Solar Wind Proton Flux Research Articles

Charge exchange near Mars: The solar wind absorption and energetic neutral atom production

Charge exchange between solar wind protons and neutral atmospheric atoms is expected to affect the solar wind interaction with Mars, but its influences and significance have only been touched upon in previous work. Here several features associated with the charge exchange process between the solar wind protons and Martian neutral upper atmospheres are described. The analysis is based on an empirical proton model derived from Phobos 2 observations interacting with the Martian atomic (H) and molecular (H2) hydrogen, and oxygen (O) upper atmospheres representing solar minimum and solar maximum conditions. The region where the largest fraction of solar wind protons is lost by the charge exchange process is found to be a thin layer above the surface of Mars on the dayside resulting from charge exchange with the thermal oxygen. In general, the magnetosheath and “magnetosphere” (where the observed plasma takes on a different character in the Phobos 2 data) produce two distinguishable regions where the loss rate of solar wind protons is highest. Increasing solar activity increases the loss rate in the magnetosheath but decreases it in the magnetosphere. No significant increase of the absorption of the solar wind was found near the “magnetopause” suggesting that the decrease of the solar wind protons observed by Phobos 2 are not due to the charge exchange process. In addition to a reduction in the solar wind density, the charge exchange reaction results in energetic neutral atom (ENA) production. This paper considers some of the detailed properties expected for the ENA population at Mars. The ENA differential fluxes were found to be typically 106–107 cm−2 s−1 keV−1 in the energy range 0.01–1 keV. During solar minimum, the ENA production rate and ENA integral fluxes were found to be highest in the magnetosheath. At solar maximum the ENA production rate is highest in the magnetosphere, and ENA integral fluxes in the dayside magnetosphere appear to become comparable to the fluxes in the magnetosheath if the proton temperature in the magnetosphere is low. It is found that 1–3% of the original solar wind proton flux converts into ENAs before the bow shock. The ENAs produced upstream are undeflected and so may precipitate into the Martian upper atmosphere, depositing an energy flux of up to 3 × 109 eV cm−2 s−1 derived from the solar wind. These results both suggest the possible benefits of observing ENA fluxes around Mars and suggest the necessary parameters for detector design.

Solar wind proton flux latitudinal variations: Comparison between Ulysses in situ data and indirect measurements from interstellar Lyman α mapping

We compare the solar wind proton flux latitude dependence derived in the past from the interstellar neutral H distribution in the inner heliosphere from Lyman α observations with recent “in situ” solar wind observations by Ulysses [Goldstein et al., 1995, Phillips et al., 1995, 1996, J.L. Phillips, private communication, 1996]. We find common features, such as a significant proton flux decrease with increasing heliographic latitude (about 30%) in the low‐latitude regions and broad “plateaus” of low particle fluxes (2.0 – 2.5 × 108 protons cm−2 s−1) around the poles. We use our model of interstellar H distribution under the influence of a multiparameter, latitude dependent solar wind to investigate the effects of a solar wind distribution matching as closely as possible the south heliographic Ulysses observations. For the first time, multiple scattering is included in such an anisotropic model. The Ulysses‐type wind is found to produce a secondary minimum of Lyman α intensity in the upwind direction, something already observed near solar minimum of activity. However, the modeled feature has a larger amplitude than the observed one, probably an indication of smoothing due to the combination of solar rotation and waviness of the neutral sheet. The total solar wind particle flux and the full Sun‐averaged ionization rate of the interstellar neutral H are estimated in various cases. For identical equatorial and polar fluxes, the existence of broad plateaus results in a significant reduction of the average neutral H ionization (and then of the ionization cavity) when comparing with models using a classical “harmonic” dependence with latitude. As a result, the downwind cavity is less depleted. This may partially explain some discrepancies between the expected and observed Lyman α emissions from the interplanetary hydrogen cavity, in particular, the excess of emission from the downwind cavity compared with the classical model.

Coupling of the coronal he abundance to the solar wind

Models of the transition region — corona — solar wind system are investigated in order to find the coronal helium abundance and to study the role played by coronal helium in controlling the the solar wind proton flux. The thermal force on α-particles in the transition region sets the flow of helium into the corona. The frictional coupling between α-particles and protons and/or the electric polarization field determines the proton flux in the solar wind as well as the fate of the coronal helium content.

Termination of the solar wind proton flow near Mars by charge exchange

One striking observation made during the Phobos-2 mission concerns the existence of a welldefined plasma boundary where the solar wind proton flux terminates. Different mechanisms have been proposed to explain this interesting new feature. In the present study, we explore the possible influence of charge exchange on the loss of solar wind protons in the Martian exosphere as the reason of such a composition boundary. Unlike the cometary interaction, the charge exchange process is effective only in the vicinity of the subsolar point of the Martian “magnetopause” where the streamlines cross the dense neutral atmosphere. The most recent model of the Martian upper atmosphere and the flow field obtained by numerical simulation are used to provide the basis for such a scenario.

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.

