C. S. Salem

Magnetic Fluctuation Power Near Proton Temperature Anisotropy Instability Thresholds in the Solar Wind

The proton temperature anisotropy in the solar wind is known to be constrained by the theoretical thresholds for pressure-anisotropy-driven instabilities. Here, we use approximately 1x10;{6} independent measurements of gyroscale magnetic fluctuations in the solar wind to show for the first time that these fluctuations are enhanced along the temperature anisotropy thresholds of the mirror, proton oblique firehose, and ion cyclotron instabilities. In addition, the measured magnetic compressibility is enhanced at high plasma beta (beta_{ parallel} greater, similar1) along the mirror instability threshold but small elsewhere, consistent with expectations of the mirror mode. We also show that the short wavelength magnetic fluctuation power is a strong function of collisionality, which relaxes the temperature anisotropy away from the instability conditions and reduces correspondingly the fluctuation power.

IDENTIFICATION OF KINETIC ALFVÉN WAVE TURBULENCE IN THE SOLAR WIND

The nature of small-scale turbulent fluctuations in the solar wind is investigated using a comparison of Cluster magnetic and electric field measurements to predictions arising from models consisting of either kinetic Alfven waves or whistler waves. The electric and magnetic field properties of these waves from linear theory are used to construct spacecraft-frame frequency spectra of (|δE|/|δB|) s/c and (|δB ∥|/|δB|) s/c , allowing for a direct comparison to spacecraft data. The measured properties of the small-scale turbulent fluctuations, found to be inconsistent with the whistler wave model, agree well with the prediction of a spectrum of kinetic Alfven waves with nearly perpendicular wavevectors.

THE SLOW-MODE NATURE OF COMPRESSIBLE WAVE POWER IN SOLAR WIND TURBULENCE

We use a large, statistical set of measurements from the Wind spacecraft at 1 AU, and supporting synthetic spacecraft data based on kinetic plasma theory, to show that the compressible component of inertial range solar wind turbulence is primarily in the kinetic slow mode. The zero-lag cross-correlation C(?n, ?B ?) between proton density fluctuations ?n and the field-aligned (compressible) component of the magnetic field ?B ? is negative and close to ?1. The typical dependence of C(?n, ?B ?) on the ion plasma beta ? i is consistent with a spectrum of compressible wave energy that is almost entirely in the kinetic slow mode. This has important implications for both the nature of the density fluctuation spectrum and for the cascade of kinetic turbulence to short wavelengths, favoring evolution to the kinetic Alfv?n wave mode rather than the (fast) whistler mode.

SOLAR WIND MAGNETOHYDRODYNAMICS TURBULENCE: ANOMALOUS SCALING AND ROLE OF INTERMITTENCY

In this paper, we present a study of the scaling properties and intermittency of solar wind MHD turbulence based on the use of wavelet transforms. More specifically, we use the Haar Wavelet transform on simultaneous 3 s resolution particle and magnetic field data from the Wind spacecraft, to investigate anomalous scaling and intermittency effects of both magnetic field and solar wind velocity fluctuations in the inertial range. For this purpose, we calculated spectra, structure functions, and probability distribution functions. We show that this powerful wavelet technique allows for a systematic elimination of intermittency effects on spectra and structure functions and thus for a clear determination of the actual scaling properties in the inertial range. The scaling of the magnetic field and the velocity fluctuations are found to be fundamentally different. Moreover, when the most intermittent structures superposed to the standard fluctuations are removed, simple statistics are recovered. The magnetic field and the velocity fluctuations exhibit a well-defined, although different, monofractal behavior, following a Kolmogorov –5/3 scaling and a Iroshnikov-Kraichnan –3/2 scaling, respectively. The multifractal properties of solar wind turbulence appear to be determined by the presence of those most intermittent structures. Finally, our wavelet technique also allows for a direct and systematic identification of the most active, singular structures responsible for the intermittency in the solar wind.

