Search

A systems science examination of the Earth’s fully interconnected dynamic magnetosphere is presented. Here the magnetospheric system (a.k.a. the magnetosphere–ionosphere–thermosphere system) is considered to be comprised of 14 interconnected subsystems, where each subsystem is a characteristic particle population: 12 of those particle populations are plasmas and two (the atmosphere and the hydrogen geocorona) are neutrals. For the magnetospheric system, an assessment is made of the applicability of several system descriptors, such as adaptive, nonlinear, dissipative, interdependent, open, irreversible, and complex. The 14 subsystems of the magnetospheric system are cataloged and described, and the various types of magnetospheric waves that couple the behaviors of the subsystems to each other are explained. This yields a roadmap of the connectivity of the magnetospheric system. Various forms of magnetospheric activity beyond geomagnetic activity are reviewed, and four examples of emergent phenomena in the Earth’s magnetosphere are presented. Prior systems science investigations of the solar-wind-driven magnetospheric system are discussed: up to the present these investigations have not accounted for the full interconnectedness of the system. This overview and assessment of the Earth’s magnetosphere hopes to facilitate (1) future global systems science studies that involve the entire interconnected magnetospheric system with its diverse time and spatial scales and (2) connections of magnetospheric systems science with the broader Earth systems science.

The hydrogen exosphere constitutes the uppermost atmospheric layer of the Earth, and its shape may reflect the last stage of the atmospheric escape process. The distribution of hydrogen in the outer exosphere remains unobserved because outer geocoronal emissions are difficult to observe from within the exosphere. In this study, we used the Lyman Alpha Imaging Camera (LAICA) onboard the Proximate Object Close Flyby with Optical Navigation (PROCYON) spacecraft, located outside the exosphere, to obtain the first image of the entire geocorona that extends to more than 38 Earth radii. The observed emission intensity distribution can be reproduced using our analytical model that has three parameters: exobase temperature, exobase density, and solar radiation pressure, which implies that hot hydrogen production in the magnetized plasmasphere is not the dominant process shaping the outer hydrogen exosphere. However, the role of the magnetic effect in determining the total escape flux cannot be ruled out

Accepted: 2017-01-11, 資料番号: SA1160265000

International audience; The Earth's hydrogen exosphere Lyman‐α radiation was mapped with the SWAN/SOHO instrument in January 1996, 1997 and 1998 (low solar activity). The use of a hydrogen absorption cell allowed to disentangle the interplanetary emission from the geocoronal one and to assign the absorbed signal almost entirely to the geocorona. The geocorona was found to extend at least up to 100 Earth Radii (RE) with an intensity of 5 Rayleigh, an unprecedented distance well exceeding the recent results of LAICA imager (∼50 RE), and encompassing the orbit of the Moon (∼60 RE). We developed a numerical kinetic model of the hydrogen atoms distribution in the exosphere which includes the solar Lyman‐α radiation pressure and the ionization. The radiation pressure compresses the H exosphere on the day side, producing a bulge of H density between 3 and 20 RE which fits observed intensities very well. The SWAN Lyman‐α distribution of intensity was compared both to LAICA (2015) and to OGO‐5 (1968) measurements. Integrated H densities of SWAN at a tangent distance of 7 RE are larger than LAICA/OGO‐5 by factors 1.1‐2.5, while we should expect a stronger effect of the radiation pressure at solar max. We discuss the possible role of H atoms in satellite orbits to explain this apparent contradiction. An onion‐peeling technique is used to retrieve hydrogen number density in the exosphere for the three SWAN observations. They show an excess of density versus models at large distances, which is likely due to non‐thermal atoms (not in the model).

Recording the tweeks with a maximum up to eight harmonics using the receiver installed at Tay Nguyen University (12.65° N, 108.02° E) during 2013–2014, we investigated the morphology of the nighttime D-region ionosphere. Tweeks were recorded on 5 quiet nights per month. The results show that the mean reflection height in 2014 ( R z = 79.3 ) is lower by 3.3 km than that in 2013 ( R z = 64.9 ). The reflection height at low latitudes is higher than that at high latitudes. The mean reference height h ′ in 2013 is higher about 0.9 km than that in 2014 and the mean sharpness factor β in 2013 is higher by 0.07 km−1 than that in 2014. The short-term variation of reflection heights for tweeks with harmonics m = 1–3 and sunspot number have the negative correlation coefficients. However, the correlations between them are not clear. On some nights, from 19:00–21:00 LT, the reflection height temporal variability shows a moderate to strong negative correlation with the tweek occurrence. This suggests that the reflection height variation may be caused by QE fields generated by lightning discharges. The variations of tweek reflection heights observed during 2013–2014, at low latitudes could be significantly caused by the ionization effect by Lyman- α and Lyman- β coming from geocorona, variation of neutral density, particle precipitations, and by direct energy coupling between lightning and lower ionosphere.

Geocoronal H emission data acquired by NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission are analyzed to quantify the H density distribution over the entire magnetosphere ionosphere thermosphere region in order to investigate the response of the atmospheric system as a whole to geomagnetic storms. It is shown that at low and mid latitudes the H density averaged over storm times in the thermosphere-exosphere transition region decreases by ∼30%, while the H density at exospheric altitudes above ∼1-2RE increases by up to ∼40% relative to quiet times. We postulate that enhanced ion-neutral charge exchange in the topside ionosphere and inner plasmasphere is the primary driver of the observed H redistribution. Specifically, charge exchange reactions between H atoms and ionospheric/plasmaspheric O+ leads to direct H loss, while those between thermal H and H+ yield kinetically energized H atoms which populate gravitationally bound satellite orbits. The resulting H density enhancements in the outer exosphere would enhance the charge exchange rates in the ring current and the associated energetic neutral atom production. Regardless of the underlying mechanisms, H redistribution should be considered as an important process in the study of storm-time atmospheric evolution, and the resultant changes in the geocoronal H emissions potentially could be used to monitor geomagnetic storms.

