OBSERVATION AND MEASUREMENT OF THE OPTICAL, THERMAL, PRESSURE AND OTHER ATMOSPHERIC PHYSICAL/CHEMICAL EFFECTS DURING THE TOTAL SOLAR ECLIPSE OF 11th AUGUST 1999 IN EUROPE: A PROPOSAL
Project prepared by
MARCOS A. PENALOZA M. (firstname.lastname@example.org) University of Essex. Institute for Environmental Research Central Campus. Wivenhoe Park. Colchester, Essex CO4 3SQ England, U. K. and Universidad de Los Andes. Facultad de Ciencias Departamento de Fisica Merida, Edo. Merida. Venezuela November 1998
Originally or historically speaking partial and total solar eclipses have been the exclusive concern of Astronomy and Astrophysics. However over the decades of this century these phenomena have been taken seriously into account by the Atmospheric and Environmental Sciences to study the response of the atmosphere during the interesting and particular circumstance in which solar light is partially or totally being blocked by the Moon (Silverman and Mullen, 1974). A total eclipse of Sun is about as close to a controlled experiment as an atmospheric research can hope for. Sunlight diminishes at a uniform and predictable rate, and near totality, the dark umbra of the Moon sweeps across the top of the atmosphere in a narrow predictable path. It is then possible to study how chemical and physical processes in the atmosphere take place owing to the absence of sunlight. An eclipse has the advantage over sunset of occurring very rapidly, and with the Sun hardly moving in the sky. Specifically, while a total eclipse is in progress typical optical, meteorological, environmental and other physical/chemical effects appear. In this project a sampling of some of these effects (Zirker, 1995) and research related to them will be considered and that could be carried out on the Earths atmosphere during the forthcoming, and the last total eclipse of the Sun of this century and millennium on 11th August 1999 to be observed from Europe (Espenak and Anderson, 1997). In particular, and taking into account the experience acquired in the observations made during the recent total solar eclipse in Venezuela on 26 February 1998 (Espenak and Anderson, 1996; James, 1998; Penaloza, 1998), this project is intended to make observations and measurements of the optical, thermal, pressure, and other atmospheric physical/chemical effects produced by this astronomical event and will be described briefly in the next section.
II. OPTICAL, METEOROLOGICAL, ENVIRONMENTAL AND OTHER
Instrumental observations of the colour and brightness of the sky, at different locations, and at different heights (zenith horizon), directions (azimuths), polarities and wavelengths of the visual spectrum, form one of the most interesting and appropriate experiments to be carried out during a total solar eclipse. It allows one to quantify the degree of darkness and polarisation attained for the sky, and to determine its colour, while the eclipse observation place is located within the Moons shadow (umbra).
Reported observations of previous eclipses have indicated that the abrupt drop of the solar light, produced during the few minutes of the total phase, and just after the solar dish has been occulted 97.8 % (Beard, 1948; Sharp et al., 1971; Velasquez, 1971), produces a sudden darkness enough as to generate appreciable changes which are characteristics of this kind of phenomena.
In good conditions it is well known that the blue colour of the sky is produced by a polarised process of Rayleigh single scattering occurring in air molecules if the air is clean and dry. In different conditions when the air of a particular place contains natural or anthropogenic aerosol, an additional Mie single scattering process has to be taken into account to describe other optical sky features.
During the solar eclipse totality, the sky darkness is not total. Probably light levels, giving to the sky a weak brightness, are similar to those light levels produced at dusk when the Sun is already some degrees below the horizon. Measurements made of the sky radiance during the total phase of a solar eclipse have shown that the darkest part corresponds to the zenith, with a bluish colour, whereas the lightest part corresponds to the horizon, with a reddish colour. These observations are predicted by the model of Gedzelman (1975) when they are considered near the horizon.
This penumbral and umbral brightness is fundamentally produced by a process of multiple scattering occurring in these eclipse zones. There must be at least a second order scattering in order that the solar photons can reach an observer or instrument installed in the totality shadow. The light quality arrived at will depend on the degree of the air transparency or opacity in the observation place. Therefore, it is not the same to carry out these kind of observations in places with contamination levels above the natural background (urban zones) than to carry them out in cleaner places (countryside). It is recommendable to carry out observations in both places in order to make comparisons and to draw conclusions on the degree of atmospheric contamination in the city involved.
