Wind Mass Flux Research Articles

Ulysses solar wind plasma observations at high latitudes

Ulysses reached its peak northerly heliolatitude of 80.2°N on July 31st, 1995, and is now moving towards aphelion at 5.41 AU which it will reach in May, 1998. We summarize measurements from the solar wind plasma experiment, SWOOPS, emphasizing northern hemispheric observations but also providing southern and equatorial results for comparison. The solar wind momentum flux during Ulysses' fast pole-to-pole transit at solar minimum was significantly higher over the poles than at near-equatorial latitudes, suggesting a non-circular cross section for the heliosphere. Furthermore, modest asymmetries in wind speed, density, and mass flux were observed between the two hemispheres during the fast latitude scan. The solar wind was faster and less dense in the north than in the south. These asymmetries persist in the most recent high- and mid-latitude data but are less pronounced. As of July, 1996 the northern fast solar wind has lacked any strong stream interactions or shocks and, although a comprehensive search has not yet been made, no CMEs have yet been identified during this interval. On the other hand, Alfvénic, compressional, and pressure-balanced features are abundant at high latitudes. The most recent data, at 4 AU and 32°N, has begun to show the effects of solar rotation modulated features in the form of recurrent compressed regions.

The problem of solar wind mass flux regulation

The problem of solar wind mass flux regulation is considered in the frame of the solar wind and corona model with selfconsistent wave energy - momentum sources. It is shown that the mass flux regulation is caused by the strong interaction between the solar wind and the solar corona. Depending on the regime of solar wind formation the limitation of the solar wind mass flux takes place either in the region near the transonic point or in the inner corona.

First Results from Swan Lyman α Solar Wind Mapper on SOHO

After one year of almost flawless operation on board the SOHO spacecraft poised at L1 Lagrange point, we report the main features of SWAN observations. SWAN is mainly dedicated to the monitoring of the latitude distribution of the solar wind by the La method. Maps of sky Lα emissions were recorded througout the year. The region of maximum emission, located in the upwind hemisphere, deviates strongly from the pattern that could be expected from a solar wind constant with latitude. It is divided into two lobes by a depression aligned with the solar equatorial plane called the Lyα groove already noted in 1976 Prognoz data. The north lobe is much brighter than the south lobe. These two characteristics can be explained qualitatively by an enhanced ionization along the neutral sheet where the slow solar wind is concentrated, which results from the higher low-latitude solar wind mass flux as measured by Ulysses. The groove is the direct imprint on the sky of the enhanced carving by the slow solar wind, at this time of solar minimum, when the tilt angle of the neutral sheet is small. The question is still pending to predict what will happen with the ascending phase of the solar cycle. Observations of comets are briefly mentioned, with the ability of SWAN to monitor the H2O production of many comets. Operations of the instrument are briefly described, including some instrumental problems which could be solved by software modifications sent to the instrument.

Observations of a sky Lyman α groove related to enhanced solar wind mass flux in the neutral sheet

We report several observations of the Lα interplanetary emission recorded by a photometer flown in 1977 on board the soviet spacecraft Prognoz‐5. The Prognoz scans sampled emission in a plane perpendicular to the sun‐spacecraft line and also traversed the Lα maximum emission region (MER), a region centered a few AU from the Sun on the upstream interstellar‐flow axis. These scans reveal the existence of a 10% decrease in intensity centered near the ecliptic plane and about 30° wide. All the dips form a new feature of the interplanetary emission, a “groove” aligned approximately with the ecliptic plane. This groove is present only near the upwind direction, and is interpreted as the result of enhanced ionisation of interstellar H by charge‐exchange with the solar wind in a sheet of approximately 30° width in latitude around the average position of the neutral sheet, at this time of solar minimum.

Inhibition of Wind-Compressed Disk Formation by Nonradial Line Forces in Rotating Hot-Star Winds

We investigate the effects of nonradial line forces on the formation of a wind-compressed (WCD) around a rapidly rotating B star. Such nonradial forces can arise both from asymmetries in the line resonances in the rotating wind and from rotational distortion of the stellar surface. They characteristically include a latitudinal force component directed away from the equator and an azimuthal force component acting against the sense of rotation. Here we present results from radiation-hydrodynamical simulations showing that these nonradial forces can lead to an effective suppression of the equatorward flow needed to form a WCD as well as a modest (~20%) spin-down of the wind rotation. Furthermore, contrary to previous expectations that the wind mass flux should be enhanced by the reduced effective gravity near the equator, we show here that gravity darkening effects can actually lead to a reduced mass loss, and thus lower density, in the wind from the equatorial region. Overall, the results here thus imply a flow configuration that is markedly different from that derived in previous models of winds from rotating early-type stars. In particular, a major conclusion is that equatorial wind compression effects should be effectively suppressed in any radiatively driven stellar wind for which, as in the usual CAK formalism, the driving includes a significant component from optically thick lines. This presents a serious challenge to the WCD paradigm as an explanation for disk formation around Be and other rapidly rotating hot stars thought to have CAK-type, line-driven winds.