[1] Satellite-based measurements of geocoronal Lyman α (Lyα) emission at 121.6 nm, created through multiple scattering of solar Lyα photons by atomic hydrogen, offer a valuable means of inferring the hydrogen abundance, [H], in the terrestrial thermosphere and exosphere on a global, long-term basis. We present initial results from an analysis of Lyα radiance measurements acquired across the Earth's limb from 2002 to 2007 by the Global UltraViolet Imager (GUVI) onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) spacecraft. This data spans nearly half of a solar cycle, and both the absolute Lyα radiance as well as its relative variation across the limb are shown to exhibit a significant dependence on solar activity. We describe sensitivities of a forward radiative transport (RT) model to key parameters governing the [H] distribution in order to assess implications for [H] estimation from the GUVI limb scan data throughout the solar cycle. Based on data-model comparisons, we conclude that the observed solar cycle variability is indicative of a decrease in dayside H density at the exobase with increasing solar activity. These results, along with additional forward RT modeling based on NRLMSISE-00 model specification of [H], are also used to assess contemporary semiempirical model accuracy.

Long-term ring current decay following a magnetic storm is mainly due to charge exchange collisions of ring current ions with geocoronal neutral atoms forming energetic neutral atoms (ENAs) that leave the ring current system. Therefore, the density distribution of these cold and tenuous neutral hydrogen atoms plays a key role in the ring current recovery. TWINS ENA images provide a direct measurement of these ENA losses and therefore insight into the dynamics of the ring current decay through interactions with the geocorona. To assess the influence of geocoronal neutrals on ring current decay, we compare the predicted ENA emission using five different geocoronal models and the HEIDI ring current model to simulate the July 22, 2009 storm. We show that for high energy H + ( ≥ 100 keV ) , all geocoronal models predict similar decay rates of the ring current ions. However, for low energy ions ( ≤ 100 keV ), the decay rate varies significantly depending on the geocoronal density model. Comparison with TWINS ENA images shows that the location of the peak ENA enhancements is highly dependent on the distribution of geocoronal hydrogen density. The ring current topology depends greatly on the hydrogen model used, therefore knowing the H-distribution is very important in understanding how the ring current recovers following a magnetic storm.

[1] The dominant neutral constituent in Earth's upper exosphere, atomic hydrogen (H), resonantly scatters solar Lyman-alpha (121.567 nm) radiation, observed as the geocorona. We report here observations of an exospheric response to geomagnetic storms obtained using measurements of the geocorona by Lyman-alpha detectors on the Two Wide-angle Imaging Neutral-atom Spectrometers mission. We introduce a new parameter, NH, the number of H atoms in the spherical shell from a geocentric distance of 3 to 8 Earth radii, to quantitatively characterize in a simplified way global exospheric conditions. Five geomagnetic storms observed during three months in the second half of 2011 are accompanied by abrupt temporary increases, spikes, of NH from 6% to 17%, lasting not longer than a day. These increases seem to show some correlation with the minimum Dst index reached during the peak of each storm.

Cascade contributions to geocoronal Balmer α airglow line profiles are directly proportional to the Balmer β/α line ratio and can therefore be determined with near simultaneous Balmer β observations. Due to scattering differences for solar Lyman β and Lyman γ (responsible for the terrestrial Balmer α and Balmer β fluorescence, respectively) there is an expected trend for the cascade emission to become a smaller fraction of the Balmer α intensity at larger shadow altitudes. Near coincident Balmer α and Balmer β data sets, obtained from the Wisconsin Hα Mapper (WHAM) Fabry–Perot, are used to determine the cascade contribution to the Balmer α line profile, and to show, for the first time, the Balmer β/α line ratio, as a function of shadow altitude. We show that this result is in agreement with direct cascade determinations from Balmer α line profile fits obtained independently by high resolution Fabry–Perot at Pine Bluff, WI. We also demonstrate with radiative transport forward modeling that a solar cycle influence on cascade is expected, and that the Balmer β/α line ratio poses a tight constraint on retrieved aeronomical parameters (such as hydrogen's evaporative escape rate and exobase density). Index Terms: Hydrogen Geocorona, Balmer-alpha Line Profile, Fabry–Perot Interferometry

The structure of energetic protons from the Earth radiation belts, averaged for a magnetically quiet period, can be explained by the equilibrium between the radial diffusion transfer, loss due to Coulomb collisions, charge exchange with ambient neutral hydrogen of the geocorona, and drift of protons under the influence of magnetospheric convection. By transfer we mean diffusion owing to fluctuations related to substorms in the large-scale electric and magnetic fields. Equatorially mirroring protons with energies of 1–750 keV have been considered, and the theoretical predictions of the proton energy spectra for L = 1.0−6.6 have been compared with the observations on several satellites.

X-ray emission due to charge transfer collisions between heavy solar wind ions and neutrals has been predicted to exist both in the heliosphere and in the geocorona. The heliospheric X-ray emission can account for roughly half of the observed soft X-ray background intensity. It was also suggested that temporal variations in the heliospheric and geocoronal soft X-ray intensities will result from solar wind variations. In this paper, a simple model of the charge exchange X-ray emission mechanism is combined with measured solar wind parameters as a function of time and used to generate predictions of the temporal variation of the X-ray intensity observed at Earth for the time periods 1990–1993 and 1996–1998. Measured solar wind proton fluxes are also directly compared with the “long-term enhancement” part of the soft X-ray background measured by the Rontgen Satellite (ROSAT). A significant positive correlation exists, which supports the existence of X-ray emission associated with the solar wind interaction with either interstellar neutrals and/or with geocoronal neutral hydrogen.

Balmer α intensity measurements made with a dual etalon Fabry–Perot spectrometer at Haleakala during two campaigns in 1988 are presented. The data from each campaign demonstrate night-to-night stability, despite variations in geophysical conditions. Analysis of these data using a nonisothermal Lyman β radiative transport code, updated solar Lyman β line-center flux estimates, and corrected thermospheric atomic hydrogen density profiles points to the resolution of the “factor of 2” problem. A careful reassessment of other mechanisms for upper atmospheric Balmer α excitation has also been carried out.

