Convective Growth Research Articles

Some aspects of Bay of Bengal cyclone of 29 January to 4 February 1987

A satellite study of an unusual tropical cyclone is made to, investigate genesis, intensification, movement and the decay over sea. Dvorak technique has been applied for its Intensity analysis. Satellite cloud imagery analysis indicates that initial genesis was spawned by cross equatorial surges and mechanical subsidence occurring in the relatively cloud free area between deep layer convective cloud masses. Subsequent intensification was favoured by high sea surface temperature and suitable synoptic settings. It appears that initial movement of the vortex was guided by b-effect and subsequent movement by its environmental steering current. New convective cloud mass growth was observed in the direction of its movement and during change of its direction of movement, its speed became slow or it remained stationary for some time. Presence of dry slot in western and southwestern side in its intense stage and elongation of associated cloud mass northeastwards towards the later part of its life indicate its increasing interaction with subtropical westerly flow. This finally led to its dissipation right over sea where sea surface temperature was also relatively less.

Trapped electrons as a free energy source for the auroral kilometric radiation

Simultaneous measurements of electromagnetic fields and particle distributions, measured during the crossing by the Swedish spacecraft Viking of an auroral kilometric radiation (AKR) source, are presented. It is shown that AKR is generated within an acceleration region characterized by an upward directed parallel electric field, as evidenced by its signature on the proton and electron distributions. From particle observations inside the AKR source it is clear that the potential drop below the spacecraft produces upward moving field‐aligned ion beams and a depletion in the density of low energy electrons. The potential drop above the spacecraft produces downward accelerated electrons. A large fraction of these electrons have small parallel velocities; they mirror above the ionosphere. These trapped electrons lie in a region of velocity space which should be empty in a simple adiabatic theory. We suggest that these electrons get trapped when they experience a time‐varying (or space‐varying) parallel electric field. This conclusion is supported by the comparison between the observed electron distribution function and a model distribution function built by applying Liouville theorem. Since trapped electrons can cause positive gradients (∂fe/∂V⊥ > 0) over a broad range of parallel velocities, around v⫽ ≈ 0, it is suggested that they are the free energy source for the AKR. This conclusion is substantiated by an evaluation of the convective growth rate, where the various input parameters have been determined by fitting particle data.

Ground‐satellite study of a Pc 1 ion cyclotron wave event

The magnetospheric generation and propagation characteristics of ion cyclotron waves associated with a Pc 1 emission observed by a network of four middle and low‐latitude ground stations are determined using ground source location techniques and ISEE 1 plasma data. The source region at L = 4.7 ± 0.7 is determined from propagation in the F2 region ionospheric duct using wave polarization characteristics at ground stations. This source is just inside the steep plasmapause seen by ISEE 1 at L = 4.9 ± 0.1. The ion cyclotron wave packet interhemispheric bounce period measured from the ground spectra increases with time from 140 to 155 s during the event but is in agreement with dispersion calculations undertaken using ISEE 1 electron density and cool to cold ion composition data in a H+ plasma with 8–10% He+ and <1% 0+ ions. The frequency of the Pc 1 emission band seen on the ground (0.5–0.8 Hz) corresponds to the propagation region between the equatorial 0+ cutoff frequency and the He+ cyclotron frequency. Linear convective growth rate calculations inside the plasmapause show significant wave amplitudes in this band. With these results it is possible to completely describe the wave generation mechanism and the magnetosphere ionosphere propagation characteristics for the Pc 1 event.

Ion heating as a limiting process in CO2 laser induced Brillouin scattering

Two complementary models invoking ion heating are presented which, together with pump depletion, quantitatively account for the principal observations (reflectivity, spectral shape, saturation) of stimulated Brillouin scattering (SBS) in a CO2 laser/plasma interaction experiment. A simple energy balance model, which incorporates ion heating and includes a consistent calculation of ion wave damping, is applicable for steady state convective growth of the ion waves (long interaction times). A more refined model, which includes a calculation of the trapped ion fraction (along with heating), is applicable for SBS in a transient regime. Strong ion wave damping has been consistently observed in both cases in our high intensity CO2 laser/plasma scattering experiments. A variety of measurements have been made to investigate the basic features of SBS and associated density fluctuations for both long pulse (40 ns) and short pulse (2 ns) CO2 laser irradiation. Measured parameters reported here include plasma density, temperature, density scalelength, SBS reflectivity, and spectral characteristics. Together with previous measurements of density fluctuations (amplitude, wavenumber spectra, spatial and temporal information), the observed behavior confirms that Brillouin scattering is dominated by strong ion trapping, heating and ion wave damping for the laser/plasma parameters prevailing in these experiments.

