639 research outputs found
Anti-sunward high-speed jets in the subsolar magnetosheath
Using 2008â2011 data from the five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft in Earth's subsolar
magnetosheath, we study high-speed jets identified as intervals when the
anti-sunward component of the dynamic pressure in the subsolar magnetosheath exceeds half of its
upstream solar wind value. Based on our comprehensive data set of 2859
high-speed jets, we obtain the following statistical results on jet properties
and favorable conditions: high-speed jets occur predominantly downstream of
the quasi-parallel bow shock, i.e., when interplanetary magnetic field cone
angles are low. Apart from that, jet occurrence is only very weakly dependent
(if at all) on other upstream conditions or solar wind variability. Typical
durations and recurrence times of high-speed jets are on the order of tens of
seconds and a few minutes, respectively. Relative to the ambient
magnetosheath, high-speed jets exhibit higher speed, density and magnetic
field intensity, but lower and more isotropic temperatures. They are
almost always super-Alfvénic, often even super-magnetosonic, and
typically feature 6.5 times as much dynamic pressure and twice as much total
pressure in anti-sunward direction as the surrounding plasma does. Consequently,
they are likely to have significant effects on the magnetosphere and
ionosphere if they impinge on the magnetopause
Supermagnetosonic jets behind a collisionless quasi-parallel shock
The downstream region of a collisionless quasi-parallel shock is structured
containing bulk flows with high kinetic energy density from a previously
unidentified source. We present Cluster multi-spacecraft measurements of this
type of supermagnetosonic jet as well as of a weak secondary shock front within
the sheath, that allow us to propose the following generation mechanism for the
jets: The local curvature variations inherent to quasi-parallel shocks can
create fast, deflected jets accompanied by density variations in the downstream
region. If the speed of the jet is super(magneto)sonic in the reference frame
of the obstacle, a second shock front forms in the sheath closer to the
obstacle. Our results can be applied to collisionless quasi-parallel shocks in
many plasma environments.Comment: accepted to Phys. Rev. Lett. (Nov 5, 2009
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Millimeter-wave signal generation using an integrated mode-locked semiconductor laser and photodiode
A compact optoelectronic integrated circuit for generation of mm-wave frequencies is demonstrated. A monolithically integrated semiconductor ring laser, optical amplifier and waveguide photodiode are used to generate electrical signals up to 85.2 GHz
In Situ Observations of a Magnetosheath High-Speed Jet Triggering Magnetopause Reconnection
Magnetosheath highâspeed jetsâlocalized dynamic pressure enhancements typically of âŒ1 Earth radius in sizeâimpact the dayside magnetopause several times per hour. Here we present the first in situ measurements suggesting that such an impact triggered magnetopause reconnection. We use observations from the five Time History of Events and Macroscale Interactions during Substorms spacecraft in a stringâofâpearls configuration on 7 August 2007. The spacecraft recorded magnetopause inâandâout motion during an impact of a magnetosheath jet (VNâŒâ300 km/s along the magnetopause normal direction). There was no evidence for reconnection for the preimpact crossing, yet three probes observed reconnection after the impact. We infer that the jet impact compressed the originally thick (60â70 di), high magnetic shear (140â160° magnetopause until it was thin enough for reconnection to occur. Magnetosheath highâspeed jets could therefore act as a driver for bursty dayside reconnection
Direct observations of a surface eigenmode of the dayside magnetopause
The abrupt boundary between a magnetosphere and the surrounding plasma, the magnetopause, has long been known to support surface waves. It was proposed that impulses acting on the boundary might lead to a trapping of these waves on the dayside by the ionosphere, resulting in a standing wave or eigenmode of the magnetopause surface. No direct observational evidence of this has been found to date and searches for indirect evidence have proved inconclusive, leading to speculation that this mechanism might not occur. By using fortuitous multipoint spacecraft observations during a rare isolated fast plasma jet impinging on the boundary, here we show that the resulting magnetopause motion and magnetospheric ultra-low frequency waves at well-defined frequencies are in agreement with and can only be explained by the magnetopause surface eigenmode. We therefore show through direct observations that this mechanism, which should impact upon the magnetospheric system globally, does in fact occur
Identification of Kelvin waves: numerical challenges
Kelvin waves are expected to play an essential role in the energy dissipation
for quantized vortices. However, the identification of these helical
distortions is not straightforward, especially in case of vortex tangle. Here
we review several numerical methods that have been used to identify Kelvin
waves within the vortex filament model. We test their validity using several
examples and estimate whether these methods are accurate enough to verify the
correct Kelvin spectrum. We also illustrate how the correlation dimension is
related to different Kelvin spectra and remind that the 3D energy spectrum E(k)
takes the form 1/k in the high-k region, even in the presence of Kelvin waves.Comment: 6 pages, 5 figures. The final publication is available at
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