Streamers, Sprites, Leaders, Lightning: Plasma Chemistry of Sprite Streamers D.D. Sentman, H.C. Stenbaek-Nielsen (University of Alaska) M.G. McHarg (U.S. Air Force Academy) J.S. Morrill (Naval Research Laboratory) Streamers, Sprites, Leaders, Lightning: From Micro- to Macroscales A Multidisciplinary Workshop on Outstanding Problems in Electrical Discharge Processes Lorentz Center Leiden, The Netherlands 8-12 October 2007

Varieties of Transient Luminous Events in the Upper Atmosphere What chemical residues are produced in TLEs? GIANT BLUE JET TROLL PIXIES (Elaboration of figure by Lyons et al. 2000)

Outline of Talk Chemical Model Optical observations of sprites – a window into lightning induced chemical modifications of the upper atmosphere Current state of optical observations 1,000 fps observations – first evidence for transient chemical modifications 10,000 fps observations – first time-resolved imagery of sprite dynamics Simplified model of a sprite streamer Based on time and space resolved observations Chemical Model 80+ species, 500+ reactions Combines Electric field-related processes (ionization, excitation) in the head Chemical reactions in the head and in the trailing region Includes reaction chains for positive ions (proton hydrates) and negative ion clusters

Sprite Gallery – Images From a Variety of Sources Obtained With Different Types of Cameras (Su – Bare CCD TV) Early sprite imagers were intensified CCD TV cameras. Recent research has simultaneously moved in two directions: (1) high speed (10,000 fps) cameras, and (2) inexpensive bare CCD imagers, both TV and integrating systems. Sprites are bright enough (>> 1 MR) that they are now considered "easy" to observe.

“Reignition” of a Sprite Implies remnant compositional effect Study Region Sequence of 1000 fps images showing a reactivated sprite, taken from Figure 2 of Stenbaek-Nielsen et al. (2000). The top row shows the initial sprite, and the second row shows the reignited event after a 44 ms break.

Sprite Reignition – 1000 fps (clip)

Sprite at 10,000 fps Streamer heads clearly resolved Dark space behind head implies E ~ 0 Trailing afterglow region chemiluminescence?

Similarity of Laboratory and Sprite Streamer Structures Laboratory streamers at various exposure times. Dendritic structures (left) are due to smearing over long (>>1 ns) exposure times. Time resolved structures at right show bright streamer heads only, with no apparent trailing columns. Streamer heads are similar Sprite streamers at 70 km, with exposure times of 50 ms. Equivalent exposure time at STP is 1 ns. [After Ebert et al., The multiscale nature of streamers, Plasma Sources Sci. Technol., 15, S118-S129, 2006.]

Simplified Streamer Model for 70 km Altitude 5 25 m Input: E0 = 5 Ek, Dt = 6 ms, M = 14, vs=107 m/s ~ 12 vte(7.5 eV) Output: densities vs time of ne, ion and active species.

Plasma Chemical Model of Sprite Streamers Electric Field Driven Processes Chemical Reactions + …

Electron Energy Distribution Function - Nonthermal Solution of Time-Stationary Kinetic (Boltzmann) Equation (2-Term SH approximation) At low electric fields n(e) in air has of a Druyvesteyn-like form [n/e1/2 ~ exp(-e2/a02)]. At reduced fields of e/p > 10 V/cm/torr a high energy tail begins to form above the e ~ 4 eV barrier in the N2 vibrational cross section. The form of this distribution function is characteristic of nitrogen. Other gases possess different equilibrium distributions. The dynamics of streamer development are determined by electrostatic processes originating in a short electric field pulse acting locally within a small region of space, and self-consistent response of the medium in response to the breakdown. The processes may be described using a simple (compared to magnetized plasmas!) kinetic model for the development of the electron energy distribution in the form shown here. The curves shown the form of the EEDF for various reduced electric fields (breakdown is at ~40 V/cm/torr). In this form a Maxwellian would be a straight line, and and Druyvesteyn distribution would be a parabola. It is obvious that the real distribution is neither. Most electron interactions with neutrals (ionization, excitation of excited states, dissociation, etc.) occur by interaction with electrons in the high energy tail, since typical energies are ~5-10 eV for dissociation or activation, and 15-18 eV for ionization (see next slide).

Ionization, Dissociative Attachment, and Vibrational Excitation Frequencies Ionization coefficient Process: e* + N2 ® N2+ + 2e Modeled by: Attachment coefficient Process: e + O2 ® O- + O Modeled by Electron mobility Defined through drift speeed vd = meE Modeled by (Ek=123 Td=32 kV/cm at STP) Vibrational excitations play a significant role in determining the form of the EEDF. In general the excitation frequencies of the vibrational modes of both ground and excited states are much larger than the ionization/attachment frequencies at all undervoltage (E < Ek) and modest (E > Ek) overvoltage fields. Ionization and attachment frequencies

