Sky twilight brightness and colour during high solar activity

With a sensitive photoelectric photometer, observations of the sky twilight brightness have been carried out at different positions in the sky during high solar activity period. The measurements have been obtained using blue and red wide band glass filters centered at 4410 and 7900 Å, respectively. The variation of the (B-R) colour index of the sky twilight with Sun's depression have been investigated at different altitudes in the sky above the horizon and various bearing angles from the solar vertical.     

Solar flare high-energy electron spectra

The Moon's umbral shadow, tangentially penetrating the Earth's atmosphere, appreciably reduces the brightness of the twilight sky at points located under the shadow axis. This should yield favourable conditions for observation of the zodiacal light for small altitudes of the Sun below the horizon. The location of the projection of the Moon's shadow axis at the Earth's surface under the above conditions is calculated for the case of the total solar eclipse of 30 June, 1992.     

Changes in thermospheric composition inferred from twilight O+(2P) emission

Measurements of the twilight enhancement of airglow emission from O⁺(²P) near 7325 Å reveal major changes which accompany geomagnetic activity, no significant distance between evening and morning and an increase in brightness paralleling the approach to solar maximum. The principal source for O⁺(²P) is direct photoionization from O(³P) but at low solar activity there appears to be a contribution from another source in early twilight which may be local photoelectron ionization into O⁺(²P). The geomagnetic and solar effects appear to reflect changes in the O and N₂ density in the thermosphere; ground based twilight measurements of O⁺ emissions thus provide a simple means for monitoring thermospheric structure from 300 km to ～ 500 km at solar minimum and to ～600 km at solar maximum.     

Further study of changes in thermospheric molecular oxygen abundance inferred from twilight 6300 Å airglow

Measurements of the [OI] 6300 Å twilight airglow during 1973 near Boulder, Colorado, show a strong dependence upon geomagnetic activity for the morning enhancement at solar depression angles where production of O¹D) is due primarily to photodissociation of O₂ and local photoelectron excitation. Analysis indicates that photodissociation is the dominant source; hence we infer a well defined magnetic dependence for the O₂/N₂ concentration ratio in the thermosphere. A seasonal variation in the twilight enhancement intensity is barely evident, in contrast with earlier observations made near solar maximum; the smaller variation is associated with a corresponding reduction in the seasonal variation of geomagnetic activity.     

Photoelectric study of the sky brightness along Sun’s meridian during the March 29, 2006 solar eclipse

Photoelectric observations of the sky brightness along Sun’s meridian have been carried out at Salloum during the March 29, 2006 total solar eclipse. The measurements have been taken at different zenith distances along the Sun’s meridian using yellow and red wide band glass filters centered at 5500 Å and 7900 Å, respectively. The present results of the sky brightness during the total solar eclipse have been compared with that of twilight, and night sky obtained by the same instrument at Abu-Simbel and Kottamia observatory sites respectively. The variation of V–R color index with zenith distance have been also studied. The visibility of planets and stars during the March 29, 2006 total solar eclipse is given.     

The troposphere-stratosphere radiation field at twilight: A spherical model

Time-dependent calculations of trace constituent distributions require as input the dissociating radiation field as a function of altitude and solar zenith angle. An isotropic, spherical, multiple scattering model of the radiation field has been developed to determine the radiation field at twilight. Comparison of the spherical model with a plane parallel model at twilight shows that: (1) for solar zenith angles less than 92°, plane parallel solutions for the source function are suitable if the initial deposition of solar energy is calculated for a spherical atmosphere; (2) for solar zenith angles greater than 92°, the plane parallel radiation field can be several orders of magnitude smaller than that calculated with the spherical model; (3) at altitudes above 40 km and at all solar zenith angles, the spherical model predicts 10–20% less radiation than the radiation field calculated with the plane parallel model. Calculations of the rate of photodissociation of NO₂ in the troposphere and stratosphere show that the spherical model yields significantly higher values at solar zenith angles greater than 92°.     

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.     

