Solar Extreme Ultraviolet Flux Research Articles

All-reflection interferometer for extreme-ultraviolet airglow studies

We describe a possible sounding rocket payload consisting of an interferometer that would observe O I 1304-Å solar and airglow emissions simultaneously and a low-resolution (15-Å) extreme ultraviolet (EUV) spectrometer with a band pass between 250 and 1050 Å to measure the solar EUV flux, a primary source for the O I 1304-Å dayglow emission. The solar measurements, the first of their kind, could provide detailed information on the column of O along the line of sight of the instrument as well as information on the full disk solar line profile, which is important to planetary as well as cometary physics. The information gained through line profile studies of the dayglow includes the relative contribution of the two main excitation mechanisms, photoelectron impact and solar resonance scattering, and a means to verify cross sections and branching ratios. Furthermore, valuable information on optically thick cascade enhancement can be ascertained. All such information will substantiate sophisticated models, electron and radiative transport, that can be utilized in the remote sensing of the thermosphere.

SOURCE: The Solar Ultraviolet Radiation and Correlative Emissions Mission

The Solar Ultraviolet Radiation and Correlative Emissions (SOURCE) mission is intended to advance our ability to specify the spectral irradiance of the Sun in the extreme ultraviolet (EUV) wavelength range through simultaneous, radiometrically accurate measurements of the solar EUV spectral irradiance and measurements, including EUV and visible images, of solar parameters that are correlated with the EUV flux. The data will be used in combination with empirical modelling to develop and validate a more accurate system of proxy, or surrogate, indices for the solar EUV flux.

"Modeled solar EUV flux during the Equinox Transition Study: September 17–24, 1984""

A new empirical model of solar extreme ultraviolet (EUV) irradiance variability has been developed based on satellite and rocket EUV observations for all levels of solar activity. Its results for the Equinox Transition Study (ETS) period of September 17–24, 1984, have been calculated, and the tabulated results of photon and energy fluxes for each day of this period are presented. The model average total EUV full‐disk energy flux for the spectral range between 1.9 and 105.0 nm for the ETS period is 2.9 ergs cm−2 s−1, with a general secular increase in the total flux level between September 17 and September 24. This modeled rise corresponds to the increased EUV irradiance variability due to active region evolution and passage across the disk. The modeled EUV values in September 1984 near solar cycle 21 minimum conditions are higher than rocket‐ and satellite‐measured EUV values near solar minimum conditions of cycle 20. However, the results are consistent with recent trends in the literature to revise the solar cycle 20 minimum flux values upward.

Solar EUV measurements at Venus based on photoelectron emission from the Pioneer Venus Langmuir Probe

The photoelectron current from the Pioneer Venus Langmuir probe has provided measurements of the solar extreme ultraviolet flux at Venus since 1979. This current is the product of the photoelectric yield of the collector and the solar spectrum at wavelengths short enough to cause emission. Calculations show that approximately 51% of the emission is due to Lyman α (1216 Å), 46% is produced by wavelengths between 550 and 1100 Å, and less than 3% is due to wavelengths longer than Lyman α. Thus the Langmuir probe provides a direct measure of the total solar EUV flux, including most of the wavelengths that produce the Venus ionosphere and heat and excite neutrals in the thermosphere. The measurement technique is described, and the daily average measurements of photocurrent obtained between 1979 and 1987 are presented. The photocurrents exhibit variations related to the solar cycle and solar rotation, as well as a major 7.2‐month periodicity. We present three indices of EUV based on the measurements: (1) the photoemission current itself, (2) the total EUV flux, and (3) an F10.7‐like solar index. These are compared with related measurements made simultaneously at Earth. Since the measurements are made at Venus, they are more useful for many Pioneer Venus investigations than are Earth or Earth satellite observations of the Sun. These data may also help solar physicists track the intensity of EUV emission regions on the Sun while they are not visible from the Earth. The EUV flux profile of a solar flare event is also illustrated. In the future the method also could be applied on a comet mission to obtain the incident solar EUV flux, to measure the EUV extinction profiles of the cometary atmosphere, and to sample directly the dust and gas environment of the comet through the ionization the dust and gas produce when they impact the collector.

Absolute, extreme-ultraviolet, solar spectral irradiance monitor (AESSIM)

AESSIM, the Absolute, Extreme-Ultraviolet, Solar Spectral Irradiance Monitor, is designed to measure the absolute Solar spectral irradiance at extreme-ultraviolet (EUV) wavelengths. The data are required for studies of the processes that occur in the Earth's upper atmosphere and for predictions of atmospheric drag on space vehicles. AESSIM is comprised of Sun-pointed spectrometers and newly-developed, secondary standards of spectral irradiance for the EUV. Use of the in-orbit standard sources will eliminate the uncertainties caused by changes in spectrometer efficiency that have plagued all previous measurements of the Solar spectral EUV flux.

Quenching of O +( 2D) by electrons in the thermosphere

A major loss process for the metastable species, O +( 2D), in the thermosphere is quenching by electrons O +( 2D) + e → O +( 4S) + e . To date no laboratory measurement exists for the rate coefficient of this reaction. Thermospheric models involving this process have thus depended on a theoretically calculated value for the rate coefficient and its variation with electron temperature. Earlier studies of the O +( 2D) ion based on the Atmosphere Explorer data gathered near solar minimum, could not quantify this process. However, Atmosphere Explorer measurements made during 1978 exhibit electron densities that are significantly enhanced over those occurring in 1974, due to the large increases that have occurred in the solar extreme ultraviolet flux. Under such conditions, for altitudes ≳ 280 km, the electron quenching process becomes the major loss mechanism for O +( 2D), and the chemistry of the N + 2 ion, from which the O +( 2D) density is deduced, simplifies to well determined processes. We are thus able to use the in situ satellite measurements made during 1978 to derive the electron quenching rate coefficient. The results confirm the absolute magnitude of the theoretical calculation of the rate coefficient, given by the analytical expression k( T e ) = 7.8 × 10 −8 ( T e /300) −0.5 cm 3 s −1. There is an indication of a stronger temperature dependence, but the agreement is within the error of measurement.

