Twilight effects of solar ionizing radiation

The absorption of solar ionizing radiation during twilight is investigated. Ion production rates are obtained as a function of altitude and twilight intensities and altitude profiles of emissions arising from the fluorescence of solar ionizing radiation are calculated for various solar depression angles. For an atmosphere with an exospheric temperature of 750°K, the predicted overhead intensity from fluorescence of the O⁺(²P — ²D) lines at 7319–7330 diminishes from 175 R at dusk to 10 R at a solar depression angle of 10°. The predicted overhead intensities from fluorescence of the N₂⁺ Meinel and first negative systems are respectively about 175 R and 20 R at dusk diminishing to respectively 1.5 R and 0.1 R at a solar depression angle of 10°.

It is suggested that a charge transfer reaction of O⁺²D in N₂ is a significant source of N₂⁺ ions. This reaction offers a possible explanation for the high apparent rotational temperatures in the first negative system observed by Broadfoot and Hunten. Other excitation and ionization mechanisms are briefly discussed.     

The Venus ionosphere at grazing incidence of solar radiation: Transport of plasma to the night ionosphere

Using a quasi-two-dimensional model of the Venus ionosphere, we calculated the ion number densities and horizontal ion bulk velocities expected for a range of solar zenith angles near the terminator (80 to 100°), and compared them with data obtained from the Pioneer Venus Orbiter retarding potential analyzer. The calculated ion bulk velocity arises entirely from the solar EUV-induced plasma pressure gradient and has a magnitude consistent with observations; ionization by suprathermal electrons is neglected in those computations. We find that while photoionization is the dominant source of ionospheric plasma for solar zenith angles less than 92°, plasma transport from the dayside is the dominant plasma source for solar zenith angles greater than 95°. We also show that the main nightside plasma peak at approximately 140 km altitude is of the F₂ type (i.e., is diffusion controlled). Its altitude and shape are thus quite insensitive to the altitude of the ion source.     

The intensity variation of the atomic oxygen red line during morning and evening twilight on 9–10 April 1969

During the evening of 9 April and the morning of 10 April 1969, the twilight zenith intensity of the atomic oxygen red line OI(³P-¹D) at 6300 Å was measured at the Blue Hill Observatory (42°N, 17°W). At the same time incoherent scatter radar data were being obtained at the Millstone Hill radar site 50 km distant. We have used a diurnal model of the mid-latitude F-region to calculate the ionospheric structure over Millstone Hill conditions similar to 9–10 April 1969. The measured electron temperature, ion temperature, and electron density at 800 km are used as boundary conditions for the model calculations. The diurnal variation of neutral composition and temperature were obtained from the OGO-6 empirical model and the neutral winds were derived from a semiempirical three-dimensional dynamic model of the neutral thermosphere. The solar EUV flux was adjusted to yield reasonable agreement between the calculated and observed ionospheric properties.This paper presents the results of these model computations and calculations of the red line intensity. The 6300 Å emission includes contributions from photoelectron excitation, dissociative recombination, Schumann-Runge photodissociation and thermal electron impact. The variations of these four components for morning and evening twilight between 90–120° solar zenith angles, and their relative contributions to the total 6300 Å emission line intensity, are presented and the total is compared to the observations. For this particular day the Schumann-Runge photodissociation component, calculated using the solar fluxes tabulated by Ackermann (1970), is the dominant component of the morning twilight 6300 Å emission. During evening twilight it is necessary to utilize a lower O₂ density than for the morning twilight in order to bring the calculated and observed 6300 Å emission rates into agreement. The implication that there may be a diurnal variation in the O₂ density at the base of the thermosphere is discussed in the light of available experimental data and current theoretical ideas.     

