Boundary layer ion composition at Jupiter during the inbound pass of the Ulysses flyby

During the inbound segment of the Ulysses flyby of Jupiter, there were multiple incursions into the dawnside low-latitude boundary layer, as identified by Bame et al. (Science257, 1539–1542, 1992) using plasma electron data. In the present study, ion composition and spectral measurements provide independent collaborative evidence for the existence of distinct boundary layer regions. Measurements are taken in the energy-per-charge range of 0.6–60 keV/e and involve mass as well as mass-per-charge identification by the Ulysses/SWICS experiment. Ion species of Jovian magnetospheric origin (including O⁺, O²⁺, S²⁺, S³⁺) and sheath origin (including He²⁺ and high charge state CNO) have been directly identified for the first time in the Jovian magnetospheric boundary layer. Protons of probably mixed origin and He⁺ of possibly sheath (ultimately interstellar pickup) origin were also observed in the boundary layer. Sheath-like ions are observed throughout the boundary layer; however, the Jovian ions are depleted or absent for portions of two boundary layer cases studied. Ions of solar wind origin are observed within the outer magnetosphere. and ions of magnetospheric origin are found within the sheath, indicating that transport across the magnetopause boundary can work both ways, at least under some conditions. Although their source cannot be uniquely identified, the proton energy spectrum in the boundary layer suggests a sheath origin for the lower energy protons.     

Ulysses observations of the planetary depletion layer at Jupiter

The repeated samplings of the Jovian magnetosheath during the Ulysses encounter with Jupiter provided an opportunity to probe the planetary depletion layer. Of the 10 complete crossings of the Jovian magnetopause, only three contained clear signatures of an overlying depletion layer. All of these occurred on the flanks of the magnetosphere near the dusk terminator; crossings on the dayside were ambiguous or clearly lacked a depletion layer signature. In this paper we present a detailed analysis of the observations by the Ulysses solar wind plasma and magnetometer experiments and discuss conditions favorable and unfavorable for depletion layer observation.     

The solar wind H and He content

Charge exchange collisions between interplanetary neutral H atoms and solar wind protons may lead to fluxes of neutral H atoms and He⁺ ions in the solar wind. Photoionization of interplanetary helium atoms may also contribute to the He⁺ flux. The expected fluxes of He⁺ ions and neutral H atoms in the solar wind are computed. A simple model is used to compute the intensity of resonantly backscattered solar Hell (λ304 Å) and Lyman α radiation.     

Energy and pitch angle distributions for auroral ions using the current sheet acceleration model

Using a dipole plus tail magnetic field model, H⁺, He⁺⁺ and O ₁₆ ⁺⁶ ions are followed numerically, backward in time, from an output plane perpendicular to the axis of the geomagnetic tail, to their point of entrace to the magnetosphere as solar wind particles in the magnetosheath. An adiabatic or guiding center approximation is used in regions where the particles do not interact directly with the current sheet. A Maxwellian distribution with bulk flow is assumed for solar wind particles in the magnetosheath. Bulk velocity, density, and temperature along the magnetopause are taken from the fluid calculations of Spreiter. Using Liouville's theorem, and varying initial conditions at the output plane, the distribution function is found as a function of energy and pitch angle at the output plane. These results are then mapped to the auroral ionosphere using guiding center theory. Results show that the total precipitation rate is sufficient only for particles which enter the magnetosphere near the edges of the current sheet. Small pitch angles are favored at the output plane, but mappings to the auroral ionosphere indicate isotropic pitch angle distributions are favored with some peaking of the fluxes parallel or at other angles to the field lines. Perpendicular auroral pitch angle anisotropies are at times produced by the current sheet acceleration mechanism. Therefore, caution must be used in interpreting all such observations as ‘loss cone-trapping’ distributions. Energy spectra appear to be quite narrow for small cross-tail electric fields, and a little broader as the electric field increases. Comparisons of these results with experimental observations are presented.     

