Solar Total Irradiance: A Reference Value for Solar Minimum

Simultaneous solar total irradiance observations performed by absolute radiometers on board satellites during the quiet-Sun period between solar cycles 21 and 22 (1985–1987), are analyzed to determine the solar total irradiance at 1 AU for the solar minimum. During the quiet-Sun period the total solar irradiance, UV irradiance, and the various solar activity indices show very little fluctuation. However, the absolute value of the solar total irradiance derived from the observations differ within the accuracy of the radiometers used in the measurements. Therefore, the question often arises about a reference value of the solar total irradiance for use in climate models and for computation of geophysical, and atmospheric parameters. This research is conducted as a part of the Solar Electromagnetic Radiation Study for Solar Cycle 22 (SOLERS22). On the basis of the study we recommended a reference value of 1367.0 ± 0.04 W m^-2 for the solar total irradiance at 1 AU for a truly quiet Sun. We also find that the total solar irradiance data for the quiet-Sun period reveals strong short-term irradiance variations.     

The ionization state of oxygen ions in anomalous cosmic rays: Results from the ANURADHA experiment in Spacelab-3

We report the first results on the determination of the ionization states of oxygen ions in the anomalous cosmic rays (ACR) from the measurements of their flux in the cosmic-ray experiment in Spacelab-3 (SL-3) mission of NASA flown at 350 km altitude during 29 April–6 May, 1985. The detectors used were specially prepared CR-39 plastics of very high sensitivity for recording tracks of ions withZ>2. The measured orbit averaged flux of ACR oxygen is (2.9±1.3)×10^–4 particles m^–2sr^–1s^–1 (MeV N^–1) at an energy of 23 MeV N^–1. We made an independent estimate of the expected ACR oxygen flux at SL-3 orbit from interplanetary data and compared this with the measured flux to infer the ionization states of ACR oxygen ions. The flux and energy spectra of ACR oxygen at 1 AU outside the magnetosphere is obtained from the data of Voyager-2, during the same epoch as the SL-3 flight, and using the measured radial intensity gradient of 15%/AU for ACR oxygen between 1–17 AU. We calculate the geomagnetic transmission factors for ACR oxygen ions of charge states O⁺¹, O⁺², etc., from the known cut-off rigidities in the world grid and using the SL-3 trajectories for 116 orbits in the 6-day mission to obtain the expected flux at SL-3 for different charge states. When these flux values are compared with our measured flux, the averge ionization state of ACR oxygen ions in the energy interval of 20–26 MeV N^–1 is obtained as O⁺¹.     

Solar radiation and its variation in time

In order to assess the variability of the solar radiation, the record of determinations of the total and spectral solar irradiance of the last 15 years is analysed. Although the datapoints for the period before 1969 suggest a slight decrease, the uncertainties of these determinations are too large to render this statement significant. Together with the results of the following period (1969–1980) which show that within the uncertainty no change is detectable, it can be concluded, that the solar constant has not changed during the last 15 years. The same result is found from the spectral distribution record, though not as conclusive as for the solar constant, due to the much larger uncertainties of the spectral data.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.     

Measurement and Uncertainty of the Long-Term Total Solar Irradiance Trend

A possible long-term trend of the total solar irradiance could be a natural cause for climate variations on Earth. Measurement of the total solar irradiance with space radiometers started in 1978. We present a new total solar irradiance composite, with an uncertainty of ± 0.35 W m⁻². From the minimum in 1995 to the maximum in 2002 the total solar irradiance increased by 1.6 W m⁻². In between the minima of 1987 and 1995 the total solar irradiance increased by 0.15 W m⁻².     

Modeled soft X-ray solar irradiances

Solar soft X-rays have historically been inaccurately modeled in both relative variations and absolute magnitudes by empirical solar extreme ultraviolet (EUV) irradiance models. This is a result of the use of a limited number of rocket data sets which were primarily associated with the calibration of the AE-E satellite EUV data set. In this work, the EUV91 solar EUV irradiance model has been upgraded to improve the accuracy of the 3.0 to 5.0 nm relative irradiance variations. The absolute magnitude estimate of the flux in this wavelength range has also been revised upwards. The upgrade was accomplished by first digitizing the SOLRAD 11 satellite 4.4 to 6.0 nm measured energy flux data set, then extracting and extrapolating a derived 3.0 to 5.0 nm photon flux from these data, and finally by performing a correlation between these derived data and the daily and 81-day mean 10.7 cm radio flux emission using a multiple linear regression technique. A correlation coefficient of greater than 0.9 was obtained between the dependent and independent data sets. The derived and modeled 3.0 to 5.0 nm flux varies by more than an order of magnitude over a solar cycle, ranging from a flux below 1×10⁸ to a flux greater than 1×10⁹ photons cm^–2 s^–1. Solar rotational (27-day) variations in the flux magnitude are a factor of 2. The derived and modeled irradiance absolute values are an order of magnitude greater than previous values from rocket data sets related to the calibration of the AE-E satellite.     