Turbulence in the troposphere of Venus: Refined characteristics and new estimates

Based on spaceborne experimental data, characteristics of turbulence are calculated for the Venusian troposphere under conditions corresponding to the planet-averaged flux of solar radiation, which is equal to its value at a solar zenith angle of 66°. Additionally, given experimental data on radiation fluxes and their numerical calculations, turbulence characteristics were calculated for a solar zenith angle of 45°. The turbulence pattern is significantly different for small and large solar zenith angles. At large solar zenith angles, there exist an anomalous downward turbulent heat flux above 7–10 km and a normal upward flux at lower heights. At small zenith angles, the turbulent flux is normal throughout the entire troposphere. The dissipation of turbulent energy contributes significantly to the atmospheric heating in a wide range of altitudes. The spectrum of the time and space scales of dissipative processes in the troposphere is very wide and changes with height.Translated from Astronomicheskii Vestnik, Vol. 39, No. 1, 2005, pp. 38–50.Original Russian Text Copyright © 2005 by Izakov.     

Line shape of the non-thermal 6300 Å O(1D) emission

The line shape of the non-thermal O(¹D) 6300 Å emission is calculated using the two population model of Schmitt, Abreu and Hays (Planet. Space Sci.29, 1095, 1981). The calculated line shapes simulate observations made from a space platform at different zenith angles and altitudes. The non-thermal line shapes observed at zenith angles other than the local vertical have been obtained by using the Addition theorem for spherical harmonics of a Legendre polynomial expansion of the non-thermal population distribution function.     

Simulation of atmospheric effects on the emergent radiation over a checkerboard type of terrain

To evalute the effect of the non-uniform surface on the radiation field, the upwelling radiation at the top of the atmosphere bounded by the checkerboard type of terrain is computed using the modified doubling method. The terrain is composed of the square Lambert surfaces with two different albedoes. The dimension of the each square is assumed to be 0.5–6 km. The radiance of the terrain is discussed with respect to the atmospheric effect. The observational site is located at altitude 30 km. The corresponding projected point on the ground is located at the center of a square. The solar and observational direction is located in the plane parallel to the checkerboard squares. The atmosphere is assumed to be homogeneous, which is composed of aerosol and molecules, where the model aerosol is of the oceanic or the water soluble types.Numerical simulation exhibits the extraordinary effect near the edge of each squares. The radiance of the terrain depends upon the difference of albedoes and size of squares. It increases with the increase of the dimension of the square. It decreases with the optical thickness. At large optical thickness, the variation of radiation with zenith direction depends upon the aerosol characteristics. It shows little dependence on the solar zenith angle if less than 20°.     

Solar EUV and Electron-Proton-Hydrogen Atom-Produced Ionosphere on Mars: Comparative Studies of Particle Fluxes and Ion Production Rates Due to Different Processes

The total photoelectron and secondary electron fluxes are calculated at different times and altitudes along the trajectory of Mars Global Surveyor passing through the nightside and dayside martian ionosphere. These results are compared with the electron reflectometer experiment on board Mars Global Surveyor. The calculated electron spectra are in good agreement with this measurement. However, the combined fluxes of proton and hydrogen atom as calculated by E. Kallio and P. Janhunen (2001, J. Geophys. Res.106, 5617-5634) were found to be 1-2 orders of magnitude smaller than the measured spectra. We have also calculated ionization rates and ion and electron densities due to solar EUV, X-ray, and electron-proton-hydrogen atom impacting with atmospheric gases of Mars at solar zenith angles of 75°, 105°, and 127°. In the vicinity of the dayside ionization peak, it is found that the ion production rate caused by the precipitation of proton-hydrogen atom is larger than the X-ray impact ionization rate while at all altitudes, the photoionization rate is always greater than either of the two. Moreover, X-rays contribute greatly to the photoelectron impact ionization rate as compared to the photoion production rate. The calculated electron densities are compared with radio occultation measurements made by Mars Global Surveyor, Viking 1, and Mars 5 spacecraft at these solar zenith angles. The dayside ionosphere produced by proton-hydrogen atom is smaller by an order of magnitude than that produced by solar EUV radiation. X-rays play a significant role in the dayside ionosphere of Mars at the altitude range 100-120 km. Solar wind electrons and protons provide a substantial source for the nightside ionosphere. These calculations are carried out for a solar minimum period using solar wind electron flux, photon flux, neutral densities, and temperatures under nearly the same areophysical conditions as the measurements.     

