scholarly journals Evidence of standing waves during a Pi2 pulsation event observed on Cluster

2006 ◽  
Vol 24 (10) ◽  
pp. 2719-2733 ◽  
Author(s):  
A. B. Collier ◽  
A. R. W. Hughes ◽  
L. G. Blomberg ◽  
P. R. Sutcliffe
Keyword(s):  
Relative Phase ◽  
Cavity Mode ◽  
Standing Waves ◽  
Phase Relationship ◽  
Wave Number ◽  
Poynting Flux ◽  
Pi2 Pulsation ◽  
Pi2 Pulsations ◽  
Low Latitudes ◽  
Azimuthal Wave Number

Abstract. Observations of Pi2 pulsations at middle and low latitudes have been explained in terms of cavity mode resonances, whereas transients associated with field-aligned currents appear to be responsible for the high latitude Pi2 signature. Data from Cluster are used to study a Pi2 event observed at 18:09 UTC on 21 January 2003, when three of the satellites were within the plasmasphere (L=4.7, 4.5 and 4.6) while the fourth was on the plasmapause or in the plasmatrough (L=6.6). Simultaneous pulsations at ground observatories and the injection of particles at geosynchronous orbit corroborate the occurrence of a substorm. Evidence of a cavity mode resonance is established by considering the phase relationship between the orthogonal electric and magnetic field components associated with radial and field-aligned standing waves. The relative phase between satellites located on either side of the geomagnetic equator indicates that the field-aligned oscillation is an odd harmonic. Finite azimuthal Poynting flux suggests that the cavity is effectively open ended and the azimuthal wave number is estimated as m~13.5.

scholarly journals Experimental Study on Coherent Structures by Particles Suspended in Half-Zone Thermocapillary Liquid Bridges: Review

Fluids ◽  
2021 ◽  
Vol 6 (3) ◽  
pp. 105
Author(s):  
Ichiro Ueno
Keyword(s):  
Coherent Structures ◽  
Three Dimensional ◽  
Wave Number ◽  
Flow Structures ◽  
Liquid Bridges ◽  
Thermocapillary Effect ◽  
Particle Accumulation ◽  
Azimuthal Wave Number ◽  
Experimental Approaches ◽  
Particle Behaviour

Coherent structures by the particles suspended in the half-zone thermocapillary liquid bridges via experimental approaches are introduced. General knowledge on the particle accumulation structures (PAS) is described, and then the spatial–temporal behaviours of the particles forming the PAS are illustrated with the results of the two- and three-dimensional particle tracking. Variations of the coherent structures as functions of the intensity of the thermocapillary effect and the particle size are introduced by focusing on the PAS of the azimuthal wave number m=3. Correlation between the particle behaviour and the ordered flow structures known as the Kolmogorov–Arnold—Moser tori is discussed. Recent works on the PAS of m=1 are briefly introduced.

scholarly journals Intermediate-<I>m</I> ULF waves generated by substorm injection: a case study

2010 ◽  
Vol 28 (8) ◽  
pp. 1499-1509 ◽  
Author(s):  
T. K. Yeoman ◽  
D. Yu. Klimushkin ◽  
P. N. Mager
Keyword(s):  
Phase Variation ◽  
Wave Activity ◽  
Wave Number ◽  
Ulf Wave ◽  
Azimuthal Wave ◽  
Azimuthal Wave Number ◽  
Source Particle ◽  
Phase Propagation ◽  
Field Lines

Abstract. A case study of SuperDARN observations of Pc5 Alfvén ULF wave activity generated in the immediate aftermath of a modest-intensity substorm expansion phase onset is presented. Observations from the Hankasalmi radar reveal that the wave had a period of 580 s and was characterized by an intermediate azimuthal wave number (m=13), with an eastwards phase propagation. It had a significant poloidal component and a rapid equatorward phase propagation (~62° per degree of latitude). The total equatorward phase variation over the wave signatures visible in the radar field-of-view exceeded the 180° associated with field line resonances. The wave activity is interpreted as being stimulated by recently-injected energetic particles. Specifically the wave is thought to arise from an eastward drifting cloud of energetic electrons in a similar fashion to recent theoretical suggestions (Mager and Klimushkin, 2008; Zolotukhina et al., 2008; Mager et al., 2009). The azimuthal wave number m is determined by the wave eigenfrequency and the drift velocity of the source particle population. To create such an intermediate-m wave, the injected particles must have rather high energies for a given L-shell, in comparison to previous observations of wave events with equatorward polarization. The wave period is somewhat longer than previous observations of equatorward-propagating events. This may well be a consequence of the wave occurring very shortly after the substorm expansion, on stretched near-midnight field lines characterised by longer eigenfrequencies than those involved in previous observations.

