Andrew F. Nagy

Collisional losses of ring current ions

The time evolution of the ring current population during the recovery phase of a typical moderate magnetic storm is studied, using a newly developed kinetic model for H+, He+ and O+ ions which includes nonequatorially mirroring particles. The bounce-averaged distribution function is defined for variables that are accessible to direct measurement, and some useful formulas for calculating the total energy and number density of the ring current are derived. The bounce-averaged kinetic equation is solved, including losses due to charge exchange with neutral hydrogen and Coulomb collisions with thermal plasma along ion drift paths. Time-dependent magnetospheric electric fields and anisotropic initial pitch angle distributions are considered. The generation of ion precipitating fluxes is addressed, a process that is still not completely understood. It is shown that both the decrease of the distribution function due to charge exchange losses and the buildup of a low-energy population caused by Coulomb collisions proceed faster for particles with smaller pitch angles. The maximum of the equatorial precipitating fluxes occurs on the nightside during the early recovery phase and is found to be of the order of 104–105 cm−2sr−1s−1keV−1. The mechanisms considered in this paper indicate that magnetospheric convection plays the predominant role in causing ion precipitation; Coulomb scattering contributes significantly to the low-energy ion precipitation inside the plasmasphere.

Three-dimensional, multispecies, high spatial resolution MHD studies of the solar wind interaction with Mars

[1] We present the results of model calculations, using our new, four-species, spherical MHD model. Our results are compared with the relevant and limited available data. The resulting comparisons help us to increase our understanding of the interaction processes between the solar wind and the Martian atmosphere/ionosphere. This new model with a spherical grid structure allowed us to use small (� 10 km) radial grid spacing in the ionospheric region. We found that the calculated bow shock positions agree reasonably well with the observed values. The calculated results vary with interplanetary magnetic field orientation, solar cycle conditions, and subsolar location. We found that our calculated ion densities, with parameters corresponding to solar cycle minimum conditions, reproduced the Viking 1 observed ion densities well. The calculated solar cycle maximum densities, above � 140 km, are also consistent with the appropriate Mars Global Surveyor radio occultation electron densities. Both the calculated solar cycle maximum and solar cycle minimum total transterminator and escape fluxes are significantly smaller than our previously published values. This decrease is due to the improved temperature values used for the recombination rates in this new model, which in turn results in lower ion densities and lower fluxes. INDEX TERMS: 2780 Magnetospheric Physics: Solar wind interactions with unmagnetized bodies; 6026 Planetology: Comets and Small Bodies: Ionospheres— composition and chemistry; 6028 Planetology: Comets and Small Bodies: Ionospheres—structure and dynamics; 2728 Magnetospheric Physics: Magnetosheath; KEYWORDS: Mars, MHD, bow shock, ionosphere, solar wind interaction

The plasma environment of Mars

When the supersonic solar wind reaches the neighborhood of a planetary obstacle it decelerates. The nature of this interaction can be very different, depending upon whether this obstacle has a large-scale planetary magnetic field and/or a well-developed atmosphere/ionosphere. For a number of years significant uncertainties have existed concerning the nature of the solar wind interaction at Mars, because of the lack of relevant plasma and field observations. However, measurements by the Phobos-2 and Mars Global Surveyor (MGS) spacecraft, with different instrument complements and orbital parameters, led to a significant improvement of our knowledge about the regions and boundaries surrounding Mars.

Kinetic model of the ring current-atmosphere interactions

Our numerical model of the ring current-atmosphere coupling (RAM) is further developed in order to include wave-particle interaction processes. The model calculates the time evolution of the phase space distribution function in the region from 2 RE to 6.5 RE, considering losses due to charge exchange, Coulomb collisions, and plasma wave scattering along ion drift paths. The spatial regions of ion cyclotron wave instability are determined by calculating the convective growth rates for electromagnetic ion cyclotron (EMIC) waves, integrating them along wave paths, and selecting regions of maximum wave amplification. The source regions are located on the duskside in agreement with the predominant occurrence of EMIC waves. A spectral power density of 1 nT2/Hz is adopted within the unstable regions. According to quasi-linear theory, the fluctuating fields are regarded as imposed on the system, and the losses due to wave-particle interactions are described with diffusive processes. The effects of the presence of heavy ion components on the quasi-linear diffusion coefficients are also considered. Resonance with ion cyclotron waves reduce the anisotropy of the proton population and the unstable regions disappear with time. Global patterns of precipitating ion fluxes are obtained and compared with observations.

