Adolfo F. Viñas

Interplanetary fast shocks and associated drivers observed through the 23rd solar minimum by Wind over its first 2.5 years

A list of the interplanetary shocks observed by Wind from its launch (in Nov 1994) to May 1997 is presented. The magnetohydrodynamic nature of the shocks is investigated, and the associated shock parameters and their uncertainties are accurately computed using two techniques. These are: 1) a combination of the “preaveraged” magnetic-coplanarity, velocity-coplanarity, and the Abraham-Schrauner-mixed methods, and 2) the Vinas and Scudder [1986] technique for solving the nonlinear least squares Rankine-Hugoniot equations. Within acceptable limits these two techniques generally gave the same results, with some exceptions. The reasons for the exceptions are discussed. The mean strength and rate of occurrence of the shocks appear to correlate with the solar cycle. Both showed a decrease in 1996 coincident with the time of the lowest ultraviolet solar radiance, indicative of solar minimum and the beginning of solar cycle 23. Eighteen shocks appeared to be associated with corotating interaction regions (CIRs). The shock normal distribution showed a mean direction peaking in the ecliptic plane and with a longitude of ∼ 200° (GSE coordinates). Another 16 shocks were determined to be driven by solar transients, including magnetic clouds. These had a broader distribution of normal directions than those of the CIR cases with a mean direction close to the Sun-Eart line. Eight shocks of unknown origin had normal orientations far off the ecliptic plane. No shock propagated with longitude ϕn ≥ 220±10°, i.e. against the average Parker spiral direction. Examination of the obliquity angle θBn (i.e., between the shock normal and the upstream interplanetary magnetic field) for the full set of shocks revealed that about 58% were quasi-perpendicular, and about 32% of the shocks oblique, and the rest quasi-parallel. Small uncertainty in the estimated angle θBn was obtained for about 10 shocks with magnetosonic Mach numbers between 1 and 2.

Ion acceleration and abundance enhancements by electron beam instabilities in impulsive solar flares

We show that a nonrelativistic electron beam in a hydrogen-helium solar flare plasma will excite H(+) electromagnetic ion cyclotron, shear Alfven, and R-X waves, in addition to waves resulting from the two-stream instability. The H(+) electromagnetic ion cyclotron and shear Alfven waves are able to selectively accelerate ambient He-3 and Fe, respectively, to MeV energies through first harmonic gyroresonance, and thereby account for the large (He-3)/(He-4) and Fe/C ratios seen in the energetic particles from impulsive solar flares. In this model, separate heating and acceleration mechanisms for either He-3 or Fe are not required, and Fe acceleration is quite efficient since it does not need to occur by second harmonic gyroresonance. The combination of the other two unstable modes is able to accelerate ions to hundreds of MeV if the particles become trapped in an electrostatic potential well of a two-stream wave.

Parametric instabilities of circularly polarized large-amplitude dispersive Alfvén waves: excitation of obliquely-propagating daughter and side-band waves

We investigate the parametric instabilities of a large-amplitude circularly polarized dispersive parallel-propagating Alfven wave. Our treatment is more general than that of previous derivations based on the two-fluid equations in that we allow for propagation of the unstable daughter and side-band waves at arbitrary angles to the background (DC) magnetic field. We present the characteristics of the decay and modulational instabilities as functions of propagation angle. We find, in addition to the well-known decay and modulational instabilities, that at oblique and perpendicular propagation there is another parametric instability, namely the filamentation instability, which is characterized by a broad band-width in wavenumber and which satisfies the condition Re (ω) ≪ γ. A second parametric process at oblique and perpendicular angles of propagation, which has not been reported before is also investigated, namely the parametric magneto-acoustic instability. This instability is distinct from the filamentation instability in that it is characterized by density perturbations with large real frequencies that satisfy the condition Re (ω) ≫ γ. Unlike the filamentation instability, the magneto-acoustic instability extends over a broad angular range, but has a very narrow band-width in wavenumber. We report the dispersive characteristics of the filamentation and magneto-acoustic instabilities as functions of plasma β dispersion η and pump amplitude η for arbitrary propagation angles.

