Solar Wind Acceleration Research Articles

A model of solar wind and corona driven by high-frequency Alfvén waves

The small-scale ejection events in the solar atmosphere detected in recent space observations have been regarded as manifestations of small-scale magnetic reconnection. But such reconnections generate high-frequency Alfvén waves. Based on this idea we set up a time-dependent, two-fluid model of coronal heating and solar wind acceleration. Numerical solutions are made, showing the time evolution, and including a discussion of their convergence and conservation properties. Under suitable assumptions regarding the generation of the waves in the transition zone, the numerical solution can explain a number of observations including coronal heating, the radial density profile of coronal holes, the acceleration of the wind near the bottom of the corona and the high-velocity streams observed at 0.3 au.

Summary of Session A: Coronal Heating and Solar Wind Acceleration

The solar corona is not easily observable from the ground; only with the coronagraphs located in high mountains or at infrequent and short occasions of total eclipses. And in any case these observations are limited to the corona beyond the solar limb. Therefore, it is natural that our understanding of the physics of the solar corona has advanced dramatically by sustained observations from space-based platforms, like Skylab in the 1970’s. These several years are particularly notable in this respect, because of a wide variety of data from Yohkoh, Ulysses, and SOHO.

Solar Wind Velocity and Anisotropic Coronal Kinetic Temperature Measured with the O VI Doublet Ratio

Doppler dimming of the O vi resonance lines (λ1032 A, λ1037 A) in an expanding corona is calculated including the pumping effect on the O vi λ1037.61 A of both C ii lines at λ1036.34 A and λ1037.02 A, and the effect of the width of the absorption profiles of the coronal oxygen ions along the incident radiation. The pumping effect of the C ii line at λ1036.34 A allows us to extend to approximately 450 km s−1 the measurement of solar wind velocities with the O vi line ratio technique. Since the emissivity ratio of the O vi doublet depends on the width of the oxygen coronal absorbing profiles, this ratio can provide an accurate measurement of the solar wind velocity in the case that the width of the absorbing profile along the direction of the incident radiation is independently determined. However, if on the one hand the ratio of the emissivities of the O vi doublet has limitations in probing the wind velocity, on the other hand it can be used as a diagnostics for inferring the velocity distribution of the coronal O vi ions along the radial, and detecting possible velocity anisotropies. This diagnostics, applied to recent observational results, allows us to infer that the velocity distribution of the oxygen ions is much broader in the direction perpendicular to the magnetic field direction, and that the acceleration of the fast solar wind in the first 2 solar radii is high.

Fractionation of minor ions in the solar wind acceleration process

In view of new observational evidence from isotope spectrometers on WIND, SOHO and ACE, we explore the efficiency of fractionation processes in the inner corona. We reinvestigate the role of Coulomb collisions in the acceleration of minor ions using a multi-fluid model. To model the main gas we study stationary solutions for the continuity and momentum equations of electrons, protons, and alpha particles. For simplicity, the energy equations are replaced by a prescribed relation between temperature and electron density. For large particle fluxes, the Coulomb friction equalizes the velocities of all species, and no fractionation takes place. For reduced fluxes, the alpha particles lag behind the protons and accumulate in the corona, leading to a depletion of helium in the solar wind. Minor ions can be filled as test particles into the main gas which follows a given profile. The behaviour of these particles depends in a complicated manner, both on their mass and on their charge, structured by the interplay of acceleration, gravitation, electromagnetic fields, Coulomb drag, and thermal diffusion.

Gamma Rays from Solar Flares

AbstractWe review recent results obtained from the analysis of the solar flare gamma ray line emission: (a)The gamma ray derived ambient elemental abundances show that the First Ionization Potential (FIP) effect already sets in at relatively low altitudes in the solar atmosphere.(b)The composition of the flare accelerated particles that produce the gamma rays exhibit heavy element and 3He abundance enhancements that are typical for impulsive flares. Unlike the solar energetic particle (SEP) observations in interplanetary space, the gamma ray method allows us to trace the time development of these enhancements.(c)Solar flare gamma ray spectroscopy provides the most direct measure of the abundances of the two very high FIP elements, He and Ne, in subcoronal regions leading to somewhat higher abundances than the generally accepted values.(d)The high intensities of the aa lines observed from a number of flares imply a high (≳ 0.1) ambient He/H and/or accelerated α/p.(e)There are indications for the isotopic fractionation of He from the photosphere to corona that has important implications on the mechanism of solar wind acceleration, the protosolar deuterium abundance and Galactic chemical evolution.

Ultraviolet Coronagraph Spectrometer Observations of Density Fluctuations in the Solar Wind

Recent Ultraviolet Coronagraph Spectrometer (UVCS) white-light channel (WLC) observations on board the Solar and Heliospheric Observatory (SOHO) indicate quasi-periodic variations in the polarized brightness (pB) in the polar coronal holes. This is the first observation of possible signatures of compressional waves high above the limb (at heliocentric distances in the range 1.9-2.45 R☉). The Fourier power spectrum of the pB time series at 1.9 R☉ shows significant peak at about 6 minutes and possible fluctuations on longer timescales (20-50 minutes). The observation at 1.9 R☉ is the only currently available WLC data set with sufficient cadence to resolve the 6 minute period. These preliminary observations may result from density fluctuations caused by compressional waves propagating in polar coronal holes. We stress that our results are preliminary, and we plan future high-cadence observations in both plume and interplume regions of coronal holes. Recently, Ofman & Davila used a 2.5 D MHD model and found that Alfven waves with an amplitude of 20-70 km s-1 at the base of the coronal hole can generate nonlinear, high-amplitude compressional waves that can contribute significantly to the acceleration of the fast solar wind. The nonlinear solitary-like waves appear as fluctuations in the density and the radial outflow velocity and contribute significantly to solar wind acceleration in open magnetic field structures. The motivation for the reported observations is the MHD model prediction.

The Lyman‐α line from the solar wind acceleration region: Results from an eight‐moment approximation two‐fluid solar wind model

We investigate neutral hydrogen in an eight‐moment, two‐fluid, high speed solar wind model. In this model the proton heat flux decreases rapidly in the solar wind acceleration region, and extended coronal heating of the proton gas leads to high coronal proton temperatures and large solar wind flow speeds. The properties of the Lyman‐α line from the neutral hydrogen in two such models are studied. We show that the temperature maximum in the corona inferred from the observed Lyman‐α line may be more than 20% lower than the proton temperature maximum.

SWICS/Ulysses observations: The three‐dimensional structure of the heliosphere in the declining/minimum phase of the solar cycle

The ESA/NASA spacecraft Ulysses provides the first in‐situ observation of the solar wind at high solar latitudes. Data obtained with the Solar Wind Ion Composition Spectrometer (SWICS) during the first orbit of Ulysses around the Sun can be used to obtain a global view of the solar wind pattern within the heliosphere during the declining/minimum phase of the solar activity cycle. This provides information on the solar corona, specifically on any latitudinal variation of coronal parameters as far as they play a role in the solar wind acceleration process. The measurements show: The 3‐dim. distribution of the solar wind appears as a pattern of basically two states. The fast solar wind with speeds of about 750 km/s emanating from the polar coronal holes dominates the heliosphere. The slow solar wind from the streamer belts is restricted to a region of about +/−20° around the heliographic equator. The boundary between the two states of the solar wind is sharply defined. The coronal hole streams are remarkably stable and homogeneous on a larger scale. The two independent coronal holes are similar in terms of average speed and spatial extent. In both coronal holes a gradual increase (≤10%) of speed with solar latitude is observed.

An estimate of large‐scale solar wind density and velocity profiles in a coronal hole and the coronal streamer belt

With the use of the total electron content data obtained by the Ulysses Solar Corona Experiment (SCE) during the first solar conjunction in summer 1991, two data sets were selected, one associated with a coronal hole and the other associated with coronal streamer belt crossings. To determine a large‐scale coronal streamer belt density profile, the electron content of the tracking passes embedded in the coronal streamer belt were corrected for the contributions from coronal hole densities. The inferred large‐scale streamer belt electron density profile has a radial falloff exponent of −2.4 for distances greater than 7 Rs implying the acceleration of the slow solar wind according to υ(r) ∼ r0.4, in qualitative agreement with SOHO results. The acceleration terminates beyond 60 Rs in agreement with Helios in situ observations. All radial electron density profiles inferred from coronal radio sounding observations, particularly during times of high solar activity, are dominated by coronal streamer contributions. They are applicable to coronal streamers, confined to a limited latitude range about the heliospheric current sheet, and they are not representative of a large‐scale mean coronal electron density profile. Because of a lack of data, similar analysis of the coronal hole electron content data was not unequivocally feasible. The coronal hole tracking passes corrected for contributions from coronal streamer areas display large electron content and density fluctuations inconsisting with the plume interpretation by Woo [1996]. Assuming that the lowest densities represent typical hole densities and comparing these with streamer densities at the same distance, we found the streamer‐to‐hole density ratio to be a factor of 10, which agrees with white light coronagraph results.

Plasma Heating by Alfven Wave Filamentation: A Relevant Mechanism in the Solar Corona and the Interstellar Medium

A mechanism for small-scale generation in magnetically dominated, homogeneous space plasmas is proposed, resulting from transverse collapse (filamentation) of weakly nonlinear, circularly polarized Alfven waves. It is argued that waves whose frequencies exceed a magnitude of about 10 Hz can significantly contribute to the heating of open coronal holes and to the acceleration of the fast solar wind. In coronal loops, this frequency lower bound reduces to 1 Hz. The importance of Alfven wave filamentation is also demonstrated in the warm ionized phase of the interstellar medium.