The solar constant

A detailed compilation of the most recent values of the solar constant is given (13 values published from 1967 to 1970). The most probable value seems to be 1.95 cal cm^–2 min^–1 or 1.36 kW m^–2 with a formal rms error of ± 0.3%. The corresponding effective temperature is 5770K.Systematic errors of the order of ± 1%, but also a possible variability of the same order cannot be excluded.     

Results of the solar constant experiment onboard Spacelab 1

A cavity type absolute radiometer was flown on Spacelab 1 in December 1983. We obtain a value of the solar constant of 1361.5 W m^–2 with an estimated accuracy of ±2.3 W m^–2 or 0.17%. When comparing this with other recent determinations, we find discrepancies which we consider indicative of metrological problems in present day absolute radiometry.     

Solar radio burst and in situ determination of interplanetary electron density

We review and discuss a few interplanetary electron density scales which have been derived from the analysis of interplanetary solar radio bursts, and we compare them to a model derived from 1974–1980 Helios 1 and 2 in situ density observations made in the 0.3–1.0 AU range. The Helios densities were normalized to 1976 with the aid of IMP and ISEE data at 1 AU, and were then sorted into 0.1 AU bins and logarithmically averaged within each bin. The best fit to these 1976-normalized, bin averages is N(R _AU) = 6.1R ^-2.10 cm^-3. This model is in rather good agreement with the solar burst determination if the radiation is assumed to be on the second harmonic of the plasma frequency. This analysis also suggests that the radio emissions tend to be produced in regions denser than the average where the density gradient decreases faster with distance than the observed R ^-2.10.NAS/NRC Postdoctoral Research Associate on leave from Laboratory Associated with CNRS No. 264, Paris Observatory, France.     

Solar irradiance from Nimbus-7 compared with ground-based photometry

We have compared total solar irradiance from Nimbus-7 with ground-based photometry from the San Fernando Observatory (SFO) for 109 days between June 1 and December 31, 1988. We have also included in some analyses NOAA-9 SBUV2 data orF10.7 radio flux. The Nimbus-7 data are from orbital samples, averaged to the mean time of observation at SFO. Using the same parameters as in Chapmanet al. (1992), the multiple regression gives anR ² = 0.9131 and a solar minimum irradiance,S ₀, = 1371.76 ± 0.18 W m^–2 for the best fit.     

The Hypothesis of Additional Acceleration in the Motion of Comet Harrington–Abell from Jupiter

By processing 494 observations of Comet Harrington–Abell, we obtained a unified system of elements that includes its turn around the Sun during which it closely approached Jupiter to a minimum distance of 0.037 AU in 1974. A study of the cometary orbit before and after the approach showed that, probably, at the approach of the comet to Jupiter, apart from the well-known gravitational perturbations, its motion was affected by an additional force. An improvement of the cometary orbit by assuming that an additional acceleration inversely proportional to the square of the distance to Jupiter exists in its motion yielded the following values: (4.57 ± 0.42) × 10^–10 and (–7.20 ± 0.42) × 10^–10 AU day^–2 for the radial and transversal acceleration components, respectively. As a plausible explanation of the changes in the cometary orbit, we additionally considered a model based on the hypothesis of partial disintegration of the cometary nucleus. The parameter that characterizes the instant displacement of the center of inertia along the jovicentric radius vector was estimated to be –1.83 ± 0.75 km. Based on a unified numerical theory of cometary motion, we determined the nongravitational parameters using Marsden's model for two periods: A ₁ = (11.68 ± 1.74) × 10^–10 AU day^–2, A ₂ = (0.53 ± 0.0357) × 10^–10 AU day^–2 for 1975–1999 and A ₁ = (5.92 ± 5.86) × 10^–10 AU day^–2, A ₂ = (0.08 ± 0.028) × 10^–10 AU day^–2 for 1955–1969, under the assumption that the nongravitational acceleration changed at the approach of the comet to Jupiter.     