Decay of equatorial ring current ions and associated aeronomical consequences

The decay of the major ion species which constitute the ring current is studied by solving the time evolution of their distribution functions during the recovery phase of a moderate geomagnetic storm. In this work, only equatorially mirroring particles are considered. Particles are assumed to move subject to E×B and gradient drifts. They also experience losses along their drift paths. Two loss mechanisms are considered: charge exchange with neutral hydrogen atoms and Coulomb collisions with thermal plasma in the plasmasphere. Thermal plasma densities are calculated with a plasmaspheric model employing a time-dependent convection electric field model. The drift-loss model successfully reproduces a number of important and observable features in the distribution function. Charge exchange is found to be the major loss mechanism for the ring current ions; however the important effects of Coulomb collisions on both the ring current and thermal populations are also presented. The model predicts the formation of a low-energy (< 500 eV) ion population as a result of energy degradation caused by Coulomb collisions of the ring current ions with the plasmaspheric electrons; this population may be one source of the low-energy ions observed during active and quiet periods in the inner magnetosphere. The energy transferred to plasmaspheric electrons through Coulomb collisions with ring current ions is believed to be the energy source for the electron temperature enhancement and the associated 6300 A (stable auroral red [SAR] arc) emission in the subauroral region. The calculated energy-deposition rate is sufficient to produce a subauroral electron temperature enhancement and SAR arc emissions that are consistent with observations of these quantities during moderate magnetic activity levels.

Dynamical behavior of thermal protons in the mid-latitude ionosphere and magnetosphere

Abstract Satellite and other observations have shown that H+ densities in the mid-latitude topside ionosphere are greatly reduced during magnetic storms when the plasmapause and magnetic field convection move to relatively low L-values. In the recovery phase of the magnetic storm the convection region moves to higher L-values and replenishment of H+ in the empty magnetospheric field tubes begins. The upwards flow of H+, which arises from O+—H charge exchange, is initially supersonic. However, as the field tubes fill with plasma, a shock front moves downwards towards the ionosphere, eventually converting the upwards flow to subsonic speeds. The duration of this supersonic recovery depends strongly on the volume of the field tube; for example calculations indicate that for L = 5 the time is approximately 22 hours. The subsonic flow continues until diffusive equilibrium is reached or a new magnetic storm begins. Calculations of the density and flux profiles expected during the subsonic phase of the recovery show that diffusive equilibrium is still not reached after an elapsed time of 10 days and correspondingly there is still a net loss of plasma from the ionosphere to the magnetosphere at that time. This slow recovery of the H+ density and flux patterns, following magnetic storms, indicates that the mid-latitude topside ionosphere may be in a continual dynamic state if the storms occur sufficiently often.

Models of Venus neutral upper atmosphere - Structure and composition

Abstract Models of the Venus neutral upper atmosphere, based on both in-situ and remote sensing measurements, are provided for the height interval from 100 to 3,500 km. The general approach in model formulation was to divide the atmosphere into three regions: 100 to 150 km, 150 to 250 km, and 250 to 3,500 km. Boundary conditions at 150 km are consistent with both drag and mass spectrometer measurements. A paramount consideration was to keep the models simple enough to be used conveniently. Available observations are reviewed. Tables are provided for density, temperature, composition (CO 2 , O, CO, He, N, N 2 , and H), derived quantities, and day-to-day variability as a function of solar zenith angle on the day- and nightsides. Estimates are made of other species, including O 2 and D. Other tables provide corrections for solar activity effects on temperature, composition, and density. For the exosphere, information is provided on the vertical distribution of normal thermal components (H, O, C, and He) as well as the hot components (H, N, C, O) on the day- and nightsides.