The electrification of New Mexico thunderstorms: 1. Relationship between precipitation development and the onset of electrification

Aircraft, radar, and surface observations were used to study the relationship between precipitation development and the onset of electrification in thunderstorms which formed near or over the Magdalena Mountains of New Mexico. The study included storms which were electrically active as well as ones in which no electrical enhancement was observed. Electric fields inside these clouds showed negligible enhancement and did not exceed 1 kV m−1 until reflectivities at 6 km above mean sea level (msl) (about −10°C) exceeded approximately 40 dBZ and cloud tops exceeded 8 km. The onset of electrification occurred during or immediately after convective growth within the cloud.

The evolution of the double-diffusive instability: Salt fingers

The finite-amplitude growth of finger convection at an interface of two uniform solutions is studied to determine the processes that govern the scale and amplitude of the convection, the growth of the fingering interface, and the transition of the convection to turbulence. A conceptual model of fingering processes is provided and details are studied by means of direct numerical simulation. The simulations obtain full finger convection and turbulence for ratios of viscosity to diffusivities in the range between 1 and 10. The study shows that the evolution of finger convection in its entirety is characterized by the scale and amplitude of the fastest growing finger mode. In the parameter range studied, the growth of the fingering interface is found to be controlled mostly by molecular diffusion. Convective flux divergence only plays a secondary role. The growth of the interface is shown to have the effect of increasing the horizontal scale of the convection while decreasing the buoyancy flux. However, the kinetic and potential energies remain statistically stationary for fully developed finger convection. Density inversion developed within finger convection is shown to play an important role in the breakup of finger structures into turbulence. The significance of ‘‘collective’’ instability in determining this transition is not substantiated by the simulation for the parameter values studied here.

The convective growth of the beam–plasma instability

Using a newly modified version of a low noise Vlasov code, a series of simulations studying the convective growth of the cold-beam, warm-plasma instability is performed. The scalings of spatial growth on beam density and velocity are in good agreement with earlier theoretical predictions. In agreement with Bers [Basic Plasma Physics (North Holland, Amsterdam, 1983), p. 451], the spatial growth rate ki λD =γ/(v1v2v3 )1/3, where γ is the temporal growth rate and vα is the linear group velocity of the three modes coupled by the instability. For the cold-beam, warm-plasma instability, this reduces to the standard formula ki λD =(3)1/2/2 (nb ve /6npvb)1/3.

Abundance of condensable species at planetary cold traps: The role of moist convection

We examine the interplay of moist convection and particle growth in the tropopause region of the atmospheres of Earth, Uranus and Neptune, in an attempt to understand the diverse vertical distributions of these planets' major condensable species. On Earth, water is undersaturated in the lower stratosphere relative to the mixing ratio expected from average temperature profiles near the tropopause cold trap. On Uranus, methane is largely depleted in the upper stratosphere by photochemistry, but plausibly close to its saturated value at the tropopause. On Neptune methane is oversaturated by 2–3 orders of magnitude in the lower stratosphere. Paradoxically, moist convection has been invoked to undersaturate the Earth's stratosphere and oversaturate Neptune's. A simple one-dimensional model of moist convection is used to address this problem, along with analytic expressions for growth timescales of particles from the vapor. For the Earth, we also derive downward fluxes for Junge layer aerosols to assess their role in dehydrating the stratosphere. We reach the following conclusions. (1) The downward flux of aerosols is not sufficient to dehydrate the Earth's stratosphere by chemical desiccation, but aerosols may play a role as nucleating particles for ice crystals in the cold anvil model of Danielsen (1982, Geophys. Res. Lett. 9, 605). (2) The source of Neptune's oversaturated stratosphere is convective penetration of the tropopause by methane moist convective columns. However, certain restrictive conditions are required to initiate and maintain moist convection. Conditions in the troposphere of Uranus, on the other hand, are not conducive to the initiation of moist convection, and gaseous methane closely follows the vapor pressure law. (3) The action of moist convection in undersaturating Earth's stratosphere while oversaturating Neptune's can be understood in terms of very different particle size distributions for the condensable on the two bodies. Methane particles remain small and are carried up through Neptune's columns; on Earth water droplets grow rapidly and fall out. Additionally, the conditions appropriate for radiative cooling of terrestrial anvils, which accounts for the undersaturation, do not apply on Neptune.

