1. 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

2. 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)

3. 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

4. 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.

5. “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.

6. Sprite Reignition – 1000 fps
(clip)

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

8. 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.]

9. Simplified Streamer Model for 70 km Altitude5
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.

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

11. 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 eV for ionization (see next slide).

12. 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

13. 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 ( 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+

14. 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.

15. N2 1P and 2P Emissions

16. 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

17. 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

18. 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

19. 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.

21. (Auroral green line)
(~ nm)
(~1.2 mm)
(~ mm)

22. 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

23. 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.