Solar Wind Anisotropies Research Articles

Water production of comet C/1999 S4 (LINEAR) observed with the SWAN instrument.

The SWAN (Solar Wind ANisotropies) Lyman-alpha all-sky camera on the SOHO spacecraft observed the hydrogen coma of comet C/1999 S4 (LINEAR) from the end of May through mid-August 2000. A systematic set of water-production rates was obtained for this well-documented event of complete fragmentation of a cometary nucleus. The observations indicate that the lower limit for the sunlit surface area of the nucleus was about 1 square kilometer before the fragmentation and that the amount of water released throughout the observing period was 3.3 x 10(9) kilograms. Evidence suggests that the activity of the comet was dominated by successive fragmentation. There were four major outbursts, occurring about every 16 days. The 21 July event led to the complete fragmentation and sublimation of what remained of the nucleus, producing the last 3 x 10(8) kilograms of water. A model where the fragment size distribution follows the power law N(R) approximately R-(2.7), where N and R are the number and radius of fragments, reproduces the observed dissipation. This distribution possibly reflects the internal structure of the nucleus.

Thermal coupling of protons and neutral hydrogen with anisotropic temperatures in the fast solar wind

The thermal coupling between the neutral hydrogen and protons in the inner corona is explored by extending the study of Allen et al. [1998] to include anisotropic proton temperature to determine what the neutral hydrogen Ly α spectral line measurements reveal about the proton temperature, temperature anisotropy, and outflow velocity in the fast solar wind. The anisotropic proton temperature is produced by ion cyclotron resonant interaction of protons with high‐frequency waves, produced by a nonlinear cascade at the Kolmogorov dissipation rate from dominant lower‐frequency Alfvén waves. As a result of the coupling between the respective parallel and perpendicular components of the neutral hydrogen and proton temperatures, a greater temperature anisotropy in the neutral hydrogen develops as compared to the case when the proton temperature is isotropic. The neutral hydrogen and proton effective temperatures (Teff), incorporating both random and wave motions of the particles, and outflow velocities, are comparable below ∼ 3 Rs. Neutral hydrogen anisotropy ratios, TH(eff)/T‖, ∼ 4 below 3 Rs are readily attained, in agreement with observations. Below ∼ 3 Rs, these reflect the proton anisotropy ratio. For plasma conditions typical of the fast solar wind, these results imply that the measured Ly α spectral line profiles, from which the neutral hydrogen temperature, anisotropy ratio, and outflow velocity are inferred, are equivalent to measurements of protons below ∼ 3 Rs. Beyond this distance the width of the measured Ly α spectral lines provides a lower limit to the proton effective temperature and temperature anisotropy in the inner corona.

Proton temperature anisotropy in the fast solar wind: A 16‐moment bi‐Maxwellian model

A 16‐moment bi‐Maxwellian model to describe the proton temperature anisotropy and heat flux density in the fast solar wind is presented. Although the proton total heat flux density in this model is found to be significantly smaller than the classical Coulomb collision‐dominated Spitzer‐Härm heat flux density, it still plays a significant role in shaping the proton temperature anisotropy. The proton total heat flux density matches the value observed between 0.3 and 1 AU when the proton temperature is also close to the observed value. A large temperature anisotropy in the inner corona, with a proton perpendicular temperature higher than the parallel temperature, as indicated by recent ultraviolet observations, leads to a higher perpendicular heat flux. Since the perpendicular heat flux conducts energy from the perpendicular to the parallel degree of freedom, an unrealistically high parallel temperature develops in interplanetary space if no cooling mechanism for protons in the direction parallel to the magnetic field is introduced. This study suggests that with a bi‐Maxwellian‐based 16‐moment description of the solar wind protons the high proton perpendicular temperature implied by the Ultraviolet Coronagraph Spectrometer on board Solar and Heliospheric Observatory in the inner corona and by in situ measurements from Helios place strong constraints on the proton heating mechanism: not only must the protons be heated in the direction perpendicular to the magnetic field, but they must also be cooled in the parallel direction.

Production de vapeur d'eau dans les comètes par émission lyman α: expérience swan à bord de soho

SWAN (Solar Wind Anisotropies) is a ultraviolet photometer onboard the international satellite SOHO (Solar Heliospheric Observatory). It is able to measure the Lyman α emission from hydrogen atoms of the cometary cloud when the water evaporating from the nucleus is photodissociated by the solar rays. Then one can deduce the vapour production rate against the heliocentric distance using a simple model of the distribu- tion of the atoms in the coma.

