Geophysical Institute University of Alaska Plasma Chemistry of Sprite Streamers D.D. Sentman, H.C. Stenbaek-Nielsen (University of Alaska) M.G. McHarg

Geophysical InstituteUniversity of Alaska

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 CenterLeiden, The Netherlands

8-12 October 2007


Varieties of Transient Luminous Events in the Upper Atmosphere

GIANTBLUEJET

(Elaboration of figure by Lyons et al. 2000)

PIXIES

TROLL

What chemical residuesare produced in TLEs?


Outline of Talk

• 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 SourcesObtained With Different Types of Cameras

(Su – Bare CCD TV)

(Sentman – Color ICCD)

(Wescott – B/W ICCD TV)

(Su – Bare CCD TV)(Sentman – Bare CCD)

(Stanley – Intensified Hi-speed Imager)

(Sentman – ICCD TV)

(Fukunishi – ICCD TV)

(Lyons – ICCD TV)

(Stenbaek-NielsenIntensified High-speed Imager)

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 SpriteImplies remnant compositional effect

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.

Study Region


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.

Sprite streamers at 70 km, with exposure times of 50 s. Equivalent exposure timeat STP is 1 ns.

Streamer heads are similar

[After Ebert et al., The multiscale nature of streamers,Plasma Sources Sci. Technol., 15, S118-S129, 2006.]


Simplified Streamer Model for 70 km Altitude

Input: E0 = 5 Ek, t = 6 s, M = 14, vs=107 m/s ~ 12 vte(7.5 eV)Output: densities vs time of ne, ion and active species.

5

25 m


Electric Field Driven Processes

Plasma Chemical Model of Sprite Streamers

Chemical Reactions

+ …


Electron Energy Distribution Function - Nonthermal

Solution of Time-Stationary Kinetic (Boltzmann) Equation (2-Term SH approximation)

{ } {3/ 2

1/ 2Inelastic

MomentumTransfer

2 2

2( ) 0

2where is energy, ( ), , is electric field, and ( )

3

m

m mm

n n mA n Q n

t M

eEn n A E

m

e ene ee

e e n n en

¶ ¶ ¶= + + =

¶ ¶ ¶

= = =

144424443

At low electric fields n() in air has of a Druyvesteyn-like form [n/1/2 ~ exp(-2/a0

2)]. At reduced fields of /p > 10 V/cm/torr a high energy tail begins to form above the ~ 4 eV barrier in the N2 vibrationalcross section.

The form of this distribution function is characteristic of nitrogen. Other gases possess different equilibrium distributions.


exp/i

i

BA

p E pa æ ö÷ç ÷= -ç ÷ç ÷çè ø

2

exp/a

a

E BA

p p E ph

- æ öæ ö ÷÷ çç ÷= -÷ çç ÷÷ ç÷ç ÷çè ø è ø

e m

Ep A

p

b

m-æ ö÷ç= ÷ç ÷÷çè ø

( )( / )

( )( / )i d e

a d e

v p E p

v p E p

Ionization coefficientProcess: e* + N2 N2

+ + 2eModeled by:

Attachment coefficientProcess: e + O2 O- + OModeled by

Electron mobilityDefined through drift speeed vd = eEModeled by

Ionization and attachment frequenciesVibrational excitations play a significant role in determiningthe form of the EEDF. In general the excitation frequenciesof the vibrational modes of both ground and excited statesare much larger than the ionization/attachment frequenciesat all undervoltage (E < Ek) and modest (E > Ek) overvoltage fields.

Ionization, Dissociative Attachment, and Vibrational Excitation Frequencies

(Ek=123 Td=32 kV/cm at STP)


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+

Coupled Chemical Scheme

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.

80+ species, 800+ reactions


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


R5: e* + N2 → e + e + N2+

impact ionizationR6: e* + O2 → e + e + O2

+ impact ionization

R19: e* + O2 → O + O- dissociative attachment

R21: e* + N2 → e + e + N+ + N dissociative ionization

R22: e* + O2 → e + e + O+ + Odissociative ionization

R26: e + N2+ → N + N

dissociative recombination R27: e + N2

+ → N + N(2D)dissociative recombination

R34: e + O2 + O2 → O2- + O2

3-body attachmentR38: e + O3 → O2

- + O dissociative attachment

R232: e + N2O2+ → N2 + O2

dissociative recombination

Electron Sources and Sinks

Principal Source: N2 ionizationPrincipal Sinks: N2O2

+, O3

Lifetime: ~1s

Princip

al S

ourc

ePrincipal

Sinks


Metastable N2(A3u+) Sources and Sinks

R8: e* + N2 → e + N2(A) impact excitation

R82: N2(A) + O2 → N2 + O2 collisional deactivation

R84: N2(A) + O2 → N2 + O + Odissociative deactivation

R89: N2(A) + O2 → N2 + O2(b) energy transfer

R96: N2(B) + N2 → N2(A) + N2 collisional quenching

R97: N2(B) → N2(A) + h(1PN2) radiative cascade

Principal Source: Radiative cascade from N2(B)Principal Sinks: collisional, dissociative deactivationLifetime: ~1 ms

PrincipalSource

PrincipalSink


Metastable O2(a1g) Sources and Sinks

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 collisional deactivation

Principal Sources: O2(b), N2(A), O2

Principal Sink: collisional deactivationLifetime: > 1000 s

PrincipalSource

PrincipalSink


Atomic Oxygen O(3P) Sources and Sinks

R16: e* + O2 → e + O + O impact dissociation

R17: e* + O2 → e + O + O(1D)impact dissociation

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 + Odissociative quenching

R101: N2(a) + O2 → N2 + O + Odissociative quenching

R124: N(2D) + O2 → NO + O atom transfer

R134: O(1D) + N2 → O + N2

collisional deactivationR135: O(1D) + O2 → O + O2(b)

energy transfer

Numerous processes contributeto creation of atomic oxygen in roughlyequal (ROM) amounts.

PrincipalSource

PrincipalSink



(Auroral green line)(~700-900 nm)

(~1.2 m)

(~0.8-1.2 m)


Nitrogen Oxide

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

Dominant source: N(2D)Dominant sink: N(4S)Lifetime: > 1000 s

Total Production:For diameter = 25 m length = 10 kmN ~ 5 x 1019 molecules/streamer


Total chemical impact of a very large sprite is likely to be much larger than for a single streamer.

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.

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?

Volume> 103 km3

Documents

Geophysical Institute University of Alaska Plasma Chemistry of Sprite Streamers D.D. Sentman, H.C. Stenbaek-Nielsen (University of Alaska) M.G. McHarg