We examined the broadband (< 1 Hz to 30 kHz) lightning sferics associated with 395 sprites observed near North America by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) onboard the FORMOSAT-2 satellite in a 12-year period from 2004 -2015. Our analysis indicates that the ISUAL dataset contains a significant fraction (69, or ~18%) of negative sprites, which were predominantly (> 80%) observed over oceanic and coastal thunderstorms mostly in tropical areas. The mean and median of impulse charge moment change (iCMC) associated with positive (negative) sprites are +346 C km (-709 C km) and +280 C km (-649 C km), respectively. The morphology and parent lightning properties (e.g., typically with high peak currents > -80 kA and large iCMCs > -300 C km) of negative sprites observed by the ISUAL are generally consistent with that documented in ground-based observations, but the ISUAL dataset does imply that sprites are sometimes produced by negative strokes with sub-critical iCMCs (less than -300 C km). Consequently, the future survey of global occurrence of sprites is desired to be based on complementary ground and space-borne observations.

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Transient luminous event (TLEs) observations have been conducted in mainland China since 2007, with a number of TLEs documented. This study analyzed a very unusual and unique positive sprite event, that may be produced jointly by two distinct positive cloud-to-ground lightning flashes (+CGs) occurring within a short time difference but with different locations separated by about 27 km. This observation is different from previous studies reporting that most of the sprites were triggered by a single +CG flash and its possible following continuous current. Detailed analysis on extremely low frequency (ELF) magnetic field shows that combined charge moment change (CMC) due to the two +CGs is smaller (~1478 C km) than those of the parent CGs for the other two sprites (1582 and 2134 C km, respectively) recorded over the same thunderstorm. The vertical extension and brightness of the sprites correspond well with the CMC of their parent CGs, namely, the larger the CMC value the brighter the sprite, and the larger the CMC value the larger the vertical extension. Negative lightning flashes dominated during the thunderstorm life cycle. The three sprites occurred during a time window in which both negative and positive flashes were active. The three sprites occurred over the thunderstorm stratiform region.

Narrow bipolar events (NBEs) are signatures in radio signals from thunderstorms observed by ground-based receivers. They are electromagnetic waves radiated by impulsive currents of electrical discharges. They come with two polarities, the positive that brings negative charge aloft, and the negative, that brings negative charge towards the earth. The sources of negative NBEs are at the very top of thunderclouds, and positive NBEs are at the upper levels, but inside the clouds. NBEs may occur at the onset of lightning, but the discharge process is not well understood. Here, we present spectral measurements by the Atmosphere-Space Interactions Monitor (ASIM) on the International Space Station that are associated with nine negative and three positive NBEs observed by a ground‐based array of receivers at the closest distance of about 100 km. We found that both negative and positive NBEs are associated with emissions at 337 nm with weak or no detectable emissions at 777.4 nm, suggesting that NBEs are associated with fast streamer breakdown. The rise times of the emissions for negative NBEs are about 10 µs, consistent with source locations at cloud tops where photons undergo little scattering by cloud particles, and for positive NBEs are ~ 1 ms, consistent with locations deeper in the clouds. For negative NBEs, the amplitude of the emissions is almost linearly correlated with the peak current of the associated NBEs. Our findings suggest that ground-based observations of radio signals provide a new means to measure the occurrences and strength of cloud-top discharges with implications for studies of perturbations of greenhouse gas concentrations at the tropopause.

 An automatic method is developed to recognize tweek atmospherics and diagnose the lower ionosphere with the machine learning method.  The differences of h and N e between the automatic and the manual method are -0.07±2.73 km, 0.03 ±0.92 cm -3 , respectively. The automatic method is capable of recognizing higher harmonic tweek sferics as well.