by Viktor DOBRYNIN, Cand. Sc. (Phys. & Math.), senior engineer, Laser Physics and Nanotechnologies Department, Institute of Physics and Technology, Irkutsk State National Technological University

The water of Baikal, the world's largest fresh-water lake, is aglow. That's what Irkutsk experts say. True, this glow is very weak, not visible to the naked eye, but is sufficient enough for being registered by multiplier phototubes (MP), the counters of photon singletons. Certain characteristics are indicative of chemiluminescence as the mechanism of spontaneous glow to a depth of 25-125 meters. However, the concrete material source and the nature of such luminescence are a sealed book yet. But what we know for certain is that this phenomenon comes from the purity of lake water.

LOOKING BACK

"Such studies are underway in seas and oceans, too. But the mechanism of the glow is quite different there-these are luminescent bacteria, algae, and other organisms responsible for a light-induced background (the phenomenon of bioluminescence). Nothing of the kind has been discovered in Lake Baikal. The pattern is different over there, we should delve into it." Such is an opinion of Nikolai Ivanov, director of the Irkutsk Institute of Physics and Technology.

We discovered the glow phenomenon* in 1982 in the course of a comprehensive field expedition headed by the RAS Institute of Nuclear Physics. Up until 1991 most of the experiments had been carried out by Irkutsk State University, Tomsk Polytechnical Institute and Institute of Chemical Physics (Moscow). The Moscow Institute of Nuclear Physics and the Limnological Institute (RAS Siberian Branch) gave tangible support to this line of

* See: M. Kuzmin, G. Khursevich, "Diatom Chronicle of Lake Baikal and Climatic Change", Science in Russia, No. 3, 2012.-Ed.

research. The data thus obtained showed that the Baikal glow phenomenon is different from oceanic biolumines-cence. The field parties measured the amplitude and spectra of the glow, they determined the distribution pattern of glow intensity at different depths and in different regions of Lake Baikal. Furthermore, they studied the luminescence characteristics in a volume of water isolated from the environment as well as certain physicochemical effects on water samples. The practical task-that of studying background conditions of muons and neutrinos* in Baikal-was accomplished with much success. But the very cause of the phenomenon was still an open question.

In 2011, after many years of recess, we resumed our research, this time with Irkutsk Technological University, and made several interesting and essentially new observations. The Angara**, an only river flowing out of Lake Baikal, became our Number One target. Apart from the academic interest, our research work was directly connected with the monitoring of Baikal and Angara waters, where luminescence comes as an indicator of environmental conditions. Since pure water is scarce in many countries, and it is going to cost more than petroleum soon, it is very important to show great care for Lake Baikal***, a unique body of renewable fresh water.

* Muon, elementary particle with a negative electric charge and 1/2 spin; neutrino, neutral elementary particle with half integer spin implicated in weak and gravitational interactions only.-Ed.

** Angara, the largest right tributary of the Yenisei flowing out of Lake Baikal.-Ed.

*** The water reserve in Baikal totals around 23,000 km³, or as much as in the five Great Lakes of North America. As a matter of fact, Baikal is the world's largest reservoir of clean fresh water. The lake accounts for one-fifth of the global reserves of surface fresh water counting in glaciers, firns and the ice of the Antarctic, Greenland and other solid-state water sources. More than 4/5 of this country's reserves of surface fresh water are in Lake Baikal.-Ed.

We at Irkutsk State University began our studies back in the fall of 1981. This work, involving the use of multiplier phototubes (MP), was initiated by the Baikal Neutrino Project. At first we measured the distribution of solar radiation multiple scattering at great depths with the aid of a bathyphotometer.* In our experiments we tried to ascertain regularities implicated in the attenuation of solar radiation in Baikal water and find out how deep it gets in. At first we supposed that at great depths the signal would depend solely on MP dark noises. But we were wrong, as we could see it half a year later in follow-up immersions. As it turned out, the effect of water self-glow was on at all depths, and it had nothing to do with insolation. In fact, the Baikal water glow was discovered quite by chance-true, with the use of up-to-date sensitive techniques.

