Some appearances of the aurora borealis in Greece

Summary Some observations of the aurora borealis in Greece are given with the sources from which they were taken. It was found that the appearances of this phenomenon are not exceptionally rare. These appearances coincide with the years of maximun of solar activity and especially 1–2 years after it, while in the years of minimum of solar activity or near these years none of the described cases belong here. Except of two appearances of this phenomenon all the others were observed in Spring and Autumn or near these seasons. The southest geographical latitude of which edges of the aurora borealis have been observed in Greece is the latitude of 35 N.     

Emergence of sheared magnetic flux tubes in an active region observed with the SVST and TRACE

The active region NOAA AR 8331 was a target of an international ground-based observational campaign in the Canaries and coordinated with space instruments (TRACE and Yohkoh). We focus our study on observations obtained with the SVST at LaPalma, and with TRACE. On 10 September 1998, arch-filament systems were observed with high spatial and temporal resolution, from the lower to the upper atmosphere of the Sun, during five hours. Flux tubes emerged with increasing shear, which apparently led to energy release and heating in the overlying corona. A model for filament formation by the emergence of U-shaped loops from the subphotosphere, as proposed by Rust and Kumar (1994), is supported by the present observations. The coronal response to these events is visualized by rising, medium-scale loop brightenings. The low-lying X-ray loops show short-lived, bright knots which are thought to result from interaction between different loop systems.     

The role of technology for achieving climate policy objectives: overview of the EMF 27 study on global technology and climate policy strategies

This article presents the synthesis of results from the Stanford Energy Modeling Forum Study 27, an inter-comparison of 18 energy-economy and integrated assessment models. The study investigated the importance of individual mitigation options such as energy intensity improvements, carbon capture and storage (CCS), nuclear power, solar and wind power and bioenergy for climate mitigation. Limiting the atmospheric greenhouse gas concentration to 450 or 550 ppm CO₂ equivalent by 2100 would require a decarbonization of the global energy system in the 21^st century. Robust characteristics of the energy transformation are increased energy intensity improvements and the electrification of energy end use coupled with a fast decarbonization of the electricity sector. Non-electric energy end use is hardest to decarbonize, particularly in the transport sector. Technology is a key element of climate mitigation. Versatile technologies such as CCS and bioenergy are found to be most important, due in part to their combined ability to produce negative emissions. The importance of individual low-carbon electricity technologies is more limited due to the many alternatives in the sector. The scale of the energy transformation is larger for the 450 ppm than for the 550 ppm CO₂e target. As a result, the achievability and the costs of the 450 ppm target are more sensitive to variations in technology availability.     

A procedure to develop scalable models for the transient response of sleepers in conventional and high-speed railway lines and implementation to the vertical vibration mode

This paper presents a procedure to develop scalable reduced models for the through-the soil interaction and traveling wave effects of distant sleepers in a long railway track. For development purposes, and, without loss of generality, the geometry of the sleepers is consistent with the UIC-60 track system commonly used in European high speed rail and the vertical vibration mode is considered. The ballast and the effects of soil layering are not considered in the present paper; however, it is the subject of ongoing research. The proposed reduced models are based on B-Spline impulse response functions (BIRF) of the sleepers only as computed through boundary element method (BEM) solutions of the full model, preserve the frequency content of the full models, and they are highly accurate within the assumptions of linear isotropic and homogeneous soil media. They are expressed in a scalable form with respect to soil properties and sleeper spacing. In particular, the BIRFs of distant sleepers can be accurately approximated by appropriate scaling operations of time and amplitude of a reference sleeper BIRF while retaining all dynamic characteristics of the full model. Three main scaling parameters are proposed: (i) the apparent propagation velocity, (ii) the geometric damping coefficient, and (iii) the soil properties of a reference soil (i.e., the shear modulus and shear wave velocity). The models are validated through comparisons with other BEM solutions, and the accuracy and efficiency are established. The proposed models are developed as part of an NSF funded research on vibrations induced by high-speed rail traffic and are consistent with the associated train and rail models and a multi-system interface coupling (MSIC) technique that were developed as a part of the project and presented in companion papers. The proposed procedure forms the framework for developing scaled reduced models for other vibration modes and different sleeper geometries and can be generalized to include any foundation type or layered soil profiles.     

Analysis of Magnetic Shear in An X17 Solar Flare on October 28, 2003

An X17 class (GOES soft X-ray) two-ribbon solar flare on October 28, 2003 is analyzed in order to determine the relationship between the timing of the impulsive phase of the flare and the magnetic shear change in the flaring region. EUV observations made by the Transition Region and Coronal Explorer (TRACE) show a clear decrease in the shear of the flare footpoints during the flare. The shear change stopped in the middle of the impulsive phase. The observations are interpreted in terms of the splitting of the sheared envelope field of the greatly sheared core rope during the early phase of the flare. We have also investigated the temporal correlation between the EUV emission from the brightenings observed by TRACE and the hard X-ray (HXR) emission (E > 150 keV) observed by the anticoincidence system (ACS) of the spectrometer SPI on board the ESA INTEGRAL satellite. The correlation between these two emissions is very good, and the HXR sources (RHESSI) late in the flare are located within the two EUV ribbons. These observations are favorable to the explanation that the EUV brightenings mainly result from direct bombardment of the atmosphere by the energetic particles accelerated at the reconnection site, as does the HXR emission. However, if there is a high temperature (T > 20 MK) HXR source close to the loop top, a contribution of thermal conduction to the EUV brightenings cannot be ruled out.