CAUSES OF THE FETCH EFFECT IN WIND EROSION

The increase of soil mass flux with distance downwind, the fetch effect for wind erosion, has been observed and reported on since 1939. This model incorporates the following three mechanisms. (1) The ‘avalanching’ mechanism in which one particle moving downwind would dislodge one or more particles upon impact with the surface. The result of a chain of such events is an increase of mass flux with distance. (2) The ‘aerodynamic feedback’ effect, suggested by P. R. Owen, in which the aerodynamic roughness height is increased by saltation of particles; the resulting increased momentum flux increases saltation. These increases define a positive feedback loop with respect to distance downwind. (3) The ‘soil resistance’ mechanism, which is largely an expression of the change with distance of threshold velocity. Change of threshold velocities may be caused by inhomogeneities of the soil or progressive destruction of aggregates and crust in the direction of saltation fetch. An experiment was run in March 1993 at Owens Lake to test this model. Detailed measurements of wind profiles and mass fluxes were taken on a line parallel to the wind direction. These data support the proposed three-mechanism model.

Correlated Variations in the Solar Neutrino Flux and the Solar Wind and the Relation to the Solar Neutrino Problem

Solar wind parameters from the Massachusetts Institute of Technology (MIT) plasma experiment on the IMP 8 spacecraft overlap ∼19 years of published neutrino flux observations from the Homestake experiment. A strong correlation is found between neutrino flux and solar wind properties, in particular, the solar wind mass flux. The correlation is significantly better than any anticorrelation with sunspot number and is comparable to those previously found with photospheric magnetic flux and shifts in p -mode frequencies. If current notions of solar structure are correct, these observations require new fundamental physics of neutrinos. For a proper choice of neutrino parameters, the level of variations is consistent with resonant conversion of electron neutrinos to a nondetected flavor eigenstate mediated by the magnetic field in the sun's convective zone. The solar wind mass flux may act as a proxy for this field, producing the solar wind-neutrino flux connection.

SWAN: A study of Solar Wind Anisotropies on SOHO with Lyman alpha sky mapping

On board the SOHO spacecraft poised at L1 Lagrange point, the SWAN instrument is mainly devoted to the measurement of large scale structures of the solar wind, and in particular the distribution with heliographic latitude of the solar wind mass flux. This is obtained from an intensity map of the sky Lymanα emission, which reflects the shape of the ionization cavity carved in the flow of interstellar H atoms by the solar wind. The methodology, inversion procedure and related complications are described. The subject of latitude variation of the solar wind is shortly reviewed: earlier Lymanα results from Prognoz in 1976 are confirmed by Ulysses. The importance of the actual value of the solar wind mass flux for the equation of dynamics in a polar coronal hole is stressed. The instrument is composed of one electronic unit commanding two identical Sensor Units, each of them allowing to map a full hemisphere with a resolution of 1°, thanks to a two-mirrors periscope system. The design is described in some details, and the rationale for choice between several variants are discussed. A hydrogen absorption cell is used to measure the shape of the interplanetary Lymanα line and other Lyman α emissions. Other types of observations are also discussed : the geocorona, comets (old and new), the solar corona, and a possible signature of the heliopause. The connexion with some other SOHO instruments, in particular LASCO, UVCS, SUMER, is briefly discussed.

Coronal heating, densities, and temperatures and solar wind acceleration

The outflow of coronal plasma into interplanetary space is a consequence of the coronal heating process. Therefore the formation of the corona and the acceleration of the solar wind should be treated as a single problem. The deposition of energy into the corona through some “mechanical” energy flux is balanced by the various energy sinks available to the corona, and the sum of these processes determines the coronal structure, i.e., its temperature and density. The corona loses energy through heat conduction into the transition region and through the gravitational potential energy and kinetic energy put into the solar wind. We show from a series of models of the chromosphere‐transition region‐corona‐solar wind system that most of the energy deposited in a magnetically open region goes into the solar wind. The transition region pressures and the coronal density and temperature structure may vary considerably with the mode and location of energy deposition, but the solar wind mass flux is relatively insensitive to these variations; it is determined by the amplitude of the energy flux. In these models the transition region pressure decreases in accordance with the increasing coronal density scale height such that the solar wind mass loss is consistent with the energy flux deposited in the corona. On the basis of the present study we can conclude that the exponential increase of solar wind mass flux with coronal temperature, found in most thermally driven solar wind models, is a consequence of fixing the transition region pressure.