Muon flux at the geographical South Pole

The muon flux at the South Pole was measured for five zenith angles, 0°, 15°, 35°, 82.13° and 85.15° with a scintillator muon telescope incorporating ice Cherenkov tank detectors as the absorber. We compare the measurements with other data and with calculations.     

Polarimetry of the twilight sky and stratospheric aerosol

We propose a modification of the method of polarimetric measurements of the twilight sky, traditionally performed in a zenith direction, to study physical properties of the stratospheric aerosol (at altitudes higher than 30 km). The measurements carried out in zenith directions as a rule limit phase angles by values of 80–100°. We suggest setting up the declination of the telescope equal to the declination of the sun and measuring the polarization degree of the twilight sky at different values of the right ascension. It will allow us not only to enhance the range of the phase angles but also to plan observations in a way to obtain data on the phase dependence of the polarization degree of the light scattered by atmospheric layers at different altitudes.     

N2 emission in the twilight

About a year's observations of the N₂⁺ band (3914 Å) at Kitt Peak (latitude 32°) are reported. Morning intensities are the same throughout the year, but there is a strong winter maximum in the evening. It is suggested that the additional ionization is produced by photoelectrons from the magnetic conjugate point. Heights are estimated by the zenith-horizon method, which gives 235 km for the constant component and 350 km during the evening enhancement. The intensity variation through twilight is therefore entirely due to changes of the N₂⁺ concentration; each ion scatters light at a constant rate. The rotational distribution resembles that for a temperature of 1600°K, much higher than the temperature of the atmosphere. It is suggested that part of the ions may be produced by charge transfer from metastable O⁺(²D). N₂⁺ concentrations resulting from photoionization are calculated; they give a fair account of the observed horizon intensities, but not the zenith. Non-local electrons from higher in the atmosphere are suggested as a possible extra source; alternatively, the zenith measurements may be perturbed by scattered horizon light. The band intensity in the nightglow cannot be measured; the upper limit is 1 R.     

Solar activity and atmospheric attenuation effects on the visibility of stars and planets during twilight

By use of the values of the sky twilight brightness deduced at sea level at Abu Simbel in 1980 during the high solar activity period, the visibility of stars and planets of magnitudes less than 5.5 during twilight are obtained. The results are given in charts for each one degree of Sun's depression below the horizon. These charts can be applied for different altitudes above the horizon and different bearing angles from direction of sunset. Tables of corrections for different values of atmospheric attenuation and solar activity are given.     

Simulation of atmospheric effect on the emergent radiation over a circular lake

The upwelling radiation at the top of the atmosphere is computed over a circular lake which is located in the uniform Lambert surface, using a modified version of the doubling-adding method. The radiance over the lake is discussed with respect to the atmospheric effect. The radius of the lake is assumed to be 0.5, 1, and 3 km. The observational site is located at altitude 30 km. The zenith of the observational site is located in the plane which is determined by the zenith of the center of the lake and incident solar direction. The zenith angle of the observational site to the center of the lake is fixed to 6.28°. The atmosphere is assumed to be homogeneous, which is composed of aerosol and molecule, where the model aerosol is of the oceanic or the water soluble types.Numerical simulation exhibits an extraordinary effect near the lake. The radiance of the lake against the surrounding depends upon the albedo of the surrounding surface. It increases with the increase of the size of the lake and decreases with the optical thickness. At large optical depth, the radiance depends upon the aerosol characteristics. It shows little dependence on the solar zenith angle if less than 60°.     