The Baikal water glow phenomenon, it became clear, was not a trivial problem, and it had to be addressed closely. Most of the work was done near the location of a deep-water neutrino telescope (on the 106th kilometer of the Roundabout Baikal Railway 3.5 km offshore). Each spring research workers pitched a camp amid ice, the site of the International Neutrino Project. Some experiments were conducted at the Vereshchagin research vessel. In 1983 we, together with a party of the Limno-logical Institute and the Institute of High-Energy Physics, sailed from the southern to the middle part of Baikal and took measurements at six deep-water stations. We observed glowing water at every point of the depths investigated. The glow intensity was much dependent on

* A device capable of registering even very weak flashes of light deep within bodies of water. It was designed in the 1970s by Soviet and foreign engineers.-Ed.

the depth. The maxima were observed at a depth of 50 to 75 meters. The glow changed 20- to 100-fold, exponentially decreasing at greater depths. This was quite surprising, since impurities in Baikal water do not vary more than 50 percent. The lowest level of glow was registered at the deepest station near the island of Olkhon*-about 100 photons per square centimeter a second.

There might be some dependence between the glow intensity and transparence, for near the neutrino telescope, at 800 m to 1,200 m, there is what we call a "transparence core", or the purest water, and its glow is weaker. We may also point at seasonal variations of luminescence, with its intensity changing 2 to 5 times in just one year.

There was also a slow attenuation phenomenon in isolated water samples, down to the irreversible disappearance of the glow, should the sample be heated to 25 °C-the intensity of the glow depending on the concentration of oxygen dissolved in the water (the glow dwindles at O₂ lower concentrations and goes up again to its previous level at O₂ higher concentrations).

Studying the Baikal microorganisms, we discovered a synchronous change of the glow intensity in a sample containing a bacterial culture. In these experiments we did not observe bioluminescence, and its level (cellular chemiluminescence) was not high enough (not above 10 percent) to explain the phenomenon. Cooperating with

* This is the largest island in Baikal, 71 km long, up to 12 km wide and 730 km² in area. Together with the northwestern shore it forms Maloye More (Little Sea) and the strait Olkhonskie Vorota (Olkhon Gate). The Baikal's deepest trench, at 1,642 meters, is nearby.-Ed.

the Moscow Institute of Chemical Physics and using its techniques, we made a study of certain physicochemical effects on the glow intensity: at 25 to 125 m depths, the greatest contribution was made by chemiluminescence processes (chain reactions triggered by the OH radical).

IT GLOWS AFTER ALL!

Even at the early stage of our work we understood that the brightest component of the glow came from chemical reactions proceeding with the participation of dissolved oxygen and, probably, of some organic substances (impurities). Such chemiluminescence is an example of the direct conversion of part of the chemical energy into luminous (light) energy.

Our latest data show: in practical terms, any water gives off light. But the glow of distilled water having no admixtures is very weak (the source of this luminescence being the natural radiation background). The tap water in our homes attenuates quickly enough. But the glow of the Baikal and Angara waters is more intensive and it may persist for a month or so. In order to "catch" light flow invisible to an unaided eye we make use of a high-sensitivity device, a laboratory photometer*, its latest version developed at Irkutsk State Technological University. The measuring module of this instrument is placed within a thermostat containing water samples. Watching the computer display, we can see intensity measurements in time. The basic and associated data

* An instrument used in measuring the intensity of light; light meter.-Ed.

(glow intensity, temperature, time, performance of the sensors) are recorded in a file at 1 min pitch.

