37 research outputs found
An investigation of the field-aligned current associated with a large-scale ULF wave in the morning sector,, Planet. Space Sci., 55, 770, 2007.
No full textPrevious work by Scoffield, H.C., Yeoman, T.K., Wright, D.M., Milan, S.E., Wright, A.N., Strangeway, R.J. [2005. An investigation of the field aligned currents associated with a large scale ULF wave using data from CUTLASS and FAST. Ann. Geophys. 23, 487-498) investigated a large-scale ULF wave, occurring in the dusk sector (-1900MLT). The wave had a period of similar to 800s (corresponding to 1.2mHz frequency), an azimuthal wave number of similar to 7 and a full-width at half-maximum (FWHM) across the resonance of 350km. IMAGE ground magnetometer and SuperDARN radar observations of the wave's spatial and temporal characteristics were used to parameterise a simple, two-dimensional field line resonance (FLR) model. The model-calculated field-aligned current (FAC) was compared with FACs derived from the FAST energetic particle spectra and magnetic field measurement. Here the authors use the same method to investigate the FAC structure of a second large-scale ULF wave, with a period of similar to 450 s, occurring the dawn sector (similar to 0500 MLT) with an opposite sense background region 1-region 2 current system. This wave has a much larger longitudinal scale (m similar to 4.5) and a smaller latitude scale (FWHM = 150 km). Unlike the dusk sector wave, which was dominated by upward FAC, FAST observations of the dawn sector wave show an interval of large-scale downward FAC of similar to 1.5 mu A m(-2). Downgoing magnetospheric electrons with energies of a few kcV were observed, which are associated with upward FACs of similar to 1 mu A m(-2). For both wave studies, downward currents appear to be carried partially by upgoing electrons below the FAST energy detection threshold (5eV), but also consist of a mixture of hotter downgoing magnetospheric electrons and upgoing ionospheric electrons of energies 30 eV-1 keV. Strong intervals of upward current show that small-scale structuring of scale similar to 50 km has been imposed on the current carriers. In general, this study confirms the findings of Scoffield, H.C., Yeoman, T.K., Wright, D.M., Milan, S.E., Wright, A.N., Strangeway, R.J. [2005. An investigation of the FACs associated with a large-scale ULF wave using data from CUTLASS and FAST. Ann. Geophys. 23, 487-498). (c) 2006 Elsevier Ltd. All rights reserved.</p
An assessment of plasma instabilities or planetary lightning as a source for the VLF bursts detected at Venus
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Electron acceleration observed by the FAST satellite within the IAR during a 3 Hz modulated EISCAT heater experiment
No full textInternational audienceA quantitative analysis is presented of the FAST satellite electric field and particle flux data during an EISCAT heating experiment run on 8 October 1998. Radio frequency heating, modulated at 3 Hz, launched ULF waves from the ionosphere into the lower magnetosphere. The ULF waves were observed in FAST data and constituted the first satellite detection of artificially excited Alfvénic ULF waves. The downward electron flux data for this event contain the first observations of electrons undergoing acceleration within the Ionospheric Alfvén Resonator (IAR) due to parallel electric fields associated with an artificially stimulated Alfvén wave. The time history and spectral content of the observed down-ward electron fluxes is investigated by considering the effects of a localised parallel electric field. Furthermore, it is demonstrated that a power law electron energy distribution describes the time-variable observed fluxes better than a Maxwellian distribution
Laboratory astrophysics : investigation of planetary and astrophysical maser emission
No full textThis paper describes a model for cyclotron maser emission applicable to planetary auroral radio emission, the stars UV Ceti and CU Virginus, blazar jets and astrophysical shocks. These emissions may be attributed to energetic electrons moving into convergent magnetic fields that are typically found in association with dipole like planetary magnetospheres or shocks. It is found that magnetic compression leads to the formation of a velocity distribution having a horseshoe shape as a result of conservation of the electron magnetic moment. Under certain plasma conditions where the local electron plasma frequency ωpe is much less than the cyclotron frequency ωce the distribution is found to be unstable to maser type radiation emission. We have established a laboratory-based facility that has verified many of the details of our original theoretical description and agrees well with numerical simulations. The experiment has demonstrated that the horseshoe distribution produces cyclotron emission at a frequency just below the local electron cyclotron frequency, with polarisation close to X-mode and propagating nearly perpendicularly to the electron beam motion. We discuss recent developments in the theory and simulation of the instability including addressing radiation escape problems, and relate these to the laboratory, space, and astrophysical observations. The experiments showed strong narrow band EM emissions at frequencies just below the cold-plasma cyclotron frequency as predicted by the theory. Measurements of the conversion efficiency, mode and spectral content were in close agreement with the predictions of numerical simulations undertaken using a particle-in-cell code and also with satellite observations confirming the horseshoe maser as an important emission mechanism in geophysical/astrophysical plasmas. In each case we address how the radiation can escape the plasma without suffering strong absorption at the second harmonic layer.Peer reviewe
Observation of chaotic fluctuations in turbulent plasma
No full textTurbulence is a prevalent phenomenon in space and astrophysical plasmas, often characterized by stochastic fluctuations. While laboratory experiments and numerical simulations have revealed chaotic behavior, in situ observations of turbulent plasmas in natural environments have predominantly shown highly stochastic signatures. Here, we present unprecedented in situ evidence of chaotic fluctuations in the turbulent solar wind plasma downstream of the Earth's bow shock. By analyzing the relative location of magnetic-field fluctuations on the permutation entropy-complexity plane, we demonstrate that turbulence in the magnetosheath plasma exhibits characteristics of chaotic fluctuations rather than stochastic behavior, diverging from the expected traits of well-developed turbulence. This finding challenges established notions of plasma turbulence and reveals the need for caution when using the magnetosheath as a laboratory for studying plasma turbulence. © 2024 Author(s)
