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.
                         (mapena@essex.ac.uk)

            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

I. INTRODUCTION

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

PHYSICAL/CHEMICAL EFFECTS

1. Optical Effects

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

1. Radiometric and photometric measurements in different directions in the sky, from the zenith down to horizon, to quantify the sky radiance, brightness in the visual or its darkness degree during the totality (Silverman, 1975). To compare these measurements with other made in previous total eclipses (Richardson and Hulburst, 1949; Batchelder et al., 1956; Sharp, Silverman and Lloyd, 1966; Dandekar, 1968; Dandekar and Turtle, 1971; Lloyd and Silverman, 1971; Sharp, Silverman and Lloyd, 1971; Velasques, 1971; Shaw, 1975, 1979).
2. To infer from these observations the extinction coefficient of the local atmosphere in different directions of the sky, and their respective optical depths by applying the model given by Schaefer (1993).
3. To make these measurements in different wavelengths in order to determine the sky colour and polarisation changes as the eclipse develops from the partial phase to the total phase (Sharp, Silverman and Lloyd, 1966; Dandekar, 1968; Dandekar and Turtle, 1971; Hall, 1971; Lloyd and Silverman, 1971; Velasquez, 1971; Shaw, 1975)
4. To compare observations from point (1) with a model of multiple scattering already published (Shaw, 1978) and to compare observations from point (2) near the horizon with a model also already published (Gedzelman, 1975).
5. To make measurements of particle mass concentration of the air in the eclipse observation site to monitor its purity. In addition, samples of particles should be taken in order to get information on their morphology and dimensions if possible. These parameters can be related with the scattering processes of the atmosphere occurring during the event.
6. To make photography observations (Pasachoff and Covington, 1993) during the partial phase to deduce, following the method of Makita (1966), the air molecules scattering function. A comparison of these observations with those of this author, could be made.
7. To make instrumental observations of temperature (air and soil), relative humidity, and pressure changes at terrain level and compare them with other observations of the same kind (Upton and Rotch, 1887; Kimball and Fergunson, 1919; Brooks et al., 1941; Klein and Robinson, 1955; Frostman and Dabbertdt, 1970; Anderson, Keefer and Myers, 1972; Hultz et al., 1974; Anderson and Keefer, 1975; Das et al., 1982; Kankane, Hazra and Sarkar, 1982). To try detecting pressure waves during the phenomenon (Jone, Miseldine and Lambourne, 1992).
8. To explain the temperatures observations (air and soil) with models already published (Brooks et al., 1941; Phillips, 1968-69).
9. To infer from the observations of relative humidity the extinction coefficient (air, ozone, and aerosol contributions) variation of the local atmosphere by applying the model given by Schaefer (1993).
10. To observe photometrically and photographically shadow bands projected on the ground and compare them with other observations of the same phenomenon published elsewhere. To understand these observations within a model context proposed by other authors (Holden, 1883; Gaviola, 1948; Burgess and Hults, 1969; Hultz et al., 1971; Quann and Daly, 1972; Stanford, 1973; Feldman, 1974; Henry, 1975; Seykora, 1979; Marschall, 1984; Marshall, Mahon and Henry, 1984).
11. To make instrumental observations of surface atmospheric electric potential, wind speed, sensible heat flux, solar flux and soil surface temperature and compare them with other observations made in previous eclipses. To relate these observations with a possible perturbation of the atmospheric surface or boundary layer as a response of it during the eclipse and the post-eclipse period (Anderson, 1972; Dolezalek, 1972; Anderson and Dolezalek, 1972; Antonia et al., 1979; Nizamuddin, 1982; Raman and Boone, 1990; Manohar, Kandalgaonkar and Kulkarni, 1995; Porch et al., 1995).
12. To make visual observations of the partial phases, before and after the totality and also during the totality trying to establish the four contacts (Mottman, 1980). These estimations will be compared with those contacts predicted in other sources.
13. To determine the behaviour of some chemical species, at boundary layer level, like sulphur-dioxide (SO2), nitrogen-dioxide (NO2), ozone (O3), ammonia (NH3) (Maske et al. 1982), and hydroxyl (OH) (Burnett and Burnett,
14. on the day of the eclipse. Also to make observations of total column ozone, via solar ultraviolet measurements (Mims and Mims, 1993).