Thermospheric density and temperature are often derived from ionospheric observables measured by incoherent scatter radar (ISR) via solutions of the F region ion energy balance equation. However, this equation, consisting of an electron heat source balancing a neutral heat sink, can break down near the exobase, where the electron heat source can be too small by as much as 40%. An additional heat source is required, and theoretical and experimental studies point to a hot neutral oxygen (hot O) geocorona near the Earth's exobase to supply it. Hot O must therefore be included in the ion energy balance equation; however, its profile shape and concentration are unknown. We develop a simple method for including hot O profiles in the ion temperature fitting equation by calculating hot O concentration as a function of altitude and including the hot O heat source in the ion energy equation. The technique is tested as a function of solar cycle for March equinox conditions, and a χ2 analysis indicates that the hot O is more likely to form a layer than a concentration profile which decreases as a function of altitude.

The Naval Research Laboratory far ultraviolet experiment on board the OGO 4 satellite accidentally detected hydrogen Lyman α emission following the passage of missiles through the upper atmosphere. The deposition of hydrogenous exhaust products above 100 km results in a local enhancement of hydrogen, which resonantly scatters Lyman α radiation from the Sun and from the geocorona, and produces a far ultraviolet contrail. The observed local enhancement of radiation was as much as 37% above the natural background. All event detections took place at time intervals of 6 min to 10 hours following the passage of a missile or spacecraft. There was no opportunity to observe prompt emissions. We describe the observations of this unusual phenomenon and the implications for hydrogen production mechanisms.

Densities of hot (100 eV{endash}40 keV) ions vary with local time at geosynchronous orbit. Typically, the density is maximum near midnight, falls off toward both terminators, and is minimum near noon. This local time dependence is routinely observed in the Los Alamos geosynchronous plasma data. We use a month of these data (June 1996) and a simple model of particle drifts within the magnetosphere to explore its origins. We first show that the nightside density profile is shaped according to the drift path topology: trapped orbits are depleted, and only particles drifting on open trajectories contribute to this population, particularly for observations taken at times of low K{sub p}. Then, we highlight spectral absorption in dayside fluxes, which occurs when the particles pass through the geocorona. A model of charge exchange accounts for the shape and energy range of the absorption signatures observed at low K{sub p} for varying pitch angles. The density absorption computed versus local time matches the observations. Hence these basic principles of particle motion in the terrestrial magnetosphere readily explain the local time dependence of the hot ion density at geosynchronous orbit. {copyright} 1998 American Geophysical Union

We present results from EUVE measurements obtained during the all-sky survey of interplanetary and geocoronal He I 584-A emission. The data consist of count rates from the long wavelength spectrometer and the long wavelength photometric band (520–750 A) of scanner C obtained over a 1-year period from July 1992 to July 1993. During this period, EUVE was in survey mode so that the scanners made 5° × 360° sweeps of the sky in a plane perpendicular to the Sun-Earth line, while the spectrometers were aligned with the antisolar direction. The interplanetary He I signal is morphologically consistent with previous observations with similar observing geometry, such as Prognoz 6 [Dalaudier et al., 1984]. However, unlike the Prognoz 6 data, the EUVE measurements were made from low Earth orbit (520 km) and so contain geocoronal emission as well. As a result, along sight lines where the relative speed between the interplanetary wind and the Earth is at a minimum, extinction of the interplanetary signal through resonance scattering by the He geocorona occurs. We believe this to be the first detection of line extinction of the local interstellar He wind emissions by the geocorona. We find that the geocoronal extinction signatures provide a new means of determining the interstellar He wind vector and emission line profile and add further constraints on the values of other interplanetary and solar He parameters and the morphology of the He geocorona. On the basis of model fits of the observed interplanetary emission and geocoronal extinction, we determine values for the interplanetary wind ecliptic longitude λ = 76.0 ± 0.4°, latitude ϕ = −5.4 ± 0.6° (downwind direction), speed υ0 = 26.4 ±1.5 km s−1, and temperature T0 = 6900 ± 600 K. In addition, assuming an interplanetary He density of 0.01 cm−3, we determine an average solar He I 584-A line center flux of 1.4 ± 0.3 × 1010 photons cm−2 s−1 A−1 for the data analyzed here.

Energetic neutral atoms are created when energetic magnetospheric ions undergo charge exchange with cold neutral atoms in the Earth's tenuous extended atmosphere (the geocorona). Since they are unaffected by the Earth's magnetic field, these energetic neutrals travel away in straight line trajectories from the points of charge exchange. The remote detection of these particles provides a powerful means through which the global distribution and properties of the geocorona and ring current can be inferred. Due to its 2 × 9 RE polar orbit, the Polar spacecraft provides an excellent platform from which to observe ENAs because it spends much of its time in the polar caps which are usually free from the contaminating energetic charged particles that make observations of ENAs more difficult. In this brief report, we present the first ENA imaging results from Polar. Storm-time ENA images are presented for a northern polar cap apogee pass on August 29, 1996 and for a southern polar cap perigee pass on October 23, 1996. As well, we show with a third event (July 31, 1996) that ENA emissions can also be detected in association with individual substorms.

Energetic neutral atoms (ENAs) were detected by the Energetic Particles and Ion Composition (EPIC) instrument on the Geotail spacecraft during a magnetic storm on October 29–30, 1994. The energetic particles could be identified as neutrals because the direction of the particle flux steadily tracked the direction of the Earth and was uncorrelated with the changing orientation of the ambient magnetic field as the spacecraft viewed the dayside magnetosphere for ∼ 16 hours. These observations provide the first composition measurements of ENAs, yielding the storm-time evolution of ENA fluxes of hydrogen, helium, and oxygen separately as well as their energy spectra. ENA fluxes and the rate of recovery of Dst are both roughly steady, consistent with charge-exchange being a significant energy loss process for the storm-time ring current. For total energies above 200 keV, the intensity of ENA oxygen is the highest, followed by hydrogen, and then by helium. These observations enable us to deduce the line-of-sight (LOS) integrals of the products nH jion, where nH = hydrogen geocorona density, jion = differential ion flux for ion species H+, He+, and O+ at energies between ∼60 and ∼600 keV.