Interplanetary Lyman α line profiles derived from SWAN/SOHO hydrogen cell measurements: Full‐sky Velocity Field

We present here an analysis of 1 year of data obtained by the solar wind anisotropies (SWAN) instrument on board the SOHO spacecraft orbiting around the Ll Lagrange point at 1.5 × 106 km sunward from Earth. This instrument is measuring the interplanetary Lyman α background due to solar photons backscattered by hydrogen atoms in the interplanetary medium. The interplanetary (IP) Lyman a line profile reflects the velocity distribution of H atoms projected onto the line of sight (LOS). Here we apply a new profile reconstruction technique using data from the two hydrogen absorption cells included in the SWAN instrument. For a LOS in a fixed celestial direction, the Doppler shift between the interplanetary emission profile and the H cell absorption profile varies by up to ±0.12 Å during 1 year, owing to the Earth's orbital velocity around the Sun, equal to 30 km s−1. Such a Doppler spectral scan across the emission line allows us to derive Lyman α line profiles, and hence the velocity distribution, in and out of the ecliptic independent of any modeling of the neutral hydrogen atom distribution in the heliosphere or of the multiple scattering of solar photons. The spatial distribution of the apparent velocity relative to the Sun as observed from the orbit of SOHO is derived for all directions, except within 5° of the ecliptic poles. This determination strongly constrains models of the interaction of the interstellar hydrogen with the solar wind. New estimates of the upwind direction (252.3° ± 0.73° and 8.7° ± 0.90° in J2000 ecliptic coordinates) show a small discrepancy by 3° – 4° with the direction of the helium flow, perhaps connected with an asymmetry of the heliosphere induced by the interstellar magnetic field. We find that the apparent velocity relative to the sun in the upwind direction is −25.4 ± 1 km/s, whereas it is equal to 21.6 ± 1.3 km s−1 in the downwind direction. A preliminary analysis shows that the Zero Doppler shift cone and the difference between the upwind and downwind velocities correspond to a ratio μ of Lyman α radiation pressure to solar gravity of 0.9–1.0. It follows that the observed upwind apparent velocity is compatible with a velocity at infinity of H atoms of the order of 21–22 km s−1. However, extensive modeling is required in order to get more definite conclusions. The velocity map presented here is the first ever obtained. For this reason, we discuss in detail the Doppler spectral scan method and the H cell data.

A 16-Moment Fast Solar Wind Model

A 16-moment fast solar wind model based on a bi-Maxwellian velocity distribution is presented. Although the proton heat flux density is found to be significantly smaller than the classical Spitzer-Harm value, it still plays a significant role in shaping the proton temperature anisotropy in the fast solar wind.

Interplanetary Lyman α remote sensing with the Ulysses Interstellar Neutral Gas Experiment

The Ulysses neutral gas instrument obtained celestial sphere maps of interplanetary Lyman α emission from neutral hydrogen in 1991–1996. These maps are unique because the spacecraft was located at a wide range of heliocentric ecliptic latitudes. Eleven of these maps are compared with the predictions of an interstellar wind hydrogen model previously used to study Galileo and Pioneer Venus Lyman α data obtained near the ecliptic plane. The model provides reasonable agreement with the Ulysses maps. The lifetime of the interstellar hydrogen atoms against charge exchange with solar wind protons is shown to be more latitudinally isotropic at solar maximum in 1991 than at solar minimum in 1994 and 1995. This result agrees with white light coronagraph data that generally show a more isotropic coronal brightness pattern at solar maximum. The transition from a more symmetric to a more asymmetric solar wind charge exchange rate also explains differences between published Galileo data from solar maximum and Solar and Heliospheric Observatory (SOHO) Solar Wind Anisotropy Experiment (SWAN) data from solar minimum.

Preliminary retrieval of solar wind latitude distribution from Solar Wind Anisotropies/SOHO observations

The Solar Wind Anisotropies (SWAN) instrument on board the SOHO spacecraft measures Lyman alpha radiation emanating mainly from neutral hydrogen gas in the solar neighborhood. This gas is part of the local interstellar cloud in which the Sun and the heliosphere are immersed. Measurements of Lyman alpha can be used to infer the local cloud characteristics like the velocity and the direction of the flow, gas temperature, and density. The strong interaction between the Sun and the neutral hydrogen gas also makes possible investigations of solar characteristics by Lyman alpha measurements. In this work we will concentrate on deriving the latitudinal distribution of solar‐induced ionization from SWAN Lyman alpha maps measured in 1996 at a time of the solar minimum. From the ionization we derive the distribution of the solar wind mass flux. SWAN Lyman alpha data show that the ionization and the mass flux are nearly flat for all solar latitudes except the narrow belt from −20° to 20° around the solar equator. In this region, ionization and the solar wind mass flux show a definite increase, which can be seen as an intensity depression in the Lyman alpha data from directions near the ecliptic. These results confirm earlier in situ measurements by Ulysses during the present minimum and Lyman alpha measurements by Prognoz satellites 20 years ago.

