Short term characteristics of ion-acoustic type radio auroral echoes

1978 ◽  
Vol 56 (2) ◽  
pp. 292-301 ◽  
Author(s):  
Christos Haldoupis ◽  
George Sofko
Keyword(s):  
Signal Amplitude ◽  
Acoustic Velocity ◽  
Velocity Range ◽  
Short Term ◽  
Backscatter Signal ◽  
Ion Acoustic ◽  
Longitudinal Plasma ◽  
Digital Demodulation ◽  
Signal Fading ◽  
Time Sequences

Digital demodulation techniques and spectral analysis are used to study the short term (<1 s) characteristics of the ion-acoustic radio auroral echoes. Examination of 0.4 s time sequences indicates that the signal amplitude undergoes a deep and quasi-periodic fading with strongly marked periodicities in the 2–10 Hz range. Evidence shows that the fading is not due to interference but to the appearance and disappearance of independent scatterers, causing a sequence of backscatter signal bursts. If the assumption is made that these scatterers are longitudinal plasma density waves, the observed signal fading can be interpreted in terms of the growth and decay of individual regions of plasma instability rather than as interference between signals from separated coexisting scattering regions. Investigation of a large number of records suggests the following features for the irregularities associated with the ion-acoustic echoes: (1) their lifetime is in the 0.05–0.25 s range. (2) their growth (or decay) rate is in the 10–60 s−1 range, (3) their velocity remains fairly constant, even during growth and decay, and is always within the ion-acoustic velocity range in the medium.

Experimental and Theoretical Study of the Cavity Growth of Spherical Fragment Penetrating Liquid-Filled Container

2020 ◽  
Vol 143 (1) ◽  
Author(s):  
Anran Chen ◽  
Xiangdong Li ◽  
Lanwei Zhou ◽  
Yangziyi Ji
Keyword(s):  
Theoretical Study ◽  
Cavity Growth ◽  
Acoustic Velocity ◽  
Velocity Range ◽  
Improved Method ◽  
Cavity Shape ◽  
Boundary Constraints ◽  
Hydrodynamic Ram ◽  
Aircraft Fuel ◽  
Liquid Type

Abstract When high-velocity penetrator impacts and penetrates a liquid-filled container such as an aircraft fuel tank, the hydrodynamic ram (HRAM) event occurs. This process could be roughly divided into four phases, each of which could cause different degrees of damage to the liquid-filled container or the surrounding equipment. Spherical fragment impacting tests of different velocities were performed on two sizes of liquid-filled containers to investigate the effect of boundary constraints on cavity growth. The velocity range in the experiment was from 600 m/s to 1400 m/s. Through theoretical analysis and experimental results, it is found that the radial disturbance range of the cavity is not constant in different containers and under different impact velocities. An improved method is presented to modeling the cavity growth in the drag-cavity phases of HRAM events. The approach quantitatively describes the radial disturbance range of the cavity and is appropriate for the calculation of the cavity growth in HRAM. Moreover, the effect of liquid type on cavity growth is studied theoretically. When the fragment velocity is less than Mach 0.5, the length and radius of the cavity are mainly affected by the density of the liquid. When the fragment velocity exceeds Mach 0.5, the characteristics of cavity shape are mainly affected by the acoustic velocity in the liquid.

Variations revealed by INFREP Radio Network in correspondence of six earthquakes with MW greater than 5.0 occurred in the Balkan Peninsula and Adriatic Sea on 26 and 27 November, 2019

2020 ◽  
Author(s):  
Pier Francesco Biagi ◽  
Alexandra Nina ◽  
Anita Ermini ◽  
Giovanni Nico
Keyword(s):  
Adriatic Sea ◽  
Technological Development ◽  
Signal Amplitude ◽  
Balkan Peninsula ◽  
Ministry Of Education ◽  
Short Term ◽  
Before And After ◽  
The Republic ◽  
Education Science