Energetic neutral atoms (ENAs) were detected by the Energetic Particles and Ion Composition (EPIC) instrument on the Geotail spacecraft during a magnetic storm on October 29–30, 1994. The energetic particles could be identified as neutrals because the direction of the particle flux steadily tracked the direction of the Earth and was uncorrelated with the changing orientation of the ambient magnetic field as the spacecraft viewed the dayside magnetosphere for ∼16 hours. These observations provide the first composition measurements of ENAs, yielding the storm-time evolution of ENA fluxes of hydrogen, helium, and oxygen separately as well as their energy spectra. ENA fluxes and the rate of recovery of Dst are both roughly steady, consistent with charge exchange being a significant energy loss process for the storm-time ring current. For total energies above 200 keV, the intensity of ENA oxygen is the highest, followed by hydrogen, and then by helium. These observations enable us to deduce the line-of-sight (LOS) integrals of the products nHjion and nHfion, where nH=hydrogen geocorona density, jion=differential ion flux, and fion=phase space density (PSD), for ion species H+, He+ and O+ at energies between ∼60 and ∼600 keV. A ∼50–60 keV Maxwellian fits the O+ LOS PSD fairly well but fits the He+ and the H+ only poorly.

Charge exchange (CE) collisions between ring current ions and geocoronal atoms erode the ring current and give rise to a pervasive flux of energetic neutral atoms (ENAs). ENAs generated in the main ring current traversing the inner magnetosphere can be reionized in several ways, converting ENAs back into ring current ions albeit on new L shells (provided the angle between the direction of ENA travel and the local magnetic field direction at the point of collision corresponds to a pitch angle outside the local loss cone). These displaced ring current ions in turn undergo subsequent CE collisions with geocoronal atoms, generating secondary ENA fluxes that can participate in further ionizing collisions. A modeling scheme for exploring the effectiveness of this mechanism in establishing and maintaining a secondary ring current at L ≲ 3 has been developed taking full account of multiple CE and ionizing collisions. It is based on a matrix solution technique for an integral form of the Boltzmann transport equation and relies on two simplifications: (1) Since gyroperiods ≪ bounce periods, gyroradii ≪ scale lengths for variations in geocoronal and plasmaspheric densities, and single-bounce collisional depths along ion gyropaths ≪ 1, it is adequate to work in terms of quantities integrated along bounce paths and averaged in gyrophase. (2) The thermal populations are not directly perturbed by the energetic populations, while the sources of the main ring current are external to the charge-exchange-coupled systems, so that the transport equation for the secondary flux is linear. The results are steady state fluxes of trapped ions as functions of L shell, equatorial pitch angle, and energy. This model is described in full, and an example of steady state secondary H+ ring current fluxes for quiet conditions is presented using the quiet time empirical model of Sheldon and Hamilton [1993] to prescribe the main proton ring current.

The Hydrogen Deuterium Absorption Cell (HDAC) is part of the Ultraviolet Imaging Spectrograph (UVIS) experiment aboard the Cassini spacecraft. During Cassini’s Earth swingby on August 18, 1999, HDAC was used as a photometer to measure solar Lyman-α radiation scattered by the neutral hydrogen atoms of the geocorona. These data provide information about the hydrogen column density along HDAC’s line of sight during the Earth swingby. The data cover a large part of the Earth’s exosphere and enable us to determine the distribution of hydrogen atoms in this region. The results are compared to predictions of models providing hydrogen densities of the Earth’s exosphere/geocorona. We found that the models overestimate the decrease of hydrogen density for increasing distance from the Earth.

A model of the oxygen geocorona near the exobase solving the nonlinear Boltzmann equation with a Monte Carlo method is used to calculate the distribution of the hot oxygen atoms during geomagnetically disturbed nighttime conditions. The precipitation of energetic O+ ions and the subsequent enhancement of the hot O corona at high latitudes is simulated for the September 17, 1971, storm conditions. It is found that in such circumstances, the O+ precipitation is a significant source of superthermal O atoms leading to important perturbations of the velocity distribution of the bulk oxygen population. The effective gas temperature near the exobase is similar to that in the undisturbed atmosphere, but the hot O density rises considerably over the quiet condition values.

Exothermic reactions involving metastable neutrals and ions were recently proposed as sources of hot oxygen atoms in addition to the classical O2+ and NO+ dissociative recombination. The Boltzmann equations for thermal and nonthermal populations of O atoms are solved with a Monte Carlo stochastic simulation method. It is shown that the calculated energy distribution functions of O atoms are significantly in nonequilibrium in the transition region between the thermosphere and the exosphere. It is found that the inclusion of additional sources leads to stronger disturbances of the energy distribution function and, as a consequence, increases the nonthermal fraction of hot O atoms. The variation of the vertical distribution of hot O between solar maximum and minimum conditions is also evaluated and shows good agreement with the available experimental evidence.

[1] High spectral resolution line profile observations indicate a reproducible semi-annual variation in the geocoronal hydrogen Balmer α effective temperature. These observations were made between 08 January 2000 and 21 November 2001 from Pine Bluff Observatory (WI) with a second generation double etalon Fabry-Perot annular summing spectrometer operating at a resolving power of 80,000. This data set spans sixty-four nights of observations (1404 spectra in total) over 20 dark-moon periods. A two cluster Gaussian model fitting procedure is used to determine Doppler line widths, accounting for fine structure contributions to the line, including those due to cascade; cascade contributions at Balmer α are found to be 5 ± 3%. An observed decrease in effective temperature with increasing shadow altitude is found to be a persistent feature for every night in which a wide range of shadow altitudes were sampled. A semiannual variation is observed in the column exospheric effective temperature with maxima near day numbers 100 and 300 and minima near day numbers 1 and 200. Temperatures ranged from ∼710 to 975 K. Average MSIS model exobase temperatures for similar conditions are approximately 1.5× higher than those derived from the Balmer α observations, a difference likely due to contributions to the observed Balmer α column emission from higher, cooler regions of the exosphere.

This paper investigates new sources of thermospheric non thermal (hot) oxygen due to exothermic reactions involving numerous minor (ion and neutral) and metastable species. Numerical calculations are performed for low altitude, daytime, winter conditions, with moderately high solar activity and low magnetic activity. Under these conditions we find that the quenching of metastable species are a significant source of hot oxygen, with kinetic energy production rates a factor of ten higher than those due to previously considered O2(+) and NO(+) dissociative recombination reactions. Some of the most significant new sources of hot oxygen are reactions involving quenching of O(+)((sup 2)D), O((sup 1)D), N((sup 2)D), O(+)((sup 2)P) and vibrationally excited N2 by atomic oxygen.

