Earth's Exosphere Research Articles

On the solar cycle dependence of the N+/O+ content in the magnetosphere and its relation to atomic N and O in the Earth's exosphere

We investigate a solar cycle dependence of singly charged Nitrogen and Oxygen over a five‐year period from 1995–2000 on the nightside of the Earth's magnetosphere using instrumentation on the WIND spacecraft. We show that N+, another heavy ion that is of terrestrial origin, is a large fraction of O+, and it contributes significantly to magnetospheric plasma. We find that the N+/O+ ratio has a solar cycle relationship with a higher value of this ratio at solar minimum than at solar maximum. We examine the N+/O+ ratio against thermospheric N/O MSIS 86 model calculations which exhibit no solar cycle variation.

Apatite weathering and the Phanerozoic phosphorus cycle

Despite widespread debate in the literature, there is still considerable uncertainty concerning which nutrient ultimately controls marine net ecosystem production (NEP) over geologic time. Geochemical arguments suggest that phosphorus is the culprit. The weathering of apatite, the primary phosphorus sink in Earth's exosphere, controls long-term phosphorus availability. If phosphorus is the ultimate controlling nutrient over geologic time scales, then long-term marine NEP is coupled to the release of phosphorus from apatite weathering. The most abundant apatite compositions found in nature are igneous fluorapatite and marine sedimentary carbonate fluorapatite. Sparse data exist on how these compositions dissolve under Earth's surface conditions. To demonstrate a need for these data and their application, we present a kinetic treatment of existing data, augmented by new results. We then use these results in a weathering model designed to illustrate the control exerted by temperature (via activation energy) and surface area on the phosphorus flux from apatite dissolution during the Phanerozoic. Our conclusion is that activation energy, and hence temperature, and apatite surface area are important parameters governing the phosphorus flux from apatite weathering and therefore marine NEP during Phanerozoic time.

Stellar Lyman-alpha emission and the local interstellar medium

Under the auspices of this ADP program, a systematic study was made of IUE archival images in order to extract spectra of the Lyman alpha region and to measure the stellar Lyman alpha flux for as many late-type stars as possible. The Lyman alpha resonance line is a powerful cooling channel for the hot chromospheres of solar-type stars, but has not been studied before in any systematic fashion across the H-R diagram. A major deterrent which has limited the use of Lyman alpha in the study of stellar chromospheres is the contamination of this spectral feature caused by the scattering of solar Lyman alpha photons in the Earth's exosphere. This scattered light is monochromatically imaged through the entrance slot of the IUE telescope and is superposed onto the stellar spectrum. In all but the shortest exposures with IUE, this 'geocoronal emission' overwhelms the stellar flux and makes it impossible to directly measure the strength of the stellar chromospheric feature. The IUESIPS processing contains no provision for correcting standard G.O. output products for this contamination. The first task was to develop a scheme for removing the geocoronal flux, specifically from low-dispersion spectra taken with the Short-Wavelength Camera of IUE. The strategy adopted was to fit a 'sky model' to the spatially-resolved geocoronal emission observed through the large science aperture of the telescope, using the spectral orders on either side of the central ones where the stellar emission is concentrated. The model emission was then subtracted from the observed image, leaving behind the corrected stellar Lyman alpha emission. The details of this fitting procedure are described. Having devised a successful method for removing the unwanted geocoronal emission, the correction procedure was applied to 366 archival images which, from inspection of the photowrites in the IUE browse file, seemed especially promising. In this survey, Lyman alpha emission were eventually detected in the spectra of 227 stars representing a wide range in age, temperature, and luminosity throughout the cool half of the H-R diagram. Previously fewer than 30 such stars had been measured, and an order of magnitude increase in the numbers of stars having Lyman alpha flux measurements is provided. Multiple measurements were made for 52 stars and upper limits on chromospheric flux were derived for another 48 stars.

