Dependence of positive and negative sprite morphology on lightning characteristics and upper atmospheric ambient conditions

Abstract

[1] Carrot sprites, exhibiting both upward and downward propagating streamers, and columniform sprites, characterized by predominantly vertical downward streamers, represent two distinct morphological classes of lightning-driven transient luminous events in the upper atmosphere. It is found that positive cloud-to-ground lightning discharges (+CGs) associated with large charge moment changes (Qh Q ) tend to produce carrot sprites with the presence of a mesospheric region where the electric field exceeds the value 0.8E k and persists for > $ 2 ms, whereas those associated with small Qh Q are only able to produce columniform sprites. Columniform sprites may also appear in the periphery of a sprite halo produced by +CGs associated with large Qh Q . For a sufficiently large Qh Q , the time dynamics of the Qh Q determines the specific shape of the carrot sprites. In the case when the sufficiently large Qh Q is produced mainly by an impulsive return stroke, strong electric field is produced at high altitudes and manifests as a bright halo, and the corresponding conductivity enhancement lowers/enhances the probability of streamer initiation inside/below the sprite halo. A more impulsive return stroke leads to a more significant conductivity enhancement (i.e., a brighter halo). This conductivity enhancement also leads to fast decay and termination of the upper diffuse region of carrot sprites because it effectively screens out the electric field at high altitudes. On the contrary, if the sufficiently large Qh Q is produced by a weak return stroke (i.e., a dim halo) accompanied by intense continuing current, the lightning-induced electric field at high altitudes persists at a level that is comparable to E k , and therefore an extensive upper diffuse region can develop. Furthermore, we demonstrate that 'negative sprites' (produced by -CGs) should be necessarily carrot sprites and most likely accompanied by a detectable halo, since the initiation of upward positive streamers is always easier than that of downward negative streamers, and -CGs are usually associated with impulsive return stroke with no continuing current. We also conjecture that in some cases, fast decaying single-headed upward positive streamers produced by -CGs may appear as bright spots/ patches. We show that the threshold charge moment changes of positive and negative sprites are, respectively,~320 and~500 C km under typical nighttime conditions assumed in this study. These different initiation thresholds, along with the different applied electric field required for stable propagation of positive and negative streamers and the fact that +CGs much more frequently produce large charge moment changes, represent three major factors in the polarity asymmetry of +CGs and -CGs in producing sprite streamers. We further demonstrate that lower mesospheric ambient conductivity leads to smaller threshold charge moment change required for the production of carrot sprites. We suggest that geographical and temporal conductivity variations in the lower ionosphere documen...

Plasma irregularities in the D-region ionosphere in association with sprite streamer initiation

“…First, the descending halo structures emphasized in the present work are due to a significant vertical dimension of the plasma irregularities leading to the effective illumination of a small portion of the plasma irregularities at each moment of time by the lightning electric field. In the case when the plasma irregularities are approximately spherical and have a compact size of tens to hundreds of metres, which are capable of initiating sprite streamers according to the previous modelling studies 16,19 , the plasma irregularities could only be recorded as a local enhancement of the halo luminosity around the origin of sprite streamers. Second, the brightness of halos depends on the impulsiveness of the causative lightning discharges 19 .…”

“…In the case when the plasma irregularities are approximately spherical and have a compact size of tens to hundreds of metres, which are capable of initiating sprite streamers according to the previous modelling studies 16,19 , the plasma irregularities could only be recorded as a local enhancement of the halo luminosity around the origin of sprite streamers. Second, the brightness of halos depends on the impulsiveness of the causative lightning discharges 19 . In some sprite events, the halo and halo structures can remain sub-visual if the charge transferring lightning current is not impulsive enough.…”

“…In our plasma fluid model 19 , the chemical reactions accounted for include the electron impact ionization of N 2 and O 2 , the electron dissociative attachment to O 2 and the electron detachment process O À þ N 2 -e þ N 2 O. Note that, although included, the effect of detachment in our simulations is negligible over a B2 ms time scale.…”

“…According to previous modelling studies 11,17,19 , the electron density in plasma irregularities needs to be B2-5 orders of magnitude higher than the ambient electron density to initiate sprite streamers, depending on the onset altitude of the sprite streamers (at lower altitudes, the difference needs to be larger). It is reasonable to believe that such strong perturbations cannot persist in a quiet lower ionosphere, and it is important to emphasize that the origin(s) of those plasma irregularities cannot be determined using the results presented in this work.…”

“…Photoionization processes are included using the three-group SP 3 model 28 . The motion of charged species is simulated by solving the drift-diffusion equations for electrons and ions coupled with Poisson's equation 19 . The transport equations for charged species are solved using a flux-corrected transport technique that combines an eighth-order scheme for the high-order fluxes and a donor cell scheme for the low-order fluxes.…”