[1] We investigated, for the first time, long-term (1976–2010) variations in reflection heights of tweek atmospherics based on very low frequency (VLF) observations at Kagoshima, Japan. The results revealed the effects of the solar cycle on the nighttime lower ionosphere at low to middle latitudes. The tweek reflection heights on geomagnetically quiet days were analyzed every month over three solar cycles by using an automated spectral fitting procedure to estimate the cutoff frequency. The average and standard deviation of the reflection height were 95.9 km and ±3.1 km, respectively. Typical periods of time variation for the reflection height were 13.3, 3.2, 1.3, 1.0, 0.6, and 0.5 years. The variations in tweek reflection heights did not show simple anticorrelation with solar activity. The correlation coefficient between tweek reflection height and sunspot number was 0.03 throughout the three solar cycles. Hilbert-Huang transform analysis successfully indicated the presence of 0.5–1.5 year and ∼10 year variations as intrinsic mode functions (IMF). The decomposed IMF with the ∼10 year variation had a positive correlation with sunspot numbers and a negative correlation with galactic cosmic rays (GCRs). We hypothesize that these variations in tweek reflection heights could be caused by coupling of several ionization effects at the D and lower E regions, effects such as geocorona, GCRs, particle precipitation, and variations in neutral density in the lower thermosphere. Among these processes, the geocorona and particle precipitation could show negative correlation, while the GCRs and neutral density could show positive correlation with solar activities.

[1] Exospheric atomic hydrogen (H) resonantly scatters solar Lyman-α (121.567 nm) radiation, observed as the geocorona. Measurements of scattered solar photons allow one to probe time-varying three-dimensional distributions of exospheric H atoms. The Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) mission images the magnetosphere in energetic neutral atom (ENA) fluxes and additionally carries Lyman-α detectors (LADs) to investigate exospheric atomic hydrogen. Knowledge of exospheric properties is essential for the interpretation of magnetospheric images in ENA fluxes produced in charge exchange between energetic ions and H atoms. We describe the process of obtaining exospheric distributions and present, as an example, a global H number density distribution for 11 June 2008. The TWINS LAD experimental data are especially sensitive to atomic hydrogen at geocentric distances from 3 to 8 Earth radii. The distribution reveals asymmetries from day to night, dawn to dusk, and north to south. We discuss the available data sets and their coverage, limitations, and promise for a study of exospheric response to seasonal, solar, and geomagnetic variations.

[1] The magnetospheric cusps separate closed dayside magnetospheric field lines from open field lines of the magnetotail mantle and lobes. All magnetospheric field lines that map to the magnetopause also pass through the cusp regions. Thus whenever magnetic reconnection occurs at the magnetopause, magnetosheath plasma can enter one or both of the cusp regions and charge exchange with the geocorona. The resulting energetic neutral atoms (ENAs) resulting from this charge exchange process propagate away from the cusps and are observed remotely by the Interstellar Boundary Explorer (IBEX). The asymmetry of the ENA intensities between the northern and southern cusps are strongly dependent upon the Earth's dipole tilt angle and are consistent with in situ cusp observations. These asymmetric fluxes in the cusp regions are suggested to be explained by the regions at the magnetopause where magnetic reconnection is expected.

A sounding rocket measurement of the ultraviolet, atomic oxygen dayglow reveals an excess of emission compared to standard thermospheric model calculations at exospheric altitudes. We explore two explanations for this discrepancy: a breakdown of the radiative transfer model due to nonlocal thermal equilibrium (non-LTE) conditions above the exobase and a hot atomic oxygen geocorona. In particular, the effects of non-LTE on the 3P2, 1, 0 sublevel populations are modeled, and a hot O component in the upper thermosphere and lower exosphere is added to investigate the effects on the modeled emissions. For both cases, the data are reanalyzed and compared with the results using a standard LTE model. A hot O geocorona having a peak density of 10(exp 6)/cc at 550 km and a temperature of 4000 K is consistent with the data and appears to be the most reasonable explanation of the high-altitude enhanced emissions observed in the data.

[1] The 11-year solar cycle is a dominant source of natural variability in the upper atmosphere, and its effect on atomic hydrogen distributions and emissions must be understood to investigate possible signs of longer-term climatic trends in this region. We present midlatitude geocoronal hydrogen Balmer a observations from solar cycle 23 (1997-2006) and three solar minimum periods, 1985, 1997, and 2006. The 1997 through 2006 observations were taken with the Wisconsin H-a Mapper Fabry-Perot (WHAM), a ground-based CCD-annular summing instrument that began observations at the Kitt Peak Observatory in Arizona in 1997. The 1985 observations were made with a similarly designed "pre-WHAM" Fabry-Perot Interferometer utilizing photomultiplier detection and located in Wisconsin. WHAM has consistently observed higher column emission intensities during solar maximum periods than during solar minimum conditions, with the ratio dependent upon the viewing geometry. The observations from three solar minimum periods agree to within 18% uncertainties over most of the shadow altitude range. An analysis of recent Fabry-Perot observations of upper atmospheric hydrogen during solar cycle 23 and during three solar minima (1985, 1997, 2006) established a reference data set of highly precise, consistently calibrated, thermospheric plus exospheric hydrogen column emission observations from northern midlatitudes that can be used to compare with future observations.

This review examines models existing in the literature which describe the magnetic field produced by the ring current (DR) at the Earth's surface based on the energy balance equation. The parameters of this equation, the injection function F and decay parameter τ are considered to depend on parameters of the interplanetary medium and the DR intensity. The existing models are shown to be able to describe the DR variations with sufficient accuracy (r.m.s. deviation δ between the experimental and modelled values of DR for 170 magnetic storms is 5 < δ < 15 nT and the correlation coefficient between the two is 0.85

[1] X rays are produced throughout the terrestrial magnetosheath as a consequence of charge transfer collisions between heavy solar wind ions and exospheric neutrals. The solar wind ions resulting from these collisions are left in highly excited states and emit extreme ultraviolet or soft X-ray photons. We previously simulated X-ray images of the magnetosheath as seen from an observation point outside the geocorona for average solar wind conditions. The locations of the bow shock and magnetopause were evident in these images, but the cusps were not taken into account. For the current paper we used dynamic three-dimensional MHD simulations of the solar wind, magnetosheath, and magnetosphere for the 31 March 2001 geomagnetic storm. A sky map was generated of the expected X-ray emissions as seen by a hypothetical X-ray detector on the IMAGE spacecraft. Modeled images as seen from an observation point well outside the geocorona were also created. The cusps can clearly be detected in both types of simulated images. Images of the magnetosheath in energy neutral atoms (ENA) also show the cusps. X-ray imaging of the magnetosheath, revealing the structure of the magnetopause and the bow shock, if carried out, could potentially make a valuable contribution to our understanding of the solar wind interaction with the magnetosheath.