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

1. Equipment Available
   • - Photodiodes (RS Components Ltd. Corby, Northants. England). RS stock No. 308-067. Responsitivity of 0.35 A/W at 633 nm, 0.2 A/W at 450 nm, 0.5 A/W at 900 nm, 0.15 A/W at 900nm. 5 mm2 of active light sensitivity area (ALSA). Thermal stability of 150mV/0C. Output dark level of 20-60 mV.
   • - A Macam photodetector, model SD10Q-Cos (No. 1738), 15 mm2 ALSA, covering wavelengths from 400 nm to 700 nm and with a maximun spectral response at 560 nm.
   • - A Skye photodetector, model SKP 210, covering wavelengths from 350 nm to 750 nm, and with maximum spectral response at 300 nm and at 700 nm, respectively.
   • - An Ealing Electro-Optics (EEO) Broad Band Silicon Detector, 0.38 cm2 ALSA, in combination with a photopic filter (400 to 700 nm), both used with an EEO Research Radiometer/Phometer unit.
   • - A Wotan Xenophot HLX quarts bulb with a tungsten filament along with a LI-COR integrating quantum / radiometer / photometer, model LI-188B, and different glass neutral density filters, for calibration purposes.
   • - A Macam ultraviolet digital radiometer, model UV103 with filters A (365 nm), B (310 nm), y C (254 nm).
   • - A Casella airborne particulate monitor, model AMS 950IS (Casella Ltd. Kempton, Bedford. England). Version 0-200. Measurement range: 0.01 - 19.99 mg/m3 or 0.1 - 199.9 mg/m^3,
   • - Particulate sampling and sizing mobile equipment: 290 Marple personal cascade impactor (Graseby & Andersen. Atlanta, Georgia). Precision aerodynamic particle size distribution from 0.5 to 20 mm. Filter collection using different types of filters (Matricel, Nucleopore, etc.), via filter packs and pumping.
   • - Pressure-plate anemometer.
   • - Three meter data loggers Grant (Grant Instruments Ltd. Cambridge, England), model Squirrel 1201, 1202 and SQ8-2U-2V, respectively.
   • - Temperature and humidity probes (to be used coupled with the Grant data loggers). Two probes (mini-thermistors type U) for temperature (Betatherm Corp. Mass.), and one for humidity (VH-L type. Vaisala, Finland).
   • - Acquisition data system for PC computers (software Esayest Lx / Keithley Asyst. AsystSoftware Technologies, Inc./ Keithley Instruments, Inc. Rochester, New York, and Grant Squirrel softwares. Grant Instruments Ltd. Cambridge, England).
2. New equipment and acquisition data systems needed

- - Total solar pyranometer YES, model TSP-700, TSP-400 or TSP-100 (Yankee Environmental Systems, Inc. Mass.).

• - Sun photometer YES, model SPUV-10 (Yankee Environmental Systems, Inc. Mass.).
• - Ultraviolet pyranometer YES, model UVA-1 (Yankee Environmental Systems, Inc. Mass.).
• - Multifilter rotating shadowband radiometer YES, model MFR-7 (Yankee Environmental Systems, Inc. Mass.).
• - YES data Acquisition System (YESDAS-2).
• - A LI-COR photometric sensor, model LI-210SB to be used with the quantum/radiometer/photometer LI-COR, model LI-188B for photometric calibration purposes in units of lux.
• - A Skye lux or photometric sensor, model SKL 310.
• - A Delta-T Devices (AT) barometric pressure sensor, type BS4.
• - Grant Squirrel data loggers, model Series 1000 and/or 1600 (Grant Instruments Ltd. Cambridge, England).
• - Grant Taskwise (Grantware data analysis software): transfer, analysis and set-up program for Squirrel data loggers, and/or Grant Squirrelwise (or Squirrelwise Plus) data analysis software: Windows data transfer and analysis programs for Squirrel data loggers (Grant Instruments Ltd. Cambridge, England).
• - A notebook computer for field trials.

  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

• University of Essex (Institute for Environmental Research). Colchester, England. Co-ordinator: Dr. Ian Colbeck [colbi@essex.ac.uk]. Participants: Marcos Penaloza [mapena@essex.ac.uk] and Meng-Chen Chung [mchung@essex.ac.uk].
• University of Los Andes (Faculty of Science. Department of Physics). Merida, Venezuela (Marcos Penaloza [mpenaloa@ciens.ula.ve]).

Fellow Organization

• International Astronomical Union - Solar Eclipses Working Group. Chairman and Co-ordinator: Dr. Jay Pasachoff. Williams College - Hopkins Observatory. Williamstown, Massachusetts. U.S.A. [jmp@williams.edu].

VII. FINAL COMMENTS

The present project has some advantages:

1. Its cost of implementation and execution is not too expensive. Scientific staff and technical personnel are already trained during the past and recent solar eclipse in Venezuela, and those based on the country host, can supply assistance. Some equipment is available to be set up in the observation site but other has to be acquired. Funds are necessary to cover travel expenses and to buy new equipment.
2. Various task groups or teams can be organised to accomplish it by parts. One group will perform the optical and radiometric measurements of the sky brightness, radiance, colour and polarisation (points 1, 3). Another group will carry out the photographic part indicated in point 6. Other group will achieve the meteorological, and environmental measurements (points 5, 7). Other group will be in charge of pursuing the part corresponding to the shadow bands (point 10). A group will be engaged with the observations and measurements indicated in point 11, while other group will make indirect visual observations specified in point 12. Finally other group will make measurements related to point 13.
3. Most of the instrumental observations and measurements will be recorded using automatic acquisition data systems. Therefore people working at the groups referred to above will have time available to enjoy the eclipse during the totality phase.
4. The optical, radiometric, environmental, and meteorological instruments need not to be pointed toward the eclipsed sun. Hence in the unfortunate case in which the eclipsed sun were covered by clouds, although other regions of the sky do not (for example the zenith and/or on the horizon), experiments related to instruments involved can continue running. And if the sky were mostly overcast some of the experiments could even take place and the data so obtained used in particular models which can be applied for these special circumstances (Fritz, 1955; Enarun and Littlefair, 1995).