The optical depth sensor (ODS) for column dust opacity measurements and cloud detection on martian atmosphere

A lightweight and sophisticated optical depth sensor (ODS) able to measure alternatively scattered flux at zenith and the sum of the direct flux and the scattered flux in blue and red has been developed to work in martian environment. The principal goals of ODS are to perform measurements of the daily mean dust opacity and to retrieve the altitude and optical depth of high altitude clouds at twilight, crucial parameters in the understanding of martian meteorology. The retrieval procedure of dust opacity is based on the use of radiative transfer simulations reproducing observed changes in the solar flux during the day as a function of 4 free parameters: dust opacity in blue and red, and effective radius and effective width of dust size distribution. The detection of clouds is undertaken by looking at the time variation of the color index (CI), defined as the ratio between red and blue ODS channels, at twilight. The retrieval of altitude and optical depth of clouds is carried out using a radiative transfer model in spherical geometry to simulate the CI time variation at twilight. Here the different retrieval procedures to analyze ODS signals, as well as the results obtained in different sensitivity analysis are presented and discussed.     

Radiation field in the troposphere and stratosphere—II. Numerical analysis

The results of calculations of the multiply-scattered solar-induced radiation field in the troposphere and stratosphere are presented for direct application to photochemical models. The enhancement factors due to multiple scattering are given for the heights, solar zenith angles and wavelengths (between 800 and 300 nm) which play a role in the photodissociation of various atmospheric constituents.     

Model of photoelectron impact ionization within the high latitude ionosphere at Mars: Comparison of calculated and measured electron density

The production rate, ion density and electron density are calculated between longitudes 0° and 360° E due to incident radiation of wavelength range 1-102.57 nm in the dayside atmosphere of Mars. These calculations are made by using global analytical yield spectrum (AYS) model at solar zenith angle 80° between latitudes 50° and 70° N for spring equinox and medium solar activity condition. These conditions are appropriate for Mars Global Surveyor (MGS) Phase 2 aerobraking period during which both the accelerometer and the radio occultation data are used. The calculated results are compared with MGS radio occultation measurements carried out at different latitudes (64.7°-67.3° N) and longitudes (0°-360° E) in December 1998 between solar zenith angle 78° and 81°. This measurement shows primary and secondary ionization peaks, which are varying with longitudes. Our calculation suggests that first peak is produced by photoionization and photoelectron impact ionization processes due to absorption of solar EUV radiation (9-102.57 nm). The second peak is produced by photoelectron impact ionization of soft X-ray photon (1-9 nm). There is a good agreement between our calculation and measurement as far as the maximum and the minimum values of primary peak altitude/peak density of electrons are concerned. However, the calculated values of secondary peak density and peak altitude are higher than the measured values by a factor of 1.5-2.0 and 1.1, respectively. The secondary peak is brought into agreement with the measurement using low X-ray flux by a factor of 2 to 3 below 9 nm. The longitudinal distribution of calculated and measured peak density and peak altitude are fitted by least-square method with 0.95 confidence limits.     

The Method of Differential Measurement of Astronomical Refraction and Results of Trial Observations

Because of the influence of atmospheric refraction the astronomical observations of the objects with the angles of elevation below 15° are generally avoided, but for the sake of the complete theoretical research the atmospheric refraction under the condition of lower angles of elevation is still worthy to be analyzed and explored. Especially for some engineering applications the objects with low angles of elevation must be observed sometimes. A new idea for determining atmospheric refraction by utilizing the differential method is proposed. A series of observations of the starry sky at different heights are carried out and by starting from the zenith with a telescope with larger field of view, the derivatives of various orders of atmospheric refraction function at different zenith distances are calculated and finally the actually observed values of atmospheric refraction can be found via numerical integration. The method does not depend upon the strict local parameters and complex precise observational instrumentation, and the observational principle is relatively simple. By the end of 2007 a simply constructed telescope with a larger field of view at Xinglong Observing Station was employed to carry out trial observations. The values of atmospheric refraction at the true zenith distances of 44.8° to 87.5° were obtained from the practical observations based on the differential method, and the feasibility of the method of differential measurement of atmospheric refraction was preliminarily justified. Being limited by the observational conditions, the accuracy of the observed result was limited, the maximal accidental error was about 6” and there existed certain systematic errors. The value of the difference between the result obtained at the zenith distance of 84° and that given in the Pulkovo atmospheric refraction table was about 15”. How to eliminate the cumulative error introduced due to the integration model error is the key problem which needs to be solved in future.     

