Clash of Lightning Bolts above the Clouds
MSTF Media reports:
Lightning flashes across the sky, briefly illuminating it before darkness reigns again. This dazzling spectacle is the typical image of lightning etched in our minds. This massive electrical discharge, however, gives rise to other amazing phenomena that are very rarely seen.
These lesser-known phenomena differ from the familiar lightning that occurs in the lowest layer of the atmosphere, the troposphere. They typically manifest above the cloud tops and in the upper layers of the atmosphere, i.e. in the mesosphere and lower ionosphere. So, it is only natural that we do not see them! These phenomena are called transient luminous events because they occur within one or a few milliseconds and then disappear. In addition, other phenomena occur during lightning that are not optically visible, but have an effect on the atmosphere.
Umran Inan, a Turkish scientist and former professor at Stanford University, is one of the scientists who, along with his team, has researched these phenomena. His area of expertise is very low frequency (VLF) electromagnetic radiation and geosciences. Red sprites, elves, and blue jets are among the mysterious names given to some of these transient luminous events; phenomena that have also been observed from space shuttles.
Red Sprites, Dancers of Fire
Red sprites are clusters of red light above the cloud tops seen after lightning. They look as if the cloud is shooting one or more red lightning bolts upwards! Of course, it would not be true to say that this phenomenon always occurs above every cloud from which lightning is emitted; rather, its occurrence depends on the conditions. Recorded videos show red sprites at altitudes of 50 to 90 kilometers above the Earth's surface. These light clusters are red from top to bottom, but their lower part has a tinge of blue. Their presence endures for mere milliseconds, vanishing almost instantly after their formation.
In Inan’s theory explaining red sprites, lightning-producing clouds are considered electric dipoles, where the top of the cloud has a positive charge, and the bottom, a negative one. In this case, there is a system around which an electric field is created, but this field does not have the ability to penetrate the high altitudes of the atmosphere above the cloud. This is because the electrical charges, or electrons, in the atmosphere act like a shield, protecting the upper atmosphere from the cloud's electric field.
After the positive charge of the cloud is discharged to the ground through lightning, the cloud changes from a dipole to a negatively charged electric monopole, and the electric field of a monopole is much stronger than that of a dipole. Then, the cloud's electric field and the electrons in the atmosphere combine to produce a strong field that can penetrate the upper atmosphere. This strong electric field exerts force on the electrons in the upper atmosphere and gives them energy. The energetic electrons collide with gas molecules in that area. During these collisions, energy is transferred to the gas molecules. Then, gas molecules get excited after receiving this energy and lose the received energy by light emission.
The red light is mostly due to the excitation of nitrogen at that height above the ground. The clustered shape of red sprites is also, according to many scientists, due to inhomogeneity in the conductivity of the mesosphere. As mentioned earlier, various light phenomena occur during lightning, one of which is red sprites. Sometimes, before the red sprites appear, a long light is seen in the sky that disappears very quickly which are known as elves.
Elves, Twinkles from Heaven to Earth
Elves are seen at altitudes of 75 to 105 kilometers above the Earth's surface, much higher than cloud level. They are horizontal, 100 to 300 kilometers long, and last less than a millisecond. Named after their shape, these phenomena are called light worm or elves. According to Inan, the most likely source of elves is low-frequency electromagnetic pulses that occur after strong thunderstorms. Pulses that propagate upward give energy to the electrons in the lower layers of the ionosphere, heating them. The energetic electrons also excite the gases in that area and cause them to emit light.
The luminous intensity of this event is very high, which means that the lower layer of the ionosphere receives a lot of heat as a result; therefore, the light creates a noticeable change in the lower region of the ionosphere. We use this region to guide radio waves, and any change in this area itself affects the propagation of radio waves. The electromagnetic pulse generated by lightning is not only a source of elves, but also has other effects on the atmosphere that affect our daily lives.
Electromagnetic Pulses, Bows into the Sky
The pulse generated by lightning can interact with the ionosphere, causing ionization and heating of its lower layer. That is, it affects an altitude of approximately 100 kilometers above the Earth and can change the physical properties of the ionosphere, including its refractive index, in a very short time. Inan has calculated that it takes much longer for the conditions to revert back to their original state. Changes in the ionosphere, which are vital for the propagation of radio waves throughout the Earth, cause changes in the characteristics of the emitted waves, such as phase and intensity. This, in turn, causes devices such as radio and television, and even the Global Positioning System (GPS)— which use electromagnetic waves— to stop working during a thunderstorm.
Also, the heating of the ionosphere and the rise in its temperature increase its conductivity, which leads to the creation of an electric current and can produce waves with very low frequencies. According to Inan, the ionization of the ionosphere through a process can generate the X-rays that are observed during lightning.
This ionized region behaves like a plasma. Studying very low frequency (VLF) electromagnetic waves and their interaction with charged particles in plasma, such as electrons, is another main focus of Inan’s work. To better understand the application of this research, we need to understand some concepts, including the Van Allen radiation belt, which will be discussed below.
Radiation Belt: Earth's Defender, Satellite Attacker
When harmful and charged solar rays reach the Earth, they are trapped by the Earth's magnetic field and cannot hit the Earth's surface. This magnetic field protects us and other living things from these rays. These particles create areas around the Earth known as the Van Allen radiation belt. This belt poses a major challenge for satellites, electromagnetic wave transmission, and GPS, because any satellite or electric circuit placed in this area will quickly wear out and fail as a result of the radiation of these particles. Also, the intensity and frequency of electromagnetic waves in this area change, disrupting communications in these areas.
By studying the propagation of low-frequency electromagnetic waves in plasma, Inan has attempted to understand the interaction of these waves with charged particles better. The VLF research team at Stanford University, led by Inan, and with more than 30 years of research at Palmer Station in Antarctica, has succeeded in making precise VLF measurements. Their studies have made it possible to study wave-particle interactions in near-Earth space. The interaction of these waves with electrons causes the electrons to accelerate and move.
It is possible that in the future, high-power VLF electromagnetic wave transmitters could be made through artificial processes, producing waves that could be used to reduce harmful electrons in the radiation belt. The results of this research and their development will change our understanding of plasma physics, astronomy, and meteorological physics.