A model of the terrestrial ionosphere in the altitude interval 50–4000 km I. Atomic ions (H+, He+, N+, O+)

An empirical model of atomic ion densities (H⁺, He⁺, N⁺, O⁺) is presented up to 4000 km altitude as a function of time (diurnal, annual), space (position, altitude) and solar flux (F10.7) — using observations of satellites (AE-B, AE-C, AE-D, AE-E, ISIS-2, OGO-6) and rockets during quiet geophysical conditions (K _p 3). The numerical treatment is based upon harmonic functions for the horizontal pattern and cubic splines for the vertical structure.The ion densities increase with increasing height up to a maximum (depending roughly on the ion mass) and decrease beyond that with increasing altitude. Above 200 km, O⁺ is the main ionic constituent being replaced at approximately 800 km (depending on latitude, local time, etc.) by H⁺. Around polar regions the light ions, H⁺ and He⁺, are depleted (polar wind) and the heavier ones enhanced. During local summer conditions the ion densities increase around polar latitudes and correspondingly decrease during local winter, except He⁺ which reflects the opposite pattern. Diurnal variations are intrinsically coupled to the individual plasma layers: N⁺ and O⁺ peak, in general, during daytime, while the amplitudes and phases of H⁺ and He⁺ change strongly with altitude and latitude. Earth, Moon and Planets Review article.     

Observations of penetrated solar wind plasma elements in the plasma mantle

PROGNOZ-7 observations of intense “magnetosheath-like” plasma deep inside the high latitude boundary layer, the plasma mantle, indicates that solar wind plasma elements may occasionally penetrate the magnetopause and form high density regions in the plasma mantle. These “magnetosheath-like” regions are usually associated with strong flow of solar wind ions (e.g. H⁺ and He²⁺) and the presence of terrestrial ions (e.g. O⁺). The magnetosheath-like structures may roughly be classified as “newly injected” or “stagnant”. The newly injected structures have characteristics very similar to those found in the magnetosheath, i.e. strong antisunward flow and magnetosheath ion composition and density. The magnetic field characteristics may, however, differ considerably from those found further out in the magnetosheath. The “stagnant” structures are characterized by a reduced plasma flow, a lower density and a different ion composition as compared to that in the magnetosheath. In a few cases newly injected structures were even found in the innermost part of the mantle (i.e. forming a “boundary region” adjacent to the lobe). These cases were also associated with fairly strong fluxes of O⁺ ions in the outer mantle. Whilst the newly injected type of magnetosheath-like structure contained almost no O⁺ ions, the stagnant regions were intermixed by an appreciable amount of ionospheric ions. The newly injected and stagnant penetration regions had both in common a diamagnetic decrease of the ambient magnetic field. The newly injected structures, however, were also associated with a considerable reorientation of the magnetic field vector. A common feature for penetration regions well separated from the magnetopause is that they are mainly observed for a southward IMF. A third category of plasma mantle penetrated events, denoted “open magnetopause” events, usually occurred when the IMF was away and northward. Characteristics for these events were a smooth transition/rotation of the magnetic field vector near the magnetopause, and fairly high ion densities in the mantle and the transition region.     

H-implantation in SO2 and CO2 ices

Ices in the solar system are observed on the surface of planets, satellites, comets and asteroids where they are continuously subordinate at particle fluxes (cosmic ions, solar wind and charged particles caught in the magnetosphere of the planets) that deeply modify their physical and structural properties. Each incoming ion destroys molecular bonds producing fragments that, by recombination, form new molecules also different from the original ones. Moreover, if the incoming ion is reactive (H⁺, Oⁿ⁺, Sⁿ⁺, etc.), it can concur to the formation of new molecules.Those effects can be studied by laboratory experiments where, with some limitation, it is possible to reproduce the astrophysical environments of planetary ices.In this work, we describe some experiments of 15-100 keV H⁺ and He⁺ implantation in pure sulfur dioxide (SO₂) at 16 and 80 K and carbon dioxide (CO₂) at 16 K ices aimed to search for the formation of new molecules. Among other results we confirm that carbonic acid (H₂CO₃) is formed after H-implantation in CO₂, vice versa H-implantation in SO₂ at both temperatures does not produce measurable quantity of sulfurous acid (H₂SO₃). The results are discussed in the light of their relevance to the chemistry of some solar system objects, particularly of Io, the innermost of Jupiter's Galilean satellites, that exhibits a surface very rich in frost SO₂ and it is continuously bombarded with H⁺ ions caught in Jupiter's magnetosphere.     