Coupling of the coronal helium abundance to the solar wind

view Abstract Citations (28) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Coupling of the Coronal Helium Abundance to the Solar Wind Hansteen, Viggo H. ; Leer, Egil ; Holzer, Thomas E. Abstract Models of the transition region-corona-solar wind system are investigated in order to find the coronal helium abundance and to study the role played by coronal helium in controlling the solar wind proton flux. The thermal force on alpha-particles in the transition region sets the flow of helium into the corona. The frictional coupling between alpha-particles and protons and/or the electric polarization field determines the proton flux in the solar wind as well as the fate of the coronal helium content. The models are constructed by solving the time-dependent population and momentum equations for all species of hydrogen and helium in an atmosphere with a given temperature profile. Several temperature profiles are considered in order to very the roles of frictional coupling and electric polarization field in the solar wind, and the thermal force in the transition region. Steady-state solutions are found for coronae with a hydrogen flux at 1 AU of 1.0 x 109/cm2/sec or larger. For coronae with lower hydrogen fluxes, the helium flux into the corona is larger than the flux 'pulled out' by the solar wind protons, and solutions with increasing coronal helium content are found. The timescale for forming a helium-filled corona, that may allow for a steady outflow, is long compared to the mixing time for the corona. Publication: The Astrophysical Journal Pub Date: June 1994 DOI: 10.1086/174293 Bibcode: 1994ApJ...428..843H Keywords: Abundance; Alpha Particles; Astronomical Models; Coronas; Coupling; Helium; Hydrogen; Proton Flux Density; Solar Wind; Equations Of Motion; Momentum; Populations; Temperature Gradients; Temperature Profiles; Astronomy; SUN: SOLAR WIND; SUN: ABUNDANCES; SUN: CORONA; SUN: TRANSITION REGION full text sources ADS |

Solar wind models

The understanding of the solar wind is based upon Parker's (1958) description of a thermally driven subsonic - supersonic outflow from a fully ionized electron-proton corona. The basic physical processes of thermally driven solar wind models are discussed. Also studied are the effect of alpha particles in the corona on the solar wind proton flux. The acceleration of the solar wind by Alfven waves is discussed.

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.

Observation of Dust Generated Hydrogen in the Solar Vicinity

Solar wind protons impinging on interplanetary dust grains are trapped, deionized, and subsequently desorbed. The steady state distribution of desorbed neutral hydrogen inside of 0.4 AU can be deduced by observation of resonantly scattered solar 121.6 nm radiation. Calculated integral intensities and spectral profiles are given, showing the clear spectral separation of the different radiation components. Given a specific solar wind proton flux, the interplanetary dust distribution can be determined. Conversely, dust density profiles from zodiacal light measurements can be used to deduce solar wind proton fluxes at heliocentric distances of 0.4 to 0.15 AU. Observations of latitudinal and short-term temporal proton flux variations seem feasible.

Constraints on the solar coronal temperature in regions of open magnetic field

It is shown that the simultaneous consideration of observed values of the solar wind proton flux density at 1 AU and of the electron pressure at the base of the solar corona leads to relatively strong constraints on the coronal temperature in the region of subsonic solar wind flow. The extreme upper limit on the mean coronal temperature in the subsonic region is found to be about 2.6 × 106 K, but this upper limit is reduced to about 2.0 × 106 K if reasonable, rather than extreme, assumptions are made; the limit on the maximum temperature is about 0.5 × 106 K greater than the limit on the mean. It is also found that the same two observations limit the rate of momentum addition possible in the region of subsonic solar wind flow.

Implications of Saito's Coronal Density Model on the polar solar wind flow and heavy ion abundances

A comparison of polar solar wind proton flux upper limits derived using a coronal density model, with Lyman alpha measurements of the length of the neutral H tail of comet Bennet at high latitudes, shows that either extended heating beyond 2 solar radii is necessary some of the time or that the model's polar densities are too low. Whichever possibility is the case, the fact that the solar wind particle flux does not appear to decrease with increasing latitude indicates that the heavy element content of the high latitude wind may be similar to that observed in the ecliptic. It was then shown that solar wind heavy ion observations at high latitudes allow a determination of the electron temperature at heights which bracket the nominal location of the coronal temperature maximum thus providing information concerning the magnitude and extent of mechanical dissipation in the intermediate corona.

Solar wind and extreme ultraviolet modulation of the lunar ionosphere/exosphere

The ALSEP/SIDE detectors routinely monitor the dayside lunar ionosphere. Variations in the ionosphere are found to correlate with both the 2800 MHz radio index which can be related to solar EUV and with the solar wind proton flux. For the solar wind, the ionospheric variation is proportionately greater than that of the solar wind. This suggests an amplification effect on the lunar atmosphere due perhaps to sputtering of the surface or, less probably, an inordinate enhancement of noble gases in the solar wind. The surface neutral number density is calculated under the assumption of neon gas. During a quiet solar wind this number agrees with or is slightly above that expected for neon accreted from the solar wind. During an enhanced solar wind the neutral number density is much higher.

The effect of asymmetric solar wind on the Lyman α sky background

The Lyman α (Ly α) sky background arises from the scattering of solar Ly α from a spatial distribution of neutral hydrogen in interplanetary space. This distribution is partially determined by the solar wind proton flux, which provides the principal mechanism of loss by charge exchange of the neutral hydrogen. By generating isophotal maps of scattered Ly α for several choices of interstellar wind direction and solar wind proton flux distributions, the results show that latitudinal variations of the solar wind proton flux can have a significant effect on the observed location and shape of the Ly α intensity maximum. This fact should aid in the interpretation of Ly α maps and also indicates a possible method for inferring values for the average solar wind proton flux out of the ecliptic plane.