scholarly journals SABRE observations of Pi2 pulsations: case studies

1997 ◽  
Vol 15 (1) ◽  
pp. 40-53 ◽  
Author(s):  
E. G. Bradshaw ◽  
M. Lester
Keyword(s):  
Case Studies ◽  
Magnetic Activity ◽  
Auroral Zone ◽  
Field Of View ◽  
Ionospheric Currents ◽  
Pi2 Pulsation ◽  
First Case ◽  
Pi2 Pulsations ◽  
Generation Region

Abstract. The characteristics of substorm-associated Pi2 pulsations observed by the SABRE coherent radar system during three separate case studies are presented. The SABRE field of view is well positioned to observe the differences between the auroral zone pulsation signature and that observed at mid-latitudes. During the first case study the SABRE field of view is initially in the eastward electrojet, equatorward and to the west of the substorm-enhanced electrojet current. As the interval progresses, the western, upward field-aligned current of the substorm current wedge moves westward across the longitudes of the radar field of view. The westward motion of the wedge is apparent in the spatial and temporal signatures of the associated Pi2 pulsation spectra and polarisation sense. During the second case study, the complex field-aligned and ionospheric currents associated with the pulsation generation region move equatorward into the SABRE field of view and then poleward out of it again after the third pulsation in the series. The spectral content of the four pulsations during the interval indicate different auroral zone and mid-latitude signatures. The final case study is from a period of low magnetic activity when SABRE observes a Pi2 pulsation signature from regions equatorward of the enhanced substorm currents. There is an apparent mode change between the signature observed by SABRE in the ionosphere and that on the ground by magnetometers at latitudes slightly equatorward of the radar field of view. The observations are discussed in terms of published theories of the generation mechanisms for this type of pulsation. Different signatures are observed by SABRE depending on the level of magnetic activity and the position of the SABRE field of view relative to the pulsation generation region. A twin source model for Pi2 pulsation generation provides the clearest explanation of the signatures observed.

The Conditions under which Opposing Motion is Seen in Transparency or as Flicker

Perception ◽  
1997 ◽  
Vol 26 (1_suppl) ◽  
pp. 31-31 ◽  
Author(s):  
M M Del Viva ◽  
M C Morrone
Keyword(s):  
Feature Tracking ◽  
Relative Phase ◽  
Phase Relationship ◽  
The Other ◽  
Spatial Frequencies ◽  
Sinusoidal Gratings ◽  
Exact Degree ◽  
Harmonic Components ◽  
Over Time ◽  
Moving Pattern

We present several examples of moving stimuli comprising several harmonic components that can be perceived either as moving independently, or together as a coherent moving pattern. A simple example is two sinusoidal gratings of equal contrast and spatial frequencies moving in opposite directions that are perceived as a single grating modulated sinusoidally over time (counterphase). However, two square waves drifting in opposite directions, while being a superposition of counterphasing pairs, are perceived as two distinct patterns drifting in transparency one over the other. Intermediate situations such as pairs of counterphase gratings can be perceived alternatively as counterphases or as drifting in two directions. We show that the relative phase of the components plays a fundamental role in whether they group or become transparent. The tendency of a component to become part of a pattern is maximal when the phase relationship is 0 deg, and gradually decreases to a minimum at 90 deg. This agrees quantitatively with previous measurements on different stimuli (paper presented at Del Viva and Morrone, ARVO96), suggesting a common computational mechanism. The data were well modelled by a nonlinear model of motion analysis based on feature tracking, showing sensitivity to the exact degree of nonlinearity.

scholarly journals Hydromagnetic Dynamo in Astrophysical Jets

1993 ◽  
Vol 157 ◽  
pp. 367-371 ◽  
Author(s):  
A. Shukurov ◽  
D.D. Sokoloff
Keyword(s):  
Magnetic Field ◽  
Cylindrical Shell ◽  
Plasma Flow ◽  
Wave Number ◽  
Astrophysical Jets ◽  
Thin Cylindrical Shell ◽  
Azimuthal Wave Number ◽  
Hydromagnetic Dynamo ◽  
Field Lines ◽  
Jet Plasma

The origin of a regular magnetic field in astrophysical jets is discussed. It is shown that jet plasma flow can generate a magnetic field provided the streamlines are helical. The dynamo of this type, known as the screw dynamo, generates magnetic fields with the dominant azimuthal wave number m = 1 whose field lines also have a helical shape. The field concentrates into a relatively thin cylindrical shell and its configuration is favorable for the collimation and confinement of the jet plasma.

