L’impact du streamer de bleu jet sur la chimie stratosphérique

L’impact du streamer de bleu jet sur la chimie
stratosphérique

 Transient Luminous Events (TLEs)

The TLE family includes high-altitude lightning flashes and large-scale optical events that last for less than a second (Figure 1-5). Although eyewitness reports of TLEs above thunderstorms have been recorded for more than a century, the first image of one was captured only in 1989, serendipitously during a test of a low-light television camera [Franz et al., 1990]. Since then, many transient luminous events (TLEs) have been documented and classified above thunderstorms [Pasko et al., 2002] from the ground [e.g. Winckler et al., 1993; Lyons, 1994], from aircraft [e.g. Sentman and Wescott, 1993; Wescott et al., 1995], from the space shuttle [e.g. Boeck et al., 1992] and from orbiting sensors [e.g. Neubert et al., 2001; Chern et al., 2003; Chen et al., 2008]. All of them show that TLEs appear over most regions of the globe. They occur above thunderstorm clouds up to the mesosphere (or lower ionosphere). They are directly related to the electrical activity in the underlying thunderstorms [Sentman et al., 1995; Neubert, 2003; Pasko, 2003] and they are luminous manifestations of the electrodynamic couplings between the atmospheric layers. Sprites develop at the base of the ionosphere and move rapidly downwards at speeds of up to 10000 km/s. They appear in the altitude range of ~40 km to 90 km above large thunderstorms [Sentman et al., 1995; Boccippio et al., 1995; Lyons, 1996; Stanley et al., 1999]. The first image of a sprite was captured by scientists from the University of Minnesota in 1989. Sprites generally appear in clusters of three or more and they can cover horizontal distances of ~50 km. They are reddish-orange in color and they have various shapes, i.e. carrot-, angel- or column-like shapes with hanging tendrils below. Sprites usually occur at altitudes of ~65 km to 75 km and their tendrils can extend down to altitudes as low as 40 km. Sprites are associated with powerful positive cloud-to-ground lightning [Barrington-Leigh et al., 1999; Bell et al., 1998]. The electromagnetic pulse (EMP) is generated above the thunderstorm cloud during an intense lightning discharge. The resulting intense electric field above the clouds can extend over a large altitude range to the upper atmosphere and is possibly responsible for atmospheric air breakdown or runaway electron ionization [Pasko et al., 1995, 1996; Bell et al., 1995]. Ionization and optical emissions that occur are manifested in the form of the visible Sprite. Elves are lightning induced TLEs, occurring at high altitude above large and intense lightning strikes in a thunderstorm. They were first identified in 1990 in space shuttle images. Their ring-like shapes are different from sprites. They can spread over 100-300 km laterally, occurring in the lower ionosphere [Fukunishi et al., 1996; Inan et al., 1997]. Similar to sprites, elves are likely caused by heated lower ionospheric electrons by EMPs [Inan et al., 1991; Taranenko et al., 1995]. Jets include gigantic jets (GJs) (e.g. Figure 1-6a), blue jets (BJs) (e.g. Figure 1-6b) and blue starters. All jets emanate from the top of thunderclouds (~15-18 km), and they are differentiated by their terminal altitudes. GJs propagate upwards towards Earth’s Chapter Ⅰ 16 ionosphere to an altitude of ~90 km [Su et al., 2003], BJs up to 40 km with moving speeds of ~100 km/s [Wescott et al., 1995, 1998, 2001; Lyons et al., 2003], while blue starters rise to a maximum altitude of ~25.5 km [Wescott et al., 1996, 2001]. Blue starters are considered as the initial stages of blue jets. As shown in Figure 1-6, GJs present a visible blue color in the lower channel, like BJs, and a red color with a streamer-like structure above 40 km. Figure 1-6. Color photographs of (a) gigantic jets, captured by Patrice Huet on 7th March 2010 [Soula et al., 2011], and (b) blue jet captured by William Nguyen Phuoc on 8th December 2015. 

Physical characteristics of jets

BJs and GJs form by the escape of conventional lightning leaders upward from thunderclouds. They are types of upper-atmospheric lightning and of cloud-to-air discharge within a thunderstorm, propagating upwards. They mainly occur over ocean [e.g. Chen et al., 2008] and in the tropics (tropopause ~17 km) and at mid-latitudes (~15 km). A typical BJ is a slow-moving, fountain-like cone with a cone angle of ~15°, which emanates from the top of thunderclouds up to an altitude of ~ 40-50 km [Wescott et al., Chapter Ⅰ 17 1995, 2001; Lyons et al., 2003], moving with an upward velocity of ~100 km/s. Its light emission duration is ~200-300 milliseconds [Wescott et al., 1996]. Due to a set of blue and near-ultraviolet emission lines from neutral and ionized molecular nitrogen, BJs are blue in color [Wescott et al., 1995, 1998, 2001]. The structure and apparent speed of a jet vary significantly during its lifetime [Pasko et al., 2002 (P2002); Su et al., 2003]. The evolution of a jet is characterized by three main stages, i.e. formation of the leading jet, fully developed jet, and trailing jet. In a GJ event captured by P2002 (Figure1-7), the leading jet (stage A) grew at the beginning with an average speed of ~60 km/s, increasing to a speed of up to ∼200 km/s (#11–12), and then decelerating (#12–13) and again brightening and accelerating to ∼500 km/s (#13–14). Then, the tip propagated at ∼500 km/s to ~48 km and ejected fully developed jets (stage B), which moved at ~1200 km/s. Meanwhile, bright diffuse spots appeared at a height of ~ 70 km. At the end of the event, the top of the trailing jet (stage C) faded away and its forked structure persisted at a height of ≤ 32 km. Figure 1-7. Video fields 7-20 (~17 ms each) during the 15 Sep 2001 GJ event that occurred ∼200 km northwest of Arecibo Observatory, Puerto Rico. Credit from Mishin and Milikh [2008] BJ and GJ events differ in their maximum altitude and their polarities [Krehbiel et al., 2008]. As shown in Figure 1-8, BJs develop up to a lower altitude than GJs. Studies found that BJs are generated by electrical breakdown between upper storm charges and the cloud top screening charge. GJs initiate as an in-cloud discharge between a midlevel dominant charge and an upper-level screening charge, and then propagate out of the storm top. From normal polarity thunderclouds (Figures 1-8a and 1-8b), BJs transport a positive charge upward, while GJs transport a negative charge upward. On inverted polarity storms (Figures 1-8c and 1-8d), BJs produce a negative charge upward, while GJs produce a positive charge upward. The mechanism that produces the jets is similar to cloud-to-ground lightning, which causes a charge imbalance in the storm

