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NASA NTRS1991

The magnetic barrier at Venus

Altitude profiles of the Venus magnetic barrier are derived here from a statistical analysis of the Pioneer Venus Orbiter magnetometer data. The outer boundary of the magnetic barrier is then compared with the obstacle expected from gasdynamic models of the bow shock, and the stagnation pressure is compared with that expected from gasdynamic theory. The magnetic barrier is strongest at the subsolar point and weakens as expected with increasing solar zenith angle. The existence of a north-south asymmetry in the barrier strength is also demonstrated. The magnetic barrier is about 200 km thick at the subsolar point and 800 km thick at the terminator. The magnetic barrier transfers most of the solar wind dynamic pressure to the ionosphere via the enhanced magnetic pressure. The convected field gasdynamic model predicts the correct bow shock location if the magnetic barrier is treated as the obstacle.

Zhang, T. L.↗

NASA NTRS1991

Ion dynamics in the Venus ionosphere

Measurement data on the ion velocity in the Venus ionosphere (mainly from the Pioneer Venus Orbiter Retarding Potential Analyzer) are summarized, and theoretical models developed to explain them are reviewed. Data and theoretical predictions are compared in extensive graphs and diagrams and discussed in detail. It is shown that the predominant flow is away from the subsolar point, at up to 3 km/sec in the terminator region. A model of axisymmetric flow based on momentum, energy, and mass conservation laws is found to reproduce the observed ion velocities at solar zenith angles less than about 140 deg, but not the high velocities and chaotic behavior seen near the antisolar point. Also discussed are significant differences between the flow above and below about 400 km and the effects of changes in the dynamic pressure of the solar wind.

Miller, K. L.↗

NASA NTRS1992

Photovoltaic array for Martian surface power

Missions to Mars will require electric power. A leading candidate for providing power is solar power produced by photovoltaic arrays. To design such a power system, detailed information on solar-radiation availability on the Martian surface is necessary. The variation of the solar radiation on the Martian surface is governed by three factors: (1) variation in Mars-Sun distance; (2) variation in solar zenith angle due to Martian season and time of day; and (3) dust in the Martian atmosphere. A major concern is the dust storms, which occur on both local and global scales. However, there is still appreciable diffuse sunlight available even at high opacity, so that solar array operation is still possible. Typical results for tracking solar collectors are also shown and compared to the fixed collectors. During the Northern Hemisphere spring and summer the isolation is relatively high, 2-5 kW-hr/sq m-day, due to the low optical depth of the Martian atmosphere. These seasons, totalling a full terrestrial year, are the likely ones during which manned mission will be carried out.

Appelbaum, J.↗

NASA NTRS1992

Photovoltaic arrays for Martian surface power

Missions to Mars will require electric power. A leading candidate for providing power is solar power produced by photovoltaic arrays. To design such a power system, detailed information on solar-radiation availability on the Martian surface is necessary. The variation of the solar radiation on the Martian surface is governed by three factors: (1) variation in Mars-sun distance; (2) variation on solar zenith angle due to Martian season and time of day; and (3) dust in the Martian atmosphere. A major concern is the dust storms, which occur on both local and global scales. However, there is still appreciable diffuse sunlight available even at high opacity, so that solar array operation is still possible. Typical results for tracking solar collectors are also shown and compared to the fixed collectors. During the Northern Hemisphere spring and summer the isolation is relatively high, 2-5 kW-hr/sq m-day due to the low optical depth of the Martian atmosphere. These seasons, totalling a full terrestrial year, are the likely ones during which manned mission will be carried out.