An experimental and theoretical study of the mean diurnal variation of O+, NO+, O2+, and N2+ ions in the mid‐latitude F1 layer of the ionosphere

Over the past decade, several models have been developed which describe the diurnal behavior of molecular and atomic ions in the altitude range where the atomic and molecular densities are comparable. However, no complete set of diurnal measurements has been available in the past with which to compare these theoretical results, because the altitude regime (130–220 km) lies below the reach of conventional satellites and rocket results give only a transient picture. The Atmosphere Explorer C, D, and E satellites have been taking measurements of ion concentrations together with related aeronomic parameters in the thermosphere for over 4 years. These data have been analyzed to extract the average diurnal variation (i.e., for solar zenith angles less than 105°) of the ion species NO+, O2+, N2+, and O+ as functions of altitude. The AE data have been used in a number of separate studies which have reduced the uncertainties associated with key reaction rate coefficients. As a test of the overall consistency of the photochemistry of the F1 layer we compare the measured ion concentrations with those calculated by using a model which incorporates our latest understanding of the chemistry and which uses, as input parameters, the measured neutral concentrations and temperature, the solar extreme ultraviolet flux, and electron and ion temperatures. The results show that a fairly simple model, in which the free parameters are minimized, reproduces the measurements to a satisfactory degree.

Intensity variation of atomic oxygen red line in the day airglow with solar activity

Rates of production of O(1D) atoms in the upper atmosphere by photodissociation of O2, dissociative recombination of O2+, NO+ and electron impact excitation of O(3P) have been calculated for low, medium and high levels of solar activity. Variations with solar activity, of neutral and ionic composition, electron and neutral temperatures of the upper atmosphere and solar extreme ultraviolet fluxes incident on it have been taken into consideration.

Altitude Distributions of and Radiations from Certain Oxygen and Nitrogen Metastable Constituents

The airglow emitted by certain metastable constituents N(2D), O(1D), O+(2P), and O+(2D) in the daytime atmosphere is investigated and their altitude distributions of density are obtained.The production of N(2D) atoms by dissociative recombination of N2+ with electrons is considered. Their quenching by collisions with O2 molecules is calculated, and it was shown that the calculated integrated intensity of NI 5199A line agrees with the observed value.Assuming collisional deactivation of O(1D) atoms by O2 molecules, the production rates of O(1D) by photo-dissociation of O2 in the Schumann-Range continuum at low, medium and high activities of the sun are calculated. The O(1D) distribution and rates of emission of the oxygen red line in the dayglow are also calculated. Calculated intensity at minimum activity agrees with the observed value. An order of magnitude increase in its intensity from minimum to maximum activity cannot be accounted for as due to the variations of molecular oxygen concentration and of solar extreme ultra violet flux incident upon the upper atmosphere. Contributions due to dissociative recombination of O2+ and by thermal electron impact at different levels of solar activity cannot be computed as the variation of ion composition of the ionosphere and that of electron temperature with solar cycle are not known.The production rates of metastable O+(2P) and O+(2D) ions by photoionization of O atoms are calculated. Their equilibrium distributions are computed after using the recent data of rate coefficients for ion-atom interchange and other ionic reactions. As a check, the zenith intensity of (7319-7330A) multiplet of O+ in the day airglow is calculated.

Long-term variation of solar extreme ultraviolet fluxes

The aeronomical importance of the 27-day variation of the incident solar EUV fluxes observed from the Oso 1 satellite over two successive solar rotations during spring 1962 has been discussed recently by Bourdeau et al. [1964] in this journal. Existing observational knowledge of variations over longer periods of time is even less satisfactory than that concerning these 27-day variations, since no continuous EUV monitoring experiments were ever made before the launch of Oso 1. Inferences of long-term variations of absolute intensities in the solar EUV spectrum based on a few short-lived rocket experiments must be treated with caution. In this letter we discuss the significance of certain long-term variations which are apparently implied by measurements variously covering the EUV range from 1300 A to 250 A and 310 A to 55 A, accomplished in rocket experiments with EUV monochromators by our group of the Air Force Cambridge Research Laboratories over the past five years. The essential results of these measurements obtained since the first successful monochromator flight of 1959 have been reported previously (except for results from the flight of December 12, 1963, which are presented here). References and data on EUV fluxes up to August 1960 were presented by Hinteregger [1961] and later work was discussed recently by Hinteregger et al. [1965].

Apparent Relation between Solar Activity and the 440-Mc/s Radar Distance of Venus

A CURIOUS correlation (Fig. 1) has been found between the systematic variations in the astronomical unit, as derived from radar observations of Venus at 440 Mc/s (68 cm)1, and the solar 20-cm flux as measured at the Heinrich Hertz Institut in Berlin-Adlers-hof2. The 20-cm flux is known to be a good indicator of solar activity, and there is evidence that solar decimetre radiation (slowly varying component) is related to both the solar extreme ultra-violet flux and the solar wind intensity3,4.