Such experiments, we are certain, have good prospects: the glow may be indicative of the quality of natural waters. Today it is the Angara that is in the focus of our attention. Since its water down to the high dam of the Irkutsk Hydraulic Power Station is similar to that of Baikal in hydrochemical and hydrobiological characteristics, it is a suitable-and, what is most important-a more accessible object. Thereby we could cull more of the substantive data and develop techniques for Baikal water experiments. Accordingly, we are planning to set up a monitoring station at the Angara's source (where it flows out from Lake Baikal) for getting regular express data on the state of Baikal water, specifically, on its glow as a quality indicator. Such findings could be supplemented with microbiological and hydrochemical analysis data.

SUPERSENSITIVE PHOTOMETERS OF A NEW GENERATION

To push ahead with our studies of the Baikal water glow we built a complex of high-sensitivity apparatuses enabling us to investigate samples and natural water alike. Aside from the laboratory photometer, we developed a deep-water counter of photons on the basis of the latest photomultiplier tube XP3540B, manufactured by PHOTON IS, a group of European companies. We made the counter jointly with Irkutsk State University quite recently, in 2012. This device, 100 times as sensitive as the bathyphotometers of the nineteen-eighties, is meant for measuring luminous fluxes.

During our ice expedition of 2012 we tested the photon counter under natural conditions and obtained new data on the fine structure of luminescence intensity and its dynamics. Formerly such measurements were made to a depth of no less than 50 meters. But now we actually worked non-stop for five days and nights, and immersed the sounder up and down, from 150 to 1,340 meters. We conducted our key experiments at night against the diminishing background of astronomical light getting in. This allowed to determine the level of water self-glow at lower depths. Each day we worked for as long as twenty hours, with the coastal part of the bathyphotometer placed within the RUNG* laboratory on ice.

Our ice camp got electric power from a diesel generator. A cable winch was employed for lowering the sounder into the ice-hole (a maina in local parlance). Data were flashed to the "coastal" center by cable. A pressure sensor fed automatic data on the depth of the sounder's immersion. Its electronic hardware was locked in a solid body supplied with a porthole from which, with the aid of a MP, we registered the water glow and the light of astronomical and man-made sources.

This way we obtained 21 vertical profiles of illumination intensity at high space and time resolution, and hit upon curious effects and variations of glow intensity at mid-depths. For the most part those were small, insignificant variations... and yet at 600-800 m depths such variations were found to rise 1.5- and 2-fold. This phenomenon needs further research so as to explain the causes of such vertical stability of Baikal water.

Camping out, we carried out supplementary investigations using artificial light sources. In one such experiment we, while determining the water glow intensity, measured also the phenomenon of radiation multiple scattering of a blue light-emitting diode, LED. We wanted to find a relationship between the number of scatterers (diffusers, in this case, suspension) and the glow intensity. We did find this dependence, and it

* A truck van.-Ed.

Dependence of the intensity of Angara water glow upon six irradiations of the sample by white-light LED: X axis-time (h); Y axis-glow intensity in MP pulses per second (MP dark noises out); 1-6-periods of sample illumination by LED; Imax, Imin-glow maxima and minima.

Seasonal dependence of Angara glow maxima (dotted line) on daylight hours (curve): X axis-time (24 h), count up as of January 1, 2012 (zero); Y axis-glow intensity in MP pulses per second, left, and daylight hours (h), right; dot-and-dash line-control of laboratory photometer readings.

proved linear. Now we've got to evaluate and interpret our data. In addition, the measurements of solar radiation multiple scattering enabled us to assess within high accuracy the optical characteristics of Baikal water 400 meters below.

