C.-F. Enell

Seeking sprite-induced signatures in remotely sensed middle atmosphere NO2: latitude and time variations

Recent research on sprites shows these and other transient luminous events can exert a local impact on atmospheric chemistry, although with minor effects at global scales. In particular, both modelling and remote sensing work suggest perturbations to the background NOx up to a few tens of per cent can occur above active sprite-producing thunderstorms. In this study we present a detailed investigation of MIPAS/ENVISAT satellite measurements of middle atmospheric NO2 in regions of high likelihood of sprite occurrence during the period August to December 2003. As a proxy of sprite activity we used ground based WWLLN detections of large tropospheric thunderstorms. By investigating the sensitivity of the analysis to the characteristics of the adopted strategy, we confirm the indication of sprite-induced NO2 enhancements of about 10% at 52 km height and tens of per cent at 60 km height immediately after thunderstorm activity, as previously reported by Arnone et al (2008b Geophys. Res. Lett. 35 5807). A further analysis showed the enhancement to be dominated by the contribution from regions north of the Equator (5 ◦ Nt o 20 ◦ N) during the first 30 to 40 days of the sample (i.e. the tail of Northern Hemisphere summer) and in coincidence with low background winds. (Some figures in this article are in colour only in the electronic version)

WACCM climate chemistry sensitivity to sprite perturbations

Transient luminous events affect Earths atmosphere between thunderstorm tops and the lower ionosphere through ion-neutral chemistry reactions. Particular emphasis has been given to sprites, with models and observations suggesting a capability of perturbing atmospheric nitrogen oxides at a local level, as it is known to occur for tropospheric lightning and laboratory air discharges. However, it is as yet unknown whether sprites can be a relevant source of nitrogen oxides for the upper atmosphere. In this paper, we study the sensitivity of the Whole Atmosphere Community Climate Model (WACCM) to sprite-like nitrogen oxide perturbations. We take a top-down approach to estimate what magnitude sprite perturbations should have to become significant as compared to other relevant atmospheric processes and study the sensitivity of the model response within the given uncertainties. We show that, based on current predictions by sprite streamer chemistry models, sprites can perturb Tropical NOx at 70 km altitude between 0.015 ppbv (buried in the background variability) and 0.15 ppbv (about 20%), adopting a local NOx production per sprite of 1.5·1023 and 1.5·1024 molecules respectively at this altitude. Below the lowest of the adopted values, sprites are irrelevant at global scales. Sprite NOx may build up to significantly larger amounts locally above active thunderstorms, further aided by other transient luminous events and possibly terrestrial gamma ray flashes. We also use model results to interpret the available observational studies and give recommendations for future campaigns.

Broadband meter‐wavelength observations of ionospheric scintillation

Intensity scintillations of cosmic radio sources are used to study astrophysical plasmas like the ionosphere, the solar wind, and the interstellar medium. Normally, these observations are relatively narrow band. With Low-Frequency Array (LOFAR) technology at the Kilpisjarvi Atmospheric Imaging Receiver Array (KAIRA) station in northern Finland we have observed scintillations over a three-octave bandwidth. “Parabolic arcs,” which were discovered in interstellar scintillations of pulsars, can provide precise estimates of the distance and velocity of the scattering plasma. Here we report the first observations of such arcs in the ionosphere and the first broadband observations of arcs anywhere, raising hopes that study of the phenomenon may similarly improve the analysis of ionospheric scintillations. These observations were made of the strong natural radio source Cygnus-A and covered the entire 30–250 MHz band of KAIRA. Well-defined parabolic arcs were seen early in the observations, before transit, and disappeared after transit although scintillations continued to be obvious during the entire observation. We show that this can be attributed to the structure of Cygnus-A. Initial results from modeling these scintillation arcs are consistent with simultaneous ionospheric soundings taken with other instruments and indicate that scattering is most likely to be associated more with the topside ionosphere than the F region peak altitude. Further modeling and possible extension to interferometric observations, using international LOFAR stations, are discussed.