ELECTRON PROPERTIES AND COULOMB COLLISIONS IN THE SOLAR WIND AT 1 AU: WIND OBSERVATIONS

The question of what controls the electron properties in the solar wind has been the subject of several extensive analyses over the past 20 years. We analyze here the electron properties of the solar wind observed by the Wind satellite at 1 AU in the ecliptic plane, during 50 days close to the last minimum of solar activity. The electron temperature anisotropy Te∥/Te⊥, which seems to depend on the wind speed Vsw, the density Np, the heliomagnetic latitude λm, or the time, actually depends mainly on the Coulomb collisions. The collisional age Ae is the number of transverse collisions suffered by a thermal electron during the expansion of the wind over the scale of the density gradient. The Ae depends on Vsw, on Np, and thus on λm; it also depends on the time because it changes strongly at the crossing of a stream interface. We show that Te∥/Te⊥ is strongly correlated with 1/Ae. The effect of Coulomb collisions on the electron heat flux are also investigated. We find that the total electron heat flux Qe displays an upper bound that is inversely proportional to the collisional age, in favor of a regulation of the heat flux by Coulomb collisions. The observed heat flux is then compared to the collisional heat flux of the classical Spitzer-Harm (SH) theory. Although earlier observations have shown that the electron heat flux in the solar wind at 1 AU is well below the values given by the SH theory, we find that the observed heat flux reaches the SH limit for the lowest values of the electron mean free paths. The Coulomb collisions thus seem to play a part in the regulation of the electron heat flux in the solar wind.

Determination of accurate solar wind electron parameters using particle detectors and radio wave receivers

We present a new, simple, and semiempirical method for determining accurate solar wind electron macroscopic parameters from the raw electron moments obtained from measured electron distribution functions. In the solar wind these measurements are affected by (1) photoelectrons produced by the spacecraft illumination, (2) spacecraft charging, and (3) the incomplete sampling of the electron distribution due to a nonzero low-energy threshold of the energy sweeping in the electron spectrometer. Correcting fully for these effects is difficult, especially without the help of data from other experiments that can be taken as a reference. We take here advantage of the fact that high-resolution solar wind electron parameters are obtained on board Wind using two different instruments: the electron electrostatic analyzer of the three-dimensional Plasma experiment (3DP), which provides 3-D electron velocity distribution functions every 99 s as well as 3-s resolution computed onboard moments, and the thermal noise receiver (TNR), which yields unbiased electron density and temperature every 4.5 s from the spectroscopy of the quasi-thermal noise around the electron plasma frequency. The present correction method is based on a simplified model evaluating the electron density and temperature as measured by the electron spectrometer, by taking into account both the spacecraft charging and the low-energy cutoff effects: approximating the solar wind electron distributions by an isotropic Maxwellian, we derive simple analytical relations for the measured electron moments as functions of the real ones. These relations reproduce the qualitative behavior of the variation of the raw 3DP electron density and temperature as a function of the TNR ones. In order to set up a precise “scalar correction” of the raw 3DP electron moments, we use the TNR densities and temperatures as good estimates of the real ones; the coefficients appearing in the analytical relations are obtained by a best fit to the data from both instruments during a limited period of time, chosen as a reference. This set of coefficients is then used as long as the mode of operation of the electron spectrometer is unchanged. We show that this simple scalar correction of the electron density and temperature is reliable and can be applied routinely to the high-resolution 3DP low-order moments. As a by-product, an estimate of the spacecraft potential is obtained. The odd-order moments of the distribution function (electron bulk speed and heat flux) cannot be corrected by the model since the distribution is assumed to be an isotropic Maxwellian. We show, however, that a better estimate of the electron heat flux can be obtained by replacing the electron velocity by the proton velocity.