Coupled Chemical Scheme 80+ species, 800+ reactions Solve the coupled set of 68 ODEs dni = Si – Li dt for the species listed below. Si is the source term and Li is loss term for species ni, each summed over RHS and LHS, resp., of all reactions in which ni appears. The numerical integration was performed using a variable step stiff ODE solver. Species followed in the simulation. Bath species N2, O2, H2O, CO2, CO and HCl. Neutral (36 + 6 bath) Negative (12) Positive (27) N2(X, v=1-4), N2(A), N2(B), N2(a’), N2(C), N2(W3, B’, a, w1, E, a’’), N(4S), N(2D), N(2P), O2(a), O2(b), O2(A), O(3P), O(1D), O(1S), O3, NO, NO2, NO3, N2O, N2O5, H, OH, OH*, HO2, H2O2, HNO3, HO2, NO2, Cl, ClO e, O-, O2-, O3, O4-, NO2-, NO3, CO3-, CO4-, OH, HCO3-, Cl- N2+, O2+, N+, O+, N3+, N4+, O4+, NO+, NO2+, N2O+, N2O2+, N2NO+, O2NO+, (H2O)O2+, (H2O)H+, (H2O)2H+, (H2O)3H+, (H2O)4H+, (H2O)OHH+, (H2O)NO+, (H2O)2NO+, (H2O)3NO+, CO2NO+, (H2O)2CO2NO+, (H2O)2N2NO+, (HO)N2NO+, (H2O)2N2NO+

Kinetic Scheme Reaction set includes 30 electric-field driven electron impact processes 10 electron-ion recombination processes 25 attachment-detachment processes 23 ground state chemistry reactions 75 active species reactions 27 ion conversion processes 23 odd-hydrogen and odd-nitrogen processes (includes hydroxyl chemistry) 30 positive ion chemistry (hydrates) reactions 35 negative ion and chlorine reactions 565 ion-ion recombination (2- and 3-body) reactions Total: 836 Focus is on basic chemical reactions – no chemistry derived from vibrational kinetics is included at this stage.

N2 1P and 2P Emissions

Electron Sources and Sinks Principal Source: N2 ionization Principal Sinks: N2O2+, O3 Lifetime: ~1s R5: e* + N2 → e + e + N2+ impact ionization R6: e* + O2 → e + e + O2+ R19: e* + O2 → O + O- dissociative attachment R21: e* + N2 → e + e + N+ + N dissociative ionization R22: e* + O2 → e + e + O+ + O R26: e + N2+ → N + N dissociative recombination R27: e + N2+ → N + N(2D) R34: e + O2 + O2 → O2- + O2 3-body attachment R38: e + O3 → O2- + O R232: e + N2O2+ → N2 + O2 Principal Source Principal Sinks

Metastable N2(A3Su+) Sources and Sinks Principal Source: Radiative cascade from N2(B) Principal Sinks: collisional, dissociative deactivation Lifetime: ~1 ms R8: e* + N2 → e + N2(A) impact excitation R82: N2(A) + O2 → N2 + O2 collisional deactivation R84: N2(A) + O2 → N2 + O + O dissociative deactivation R89: N2(A) + O2 → N2 + O2(b) energy transfer R96: N2(B) + N2 → N2(A) + N2 collisional quenching R97: N2(B) → N2(A) + hn(1PN2) radiative cascade Principal Sink Principal Source

Metastable O2(a1Dg) Sources and Sinks Principal Sources: O2(b), N2(A), O2 Principal Sink: collisional deactivation Lifetime: > 1000 s R12: e* + O2 → e + O2(a) impact excitation R88: N2(A) + O2 → N2 + O2(a) energy transfer R90: N2(A) + O2(a) → N2(B) + O2 energy pooling R111: O2(a) + O2 → O2 + O2 collisional deactivation R117: O2(b) + N2 → O2(a) + N2 Principal Sink Principal Source

Atomic Oxygen O(3P) Sources and Sinks R16: e* + O2 → e + O + O impact dissociation R17: e* + O2 → e + O + O(1D) R61: N + NO → N2 + O atom transfer R79: O + O2 + N2 → O3 + N2 3-body association R84: N2(A) + O2 → N2 + O + O dissociative quenching R99: N2(B) + O2 → N2 + O + O R101: N2(a¢) + O2 → N2 + O + O R124: N(2D) + O2 → NO + O R134: O(1D) + N2 → O + N2 collisional deactivation R135: O(1D) + O2 → O + O2(b) energy transfer Principal Sink Principal Source Numerous processes contribute to creation of atomic oxygen in roughly equal (ROM) amounts.

(Auroral green line) (~700-900 nm) (~1.2 mm) (~0.8-1.2 mm)

N ~ 5 x 1019 molecules/streamer Nitrogen Oxide Dominant source: N(2D) Dominant sink: N(4S) Lifetime: > 1000 s Total Production: For diameter = 25 m length = 10 km N ~ 5 x 1019 molecules/streamer R59: N + O2 → NO + O R61: N + NO → N2 + O R124: N(2D) + O2 → NO + O R125: N(2D) + O2 → NO + O(1D) R177: N+ + O2 → O+ + NO

Total chemical impact of a very large sprite is likely to be much larger than for a single streamer. Volume > 103 km3 Our calculation was for a single one of these streamers … but what’s the total impact of the entire event? What’s the impact of a thunderstorm? The totality of thunderstorms over the earth? It is unknown at this point what the total chemical impact of sprite-induced perturbations on the larger atmospheric chemical system is. Further observations and modeling are warranted.