Images of the Earth's aurora and geocorona from the dynamics explorer mission

Several results from analyses of auroral and geocoronal images from the Dynamics Explorer Mission are summarized. (1) The motion of the transpolar arc of a theta aurora is found to be correlated with the y-component of the interplanetary magnetic field. The arc motion is in the general direction of the y-component. (2) A sequence of global images of a small auroral substorm shows the initial development of intense luminosities in a relatively small spatial region, or ‘bright spot’, in the pre-midnight sector of the auroral oval and a subsequent appearance of an expanding area of lesser intensities at lower latitudes and contiguous to the midnight boundary of the bright spot. This evolution of auroral luminosities is interpreted in terms of acceleration of electrons in the boundary layer of the magnetotail plasma sheet to produce the bright spot and subsequent injection into, and eastward drift within, the plasma sheet to form the diffuse area of lesser intensities. (3) A series of images of the Earth's geocorona in scattered solar Ly α emissions is used to obtain a best-fit spherical model of atomic hydrogen densities in the Earth's exosphere. A Chamberlain model provides an adequate fit to radial distances of 4.5 R E, beyond which an exponential fit is used. The geocoronal tail is detected as an asymmetric increase in scattered Ly α intensities in the anti-solar direction.

The effect of the charge exchange source on the velocity and ‘temperature’ distributions and their anisotropies in the Earth's exosphere

The velocity distribution of atomic hydrogen in the earth's exosphere has been calculated as a function of altitude and direction using a Monte Carlo simulation which includes the classical exobase source and the higher‐altitude plasmaspheric charge exchange source. The charge exchange ‘hot’ hydrogen atom source produces the satellite component and ‘hot’ ballistic and escape components. The velocity distribution functions F(υ) have been plotted as log F(υ) against υ². For a Maxwellian velocity distribution the slope would be constant with velocity υ and inversely proportional to the defined temperature. For the classical exobase source alone the slope is constant only for the upward radial velocity component, and it increases dramatically as the altitude increases above one earth radius for both the incoming radial and the two transverse velocity components. This ‘temperature decrease’ is the same effect as indicated in the analytical work of Chamberlain (1963), due to the absence of the incoming‐hyperbolic and flyby components of the particle distributions. The effect of charge exchange is to enhance the wings of the velocity distributions, especially for the upward‐radial component. Charge exchange does not, however, succeed in overcoming the ‘temperature decreases’ in the incoming‐radial and transverse directions at altitudes above one earth radius. The gradients of the ‘temperatures’ and their anisotropies are great enough to engender some lack of confidence in the results of radiative transfer treatments for Lyman α, which have been carried out so far on the basis of isothermal exospheric hydrogen models.

A calculated hydrogen distribution in the exosphere

The density distribution in a planetary exosphere is a function of the boundary conditions, i.e. the density and the temperature distributions at the exobase. We established, with the use of Liouville's theorem, that the exospheric density may be calculated with a triple integral. The results of a numerical evaluation of this integral are presented for three different exobase models for the case of atomic hydrogen in the Earth's exosphere. They all satisfy the zero net ballistic flux condition, but correspond to different levels of solar activity. A constant density level was found at 1.8 earth radii, below which the computed density distribution may be approximated by a local spherical Chamberlain model. Above the constant density level, a simple relation was found to fit the computed density distribution within ±5 per cent.

Deuterium in the Earth's exosphere

Lyman alpha transition line in exosphere deuterium line implying enrichment factor ratio to earth surface deuterium

Charged-particle temperatures and concentrations in the Earth's exosphere

Under the assumption that the earth's magnetospheric plasma between 800 km and the plasmapause is in diffusive equilibrium, charged-particle temperatures and concentrations throughout the region are estimated from experimental electron concentration and scale-height data near the base of the region. Using various plausible models of electron and ion temperature distributions, relative abundances of oxygen, helium, and hydrogen ions near the base level as functions of dip latitude are computed. Ion temperatures and temperature gradients are also deduced. Experimental data employed in the analysis include Alouette 1 midlatitude electron concentration profiles, Explorer 22 electron temperature data, and equatorial electron concentration profiles deduced from whistler observations. It is found that daytime temperature gradients of 1–2°K/km near 30° dip latitude at 1000 km and decreasing with latitude must exist along the field lines for the experimental data to be compatible with thermal equilibrium and a constant temperature gradient above 800 km. For unequal electron and ion temperatures, lower temperature gradients suffice. The relative abundance of hydrogen (oxygen) ions at 1000 km decreases (increases) dramatically with latitude, while the helium abundance shows little systematic variation. The results compare favorably with independent measurements of ion temperature and composition, lending confidence in the basic diffusive equilibrium assumption.