Generation of Electron Suprathermal Tails in the Upper Solar Atmosphere: Implications for Coronal Heating

We present a mechanism for the generation of non-Maxwellian electron distribution function in the upper regions of the solar atmosphere in the presence of collisional damping. It is suggested that finite-amplitude, low-frequency, obliquely propagating electromagnetic waves can carry a substantial electric field component parallel to the mean magnetic field that can be significantly larger than the Dreicer electric field. This long-wavelength electric fluctuation is capable of generating high-frequency electron plasma oscillations and low-frequency ion acoustic-like waves. The analysis has been performed using 1-1/2D Vlasov and PIC numerical simulations in which both electrons and ions are treated kinetically and self consistently. The simulation results indicate that high-frequency electron plasma oscillations and low-frequency ion acoustic-like waves are generated. The high-frequency electron plasma oscillation drives electron plasma turbulence, which subsequently is damped out by the background electrons. The turbulence damping results in electron acceleration and the generation of non-Maxwellian suprathermal tails on timescales short compared to collisional damping. Bulk heating also occurs if the fluctuating parallel electric field is strong enough. This study suggests that finite-amplitude, low-frequency, obliquely propagating electromagnetic waves can play a significant role in the acceleration and heating of the solar corona electrons and in the coupling of medium and small-scale phenomena.

Constraints on the O+5 Anisotropy in the Solar Corona

Velocity distributions of O+5 ions derived from Ultraviolet Coronagraph Spectrometer (UVCS) observations in coronal holes indicate that the O+5 ions are highly anisotropic (T⊥i/T∥i ≈ 30-300 at 3.5 R☉). The observations provide empirical values for the electron density and the ion temperatures. It is well known that the electromagnetic ion cyclotron instability is driven by temperature anisotropy. The instability leads to the rapid decrease of anisotropy and transfer of part of the kinetic energy of the particles into the magnetic field fluctuations. Here we use linear theory and hybrid simulations combined with the empirical values of the densities and the temperatures to investigate the ion cyclotron instability of the anisotropic minor ions in the coronal hole plasma. We find that an initial O+5 anisotropy of 50 decreases by an order of magnitude within ~300-900 proton cyclotron periods. Thus, the ion cyclotron instability constrains the anisotropy of O+5 ions that can be sustained in the solar corona without continuous perpendicular heating.

PLASMA AND MAGNETIC FIELD INSIDE MAGNETIC CLOUDS: A GLOBAL STUDY

Data observed during spacecraft encounters with magnetic clouds have been extensively analyzed in the literature. Moreover, several models have been proposed for the magnetic topology of these events, and fitted to the observations. Although these interplanetary events present well-defined plasma features, none of those models have included a simultaneous analysis of magnetic field and plasma data. Using as a starting point a non-force-free model that we have developed previously, we present a global study of MCs that include both the magnetic field topology and the plasma pressure. In this paper we obtain the governing equations for both magnitudes inside a MC. The expressions deduced are fitted simultaneously to the measurements of plasma pressure and magnetic field vector. We perform an analysis of magnetic field and plasma WIND observations within several MCs from 1995 to 1998. The analysis is confined to four of these events that have high-quality data. Only in one fitting procedure we obtain the orientation of the magnetic cloud relative to the ecliptic plane and the current density of the plasma inside the cloud. We find that the equations proposed reproduce the experimental data quite well.