Appelbaum, Joseph↗

NASA NTRS2010

Use of an Improved Radiation Amplification Factor to Estimate the Effect of Total Ozone Changes on Action Spectrum Weighted Irradiances and an Instrument Response Function

Multiple scattering radiative transfer results are used to calculate action spectrum weighted irradiances and fractional irradiance changes in terms of a power law in ozone OMEGA, U(OMEGA/200)(sup -RAF), where the new radiation amplification factor (RAF) is just a function of solar zenith angle. Including Rayleigh scattering caused small differences in the estimated 30 year changes in action spectrum-weighted irradiances compared to estimates that neglect multiple scattering. The radiative transfer results are applied to several action spectra and to an instrument response function corresponding to the Solar Light 501 meter. The effect of changing ozone on two plant damage action spectra are shown for plants with high sensitivity to UVB (280-315 run) and those with lower sensitivity, showing that the probability for plant damage for the latter has increased since 1979, especially at middle to high latitudes in the Southern Hemisphere. Similarly, there has been an increase in rates of erythemal skin damage and pre-vitamin D3 production corresponding to measured ozone decreases. An example conversion function is derived to obtain erythemal irradiances and the UV index from measurements with the Solar Light 501 instrument response function. An analytic expressions is given to convert changes in erythemal irradiances to changes in CIE vitamin-D action spectrum weighted irradiances.

Herman, Jay R.↗

NASA NTRS1990

A model study of the response of mesospheric ozone to short-term solar ultraviolet flux variations

An investigation is conducted in order to determine the relative importance of several modeled processes in controlling the magnitude and phase of the mesospheric ozone response. A detailed one-dimensional modeling study of the mesospheric ozone response to solar UV flux variations is conducted to remove some of the deficiencies in previous studies. This study is also used to examine specifically the importance of solar zenith angle, self-consistent calculation of water vapor abundance, and temperature feedback with a nonlocal thermodynamic equilibrium radiation model. The photochemical model is described, and the assumptions made for the purpose of comparing model results with the observed ozone response obtained from a statistical analysis of Solar Mesosphere Explorer data (Keating et al., 1987) are discussed. The numerical results for the theoretical ozone response are presented. The results of selected time-dependent calculations are considered to illustrate the degree to which a relatively simple model of the mesosphere is able to capture the major characteristics of the observed response.

Summers, M. E.↗

NASA NTRS1980

Model calculations of the dayside ionosphere of Venus - Energetics

A model of the energy balance of the dayside ionosphere of Venus is presented. Calculations of the dayside electron and ion temperature profiles are carried out and compared with data from experiments on the Pioneer Venus orbiter. The coupled heat conduction equations for electrons and ions are solved for several values of the solar zenith angle. It is shown that thermal conductivities are inhibited by the presence of a horizontal magnetic field. A realistic model of the magnetic field that includes fluctuations is employed in deriving an appropriate expression for the thermal conductivity. The contributions of photoelectrons, ion chemistry, Joule heating, and solar wind heating to the energy balance of the ionosphere are considered.

Cravens, T. E.↗

NASA NTRS2021

Ionization Efficiency in the Dayside Ionosphere of Mars: Structure and Variability

Ionization efficiency, the ratio between photoelectron impact ionization and primary photoionization, is an important quantity from the perspective of ionospheric energy balance and as a quantity to be parameterized for use in global simulations. We investigate ionization efficiency using MAVEN in situ measurements of solar extreme ultraviolet (EUV) flux, suprathermal electron flux, and neutral density. Its behavior with respect to pressure and solar zenith angle (SZA) is explained by ionization cross-sections and photoabsorption, plus photoelectron transport near the terminator. We find similar ionization efficiencies between the species CO 2 , O, N 2 , and CO, with Argon ∼40% higher, explained largely by its higher ionization potential. Efficiency depends positively on solar activity, increasing by ∼50–100% (depending on season) from low to moderate solar EUV levels, before flattening at higher EUV levels. EUV spectral hardening appears to be responsible for only ∼15% of this increase, the remainder best explained by variability in neutral composition. We find a negligible dependence of ionization efficiency on the strength and direction of crustal magnetic fields. Efficiencies agree poorly with the theoretical model of Nicholson et al. (2009), https://doi.org/10.1111/j.1365-2966.2009.15463.x, possibly reflecting differences in model versus measured solar EUV spectrum and neutral densities. We fit the data to the empirical parameterization of Mendillo et al. (2011), https://doi.org/10.1029/2011ja016865 but find that a single fit cannot capture ionization efficiency behavior across all dayside SZA values. We believe that an empirical model of Mars ionization efficiency will require MAVEN data and validated transport modeling to reach to sufficiently high pressures to capture Mars' photochemical peaks M1 and M2

