This paper provides the overview of an inte- grated, multi-disciplinary effort to study the effects of the 29 March 2006 total solar eclipse on the environment, with special focus on the atmosphere. The eclipse has been visible over the Eastern Mediterranean, and on this occasion several research and academic institutes organised co-ordinated ex- perimental campaigns, at different distances from eclipse to- tality and at various environments in terms of air quality. De- tailed results and findings are presented in a number of com- ponent scientific papers included in a Special Issue of Atmo- spheric Chemistry and Physics. The effects of the eclipse on meteorological parameters, though very clear, were shown to be controlled by local factors rather than the eclipse mag- nitudes, and the turbulence activity near surface was sup- pressed causing a decrease in the Planetary Boundary Layer. In addition to the above, the decrease in solar radiation has caused change to the photochemistry of the atmosphere, with night time chemistry dominating. The abrupt "switch off" of the sun, induced changes also in the ionosphere (140 up to 220 km) and the stratosphere. In the ionosphere, both photochemistry and dynamics resulted to changes in the re- flection heights and the electron concentrations. Among the most important scientific findings from the experiments un- dertaken has been the experimental proof of eclipse induced thermal fluctuations in the ozone layer (Gravity Waves), due to the supersonic movement of the moon's shadow, for the first time with simultaneous measurements at three altitudes namely the troposphere, the stratosphere and the ionosphere.

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The Total Solar Eclipse of March 2006: overview

This work completes a two-part review on waves in gases, of which the first part [Rev. Mod. Phys. 58, 117 (1986)] dealt with the modern aspects of acoustics of jets, turbulence, and ducts; this second part extends the range of topics from sound to magnetic, internal, and (to a lesser extent) inertial waves, thus considering all four restoring forces (pressure, gravity, and Lorentz and Coriolis forces). The motivations for the study of these waves were outlined in the introduction to Part I. Part II reviews the coupling of acoustic, magnetic, and internal waves, in four stages: in Sec. I dispersion relations are used to study the propagation and radiation of magneto-acoustic-gravity-inertial waves in media for which the wave speeds and scattering scales are constant; in Sec. II the case of linear waves in stratified media, with nonuniform propagation velocity, is then discussed by means of special functions, appearing as exact solutions of second-order problems; in Sec. III the study of linear waves with variable propagation speeds is extended to certain classes of higher-order problems including a discussion of cutoff frequencies, critical levels, partition of energy, mode coupling and conversion, etc; in Sec. IV the preceding studies are extended to damped and nonlinear waves, to include dissipation with variable damping scales and large disturbances in media under nonuniform external forces, such as magnetic flux tubes. The conclusion (Sec. V) sums up both parts of the review, in the sense that it deals with all types of waves in fluids; it mentions a few currently controversial topics, points out some directions for future research, and indicates methods available to address these issues.

On waves in gases. Part II: Interaction of sound with magnetic and internal modes

We present an analysis of the gravity wave structure and the associated forcing of the middle atmosphere induced by the screening of the ozone layer from solar heating during a solar eclipse. Fourier integral techniques and numerical evaluation of the integral solutions were used to assess the wave field structure and to compute the gravity wave forcing of the atmosphere at greater heights. Our solutions reveal dominant periods of a few hours, characteristic horizontal and vertical scales of about 5000 to 10,000 km and 200 km, respectively, and an integrated momentum flux in the direction of eclipse motion of about 5.6 x 10 exp 8 N at each height above the forcing level. These results suggest that responses to solar eclipses may be difficult to detect above background gravity wave and tidal fluctuations until well into the thermosphere. Conversely, the induced body forces may penetrate to considerable heights because of the large wave scales and will have significant effects at levels where the wave field is dissipated.