[1] The role of the auroral sources induced by the electron and proton precipitation in the formation of the hot oxygen corona in the polar upper atmosphere is studied. It is found that both electron precipitation through exothermic chemistry and proton precipitation through atmospheric sputtering significantly contribute to the population of the hot oxygen geocorona. It is also found that only atmospheric sputtering results in the formation of the escape flux of energetic oxygen atoms, providing an important source of heavy atoms for the magnetosphere. The exothermic chemistry induced by the electron precipitation and/or by the absorption of the solar UV radiation is operating continuously in the polar upper atmosphere and results in a steady population of the very near-Earth environment by suprathermal oxygen atoms with energies below a few eV. By contrast, atmospheric sputtering by magnetospheric protons provides a more variable contribution, strongly coupled with the cusp region. It produces the more energetic oxygen atoms that populate the external regions of the hot oxygen geocorona. The results of calculations are in a good agreement with the analysis of the low-latitude perigee Low Energy Neutral Atom (LENA) images showing that the instrument signal consists of low to medium energy (5–30 eV) oxygen atoms produced in and near the cusp region. The more energetic (>30 eV) fraction of energetic oxygen atoms produced by the ioninduced atmospheric sputtering could be responsible for the energetic neutrals observed by the instrument far away from the cusp or oval regions. The total escape flux of oxygen atoms associated with atmospheric sputtering by protons is found about 8 � 10 23 s � 1 ; therefore this mechanism may provide a substantial contribution to the magnetospheric oxygen population.

[1] The high-energy proton-hydrogen (H+/H) beam associated with proton auroral precipitation transfers momentum in elastic and inelastic collisions with ambient thermal atomic oxygen in the high latitude thermosphere. This process provides a localized novel source of hot oxygen atoms in addition to exothermic photochemistry, charge exchange and momentum transfer from O+ ion precipitation and charge exchange with accelerated ionospheric O+ ions. We suggest that this source contributes to the population of the hot oxygen geocorona and to the flux of escaping oxygen atoms. For an incident proton energy flux of 1 mW m−2 and a mean energy Emean ≈ 5 keV, we calculate a density of hot oxygen atoms with energy above 1 eV of 2.0 × 103 cm−3 and a mean kinetic energy of about 3.5 eV at 700 km. The total upward flux of hot oxygen atoms with energies higher 1 eV is estimated as 3.5 × 108 cm−2 s−1.

[1] Analysis of ENA data from the LENA instrument on the IMAGE spacecraft shows that the terrestrial atmosphere is a copious emitter of energetic neutral atoms (

[1] Radiative recombination of O+ generates a variety of O-atom Rydberg levels, which then radiatively cascade to the ground state. The last two steps in the cascade produce 844.6 nm and 130.2–130.6 nm radiation in the triplet set of Rydberg levels and 777.2–777.5 nm and 135.6 nm radiation in the quintets. These four emissions are well known from ionospheric observations, and the higher members of the cascades are sometimes seen in auroral spectra but rarely in the nightglow. Sky spectra taken in 2000 at the Keck II telescope with the ESI echelle spectrograph reveal 20 members of the Rydberg transitions in the 390–930 nm region not previously observed in the nightglow. Most of these are not seen in earlier Keck spectra, taken at solar minimum. We are able to quantify the relative intensities of the emissions across the entire wavelength region using standard star spectra, allowing comparisons to be made with theoretical calculations. As the atmosphere is optically thin for the quintet transitions, corrections for radiative scattering are not necessary, and relative agreement between theory and observations is excellent. For the triplets the data agree more closely with the optically thick calculations. The relationship between the intensities of the quintet 3d-3p (926–926.6 nm) and the triplet 3p-3s (844.6 nm) multiplets is investigated and found to be linear. Accurate wavelengths are determined for the Rydberg quintets on the basis of observations with the high-resolution (∼40,000) Keck I/HIRES system. For the triplets the multiplets are incompletely resolved. Examples of spectra are presented where the 844.6 nm multiplet is the only OI Rydberg emission observed. We argue that the cause is Bowen fluorescence, a consequence of photoexcitation of the OI 3d-2p transition by H Lyman-β radiation in the geocorona. The fact that so many of the oxygen Rydberg lines have been identified in this study and that their intensities are consistent with cascade models of radiative recombination is a strong confirmation of the importance of this process in the tropical nightglow near solar maximum.

[1] X-rays are generated throughout the terrestrial magnetosheath as a consequence of charge transfer collisions between heavy solar wind ions and geocoronal neutrals. The solar wind ions resulting from these collisions are left in highly excited states and emit extreme ultraviolet or soft X-ray photons. A model has been created to simulate this X-ray radiation. Published terrestrial exospheric hydrogen distributions and solar wind speed, density and temperature distributions were used in this model. Simulated images were created as seen from an observation point outside the geocorona. The locations of the bow shock and magnetopause are evident in these images. Perhaps this X-ray emission can be used to remotely sense the solar wind flow around the magnetosphere. Since similar X-rays are produced in the heliosphere, the challenge will be, however, to eliminate this background emission.

[1] During geomagnetic storms a well defined belt of trapped protons and ENAs (energetic neutral atoms) is observed around geomagnetic equator at low L-values. Their source is RC (ring current) protons existing at larger L-values. Through charge exchange with the geocorona RC protons become ENAs and if subjected to a new charge exchange become trapped protons. From low latitude particle observations at four different local times we follow; the RC injection region, the drift of RC-particles through the evening/afternoon into the morning sector, the RC-asymmetry and convection loss to the dayside during the storm initial and main phase, and its development into a symmetric RC in the recovery phase of the storm. INDEX TERMS: 2778 Magnetospheric Physics: Ring current; 2720 Magnetospheric Physics: Energetic particles, trapped; 2730 Magnetospheric Physics:Magnetosphere— inner; 2788 Magnetospheric Physics: Storms and substorms. Citation: Soraas, F., K. Oksavik, K. Aarsnes, D. S. Evans, and M. S. Greer, Storm time equatorial belt – an ‘‘image’’ of RC behavior, Geophys. Res. Lett., 30(2), 1052, doi:10.1029/ 2002GL015636, 2003.