Robert Lillis↗

NASA NTRS1986

Non-Lambertian effects on remote sensing of surface reflectance and vegetation index

This paper discusses the effects of non-Lambertian reflection from a homogeneous surface on remote sensing of the surface reflectance and vegetation index from a satellite. Remote measurement of the surface characteristics is perturbed by atmospheric scattering of sun light. This scattering tends to smooth the angular dependence of non-Lambertian surface reflectances, an effect that is not present in the case of Lambertian surfaces. This effect is calculated to test the validity of a Lambertian assumption used in remote sensing. For the three types of vegetations considered in this study, the assumption of Lambertian surface can be used satisfactorily in the derivation of surface reflectance from remotely measured radiance for a view angle outside the backscattering region. Within the backscattering region, however, the use of the assumption can result in a considerable error in the derived surface reflectance. Accuracy also deteriorates with increasing solar zenith angle. The angular distribution of the surface reflectance derived from remote measurements is smoother than that at the surface. The effect of surface non-Lambertianity on remote sensing of vegetation index is very weak. Since the effect is similiar in the visible and near infrared part of the solar spectrum for the vegetations treated in this study, it is canceled in deriving the vegetation index. The effect of the diffuse skylight on surface reflectance measurements at ground level is also discussed.

Lee, T. Y.↗

NASA NTRS1995

Effect of stratospheric aerosol on ozone profile from BUV measurements

Results of a simulation study on the effects of optically thick stratospheric sulfate aerosol layers on the backscattered ultraviolet radiation (buv) in the range 256-340 nm are presented. In general, the increased Mie scattering produced by the aerosols results in an enhancement of the buv radiation. The increase is approximately linear with optical depth and strongly depends on solar zenith angle and aerosol layer altitude in relation to the ozone maximum. The effect is greatest at those wavelenghts whose contribution functions peak in the vicinity of the densest part of the aerosol layer. The aerosol induced perturbation of the buv field affects the ozone profile retrieval from space measurements by the Solar Backscatter Ultraviolet Experiment (SBUV) experiment. In the tropical stratosphere, the retrieved ozone between 25 and 45 km is underestimated as a result of increased Mie scattering. On the other hand, an algorithm related effect causes the retrieved ozone below 25 km to be overestimated by an amount similar to the stratospheric deficit.

Torres, Omar↗

NASA NTRS1980

Electron temperature and heat flow in the nightside Venus ionosphere

A steady-state two-dimensional heat balance model is used to analyze the night side Venusian ionospheric electron temperatures given by the Pioneer Venus orbiter electron temperature probe. The energy calculation includes the solar EUV heating at the terminator, electron cooling to ions and neutrals, and heat conduction within the ionospheric plasma. An optimum magnetic field is derived by solving for the heat flux directions which force energy conservation while constrained by the observed temperatures within the range of 80-170 deg solar zenith angle and 160-170 km. The heat flux vectors indicate a magnetic field that connects the lower night side ionosphere to the day side ionosphere, and connects the upper ionosphere to the ionosheath. The lower ionosphere is heated through conduction of heat from the dayside, and the upper ionosphere is heated by the solar wind in the ionosheath with heat flowing downward and from the nightside to the day side.

Hoegy, W. R.↗

NASA NTRS1973

Daytime ion chemistry of N2+.

Rocket measurements of the emission of the (0, 0) first negative band of N2+ at 3914 A in the day airglow have been made between 120 and 300 km. The solar zenith angle at the time of the flight was 60 deg, and the zenith intensity above 120 km was 1.6 kR averaged over both legs of the flight. The data are compared with a model in which charge exchange of N2 with metastable O+(2D) ions is included as an additional source of N2 ionization. Above 240 km this mechanism is dominant over the photoionization of N2 by solar extreme ultraviolet radiation. The derived N2+ density is consistent with the results of rocket-borne mass spectrometric observations.

