Analysis of nighttime streamer corona discharges in storm clouds with ASIM data

Zusammenfassung

The main objective of this doctoral thesis is to exhaustively study the corona-type electrical discharges that take place in storm clouds. Corona discharges are characterized, among other things, by transient optical emissions in the near ultraviolet and visible blue range (280 - 450 nm). This is why they have been called Blue LUminous Events (BLUEs). This thesis presents the first worldwide nighttime climatology of corona discharges in storm clouds by combining two years of BLUEs data recorded by MMIA instrument of ASIM on-board the ISS (Neubert et al., 2019). This doctoral thesis focuses on investigating the physics of BLUEs, with the aim of un- derstanding their nature and determining the mechanisms that produce them. Addition- ally, the thesis addresses the detailed analysis of the geographic and seasonal distribution of BLUEs is addressed from two years of unique data obtained by the Modular Multispec- tral Imaging Array (MMIA) of the European Space Agency’s (ESA) Atmosphere Space Interaction Monitor (ASIM) instrument that was launched into space on April 18, 2018 and has been operational at the International Space Station (ISS) since then. MMIA has three fast photometers (100 kHz) and two cameras. The photometers have interferential filters centered on the wavelengths 337.0 nm (with a width of 4 nm), 777.4 nm (with a width of 5 nm) and a wide filter between 180 nm and 230 nm. The two available cameras use filters centered at 337.0 nm and 777.4 nm with the same characteristics as the light meter filters. The doctoral thesis is presented as a collection of three scientific articles in which I am the first author. These articles have been published in high-impact journals in the field of atmospheric sciences (atmospheric electricity). The articles are preceded by a thesis index, a summary (in Spanish and English), an introduction, the motivation of the studies carried out, a brief description of the methodology that has been followed and a concise presentation of the BLUEs. In my first article (published in Journal of Geophysical Research - Atmospheres, 2020, and highlighted by the editor) the structure of the BLUEs is described based on the observations of a storm recorded by ASIM on May 14, 2019 over Indonesia. The events were observed using the 337.0 nm photometer, during periods of little or no concurrent activity in the photometer centered on the 777.4 nm band (which is typical for lightning optical emissions). Through radio measurements made from the ground, it was concluded that 7 of the 10 cases analyzed corresponded to positive narrow bipolar events (NBE). NBEs are characterized by being very strong radio frequency (RF) pulses with three important features: (1) their very short duration (about 20 microseconds), (2) a bipolar appearance (very fast rise and fall of two radio pulses of positive and negative polarity or vice versa) of the waveform in the VLF / LF frequency range (10 - 400 kHz) and (3) followed by extremely powerful bursts of radiation. on VHF / HF (3 - 300 MHz). NBEs, when detected in the VLF/LF range, are typically ten times more intense than radio emissions in the high frequency (HF) range typically emitted by normal intracloud and/or intercloud lightning.. In addition, recent observations (2016 and 2017) suggest that NBEs are the result of very fast processes of electrical breakdown in air. Fast breakdown is associated to cold electrical discharges, that is, those that do not heat the surrounding air (unlike normal lightning) and in which the temperature of the ambient electrons reaches very high values of up to 80000 ◦C (about 7 eV) (electron volts). The fact that electrons have a small mass (2000 times lighter than the lightest atom) prevents them from heating the surrounding air. In this first work, a new and innovative method was developed to determine the height of BLUEs in storm clouds from the analysis of their optical signals. In particular, the method is based on fitting the 337.0 nm light curves captured by the MMIA 337.0 nm photometer to a model of light diffusion in storm clouds. The height obtained agrees well with measurements carried out with methods based on ground-based radio detection. The altitude at which these NBEs occur ranges between 8.5 km and 14 km. Obser- vations indicate that single-pulse blue flashes are caused by streamers (air plasma darts with strong head ionization) which, as discussed above, do not heat the air (they are cold discharges). The findings discussed in the first paper led us to conclude that positive NBEs are corona discharges made up of hundreds of millions of positive streamers (the head of the streamer carries positive electrical charge) that form in thunderclouds and that the BLUEs are the optical manifestation of the NBEs. My second article (published in Geophysical Research Letters, 2021), focuses on the global analysis of the blue pulses observed in storm clouds, focusing on studying the relationship of the BLUEs with lightning, both annually and seasonally. For this, a new algorithm has been developed and used to be able to filter the raw information from MMIA, and systematically obtain only the activity of BLUEs, excluding the rest of electrical interactions in storm clouds. The result of this comprehensive work is the first, and so far the only, nocturnal climatology of BLUEs (corona discharge activity in storm clouds) obtained from data from the ASIM MMIA instrument. Our research concluded that approximately 11 BLUEs occur globally every second at local midnight, and the average number of BLUEs on land/sea is 7 to 4. Geographically, the peak of the distribution of BLUEs is located in a region in northwestern Colombia. Three main zones of atmospheric electrical activity are shown in this distribution: America, Europe / Africa, and Asia / Australia. A zone of weaker BLUE electrical activity has also been found in the Pacific. My third paper (published in Journal of Geophysical Research - Atmospheres, 2022) delves into the results of the second, and focuses on distinguishing different types of blue pulses based on the goodness of their fit with the ”first hitting model” or FHM (model of diffusion of light in storm clouds) and in extracting its main characteristics such as optical power density (μW m−2) of the 337.0 nm pulse maximum, rise time of maximum, total duration, brightness, etc, based on the fit and raw data. In addition, we also found two types of global distributions of BLUEs, depending on whether an extra criteria is applied (or not) to the algorithm of the previous work (paper two) on the South Atlantic Magnetic Anomaly (SAA). This additional criteria is included because a clear oversensing of cosmic rays was observed with ASIM in the SAA region. The extra criteria aims to limit the number of cosmic rays detected by putting a limit on the duration of the events detected in the SAA. It is concluded that the real number of BLUEs would range between the two distributions found in this third work. The analysis of the characteristics of the BLUEs indicates that around 10% of the events are clearly BLUEs with a clear maximum. Most of them have a peak optical power density below 25 μW/m2, occur up to 4 km below the upper cloud boundary and are made up of hundreds of millions of streamers (up to 3 × 109).