Excitation of backward doppler-shifted cyclotron radiation in a magneto-active plasma by an electron stream

The radiative instability of an electron-stream magnetoactive plasma system is investigated for a cold ambient plasma and a helical electron stream which has a delta function momentum distribution. Using parameters appropriate to the earth's exosphere, the growth rate of backward Doppler-shifted cyclotron radiation is evaluated numerically as a function of the emission or wave normal angle θ. It is shown that the rate of growth is maximum for θ = 180° and that only frequencies close to that emitted at θ = 180° are amplified appreciably. The application of the theory to VLF discrete emission is discussed.

Studies of planetary atmospheres: 1. The distribution of electrons and ions in the Earth's exosphere

The factors which govern the distribution of electrons and ions in a planet's exosphere under diffusive equilibrium are discussed. The theory takes into account the effect of the electric field that arises from charge separation, the centrifugal force arising from the rotation of the planet, and the effect of the planet's gravitational field. It is assumed that the charged particles are constrained to move only along the direction of the planet's magnetic lines of force. The modifications that result in the electron and ion distributions when a temperature variation is assumed along a line of force are also considered. The results predicted by the theory are compared with actual experimental observations of the electron density distribution in the earth's exospheric plasma which have been obtained in recent years from whistler data and from topside ionograms made by the Alouette satellite.

A hydromagnetic theory of geomagnetic storms and auroras

A theory of geomagnetic storms, auroras and associated effects is further developed. It depends on motions in the Earth's exosphere or magnetosphere initiated by a combination of pressure and frictional drag of the solar wind and modified and extended by electric fields and currents in the ionosphere. Motion may be non-divergent, streamline flow opposed only by Lorentz forces in the ionosphere and not propagating to Earth, or divergent, non-streamline motion opposed by Lorentz forces in the Earth. The two types of motion are coupled in the E region where the former is identified with free flow of Hall current and the generation of non-streamline motion. The latter is identified with blockage of Hall current, the creation of a polarization field and hence the generation of streamline motion. A theory of all components of a geomagnetic storm is given in terms of combinations of these motions, and their distant, ionospheric and earth currents. This includes a new theory of the preliminary reverse part of the DS field and the transition from the sudden commencement to the main phase of the DS field. It is extended to introduce briefly a theory of auroras based mainly on ionospheric drifts caused by the magnetospheric motions.

A possibility of the generation of V.L.F. emissions in the outer Earth's exosphere

A possibility of the generation of V.L.F. emissions in the outer Earth's exosphere

Variations of the earth's exosphere and geomagnetic disturbances during the geomagnetic storm of 4 September 1958

The time variations of D s and D st for the geomagnetic storm of 4 September 1958 have been calculated using hourly values of the horizontal component of the geomagnetic field over the world. These D st and D s have been compared with the observation of the earth's exosphere by Explorer IV and that of auroral radio echoes during the same period, During this geomagnetic storm marked decrease in the intensity of trapped radiations was observed in the outer zone and brilliant auroral displays were observed at latitudes well below the auroral zone. The time variations of D s at geomagnetic latitudes of 57–58° have indicated the largest deviation of all ones at other latitudes. All peaks of D s at high latitudes and auroral radio echoes at Jodrell Bank occurred for the main phase of this geomagnetic storm. The time variations of D s at low latitudes have shown only a little deviation and have been almost relatively unvariable compared with ones at high latitudes during the geomagnetic storm. From the analyses of D s at high latitudes it is deduced that the exospheric region within the inner boundary of the outer zone, where is is located along the geomagnetic lines of force intersecting the earth's surface at geomagnetic latitudes of about 50°, was not so much disturbed by the invasion of solar plasma cloud. It is considered that the dumping of part of the contents of the outer earth's exosphere, which interprets well the reduction in the intensity of trapped radiations in the outer exosphere, produces regions of comparatively high ionization which are detectable as auroral radio echoes and D s of geomagnetic disturbances. It has been shown that the time variations of D s at the auroral and the sub-auroral zones represent reasonably well the extent of disturbance of trapped radiations in the outer exosphere during the geomagnetic storm. The equatorward shifting of latitude of the maximum strength of chorus and hiss during the geomagnetic storm may be due to the inaction of generation mechanism of v.l.f. emissions in the exospheric region where the geomagnetic lines of force intersecting the auroral zone pass through during the geomagnetic quiet period, in addition to the equatorward shifting of precipitation of auroral particles during the geomagnetic storm. Statistical results of v.l.f. emission suggest that the electron density distribution given by the gyrofrequency model may not exist along the geomagnetic lines of force intersecting the earth's surface at the auroral zone during the geomagnetic storm. D st at the auroral zone has shown larger deviation from the prestorm level for the main phase, sharper recovery after the main phase, and earlier recovery for the recovery phase than ones at the middle and the equatorial zones. This slow and late recovery of equatorial D st and small variation of equatorial D s may support the existence of the equatorial ring current around the earth. A simple model of the state of earth's exosphere during the main phase of the storm has been proposed.

