105 research outputs found
The Nonlinear Evolution of Instabilities Driven by Magnetic Buoyancy: A New Mechanism for the Formation of Coherent Magnetic Structures
Motivated by the problem of the formation of active regions from a
deep-seated solar magnetic field, we consider the nonlinear three-dimensional
evolution of magnetic buoyancy instabilities resulting from a smoothly
stratified horizontal magnetic field. By exploring the case for which the
instability is continuously driven we have identified a new mechanism for the
formation of concentrations of magnetic flux.Comment: Published in ApJL. Version with colour figure
Moored observations of mesoscale features in the Cape Basin: characteristics and local impacts on water mass distributions
The eastern side of the South Atlantic Meridional overturning circulation
Basin-wide Array (SAMBA) along 34.5° S is used to assess the
nonlinear, mesoscale dynamics of the Cape Basin. This array presently
consists of current meter moorings and bottom mounted Current and Pressure recording Inverted Echo
Sounders (CPIES)
deployed across the continental slope. These data, available from September
2014 to December 2015, combined with satellite altimetry allow us to
investigate the characteristics and the impact of mesoscale dynamics on local
water mass distribution and cross-validate the different data sets. We
demonstrate that the moorings are affected by the complex dynamics of the
Cape Basin involving Agulhas rings, cyclonic eddies and anticyclonic eddies
from the Agulhas Bank and the South Benguela upwelling front and filaments.
Our analyses show that exchange of water masses happens through the advection
of water by mesoscale eddies but also via wide water mass intrusions
engendered by the existence of intense dipoles. These complex dynamics induce
strong intra-seasonal upper-ocean velocity variations and water mass
exchanges between the shelf and the open ocean but also across the
subantarctic and subtropical waters. This work presents the first independent
observations comparison between full-depth moorings and CPIES data sets
within the eastern South Atlantic region that gives some evidence of eastern
boundary buoyancy anomalies associated with migrating eddies. It also
highlights the need to continuously sample the full water depth as
inter-basin exchanges occur intermittently and affect the whole water column.</p
Controlling the Frequency-Temperature Sensitivity of a Cryogenic Sapphire Maser Frequency Standard by Manipulating Fe3+ Spins in the Sapphire Lattice
To create a stable signal from a cryogenic sapphire maser frequency standard,
the frequency-temperature dependence of the supporting Whispering Gallery mode
must be annulled. We report the ability to control this dependence by
manipulating the paramagnetic susceptibility of Fe3+ ions in the sapphire
lattice. We show that the maser signal depends on other Whispering Gallery
modes tuned to the pump signal near 31 GHz, and the annulment point can be
controlled to exist between 5 to 10 K depending on the Fe3+ ion concentration
and the frequency of the pump. This level of control has not been achieved
previously, and will allow improvements in the stability of such devices.Comment: 17 pages, 10 figure
Creating traveling waves from standing waves from the gyrotropic paramagnetic properties of Fe ions in a high-Q whispering gallery mode sapphire resonator
We report observations of the gyrotropic change in magnetic susceptibility of
the Fe electron paramagnetic resonance at 12.037GHz (between spin states
and ) in sapphire with respect to applied magnetic field.
Measurements were made by observing the response of the high-Q Whispering
Gallery doublet (WGH) in a Hemex sapphire resonator cooled to 5
K. The doublets initially existed as standing waves at zero field and were
transformed to traveling waves due to the gyrotropic response.Comment: Accepted for publication in Phys. Rev.