On two‐component models of solar wind fluctuations

Matthaeus et al. first suggested a two‐component scenario for solar wind fluctuations which contains both parallel propagating Alfvén waves together with a population of quasi‐two‐dimensional turbulence. On the basis of these earlier ideas and further evidence, Tu and Marsch developed a two‐component model, in which small‐scale incompressible fluctuations are decomposed into Alfvén waves and convective structures, in an attempt to account for observed radial evolutions of energy density, normalized cross helicity σc, and Alfvén ratio rA of solar wind fluctuations. According to a recent analysis on two‐dimensional Alfvénic disturbances in the equatorial solar wind with a spiral magnetic field, we discuss the theoretical basis of this important two‐component concept and further indicate necessary ingredients that a more self‐consistent two‐component turbulence model should contain. From the perspective of Alfvénic disturbances in the solar wind, the wave and convective components coexist in a naturally intermixed manner. Our analysis specifically describes behaviors of Alfvénic disturbances of all scales, including the intermediate ones. In a fully developed spiral magnetic field at large radii, the criterion for separating radially propagating Alfvén waves and radially convective Alfvénic structures depends on whether |ƒ + mƒS| is greater or less than the characteristic frequency ƒc ≡ (FMU³/FB²)1/2, where ƒ is the perturbation frequency, ƒS is the solar rotation frequency, m is an integer for the azimuthal wavenumber, FM is the solar wind mass flux, FB is the radial magnetic flux, and U is the asymptotic wind speed. For typical solar wind parameters, the timescale corresponding to ƒc is ≳ 15 hours.

Mass Loss from Rotating Hot-stars: Inhibition of Wind Compressed Disks by Nonradial Line-forces

We review the dynamics of radiatively driven mass loss from rapidly rotating hot-stars. We first summarize the angular momentum conservation process that leads to formation of a Wind Compressed Disk(WCD) when material from a rapidly rotating star is driven gradually outward in the radial direction. We next describe how stellar oblateness and asymmetries in the Sobolev line-resonance generally leads to nonradialcomponents of the driving force is a line-driven wind, including an azimuthal spin-down force acting against the sense of the wind rotation, and a latitudinal force away from the equator. We summarize results from radiation-hydrodynamical simulations showing that these nonradial forces can lead to an effective suppressionof the equatorward flow needed to form a WCD, as well as a modest (∼ 25%) spin-downof the wind rotation. Furthermore, contrary to previous expectations that the wind mass flux should be enhanced by the reduced effective gravity near the equator, we show here that gravity darkening effects can actually lead to a reducedmass loss, and thus lower density, in the wind from the equatorial region. Finally, we examine the equatorial bistability model, and show that a sufficiently strong jump in wind driving parameters can, in principle, overcome the effect of reduced radiative driving flux, thus still allowing moderate enhancements in density in an equatorial, bistability zone wind.

Mass flux in the ecliptic plane and near the Sun deduced from Doppler scintillation

During the late declining phase of the solar cycle, the tilt of the solar magnetic dipole with respect to the Sun's rotation axis leads to large‐scale organization of the solar wind, such that alternating regions of high‐ and low‐speed solar wind are observed in the ecliptic plane. In this paper, we use Doppler scintillation measurements to investigate mass flux of these two types of solar wind in the ecliptic plane and inside 0.3 AU, where in situ measurements have not been possible. To the extent that Doppler scintillation reflects mass flux, we find that mass flux in high‐speed streams: (1) is lower (by a factor of approximately 2.2) than the mass flux of the average solar wind in the heliocentric distance range of 0.3–0.5 AU, (2) is lower still (by as much as a factor of about 4) than the mass flux of the slow solar wind associated with the streamer belt, and (3) appears to grow with heliocentric distance. These Doppler scintillation results are consistent with the equator to pole decrease in mass flux observed in earlier spectral broadening measurements, and with trends and differences between high‐ and low‐speed solar wind observed by in situ measurements in the range of 0.3–1.0 AU. The mass flux results suggest that the solar wind flow in high‐speed streams is convergent towards the ecliptic near the Sun, becoming less convergent and approaching radial with increasing heliocentric distance beyond 0.3 AU. The variability of mass flux observed within equatorial and polar high‐speed streams close to the Sun is strikingly low. This low variability implies that, as Ulysses currently ascends to higher latitudes and spends more time in the south polar high‐speed stream after crossing the heliospheric current sheet, it can expect to observe a marked decrease in variations of both mass flux and solar wind speed, a trend that appears to have started already.

