The most common phenomena are ELVES – Emission of Light and VLF perturbations due to Electromagnetic Pulse Sources . They occur at the lower ionosphere, at altitudes of about 90 km. They are concentric rings of light expanding with speed close to the speed of light. The diameter of the elves disk may range from 200 to 500 km. The whole process of expansion takes up to a millisecond.
The possible mechanism of production of an elve is as follows. Firstly, a strong lightning occurs - which translates into a rapid change in the electric field in the atmosphere. This produces an electromagnetic pulse propagating upwards through the atmosphere and eventually reaches the ionosphere where it causes production of light observed as an elve. To estimate the image viewed by the JEM–EUSO detector while observing an elve example, we have estimated the parameters of the average elve basing our assumptions on the results provided by the ISUAL experiment and presented in . The occurrence rate of the elves is 35 per minute around the world. The spatially averaged brightness of an elve presented in  is: 0.17±0.08 M
R=1.36⋅1014 ph sr−1 m−2 s−1. The unit R stands for “Rayleigh” (unit of photon flux) typically used for these atmospheric phenomena. We assume an elve with the diameter of 300 km and the timescale of 2 ms = 800 GTU, which should allow for recording of the whole event and the observation directly in the JEM-EUSO detector in the nadir mode. This is the least favorable scenario in the sense that the geometry of the system allows for the maximum amount of light reaching the focal surface of the detector.
With all these assumptions, we get 9.25⋅1011 photons, which is an estimate for the whole event, in all wavelength channels. As elves are ionospheric phenomena, they are mostly red. The contribution of “blue” wavelengths is marginal. If we take 13 % of the obtained photons fitting our wavelength requirements (second positive line of Nitrogen), we will get the estimated number as: 1.2⋅1011 photons. This number of photons will be recorded as a total number by all pixels of the focal surface throughout the duration of the event.
The number of pixels in the JEM-EUSO detector is 315 648, it equals to about 40 000 KIs. The distribution of the photons among the separate KIs has not been considered here. Similarly, the time distribution of the incoming photons has not been taken into account in this work.
Both these factors will reduce the number of photons reaching a KI in one GTU by a few orders of magnitude. It is also important to note that the estimation relates to one such event and not to the total number of photons coming from other sources in the atmosphere. The calculations here presented are based on averaged results, while the most dangerous for the photomultipliers are the extreme cases of events producing light, for which the order of magnitude of the brightness may be a few hundred times greater.
The observed characteristics of different types of TLE can be found in the Table 1.
Sprites are associated with giant storm clouds with dimensions over 1000 km producing strong electric field in the mesosphere in a volume of over 104 km3. Sprites are massive but weak luminous flashes, appearing at altitudes of 40 – 90 km. The heads of sprites are predominantly red. The brightest region lies in the altitude range 65 – 75 km, above which there is often a faint red glow structure that extends to about 90 km. Below the bright red region, blue tendril-like filamentary structures often extend downward to 40 km. Sprites rarely appear singly, usually occurring in groups – two, three or more. The duration of sprites is of the order of milliseconds. Currents of the lightning strokes associated with sprites reach the intensity of more than 100 kA . The pictures of sprites obtained from the ground and from aircraft show complex structures that assume a variety of forms. Early research reports for these events called them “upward lightning”, “upward discharges”, “cloud – to – stratosphere discharges” and “cloud – to – ionosphere discharges”. Now they are called sprites. The tendril – streamer structure under the upper part of the phenomenon – the so called head of a sprite shows extremely interesting and as yet not fully explained phenomenon of merging streamers, which form bright spherical structures – the beads. Measurements of microsecond time resolution may help to explain the mechanism behind the phenomenon. The brightness measured by ISUAL is 1.5±1.1 MR, which translates into some 3.48⋅1011 ph in total and 1.46⋅1011 in the wavelength region detected by the JEM-EUSO. This number is the spatially integrated amount of photons recorded throughout the whole event duration.
Other types of TLE
Blue jets are optical ejections from the top of the electrically most active regions of thunderstorms. Following their emergence from the top of the thundercloud, they typically propagate upward in narrow structure in form of cones of about 15 degrees full width at vertical speeds of roughly 100 km/s, fanning out and disappearing at heights of about 40 – 50 km. At the base their diameter is about 400 m. Blue jets are rather rare events. Their appearance rate is much lower than that of sprites. Blue jets are not aligned with the local magnetic field. They are triggered by the electric field inside the cloud. Their appearance is not connected to a cloud–to–ground discharge. The brightness of blue jets is estimated as 0.5 MR .
They are much larger than the Blue Jets. Gigantic Jets originate form the cloud top and develop into a tree-like structure reaching the lower ionosphere. Their propagate upward at the speed of about 100 km/s. The ISUAL measured only 8 such events. The brightest one was 0.8 MR.