Amplification of whistler waves propagating through inhomogeneous, anisotropic, mirror-confined hot-electron plasmas

A fully relativistic local dispersion relation for whistler waves has been solved at closely spaced points along the magnetic field lines of a 2:1 magnetic mirror in which a highly anisotropic, spatially inhomogeneous, hot-electron plasma is confined. The limiting plasma parameters for convective (spatial) growth have been determined numerically and used to identify plasma conditions leading to maximum amplification of input microwave signals introduced in the form of whistler waves. The maximum gain has been evaluated numerically for a range of values of the hot-electron plasma within which all major stability criteria are satisfied. Very high gains (∼40 dB) are indicated over the entire range of beta investigated.

Oscillator priming and preoscillation noise in a gyrotron

Phase control is achieved in a pulsed gyrotron oscillator both by applying an external priming signal directly to the oscillator and by applying the signal to a prebunching cavity. A pulse-to-pulse phase jitter of greater than 2.5 degrees is achieved in the gyrotron at drive-to-oscillator power ratios of -36.6 dB (drive signal-to-noise power ratio of 36 dB) in the direct injection case and -71 dB (drive signal-to-noise power ratio of 22 dB) in the prebunched case. A lumped-element theory is compared to the experimental results. The theoretical description seems valid when the drive frequency is within about 5 MHz of that of the oscillator. Preoscillation noise in the gyrotron is about 10 mu W, larger than expected from either shot noise thermal noise but in the vicinity of spontaneous cyclotron emission. Convective radio-frequency noise growth is investigated. No evidence of the electrostatic cyclotron instability was seen. All growth observed is attributed to the gyroklystron amplification mechanism. However the noise growth per unit length is not as large as that of a narrow-band drive signal. >

Comparative study of the effect of cold ions and thermal anisotropy on the convective electromagnetic proton cyclotron instability

Although the role played by cold ions on the electromagnetic proton-cyclotron instability has been thoroughly investigated, there are some aspects which deserve further attention. Moreover, since the presence of ion cyclotron waves in the magnetosphere has been experimentally established it is important to have a full understanding of the role played by cold ions. Thus, it is shown here that the main effect of cold ions on the proton-cyclotron instability is to shift the unstable spectrum. If the only cold species are protons, then the shift of the spectrum is always towards lower frequency values, i.e. away from the marginal mode. If in the absence of cold protons the maximum convective growth rate lies far from the marginal mode, the only effect of cold protons is to shift the spectrum further away from the marginal mode. However, if the maximum convective growth rate in the absence of cold protons lies close to the marginal mode, then the shift of the spectrum leads also to a large amplification. The reason for this behaviour lies in the fact that as the thermal anisotropy decreases, the frequency range of the unstable spectrum also decreases, stabilizing high frequency waves. Since unstable higher frequency waves occur in the absence of cold protons, these are the first to be stabilized. The observed amplification due to cold protons is the result of comparing waves which have suffered some stabilization with waves which, due to the shift of the unstable spectrum, have not yet been affected by the stabilization which follows the reduction of the thermal anisotropy. A consequence of these results is that, since the thermal anisotropy decreases as the instability develops, the instability lasts longer in the presence of cold protons than in their absence. If more than one cold species are present, the situation is more complex but the basic mechanism is the same. It is conjectured that the catalytic role played by cold He + ions in the generation of ion cyclotron waves in the magnetosphere consists of shifting the unstable spectrum away from a forbidden or sta- bilization region.

Fundamental Study on Generation and Growth of Natural Convection around Disposal Canister by Decay Heat of Radioactive Nuclides Waste.

