Despite the geophysical importance of solar ultraviolet radiation, specific aspects of its temporal variations have not yet been adequately determined experimentally, nor are the mechanisms for the variability completely understood. Satellite observations have verified the reality of solar ultraviolet irradiance variations over time scales of days and months, and model calculations have confirmed the association of these short-term variations with the evolution and rotation of regions of enhanced magnetic activity on the solar disc. However, neither rocket nor satellite measurements have yet been made with sufficient accuracy and regularity to establish unequivocally the nature of the variability over the longer time of the 11-year solar cycle. The comparative importance for the long-term variations of local regions of enhanced magnetic activity and global scale activity perturbations is still being investigated. Solar ultraviolet irradiance variations over both short and long time scales are reviewed, with emphasis on their connection to solar magnetic activity. Correlations with ground-based measures of solar variability are examined because of the importance of the ground-based observations as historical proxies of ultraviolet irradiance variations. Current problems in understanding solar ultraviolet irradiance variations are discussed, and the measurements planned for solar cycle 22, which may resolve these problems, are briefly described.

Solar ultraviolet irradiance variations: A review

Modele du cycle d'activite solaire. Niveaux d'activite chromospherique stellaire moyennes dans le temps. Fluctuations de l'activite a long terme d'etoiles froides

Stellar Activity Cycles

We employ the JPL long ephemeris DE-102 to study the inertial motion of the Sun for the period A.D. 760–2100. Defining solar orbits with reference to the Sun's successive close approaches to the solar system barycenter, occurring at mean intervals of 19.86 yr, we find simple relationships linking the inertial orientation of the solar orbit and the amplitude of the precessional rotation of the orbit with the occurrence of the principal prolonged solar activity minima of the current millenium (the Wolf, Sporer, and Maunder minima). The progression of the inertial orientation parameter is controlled by the 900-yr ‘great inequality’ of the motion of Jupiter and Saturn, while the precessional rotation parameter is linked with the 179-yr cycle of the solar inertial motion previously identified by Jose (1965). A new prolonged minimum of solar activity may be imminent.

Prolonged minima and the 179-yr cycle of the solar inertial motion

Assessing the relationship between solar activity and some large scale climatic phenomena

Atmospheric turbidity over Egypt

Application of the non-integer technique of power spectral analysis has yielded evidence for long-term periodicities in the Zurich sunspot series. Although the spectrum shows significant power in the wavelength range 9 to 12 yr, additional peaks corresponding to cycle lengths within the ranges 80–100 and 170–180 yr are also found to be significant. The reality of these peaks is tested extending the historical record into an independent future period. The results indicate that the next 60 yr will be characterized by a relatively low sunspot activity.

On the existence of long-term periodicities in solar activity

Direct solar radiation measurements were used to determine the aerosol optical depth (AOD) increase associated with the presence of aerosol and large particles (ash) originating from the 28 March to 4 April 1982 El Chichon eruptions (17.5oN, 93.3oW; Mexico) on Mexico City's polluted atmospheric aerosol layer. The results are compared with those obtained at Vancouver, British Columbia, revealing that a first AOD increase occurred in both locations during May, June, and July 1982. However, the Mexico City AOD decay is more extended, not reaching normal climatological values until February 1983. Meanwhile, Vancouver's AOD reached minimum values in September 1982, which subsequently increased over a period from October 1982 to September 1983. This secondary maximum was recorded in Mexico City from March to August 1983. Results suggest that the first AOD increases in May, June and July 1982, both at Vancouver and Mexico City, are due chiefly to short life-time volcanic ash particles being located near the surface. However, the second AOD increases, associated with anomalously colored twilights, corresponds to stratospheric volcanic aerosols.

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Determination of the atmospheric optical depth due to the El Chichon stratospheric aerosol cloud in the polluted atmosphere of Mexico City

Resumen Con el fin de mejorar la calidad productiva de los tepetates y con ello su habilitacion, el objetivo de este estudio fue evaluar el efecto del durazno ( Prunus persica (L)Batsch), abonos organicos y micorriza en las caracteristicas fisicas, quimicas y bioquimicas de un tepetate fragmentado. Este trabajo se realizo en invernadero en el Instituto de Geologia, Universidad Nacional Autonoma de Mexico (UNAM). El tepetate se recolecto en el municipio de Tetela del Volcan, Morelos, se roturo (2 a 10 mm diametro) y se coloco en macetas sin perforaciones, adicionandose las enmiendas: estiercol bovino, composta y micorriza. Las plantas de durazno se trasplantaron de tres meses de edad; al final del experimento (un ano) se cortaron, se saco el tepetate de la maceta y se seco al aire para su analisis en el laboratorio. El diseno experimental fue completamente al azar con dos factores y tres niveles cada uno (9 tratamientos), con cuatro replicas (36 unidades experimentales). Con los datos se hizo un analisis de varianza y se uso la prueba de Tukey (p ≤ 0.05) en las medias. Los valores de agregacion presentaron el mayor porcentaje en la fraccion de 5 a 10 mm y la de menor porcentaje fue la > 10 mm; sin embargo, en las distribuciones de los tratamientos relativas a la distribucion original del material utilizado, se observa una modificacion importante, comparada con la experimentada en el testigo absoluto. En la estabilidad, la fraccion con mayor porcentaje fue la > 5 mm y la de menor de 3 a 5 mm. El pH fue de ligera a medianamente alcalino, el carbono organico aumento. De las bases intercambiables el sodio (Na

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Gerardo Zenteno

Papers

On the existence of long-term periodicities in solar activity

Determination of the atmospheric optical depth due to the El Chichon stratospheric aerosol cloud in the polluted atmosphere of Mexico City

Enmiendas orgánicas y durazno, Prunus persica (L.) Batsch, en el mejoramiento de una capa endurecida, tepetate tipo duripán

Spectroscopic observations of comet Giacobini-Zinner during the ice encounter