 Chemistry associated with TLEs

It is well known that electric discharge in the middle atmosphere produces NOx (=NO+NO2) as a result of intense heating and /or shock wave from a lightning channel [Chameides et al., 1987], by recombination reactions and ion-neutral reactions [Griffing, 1977; Kossyi et al., 1992] of atomic oxygen and nitrogen. Thus, the lightning produces nitrogen oxides. The production of NOx in TLEs has the same reaction steps as in tropospheric discharges [Hiraki, 2004]. The discharge generates an electric field which drives electron impact on air molecules and atoms by ionization, dissociation and excitation processes. Goldenbaum and Dickerson [1993] proposed that NOx formation in thunderstorm discharges starts with the dissociation of oxygen and nitrogen molecules through: e − + N2 → N + N + e − (1-29) e − + O2 → O + O + e − (1-30) NO is then produced by the single replacement of an oxygen or nitrogen atom with nitrogen or oxygen molecules: N2 + O → NO + N (1-31) O2 + N → NO + O (1-32) Chapter Ⅰ 20 Finally, the reaction ends with a recombination of nitrogen and oxygen atoms: N + N → N2 (1-33) O + O → O2 (1-34) For N2 and O2, which are common air constituents, the principal ionization reactions are: e − + N2 → N2 + + 2e − (1-35) e − + N2 → N + + N + 2e − (1-36) e − + O2 → O2 + + 2e − (1-37) e − + O2 → O + + O + 2e − (1-38) The oxygen and nitrogen atoms that are produced in these reactions can be in their ground states or in excited states. An example for O2 is schematically shown in Figure 1-10. The excitation of N2 or O2 is caused by electron collisions, which produce electronically excited N2 or O2 molecules through vibration. Figure 1-10. O2 transformation process induced by discharge As the upward discharges from thunderstorms or discharges in the stratosphere, the chemistry effects of TLEs have recently received significant attention. The study by Sentman and Stenbaek-Nielsen [2009], for example, determined the concentrations of species impacted by streamers. Compared to no-discharge, the study showed that  streamer impact leads to an increase in oxygen atom, O(1D) and N2(A 3 ∑ ) + u by a factor of 2 and to an increase in O2(a) by a factor of 3. The study by Gordillo-Vázquez [2008] further concluded that H2O affects the property of CO4 − anions, but that there is no significant effect on NOx formation or destruction. Stratospheric NOx contributes to O3 depletion [Callis, 2002] and the high concentration of NO in the mesosphere can transport downward to the stratosphere in the polar winter. TLEs are thus important sources of middle atmosphere NOx, while the effect of TLEs on NOx production at higher altitudes still remains largely unexplored [Pasko et al., 2012]. The chemical effects of sprites have been investigated in a number of model studies, e.g. those by Hiraki et al. [2008], Evtushenko et al. [2013], and Parra-Rojas et al. [2013]. In addition, prior studies have suggested that NOx in the middle atmosphere are locally significantly impacted by TLEs, based either on calculation-based estimates [Neubert et al., 2008; Gordillo-Vazquez, 2008; Sentman et al., 2008; Enell et al., 2008; Arnone et al., 2014; Perez-Invernon et al., 2019], on observation-based estimates [Arnone et al., 2008], or on laboratory experiments [Peterson et al., 2009]. Enell et al. [2008] inferred that NOx increases by about 1 order of magnitude within streamers. The NOx produced is significant in its local concentration and distribution, especially at high altitudes. The long-term behaviors of NOx, Ox and HOx species impacted by night sprites were calculated by Hiraki et al. [2008]. They noted that sprites can impact local chemistry for hours at high altitudes (40-70 km). Peterson et al. [2009] estimated that the NOx production impacted by each event comprises 1.7×1022 -7.4×1026 molecules in blue jets and 6.8×1023 -6.3×1027 molecules in red sprites. Perez-Invernon et al. [2019] showed that there are about 3.8 Tg (1Tg=1012 grams) N2O-N/year and 0.07 Tg NO-N/year produced by BJs near the stratosphere by using a global climate model.