Variations in the solar wind speed along the earth's orbit

Comparison of solar wind speed data obtained from the Pioneer 6 and 7 and Vela 3, 4, and 5 satellites from January 1969 through July 1970 has been undertaken. The distribution of measured speeds is similar for all satellites, despite wide separations along the Earth's orbit. For satellite separations (along the Earth's orbit) of 0.5 AU or less, the speeds measured by different satellites are closely correlated, i.e., it is usually possible to predict (to within ± 100 km sec^–1) the arrival of a particular solar wind speed at one satellite on the basis of earlier measurements at another. For separations larger than 1.0 AU it is usually not possible to make accurate predictions in this manner. This appears to be evidence that: (1) the boundary conditions on the coronal expansion at the base of the corona are a sensitive function of latitude and/or (2) the boundary conditions at any one point on the Sun evolve sufficiently in 4 days to alter significantly the speed of the wind at 1 AU.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

A search for solar neutrons during solar flares

The upper limit on the solar neutron flux from 1–20 MeV has been measured, by a neutron detector on the OGO-6 satellite, to be less than 5 × 10^–2 n cm^–2 s^–1 at the 95% confidence level for several flares including two flares of importance 3B and a solar proton event of importance 3B. The measurements are consistent with the models proposed by Lingenfelter (1969) and by Lingenfelter and Ramaty (1967) for solar neutron production during solar flares. The implied upper limit on the flux of 2.2 MeV solar gamma rays is about the same as the 2.2 MeV flux observed by Chupp et al. (1973).     

Evidence for electron excitation of type III radio burst emission

Type III radio bursts observed at kilometric wavelengths ( 0.35 MHz) by the OGO-5 spacecraft are compared with > 45 keV solar electron events observed near 1 AU by the IMP-5 and Explorer 35 spacecraft for the period March 1968–November 1969.Fifty-six distinct type III bursts extending to 0.35 MHz ( 50 R equivalent height above the photosphere) were observed above the threshold of the OGO-5 detector; all but two were associated with solar flares. Twenty-six of the bursts were followed 40 min later by > 45 keV solar electron events observed at 1 AU. All of these 26 bursts were identified with flares located west of W 09 solar longitude. Of the bursts not associated with electron events only three were identified with flares west of W 09, 18 were located east of W 09 and 7 occurred during times when electron events would be obscured by high background particle fluxes.Thus almost all type III bursts from the western half of the solar disk observed by OGO-5 above a detection flux density threshold of the order of 10^–13 Wm^–2 Hz^–1 at 0.35 MHz are followed by > 45 keV electrons at 1 AU with a maximum flux of 10 cm^–2 s^–1 ster^–1. If particle propagation effects are taken into account it is possible to account for lack of electron events with the type III bursts from flares east of the central meridian. We conclude that streams of 10–100 keV electrons are the exciting agent for type III bursts and that these same electrons escape into the interplanetary medium where they are observed at 1 AU. The total number of > 45 keV electrons emitted in association with a strong kilometer wavelength type III burst is estimated to be 5 × 10³².     

The observation of the solar irradiance and its variations,challenging space metrology

The problems associated with the accurate determination of the total and spectral irradiances of the Sun are discussed. It is estimated that an ultimate accuracy of the order of 2 to 5 × 10^–4 should be aimed at and be feasible for total solar irradiance measurements made with second generation objectively characterised absolute radiometers.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.     

Active cavity radiometer type IV

A new cavity pyrheliometer, the active cavity radiometer type IV (ACR IV), has been developed for the measurement of total solar optical irradiance. Analysis predicts its ability to measure at the solar constant level with 0.1% uncertainty in SI units. In comparison tests ACR IVs have consistently demonstrated 0.5% higher results than the World Radiometrie Reference scale. A prototype has been tested, and a flight instrument has been developed and flown in a sounding rocket experiment to determine the solar constant. ACR IV instrumentation is being developed for flight experiments on the Spacelab 1 and Solar Maximum Missions to monitor the total solar output of optical radiation as part of a long-term program to detect variations of climatological significance.The full paper appeared in Appl. Opt. 18 (1979), 179.     

