122 research outputs found
INTERMITTENCY IN SPACE PLASMA TURBULENCE
No full textWe present a short overview on theoretical aspects and observational findings about intermittency in solar wind MHD turbulence. This phenomenon, which became a remarkable topic in the past few years, is due to the fact that solar wind fluctuations are neither isotropic nor scale-invariant, two of the fundamental hypotheses at the basis of K41 Kolmogorov's theory. As a matter of fact, intermittency reflects in an inhomogeneous energy transfer during the turbulent cascade. In such conditions, time series alternate intervals of very high activity to intervals of quiescence. We will first introduce the problem of intermittency from a theoretical point of view, mentioning also different models apt to reproduce statistical features of turbulent intermittent behaviour. Afterwards, we will show how intermittency affects solar wind parameters, how intermittency evolves with heliocentric distance from the sun and how typical interplanetary events causing intermittency look like
Evaluation Of Variable Speed Limits For Real-Time Freeway Safety Improvement
No full textUse of various variable speed limit (VSL) strategies as a tool for safety improvement on freeways was evaluated using simulation of a section of Interstate 4 in Orlando, FL. Real-time crash likelihood was calculated based on models developed in previous research by the first author [Abdel-Aty, M., Uddin, N., Pande, A., January 2005. Split models for predicting multi-vehicle crashes during high-speed and low-speed operating conditions on freeways. In: Presented at the 84th Annual Meeting of the Transportation Research Board, Washington, DC]. VSL implementation produced safety improvement by simultaneously implementing lower speed limits upstream and higher speed limits downstream of the location where crash likelihood is observed in real-time. This improvement was realized in the case of medium-to-high-speed regimes on the freeway, but no benefit was achieved in low-speed situations (no substantial safety benefit from implementing VSL in congested situation\u27s simulation). The final recommendations for implementing VSL are:gradually introducing speed limit changes over time (5 mph every 10 min);abruptly changing speed limit in space (no gap distance);reducing speed limits upstream and increasing speed limits downstream of location of interest;the speed limit changes up- and downstream should be large in magnitude (15 mph) and implemented within short distances (2 miles) of the location of interest. In addition to the safety benefit, this final strategy also produced travel time savings
Reply to "Comment on 'Detuning effects in the one-photon mazer' "
No full textpeer reviewedWe refute in this Reply the criticisms made by Abdel-Aty. We show that none of them are founded and we demonstrate very explicitly what is wrong with the arguments developed by this author
Obliquely propagating electromagnetic drift ion cyclotron instability
No full textIdentification of wave characteristics during current disruption events in the near-Earth geomagnetic tail region is important for understanding the substorm onset mechanism. The present paper discusses a linear stability analysis in the ion cyclotron frequency range of a plasma possessing the temperature anisotropy and cross-field flow. It is found that the ion cyclotron drift waves propagating in quasi-perpendicular direction with respect to the ambient magnetic field are characterized by low frequencies (omega less than or similar to 0.5 Omega(i)), while quasi-parallel waves have frequencies close to the ion cyclotron frequency (omega similar to Omega(i)). This finding is consistent with the observation by THEMIS spacecraft of a current disruption event in which similar high-and low-frequency band structures are present. The interpretation of the low-frequency mode as the quasi-perpendicular mode is consistent with a recent data analysis. The present paper shows that the high-frequency quasi-parallel mode can simultaneously be excited with the low-frequency quasi-perpendicular mode.X117sciescopu
Global two-fluid stability of bifurcated current sheets
No full text[1] Electromagnetic stability of modified Harris model, featuring a bifurcated current density profile, against lower-hybrid drift perturbation is investigated. The bifurcated current sheet is characterized by temperature and cross-field flow velocity profiles on different spatial scales across the current sheet. It is found that the eigenmodes of unstable solutions are mainly confined to the inner slopes of the bifurcated current sheet. That is, the fluctuations are trapped within the two current layers. This finding shows that the source of anomalous resistivity is focused near the neutral sheet, thus indicating that the excitation of electromagnetic lower-hybrid drift instability in a bifurcated current sheet may be a major cause of magnetic reconnection onset.X1132sciescopu
Inverse cascade feature in current disruption
No full textWe investigate the origin of waves leading to current disruption and dipolarization observed by a THEMIS satellite at XGSM similar to -8 R-E in the magnetotail near a substorm expansion onset on 29 January 2008. Continuous wavelet transform of the magnetic activity associated with current disruption shows a clear inverse cascade feature of waves starting at frequencies slightly below the ion cyclotron frequency to waves at low frequencies that correspond to the time scale of dipolarization. There was no other low-frequency wave associated with the time scale of dipolarization. On the basis of this inverse cascade feature, we provide a preliminary theoretical analysis to propose that the initial excited waves causing eventual dipolarization in this event may originate from the drift-driven electromagnetic ion cyclotron instability, which is an electromagnetic version of the Weibel instability, or an ordinary mode instability similarly driven by the cross-field ion drift.X1136sciescopu
Electron source associated with dipolarization at the outer boundary of the radiation belts: Non-storm cases
No full textAn investigation on directionally dependent fracture toughness behavior of monolithic nickel gradient material synthesized from electroplating
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