Solar wind mass and momentum flux variations at 0.3 AU

In the past we have used electron Thomson scattering brightness observations, obtained with the zodiacal-light photometers on board the spacecraft Helios 1 and Helios 2, to study the global density structure of the quiet corona and inner heliosphere (greater than 17 solar radii). This was done by means of a comparison of synoptic maps based on these Thomson scattering observations and synoptic maps based on other solar/heliospheric data, such as IPS velocity, K-coronameter brightness and magnetic source surface data. In this paper we continue this approach by combining the Helios Thomson scattering maps (which provide density information) with IPS solar wind velocity maps to map out variations in mass and momentum flux of the solar wind as a function of latitude and phase of the solar cycle. The method used to construct the Helios and IPS synoptic maps emphasizes the global, persistent (as opposed to transient) structures, and thus can be viewed as approximating conditions in the quiet corona and inner heliosphere.

The solar cycle variation of coronal mass ejections and the solar wind mass flux

Coronal mass ejections (CMEs) are an important aspect of coronal physics and a potentially significant contributor to perturbations of the solar wind, such as its mass flux. Sufficient data on CMEs are now available to permit study of their longer‐term occurrence patterns. Here we present the results of a study of CME occurrence rates over more than a complete 11‐year solar sunspot cycle and a comparison of these rates with those of other activity related to CMEs and with the solar wind particle flux at 1 AU. The study includes an evaluation of corrections to the CME rates, which include instrument duty cycles, visibility functions, mass detection thresholds, and geometrical considerations. The main results are as follows: (1) The frequency of occurrence of CMEs tends to track the solar activity cycle in both amplitude and phase; (2) the CME rates from different instruments, when corrected for both duty cycles and visibility functions, are reasonably consistent; (3) considering only longer‐term averages, no one class of solar activity is better correlated with CME rate than any other; (4) the ratio of the annualized CME to solar wind mass flux tends to track the solar cycle; and (5) near solar maximum, CMEs can provide a significant fraction (i.e., ≈ 15%) of the average mass flux to the near‐ecliptic solar wind.

Latitudinal variation of speed and mass flux in the acceleration region of the solar wind inferred from spectral broadening measurements

Spectral broadening measurements conducted at S‐band (13‐cm wavelength) during solar minimum conditions in the heliocentric distance range of 3–8 Ro by Mariner 4, Pioneer 10, Mariner 10, Helios 1, Helios 2, and Viking have been combined to reveal a factor of 2.6 reduction in bandwidth from equator to pole. Since spectral broadening bandwidth depends on electron density fluctuation and solar wind speed, and latitudinal variation of the former is available from coherence bandwidth measurements, the remote sensing spectral broadening measurements provide the first determination of the latitudinal variation of solar wind speed in the acceleration region. When combined with electron density measurements deduced from white‐light coronagraphs, this result also leads to the first determination of the latitudinal variation of mass flux in the acceleration region. From equator to pole, solar wind speed increases by a factor of 2.2, while mass flux decreases by a factor of 2.3. These results are consistent with measurements of solar wind speed by multi‐station intensity scintillation measurements, as well as measurements of mass flux inferred from Lyman α observations, both of which pertain to the solar wind beyond 0.5 AU. The spectral broadening observations, therefore, strengthen earlier conclusions about the latitudinal variation of solar wind speed and mass flux, and reinforce current solar coronal models and their implications for solar wind acceleration and solar wind modeling.

On the relation between coronal heating, flux tube divergence, and the solar wind proton flux and flow speed

A one-fluid solar wind model is used to investigate some relations between coronal heating, the flux tube divergence near the Sun, and the solar wind proton flux and flow speed. The effects of energy addition to the supersonic region of the flow are also studied. We allow for a mechanical energy flux that heats the corona, and an Alfven wave energy flux that adds energy, mainly to the supersonic flow, both as momentum and as heat. We find that the mechanical energy flux determines the solar wind mass flux, and in order to keep an almost constant proton flux at the orbit of Earth with changing flow geometry, that the mechanical energy flux must vary linearly with the magnetic field in the inner corona. This thermally driven wind generally has a low asymptotic flow speed. When Alfven waves are added to the thermally driven flow, the asymptotic flow speed is increased and is determined by the ratio of the Alfven wave and the mechanical energy fluxes at the coronal base. Flow speeds characteristic of recurrent high-speed solar wind streams can be obtained only when the Alfven wave energy flux, deposited in the supersonic flow, is larger than the mechanical energy flux heating the corona.