Another interesting optical phenomenon produced at terrain level, is the appearance of the so-called show bands on the surface while the total phase is in progress. Many authors have tried to explain this phenomenon which has been observed photographically and photometrically (Feldman, 1974; Hultz et al., 1971; Marshall, 1984; Seykora, 1979; Stanford, 1973; etc.).
b. Thermal and Pressure Effects
Atmospheric temperature and pressure observations made in other total solar eclipses have shown a rapid descent of temperature and pressure changes which produce meteorological anomalies typical in this kind of phenomena (Anderson et al., 1972; Anderson and Keefer, 1975; Klein and Robison, 1952; Hultz et al., 1974; etc.). Among them are the appearance of winds which would eventually influence in a positive way (positive feedback) cleaning the air pollution if the eclipse is being observed from a contaminated area. To some extent it is still unknown. These observations could be compared with those already published elsewhere and with a model presented by Phillips (1968). Presumably, due to a total solar eclipse, pressure waves can be observed. An attempt to detect them have been done in past eclipses (Bertin, Hughes and Kersley, 1977; Jones, Miseldine and Lambourne, 1992, etc.).
c. Effects on the Atmospheric Surface Boundary Layer
An ideal experiment to study the evolution of the Atmospheric Surface Boundary Layer would be to switch off and then switch back on solar radiation abruptly. This would produce significant variations in boundary layer characteristics making it perfect to study the dynamics and evolution of the atmospheric surface boundary layer (Antonia et al., 1979; Raman, Boone and Rao, 1990; Porch et al., 1995). A total solar eclipse would be give the right setting for this experiment providing near instantaneous cut-off of solar radiation with sudden change in surface cooling. Unfortunately a patch of clouds covering the Sun will not satisfy the appropriate conditions due to complex reflection and light scattering produced in this case. Solar eclipses in clear skies creates an unusually clean and rapid change in the solar radiation which is the main cause driving the boundary layer. In the few studies dealing with boundary layer studies during solar eclipses, important findings are reported. In general, these indicate significant changes in the atmospheric stability during the eclipse with slightly stable conditions present after the second contact (Raman, Boone and Rao, 1990). More research on this area is required in order to have a more complete assessment of this effect.
d. Other Physical Effects
The impact of a total solar eclipse on surface atmospheric electricity is another of the effects which can be studied during this phenomenon. In particular parameters like surface electrical potential gradient and conductivity are recorded as long as the eclipse is in progress. Research on this point shows that as a result of surface atmospheric turbulence change, the surface electrical conductivity is enhanced, while the potential gradient is reduced. This result is a general observation in the majority of the eclipse events. Nevertheless, in spite of the results obtained in past eclipses a complete understanding of the processes which link the observed effects is not yet clear. It is highly recommendable to span the time observation of the parameter involved, 3-4 hours beyond the eclipse duration in order to assess properly the effects on surface atmospheric potential gradient (Manohar, Kandalgaonkar and Kulkarni, 1995).
e. Chemical Effects
A sizeable part of the radiation over the whole spectrum range is absorbed at various levels of the atmosphere by the different constituent molecules and atoms at their characteristic wavelengths. Therefore it is well known that during the day there are some reactions that take place in the atmosphere depending on the action of solar radiation; at nights these reactions do not occur. During a solar eclipse the amount of this radiation, at all wavelengths, reaching the Earths atmosphere decreases as long as the solar disc is being partially/totally blocked by the moon. Thus the disturbances produced on these photochemical reactions, and consequently the perturbations on the concentration variation of the chemical species involved in these photochemical processes, by a diminishing of the solar radiation during the partial phases and totality of a solar eclipse, have theoretical and observational relevance to atmospheric chemistry studies (Saha, 1982).
In particular, atmospheric hydroxyl response to a partial eclipse has been studied by Burnett and Burnett (1985). Fluctuations in column ozone during a total solar eclipse have been studied as well (Mims and Mims, 1993). A theoretical study of the changes occurring in the ozonosphere has been carried out by Hunt (1965) during the obscuration of the sun, and changes produced in the concentration of mesospheric ozone have been observed by Randhawa (1968), and by Agashe and Rathi (1982) under the same circumstance; other ozone measurements have been made by Stransz (1961), and Chatterjee et al. (1982). In the troposphere, near the ground, measurements of ozone concentration have been made by Srivastava et. al (1982), and Chatterjee et al. (1982). In the stratosphere, a theoretical study tracing species variations during a solar eclipse has been conducted by Wuebbles and Chang (1979), and at 19.8 km Starr et al. (1980) have measured NO2 and O3. The response of stratospheric constituents to a solar eclipse in the specific situation of sunrise, and sunset have been considered by Herman (1979). Additional studies have been made using rockets to reach high altitude levels of the atmosphere (Subbaraya and Lal, 1982). Finally, tropospheric pollutants like sulphur-dioxide (SO2) and ammonia (NH3), besides nitrogen-dioxide and ozone, have been monitored by Maske et al. (1982) using different chemical methods.
>From the above exposition, presented in a reviewed manner, a project of observation and measurement of the optical, thermal, pressure and other atmospheric physical/chemical effects, would be achieved during the total solar eclipse on 11th February 1999 in Europe (Espenak and Anderson, 1997) under the objectives specified in the following section.