Effects of High Ion Temperatures on Spectral Line Diagnostics in the Source Region of the High‐Speed Solar Wind

Theoretical work on solar wind acceleration has suggested that the proton and electron temperatures at the base of the solar coronal hole region are not thermally equilibrated but that instead the proton temperature may exceed the electron temperature significantly. Recently, Kohl, Strachan, & Gardner have measured both broad and narrow components of the H Lyα profile and have suggested that the broad component may be the signature of high proton temperatures, ~4-6 × 106 K or more. Since proton impact excitation can contribute to the emission line excitation processes, high proton temperatures have important implications for some spectral line emissivities that are useful for electron temperature and density determinations. The diagnostics most affected are those which are sensitive to the distribution of population within the fine structure of the ground state configuration. We discuss selected case studies. For modest proton temperatures (Tp = 3Te) we have found effects on temperature diagnostics to be less than 30% but effects on some density diagnostics may be as large as 1 order of magnitude.

Solar Wind Acceleration by Solitary Waves in Coronal Holes

Coronal holes are well-known sources of the high-speed solar wind; however, the exact acceleration mechanism of the fast wind is still unknown. We solve numerically the time-dependent, nonlinear, resistive 2.5-dimensional MHD equations and find that solitary waves are generated in coronal holes nonlinearly by torsional Alfven waves. The solitary wave phase velocity was found to be slightly above the sound speed in the coronal hole; for example, with the driving Alfven wave amplitude vd ≈ 36 km s-1 and plasma β = 5%, the solitary wave phase speed is ~185 km s-1. We show with a more simplified analytical model of the coronal hole that sound waves are generated nonlinearly by Alfven waves. We find numerically that these waves steepen nonlinearly into solitary waves. In addition, ohmic heating takes place in the coronal hole inhomogeneities owing to phase-mixing of the torsional Alfven waves. When solitary waves are present, the solar wind speed and density fluctuate considerably on timescales of ~20-40 minutes in addition to the Alfvenic fluctuations. The solitary wave-driven wind might be in better qualitative agreement with observations than the thermally driven and WKB Alfven wave solar wind models.

Do First Results from [ITAL]SOHO[/ITAL] UVCS Indicate that the Solar Wind is Accelerated by Solitary Waves?

The Ultraviolet Coronagraph Spectrometer (UVCS) on board the recently launched US-European Solar and Heliospheric Observatory (SOHO) satellite has found O VI and H I emission lines with a broad component that corresponds to ~300 km s-1 unresolved motions at about 0.7 solar radii above the photosphere. These motions appear to be independent of ion mass. We suggest that the large Doppler broadening of the ion emission lines observed by the UVCS are signatures of solitary waves in the solar wind plasma. According to our recent 2.5-dimensional (i.e., three-dimensional with azimuthal symmetry) MHD simulations, these waves may contribute significantly to the solar wind acceleration and may generate velocity fluctuations with a magnitude that agrees with the above observations.

Generalized sonic points and acceleration of multi-ion solar wind

The acceleration of the solar wind plasma near the Sun is governed by the critical (sonic point) structure of the appropriate fluid equations. For a multi-ion solar wind plasma with the components coupled by electric field and other interactions the generalized sonic points cannot be associated with any of the components alone. In this contribution the critical solution describing the passage of two-ion plasma from subsonic to supersonic region is obtained assuming sharply peaked ion temperature profiles, of interest in view of recent SOHO data. The topology of the generalized critical points follows as a by-product of the method.

MHD Waves in Energy Balance of the Solar Wind and the Solar Corona

Estimations are presented concerning the contribution of MHD waves propagating from the low boundary of chromosphere-corona transition layer in the energy balance of magnetically open outer solar atmosphere. The corresponding energy-momentum sources are shown to be sufficient for coronal heating and solar wind acceleration. Selfconsistent model of wave driven solar corona and solar wind is constructed, in the frame of which the locations of coronal temperature maximum and solar wind transonic region coincide with the regions of effective wave damping. Wave energy flux into the corona is controlled by the value of magnetic field induction at the coronal base, and the wave energy distribution into the main channels of losses is determined by the magnetic field structure in the inner and middle corona.

