141 research outputs found
STARE velocity at large flow angles: is it related to the ion acoustic speed?
International audienceThe electron drift and ion-acoustic speed in the E region inferred from EISCAT measurements are compared with concurrent STARE radar velocity data to investigate a recent hypothesis by Bahcivan et al. (2005), that the electrojet irregularity velocity at large flow angles is simply the product of the ion-acoustic speed and the cosine of an angle between the electron flow and the irregularity propagation direction. About 3000 measurements for flow angles of 50°?70° and electron drifts of 400?1500 m/s are considered. It is shown that the correlation coefficient and the slope of the best linear fit line between the predicted STARE velocity (based solely on EISCAT data and the hypothesis of Bahcivan et al. (2005)) and the measured one are both of the order of ~0.4. Velocity predictions are somewhat better if one assumes that the irregularity phase velocity is the line-of-sight component of the E×B drift scaled down by a factor ~0.6 due to off-orthogonality of irregularity propagation (nonzero effective aspect angles of STARE observations)
Volume cross section of auroral radar backscatter and RMS plasma fluctuations inferred from coherent and incoherent scatter data: a response on backscatter volume parameters
Norway and Finland STARE radar measurements in the eastward auroral
electrojet are combined with EISCAT CP-1 measurements of the electron
density and electric field vector in the common scattering volume to
investigate the variation of the auroral radar volume cross section (VCS)
with the flow angle of observations (radar look direction with respect to
the <I><B>E</B></I>×<I><B>B</I></B> electron drift). The data set available consists of ~6000 points
for flow angles of 40–85° and electron drifts between 500
and 2000 m s<sup>−1</sup>. The EISCAT electron density <I>N(h)</I>-profile data are used
to estimate the effective electron density, aspect angle and thickness of
the backscattering layer. It is shown that the flow angle variation of the
VCS is rather weak, only ~5 dB within the range of the considered
flow angles. The VCS values themselves respond almost linearly to the square
of both the electron drift velocity magnitude and the effective electron
density. By adopting the inferred shape of the VCS variation with the flow
angle and the VCS dependence upon wavelength, the relative amplitude of
electrostatic electron density fluctuations over all scales is estimated.
Inferred values of 2–4 percent react nearly linearly to the electron drift
velocity in the range of 500–1000 m s<sup>−1</sup> but the rate of increase slows
down at electron drifts >1000 m s<sup>−1</sup> and density fluctuations of ~5.5
percent due to, perhaps, progressively growing nonlinear wave losses
Instability and Chaos in Non-Linear Wave Interaction: a simple model
We analyze stability of a system which contains an harmonic oscillator
non-linearly coupled to its second harmonic, in the presence of a driving
force. It is found that there always exists a critical amplitude of the driving
force above which a loss of stability appears. The dependence of the critical
input power on the physical parameters is analyzed. For a driving force with
higher amplitude chaotic behavior is observed. Generalization to interactions
which include higher modes is discussed.
Keywords: Non-Linear Waves, Stability, Chaos.Comment: 16 pages, 4 figure
Using the Uncharged Kerr Black Hole as a Gravitational Mirror
We extend the study of the possibility to use the Schwarzschild black hole as
a gravitational mirror to the more general case of an uncharged Kerr black
hole. We use the null geodesic equation in the equatorial plane to prove a
theorem concerning the conditions the impact parameter has to satisfy if there
shall exist boomerang photons. We derive an equation for these boomerang
photons and an equation for the emission angle. Finally, the radial null
geodesic equation is integrated numerically in order to illustrate boomerang
photons.Comment: 11 pages Latex, 3 Postscript figures, uufiles to compres
HF radar observations of high-aspect angle backscatter from the E-region
International audienceWe present evidence for the observation of high-aspect angle HF radar backscatter from the auroral electrojets, and describe the spectral characteristics of these echoes. Such backscatter is observed at very near ranges where ionospheric refraction is not sufficient to bring the sounding radio waves to orthogonality with the magnetic field; the frequency dependence of this propagation effect is investigated with the Stereo upgrade of the CUTLASS Iceland radar. We term the occurrence of such echoes the "high-aspect angle irregularity region" or HAIR. It is suggested that backscatter is observed at aspect angles as high as 30°, with an aspect sensitivity as low as 1dB deg–1. These echoes are distinguished from normal electrojet backscatter by having low Doppler shifts with an azimuthal dependence that appears more consistent with the direction of the convection electric field than with the expected electron drift direction. This is discussed in terms of the linear theory dispersion relation for electrojet waves. Key words. Ionosphere (ionospheric irregularities; plasma waves and instabilities; auroral ionosphere
Sphere rolling on the surface of a cone
We analyse the motion of a sphere that rolls without slipping on a conical
surface having its axis in the direction of the constant gravitational field of
the Earth. This nonholonomic system admits a solution in terms of quadratures.
We exhibit that the only circular of the system orbit is stable and furthermore
show that all its solutions can be found using an analogy with central force
problems. We also discuss the case of motion with no gravitational field, that
is, of motion on a freely falling cone.Comment: 12 pages, 2 figures, to be published in Eur J Phy
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