High resolution observations of the 6300 Å oxygen line in the day airglow

Ground based high resolution (R ～ 120,000) spectra of the zenith day sky near 6300 Å were obtained with a PEPSIOS. When compared with the solar spectrum taken with the same spectrometer, the 6300.3 Å line of atomic oxygen was clearly present in emission. The apparent emission rate averaged 6 to 8 kR for solar zenith angles of 50 to 60 deg and decreased smoothly to about 1 kR as the solar zenith angle increased to 95 deg. The average emission line is somewhat different in width than the thermal line width expected with the Jacchia (1971) model for a 250 km altitude.     

Distribution of HCN in Titan’s upper atmosphere from Cassini/VIMS observations at 3 μm

Cassini/VIMS limb observations have been used to retrieve vertical profiles of hydrogen cyanide (HCN) from its 3 μm emission in the region from 600 to 1100 km altitude at daytime. While the daytime emission is large up to about 1100 km, it vanishes at nighttime at very low altitudes, suggesting that the daytime emission originates under non-LTE conditions. The spectrally integrated radiances around 3.0 μm shows a monotonically decrease with tangent altitude, and a slight increase with solar zenith angle in the 40-80° interval around 800 km.A sophisticated non-LTE model of HCN energy levels has been developed in order to retrieve the HCN abundance. The population of the HCN 0 0⁰ 1 energy level, that contributes mostly to the 3.0 μm limb radiance, has been shown to change significantly with the solar zenith angle (SZA) and HCN abundance. Also its population varies with the collisional rate coefficients, whose uncertainties induced errors in the retrieved HCN of about 10% at 600-800 km and about 5% above. HCN concentrations have been retrieved from a set of spectra profiles, covering a wide range of latitudes and solar zenith angles, by applying a line-by-line inversion code. The results show a significant atmospheric variability above ∼800 km with larger values for weaker solar illumination. The HCN shows a very good correlation with solar zenith angles, irrespective of latitude and local time, suggesting that HCN at these high altitudes is in or close to photochemical equilibrium. A comparison with UVS and UVIS measurements show that these are close to the lower limit (smaller SZAs) of the VIMS observations above 750 km. However, they are in reasonable agreement when combining the rather large UV measurement errors and the atmospheric variability observed in VIMS. A comparison of the mean profile derived here with the widely used profile reported by Yelle and Griffith (Yelle R.V., Griffith, C.A. [2003]. Icarus 166, 107-115) shows a good agreement for altitudes ranging from 850 to 1050 km, while below these altitudes our result exhibits higher concentrations.     

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

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

Energy deposition and primary chemical products in Titan’s upper atmosphere

Cassini results indicate that solar photons dominate energy deposition in Titan’s upper atmosphere. These dissociate and ionize nitrogen and methane and drive the subsequent complex organic chemistry. The improved constraints on the atmospheric composition from Cassini measurements demand greater precision in the photochemical modeling. Therefore, in order to quantify the role of solar radiation in the primary chemical production, we have performed detailed calculations for the energy deposition of photons and photoelectrons in the atmosphere of Titan and we validate our results with the Cassini measurements for the electron fluxes and the EUV/FUV emissions. We use high-resolution cross sections for the neutral photodissociation of N₂, which we present here, and show that they provide a different picture of energy deposition compared to results based on low-resolution cross sections. Furthermore, we introduce a simple model for the energy degradation of photoelectrons based on the local deposition approximation and show that our results are in agreement with detailed calculations including transport, in the altitude region below 1200 km, where the effects of transport are negligible. Our calculated, daytime, electron fluxes are in good agreement with the measured fluxes by the Cassini Plasma Spectrometer (CAPS), and the same holds for the measured FUV emissions by the Ultraviolet Imaging Spectrometer (UVIS). Finally, we present the vertical production profiles of radicals and ions originating from the interaction of photons and electrons with the main components of Titan’s atmosphere, along with the column integrated production rates at different solar zenith angles. These can be used as basis for any further photochemical calculations.