Feldman, P. D.↗

NASA NTRS1975

Signature extension for sun angle, volume 1

The author has identified the following significant results. Within a restricted zenith sun angle range of 35 - 50 degrees, it was empirically observed that canopy reflectance is mainly Lambertian. Reflectance changes with crop stage were simple shifts in scale in the sun angle range. It was noted that sun angle variations depend on canopy characteristics. Effects of the vegetative canopy were most pronounced at the larger solar zenith angles (20 %). The linear sun angle correction coefficients demonstrate a dependency on both crop stage (15-20 %) and crop type (10-20 %). The use of canopy reflectance modeling allowed for the generation of a simulated data set over an extremely broad envelope of sun angles.

Smith, J. A.↗

NASA NTRS2003

MGS Radio Science Electron Density Profiles: Interannual Variability and Implications for the Martian Neutral Atmosphere

Martian electron density profiles provided by the Mars Global Surveyor (MGS) Radio Science (RS) experiment over the 95-200 km altitude range indicate that the height of the electron peak and the longitudinal structure of the peak height are sensitive indicators of the physical state of the Mars lower atmosphere. The present analysis is carried out on five sets of occultation profiles, all at high solar zenith angles (SZA). Variations spanning 2-Martian years are investigated near aphelion conditions at high Northern latitudes (64.7-77.6N). A mean ionospheric peak height of 133.5-135 km was obtained for all aphelion profiles near SZA = 78-82; a corresponding mean peak density of 7.3-8.5 x 10(exp 4)/cu cm was also measured, reflecting solar moderate conditions. Strong wave 2-3 oscillations in peak heights were observed as a function of longitude over both Martian seasons. The Mars Thermospheric General Circulation Model (MTGCM) is exercised for Mars aphelion conditions. The measured interannual variations in the mean and longitude structure of the peak heights are small (consistent with MTGCM simulations), signifying the repeatability of the Mars atmosphere during aphelion conditions. A non-migrating (semi-diurnal period, wave#l eastward propagating) tidal mode is likely responsible for the wave#3 longitude features identified. The height of this photochemically driven peak can be observed to provide an ongoing monitor of the changing state of the Mars lower atmosphere. The magnitudes of these same peaks may reflect more than changing solar EUV fluxes when they are located in the vicinity of Mars crustal magnetic field centers.

Bougher, Stephen W.↗

NASA NTRS1982

Observations of energetic ions near the Venus ionopause

Ions (primarily O/+/) with spacecraft rest frame energies greater than 40 eV have been observed by the Pioneer Venus Neutral Mass Spectrometer. The signature occurs in about 13% of the 700 orbits examined, primarily near the ionopause and at all solar zenith angles. The energetic ions coincide in location with superthermal ions observed by the Ion Mass Spectrometer and more rarely occur in some of the plasma clouds observed by the Electron Temperature Probe. These observations in conjunction with measurements by the Plasma Wave Instrument near the ionopause suggest that the ions are accelerated out of ionospheric plasma by the shocked solar wind through plasma wave-particle interactions.

Kasprzak, W. T.↗

NASA NTRS1984

Dynamics of the Venus ionosphere - A two-dimensional model study

A two-dimensional model of the ionosphere of Venus which simulates ionospheric dynamics by self-consistently solving the plasma equations of motion, including the inertial term, in finite difference form has been constructed. The model, which is applied over the solar zenith angle range extending from 60 to 140 deg and the altitude range 100 to 480 km, simulates the measured horizontal velocity field quite satisfactorily. The ion density field is somewhat overestimated on the dayside, because of the choice model neutral atmosphere, and underestimated on the nightside, because of setting the ionopause height at too low an altitude. It is concluded that solar photoionization on the dayside and ion recombination on the nightside are the processes mainly responsible for accelerating the plasma to the observed velocities. The plasma flow appears to be sufficient to maintain the nightside ionosphere at or near the observed median level of ion densities.