Solar wind and chromospheric network

A physical model of the transition region, including upflow of the plasma in magnetic field funnels that are open to the overlying corona, is presented. A numerical study of the effects of Alfven waves on the heating and acceleration of the nascent solar wind originating in the chromospheric network is carried out within the framework of a two-fluid model for the plasma. It is shown that waves with reasonable amplitudes can, through their pressure gradient together with the thermal pressure gradient, cause a substantial initial acceleration of the wind (on scales of a few Mm) to locally supersonic flows in the rapidly expanding magnetic field ‘trunks’ of the transition region network. The concurrent proton heating is due to the energy supplied by cyclotron damping of the high-frequency Alfven waves, which are assumed to be created through small-scale magnetic activity. The wave energy flux of the model is given as a condition at the upper chromosphere boundary, located above the thin layer where the first ionization of hydrogen takes place. Among the new numerical results are the following: Alfven waves with an assumed f -1 power spectrum in the frequency range from 1 to 4 Hz, and with an integrated mean amplitude ranging between 25 and 75 km s4, can produce very fast acceleration and also heating through wave dissipation. This can heat the lower corona to a temperature of 5× 105 K at a height of h=12,000 km, starting from 5× 104 K at h=3000 km. The resulting thermal and wave pressure gradients can accelerate the wind to speeds of up to 150 km s-1 at h=12,000 km, starting from 20 km s-1 at h=3000 km in a rapidly diverging flux tube. Thus the nascent solar wind becomes supersonic at heights well below the classical Parker-Type sonic point. This is a consequence of the fact that any large wave-energy flux, if it is to be conducted through the expanding funnel to the corona, implies the building-up of an associated wave-pressure gradient. Because of the diverging field geometry, this might lead to a strong initial acceleration of the flow. There is a multiplicity of solutions, depending mainly on the coronal pressure. Here we discuss two new (as compared with a static transition region model) possibilities, namely that either the flow remains supersonic or slows down abruptly by shock formation, which then yields substantial coronal heating up to the canonical 106 K for the proton temperature.

EISCAT measurements of the solar wind: Measurements of the fast and slow streams

EISCAT observations of interplanetary scintillation have been used to measure the velocity of the solar wind at distances between 15 and 130 solar radii (R). The results show that away from transients and interaction regions the solar wind consists of two distinct components - a fast stream with a velocity of ∼700–800 km/s and a slow stream at ∼300–400 km/s. Both the fast and slow streams finish their acceleration much closer to the Sun than expected, with the acceleration much closer to the Sun than expected, with the acceleration of the fast high latitude solar wind being effectively complete inside 10 R. The flow vector of the solar wind is normally purely radial to the Sun, although one set of observations suggests a small but significant deviation from radial flow.

Limits of the efficiency of isotope fractionation processes in the solar wind derived from the magnesium isotopic composition as observed with the WIND/MASS experiment

From geochemical evidence it is assumed that the isotopic composition of solar magnesium agrees within fractions of a per mill with terrestrial magnesium. This provides a unique opportunity to derive upper limits on the efficiency of isotope fractionation processes in the solar wind. We analyze results from the first year of operation of the WIND/MASS instrument, and we find that the isotopic composition of magnesium in the solar wind agrees within the experimental uncertainty of a few percent with the terrestrial composition. We observe no significant variations between different solar wind regimes. Our results are discussed in the context of recent theoretical models of solar wind feeding and acceleration.

SOHO, its day in the Sun

SOHO, the Solar and Heliospheric Observatory, is a project of international cooperation between ESA and NASA to study the Sun, from its deep core to the outer corona, and the solar wind. Three helioseismology instruments are providing unique data for the study of the structure and dynamics of the solar interior, from the very deep core to the outermost layers of the convection zone. A set of five complementary remote sensing instruments, consisting of EUV, UV and visible light imagers, spectrographs and coronagraphs, give us our first comprehensive view of the outer solar atmosphere and corona, leading to a better understanding of the enigmatic coronal heating and solar wind acceleration processes. Finally, three experiments complement the remote sensing observations by making in-situ measurements of the composition and energy of the solar wind and charged energetic particles, and another instrument maps the neutral hydrogen in the heliosphere, and its dynamic change of the Solar Wind. This paper highlights some of the first results from SOHO.

Radio Maps of the Solar Wind Transition Region

Occultation studies of near-Sun plasmas using several natural sources simultaneously result in large-scale patterns, radio maps of the solar wind flow. Large radio telescopes of the P.N. Lebedev Physical Institute, Pushino, were used. Previously it had been shown that the plasma acceleration and traverse of the sound barrier proceed in an extended region, the transition region of the solar wind, located at radial distances of about 10–40 solar radii from the Sun. The 1989–1994 experiments showed that the evolution of the transition region geometry is very close to that of the optical corona. On the other hand, the plasma flow structures characteristic of the transition region persist in the course of the 11-year cycle, which demonstrates the existence of some specific mechanism of the solar wind acceleration, independent of wide variations of the general solar activity state. These experimental facts are discussed in connection with the existing theoretical approaches.