also performed in this study that may be useful for the development/improvement of regional weather and climate models to reduce the uncertainty in the prediction.