A. J. Willes

Dynamics of beam-driven Langmuir and ion-acoustic waves including electrostatic decay

The evolution of Langmuir waves and ion-acoustic waves stimulated by a hot electron beam in an initially homogeneous plasma is investigated numerically in time, position, and wave number space. Quasilinear interactions between the beam particles and Langmuir waves, nonlinear interactions between the Langmuir and ion-acoustic waves through Langmuir decay processes, and spontaneous emission are taken into account in the kinetic theory employed. For illustrative parameters of those in the solar wind near 1 a.u., nonlinear Langmuir decays are observed to transfer the beam-driven Langmuir waves rapidly out of resonance. The scattered Langmuir waves then undergo further decays, moving sequentially toward small wave numbers, until decay is kinematically prohibited. The main features of the evolution of Langmuir and ion-acoustic waves are spatially inhomogeneous. The scattered Langmuir spectra increase and eventually reach or exceed the beam-driven Langmuir spectra at a given spatial location (except in regions w...

Dynamics of fundamental electromagnetic emission via beam-driven Langmuir waves

The nonlinear process of electromagnetic Langmuir decay, which leads to radio emission near the plasma frequency, is studied for situations in which Langmuir waves are directly driven by an electron beam and indirectly generated via electrostatic Langmuir decays. The electromagnetic Langmuir decay is stimulated by the presence of ion-acoustic waves. An approximate method is devised for studying this emission process with axial symmetry (along the direction of beam propagation) in three spatial dimensions, based upon the Langmuir and ion-acoustic wave dynamics in one spatial dimension. Numerical studies of the fundamental electromagnetic emission starting from electron dynamics are then carried out via quasilinear theory, and the results are explored for illustrative parameters. The evolution of the fundamental transverse waves shows the combined effects of local emission and propagation away from the source. At a given location, the emission rate shows a series of peaks associated with successive electrom...

Second harmonic electromagnetic emission via beam-driven Langmuir waves

The linked nonlinear processes of electrostatic Langmuir decay and electromagnetic emission at the second harmonic plasma frequency are studied for situations in which Langmuir waves are driven by an electron beam. An approximate method for studying wave decay and emission in three spatial dimensions is developed, based on the Langmuir and ion-acoustic wave dynamics in one spatial dimension. The numerical solutions of quasilinear equations to study electromagnetic emission starting from the electron dynamics are carried out. The numerical results are explored for illustrative parameters. The evolution of the transverse waves shows the combined effects of local emission and propagation away from the source. At a given location, the emission rate shows a series of peaks associated with coalescences of Langmuir waves driven by the beam and those produced by successive decays. The emission rate for a given coalescence decreases with time, following an initial increase. The effects of transverse wave propagati...

Generalized Langmuir waves in magnetized kinetic plasmas

The properties of unmagnetized Langmuir waves and cold plasma magnetoionic waves (x, o, z and whistler) are well known. However, the connections between these modes in a magnetized kinetic plasma have not been explored in detail. Here, wave properties are investigated by numerically solving the dispersion equation derived from the Vlasov equations both with and without a beam instability present. For ωp>Ωe, it is shown that the generalized Langmuir mode at oblique propagation angles has magnetic z-mode characteristics at low wave numbers and thermal Langmuir mode characteristics at high wave numbers. For ωp<Ωe, it is shown that the (oblique) Langmuir mode instead connects to the whistler mode at low wave numbers. The transition from the Langmuir/z mode to the Langmuir/whistler mode near ωp=Ωe is rapid. In addition, the effects on wave dispersion and polarization after adding a beam are investigated. Applications of this theory to magnetized Langmuir waves in Earth’s foreshock and the solar wind, to waves ...

A z mode electron-cyclotron maser model for bottomside ionospheric harmonic radio emissions

A model is proposed for recent ground-based observations of auroral roar emissions, detected at 2Ωe and 3Ωe, where Ωe is the local electron cyclotron frequency in the source region, between 200 and 500 km above the Earths surface. Electron cyclotron maser emission is a likely mechanism to account for these emissions because it naturally produces coherent radiation at harmonics of Ωe. A theory for auroral roar emissions has already been proposed, whereby maser-generated second (X2) and third (X3) harmonic x mode radiation is amplified in the source region by multiple reflections off the walls of the density cavity in which they are produced. After many reflections the X2 and X3 waves propagate along the density cavity to a ground-based observer. However, it is demonstrated here with ray-tracing calculations that it is highly probable that maser-generated X2 and X3 radiation is reabsorbed at lower altitudes and thus cannot be detected at the ground. An indirect maser mechanism is proposed instead, where maser-generated z mode waves at Ωe grow to high levels in the source region and then undergo repeated nonlinear wave-wave coalescence to produce second- and third-harmonic waves that propagate directly to the ground. The z mode waves must satisfy the necessary kinematic constraints to produce observable second- and third-harmonic radiation. The dependence of the z mode maser on the temperature and functional form of the unstable electron distribution is discussed, along with the conditions required for the coalescence processes to proceed and produce the observed levels of radiation.