Fundamental Study on Generation and Growth of Natural Convection around Disposal Canister by Decay Heat of Radioactive Nuclides Waste.

Convective growth of broadband turbulence in the plasma sheet boundary layer

Convective growth of slow and fast beam acoustic waves in the plasma sheet boundary layer (PSBL) is investigated. It has been shown previously that a cold ion population must be present in order to excite beam acoustic waves in the PSBL. However, growth rates are significantly enhanced when warm plasma sheet boundary layer ions are present. Net wave growth along a ray path is determined by convective growth. This quantity is calculated for particle distribution models consistent with the PSBL where the intensity of broadband turbulence is observed to peak. Total number density dependence on beam acoustic convective growth is evaluated, and it is found that even for low density conditions of ∼0.01 cm−3, a measurable level of broadband turbulence is expected. Relative drift effects between cold and warm ion populations are also considered. In particular, it is found that slow mode convective growth can be enhanced when slowly streaming cold ions are present, compared to fast ion streams.

Wave dispersion and ray propagation in a weakly relativistic electron plasma: Implications for the generation of auroral kilometric radiation

Relativistic modifications to wave dispersion near the electron gyrofrequency are known to be important for sufficiently energetic electrons. In general, if the characteristic momentum of the energetic particles (po/moc) is greater than the ratio of the plasma frequency to the gyrofrequency (ωp/Ωe), wave propagation can be markedly different than the classical cold plasma limit. We investigate the wave dispersion in a weakly relativistic electron plasma, specifically in the context of the generation of auroral kilometric radiation (AKR). To do this, we have used a simple electron distribution consisting of a ring of energetic electrons at constant perpendicular momentum together with a stationary cold component. It is found that for such an electron distribution, where the two electron components have different gyrofrequencies due to relativistic effects, a new wave mode is introduced. This wave is trapped between the two electron gyrofrequencies. We find that the group velocity for such a wave is small; convective growth lengths for the wave are calculated to be of the order of 1 km. Consequently, ray propagation in a simple model for the density cavity present on auroral zone field lines is studied. We find that the ray paths tend to bring waves to altitudes where the growth rate maximizes; at these altitudes the waves propagate primarily across the ambient magnetic field. This could result in substantial amplification of the wave, since damping is minimized, and the waves may undergo multiple transitions across the auroral arc. However, the wave is trapped and must undergo mode conversion to escape from the auroral arc. Since the wave polarization is found to be very similar to the R‐X mode polarization, we deduce that coupling to the R‐X mode at the edge of the auroral arc may be reasonably efficient.

A Triple-Doppler Radar Analysis of a Discretely Propagating Multicell Convective Storm

Abstract A triple-Doppler radar case study of a regularly propagating multicell convective storm has been performed. The motion of the storm, observed on 7 June 1978 during Project NIMROD, was somewhat unusual with cell translation and propagation directed opposite to one another. New cell formation is believed to have been caused by a directional outflow from existing cells which created a zone of low-level convergence favorable for convective growth. The unidirectional low to midlevel vertical shear of the growth environment, oriented roughly normal to the direction of cell translation, was found to strongly influence storm dynamics and structure.

Roll-pattern evolution in finite-amplitude Rayleigh–Beńard convection in a two-dimensional fluid layer bounded by distant sidewalls

This paper considers the temporal evolution of two-dimensional Rayleigh–Bénard convection in a shallow fluid layer of aspect ratio 2L ([Gt ] 1) confined laterally by rigid sidewalls. Recent studies by Cross et al. (1980, 1983) have shown that for Rayleigh numbers in the range R = R0 + O(L−1) (where R0 is the critical Rayleigh number for the corresponding infinite layer) there exists a class of finite-amplitude steady-state ‘phase-winding’ solutions which correspond physically to the possibility of an adjustment in the number of rolls in the container as the local value of the Rayleigh number is varied. It has been shown (Daniels 1981) that in the temporal evolution of the system the final lateral positioning of the rolls occurs on the long timescale t = O(L2) when the phase function which determines the number of rolls in the system satisfies a one-dimensional diffusion equation but with novel boundary conditions that represent the effect of the sidewalls. In the present paper this system is solved numerically in order to determine the precise way in which the roll pattern adjusts after a change in the Rayleigh number of the system. There is an interesting balance between, on the one hand, a tendency for the number of rolls to change by the least number possible and, on the other, a tendency for the even or odd nature of the initial configuration to be preserved during the transition. In some cases this second property renders the natural evolution susceptible to arbitrarily small external disturbances, which then dictate the form of the final roll pattern.The complete transition involves an analysis of the motion on three timescales, a conductive scale t = O(1), a convective growth scale t = O(L) and a convective diffusion scale t = O(L2).

