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Richards, P. G.

Publications and source records attributed to Richards, P. G..

72 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

The altitude variation of the ionospheric photoelectron flux A comparison of theory and measurement

The 145 to 300-km altitude variation of the measured photoelectron flux in the 13 to 18 eV, 28 to 34 eV, and 50 to 55 eV energy regions are compared with the variations expected from theory. There is a strong linear relationship between the measured photoelectron flux and the attenuation of the solar EUV flux at these energies. Therefore, the photoelectron flux is sensitive to changes in the solar zenith angle, neutral density scale height, and total neutral density. However, contrary to previous assertions, the photoelectron flux at most energies is not sensitive to the relative densities of the neutral constituents. In addition, good agreement between theory and measurement is obtained. By using the concept of photoelectron production frequencies, the usually complex evaluation of the local equilibrium photoelectron flux is reduced to a trivial calculation so that the steps in the calculation can be readily verified.

Richards, P. G.↗
NASA NTRS1984

Results of a comprehensive study of the photochemistry of N2(+) in the ionosphere

The improved match that can be obtained between the Atmospheric Explorer (AE) data on the ionospheric F layer N2(+) abundance and theoretical predictions by changing the dissociative recombination rate coefficient is demonstrated. Historically, models have overestimated the N2(+) concentration. It is shown that the calculated enhancement is due to charge exchange between 0(+)(2D) with N2. Increasing alpha by a factor of 2-3, at least in the orbits examined, can augment the destruction of vibrationally excited N2(+) ions. However, the validity of the correction is dependent on the availability of further laboratory data.

Abdou, W. A.↗
NASA NTRS1984

An investigation of the consistency of the ionospheric measurements of the photoelectron flux and solar EUV flux

This paper demonstrates that there is an inconsistency between ionospheric measurements of the photoelectron flux and the solar EUV flux. The problem which occurs for EUV wavelengths near 250 A or photoelectron energies near 35 eV has serious implications for theoretical photoelectron flux calculations and could help to explain up to 40 percent of the difference between calculated and measured photoelectron fluxes for electron energies below 35 eV. It is also shown that: (1) only photons with wavelengths shorter than 310 A are of importance in producing photoelectrons; (2) above 20 eV, the photoelectron flux is controlled by the production of primary photoelectrons, while below 20 eV the flux is controlled by degraded photoelectrons; (3) the calculated cascading production rate is insensitive to the cross sections used for the calculation; and (4) the photoelectron flux is inversely proportional to the total inelastic cross sections.

Richards, P. G.↗
NASA NTRS1983

Comparison of measured and calculated low latitude ionospheric properties

Measurements of ionospheric parameters above Arecibo, Puerto Rico, have been compared with a computer simulation for a variety of conditions. Agreement was found between the measured and calculated electron concentration during geomagnetically quiet conditions. Comparisons for more active conditions indicate a significant upward flow of ionization during the mid-afternoon. Calculated electron temperatures were found to be consistently lower than measured temperatures during the daytime. Calculated values of NmF2 and h(max) agreed with measured results except during the post-midnight period. Calculated values of the ion flux indicate a 24-hour net flow of ionization from the northern to the Southern Hemisphere amounting to 12 percent and 6 percent of the equilibrium flux tube content above 1000 km for the winter solstice and equinox cases, respectively.

Chandler, M. O.↗
NASA NTRS1983

Large-scale counterstreaming of H(+) and He(+) along plasmaspheric flux tubes

An interhemispheric plasma transport model is used to study the flow characteristics of H(+), He(+), and O(+) along closed geomagnetic field lines for solstice conditions. The model corresponds to a time-dependent solution of the coupled continuity, momentum, and energy equations for the ions and electrons. The equations are solved along an entire flux tube from 120 km in one hemisphere to 120 km in the other hemisphere. The calculations are carried out for noon conditions for the flux tube passsing through Millstone Hill, at L = 3.2. The main conclusion is that H(+)-He(+) counterstreaming can be expected along a large segment of a plasmaspheric flux tube at solstice. For both symmetric and asymmetric wind patterns, the He(+) flow is from the winter to the summer ionosphere not only in the steady state, but during flux tube refilling due to the winter helium bulge and the depletion of N2.