Whitten, R. C.↗

NASA NTRS2000

Aerosol Climate Effects: Local Radiative Forcing and Column Closure Experiments

In an effort to reduce uncertainties in climate change predictions, experiments are being planned and conducted to measure anthropogenic aerosol properties and effects, including effects on radiative fields. The global average, direct anthropogenic aerosol effect on upwelling shortwave fluxes is estimated to be about +1/2 W/sq m, whereas errors in flux changes measured with airborne and spaceborne radiometers are 2 to 8 W/sq m or larger. This poses the question of whether flux changes expected in field experiments will be large enough to measure accurately. This paper obtains a new expression for the aerosol-induced change in upwelling flux, compares it to two-stream and adding-doubling (AD) results, and uses all three methods to estimate expected flux changes. The new expression accounts for the solar zenith angle dependences of aerosol transmission and reflection, as well as of surface albedo, all of which can have a strong effect in determining flux changes measured in field experiments. Despite its relative simplicity, the new expression gives results similar to previous two-stream results. Relative to AD results, it agrees within a few watts per square meter for the intermediate solar elevation angles where the flux changes peak (roughly 10 to 30 degrees), but it has negative errors for higher Sun and positive errors for lower Sun. All three techniques yield aerosol-induced changes in upwelling flux of +8 to +50 W/sq m for aerosol midvisible optical depths of 0.1 to 0.5. Because such aerosol optical depths occur frequently off the U.S. and European Atlantic coasts in summer, the flux changes they induce should be measurable by airborne, and possibly by spaceborne, radiometers, provided sufficient care is taken in experiment design (including measurements to separate aerosol radiative effects from those of absorbing gases). The expected flux changes are about 15 to 100 times larger than the global average flux change expected for the global average anthropogenic sulfate optical depth of 0.04. Reasons for the larger flux changes include the larger optical depths considered here (factor 2.5 to 12), plus restricting the measurements to cloud-free, daytime conditions over the ocean (factor 5 to 9).

Russell, P. B.↗

NASA NTRS2004

MGS Radio Science Electron Density Profiles: Interannual Variability and Implications for the Martian Neutral Atmosphere

Martian electron density profiles provided by the Mars Global Surveyor (MGS) Radio Science (RS) experiment over the 95-200 km altitude range indicate what the height of the electron peak and the longitudinal structure of the peak height are sensitive indicators of the physical state of the Mars lower and upper atmospheres. The present analysis is carried out on five sets of occultation profiles, all at high solar zenith angles (SZA). Variations spanning 2 Martian years are investigated near aphelion conditions at high northern latitudes (64.7 - 77.6 N) making use of four of these data sets. A mean ionospheric peak height of 133.5 - 135 km is obtained near SZA = 78 - 82 deg.; a corresponding mean peak density of 7.3 - 8.5 x l0(exp 4)/ qu cm is also measured during solar moderate conditions at Mars. Strong wave number 2 - 3 oscillations in peak heights are consistently observed as a function of longitude over the 2 Martian years. These observed ionospheric features are remarkably similar during aphelion conditions 1 Martian year apart. This year-to-year repeatability in the thermosphere-ionosphere structure is consistent with that observed in multiyear aphelion temperature data of the Mars lower atmosphere. Coupled Mars general circulation model (MGCM) and Mars thermospheric general circulation model (MTGCM) codes are run for Mars aphelion conditions, yielding mean and longitude variable ionospheric peak heights that reasonably match RS observations. A tidal decomposition of MTGCM thermospheric densities shows that observed ionospheric wave number 3 features are linked to a non-migrating tidal mode with semidiurnal period (sigma = 2) and zonal wave number 1 (s = -1) characteristics. The height of this photochemically determined ionospheric peak should be monitored regularly.

Bougher, S. W.↗