Space and Ground Observations of Thunderstorms

We have developed Spritewatch II, two new remote controlled ground based systems for observation of lightning and transient luminous events (TLEs) above thunderstorms. In this thesis I describe the systems and some of the resulting observations. In addition, I present two multiinstrumental studies of TLEs, one of which includes images made with Spritewatch II and one
by colleagues from the Paul Sabatier University in Toulouse (UPST).

The advantage of the Spritewatch II systems is that they are based on a low-cost high-speedcamera (Chronos 1.4, imaging at 1057-38000 fps, referred to as HSC). Also, the design is suchthat all components are placed inside a camera housing, including power supply, instruments and a mini PC for controlling and data handling. In addition to the HSC, both systems have a low-light Watec camera commonly used for TLE observations and one of the systems also includes a Hamamatsu R2949 photometer sampled at 122 kHz. The systems are partly autonomous because most operations are automatized and the camera pointing is controlled by a storm tracking algorithm. They were installed at three locations in Southern France: the first system (SW1) in Rustrel, the second system (SW2) in Lannemezan and later at Pic du Midi. They have a range of 800 km and are in the vicinity of two lightning mapping arrays (LMA) allowing for 3D mapping of lightning inside the clouds. Before moving SW2 to Pic du Midi, the HSC was equipped with an image intensifier necessary to achieve detailed sprite observations.

Results from observations with the intensified HSC include the evolution of the complex sprite morphology known as "angel sprite". To our knowledge, this is the first high speed  observation of an angel sprite and it shows the development of two wings: one that emanated from a bead and another that emanated 2-3 ms later from the upper tip of the long lasting glow in the sprite body. Also, we show that the HSC can be used to resolve timing distribution of the phases of sprite features, the persisting luminosity as well as the appearance of individual sprites in a dancing sprite event.

The first multi-instrumental study presented in this PhD thesis is based on 63 elves that were observed by SW1 to occur above an almost stationary storm. From lightning data, we found that the strokes that produced elves had one order of magnitude higher power and three times higher impulse charge moment change than strokes of similar peak current without observed elves. From a very low frequency radio signal crossing the storm, we found that the elves were associated with the longest types of perturbations ( 10 min duration) seen in the signals, whereas the shorter types of perturbations ( 1 min duration) occurred without optical emission. Our results suggests, that these were a result of density changes at 70 km altitude caused by electron attachment in slowly rising electric fields from lightning.

The second multi-instrumental study focused on dancing sprites: sequences of sprites that appear in succession with time intervals of no more than a few hundred milliseconds. The study involved ground based images of 19 dancing sprites (observed by colleagues from UPST), LMA data and meteorological data. The results showed that the dancing sprites were produced by extremely long flashes with average length of 156 km and 2.9 s duration in a 20 hour duration thunderstorm which developed in strong convective conditions. We argue that two of the events could be a new type of dancing sprites in which the individual sequences are not resolvable at video frame rates of 25 frames per second leading to exceptionally wide and bright sprites that do not match any of the currently reported categories of sprite morphology.