Effects of energetic heavy ions on electromagnetic ion cyclotron wave generation in the plasmapause region

An expression for the linear electromagnetic ion cyclotron convective growth rate has been derived, considering multiple ions in the energetic anisotropic component of the plasma (which provides the free energy for the instability) as well as in the cold component of the plasma. This represents a modification of recent treatments investigating electromagnetic ion cyclotron growth rates which have considered only hydrogen ions in the energetic component. Four major effects on the growth and propagation characteristics result from inclusion of heavy ions in the energetic component. Some wave growth occurs at low frequencies below the corresponding marginally unstable wave mode for each heavy ion. Enhanced quasi‐monochronomatic peaks in the convective growth rate appear just below the O+ and He+ gyrofrequency and can be quite pronounced for certain plasma conditions. Stop bands, decreased group velocity and other effects normally attributed to cold heavy ions can be produced or enhanced by heavy ions in the energetic plasma component. Partial or complete suppression of wave growth at frequencies above the marginally unstable wave mode for a particular energetic heavy ion can greatly alter the growth rates that would occur in the absence of this energetic heavy ion. The expression for the linear electromagnetic ion cyclotron convective growth rate along with appropriate plasma parameters was used to investigate the nature of linear wave growth in the plasmapause region. The frequencies of peaks in the convective growth rate given by this model compare favorably with wave measurements in this region. It is conceivable that through wave‐particle interactions, electromagnetic ion cyclotron waves could supply the energy source for various plasmapause region phenomena such as the O+ torus, the plasma cloak, and stable auroral red arcs.

Upper hybrid loss-cone instability in tandem mirror thermal barriers

Electrostatic instabilities with frequencies in the vicinity of the upper hybrid frequency are studied. Such instabilities may be excited in electron cyclotron resonance heating (ECRH) plasmas where a warm Maxwellian and a hot loss-cone component may exist simultaneously. Temporal and convective growth rates are calculated for parameters relevant to present-day tandem mirror and ELMO Bumpy Torus (EBT) devices. It is found that such modes may grow convectively to significant levels in the thermal barrier region of tandem mirrors.

Generation of Alfven‐ion cyclotron waves on auroral field lines in the presence of heavy ions

Observation of electromagnetic waves in the low‐altitude auroral zone at frequencies between the proton and helium gyrofrequencies suggests that Alfven‐ion cyclotron waves modified by the presence of helium ions are being excited. Estimates of the growth rates for this mode indicate that the auroral electron beam can provide the free energy for the instability. The effect of the heavy ions is to decrease the group velocity of the waves, leading to larger convective growth. Theoretical wave spectra are computed in the local approximation, which assumes that the gradient scale lengths in density and magnetic field are constant over the ray paths. Narrow banded spectral peaks similar to observations may be produced when the thickness of the electron beam is small (200 m at 3000 km altitude). Narrow beams also limit growth of whistler mode waves, which compete for the free energy of the electron beam.

Convective growth rate of ion cyclotron waves in a H+−He+ and H+−He+−O+ plasma

The behavior of the convective growth rate of the electromagnetic proton‐cyclotron instability is investigated in detail in a H+, He+ plasma and when a third (minority) cold ion component such as O+ is taken into account. It is shown that amplification is the result of an interplay between the cold species and the thermal anisotropy of the energetic protons. The results seem to be in good agreement with the observations performed on board GEOS 1 and 2 concerning the generation of ULF waves, below the proton gyrofrequency. It is also shown that a small amount of O+ can strongly affect the behavior of the convective growth rate of the instability below the He+ gyrofrequency, even far away from the O+ stop band.