D. S. Orlowski

Damping and spectral formation of upstream whistlers

Previous studies have indicated that damping rates of upstream whistlers strongly depend on the details of the electron distribution function. Moreover, detailed analysis of Doppler shift and the whistler dispersion relation indicate that upstream whistlers propagate obliquely in a finite band of frequencies. In this paper we present results of a kinetic calculation of damping lengths of wideband whistlers using the sum of seven drifting bi-Maxwellian electron distributions as a best fit to the ISEE 1 electron data. For two cases, when upstream whistlers are observed, convective damping lengths derived from ISEE magnetic field and ephemeris data are compared with theoretical results. We find that the calculated convective damping lengths are consistent with the data and that upstream whistlers remain marginally stable. We also show that the slope of plasma frame spectra of upstream whistlers, obtained by direct fitting of the observed spectra, is between 5 and 7. The overall spectral, wave, and particle characteristics, proximity to the shock, as well as propagation and damping properties indicate that these waves cannot be generated locally. Instead, the observed upstream whistlers arise in the shock ramp, most likely by a variety of cross-field drift and/or anisotropy driven instabilities.

Ulf waves upstream of the Venus bow shock: Properties of one‐hertz waves

Pioneer Venus orbiter data are used to examine the properties of a class of ULF upstream waves with relatively high observed frequencies (1.1–1.3 Hz). In the spacecraft frame these waves are most often left-hand elliptically polarized. They have amplitudes up to 3 nT (near the bow shock) and propagate obliquely to the magnetic field at angles from 10° to 50°. These waves show significant similarity in their properties to “one-Hertz” waves identified at the Earth in the ISEE 1 and 2 observations and the whistler waves identified earlier with IMP 6 observations. The waves appear almost immediately after the spacecraft crosses the magnetic field tangent line to the bow shock surface into the region of connected field lines. The amplitude of these waves decreases with distance from the shock measured along the magnetic field line. We have used the cold plasma dispersion relation in order to study the propagation of these waves and to explain their observed polarization. Calculated group velocities indicate that those waves have sufficient upstream velocities to propagate from the shock into the solar wind. The totality of observations, including the observed wave damping and the observation of right-handed waves, seems best explained by a source of right-handed whistler mode waves at the bow shock.

Wave phenomena in the upstream region of Saturn

The magnetic field data returned from the Voyager 1 and 2 transit through the upstream region of Saturn have been examined from 0.1 mHz to 0.25 Hz to search for the signatures of waves associated with both the electron and ion foreshocks. In contrast to the Earth, in the ion foreshock of Saturn we find two distinct bands of wave activity at frequencies around 0.5 and 2-mHz in the spacecraft frame. These two waves differ considerably in their properties. The 0.5-mHz waves are right handed, strongly elliptically polarized, while the 2-mHz waves are elliptically or circularly polarized in the left-handed sense. These two different waves may be associated with two different backstreaming particle distributions, possibly those reflected from the shock and those leaking from the hot magnetosheath. We also observe right-hand elliptically polarized upstream waves propagating obliquely to the magnetic field at frequencies between 90 and 150 mHz, which are above the proton gyrofrequency in the spacecraft frame. The frequency of these waves appears to be dependent on the interplanetary magnetic field orientation. Such dependence is characteristic of the 1-Hz waves at Earth. Discrete wave packets associated with shocklets are for the first time observed in the foreshock of an outer planet.

A test of the Hall-MHD Model: Application to low-frequency upstream waves at Venus

Early studies suggested that in the range of parameter space where the wave angular frequency is less than the proton gyrofrequency and the plasma beta, the ratio of the thermal to magnetic pressure, is less than 1 magnetohydrodynamics provides an adequate description of the propagating modes in a plasma. However, recently, Lacombe et al. [1992] have reported significant differences between basic wave characteristics of the specific propagation modes derived from linear Vlasov and Hall-MHD theories even when the waves are only weakly damped. In this paper we compare the magnetic polarization and normalization magnetic compression ratio of ULF upstream waves at Venus with magnetic polarization and normalized magnetic compression ratio derived from both theories. We find that while the “kinetic” approach gives magnetic polarization and normalized magnetic compression ratio consistent with the data in the analyzed range of beta (0.5 < beta < 5) for the fast magnetosonic mode, the same wave characteristics derived from the Hall-MHD model strongly depend on beta arid are consistent with the data only at low beta for the fast mode and at high beta for the intermediate mode.