Latitudinal distribution of solar wind as deduced from Lyman alpha measurements: An improved method

In this work we examine the possibility of deducing the latitudinal distribution of the solar ionization rate using Prognoz 6 Lyman alpha data in a more general and flexible way than previously examined. Using so‐called hot model for the hydrogen distribution and the optically thin model for the resonance scattering, theoretical Lyman alpha intensity for the interstellar hydrogen is calculated and compared with the intensity data measured by Prognoz 6. Varying the latitudinal dependence of the ionization rate, the distributions, which produce the best fit with the data, are analyzed for four different measuring sessions. As a result, we get four ionization rate distributions that have two common features. The ionization rate is enhanced near the solar equator, and large broad plateaus exist around heliographic latitudes ±30° to ±70°. The latitudinal distribution of the average ionization rate about the solar minimum deviates clearly from the spherically symmetric and sinusoidally (harmonically) with the latitude‐varying models used so far. The growth of the solar wind mass flux from the solar polar areas toward the equator corresponds to the earlier results concluded from Lyman alpha measurements. The method used in this work allows a higher latitudinal resolution of the ionization rates. However, there are several uncertainties both in the simulations and in the measurements. The exclusion of time‐dependent effects as well as multiple scattering requires that the results be considered only suggestive.

Three‐dimensional steady compressible perturbations in the magnetohydrodynamic solar wind

We present a steady three‐dimensional magnetohydrodynamic (MHD) perturbation formulation for inhomogeneous solar wind stream structures. We derive analytical solutions for asymptotic behaviors of three‐dimensional compressible MHD perturbations at a large radial distance r in a background radial wind of spherical symmetry. There are three independent perturbation solutions at large r. In contrast with a three‐dimensional propagation of Alfvénic perturbations in the same kind of background wind, the magnetic field perturbation associated with a steady compressible MHD perturbation does not dominate the background radial magnetic field at large r. Since the three independent compressible MHD perturbation solutions are related by the two regularity conditions at the MHD slow and fast critical points, the leading asymptotic radial scalings of the relevant compressible MHD perturbation variables are the same as those of the background variables at large r. In other words, while the radial variations of compressible MHD perturbations gradually merge into the background MHD wind at large r, the distinct angular variations of the perturbations persist throughout the wind. To complement these analytical analyses, we obtain complete numerical solutions in the entire radial range, which smoothly pass through the MHD slow and fast critical points. We provide numerical examples in which the perturbation increase (or decrease) of the radial wind speed can be as large as ∼ 100 km s−1 due to the perturbation increase (or decrease) of the temperature above the coronal base while the solar wind mass flux variation remains sufficiently small. This is possible because of our three‐dimensional approach and because of the regulation mechanism of the magnetic field in the wind. We also present an axisymmetric model for steady MHD solar wind structures in the presence of solar differential rotation.

Adiabatic cooling of solar wind electrons

In thermally driven winds emanating from regions in the solar corona with base electron densities of n0 ≥ 108 cm−3, a substantial fraction of the heat conductive flux from the base is transferred into flow energy by the pressure gradient force. The adiabatic cooling of the electrons causes the electron temperature profile to fall off more rapidly than in heat conduction dominated flows. Alfvén waves of solar origin, accelerating the basically thermally driven solar wind, lead to an increased mass flux and enhanced adiabatic cooling. The reduction in electron temperature may be significant also in the subsonic region of the flow and lead to a moderate increase of solar wind mass flux with increasing Alfvén wave amplitude. In the solar wind model presented here the Alfvén wave energy flux per unit mass is larger than that in models where the temperature in the subsonic flow is not reduced by the wave, and consequently the asymptotic flow speed is higher.

The solar wind mass flux

One of the unexplained observations of the solar wind at 1 AU is the relative constancy of the solar wind mass flux. Calculations using a radiative energy balance model with a thermally driven wind suggest that this near constancy of the mass (particle) flux is due to two factors: (1) the role of inward conduction and radiation in cooling the corona, and (2) the finding that the particle flux is proportional to the gas pressure in the outer corona and beyond, where the solar wind flow becomes radial.