&lt;p&gt;In this work we analyse variations in VLF/LF radio signal amplitudes recorded by the INFREP network in the period 16 November &amp;#8211; 6 December, 2019 characterized by very intensive seismic activities in the Balkan peninsula, Crete, and Adriatic, Aegean and Black seas. Namely, 38 earthquakes with magnitude greater than 4.0 occurred in this area during the noticed period; the most intensive of them occurred on 26 and 27 November: three events in Albania (M&lt;sub&gt;w&lt;/sub&gt;= 6.4, 5.3, 5.1), one in Crete (M&lt;sub&gt;w&lt;/sub&gt;= 6), one in Bosnia and Herzegovina (M&lt;sub&gt;w&lt;/sub&gt;= 5.4) and two in Adriatic sea (M&lt;sub&gt;w&lt;/sub&gt;= 5.4, 5.3). We study both long- and short- term variations that are already recorded in earlier studies. The long-term variations relate to changes in the amplitude intensities in periods of several days and their existence is shown in many previous studies. The recent analyses also indicate short-term variations in signal amplitude noises started about several tents of minutes before the earthquake (Nina et al. 2020). In this work, we analyse different areas using INFREP network, which allow us to study local changes in the atmosphere. In order to examine possible precursors we considered longer time started and ended 10 days before and after the most intensive of the considered earthquakes, respectively.&lt;/p&gt;&lt;p&gt;This research is supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia, under the projects 176002 and III44002.&lt;/p&gt;&lt;p&gt;References&lt;/p&gt;&lt;p&gt;Nina, A., S. Pulinets, P. F. Biagi, G. Nico, S. T. Mitrovi&amp;#263;, M. Radovanovi&amp;#263; and L. &amp;#268;. Popovi&amp;#263;. Science of the Total Environment 710 (2020) 136406&lt;/p&gt;

Shielding of moving test particles in warm, isotropic plasma

1973 ◽  
Vol 9 (3) ◽  
pp. 311-324 ◽  
Author(s):  
Liu Chen ◽  
A. Bruce Landon ◽  
M. A. Lieberman
Keyword(s):  
Logarithmic Singularity ◽  
Field Potential ◽  
Acoustic Velocity ◽  
Ion Distribution ◽  
Potential Wells ◽  
Test Particles ◽  
Ion Acoustic ◽  
Cone Surface ◽  
Cone Potential ◽  
A Charge

Shielding of test charges in warm, isotropic electron and electron–ion (Te ≫ Ti) plasmas is studied analytically and numerically. For a plasma with hot Maxwellian electrons and cold mobile ions, the potential due to a charge moving faster than the ion acoustic velocity has an ion acoustic Cerenkov cone. Ahead of the particle, the shielding is the usual electron Debye type with a modified longer shielding length. Potential wells with γ−1 dependence exists inside the cone. The potential falls off as along the cone surface. Outside the cone, the potential decays exponentially. A charge moving slower than the ion acoustic velocity also creates a cone, with potential decay as γ−3 outside the cone, potential wells decaying as γ−1 inside the cone, and potential wells falling off as along the cone surface. In both cases a radial logarithmic singularity exists along the trailing axis. Using a mono-energetic ion distribution, the singularity is removed and an ion thermal Cerenkov cone appears. For a monoenergetic electron plasma, assuming immobile ions, a test charge moving faster than the electron thermal velocity excites a thermal Cerenkov cone. Outside the cone, the far-field potential falls off in quadrupole form as γ−3. Inside the cone, potential wells decay as γ−1.

Conceptual short-term memory (CSTM) supports core claims of Christiansen and Chater

2016 ◽  
Vol 39 ◽  
Author(s):  
Mary C. Potter
Keyword(s):  
Working Memory ◽  
Rapid Serial Visual Presentation ◽  
Language Comprehension ◽  
Short Term Memory ◽  
Visual Presentation ◽  
Long Term Memory ◽  
Short Term ◽  
Term Memory ◽  
Use Of Knowledge

AbstractRapid serial visual presentation (RSVP) of words or pictured scenes provides evidence for a large-capacity conceptual short-term memory (CSTM) that momentarily provides rich associated material from long-term memory, permitting rapid chunking (Potter 1993; 2009; 2012). In perception of scenes as well as language comprehension, we make use of knowledge that briefly exceeds the supposed limits of working memory.

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