Ultra-Low Noise Microwave Extraction from Fiber-Based Optical Frequency Comb
In this letter, we report on all-optical fiber approach to the generation of
ultra-low noise microwave signals. We make use of two erbium fiber mode-locked
lasers phase locked to a common ultra-stable laser source to generate an 11.55
GHz signal with an unprecedented relative phase noise of -111 dBc/Hz at 1 Hz
from the carrier.The residual frequency instability of the microwave signals
derived from the two optical frequency combs is below 2.3 10^(-16) at 1s and
about 4 10^(-19) at 6.5 10^(4)s (in 5 Hz bandwidth, three days continuous
operation).Comment: 12 pages, 3 figure
Alpha effect due to buoyancy instability of a magnetic layer
A strong toroidal field can exist in form of a magnetic layer in the
overshoot region below the solar convection zone. This motivates a more
detailed study of the magnetic buoyancy instability with rotation. We calculate
the alpha effect due to helical motions caused by a disintegrating magnetic
layer in a rotating density-stratified system with angular velocity Omega
making an angle theta with the vertical. We also study the dependence of the
alpha effect on theta and the strength of the initial magnetic field. We carry
out three-dimensional hydromagnetic simulations in Cartesian geometry. A
turbulent EMF due to the correlations of the small scale velocity and magnetic
field is generated. We use the test-field method to calculate the transport
coefficients of the inhomogeneous turbulence produced by the layer. We show
that the growth rate of the instability and the twist of the magnetic field
vary monotonically with the ratio of thermal conductivity to magnetic
diffusivity. The resulting alpha effect is inhomogeneous and increases with the
strength of the initial magnetic field. It is thus an example of an
"anti-quenched" alpha effect. The alpha effect is nonlocal, requiring around
8--16 Fourier modes to reconstruct the actual EMF based on the actual mean
field.Comment: 14 pages, 19 figures 3 tables (submitted to A & A
Linear dynamics of weakly viscous accretion disks: A disk analog of Tollmien-Schlichting waves
This paper discusses new perspectives and approaches to the problem of disk
dynamics where, in this study, we focus on the effects of viscous instabilities
influenced by boundary effects. The Boussinesq approximation of the viscous
large shearing box equations is analyzed in which the azimuthal length scale of
the disturbance is much larger than the radial and vertical scales. We examine
the stability of a non-axisymmetric potential vorticity mode, i.e. a
PV-anomaly. in a configuration in which buoyant convection and the
strato-rotational instability do not to operate. We consider a series of
boundary conditions which show the PV-anomaly to be unstable both on a finite
and semi-infinite radial domains. We find these conditions leading to an
instability which is the disk analog of Tollmien-Schlichting waves. When the
viscosity is weak, evidence of the instability is most pronounced by the
emergence of a vortex sheet at the critical layer located away from the
boundary where the instability is generated. For some boundary conditions a
necessary criterion for the onset of instability for vertical wavelengths that
are a sizable fraction of the layer's thickness and when the viscosity is small
is that the appropriate Froude number of the flow be greater than one. This
instability persists if more realistic boundary conditions are applied,
although the criterion on the Froude number is more complicated. The unstable
waves studied here share qualitative features to the instability seen in
rotating Blasius boundary layers. The implications of these results are
discussed. An overall new strategy for exploring and interpreting disk
instability mechanisms is also suggested.Comment: Accepted for publication in Astronomy and Astrophysics. 18 pages.
This version 3 with corrected style fil
Measurement of fundamental thermal noise limit in a cryogenic sapphire frequency standard using bimodal maser oscillations
We report observations of the Schawlow-Townes noise limit in a cryogenic
sapphire secondary frequency standard. The effect causes a fundamental limit to
the frequency stability, and was measured through the novel excitation of a
bimodal maser oscillation of a Whispering Gallery doublet at . The
beat frequency of between the oscillations enabled a sensitive probe
for this measurement of fractional frequency instability of
with only 0.5 of output power.Comment: Published in PRL 100, 233901 (2008
ELISA: a cryocooled 10 GHz oscillator with 10-15 frequency stability
This article reports the design, the breadboarding and the validation of an
ultra-stable Cryogenic Sapphire Oscillator operated in an autonomous
cryocooler. The objective of this project was to demonstrate the feasibility of
a frequency stability of 3x10-15 between 1 s and 1,000 s for the European Space
Agency deep space stations. This represents the lowest fractional frequency
instability ever achieved with cryocoolers. The preliminary results presented
in this paper validate the design we adopted for the sapphire resonator, the
cold source and the oscillator loop.Comment: 13 pages, 10 figure
- âŠ