[1] During the summer of 2000 the LENA instrument on the IMAGE spacecraft collected a unique data set of low-energy, neutral atom emissions. The data suggests an ever-present flux of low energy (

[1] X-rays are generated throughout the heliosphere and the terrestrial magnetosheath as a consequence of charge transfer collisions between heavy solar wind ions and interstellar and geocoronal neutrals, respectively. Simple models of this X-ray emission have been presented in the past, but results from a more sophisticated model are described in this paper. In particular, to obtain the densities of interstellar neutrals our X-ray model uses to determine X-ray intensities, we use the Fahr [1971] hot model although with recently determined input parameters. The geocoronal X-ray model we use in this paper has also been improved, as described in a recent paper by Robertson and Cravens [2003]. In previous papers we demonstrated that there is significant correlation between the “long-term enhancement” part of the soft X-ray background measured by the Rontgen Satellite (ROSAT) and the solar wind proton flux. In the current paper we determine the steady-state X-ray intensities due to the interaction between the solar wind and both interstellar neutrals and the geocoronal neutrals as a function of look direction and time of year. X-ray intensity maps are shown for both a spherically symmetric solar wind and for a latitude-dependent solar wind (i.e., fast and slow solar wind regions). In all cases, the X-ray intensity is highest when the view direction is towards the Sun, but the intensity is also relatively high for view directions intersecting the gravitational focusing cone of interstellar helium. We also show a heliospheric/geocoronal X-ray intensity map for the conditions used by Snowden et al. [1995] in producing the 1/4 keV channel soft X-ray background map in galactic coordinates. Our preliminary conclusion is that very roughly 50% of the total background soft X-ray intensity in the galactic plane and 25% at high galactic latitudes can be attributed to the charge transfer process operating within the solar system, with the remaining emission coming from outside our heliosphere.

[1] In a previous study we performed a statistical analysis of the geosynchronous plasma environment from measurements of the magnetospheric plasma analyzer instruments aboard the Los Alamos geosynchronous spacecraft. In that study the mean spin-averaged particle fluxes were determined as a function of local time and Kp index, averaged over an entire year of observations. Particles on open drift trajectories should cross geosynchronous orbit twice during their drift from the nightside plasma sheet, through the inner magnetosphere, and out to the dayside magnetopause. The ratio of incoming plasma sheet phase space density and outgoing dayside phase space density of every drift path thus contains information about particle losses during the drift through the near-Earth region. For ions the losses are largely caused by charge exchange reactions with hydrogen atoms. Applying tomographic inversion techniques, we use the observed statistical losses inside the geosynchronous orbit region to infer the spatial distribution of exospheric neutral hydrogen. The particle drift paths from the nightside of geosynchronous orbit to the dayside are calculated from electric and magnetic field models. To test the sensitivity of the tomography to the convection model, we invert the geosynchronous particle observations using various field model combinations and compare the results. We find that the neutral hydrogen densities obtained from the inversion disagree with existing models of the exosphere, mainly in the near-Earth region. These differences are due to lower-than-expected losses of lower-energy particles that nominally drift through the inner region and/or particle sources not considered in the study.

Kinetic modeling results are analyzed to examine the transport of photoelectrons through the nightside inner magnetosphere. Two sources are considered, those on the dayside from direct solar illumination and those across the nightside from light scattered by the upper atmosphere and geocorona. A natural filter exists on the nightside for the dayside photoelectrons. Coulomb collisions erode the distribution at low energies and low L shells, and magnetospheric convection compresses the electrons as they drift toward dawn. It is shown that for low-activity levels a band of photoelectrons forms between L = 4 and 6 that extends throughout the nightside local times and into the morning sector. For the scattered light photoelectrons the trapped zone throughout the nightside is populated with electrons of E less than 30 eV. At high L shells near dawn, convective compression on the nightside yields an accelerated population with electrons at energies up to twice the ionospheric energy maximum (that is, roughly 1200 eV for dayside photoelectrons and 60 eV for scattered light electrons). Modeled energy and pitch angle distributions are presented to show the features of these populations.

The impact of losses due to wave scattering on the global ring current evolution is an important unresolved question. The present study attempts to address this question by incorporating wave scattering (specifically due to ring current proton resonance with ion cyclotron waves) into the Ring Current - Atmosphere Interaction Model (RAM). RAM is a drift-loss model that follows the evolution of three major ring current ion species (H + , He + , and O + ) considering adiabatic drift motions, collisional interactions with the hydrogen geocorona and with a time-dependent plasmasphere, and pitch-angle scattering of protons in the fields of ion cyclotron waves. A time-dependent global model of convective wave gain was constructed to be consistent with velocity space distributions in the RAM model and with simulations of wave propagation and amplification in the HOTRAY warm plasma ray tracing code. The plasmapause density gradient was found to have a major impact on the path-integrated gain. A simple algorithm to connect wave gain to wave amplitudes was devised, based upon observed amplitudes of Pcl waves in the inner magnetosphere. A region of strong ion cyclotron wave activity forms just inside and along the plasmapause. The integrated energy loss from the ring current during the one-hour simulation interval, due to the scattering of protons into the loss cone, caused an additional ∼8 nT recovery in the Dst index and thus was found to be important to the global energy balance of the ring current. The corresponding globally-averaged energy loss time scale for H + due to wave scattering is ∼11 hours, somewhat longer than globally-averaged collisional loss time scales for this storm. A careful consideration of feedback of the waves on the ion distribution function is required before the full temporal history of the wave scattering losses can be estimated. The result depends sensitively on the assumed waved amplitudes.