Shocklets, SLAMS, and field‐aligned ion beams in the terrestrial foreshock

We present Wind spacecraft observations of ion distributions showing field-aligned beams (FABs) and large-amplitude magnetic fluctuations composed of a series of shocklets and short large-amplitude magnetic structures (SLAMS). We show that the SLAMS are acting like a local quasi-perpendicular shock reflecting ions to produce the FABs. Previous FAB observations reported the source as the quasi-perpendicular bow shock. The SLAMS exhibit a foot-like magnetic enhancement with a leading magnetosonic whistler train, consistent with previous observations. The FABs are found to have T_b ~ 80-850 eV, V_b/V_sw ~ 1-2, T_{b,perp}/T{b,para} ~ 1-10, and n_b/n_i ~ 0.2-14%. Strong ion and electron heating are observed within the series of shocklets and SLAMS increasing by factors \geq 5 and \geq 3, respectively. Both the core and halo electron components show strong perpendicular heating inside the feature.

Residual Energy Spectrum of Solar Wind Turbulence

It has long been known that the energy in velocity and magnetic field fluctuations in the solar wind is not in equipartition. In this paper, we present an analysis of 5 yr of Wind data at 1 AU to investigate the reason for this. The residual energy (difference between energy in velocity and magnetic field fluctuations) was calculated using both the standard magnetohydrodynamic (MHD) normalization for the magnetic field and a kinetic version, which includes temperature anisotropies and drifts between particle species. It was found that with the kinetic normalization, the fluctuations are closer to equipartition, with a mean normalized residual energy of σr = –0.19 and mean Alfven ratio of r A = 0.71. The spectrum of residual energy, in the kinetic normalization, was found to be steeper than both the velocity and magnetic field spectra, consistent with some recent MHD turbulence predictions and numerical simulations, having a spectral index close to –1.9. The local properties of residual energy and cross helicity were also investigated, showing that globally balanced intervals with small residual energy contain local patches of larger imbalance and larger residual energy at all scales, as expected for nonlinear turbulent interactions.

Frame Dependence of the Electric Field Spectrum of Solar Wind Turbulence

We present the first survey of electric field data using the ARTEMIS spacecraft in the solar wind to study inertial range turbulence. It was found that the average perpendicular spectral index of the electric field depends on the frame of measurement. In the spacecraft frame it is –5/3, which matches the magnetic field due to the large solar wind speed in Lorentz transformation. In the mean solar wind frame, the electric field is primarily due to the perpendicular velocity fluctuations and has a spectral index slightly shallower than –3/2, which is close to the scaling of the velocity. These results are an independent confirmation of the difference in scaling between the velocity and magnetic field, which is not currently well understood. The spectral index of the compressive fluctuations was also measured and found to be close to –5/3, suggesting that they are not only passive to the velocity but may also interact nonlinearly with the magnetic field.

Whistler waves, Langmuir waves and single loss cone electron distributions inside a magnetic cloud: Observations

Whistler waves propagating along the ambient magnetic field are observed within a coronal mass ejection (on January 10, 1997) associated in time with Langmuir waves and electron distributions of a single loss cone type. In addition, background observations are made on the plasma wave activity in the sheath and foreshock regions that precede the magnetic cloud, on the observed radio emissions (including a type II radio burst) and on the geometry of the cloud. All the data comes from the WIND spacecraft. The whistler waves are identified using full magnetic waveforms while possible evidence of coexisting parallel, and antiparallel propagating Langmuir modes are found in the waveform and spectral wave data from the WAVES experiment. A few hundred low energy electron distributions from the Three-Dimensional Plasma (3DP) experiment are investigated. Finally, we tentatively suggest that this type of plasma wave particle activity is linked to the type II emission observed, i.e., that the emission mechanisms are proceeding and taking place within the magnetic cloud instead of at the shock region as usually thought. The extra suprathermal electrons could source from electrons accelerated at reconnection sites between the magnetic cloud and the ambient interplanetary magnetic field. A linear instability study using observed properties of the electron distributions is to be presented in a following paper.