Blue corona discharges are bursts of streamers often observed at the top of thunderclouds, but the cloud conditions that facilitate them are not well known. Here we present observations by the Atmosphere-Space Interactions Monitor of 92 corona discharges as it passed over cyclone Fani in the Bay of Bengal. The discharges formed in convective cells of unstable air carried from land over the Indian Ocean, with CAPE reaching ~6000 J kg-1. The CALIPSO satellite passed over one of the cells ~12 min after ASIM, taking the first measurements of the microphysics at the top of a cloud generating corona discharges. We find the discharges occur in a region of strong convection, the cloud reaching into the stratosphere with ice/water content ~0.1 g m-3, photon mean free path ~ 3 m and ice crystal number density ~5e7 m-3. Measurements by a lightning detection network suggest the charge structures are folded.

We report on observations of corona discharges at the uppermost region of clouds characterized by emissions in a blue band of nitrogen molecules at 337 nm, with little activity in a red band of lightning leaders at 777.4 nm. Past work suggests they are generated in cloud tops reaching the tropopause and above. Here we explore their occurrence in two convection environments of the same storm: one is developing with clouds reaching above the tropopause, and one is collapsing with lower clouds. We focus on those that form a distinct category with fast risetimes below 20 µs, signifying they are at the very top of the clouds. The discharges are observed in both environments. In the collapsing cells they are related to substructures of convection. The observations suggest that a range of storm environments may generate corona discharges, and that they may be quite common in convective surges.

The Atmosphere-Space Interactions Monitor measures Terrestrial Gamma-Ray Flashes (TGFs) simultaneously with optical emissions from associated lightning activity. We analyzed optical measurements at 180-230 nm, 337 nm and 777.4 nm related to 69 TGFs observed between June 2018 and October 2019. All TGFs are associated with optical emissions with 90% at the onset of a large optical pulse, suggesting that they are connected with the initiation of current surges. A simple model of photon delay induced by cloud scattering suggests that the sources of the optical pulses are from 0.7 ms before to 4.4 ms after the TGFs, with a median of -10±80 μs, and 1-5 km below the cloud top. The pulses have rise times comparable to lightning without identified TGFs, while the FWHM is twice as long. Pulse amplitudes at 337 nm are ∼3 times larger than at 777.4 nm. The results support the leader-streamer mechanism for TGF generation.

The electromagnetic and electrostatic fields from powerful lightning heat and ionize the lower ionosphere. The disturbances appear as halos, sprites and elves, and are also observed as perturbations in crossing radio signals. The characteristic of the lightning discharges leading to the various types of perturbations is not fully understood. Here we present an analysis of 63 elves and corresponding VLF and MF signal perturbations from an almost stationary thunderstorm that allows us to untangle some of the dependencies of perturbations on the lightning characteristics. We characterize the perturbations to a VLF-transmitter signal as “long-recovery-early-events” (LOREs), “early” events, or “rapid-onset-rapid-decay” (RORD) events. We find that LOREs are related to high lightning current and bright elves, and their amplitude and sign depend on their location along the signal path. With observations in the ELF and MF band, we find that lightning with elves has three times the impulse charge moment change (iCMC) and ten times the power than lightning of similar peak current without elves. Attenuation in MF links appear in a higher proportion and longer duration observed with elves than with high peak current lightning without elves. The remaining types of VLF perturbations occur without TLEs but with sequences of lightning that produce slowly rising CMCs reaching high values (up to ~3500 C km within ~500 ms). Slower rise times lead to lower fields in the mesosphere that may not create significant ionization but instead drive dissociative attachment of free electrons. The depletions can result in perturbations to crossing VLF signals.

Lightning is observed to incept in thundercloud electric fields below the threshold value $E_k$ for discharge initiation. To explain this, the local enhancement of the electric field by hydrometeors is considered. The conditions for the onset of positive corona discharges are studied in air for ellipsoidal geometries. A hydrometeor is simulated as an individual charged conductor in zero ambient field; there is only a field generated by the charge on the hydrometeor surface. By doing so, the feasibility of corona inception from ellipsoidal hydrometeors can be formulated based on the self-sustaining condition of electron avalanches. For representative hydrometeor volumes and typical thundercloud pressure, values between $1.2\,E_k$ and $37\,E_k$ were found for the onset electric field at the tip of the ellipsoid. From simulations the required ambient electric field for corona onset from an uncharged hydrometeor can then be derived. This results in values between $0.07\,E_k$ and $0.8\,E_k$ for semi-axes aspect ratios between 0.01 and 1. The charge required on the hydrometeor surface for corona onset is minimum for semi-axes aspect ratios between 0.04 and 0.07 depending on the considered hydrometeor volume. For the simulated hydrometeors, the values of this onset charge for typical pressures are between 1500\,pC and 3200\,pC. Including a size-correction for comparison to in situ measurement shows agreement with measured precipitation charges. From the results it is concluded that corona onset from ellipsoidal hydrometeors of a realistic volume can be achieved in thundercloud conditions for certain aspect ratios.

We present a multi-instrumental analysis of a 20-hour duration Mesoscale Convective System (MCS) over the northwestern Mediterranean sea on September 21, 2019, that produced 21 sprites recorded with a video camera, of which 19 (90$\%$) were dancing sprites. The asymmetric trailing stratiform MCS developed in strong convective conditions (3500 J kg$^{-1}$) and formed a large and persistent overshoot with several convective cores (up to 25,000 km$^2$ with cloud top temperature $<$ -66$^{\circ}$C). It exhibited a bow echo structure with a probable inverted charge dipole within the convective region before the main sprite production period and associated with a large decrease of the negative cloud-to-ground (CG) flash rate. The sprite producing positive cloud-to-ground (SP+CG) flashes mainly initiated at the edge of the convective line on the side of the stratiform region. The flashes propagated over long distances (up to $\sim$ 200 km) across it producing both positive and negative CG strokes. Some parent flashes initiated within the stratiform region close to convective structures and propagated reversely. The 19 dancing sprite events included 49 sequences, of which 46 (94$\%$) were associated with distinct SP+CG strokes and 3 with surges during the continuing current. An especially bright and wide sprite sequence was produced by three distinct SP+CG strokes that occurred within 3 ms and separated by 54 km. This sprite sequence could be classified as a new sprite category resembling to a â\euroœwallâ\euro? but structured in three groups, each associated with one of the +CG strokes, but not separated by the video imagery.