Helium ion outflow from the terrestrial ionosphere

Extensive calculations have been made of the behaviour of He⁺ for situations where ion outflow occurs from the topside ionosphere. For these circumstances, steady state solutions for the He⁺ continuity, momentum and energy equations have been obtained self-consistently, yielding density, velocity and temperature profiles of He⁺ from 200 to 2000 km altitude. To model the high latitude topside ionosphere, a range of background H⁺O⁺ ionospheres was considered with variations in the H⁺ outflow velocity, the presence of a perpendicular electric field and different peak O⁺ densities. In addition, the atmospheric density of neutral helium was chosen to model typical observed winter and summer densities. From our studies we have found that: (a) The outflowing He⁺ has density profiles of similar shape to those of H⁺, for basically different reasons; (b) The effect of the perpendicular electric field differs considerably for H⁺ and He⁺. This difference stems from the fact that an electric field acts to alter significantly the O⁺ density at high altitudes and this, in turn, changes the H⁺ escape flux through the O⁺+H charge exchange reaction. A similar situation does not occur for He⁺ and therefore the He⁺ escape flux exhibits a negligibly small change with electric field; (c) The fractional heating of He⁺ due to the He⁺O⁺ relative flow is not as effective in heating He⁺ as the H⁺O⁺ relative flow is in heating H⁺; (d) During magnetospheric disturbances when the N₂ density at the altitude of the He⁺ peak (600 km) can increase by a factor as large as 50, the He⁺ peak density decreases only by approximately a factor of 2; and (e) The He⁺ escape flux over the winter pole is approximately a factor of 20 greater than the He⁺ escape flux over the summer pole. Consequently, on high latitude closed field lines there could be an interhemispheric He⁺ flux from winter to summer.     

Solar wind-magnetosphere interaction as simulated by a 3-D em particle code

We have studied the solar wind-magnetosphere interaction using a 3-D electromagnetic particle code. The results for an unmagnetized solar wind plasma streaming past a dipole magnetic field show the formation of a magnetopause and a magnetotail, the penetration of energetic particles into cusps and radiation belt and dawn-dusk asymmetries. The effects of interplanetary magnetic field (IMF) have been investigated in a similar way as done by MHD simulations. The simulation results with a southward IMF show the shrunk magnetosphere with great particle entry into the cusps and nightside magnetosphere. This is a signature of a magnetic reconnection at the dayside magnetopause. After a quasi-stable state is established with an unmagnetized solar wind we switched on a solar wind with an northward IMF. In this case the significant changes take place in the magnetotail. The waving motion was seen in the magnetotail and its length was shortened. This phenomena are consistent with the reconnections which occur at the high latitude magnetopause. In our simulations kinetic effects will determine the self-consistent anomalous resistivity in the magnetopause that causes reconnections.Deceased January 24, 1993; R. Bunemanet al. 1993.     