Comparison of properties of upstream whistlers at different planets

Whistler mode waves have been recorded in the upstream region of Mercury, Venus, Earth and Saturn. They are elliptically polarized and observed typically at frequencies between 0.1 to 4 Hz. These intrinsically right handed waves can be left-hand polarized in the spacecraft frame as a result of strong negative Doppler shift. The waves propagate at an angle between 10 and 60 deg to the background magnetic field, with ΔBB rarely exceeding 0.1. Comprehensive studies of these waves at Earth and Venus indicate that upstream whistlers are generated at the shock rather than locally in the foreshock. In this paper we compare properties of upstream whistlers at all these planets. We also discuss the utilization of selected properties of these waves to evaluate the effective Alfvenic Mach number and the shock thickness at Mercury where solar wind measurements are not available.

Coherence lengths of upstream ulf waves : dual ISEE observations

Waves are generated in front of planetary bow shocks by a variety of plasma instabilities and they occur over a wide range of frequencies and wavelengths. In the strongly Doppler shifting environment of solar wind, multiple spacecraft observations are needed to determine unambiguously the wave properties such as frequency, phase velocity, and wavelength. The separations of the spacecraft required for these studies are restricted because the waves have finite coherence lengths. The multiple spacecraft must be located within a coherence length of each other in order that they are studying the same wave. In this study, we have used high time resolution simultaneous magnetic field data from the dual ISEE spacecraft to study the coherence lengths of upstream ULF waves. We examine the cross-correlation between ISEE 1 and 2 observations for different spacecraft separations and determine the coherence lengths for upstream 30-second waves, 3-second waves and one-Hz waves. We find that the observed coherence lengths are consistent with those estimated from the bandwidth of the spectral peak, and that these lengths vary markedly from less than 100 km to over 1 RE. In order to study all these wave phenomena, a multiple spacecraft mission such as the upcoming ESA Cluster mission would need to be capable of assuming a wide variety of possible separations.

Propagation and damping of broadband upstream whistlers

Abstract Previous studies indicated that damping rates of upstream whistlers strongly depend on the details of the electron distribution function. Moreover, detailed analysis of Doppler-shift and whistler dispersion relation indicated that upstream whistlers propagate obliquely in a broad band. In this paper we present results of a kinetic calculation of damping lengths of wide-band whistlers using the sum of 7-drifting bi-Maxwellian electron distributions as a best fit to the ISEE 1 electron data. For 2 cases, when upstream whistlers are observed, convective damping lengths derived from ISEE magnetic field and ephemeris data are compared with theoretical results. We find that the calculated convective damping lengths are consistent with the data and that upstream whistlers remain marginally stable. We also show that the slope of plasma frame spectra of upstraem whistlers, obtained by direct fitting of the observed spectra is between 5 and 7 with a sharp lower frequency cutoff corresponding to a wavelength of about one ion inertial length. When the solar wind velocity is directed largely along the wave normal of the upstream whistlers the polarization of the right hand waves becomes reversed and low frequencies are switched to high resulting in a peaked spectrum with a strong high frequency cutoff. The overall spectral, wave and particle characteristics, proximity to the shock as well as propagation and damping properties indicate that these waves cannot be generated locally. Instead the observed upstream whistlers arise in the shock ramp most likely by a variety of cross-field drift and/or anisotropy driven instabilities.

Experimental studies of the properties of 'simulated' upstream turbulence using a statistical multipoint method

Abstract In this report we present a different approach to the multipoint measurement of magnetic fields and plasma. This is called the multi-spacecraft ensemble technique, MET, essentially free of process restrictions, such as linearity and stationarity. We comprehensively discuss the other conditions and limitations intrinsic to this statistical method. We also show the results of the application of the ensemble method to the synthetic data obtained from a hybrid simulation in the region upstream of a quasi-parallel shock. The important implications of the above approach for the CLUSTER mission are discussed.