On Estimates for Growth Rates of Unstable Azimuthal Disturbances in the Stability Problem of Swirling Flows with Radius- Dependent Density

2017 ◽  
Vol 13 (3) ◽  
pp. 1-12
Author(s):  
Halle Dattu Malai Subbiah
Keyword(s):  
Growth Rate ◽  
Growth Rates ◽  
Stability Problem ◽  
Variable Density ◽  
Swirling Flows ◽  
Wave Number ◽  
Two Dimensional ◽  
Azimuthal Wave Number ◽  
The Stability ◽  
Dependent Density

Estimates for the growth rate of unstable two-dimensional disturbances to swirling flows with variable density are obtained and as a consequence it is proved that the growth rate tends to zero as the azimuthal wave number tends to infinity for two classes of basic flows.

scholarly journals A new paradigm for intensity modification of tropical cyclones: thermodynamic impact of vertical wind shear on the inflow layer

2010 ◽  
Vol 10 (7) ◽  
pp. 3163-3188 ◽  
Author(s):  
M. Riemer ◽  
M. T. Montgomery ◽  
M. E. Nicholls
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclones ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Shear ◽  
Wave Number ◽  
Carnot Cycle ◽  
Storm Intensity ◽  
Azimuthal Wave Number ◽  
Intensity Evolution

Abstract. An important roadblock to improved intensity forecasts for tropical cyclones (TCs) is our incomplete understanding of the interaction of a TC with the environmental flow. In this paper we re-visit the canonical problem of a TC in vertical wind shear on an f-plane. A suite of numerical experiments is performed with intense TCs in moderate to strong vertical shear. We employ a set of simplified model physics – a simple bulk aerodynamic boundary layer scheme and "warm rain" microphysics – to foster better understanding of the dynamics and thermodynamics that govern the modification of TC intensity. In all experiments the TC is resilient to shear but significant differences in the intensity evolution occur. The ventilation of the TC core with dry environmental air at mid-levels and the dilution of the upper-level warm core are two prevailing hypotheses for the adverse effect of vertical shear on storm intensity. Here we propose an alternative and arguably more effective mechanism how cooler and drier (lower θe) air – "anti-fuel" for the TC power machine – can enter the core region of the TC. Strong and persistent, shear-induced downdrafts flux low θe air into the boundary layer from above, significantly depressing the θe values in the storm's inflow layer. Air with lower θe values enters the eyewall updrafts, considerably reducing eyewall θe values in the azimuthal mean. When viewed from the perspective of an idealised Carnot-cycle heat engine a decrease of storm intensity can thus be expected. Although the Carnot cycle model is – if at all – only valid for stationary and axisymmetric TCs, a close association of the downward transport of low θe into the boundary layer and the intensity evolution offers further evidence in support of our hypothesis. The downdrafts that flush the boundary layer with low θe air are tied to a quasi-stationary, azimuthal wave number 1 convective asymmetry outside of the eyewall. This convective asymmetry and the associated downdraft pattern extends outwards to approximately 150 km. Downdrafts occur on the vortex scale and form when precipitation falls out from sloping updrafts and evaporates in the unsaturated air below. It is argued that, to zero order, the formation of the convective asymmetry is forced by frictional convergence associated with the azimuthal wave number 1 vortex Rossby wave structure of the outer-vortex tilt. This work points to an important connection between the thermodynamic impact in the near-core boundary layer and the asymmetric balanced dynamics governing the TC vortex evolution.

scholarly journals Upper tropospheric energetics of standing eddies In wave number domain during contrasting Monsoon activity over India

MAUSAM ◽  
2021 ◽  
Vol 43 (4) ◽  
pp. 403-410
Author(s):  
S.S. BAWISKAR ◽  
S.M. SINGH
Keyword(s):  
Energy Source ◽  
Kinetic Energy ◽  
Interaction Analysis ◽  
Standing Waves ◽  
Wave Interaction ◽  
Wave Number ◽  
Mean Flow ◽  
S Wave ◽  
Flow Interaction ◽  
Energy Equations

The upper tropospheric energetics of the standing eddies in wave number domain during contrasting monsoon' activity over India have been investigated. Two normal monsoon years (1970. 1971) and two drought monsoon years (1972, 1979) are considered for a comparative study, Energy equations of Saltzman (1957) are used to compute wave-wave Interaction and wave to zonal mean flow Interaction. Analysis of the results show that the standing eddies in the region of tropical easterlies (5°S-24 .2°N) have larger kinetic energy than those in the region of southern hemispheric, westerlies (24.2°S-5°S). Wave to zonal mean flow interaction of all waves (waves 1-15) Indicate that the standing eddies are a source of kinetic energy to zonal mean flow ID the region of easterlies and there sink of kinetic energy to zonal mean flow in the region of westerlies. In the region of easterlies planetary standing waves (waves 1-2) are the major kinetic energy source to other standing waves and wave-wave Interaction of all waves leads to positive Imbalance of kinetic energy during normal monsoon years (1970, 1971) and negative imbalance, of kinetic, energy during drought monsoon years (1972, 19~9). In the region of westerlies the imbalance of kinetic energy IS negative during normal monsoon years and positive during drought monsoon years.

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