High up in the vast blue sky, are the bright and fast elves and sprites, almost mystical, like mythical creatures. Can you guess what I am talking about? A lightning strike!
Sprites and elves are upper atmosphere lightning phenomena, also known as transient luminous events (TLEs). Interestingly, the origin of these nomenclatures, viz., sprites and elves are acronyms, “Stratospheric/mesospheric Perturbations Resulting from Intense Thunderstorm Electrification” and “Emissions of Light and VLF perturbations from EMP events.” While lightning fascinates some and terrifies others, this article is about the fascinating features of a specific kind of lightning, the volcanic lightning.
Do you remember the climax of the film ‘Thor Ragnarok’ where the God of Thunder jumps nearly a hundred feet with Led Zeppelin’s Immigrant playing in the background? Lightning strikes mark his trajectory until he sets his feet on the ground. Surges of electricity emanates from his eyes, coursing through his armour, only to scorch or paralyse his enemies. Such is the personification of a lightning strike - powerful and dangerous.
A lightning strike is dangerous to human life only if it hits the ground as in the case of a cloud-to-ground lightning (CG). Such strikes occur only once in five to ten strikes. In most cases, the lightning never even reaches the ground and poses a threat to only communication systems. In most of those cases, the lightning originates in a cloud and remains within the clouds, which is known as intra-cloud lightning (IC). In other cases, it has a channel extending into the air between parts of the cloud, i.e. cloud-to-air lightning (CA). But there is more to a bolt of lightning than mere destruction or threat.
Lighting flashes are generally white, but sometimes they can be of various other colours as well. Lightning colours can cover almost the entire spectrum of colours in a rainbow, with each colour indicating a different condition of the environment and temperature range of the lightning. When there is a saturation of water droplets in the clouds, it indicates impending heavy precipitation with a fair chance of a hail storm. These droplets scatter the incoming solar radiation, creating a flash of blue lightning. In case of supersaturation, the colour turns violet. Furthermore, when the amount of water vapour in the atmosphere is significantly low, only the dust particles scatter light, and we see yellow or orange coloured lightning bolt. Red coloured lightning flashes only appear in the upper atmosphere and are called sprites. The hottest and the most dangerous kind is the pure white bolt. Green is the rarest colour to be seen in a bolt of lightning and only arises due to the energy released by supercharged oxygen molecules in the atmosphere. This green lightning bolt is an exotic phenomenon and is often referred to as volcanic lightning.
While volcanoes are associated with movements under the earth’s crust, volcanic lightning owes its origin to a totally unrelated process. Among other kinds of volcanic eruptions, the ones that emit a column of volcanic ash give rise to what we call volcanic lightning. Other kinds of volcanoes merely eject hot lava, which simply flows down the slope. The studies on volcanic lightning were first conducted in the Vesuvian Observatory, under the leadership of Prof Luigi Palmieri. His detailed studies on volcanic lightning were based on the eruptions of Mt. Vesuvius in 1858,1861, 1868, and 1872.
So, what is the origin of these lightning flashes? To understand this, let us recall a simple experiment from our childhood. Do you remember rubbing a plastic ruler on your hair and bringing small pieces of paper close to it? If you do, you would remember that those little pieces of paper would stick to the ruler surface. This is because, the event of rubbing leads to the transfer of some electrons from the hair to the ruler, leaving the ruler with an excess of electrons. Electrons are negatively charged particles and when this negatively charged ruler is brought in the vicinity of the paper pieces with no charge, the paper acquires a positive charge, and thus is attracted towards the ruler.
Beneath a volcano, all materials are compressed under very high pressure. When the volcano erupts, the tiny particles that make up a volcanic plume, are violently ejected into a low-pressure environment (atmosphere). During this process, they collide with each other, leading to the ‘charging’ of the particles i.e., gaining or losing electrons, as in the case of the ruler and hair. This process, also known as triboelectric or frictional charging, is considered one of the most efficient charge exchange methods in nature. Another typical process of charging is called fracto-emission. This method involves the breaking or fragmentation of rocks and rock particles close to the vent of the erupting volcano.
In both the cases, once the charge separation becomes too large, we have an electrical breakdown in the air around the ash plume creating flashes of lightning that connect the opposite charges. Many years of extensive studies by various volcanologists have revealed that temperature and plume height also influence the method of charging. Higher plume heights result from very violent eruptions, and hence triboelectric charging is the most feasible method in that case. On the other hand, less violent eruptions form only a small ash plume and fracto-emission at the volcano vent appear to be the common charging method. Low temperatures around the ash plume result in faster cooling of the ejected particles, increasing the electrical activity.
What is more interesting about this unique type of lightning, i.e. volcanic lightning, is its potential utility in monitoring volcanic eruptions. A 2019 study published in the Journal of Volcanology and Geothermal Research explored volcanic lightning as a cost-effective tool to track the dangers of a volcanic eruption. Today, the study of a large number of impending volcanic eruptions depends mostly on the tremor data collected from the series of 152 seismometers installed as a part of the Global Seismographic Network (GSN). A major problem of this detection method is the cost involved in installation and maintenance of the setup, that allows the tagging of only a fraction of the volcanoes that dot the Earth’s surface. This only adds to the need to shift to a more cost-effective yet efficient tracking method. Future research endeavours in volcanic lightning and related phenomena can open new avenues into understanding and tracking of volcanic activities.
This article was originally published in the 1 Year Anniversary Issue of Cogito137.
Issue 6 February 2021 http://scicomm.iiserkol.ac.in/docs/v6.pdf
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