Solar wind decrease at high heliographic latitudes detected from Prognoz interplanetary lyman alpha mapping

New evidence for a latitudinal decrease of the solar wind mass flux is presented from observations of the interplanetary Lyman alpha emission collected in 1976 and 1977 with satellites Prognoz 5 and 6. The flow of interstellar hydrogen atoms in the solar system is ionized by EUV solar radiation and charge exchange with solar wind protons which accounts for about 80% of the total ionization rate. The resulting gradual decrease of the neutral H density from the upwind region down to the downwind region observed from Ly α intensity measurements allowed the determination of the absolute value of the total ionization rate β for one H atom at 1 AU against ionization. Collected in 1976 and 1977 at five places in the solar system, the measurements are first compared to a model which assumes isotropy of the EUV and solar wind. Strong departures are obvious toward high‐latitude regions, especially when the observer is in the downwind region where the solar wind ionization has had more time to act (cumulative effect). A model was constructed which included a decrease of the ionization rate with heliographic latitude. The adjustment of data allowed for the measurement of the absolute value of the total ionization rate and implies a 50% latitude decrease of the ionization rate due to charge exchange with the solar wind, from βsw = (3.9±0.5) × 10−8; s−1 at the equator to βsw = (2.0±0.5) × 10−8 s−1 at the pole. The corresponding absolute value of the solar wind proton flux is (2.4‐3.6) × 108 cm−2 s−1 at the equator and twice less at the pole if a constant velocity is assumed for the solar wind. Even if the solar wind velocity increases from 400 to 800 km s−1, which would decrease the charge exchange cross section by 25%, there is still a decrease by about 30% of the solar wind mass flux from equator to pole. The Lyman alpha data from Mariner 10 (Kumar and Broadfoot, 1978) had already shown a similar trend in 1974, showing a persistence of the solar wind anisotropy for 3–4 years during a solar minimum. The large‐scale properties of the solar wind mass flux can therefore be monitored at all latitudes by remote sensing of Ly α interplanetary emission, since the solar wind is carving the flow of interstellar H and its anisotropies are ''printed'' on the interplanetary H distribution. With uncalibrated Ly α measurements, an absolute value of the solar wind flux can be determined at all latitudes averaged over a typical 1‐year period.

Energy and momentum exchange in transverse plasma waves

We calculate, by a perturbation analysis, the energy and momentum changes both of a single particle and of a distribution of particles moving in a transverse electromagnetic wave propagating parallel to the direction of the average magnetic field. It is shown that energy and momentum conservation of the particles and of the electromagnetic field leads to the same dispersion relation as has been obtained by Stix (1962) from solution of the Vlasov equation. More importantly, we are able to discuss the transfer of energy and momentum between particles and fields, in the context of stability or instability of the plasma. We emphasize that it is generally not sufficient to attempt to deduce the stability properties of a plasma from considerations of the energy gained or lost by the resonant particles alone. In general, one must also consider the additional constraint of momentum conservation and the effects of the nonresonant particles; this conclusion persists even in the limit of zero growth rate, when the resonant particles would be expected to play the dominant role. Two illustrations are provided. The real part of the frequency of the fire-hose instability is shown to be determined by momentum conservation, whereas the growth rate follows from energy conservation. For the new proton-resonant modes (Hollweg and Volk, 1970b), on the other hand, we find that the growth rate follows from momentum conservation, whereas energy conservation yields the real part of the frequency. We show that the earlier classification of the proton-resonant modes in accord with the sign of (Ap±ωr/Ωp) is, in fact, a classification assigned in accord with the gain or loss of energy by the protons. The small-wavelength extension of the fire hose leads to absorption of energy by the protons, and this mode could simultaneously cool the electrons and heat the protons in the solar wind near 1 AU; the other two proton-resonant modes lead to loss of energy by the protons and would aggravate the discrepancy between the two-fluid solar-wind model and observation. Finally, it is shown that the small-wavelength extension of the fire hose tends to reduce solar-wind proton thermal anisotropies to the observed values only for frequencies less than about 4Ωp, whereas the other proton-resonant instabilities lead in all instances to reduction of the anisotropy.