Richards, P. G.↗
NASA NTRS1983

A simple theoretical model for calculating and parameterizing the ionospheric photoelectron flux

A method for calculating the ionospheric photoelectron flux is developed which uses the concept of average electron energy loss to simplify the calculation of the degraded electron spectrum. This method requires only a knowledge of the total inelastic electron impact cross sections and can be used for the calculation of all secondary ion and excited state production rates. The simple calculation reduces the computing time by a factor of 10 and considerably reduces storage requirements. It is found that the ionospheric photoelectron flux in the local equilibrium region is directly proportional to the attenuated solar EUV flux, which is a function only of the total neutral column density, and is independent of the neutral density composition. Thus, it is shown that electron impact cross section can be chosen so that this method may also be used to parameterize the measured ionospheric photoelectron fluxes.

Richards, P. G.↗
NASA NTRS1982

Determination of photoionization branching ratios and total photoionization cross sections at 304 A from experimental ionospheric photoelectron fluxes

High-resolution measurements of the ionospheric photoelectron spectrum are used to derive photoionization cross sections and branching ratios for N2 and O at 304 A, the wavelength of the intense He II solar radiation. Based on a theory in which the measured fluxes at energies 22.5, 24.5, 25.5 and 27.5 eV are determined by the ratio of the production rate and the loss rate coefficient, values are obtained for the ratio of molecular nitrogen density to atomic oxygen density as a function of altitude, the sum of the total photoionization cross sections, the 2P/4S and 2D/4S branching ratios for O and the B/X and A/X branching ratios for N2 photoionization and the ratios and the interspecies cross section ratios 2D/A, 4S/X and 2P/B. The values obtained are in accord with the photoionization cross sections of Kirby et al. (1979).

Richards, P. G.↗
NASA NTRS1982

The seasonal effect of nitric oxide cooling on the thermospheric U.V. heat budget

The effects of (1) the radiative cooling of vibrationally-excited NO at 5.3 microns and (2) radiation entrapment at 63 microns on the UV heating efficiency 'epsilon' are examined. It is noted that the previously used heating efficiency definition contradicts its logical application, and that the two processes mentioned should be included as cooling phenomena in the energy equations. Comparisons are made with previous work by including O and NO radiative cooling in the calculation of epsilon, which is found to vary from 45 to 60%, depending on how the two radiative cooling mechanisms are included in the calculation. It is also found that the shape of the heating efficiency altitude profile varies with season, while the peak value is nearly invariant.

Richards, P. G.↗
NASA NTRS1981

A formula for calculating theoretical photoelectron fluxes resulting from the He/+/ 304 A solar spectral line

A simplified method for the evaluation of theoretical photoelectron fluxes in the upper atmosphere resulting from the solar radiation at 304 A is presented. The calculation is based on considerations of primary and cascade (secondary) photoelectron production in the two-stream model, where photoelectron transport is described by two electron streams, one moving up and one moving down, and of loss rates due to collisions with neutral gases and thermal electrons. The calculation is illustrated for the case of photoelectrons at an energy of 24.5 eV, and it is noted that the 24.5-eV photoelectron flux may be used to monitor variations in the solar 304 A flux. Theoretical calculations based on various ionization and excitation cross sections of Banks et al. (1974) are shown to be in generally good agreement with AE-E measurements taken between 200 and 235 km, however the use of more recent, larger cross sections leads to photoelectron values a factor of two smaller than observations but in agreement with previous calculations. It is concluded that a final resolution of the photoelectron problem may depend on a reevaluation of the inelastic electron collision cross sections.

Richards, P. G.↗
NASA NTRS1981

Ionospheric composition: The seasonal anomaly explained

The main photochemical processes of the ionosphere are reanalyzed in the light of laboratory measurements of rate coefficients, using the Atmosphere Explorer data. Major changes to the chemistry include the transfer of nearly all metastable 0(+) ions to N2(+) via charge exchange with N2. The N2(+) ions become vibrationally excited by resonant fluorescence of solar near UV and near infrared radiation, leading to a return transfer of N2(+) ionization to 0(+) by charge exchange or vibrationally excited N2(+) with atomic oxygen. With this chemistry the seasonal variations in the peak electron densities are then shown to be caused primarily by anomalous seasonal variations in neutral composition. The required neutral composition variations are empirically produced by the MSIS model atmosphere. The circulation derived from recent 3D models of the global thermosphere qualitatively accounts for the seasonal variations in neutral composition predicted by the MSIS model. In addition to the composition effect, vibrationally excited N2 is found to contribute a 20% effect to the anomalous seasonal behavior at solar maximum.