We got some new results on the glow of Angara and Baikal waters on our laboratory photometer, one of our latest discoveries: we learned the nature of ultrasound and external radiation effects on the intensity of sponta-neous luminescence in samples-ultrasound with well-developed cavitation* and hard ultraviolet (wavelength, <300 nm) quench the glow irreversibly, while the action of outer illumination is reversible. We found that blue light exerts a quenching effect on the spontaneous glow in Baikal and the Angara. For instance, powerful lasers

* Formation of cavities (cavitational vesicles, or caverns) filled with vapor. This effect arises due to a local drop of pressure in the liquid.-Ed.

of red and green light as well as LEDs of the same colors had no appreciable effect. However, white and blue LEDs cut drastically spontaneous luminescence in natural water by half and three-fourths (the magnitude depends on the power of a light-emitting diode and the time of exposure), whereupon it rises almost back to the original value. It is the blue component of a white light diode that exerts a quenching effect on the spontaneous glow of natural water.

Proceeding from such evidence, we suggested that light in our water samples could be generated by blue-sensitive microorganisms. In all likelihood, those were cyanobacteria having a photosynthetic apparatus not as perfect as in higher plants or algae. It is quite possible that the relatively low efficiency of cyanobacteria results in a loss of part of their energy in luminous radiation-that's what we observed. In our follow-up experiments we want to learn more about the input of the "living component" (microorganisms) in the aggregate water glow; also, we are going to elucidate the species-specific composition of microorganisms generating light in the process of their vital activity (phenomenon of cell che-miluminescence).

Our longtime studies of glow in Angara water samples are indirect proof of this hypothesis, for we observe a clearly seasonal pattern of luminous intensity (apparently related to change in the daylight hours and its impact on the life cycles of water organisms). From September of 2011 to January 2012 the intensity of luminescence in Angara water samples dropped dramatically, to a 1/50th of its previous level. The glow minimum concurs with the winter solstice, on or about December 22. But after January 19, 2012, we observed an exponential rise in the glow of samples which, as subsequent measurements showed, was followed by yet another downturn. Most likely, in this case we observe growth cycles of the hydrobiota coupled to changing external conditions (temperature, illumination, and other effectors).

Alternative hypotheses are developed to explain the mechanisms implicated in the glow of Angara and Baikal waters. When formulated, these hypotheses will be up for experimental proof.

AT THE INTERNATIONAL LEVEL

We reported on the results of our work to the 8th Inter-national Symposium (Optics of the Atmosphere and Ocean; Physics of the Atmosphere) held early in July of 2012 in Irkutsk at the Solar-Terrestrial Institute (RAS Siberian Branch). This forum was organized jointly with the Zuyev Institute of Atmospheric Optics under the aegis of the Federal Ministry of Education and Science, and the Russian Foundation of Basic Research.

We made one oral and four reports on visual stands dealing with the glow of natural waters. We also presented a summary report coauthored with Irkutsk State University about our joint product, a photon counter for deep-water studies.

Our detailed data elicited a genuine interest among the international scientific community. We forged quite a few business contacts. Thus, we were invited to carry out joint Baikal experiments at the Bolshiye Koty station (Irkutsk Region); this proposal came from Mikhail Panchenko, laboratory head of the Tomsk Atmospheric Optics Institute.

BAIKAL GLOW THROUGH CINECAMERA

Roskosmos*, the Russian Space Agency, got interested in our research data culled in 2012. Its cameramen visited the technopark of Irkutsk Technological University to shoot a film on the glow of Baikal waters. The shootings were made in the nanoluminescence laboratory and right in Lake Baikal.

Our inquisitions became a central topic of the sixth expedition of Roskosmos experts to Baikal. Earlier they took an active part in another Baikal project**, and even went down in bathyscaphs to the bottom of the lake to shoot a reel on the operation of the neutrino telescope. This group had no prior knowledge of the glow.

We filled in our guests on our new project and showed them around our laboratory, telling what our glow studies were really like.

* See: M. Yevgenyeva, "Space Attack", Science in Russia, No. 4, 2011; M. Khalizeva, "'Radioastron' Brings Deep Space Closer", Science in Russia, No. 1, 2012.--Ed.

** See: M. Khalizeva, "Benthal Routes of 'MIRs'", Science in Russia, No. 1, 2009.--Ed.