Shadia Rifai Habbal

UVCS/SOHO Empirical Determinations of Anisotropic Velocity Distributions in the Solar Corona

We present a self-consistent empirical model for several plasma parameters of a polar coronal hole near solar minimum, derived from observations with the Solar and Heliospheric Observatory Ultraviolet Coronagraph Spectrometer. The model describes the radial distribution of density for electrons, H0, and O5 + and the outflow velocity and unresolved most probable velocities for H0 and O5 + during the period between 1996 November and 1997 April. In this Letter, we compare observations of H I Lyα and O VI λλ1032, 1037 emission lines with spatial models of the plasma parameters, and we iterate for optimal consistency between measured and synthesized observable quantities. The unexpectedly large line widths of H0 atoms and O5 + ions at most radii are the result of anisotropic velocity distributions, which are not consistent with purely thermal motions or the expected motions from a combination of thermal and transverse wave velocities. Above 2 R, the observed transverse, most probable speeds for O5 + are significantly larger than the corresponding motions for H0, and the outflow velocities of O5 + are also significantly larger than the corresponding velocities of H0. We discuss the constraints and implications on various theoretical models of coronal heating and acceleration.

An Empirical Model of a Polar Coronal Hole at Solar Minimum

We present a comprehensive and self-consistent empirical model for several plasma parameters in the extended solar corona above a polar coronal hole. The model is derived from observations with the SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) during the period between 1996 November and 1997 April. We compare observations of H I Lyα and O VI λλ1032, 1037 emission lines with detailed three-dimensional models of the plasma parameters and iterate for optimal consistency between measured and synthesized observable quantities. Empirical constraints are obtained for the radial and latitudinal distribution of density for electrons, H0, and O5+, as well as the outflow velocity and unresolved anisotropic most probable speeds for H0 and O5+. The electron density measured by UVCS/SOHO is consistent with previous solar minimum determinations of the white-light coronal structure; we also perform a statistical analysis of the distribution of polar plumes using a long time series. From the emission lines we find that the unexpectedly large line widths of H0 atoms and O5+ ions at most heights are the result of anisotropic velocity distributions. These distributions are not consistent with purely thermal motions or the expected motions from a combination of thermal and transverse wave velocities. Above 2 R☉, the observed transverse most probable speeds for O5+ are significantly larger than the corresponding motions for H0, and the outflow velocities of O5+ are also significantly larger than the corresponding velocities of H0. Also, the latitudinal dependence of intensity constrains the geometry of the wind velocity vectors, and superradial expansion is more consistent with observations than radial flow. We discuss the constraints and implications on various theoretical models of coronal heating and acceleration.

The Ultraviolet Coronagraph Spectrometer for the Solar and Heliospheric Observatory

The SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) is composed of three reflecting telescopes with external and internal occultation and a spectrometer assembly consisting of two toric grating spectrometers and a visible light polarimeter. The purpose of the UVCS instrument is to provide a body of data that can be used to address a broad range of scientific questions regarding the nature of the solar corona and the generation of the solar wind. The primary scientific goals are the following: to locate and characterize the coronal source regions of the solar wind, to identify and understand the dominant physical processes that accelerate the solar wind, to understand how the coronal plasma is heated in solar wind acceleration regions, and to increase the knowledge of coronal phenomena that control the physical properties of the solar wind as determined by in situ measurements. To progress toward these goals, the UVCS will perform ultraviolet spectroscopy and visible polarimetry to be combined with plasma diagnostic analysis techniques to provide detailed empirical descriptions of the extended solar corona from the coronal base to a heliocentric height of 12 solar radii.

Plasma Properties in Coronal Holes Derived from Measurements of Minor Ion Spectral Lines and Polarized White Light Intensity