Three‐dimensional multispecies MHD studies of the solar wind interaction with Mars in the presence of crustal fields

interaction of the solar wind with Mars. The three ions considered are H + ,O 2 , and O + , representing the solar wind and the two major ionospheric ion species, respectively. The calculations indicate that the presence of a hot oxygen corona does not, within the resolution and accuracy of the model, lead to any significant effect on the dayside bow shock and ionopause positions. Next the trans-terminator fluxes and escape fluxes down the tail were calculated neglecting the effects of the crustal magnetic field. The calculated flux values are consistent with the measured escape fluxes and the calculated limiting fluxes from the dayside ionosphere. Finally, a 60-order harmonic expansion model of the measured magnetic field was incorporated into the model. The crustal magnetic field did not cause major distortions in the bow shock but certainly had an important effect within the magnetosheath and on the apparent altitude of the ionopause. The model results also indicated the presence of ‘‘minimagnetocylinders,’’ consistent with the MGS observations. We also recalculated the trans-terminator and escape fluxes, for the nominal solar wind case, in the presence of the crustal magnetic field and found, as expected, that there is a decrease in the calculated escape flux; however, it is still reasonably close to the value estimated from the Phobos-2 observations. INDEX TERMS: 2780 Magnetospheric Physics: Solar wind interactions with unmagnetized bodies; 2459 Ionosphere: Planetary ionospheres (5435, 5729, 6026, 6027, 6028); 5440 Planetology: Solid Surface Planets: Magnetic fields and magnetism; 2728 Magnetospheric Physics: Magnetosheath; KEYWORDS: Mars, MHD, bow shock, escape flux, solar wind interaction, crustal magnetic field Citation: Ma, Y., A. F. Nagy, K. C. Hansen, D. L. DeZeeuw, T. I. Gombosi, and K. G. Powell, Three-dimensional multispecies MHD studies of the solar wind interaction with Mars in the presence of crustal fields, J. Geophys. Res., 107(A10), 1282, doi:10.1029/2002JA009293, 2002.

Solar cycle variability of hot oxygen atoms at Mars

The population of hot oxygen atoms in the Martian exosphere is reexamined using newly calculated hot O production rates for both low and high solar cycle conditions. The hot oxygen production rates are assumed to result from the dissociative recombination of O2+ ions. These calculations take into account the calculated vibrational distribution of O2+ and the new measured branching ratios. Furthermore, these calculations also consider the variation of the dissociative recombination cross section with the relative speed of the participating ions and electrons, the rotational energy of the O2+ ions, and the spread of the ion and electron velocities. These production rates were next used in a two-stream model to obtain the energy dependent flux of the hot oxygen atoms as a function of altitude. Finally, the calculated flux at the exobase was input into an exosphere model, based on Liouvilles theorem, to calculate the hot oxygen densities as a function of altitude in the exosphere and the resulting escape flux. It was found that hot oxygen densities vary significantly over the solar cycle; the calculated densities vary from about 2×103 to 6×103 cm−3 at an altitude of 1000 km. The escape flux also varies from about 3×106 to 9×106 cm−2s−1.

The ancient oxygen exosphere of Mars: Implications for atmosphere evolution

The evolution of the Martian atmosphere, particularly with regard to water, is influenced by (1) “nonthermal” escape of oxygen atoms created by dissociative recombination and (2) by oxygen ion pickup by the solar wind. Both processes depend on the intensity of solar EUV radiation, which affects atmosphere temperatures (scale heights) and photoionization rates, and thereby the exosphere and the fluxes of escaping atoms and ions. This study involves the calculation, by the two-stream model method of Nagy and Cravens (1988), of the exospheric hot oxygen densities for “ancient” atmospheres and ionospheres (e.g., for different EUV fluxes), and the associated neutral escape fluxes. The ion production rates above nominal ionopause altitudes are also computed and are considered to be the upper limits to losses by direct solar wind pickup. Since we do not consider the pickup ion precipitation and additional neutral escape due to the sputtering process described by Luhmann and Kozyra (1991), the results presented here represent conservative estimates of the neutral escape fluxes, but somewhat generous estimates of ion loss rates. We find that when the inferred increased solar EUV fluxes of the past are taken into account, oxygen equivalent to that in several tens of meters of water, planet-wide, should have escaped to space over the last 3 Gyr.