A search for neutrons of solar origin using balloon borne detectors 1967–69

A series of telescopes having approximately a 30° half opening angle and responding to neutrons in the energy range 50 MeV to 350 MeV has been flown to the top of the atmosphere on balloons released from an equatorial launching site at Kampala, Uganda, between 1967 and 1969. The aim of the experiment was to attempt to detect solar neutrons during periods of enhanced solar activity. No neutrons of solar origin were detected, but an upper limit of the order of 30 neutrons m^–2 s^–1 at the Earth has been placed on the continuous solar neutron flux in the above energy range, and a limit of four photons m^–2 s^–1 has also been placed on the corresponding -ray flux above 80 MeV. Limits have likewise been placed on the total emission from various flares. For a 1B flare the values were 23 × 10⁴ neutrons m^–2 and 6 × 10⁴ photons m^–2.     

Frequencies of low-degree solar acoustic oscillations and the phase of the solar cycle

A study of the solar total irradiance data of the Active Cavity Radiometer Irradiance Monitor (ACRIM) on the Solar Maximum Mission (SMM) satellite shows a small but formally significant shift in the frequencies of solar acoustic (p-mode) oscillations between the epochs of maximum and minimum solar activity. Specifically, the mean frequency of the strongest p-mode resonances of low spherical-harmonic degree (l = 0–2) is approximately 1.3 parts in 10⁴ higher in 1980, near the time of sunspot maximum, than in 1985, near sunspot minimum. The observed frequency shift may be an 11-yr effect but the precise mechanism is not clear.     

Cosmic-Ray Decreases and Interplanetary Disturbances During the Solar Magnetic Polarity Reversal in Solar Cycle 20

Using the cosmic-ray intensity data recorded with ground-based monitors at Mt. Washington and Deep River, and with cosmic-ray telescopes on Pioneer 8 and 9 spacecraft as well as the 2-hour averages of the IMF (magnitude and direction) and the solar wind bulk speed and density at 1 AU, the cosmic-ray decreases and interplanetary disturbances, that occurred during the period of solar magnetic polarity reversal in solar cycle 20, were investigated.We observed a two-step Forbush decrease on 22–23 November 1969, and a Forbush decrease on 26 November 1969, which are respectively consistent with the model of Barnden (1973), and of Parker (1963) and Barnden (1973). Only one Forbush decrease event was observed in December 1969, a period during which there was a solar magnetic polarity reversal; the Forbush decrease was attributed to a long-lived corotating high-speed solar wind stream. This is indicative that at heliolongitudes from 43° E to 70° W of S–E radial, covered by the observations, the solar magnetic polarity reversal in solar cycle 20 was not carried by, nor related to, individual transient structures, and that the reversal most probably evolved gradually.     

Chemical response of the middle atmosphere to solar variations

The possible variation of the trace species concentration in the middle atmosphere related to long term solar irradiance variability is estimated by means of a one-dimensional numerical model.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.Aspirant au Fonds National de la Recherche Scientifique.     

Some requirements of a colliding comet source of gamma ray bursts

Colliding comets in the Solar System may be an important source of gamma ray bursts. The spherical gamma ray comet cloud required by the results of the Venera Satellites (Mazets and Golenetskii, 1987) and the BATSE detector on the Compton Satellite (Meeganet al., 1992a, b) is neither the Oort Cloud nor the Kuiper Belt. To satisfy observations ofN(>P _max) vsP _max for the maximum gamma ray fluxes,P _max > 10^–5 erg cm^–2 s^–1 (about 30 bursts yr^–1), the comet density,n, should increase asn a ¹ from about 40 to 100 AU wherea is the comet heliocentric distance. The turnover above 100 AU requiresn a ^–1/2 to 200 AU to fit the Venera results andn a ^1/4 to 400 AU to fit the BATSE data. Then the masses of comets in the 3 regions are from: 40–100 AU, about 9 earth masses,m _E; 100–200 AU about 25m _E; and 100–400 AU, about 900m _E. The flux of 10^–5 erg cm^–2 s^–1 corresponds to a luminosity at 100 AU of 3 × 10²⁶ erg s^–1. Two colliding spherical comets at a distance of 100 AU, each with nucleus of radiusR of 5 km, density of 0.5 g cm^–3 and Keplerian velocity 3 km s^–1 have a combined kinetic energy of 3 × 10²⁸ erg, a factor of about 100 greater than required by the burst maximum fluxes that last for one second. Betatron acceleration in the compressed magnetic fields between the colliding comets could accelerate electrons to energies sufficient to produce the observed high energy gamma rays. Many of the additional observed features of gamma ray bursts can be explained by the solar comet collision source.