Mode Conversion and Reflection of Langmuir Waves in an Inhomogeneous Solar Wind

Beam-driven Langmuir waves in the solar wind are generated just above the electron plasma frequency, which fluctuates in the inhomogeneous solar wind plasma. Consequently, propagating Langmuir waves encounter regions in which the wave frequency is less than the local plasma frequency, where they can be reflected, mode converted to transverse electromagnetic waves, and trapped in density wells. The aim here is to investigate Langmuir wave reflection and mode conversion at a linear density gradient for typical solar wind parameters. It is shown that higher mode conversion efficiencies are possible than previously calculated, but that mode conversion occurs in a smaller region of parameter space. In addition, the possibility of detecting mode conversion with in situ spacecraft Langmuir wave observations is discussed.

Banded frequency structure from linear mode conversion in inhomogeneous plasmas

Linear mode conversion of unmagnetized Langmuir waves into transverse electromagnetic waves in an inhomogeneous plasma is investigated numerically. It is shown that the presence of a tunneling region and density well near the mode conversion region can significantly influence the mode conversion efficiency. The density cavity acts as a resonator, such that the mode conversion efficiency shifts from zero to a local maximum (up to 100% efficiency) with variation of the cavity width on scales of order the Langmuir wavelength. In this way, Langmuir waves with a smooth (structureless) frequency dependence can be selectively mode converted to produce narrow frequency bands of electromagnetic radiation. Applications are discussed to banded frequency fine structure in electromagnetic emissions at the electron plasma frequency in Earth’s foreshock and the solar wind, type III solar radio bursts, and lower ionospheric auroral roar emissions.

Angle-averaged efficiencies for linear mode conversion between langmuir waves and radiation in an unmagnetized plasma

Linear mode conversion of Langmuir waves propagating in density irregularities leads to radio waves near the electron plasma frequency. Most previous work on this mechanism is for a single Langmuir wave interacting with a single planar density structure. This paper presents analytic and numerical estimates of the average conversion efficiency resulting from averaging over the distributions of the incoming Langmuir wave vectors and the orientations and length scales of density irregularities in an unmagnetized plasma. Significant mode conversion requires Langmuir wave vectors very closely aligned with the density gradient, within an angular width of order Ve∕c radians (Ve is the electron thermal speed and c the speed of light), and this width plays a major role in determining the average conversion efficiencies. Specifically, the two-dimensional (2D) averaged efficiency for planar irregularities with a uniform distribution of orientations is a factor of order Ve∕(2πc) smaller than the peak values ∼50% in t...

Long-duration Coherent Radio Emission from the dMe Star Proxima Centauri

The Australia Telescope and Anglo-Australian Telescope were used in May 2000 to record the radio and optical emissions from the dMe flare star Proxima Centauri. Eight bright optical flares over a two-day interval resulted in no detectable excess short-term radio emission at 1.38 and 2.50 GHz. However, a slowly declining 1.38 GHz emission over the two-day interval was nearly 100% right circular polarised and was restricted to a relatively narrow bandwidth with total intensity (I) and circular polarisation (V) varying significantly over the 104 MHz receiver bandwidth. These are the first observations to show that highly-polarised narrowband flare star emission can persist for several days. This signature is attributed to sources of coherent radio emission in the stars corona. Similarities with various solar radio emissions are discussed; however, it is not possible with the existing observations to distinguish between fundamental plasma emission and electron–cyclotron maser emission as the responsible mechanism.

Radio Emission from Ultrashort-Period Double Degenerate Binaries

Timing measurements of periodic X-ray pulses from two ultrashort-period double degenerate binaries, RX J1914+24 and RX J0806+15, show that the rates of change of their orbital periods are consistent with gravitational radiation losses. This contradicts the predictions of models which invoke mass transfer between the two white dwarfs. The X-ray emission is, therefore, unlikely to be powered by accretion processes. The unipolar inductor model explains the source of X-ray emission as electrical dissipation at the base of a flux tube, which connects the magnetic white dwarf to its companion. This model is most consistent with the observed X-ray pulse properties. A similar current system exists in the Jupiter–Io system, where a mildly relativistic electron current produces an auroral footprint at the base of the Io flux tube and highly polarized beamed radio emission by means of the electron cyclotron maser mechanism. Detection of radio emission from RX J1914+24 and RX J0806+15 would thus provide further support for the unipolar inductor model. We present theoretical predictions, based on a loss-cone-driven electron cyclotron maser model, of radio fluxes from systems with parameters similar to RX J1914+24 and RX J0806+15.