Theory of 2ω pe radiation induced by the bow shock

A new radiation emission mechanism is proposed to explain electromagnetic radiation observed at twice the electron plasma frequency, 2ωpe, in the upstream region of the Earths bow shock. This radiation has its origin at the electron foreshock boundary where energetic electron beams and intense narrow-band Langmuir waves are observed. The proposed emission mechanism results from the interaction of the electron beam and Langmuir waves that are backscattered off thermal ions. This interaction is described by a nonlinear dispersion equation which incorporates an effect owing to electron trajectory modulation by the backscattered Langmuir waves. Subsequent analysis of the dispersion equation reveals two important consequences. First, a long-wavelength electrostatic quasi-mode with frequency at 2ωpe is excited, and second, the quasi-mode and the electromagnetic mode are nonlinearly coupled. The implication is that, when the excited 2ωpe quasi-mode propagates in an inhomogeneous medium with slightly decreasing density, the quasi-mode can be converted directly into an electromagnetic mode. Hence the electromagnetic radiation at twice the plasma frequency is generated. Numerical solutions of the dispersion equation with the choice of parameters that describe physical characteristics of the electron foreshock are presented, which illustrates the viability of the new mechanism.

Tsallis statistics of the magnetic field in the heliosheath

The spacecraft Voyager 1 crossed the termination shock on 2004 December 16 at a distance of 94 AU from the Sun, and it has been moving through the heliosheath toward the interstellar medium since then. The distributions of magnetic field strength B observed in the heliosheath are Gaussian over a wide range of scales, yet the measured profile appears to be filamentary with occasional large jumps in the magnetic field strength B(t). All of the probability distributions of changes in B, dBn ≡ B(t + τ) - B(t) on scales τ from 1 to 128 days, can be fit with the symmetric Tsallis distribution of nonextensive statistical mechanics. At scales ≥32 days, the distributions are Gaussian, but on scales from 1 through 16 days the probability distribution functions have non-Gaussian tails, suggesting that the inner heliosheath is not in statistical equilibrium on scales from 1 to 16 days.

Electromagnetic fluctuations of the whistler‐cyclotron and firehose instabilities in a Maxwellian and Tsallis‐kappa‐like plasma

Observed electron velocity distributions in the Earths magnetosphere and the solar wind exhibit a variety of nonthermal features which deviate from thermal equilibrium, for example, in the form of temperature anisotropies, suprathermal tail extensions, and field-aligned beams. The state close to thermal equilibrium and its departure from it provides a source for spontaneous emissions of electromagnetic fluctuations, such as the whistler. Here we present a comparative analysis of the electron whistler-cyclotron and firehose fluctuations based upon anisotropic plasma modeled with Maxwellian and Tsallis-kappa-like particle distributions, to explain the correspondence relationship of the magnetic fluctuations as a function of the electron temperature and thermal anisotropy in the solar wind and magnetosphere plasmas. The analysis presented here considers correlation theory of the fluctuation-dissipation theorem and the dispersion relation of transverse fluctuations, with wave vectors parallel to the uniform background magnetic field, in a finite temperature anisotropic thermal bi-Maxwellian and nonthermal Tsallis-kappa-like magnetized electron-proton plasma. Dispersion analysis and stability thresholds are derived for these thermal and nonthermal distributions using plasma and field parameters relevant to the solar wind and magnetosphere environments. Our results indicate that there is an enhancement of the fluctuations level in the case of nonthermal distributions due to the effective higher temperature and the excess of suprathermal particles. These results suggest that a comparison of the electromagnetic fluctuations due to thermal and nonthermal distributions provides a diagnostic signature by which inferences about the nature of the particle velocity distribution function can be ascertained without in situ particle measurements.

Asymmetric Solar Wind Electron Superthermal Distributions

Electron distributions with various degrees of asymmetry associated with the energetic tail population are commonly detected in the solar wind near 1 AU. By numerically solving one-dimensional electrostatic weak turbulence equations the present paper demonstrates that a wide variety of asymmetric energetic tail distributions may result. It is found that a wide variety of asymmetric tail formation becomes possible if one posits that the solar wind electrons are initially composed of thermal core plus field-aligned counterstreaming beams, instead of the customary thermal population plus a single beam. It is shown that the resulting nonlinear wave-wave and wave-particle interactions lead to asymmetric nonthermal tails. It is found that the delicate difference in the average beam speeds associated with the forward versus backward components is responsible for the generation of asymmetry in the energetic tail.