Recent observations of the Lyα λ1216, Mg X λ625, and O VI λ1038 spectral lines carried out with the Ultraviolet Coronagraph Spectrometer (UVCS) on board SOHO at distances in the range 1.35-2.1 RS in the northern coronal hole are used to place limits on the turbulent wave motions of the background plasma and the thermal motions of the protons and Mg+9 and O+5 ions. Limits on the turbulent wave motion are estimated from the measured line widths and electron densities derived from white light coronagraph observations, assuming WKB approximation at radial distances covered by the observations. It is shown that the contribution of the turbulent wave motion to the widths of the measured spectral lines is small compared to thermal broadening. The observations show that the proton temperature slowly increases between 1.35 and 2.7 RS and does not exceed 3×10 K in that region. The temperature of the minor ions exceeds the proton temperature at all distances, but the temperatures are neither mass proportional nor mass-to -charge proportional. It is shown, for the first time, that collision times between protons and minor ions are small compared to the solar wind expansion times in the inner corona. At 1.35 RS the expansion time exceeds the proton Mg+9 collision time by more than an order of magnitude. Nevertheless, the temperature of the Mg ions is significantly larger than the proton temperature, which indicates that the heating mechanism has to act on timescales faster than minutes. When the expansion time starts to exceed the collision times a rapid increase of the O+5 ion spectral line width is seen. This indicates that the heavier and hotter ions lose energy to the protons as long as collision frequencies are high, and that the ion spectral line width increases rapidly as soon as this energy loss stops.

The effect of temperature anisotropy on observations of doppler dimming and pumping in the inner corona

Recent observations of the spectral line profiles and intensity ratio of the O VI λλ1032 and 1037.6 doublet by the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO), made in coronal holes below 3.5 RS, provide evidence for Doppler dimming of the O VI λ1037.6 line and pumping by the chromospheric C II λ1037.0182 line. Evidence for a significant kinetic temperature anisotropy of O5+ ions was also derived from these observations. We show in this Letter how the component of the kinetic temperature in the direction perpendicular to the magnetic field, for both isotropic and anisotropic temperature distributions, affects both the amount of Doppler dimming and pumping. Taking this component into account, we further show that the observation of the O VI doublet intensity ratio less than unity can be accounted for only if pumping by C II λ1036.3367 in addition to C II λ1037.0182 is in effect. The inclusion of the C II λ1036.3367 pumping implies that the speed of the O5+ ions can reach 400 km s-1 around 3 RS, which is significantly higher than the reported UVCS values for atomic hydrogen in polar coronal holes. These results imply that oxygen ions flow much faster than protons at that heliocentric distance.

Heating and cooling of protons by turbulence‐driven ion cyclotron waves in the fast solar wind

The effects of parallel propagating nondispersive ion cyclotron waves on the solar wind plasma are investigated in an attempt to reproduce the observed proton temperature anisotropy, namely, Tp⊥ ≫ Tp‖ in the inner corona and Tp⊥ < Tp‖ at 1 AU. Low-frequency Alfven waves are assumed to carry most of the energy needed to accelerate and heat the fast solar wind. The model calculations presented here assume that nonlinear cascade processes, at the Kolmogorov and Kraichnan dissipation rates, transport energy from low-frequency Alfven waves to the ion cyclotron resonant range. The energy is then picked up by the plasma through the resonant cyclotrou interaction. While the resonant interaction determines how the heat is distributed between the parallel and perpendicular degrees of freedom, the level of turbulence determines the net dissipation. Ion cyclotron waves are found to produce a significant temperature anisotropy starting in the inner corona, and to limit the growth of the temperature anisotropy in interplanetary space. In addition, this mechanism heats or cools protons in the direction parallel to the magnetic field. While cooling in the parallel direction is dominant, heating in the parallel direction occurs when Tp⊥ ≫ Tp‖. The waves provide the mechanism for the extraction of energy from the parallel direction to feed into the perpendicular direction. In our models, both Kolmogorov and Kraichnan dissipation rates yield Tp⊥ ≫ Tp‖ in the corona, in agreement with inferences from recent ultraviolet coronal measurements, and predict temperatures at 1 AU which match in situ observations. The models also reproduce the inferred rapid acceleration of the fast solar wind in the inner corona and flow speeds and particle fluxes measured at 1 AU. Since this mechanism does not provide direct energy to the electrons, and the electron-proton coupling is not sufficient to heat the electrons to temperatures at or above 106 K, this model yields electron temperatures which are much cooler than those currently inferred from observations.

Origins of the Slow and the Ubiquitous Fast Solar Wind

We present in this Letter the first coordinated radio occultation measurements and ultraviolet observations of the inner corona below 5.5Rs, obtained during the Galileo solar conjunction in 1997 January, to establish the origin of the slow solar wind. Limits on the flow speed are derived from the Doppler dimming of the resonantly scattered component of the oxygen 1032 and 1037.6 A lines as measured with the ultraviolet coronagraph spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO). White light images of the corona from the large-angle spectroscopic coronagraph (LASCO) on SOHO taken simultaneously are used to place the Doppler radio scintillation and ultraviolet measurements in the context of coronal structures. These combined observations provide the first direct confirmation of the view recently proposed by Woo & Martin that the slow solar wind is associated with the axes, also known as stalks, of streamers. Furthermore, the ultraviolet observations also show how the fast solar wind is ubiquitous in the inner corona and that a velocity shear between the fast and slow solar wind develops along the streamer stalks.