Torr, D. G.↗
NASA NTRS1981

Solar EUV energy budget of the thermosphere

The considered study represents an extension of investigations reported by Torr et al. (1980). The earlier investigations are concerned with a quantification of thermospheric UV energy partitions for steady state conditions. The heating efficiency was determined as a function of altitude for selected conditions. Its value was found to vary dynamically as a function of altitude, season, latitude, and solar cycle. The steady state solution reported earlier is extended in the current study to a time dependent solution over a diurnal cycle. It is pointed out that the reported results will make necessary a significant revision of thermospheric models. Thus, estimates of additional sources such as Joule heating obtained from global thermospheric circulation models will require reevaluation.

Torr, M. R.↗
NASA NTRS1980

A new determination of the ultraviolet heating efficiency of the thermosphere

Experimental measurements of relevant parameters over the last five years have made possible an accurate reassessment of the thermospheric heating efficiency for solar ultraviolet photons. The outcome of a steady state calculation applicable to mid-latitudes is reported here. The results are found to be significantly different from an earlier determination, both in magnitude and in variation with altitude. The heating efficiency is approximately 50% near the peak of the extreme ultraviolet energy deposition and falls to approximately 10% near 400 km. The implications for the understanding of the thermospheric energy budget are significant. The various energy channels by which solar energy is transferred to the neutral atmosphere are quantified and summer/winter and solar maximum/solar minimum conditions are compared.

Torr, M. R.↗
NASA NTRS1980

Causes of the F region winter anomaly

Measurements made by the Atmosphere Explorer satellites are used to determine the causes of the F-region winter anomaly, i.e., seasonal variations which result in peak electron densities which are larger in winter than in summer. It is found that for the data examined there are three causes of the winter anomaly: (1) seasonal changes in neutral composition; (2) an increase in the vibrational temperature of N2 and hence in the rate coefficient for the reaction O(+)(4S) + N2 yields NO(+) + N; (3) an increase in winter in the production of O(+)(4S) due to enhanced electron quenching of O(+)(2D). The presence of vibrational excitation is inferred from an increase in the rate coefficient for the reaction O(+) + N2 yields NO(+) + N from winter to summer at heights above 260 km. It is demonstrated quantitatively that the initial increase in electron density produced in winter by causes (1) and (2) results in an increase in the quenching of O(+)(4S), thereby enhancing the source of O(+)(4S) ions. It is estimated that causes (1) to (3) account for approximately 55%, 20% and 25% respectively of the seasonal change in O(+)(4S) from summer to winter at solar minimum.

Torr, D. G.↗
NASA NTRS1980

The solar ultraviolet heating efficiency of the midlatitude thermosphere

In this paper we quantify the major channels by which solar UV energy is transferred to the atmosphere, and determine the UV heating efficiency for the thermosphere. Current knowledge of the solar ultraviolet fluxes, thermospheric chemistry and neutral atmosphere is used in a steady state, interhemispheric solution of the ionospheric continuity, momentum and energy equations. The heating efficiency is found to be a strong function of altitude, and peaks near the altitude of maximum EUV deposition at a value of approximately 50%.

Torr, M. R.↗
NASA NTRS1980

Destruction of N/2D/ by O2 - A major source of 6300 A dayglow emission

The paper discusses the N(2D) + O2 yields NO + O(1D), for k1 reaction, and examines under different geophysical conditions, the effect of this source of 6300 A surface brightness on the dayglow redline. In calculating the 6300 A volume emission rate, a computer code is used which provides the thermal electron density, photoelectron flux, ion concentrations, electron and ion temperatures and odd nitrogen densities required to evaluate the O(1D) sources and sinks. It is found that under most conditions this source is the dominant mechanism responsible for the production of the 6300 A dayglow above 150 km. The volume emission rate of this source in summer is 2.5 times larger than the sum of all other sources at 180 km, and 1.5 times as large in winter. In addition, when the N(2D) source of O(1D) is taken into account, the rate coefficient for quenching of O(1D) by N2 is estimated to be 4 plus or minus 1 x 10 to the -11th cu cm per sec.

Torr, D. G.↗
NASA NTRS1979

Counterstreaming of O+ and H+ ions in the plasmasphere

Although the ion flux along plasmaspheric flux tubes has been both measured and numerically modeled for many years, it is only recently that H(+)-O(+) counterstreaming, with O(+) upwards, has been studied. The theoretical studies indicate that counterstreaming occurs under special conditions primarily near twilight. It is shown that such counterstreaming arises not only from a combination of particular conditions but is a more fundamental characteristic of the flow of plasma in the closed field-line magnetosphere. The causes of counterstreaming are analyzed showing that it must occur under steady state conditions; it is also shown how diurnal variations modify the steady state pattern.

Young, E. R.↗