Methods for Monte Carlo simulation of planetary exospheres have evolved from early work on the lunar atmosphere, where the regolith surface provides a well defined exobase. A major limitation of the successor simulations of the exospheres of Earth and Venus is the use of an exobase surface as an artifice to separate the collisional processes of the thermosphere from a collisionles exosphere. In this paper a new generalized approach to exosphere simulation is described, wherein the exobase is replaced by a barometric depletion of the major constitents of the thermosphere. Exospheric atoms in the thermosphere-exosphere transition region, and in the outer exosphere as well, travel in ballistic trajectories that are interrupted by collisons with the background gas, and by charge exchange interactions with ionospheric particles. The modified simulator has been applied to the terrestrial hydrogen exosphere problem, using velocity dependent differential cross sections to provide statistically correct collisional scattering in H-O and H-H(+) interactions. Global models are presented for both solstice and equinox over the effective solar cycle range of the F(sub 10.7) index (80 to 230). Simulation results show significant differences with previous terrestrial exosphere models, as well as with the H distributions of the MSIS-86 thermosphere model.

A model of the hot oxygen geocorona in the transition region near the exobase is described. It is based on a Monte Carlo solution of the nonlinear Boltzmann equation for hot oxygen atoms produced by chemical processes usually considered as a source of hot oxygen (photodissociation of O2 and dissociative recombination of O2+ and NO+ ions). The evolution of the system is described stochastically as a series of random Markovian processes. The energy distribution function of the thermal and non-thermal O(³P) atoms and of the nonthermal O(¹D) atoms is calculated from the thermospheric collision-dominated region to the exosphere where the gas flow is virtually collisionless. The model is applied to equatorial latitudes for conditions of low solar and geomagnetic activity. Numerical simulations show that the distribution function of thermal oxygen is increasingly perturbed by collisions with the hot oxygen population at high altitudes and departs significantly from a Maxwellian distribution at all altitudes. The number density and temperature of the nonthermal oxygen atoms are derived from their microscopic distribution function and found to be in qualitative agreement with previous theoretical and experimental estimates.

The polarization of the 584- and 304-A emissions of helium that have been resonantly scattered in the earth's atmosphere and in the interplanetary medium is discussed. It is shown that in the geocorona the simultaneous measurement of altitude profiles of the 584-A He dayglow intensity and polarization will test the hypothesis that the 584-A dayglow is excited by resonance scattering of sunlight from geocoronal helium. If this is indeed the major source of the 584-A dayglow emission, a simultaneous measurement of the polarization and the altitude profile of the intensity in the exosphere will uniquely determine the helium number density distribution as a function of altitude, and in the thermosphere it will alow a study of nondiffusive equilibrium processes. An attractive feature of this method is that the interpretation does not require a knowledge of the solar 584-A line center flux or line profile.

Data on ion and electron temperatures and concentrations to several thousand kilometers of altitude were obtained from the Atmosphere Explorer C satellite for 1974 and to 850 km from Arecibo incoherent scatter radar measurements. These data were used to normalize diffusive equilibrium profiles. From these profiles and by using the neutral atmospheric model of Jacchia (1971) and a new hydrogen model, the charge-exchange-induced neutral hydrogen escape fluxes for equatorial and middle latitudes were calculated. The data confirm earlier estimates that the charge exchange loss is more important than Jeans escape for the earth. It is also found that inside the plasmapause this charge exchange process with hot plasmapheric ions is the major production and loss process for the satellite population in the hydrogen geocorona.

Measurements of polarization and absorption by a hydrogen cell of the Lyman alpha geocoronal emission, performed on board the French spacecraft D2-A Tournesol over the working period of the tape recorder (April 19 to May 2, 1971), are compared with theoretical predictions given by spherical models of transfer using Kockarts-Nicolet and Chamberlain profiles. From polarization measurements the value of the hydrogen density at the exobase ρ500 and its latitudinal-diurnal variation are inferred. From absorption measurements, by using the exospheric temperature Tex of Jacchia, another independent way of deducing ρ500 variations is presented. Owing to calibration problems, an absolute measurement of Tex derived from absorption measurements is no longer available. However, Tex departures from the Jacchia temperatures are deduced from departures between the two values of ρ500 deduced from absorption and polarization measurements. Data obtained over the considered period of time provided ρ500 measurements independent of the solar flux. For example, the ρ500 density at noon near the equator is equal to (10 ± 2) 104 atoms cm−3. Intensity measurements at Ly α and 1304 A are also reported and are compared with the Oso 4 and 5 results. Measurements of the extrageocoronal Ly α emission are first made from the core of the geocorona. The values obtained during a satellite spin vary between 430 and 780 R. These measurements permit a ρ500 midnight density near the equator equal to (11 ± 3)104 atoms cm−3 to be deduced. The measured latitudinal-diurnal variation of ρ500 over 24 hours LT is compared with the previous experimental and theoretical values. The agreement is roughly good over 16 hours between 0900 LT and 0100 LT: the maximum occurs around 0900 LT, the minimum occurs near 1800 LT, and the midnight to midday ratio is around 1.1. However, the maximum to minimum density ratio ρmax/ρmin is higher than the expected one: 2.7 ± 0.8 as deduced from absorption measurements and 3.8 ± 1.2 from polarization measurements. The discrepancy between these two values can be interpreted by a ∼50°K departure from the Jacchia 71 temperature at southern latitudes in the afternoon. In the second part of the night, between 0100 LT and 0900 LT, a steep and large depression is observed; it can be interpreted as a local one around 35°N or a remote one, above the northern geomagnetic pole. This northern depression, the weak values of densities measured at southern latitudes, and the high density ratio ρmax/ρmin suggest the existence of a hydrogen bulge whose axis seems to be located around the sun-earth axis and to be linked to depressions over the poles.

Several series of auroral and geocoronal images from th Dynamics Explorer Mission are presented and analyzed. It is found that the motion of the transpolar arc of a theta aurora is correlated with the y-component of the IMF; the arc motion is in the general direction of the y-component. In addition, 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 premidnight sector of the auroral oval and a subsequent appearance of an expanding area of lesser intensities at low latitudes and contiguous to the midnight boundary of the bright spot. Finally, a series of images of the geocorona in scattered solar Ly-alpha 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 earth-radii, beyond which an exponential fit is used. The geocoronal tail is detected as an asymmetric increase in scattered Ly-alpha intensities in the antisolar direction.