Electro-hydromagnetic waves in a fully ionized gas-II

The propagation of hydromagnetic and low frequency radio waves in all directions in a fully ionized gas containing a magnetic field is examined. For longitudinal and transverse propagation the addition of one extra term in the magneto-ionic formulae (without collisions) accounts for the presence of heavy ions. The partition of energy of disturbance between kinetic ( K) and magnetic ( M) for longitudinal propagation of all frequencies is given by K=V 2 k 2 σ 2 M 1− σ 2 k 2c 2 2 where V is the Alfvén speed. Thus approximate equipartition may exist for some audio- and radio-frequencies in the Earth's exosphere. Some errors in Paper I of this series are corrected.

Solar-stream distortion of the geomagnetic field and polar electrojets

Distortion of the geomagnetic field by an ionized solar stream has often been thought to be related to the occurrence of polar electrojet current systems and aurora [Chapman and Bartels, 1951; and others]. Such distortion has been considered by Chapman and Ferraro [1931], Chapman [1960], and Ferraro [1960], Martyn [1951], and more recently by Piddington [1959, 1960], who considers the effects of the plasma content of the earth's exosphere, and suggests the possibility of a geomagnetic tail due to interaction with a solar stream. Distension of the geomagnetic field due to trapped particle currents [Dessler and Parker, 1959; Akasofu, 1960] may also contribute to such distortion. Chapman [1950] formulated an experiment to clarify the physical effects that involved motion of an ionized gas past a magnet, but scaling difficulties prevented such an approach.

On the Origin of V. L. F. Noise in the Earth's Exosphere

At present, there are some theories being able to explain the frequency characteristic of dawn chorus. In these theories, the natural thermal noise excited in a manner similar to the operation of a travelling-wave tube, the Doppler shifted proton cyclotron radiation and the Cerenkov radiation are considered as the origin of V. L. F. noise in the earth's exosphere respectively. This paper indicates that the Cerenkov radiation will be the most effective with respect to the emission energy in the above mentioned mechanisms in the earth's exosphere and that Cerenkov radio emissions generated by high speed protons (1.4×104km/sec) precipitating into the ionized exosphere along the earth's magnetic field may be excited simultaneously by the operation of the mechanism similar to a travelling-wave tube due to charged particles different from those causing the Cerenkov radiation relatively near the earth less than about 4 earth radii from the center of the earth and then propagate toward the earth in the whistler mode.

Structure of the Earth's exosphere

The main components of the earth's exosphere are neutral oxygen, neutral hydrogen atoms, ionized oxygen, and ionized hydrogen. The position of the base of the exosphere is established at an altitude of 530 km, from an analysis of density data obtained from satellite drag observations. The relative distribution of both neutral oxygen and neutral hydrogen is derived from a theory of the exosphere. The normalization of oxygen is accomplished by means of satellite drag data. The normalization of hydrogen is accomplished by means of the Lyman-α technique of Johnson, Friedman, and Tousey. The distribution of ionized oxygen can be established jointly from the rocket measurements of Berning and from an analysis of radiation belt data obtained from satellites. They give a consistent picture of ionized oxygen becoming the most important constituent of the atmosphere at about 1500 km. The distribution of ionized hydrogen is only imperfectly known.

