K. Stasiewicz

Cavity resonators and Alfvén resonance cones observed on Freja

Multiresolution wavelet analysis of magnetic field, electric field, and plasma density records taken on Freja during strong auroral events shows evidence for cavity Alfven resonators in the topside ...

Density depletions and current singularities observed by Freja

High time resolution data collected by the Freja spacecraft in the topside ionosphere (h ∼ 1500 km) reveal that the low-frequency turbulence observed commonly below the ion gyrofrequency consists of a large number of coherent plasma structures which represent intense field-aligned currents (j∥ ∼ 30 - 300 μAm -2 ) accompanied by plasma density depletions (cavities). The electromagnetic fluctuations are identified with inertial electron Alfven waves (IEAW) at perpendicular wavelengths ≤ 6 km. The coherent structures with singularly strong j∥ are consistent with being the resonance cones of IEAW generated by localized sources at h ≥ 1 R E . The density cavities are observed to be directly related to the magnetic field gradient, i.e., intensity of field-aligned current. Using multiresolution wavelet analysis we have investigated also the wave impedance and the polarization of the electromagnetic structures at different scales. The polarization records show phase discontinuities at small-scale electromagnetic structures which is consistent with the hypothesis that they represent resonance cones or interference patterns of IEAW.

Magnetic bubbles and kinetic Alfvén waves in the high-latitude magnetopause boundary

We present a detailed analysis of magnetic bubbles observed by the Polar satellite in the high-latitude magnetopause boundary. The bubbles which represent holes or strong depressions (up to 98%) of the ambient magnetic field are filled with heated solar wind plasma elements and observed in the vicinity of strong magnetopause currents and possibly near the reconnection site. We analyze the wave modes in the frequency range 0-30 Hz at the magnetopause (bubble) layer and conclude that the broadband waves in this frequency range represent most likely spatial turbulence of kinetic Alfven waves (KAW), Doppler-shifted to higher frequencies (in the satellite frame) by convective plasma flows. We present also results of a numerical simulation which indicate that the bubbles are produced by a tearing mode reconnection process and the KAW fluctuations are related to the Hall instability created by macroscopic pressure and magnetic field gradients. The observed spatial spectrum of KAW extends from several ion gyroradii (∼ 500 km) down to the electron inertial length (∼ 5 km).

Wave environment inside an inverted‐V electron event observed by Freja

We present an analysis of the wave environment observed inside an inverted-V electron event at an altitude of 1700 km, occurring in the upward directed current with intensity of a few µAm−2. During the analyzed event we observe that precipitating auroral electrons (∼ 2 keV) generate intense (40 mV/m) auroral hiss with peak power concentrated at the lower hybrid frequency flh ≈ fpi, and weak Langmuir waves at fpe. The low frequency turbulence observed within the event is found to be composed of a mixture of Alfven waves, O+ cyclotron waves, multi-ion resonances, and ion sound waves. The LH waves appear to interact modulationally with O+ and ion sound waves. From the observed values of flh and multi-ion resonance frequencies it is possible to estimate three-ion composition of plasma with representative values of [23, 19, 58] percent for normalized number densities of [H+, He+, O+], respectively.

Parametric decay to lower hybrid waves as a source of modulated Langmuir waves in the topside ionosphere

Langmuir emissions generated by electron beams in space plasmas usually appear as chains of strongly modulated wave packets. In this paper, we present a quantitative analysis of three-wave interaction between Langmuir and lower hybrid waves, L 1 ↔ L 2 +LH, which explains many details of recent Freja observations in the topside ionosphere. The packet-like waveforms are generated as the beating of several Langmuir modes. The primary Langmuir mode (L 1 ) is produced by beam-plasma instability, and the other modes are produced as a result of parametric decay to secondary Langmuir waves (L 2 ) and lower hybrid modes (LH). We show that the decay instability has a very low threshold and high growth rate. The limited transverse dimensions of electron beams in the polar ionosphere cause radiation losses from the beam region which influence spectra of the beam-plasma and parametric instabilities.

Analysis of the turbulence observed in the outer cusp turbulent boundary layer

One of the prominent features of the cusp Turbulent Boundary Layer (TBL) is a persistent low frequency electromagnetic turbulence that extends from <1Hz up to the electron cyclotron frequency, accompanied by what appears to be purely electrostatic noise above this frequency range. The Plasma Wave Instrument onboard Polar obtained plasma wave measurements in the cusp TBL in the form of waveform captures simultaneously from 6 different sensors (3 each orthogonal electric and magnetic) in the frequency range 1 Hz up to 25 kHz. This allowed us to directly calculate the phase velocity from the measured ratio of |dE| to |dB| and compare it to theoretical values for various modes. Using this technique, we have gained some insight into the mode of the electromagnetic turbulence that extends in frequency from ∼ 1 Hz up to the electron cyclotron frequency (several hundred Hz to a few kHz) in the TBL. The whistler and kinetic Alfven wave modes are discussed as the possible modes of this turbulence. By analyzing the high time resolution waveforms, we isolate and identify some of these modes. The electrostatic turbulence above the electron cyclotron frequency is associated with pulses and quasi-sinusoidal waveforms observed in the measured time series. These do not fit any known mode, although work is continuing in this area to show that some of them may be associated with electron holes or with downshifted Langmuir waves produced through a two-stream instability.

Langmuir wave structures registered by FREJA: analysis and modeling

Abstract A large set of observations of Langmuir emissions generated by super-thermal electron beams in the topside polar ionosphere has been obtained during the recent FREJA mission. Langmuir emissions appeared as a chain of strongly modulated wave packets; in many cases the packet envelopes were modulated periodically. In present report we suggest the theory which explains these observations. We suggest that packet-like waveforms are the beatings of several Langmuir modes. One of modes (the primary mode) appears as a result of beam-plasma instability, other modes — as a result of parametric decay of primary mode to the secondary Langmuir wave and lower-hybrid wave (or as a result of decay cascade). We present the quantitative theory of three-wave interaction L→L+LH which explain many details of FREJA observations.

Reply to Comment on “Identification of widespread turbulence of dispersive Alfvén Waves”

We welcome the comment made by Lund [this issue] as this provides us an opportunity to clarify some important implications of our paper [Stasiewicz et al., 2000a], hereafter referred to as S2000a. In his comment, Lund claims that results of S2000a are questionable because: (1) the derivation of our expression for δE⊥/δB⊥ is incorrect, and that (2) we do not prove the uniqueness of our solution. In this reply we argue that the conclusions of our paper hold and represent important new findings in space physics.