Flow properties of the solar wind derived from a two-fluid model with constraints from white light and in situ interplanetary observations

We derive the flow properties of the solar wind in coronal holes using a two-fluid model constrained by density profiles inferred from simultaneous space-based SPARTAN 201–01 and ground-based Mauna Loa White Light coronagraph observations, and by in situ interplanetary measurements. Also used as a guide is the hydrostatic temperature profile derived from the density gradient. Density profiles are inferred between 1.16 and 5.5 Rs, for two different density structures observed along the line of sight in a polar coronal hole. The model computations that fit remarkably well the empirical constraints yield a supersonic flow at 2.3 Rs for the less dense ambient coronal hole, and at 3.4 Rs for the denser structures. The novel result that emerges from these fits is a proton temperature twice as large as the electron temperature in the inner corona, reaching a peak of 2 × 106 K at 2 Rs.

Resonant acceleration and heating of solar wind ions by dispersive ion cyclotron waves

We investigate the preferential acceleration and heating of solar wind alpha particles by the resonant cyclotron interaction with parallel-propagating left-hand-polarized ion cyclotron waves. The Alfven wave spectrum equation is generalized to multi-ion plasmas and a Kolmogorov type of cascade effect is introduced to transfer energy from the low-frequency Alfven waves to the high-frequency ion cyclotron waves, which are assumed to be entirely dissipated by the wave-particle interaction. In order to distribute the dissipated wave energy among the alphas and protons, the quasi-linear theory of the wave-particle interaction is used along with the cold plasma dispersion relation, and a power law spectrum of the ion cyclotron waves is assumed, with the spectral index as a free parameter of the model. The set of three-fluid solar wind equations and the Alfven wave spectrum equation are then solved in order to find fast solar wind solutions. It is found that the effect of the alpha particles on the dispersion relation, omitted in most previous wave-driven solar wind models, has a significant influence on the preferential acceleration and heating of the alphas, especially in the region close to the Sun. With this effect included, the alpha particles can be accelerated to a bulk flow speed faster than the protons by a few hundred kilometers per second and heated by the resonant cyclotron interaction to more than mass-proportional temperature values at several solar radii. However, this mechanism does not yield a differential speed of the order of an Alfven speed and a mass-proportional temperature for the alphas beyond 0.3 AU, as observed, which confirms the same conclusion reached previously by Isenberg and Hollweg [1983] for nondispersive ion cyclotron waves.

A fast solar wind model with anisotropic proton temperature

We explore the energy requirements for the fast solar wind when the anisotropy in the proton temperature is taken into account. Using a one-dimensional, two-fluid model with anisotropic proton temperature, we present high-speed solar wind solutions which meet most of the empirical constraints currently available from in situ measurements in interplanetary space and very recent remote sensing observations of the inner corona. Included in the model is the momentum exerted on the flow by Alfven waves, as well as heating due to their damping. However, to produce solutions consistent with these empirical constraints, additional heat input to both electrons and protons, as well as momentum addition to the protons, are found to be needed. These are described by ad hoc functions with adjustable parameters. While classical thermal conduction is adopted for both electrons and protons in the inner corona in the model computations, the corresponding heat fluxes in the outer corona are limited to values comparable to current observations. The fast solar wind solutions thus obtained differ from each other mainly in their thermal properties within 0.3 AU from the Sun, a region that is still poorly probed by in situ and remote sensing measurements. To satisfy observational constraints, we find that the inclusion of a proton temperature anisotropy in the modeling of the solar wind requires that either the protons be highly anisotropic in the inner corona or that there exist a mechanism, in addition to adiabatic expansion, to cool them in the direction parallel to the magnetic field. Given these observational constraints and in the absence of knowledge of an efficient cooling mechanism, our model computations imply that the maximum temperature of the protons in the parallel direction has to be limited to 106 K in the corona. Furthermore, because of the strong coupling between electrons and protons, and between the parallel and perpendicular motions, at the coronal base, the electron temperature as well as the perpendicular proton temperature cannot be much higher than 106 K there. Although thermal anisotropy of the protons is found to have little influence on the dynamics of the fast solar wind, its inclusion imposes new requirements on the unknown coronal heating mechanisms.