The exosphere and upperFregion

The view is presented that the outer portion of the earth's ionosphere, through which radio whistlers propagate, has its origin in the earth's atmosphere rather than in interplanetary space. The outer portion of the earth's neutral-particle exosphere is dominated by neutral atomic hydrogen which is escaping steadily from the earth's atmosphere. Some of this neutral atomic hydrogen reacts with atomic oxygen ions near the base of the exosphere, giving rise to hydrogen ions. Diffusive equilibrium is the dominant factor controlling the relative concentration of ions and neutral particles near the base of the exosphere. At higher levels, the geomagnetic field exerts an influence in that it causes the ions and electrons to rotate with the earth, and the centrifugal force modifies the force of gravitational attraction appreciably. The ion electron plasma through which the radio whistlers propagate is hydrostatically supported within the magnetic tubes of force.

Properties of the upper atmosphere and their relation to the radiation belts of the earth

A new theory has been developed which gives the distribution of density with altitude for a planetary exosphere in the absence of thennodynamic equilibrium. The results differ considerably from those calculated on the basis of a hydrostatic equation. To apply this theory to the earth's exosphere, we first fix the temperature of the base of the exosphere, which is located at 530 km, by using satellite drag data between 500 and 1000 km. A temperature of 1500° gives a reasonable fit to the rather sparse data and allows us to calculate the atomic oxygen distribution with altitude. From the cosmic ray neutron albedo theory of the inner (cosmic ray) radiation belt, we can set an upper limit of about 10 6 hydrogen atoms per cm 3 at an altitude of 1000 km, but their concentration could be less. However, the total density including the ionized component must be of the order of 10 −19 g/cm 3 at 1000 km. The existence of a minimum between the two observed radiation belts, the so-called “slot”, is accounted for in terms of the breakdown of the adiabaticity of the magnetic moment of trapped particles. This same theory also allows one to make statements concerning the mean energy of the trapped protons in the inner radiation belt. It should become progressively softer with increasing altitude. The outer (auroral) radiation belt, which is centered at 3 earth radii, has a lifetime which is controlled primarily by the presence of a scattering atmosphere. From a theory which takes into account both energy loss and scattering, one can calculate the intensity contours for the quasi-equilibrium state. Special techniques give the transient case following the injection of particles into this region. The initial injection of solar corpuscular radiation consists mainly of protons and electrons with velocities of 2 × 10 8 cm/ sec. These 20 keV protons form a third and rather transient ( $ ̃ 1 day) radiation belt at 4–8 earth radii; they account for most of the effects observed during magnetic storms. Their initial diamagnetie effects produce the “reverse sudden commencement” ( SC ∗ ). Their increase of upper atmosphere ionization accounts for the sudden commencement currents while their drift in longitude accounts well for the main phase of magnetic storms. In comparing various removal processes for the injected protons, we find that charge exchange is about 100 times more effective than atmospheric scattering. Using our theoretical distribution for the hydrogen exosphere, we can therefore calculate the complete development of a magnetic storm as a function of time, including the distribution around the earth of trapped protons as well as their magnetic effects.

Statement of agreement regarding the ring-current effect

In an earlier communication [Hines and Parker, 1958], we recorded discrepant views regarding the possible magnetic effects at the ground caused by a hypothetical ring current in the earth's exosphere. This followed upon earlier statements by Parker [1956, 1958a and b] to the effect that only an increase of geomagnetic field could be produced in a reasonably short time by such a current, as a consequence of exospheric shielding, and conflicting statements by Hines [1957] and Hines and Storey [1958] to the effect that geomagnetic field changes of either sense could be propagated rapidly through the exosphere by hydromagnetic waves. In view of the interest that this controversy appears to have raised, and because of the basic importance of the consequences in geomagnetic storm theory, we wish now to record a reconciliation of views, to the effect that circulating charge high in the exosphere can indeed be associated with a decrease of geomagnetic field at ground level in a reasonably short time and, therefore, may indeed be responsible for the main phase of a geomagnetic storm. In order to clarify the basic features of the controversy most readily, the original discrepant views will be restated briefly in application to very extreme models where each in turn undoubtedly applies. It should be noted, however, that each view can in turn be justified for other models which bear a much closer resemblance to the circumstances of nature, and the separate conclusions were originally formulated with these other models in mind.