Ion flow and momentum transfer in the Venus plasma environment

An analysis of ion data from 390 Venus Express, VEX, orbits demonstrates that the flow of solar wind- and ionospheric ions near Venus is characterized by a marked asymmetry. The flow asymmetry of solar wind H⁺ and ionospheric O⁺ points steadily in the opposite direction to the planet’s orbital motion, and is most pronounced near the Pole and in the tail/nightside region. The flow asymmetry is consistent with aberration forcing, here defined as lateral forcing induced by the planet’s orbital motion. In addition to solar wind forcing by the radial solar wind expansion, Venus is also subject a lateral/aberration forcing induced by the planet’s orbital motion transverse to the solar wind flow.The ionospheric response to lateral solar wind forcing is analyzed from altitude profiles of the ion density, ion velocity and ion mass-flux. The close connection between decreasing solar wind H⁺ mass-flux and increasing ionospheric O⁺ mass-flux, is suggestive of a direct/local solar wind energy and momentum transfer to ionospheric plasma. The bulk O⁺ ion flow is accelerated to velocities less than 10 km/s inside the dayside/flank Ionopause, and up to 6000 km in the tail. Consequently, the bulk O⁺ outflow does not escape, but remains near Venus as a fast (km/s) O⁺ zonal wind in the Venus polar and nightside upper ionosphere. Furthermore, the total O⁺ mass-flux in the Venus induced magnetosphere, increases steadily downward to a maximum of 2 × 10⁻¹⁴ kg/(m² s) at ≈400 km altitude, suggesting a downward transport of energy and momentum. The O⁺, and total mass-flux, decay rapidly below 400 km. With no other plasma mass-flux as replacement, we argue that the reduction of ion mass-flux is caused by ion-neutral drag, a transfer of ion energy and momentum to neutrals, implying that the O⁺ plasma wind is converted to a neutral (thermosphere) wind at Venus. Incidentally, such a neutral wind would go in the same direction as the Venus atmosphere superrotation.     

Observation by Pioneer 7 of He+ in the distant coma of Halley's comet

During the approach of Pioneer 7 to a distance of 12.1 × 10⁶ km from the nucleus of Comet P/Halley on March 20, 1986, features were observed in plasma analyzer data that we interpret as He⁺ produced by charge exchange of solar wind He⁺⁺ with neutral cometary material. The maximum flux of the He⁺ was observed several hours after the closest approach of the spacecraft to the nucleus, and is unexpectedly large. Remarkable large discontinuous flux changes were also observed.     

A theoretical model of the bow shock and the magnetosphere of Jupiter

Using the data obtained from the Pioneer 10 and 11 observations, a theoretical model is proposed for the bow shock and the magentosphere of Jupiter. This indicates that the distance of the magnetopause from Jupiter on the sunlit side is (50–55) × r_J (r_J: Jupiter radius, = 7 × 10⁹ cm) and that the ratio of the stand-off distance to this distance is about equal to or slightly larger than unity. Hence the Mach number of the solar wind seems to be less than 1.5 at Jupiter's orbit. This result necessarily leads to a blunt body model of the Jovian magnetosphere, the tail region of which is not as extended as observed in the Earth's case.     

On solar wind interaction with the earth's magnetosphere

Hydrodynamic and electrodynamic problems of solar wind interaction with the Earth's magnetosphere on the day-side are investigated.The initial fact, well established, is that the density of the magnetic field energy in the solar wind is rather small. Magnetic field intensity and orientation are shown to determine the character of the solar wind flow around the magnetosphere. For mean parameters of the wind, if the tangential component of the magnetic field is more or equal 5γ, the flow in the magneto-sheath will be laminar. For other cases the flow is of a turbulent type.For turbulent flow, typical plasma parameters are estimated: mean free path, internal scale of inhomogeneities and dissipated energy. The results obtained are compared with experimental data.For the case of laminar flow, special attention is paid to the situation when magnetic fields of the solar wind and Earth are antiparallel. It is suggested, on the basis of solid arguments, that the southward interplanetary field diffuses from the magnetosheath into the Earth's magnetosphere. These ideas are used for the estimation of the distance to the magnetopause subsolar point. A detailed comparison with results of observation is made. The coincidence is satisfactory. Theoretical investigation has been made to a great extent for thin magnetopause with thickness δ ～ R_He-gyroradius of an electron.It is shown that during magnetospheric substorms relaxation oscillations with the period τ = 100–300 sec must appear. A theorem is proved about the appearance of a westward electrical field during the substorm development, when the magnetosphere's day-side boundary moves Earthward and about the recovery phase, when the magnetopause motion is away from the Earth, when there is an eastward electrical field.In the Appendix, plasma wave exitation in the magnetopause is considered and conductivity magnitudes are calculated, including the reduction due to the scattering by plasma turbulence.     