II. SPECIFIC OBJECTIVES
III. SCIENTIFIC METHODOLOGY AND OBSERVATIONAL APPROACH
To develop and to prepare the points (1), (3), (5), (6), (7), (11) and (13) instrumental observations and measurements can be achieved during the days prior to the eclipse to check the equipment and to monitor the meteorological and environmental conditions of the observation place. These previous measurements can be extended to a period covering sunset, dusk and twilight trying to simulate as much as possible similar conditions of light level using the occultation of the sun disk just above, on and below the horizon. In particular the optical and radiometric measurements will be based on an combined arrangement of photodiodes previously pre-calibrated and calibrated, pointed at different directions on the horizon (azimuths), zenith distances or heights including the zenith, along with three radiometers (ultraviolet and solar), two pyranomaters (ultraviolet and total solar), and one sun photometer. An estimation of particle mass concentration in the air on the site of observation is also very important. The sampling of this air with filters allows the identification, via scanning electron microscopy, of the shape and dimension of the particles collected (point 5). These parameters could be related to the scattering processes taking place in the atmosphere. The photographic observations referred to in point (6) will be prepared taking pictures of the Sun disk in the preceding days of the eclipse, but at the same position and time as it will be seen during the event. Photographic and photometric observations referred to in point (10) will be prepared taking pictures and photometric records of the ground in the preceding days of the eclipse under the same conditions relative to the Sun position and time as it will presented throughout the eclipse. To carry out point (12) the methods suggested by Handojo (1989), to make indirect visual observations, will be applied.
IV. EQUIPMENT AND ACQUISITION DATA SYSTEMS
- - Total solar pyranometer YES, model TSP-700, TSP-400 or TSP-100 (Yankee Environmental Systems, Inc. Mass.).
V. PROBABLE OBSERVATION SITES BASED ON WEATHER PROSPECTS
According to Espenak and Anderson (1997), and Espenak (1998), the total solar eclipse on Wednesday, 11 August 1999, will be visible from U. K., central and south-eastern Europe, and the Near and Middle East (Figs. 1-2). The Moon umbral path will touch British territory at the Cornwall peninsula in Englands south-western coast during the midmorning, where the totality will last 2 minutes (Fig. 3), and passing over Plymouth at approximately 12 km distance from the totality line. After having passed over the English Channel the umbra reaches continental Europe along Frances Normandy coast (Fig. 3), running across northern France (eclipse southern limit just 30 km north of Paris), southern Belgium, Luxembourg (Fig. 4) and Germany (passing over Sttugart and Munich), respectively (Fig. 5). Then, the shadow begins to swing across central Austria (passing over Salzburg) and Hungary (Fig. 6). The shadow briefly sweeps through northern Yugoslavia (Fig. 6) before continuing on to Romania (passing over Bucharest) and where the totality will reach the greatest duration (2 min 23 sec at Rimnicu-Vilcea). The shadow also will enter northern Bulgaria before going to across the Black Sea (Fig. 7). Finally, the eclipse path will touch the Near East in Turkeys northern coast on the Black Sea and continues across this country Southeast direction passing, then, over Irak and Iran (and also eastern Syria) where the shadows trajectory begins to narrow and the duration to drop. The ending stages of the phenomenon will take place late afternoon over southern Pakistan (passing over Karachi) and central India.
Along this stretched path there are multiple diversity of climatologies, meteorological conditions, landscapes, topographical characteristics, etc., which make the local eclipse circumstances, based on weather prospects, very complicated at the moment to choose the most convenient observation site. However, Espenak and Anderson (1997), and Espenak (1998), provide some basic useful information as guidelines in order to have an idea over eclipse viewing chances. According to them the probability of seeing the eclipse, over dry land, increases from Lands End (~45%), in England, to Esfahan (~95%), in Iran.
By technical and logistic convenient reasons the observation site must be restricted in somewhere between Austria and Romania. The probability of seeing the eclipse from countries in this sector of the shadow path ranges from ~53% to ~63%. Among these reasons are: (1) Positions of the sun in the sky (local time) during the event. (2) Duration of the totality at the centre line. (3) Moderate travel expenses. (4) Existence of national committees for the eclipse providing strategic support (facilities) and useful local information. (5) Invitation received from the International Astronomical Union (IAU) Working Group on Solar Eclipses to participate with them in the eclipse observation (from Romania). The reason No. 2, along with the highest probability within the limited interval referred to above, indicate that Romania would probably be the best country to go. In any case, this project has to be confined to somewhere in the countryside of the country selected, faraway from cities, to avoid artificial illumination (light pollution) and big obstacles in the sight line (mainly to the horizon). These are restrictions associated with the radiometric and photometric requirement measurements. In the particular case of Romania, the capital Bucharest and Timisora (where the Romanian observatories are located), are ruled out because they are highly polluted. Then the possible sites are: Retezat National Park, on the line of centrality (about 2400m), the heal region near Ramnicu Valcea (where the maximum will be observed), somewhere in the countryside of Romanian Plain (depending on the weather prospects), and on the Black Sea coast. In these sites observation camps will specially be set up by the Romanian Astronomical Society of Meteor Observers for scientific research activities.
Considering the many factors involved on this matter, the final decision on the exact spot to be selected will be taken later. Yet the local astronomical circumstances of the site, wherever it is, can be found in Espenak and Anderson (1997) or obtained from these authors after the eclipse.
VI. INSTITUTIONS INVOLVED IN THE PROJECT
VII. FINAL COMMENTS
The present project has some advantages: