"… We humans on Earth are too small to clear our volcanoes. That is why they are causing us so much trouble."
Antoine de Saint-Exupery "The Little Prince"
You've probably all seen this type of lightning. An interesting phenomenon! All sorts of fantastic films immediately come to mind … "The Lord of the Rings" for example:-)
I propose to see a selection of this riot of nature and the bowels of the earth. Almost all photos are clickable.
The reason for the occurrence of ordinary lightning during a thunderstorm remains the subject of research, and the nature of volcanic lightning is even less understood. One hypothesis suggests that ejected bubbles of magma or volcanic ash are electrically charged and that they move to create such separated areas. However, volcanic lightning can also be caused by charging collisions in volcanic dust.
Promotional video:
Scientists were able to record electrical activity in a cloud of volcanic ash with unprecedented resolution and identify two types of lightning that occur during an eruption. The eruption of Redout Volcano, located in Alaska, was preceded by characteristic seismic activity, which allowed a group of scientists from the New Mexico Institute of Mining to have time to establish a network of miniature observation stations near the crater in advance.
They were provided with ultra-shortwave radio detectors, which recorded lightning strikes in the ash cloud that was thrown out. During the eruption, volcanologists observed 16 powerful storms, which provided them with a large amount of data for subsequent analysis.
As a result, scientists were able to discover that volcanic lightning is divided into two types: relatively small, occurring directly near the crater, and powerful, observed high in a cloud of ash. According to scientists, both are of a different nature. Small low lightning bolts are the result of electrical processes in magma as it breaks up into many small particles. Large lightning bolts in an ash cloud occur when the temperature drops below -20 degrees Celsius, when supercooled water droplets freeze. Similar processes are caused by discharges in the clouds during thunderstorms. Scientists have also found a correlation between the height of the ash cloud and the power and frequency of lightning strikes.
The main physical processes responsible for the electrification of a gas-heat cloud above a volcano are considered. Some features of the mechanics of volcanic aerosol and its gravitational separation are analyzed. It is shown that the most important among the many physical and physicochemical processes of the generation and separation of charges in a volcanic cloud are thermionic emission and thermoelectricity. The main laws governing the electrification of aerosol particles during these processes are calculated. It was found that for the formation of lightning in a volcanic cloud, the ejection material must contain a noticeable amount of a fine fraction (1-30 microns). The possibilities of participation of other physical processes in the electrification of aerosol particles and the volcanic cloud as a whole are briefly analyzed. The kinetics of charge separation and the conditions for the formation of lightning in volcanic clouds are also considered. The relationship between the intensity of electrical processes and the energy and power of the eruption is shown. It is concluded that it is necessary to measure the electrical activity of heat clouds together with a study of the kinetics of mass removal and determination of the initial temperature of the ejection material.
Electrical phenomena in aerosols are very diverse both in form and in intensity. Electrical processes in natural aerosols are most grandiose at large volumes (tens and hundreds of thousands of cubic meters) and high voltages (up to hundreds of megavolts) [1, 2]. The frequency of lightning in thunderclouds sometimes reaches 0.05 - 0.2 s-1. However, the highest intensity of electrical processes is observed in dry gas-heat clouds above volcanoes (see bibliography in [3]). Every second large lightning (one of which is shown in Fig. 1), much more frequent small spark discharges 8-10 m long, intense and prolonged corona glow in areas covered by a volcanic cloud - this is a short list of those phenomena that were observed during volcanic eruption …
Not every eruption is accompanied by lightning. This means that the intensity of electrification of the volcanic aerosol essentially depends on the characteristics of the eruption. Generally speaking, the electrification of aerosol particles can occur for many reasons associated with physical and physicochemical processes in a gas-slag-heat cloud [3, 4]. However, in view of the fact that the intensity of electrification of the volcanic aerosol is much higher than that of all other known aerosols [3 - 6], it is possible to distinguish a number of specific processes that play the main role in the volcanic cloud.
- The most significant features of volcanic aerosol are:
- very high fever;
- a large difference in the temperature of solid aerosol particles both among themselves and in relation to the surrounding gas;
- strong nonstationarity of the system of volcanic ash particles suspended in gas. If ordinary aerosols are older than 1 min and the calculated concentrations of such an aerosol can no longer exceed na = 103 part / cm3, then the processes of electrification of volcanic aerosol proceed at concentrations n »107 - 109 part / cm3 and, as will be shown below, practically end by the end of the second second of the aerosol existence;
- volcanic aerosol, unlike all others, includes ash, lapilli, slag and even volcanic bombs, i.e. the entire mass spectrum from ~ 10-12 to> 103 g.
In this work, two mechanisms of electrification of ash-ash volcanic particles are considered, namely, thermoemission of electrons and thermoelectricity. Calculation of the thermionic emission process makes it possible to determine the minimum initial temperature Tmin of the ejection material, below which the thermionic emission intensity is so low that it is no longer able to provide noticeable electrification. The duration of the action of the thermionic mechanism is determined by the cooling time of the particles from the initial temperature to a fixed Tmin and can vary from ~ 0.1 to ~ 10 s. It is also shown that the thermoelectric mechanism of electrization of volcanic aerosol particles does not have a temperature "threshold", therefore, the range of action of this mechanism in temperature is greater than that of thermionic, and the time interval is due to the aerosol dilution time and is almost constant (~ 1.5 s).
Although the thermoelectric mechanism of electrification is sometimes inferior to the thermoemission one in terms of the rate of charge generation, it is much wider in the range of action, since it functions in any aerosols if there is a temperature difference of the contacting particles DT ~ ~ 10 K and higher. It has also been shown that other mechanisms of electrification discussed in the literature (piezoelectricity, balloelectric effect, friction of particles and gas jets, etc.) cannot play a significant role in the formation of electric charges and lightning over volcanoes, primarily due to the lack of directionality of these processes necessary for the accumulation and separation of charge on a macroscopic scale. Let us recall that two processes are necessary for the occurrence of lightning: electrification of particles on a microscopic scale and separation of charges on the scale of the entire cloud. The second one is longer,therefore, lightning occurs much later than the start of the ejection.
Macroscopic processes are considered in this work more concisely. The complexity of the processes of sedimentation and separation of charged aerosol under conditions of turbulent mixing of different-scale clouds of a volcanic cloud does not allow a rigorous calculation, so we limited ourselves to using (where possible) analogies with processes in thunderclouds. As a result, the criteria were formulated, the fulfillment of which is necessary for the occurrence of lightning of different scales.