Measurements of positive ion conversion and removal reactions relating to the Jovian ionosphere

Measured rates are presented for the reaction of He⁺ ions with H₂ (and D₂) molecules to form H⁺, H₂⁺, and HeH⁺ ions, as well as for the subsequent reactions of H⁺ and HeH⁺ ions with H₂ to form H₃⁺. The neutralization of H₃⁺ (and H₅⁺) ions by dissociative recombination with electrons is shown to be fast. The reaction He⁺ + H₂ is slow (k = 1.1 × 10^?13 cm³/sec at300°K) and produces principally H⁺ by the dissociative charge transfer branch. It is concluded that there may be a serious bottleneck in the conversion of two of the primary ions of the upper Jovian ionosphere, H⁺ and He⁺ (which recombine slowly), to the rapidly recombining H₃⁺ ion (α[H₃⁺]?3.4 × 10^?7 cm³/sec at 150°K).     

Signatures of the Interplanetary Helium Cone Reflected by Pick-up Ions

As known for a long time, interstellar wind neutral helium atoms deeply penetrate into the inner heliosphere and, when passing through the solar gravity field, form a strongly pronounced helium density cone in the downwind direction. Helium atoms are photoionized and picked-up by the solar wind magnetic field, but as pick-up ions they are not simply convected outwards with the solar wind in radial directions as assumed in earlier publications. Rather they undergo a complicated diffusion-convection process described here by an appropriate kinetic transport equation taking into account adiabatic cooling and focusing, pitch angle scattering and energy diffusion. In this paper, we solve this equation for He⁺pick-up ions which are injected into the solar wind mainly in the region of the helium cone. We show the resulting He⁺pick-up ion density profile along the orbit of the Earth in many respects differs from the density profile of the neutral helium cone: depending on solar-wind-entrained Alfvénic turbulence levels, the density maximum when looking from the Earth to the Sun is shifted towards the right side of the cone, the ratio of peak-densities to wing-densities varies and a left-to-right asymmetry of the He⁺-density profile is pronounced. Derivation of interstellar helium parameters from these He⁺-structures, such as the local interstellar medium (LISM) wind direction, LISM velocity and LISM temperature, are very much impeded. In addition, the pitch-angle spectrum of He⁺pick-up ions systematically becomes more anisotropic when passing from the left to the right wing of the cone structure. All effects mentioned are more strongly pronounced in high velocity solar wind compared to the low velocity solar wind.     

Dependence of the magnetopause position on the southward interplanetary magnetic field

The distance to the dayside magnetopause is statistically analyzed in order to detect the possible dependence of the dayside magnetic flux on the polarity of the interplanetary magnetic field. The effect of changing solar wind pressure is eliminated by normalizing the observed magnetopause distances by the simultaneous solar wind pressure data. It is confirmed that the normalized size of the dayside magnetosphere at the time of southward interplanetary magnetic field is smaller than that at the time of northward interplanetary magnetic field. The difference in the magnetopause position between the two interplanetary field polarity conditions ranges from 0 to 2R_E. Statistics of the relation between the magnetopause distance and the magnetic field intensity just inside the magnetopause testifies that the difference in the magnetopause position is not due to a difference in the magnetosheath plasma pressure. The effect of the southward interplanetary magnetic field is seen for all longitudes and latitudes investigated (|λ_GM|? 45°, |φ_SM|? 90°). These results strongly suggest that a part of the dayside magnetic flux is removed from the dayside at the time of southward interplanetary magnetic field.     