Gravity wave forcing in the middle atmosphere due to reduced ozone heating during a solar eclipse

This study aims at providing experimental ev- idence, to support the hypothesis according to which the movement of the moon's shadow sweeping the ozone layer at supersonic speed, during a solar eclipse, creates gravity waves in the atmosphere. An experiment was conducted to study eclipse induced thermal fluctuations in the ozone layer (via measurements of total ozone column, ozone pho- tolysis rates and UV irradiance), the ionosphere (Ionosonde Total Electron Content - ITEC, peak electron density height - hmF2), and the troposphere (temperature, relative humid- ity), before, during and after the total solar eclipse of 29 March 2006. We found the existence of eclipse induced dominant oscillations in the parameters related to the ozone layer and the ionosphere, with periods ranging between 30- 40 min. Cross-spectrum analyses resulted to statistically significant square coherences between the observed oscilla- tions, strengthening thermal stratospheric ozone forcing as the main mechanism for GWs. Additional support for a source below the ionosphere was provided by the amplitude of the oscillations in the ionospheric electron density, which increased upwards from 160 to 220 km height. Even though similar oscillations were shown in surface temperature and relative humidity data, no clear evidence for tropospheric in- fluence could be derived from this study, due to the modest amplitude of these waves and the manifold rationale inside the boundary layer.

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Evidence of gravity waves into the atmosphere during the March 2006 total solar eclipse

This article reviews atmospheric changes associated with 44 solar eclipses, beginning with the first quantitative results available, from 1834 (earlier qualitative, accounts also exist). Eclipse meteorology attracted relatively few publications until the total solar eclipse of 16 February 1980, with the 11 August 1999 eclipse producing the most papers. Eclipses passing over populated areas such as Europe, China and India now regularly attract scientific attention, whereas atmospheric measurements of eclipses at remote locations remain rare. Many measurements and models have been used to exploit the uniquely predictable solar forcing provided by an eclipse. In this paper we compile the available publications and review a sub-set of them chosen on the basis of importance and novelty. Beyond the obvious reduction in incoming solar radiation, atmospheric cooling from eclipses can induce dynamical changes. Observations and meteorological modelling provide evidence for the generation of a local eclipse circulation which may be the origin of the "eclipse wind". Gravity waves set up by the eclipse can, in principle, be detected as atmospheric pressure fluctuations, though theoretical predictions are limited, and many of the data are inconclusive. Eclipse events providing important early insights into the ionisation of the upper atmosphere are also briefly reviewed.

Atmospheric changes from solar eclipses

During a solar eclipse the Moon's shadow moves at supersonic speed through the Earth's atmosphere. Chimonas and Hines1,2 suggested that the resultant cooling of the atmosphere would generate a bow wave of atmospheric gravity waves, which may be detectable as a travelling ionospheric disturbance or as a ground-level atmospheric pressure variation. Although the evidence for gravity waves in the ionosphere from a solar eclipse is still weak, they may have been detected3. Ground-level pressure changes during solar eclipses occur near the region of totality4, but the measured wave velocity and period do not agree with that of the predicted disturbance5,6. We now report the detection of a ground-level pressure wave detected at three stations in India and one station in Java, Indonesia. These data may provide the first direct observation of eclipse generated gravity waves over a very long range. The most distant station in India was 6,600 km from the eclipse centre line. The microbarometer recordings indicate that a wave disturbance was recorded at each station with a quasi-period of ∼4 h and a wave velocity of ∼320 m s−1.

Evidence of atmospheric gravity waves produced during the 11 June 1983 total solar eclipse

LITTLE1 has proposed a mechanism by which an electron–electron (e–e) interaction in certain organic polymers and macromolecules might result in a high superconducting transition temperature. This e–e mechanism requires a combination of two systems consisting of a conducting or semiconducting polymeric compound forming a spine, with highly polarisable side chains attached to the spine. According to Little, the addition of such side chains can increase the e–e attraction in the spine to the point where it becomes energetically favourable to enter the superconducting state. This e–e mechanism of superconductivity has led to lively discussions, primarily concerned with superconductivity in quasi one and two-dimensional systems2,3, and the magnitude of the e–e attraction4,5. It has been shown theoretically3 that, depending on, the nature of the interaction between the electrons, a quasi one-dimensional system may exist in different states: metallic, dielectric, antiferromagnetic and superconducting. It has also been shown experimentally that the organic charge transfer salt N-methyl-pehnazinium tetracyanoqui-nodimethan has ‘metallic’ properties at temperatures above 200 K with a continuous transition to a small-band-gap insulator below 200 K (ref. 6).

E. J. Seykora

Papers

Evidence of atmospheric gravity waves produced during the 11 June 1983 total solar eclipse

An organic `metal'?