High-Energy Particle Physics in Thunderstorms

 (the image above depicts a blue jet, a form of upper atmospheric electrical discharge: By Gemini Observatory / AURA - Gemini Observatory / Association of Universities for Research in Astronomy (AURA)., Public Domain, https://commons.wikimedia.org/w/index.php?curid=61319099 )
It's September, a time when new students join our research group. This year we have three and, for the first time, all those students hail from a Physics & Astronomy background. Therefore it is appropriate that one of our new MScs, Justin Kerr, gets first crack at talking about an interesting intersection between high-energy particle physics and planetary science. By way of introduction, I'd love tell you more about the article, but I wouldn't want to steal his thunder! You'll just have to read on...

by Justin Kerr

    Here in Toronto, its currently thunderstorm season. To go along with the high humidity, we’ve had severe storms nearly every week recently reignited my long-held interest. So, when I saw a new paper posted on arXiv last week that covered a new sensor network observing thunderstorms using methods related to my previous research area of particle physics, I simply couldn’t resist digging in to the topic for this week’s post. While it might be surprising to many that high-energy particle physicists would have work to do with thunderstorms, the link between the two fields was experimentally confirmed in the 1980s. Even so, most of the knowledge about exactly how thunderstorms produce high energy particles is still hypothetical due to the difficulty of observing large thunderstorms. This is something the new paper sought to address.
    But first, let’s take a look what high-energy particles are produced by thunderstorms and how that might be happening. The first type of high-energy event being produced by thunderstorms is the terrestrial gamma-ray flash (TGF), which are rapid bursts lasting less than a millisecond. TGFs are expected to occur when an already energetic electron (likely produced by a cosmic ray entering the thunderstorm) is accelerated to near the speed of light by the intense electric field created by a lightning strike. This relativistic electron then begins a process known as a relativistic runaway electron avalanche (RREA). The RREA process consists of the initial electron colliding with the electrons surrounding atoms in the atmosphere with enough energy to knock them off. These electrons are then accelerated themselves and go on to knock off even more electrons in a chain reaction, creating a massive “avalanche” of high-energy electrons. As the electrons produced by RREA are slowed down by collisions with atomic nuclei in the atmosphere, they produce gamma rays via the Bremsstrahlung process. The gamma rays then escape upwards, where they can be detected by aircraft or satellites. 

Figure 1: Expected method of TGF formation at the top of a thundercloud. Source: https://en.wikipedia.org/wiki/Terrestrial_gamma-ray_flash#/media/File:TGF_production_by_quasi-static_fields.svg

    Several satellites have now recorded measurements of TGFs. They were first discovered by the Compton Gamma-Ray Observatory in 1994. In the 2000s, the RHESSI experiment showed the electrons could reach energies exceeding 20 MeV – which is in the range of the highest energies produced in linear accelerators for medical radiotherapy. Even more recently, the Fermi Gamma-ray Space Telescope discovered evidence of antimatter production from TGFs in 2009. When a positron meets an electron, the two annihilate and produce two photons each with an energy of 0.511 MeV (the energy equivalent of the rest mass of an electron/positron). The positrons have been experimentally shown to be produced when photons of energies exceeding 10 MeV trigger photonuclear reactions in atmospheric nuclei, creating radioactive nuclei and free neutrons. When positrons created in the TGF process either traveled directly upwards or rode along Earth’s magnetic field to the spacecraft they annihilated with electrons in the spacecraft, allowing it to see the telltale 0.511 MeV photons in its gamma ray detector. The Fermi team was able to trace these incoming positrons back along Earth’s magnetic field lines to specific thunderstorms producing them. With Fermi’s detection of numerous TGFs, it is now estimated that about 500 TGFs occur every day worldwide. Recently some rare TGFs have even been detected going downwards towards the ground, potentially opening up new opportunities to better study their formation.
    The second type of high-energy event known to be occurring in thunderstorms is gamma-ray glow. Gamma-ray glow was discovered by x-ray observations from aircraft above thunderstorms in the 1980s. This glow is much longer lasting than a TGF, with events potentially lasting tens of minutes. Energetic particles in a glow can reach similar energies to that of TGFs and are expected to be produced by the same RREA process. Unfortunately, the production of gamma-ray glow is somewhat less understood than that of TGFs. The glow seems to be produced by long-lasting stable electric fields in thunderclouds, and it is usually terminated when a lightning strike dissipates the stable field.
    The good news is that the ground-directed nature of gamma-ray glow is now providing new opportunities to study high-energy processes in thunderstorms without the need for difficult and expensive observations from above. While gamma ray photons are normally absorbed by the atmosphere in about 1km and thus long before they reach the ground from a typical thunderstorm, the Thundercloud Project in Japan has a way around this. In the winter, Japan has thunderstorms with cloud bases at heights of less than 1km and so ground based observations of their high-energy particles can be made. By assembling a network of sensors around Ishikawa Prefecture and Niigata Prefecture in Japan, they were able to record 51 gamma-ray events in the last four years. From this data, they have been able to show distinctive evidence of gamma-glow termination by lightning strikes. They were also able to detect a few downward TGFs, which show time-resolved proof of photonuclear reactions producing free neutrons and positrons. Their full results are currently in pre-print and set to be published in Progress of Theoretical and Experimental Physics.
    With the new results from the Thundercloud Project and hopefully more studies to come, we will be able to significantly advance our understanding of the processes involved in the high-energy phenomena of thunderstorms. It is certainly amazing how much there still is to learn about thunderstorms occurring right here on Earth, and how something so standard in our daily lives can be utilizing processes which we previously thought were restricted to particle accelerators or high-energy astrophysical phenomena. So the next time you hear an approaching thunderstorm, remember that you don’t need to travel to CERN or Fermilab to witness high-energy physics in action!

Sources: https://www.nasa.gov/mission_pages/GLAST/news/fermi-thunderstorms.html,
https://arxiv.org/pdf/2007.13618.pdf,
https://www.nature.com/articles/s42005-019-0168-y,
https://en.wikipedia.org/wiki/Terrestrial_gamma-ray_flash