Quasiperiodic Jovian Radio bursts: observations from the Ulysses Radio and Plasma Wave Experiment

The Ulysses flyby of Jupiter has permitted the detection of a variety of quasiperiodic magnetospheric phenomena. In this paper, Unified Radio and Plasma Wave Experiment (URAP) observations of quasiperiodic radio bursts are presented. There appear to be two preferred periods of short-term variability in the Jovian magnetosphere, as indicated by two classes of bursts, one with 40 min periodicity, the other with 15 min periodicity. The URAP radio direction determination capability provides clear evidence that the 40 min bursts originate near the southern Jovian magnetic pole, whereas the source location of the 15 min bursts remains uncertain. These bursts may be the signatures of quasiperiodic electron acceleration in the Jovian magnetosphere; however, only the 40 min bursts occur in association with observed electron bursts of similar periodicity. Both classes of bursts show some evidence of solar wind control. In particular, the onset of enhanced 40 min burst activity is well correlated with the arrival of high-velocity solar wind streams at Jupiter, thereby providing a remote monitor of solar wind conditions at Jupiter.     

The influence of neutral winds on He+ distributions in the equatorial ionosphere

The effects of F-region neutral winds on the distribution of He⁺ in the equatorial ionosphere have been examined using a theoretical model and an observational data set. It is shown by the model that components of neutral wind in the magnetic meridian up to only 50 m s^? can produce He⁺ gradients in the northern and southern sectors of a flux tube that differ by more than 80%. This is associated with interhemisphere transport velocities of He⁺ as large as 15 m s^?1 at 800 km. A substantial latitude gradient in the He⁺ distribution across the dip equator also results from the redistribution of He⁺ The changes in the He⁺ concentration at the dip equator and the latitude distribution of He⁺ in response to different neutral wind components is determined from the model and used to construct longitude distributions of He⁺ to compare with observations made at equinox. Good agreement between the calculations and observations is obtained both at the geographic and geomagnetic equators using the relationship between neutral winds, interhemispheric transport velocity and He⁺ concentration derived from the model. If these relationships can be extrapolated to accommodate the different conditions expected during solstice, we can also discuss the He⁺ distributions expected during this season.     

Mercury's atmosphere: A perspective after Mariner 10

Measurements made during the Mariner 10 flybys of Mercury have shown that this planet has a tenuous atmosphere, somewhat similar to that of the Moon, which consists of at least helium and can be classified as an exosphere. The amount of helium observed can be supplied by either the accretion of only a fraction of the solar wind He²⁺ diffusing across the magnetopause, or from outgassing of radiogenic helium from the planetary crust. The role of solar wind in the maintenance and depletion of Mercury's atmosphere is discussed in view of the density upper limits established from Mariner 10. The argon supply rate on Mercury is probably not more than that on the Earth, but it is difficult to say whether Mercury is deficient in potassium or not on the basis of the present data. The global outgassing of CO₂ and H₂O from the planet interior is estimated to be at least four orders of magnitude smaller than for Earth which indicates that either Mercury is deficient in volatiles or that this planet is very inactive.     

The structure of the magnetopause

The solar wind is a magnetized flowing plasma that intersects the Earth's magnetosphere at a velocity much greater than that of the compressional fast mode wave that is required to deflect that flow. A bow shock forms that alters the properties of the plasma and slows the flow, enabling continued evolution of the properties of the flow on route to its intersection with the magnetopause. Thus the plasma conditions at the magnetopause can be quite unlike those in the solar wind. The boundary between this “magnetosheath” plasma and the magnetospheric plasma is many gyroradii thick and is surrounded by several boundary layers. A very important process occurring at the magnetopause is reconnection whereby there is a topological change in magnetic flux lines so that field lines can connect the solar wind plasma to the terrestrial plasma, enabling the two to mix. This connection has important consequences for momentum transfer from the solar wind to the magnetosphere. The initiation of reconnection appears to be at locations where the magnetic fields on either side of the magnetopause are antiparallel. This condition is equivalent to there being no guide field in the reconnection region, so at the reconnection point there is truly a magnetic neutral or null point. Lastly reconnection can be spatially and temporally varying, causing the region of the magnetopause to be quite dynamic.