Synoptic charts of solar 9.1 cm and coronal hole data

Synoptic charts for Carrington rotations 1601–1605 (May–August, 1973) were prepared using the central meridian column of the daily 9.1 cm Stanford solar radio maps. These charts were especially contoured to emphasize temperatures near the quiet solar disk level. Synoptic charts of coronal holes from the ATM-Skylab were superimposed on the radio data to investigate the ability of the radio charts to show coronal holes. This brief period is unfortunately the only interval for which both sets of data are available. The conclusion reached is that in spite of certain problems due to active regions, side-lobe effects and a rather large beamwidth, the 9.1 cm synoptic charts can be of substantial value in identifying large coronal holes, especially during periods of low solar activity. Such synoptic charts, therefore, for the years 1962–1973 that Stanford data are available, could enhance significantly the meagre data pool for coronal holes prior to the Skylab mission.     

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 measurements of sky brightness on lunokhod-2

An astrophotometer was used for measurements of lunar sky brightness in visible and ultraviolet range during day and night. The data obtained showed unexpectedly high values of brightness during the lunar day in the visible region. From measurements during lunar ‘twilight’ conditions and from the dependence of excessive flux on cosZ⊙ we have concluded that the effect is due to scattering of solar radiation by dust particles above the surface of the Moon. Some evidence in favour of dust clouds around the Moon is presented.     

A forecast of solar activity for the 21st solar cycle

Predicted values of some main indices of solar activity for the 21st solar cycle are given. The epoch of maximum of solar activity has been placed in 1980.8±0.1. The predicted peak values of the relative sunspot numbers published by other authors are also given.     

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.     

Carrington Coordinates and Solar Maps

Solar synoptic charts are normally displayed using Carrington Coordinates with each Carrington rotation being centered at a Carrington longitude of 180^° and with a full 360^° of solar surface properties included. For the case of reproducing solar magnetic fields in the corona and heliosphere, these maps are wrapped onto the solar surface to provide the boundary conditions for a solution to a set of modeling equations such as the potential field theory equations. Due to differential rotation, the full solar surface cannot be reproduced in this fashion since different parts of the solar surface are observed at different times. We describe here the proper technique for combining observations of the solar magnetic or velocity fields made at different times into a representation of the whole solar surface at a particular specified time that we refer to as a “snapshot heliographic map“.     

On the presence of cosmic dust in the upper atmosphere

The activity of Librids on 4 April 1973 was detected by the rise of the twilight luminosity. The ratio of circumzenithal luminosity in the antisolar to solar azimuth also increases in the presence of cosmic dust in the upper atmosphere.     

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.     

Large-Scale Magnetic Field and Sunspot Cycles

Hα magnetic synoptic charts of the Sun are processed for 1915–1999 and the spherical harmonics are calculated. It is shown that the polarity distribution of the magnetic field on Hα charts is similar to the polarity distribution of the Stanford magnetic field observations during 1975–1999. The index of activity of the large-scale magnetic field A(t), representing the sum of the intensities of dipole and octupole components, is introduced. It is shown that the cycle of the large-scale magnetic field of the Sun precedes on the average by 5.5 years the sunspot activity cycle, W(t). This means that the weak large-scale magnetic fields of the Sun do not result from decay and diffusion of strong fields from active regions as it is supposed in all modern theories of the solar cycle. On the basis of the new data the intensity of the current solar cycle 23 is predicted and some aspects of the theory of the solar cycle are discussed.     

On the Possibility of Forecasting the Sun's Activity Using Radiocarbon Solar Proxy

Group sunspot number (GSN) values, averaged over decades, were reconstructed for a time interval 8505 BC–AD 1945 using data on the concentration of radiocarbon in tree rings. The prediction of an average level of solar activity was made for the future four decades by means of a nonlinear forecasting method. It was shown that the average activity of the Sun during 2005–2045 would be lower than at present. The given result is compared to the long-term forecasts by other authors. The importance of a paleoastrophysical approach for obtaining a long-term solar prognosis and for revealing the basic characteristics of solar activity change was confirmed.