THUNDERSTORMS AND LIGHTNING GENERATING SPRITES Rumjana Mitzeva

Outline Thunderstorms and lightning Field observations and results Physical mechanism Outstanding problems

Answer to questions What is the relation between lightning and sprites? Which types of thunderstorms produce sprites and in which part? Which conditions are conductive (favorable) for the required lightning activity in these regions? Why? How do these microphysical conditions develop?

Transient luminous events (TLEs) Optical signatures of electrical breakdown in the upper atmosphere due to rapid charge rearrangement in underlying thunderclouds (Adapted from Lyons et al. 2000)

Thunderstorms and Lightning a) Stratiform region of MCS -Positive CG spider lightning b) Ordinary thundercloud -Negative CG lightning c) Tilted thundercloud -Positive CG lightning d)Supercell thunderstorms with inverted polarity - Positive CG lightning

Field Studies USA – 1994, 1996 – first field study USA - Severe Thunderstorm Electrification and Precipitation Study (STEPS) - High Plains, near the Colorado-Kansas border May – J uly 2000 EUROPE - first field study 2000 EUROSPRITE – start 2003– Southern Europe and South Africa Brazil Sprite Campaign – Chile - Argentina – January 2005 Spain - Catalonia 2003, 2004 Japan – over Sea of Japan in Winter Bulgaria - ?

First analyses Boccippio et al., Analyzed 42 sprites in July 12, 1994 and 55 sprites on September 7, CG lightning precedes most sprites (85%) by approximately min Sabbas [1999] studied 746 sprite events from 7 days from the Sprites’96 Campaign, 75% of sprites were preceded by +CG lightning.

STEPS STEPS Severe Thunderstorm Electrification and Precipitation Study High Plains, near the Colorado-Kansas border May-July Colorado State University, Fort Collins, CO 2.National Center for Atmospheric Research, Boulder, CO 3.National Weather Service, Lincoln, IL 4.South Dakota School of Mines and Technology, Rapid City, SD 5.Colorado Climate Center, Fort Collins, CO 6.New Mexico Institute of Mining and Technology, Socorro, NM 7.FMA Research, Inc., Fort Collins, CO 8.National Severe Storms Laboratory, Norman, OK

EUROSPRITE Danish National Space Center, Juliane Maries Vej 30, Copenhagen 2100, Denmark 2.Oersted-DTU, Danish Technical University, Kgs. Lyngby, Denmark 3.'Commissariat Energie Atomique, Bruyeres-le-Chatel, France 4.Department of Physics, University of Crete, Heraklion, Greece 5.Laboratoire d'Aerologie, Universite Paul Sabatier, Toulouse, France 6.Space Telecommunications and Radio Science Laboratory, Stanford University, Stanford, USA 7.Geodetic and Geophysical Research Institute, Sopron, Hungary 8.Space Physics Research Institute, University of Natal, Durban, South Africa

Type of measurements Multiple-Doppler and polarimetric radar network Time-of-arrival VHF lightning mapping system Research aircraft Electric field meters carried on balloons Instruments to detect and classify transient luminous events over thunderstorms

RESULTS from STEPS More than 1200 transient luminous events (TLEs; mostly sprites) Sprite parent +CG flashes, most often within the stratiform precipitation region of larger MCSs Sprites typically accompany only a small percentage of +CG flashes Supercells rarely produce sprites, except during their dissipating stage, as stratiform debris cloud develops M=600 C-km – 10% probability of sprite occurrence M>600 C-km – 90% probability of sprite occurrence

STEPS STEPS Charge reservoir for SP+CGs would be found within the lower portions of the MCS stratiform region Large charge moment Mq values appear to be necessary, though perhaps not sufficient condition for sprite generation.

EUROSPRITE 2003 Similar results to STEPS + Infrasound from sprite – first clear identification Sprites generated by intra-cloud lightning - first detections No signatures of relativistic electrons were identified

Sprite Parent Thunderstorms In Summer Stratiform regions of mid latitude Mesoscale convective systems (MCSs) -150 km 2 Mature and dissipating organized convection Not found over ordinary isolated thunderclouds In Winter Over Sea of Japan

Sprite Parent Lightning Mainly positive CG lightning spider lightning Charge moment greater than 500 C-km Total charge transfer > 100 C

Physical mechanisms of sprite generation Wilson, the electrostatic field change of the lightning flash is sufficient to exceed the dielectric strength of the mesosphere and initiate the sprite – electrical breakdown The critical lightning source property is the vertical charge moment — the product of total charge transfer and the height above ground from which the charge is removed.

Electron runway breakdown versus conventional electrical breakdown

Why +CG lightning in stratiform regions of MCS generates sprites? +CG - larger lightning charge moment +CG - longer duration of the parent lightning currents -CG frequently exhibit multiple strokes, each with current cutoff and no continuing current, whereas +CG flashes frequently show single-stroke behavior with a continuing current

Polarity asymmetry Mobility contrast between electrons and positive ions Threshold for propagation of “+” end < for “-” end Mobile electrons are convergent on one end (the ‘easy’ direction) and divergent at the other (‘hard’ direction)

CONCLUSIONS Sprites are thought to be generated by the electric field pulse that travels upward toward the ionosphere predominantly from a positive cloud- to-ground (+CG) stroke of lightning from stratiform regions of MCS - often by spider lightning

Outstanding questions Conventional electrical breakdown or runway electrons? Why mainly positive lightning in stratiform regions of MCSs generates sprites? Do sprites and jets affect the atmosphere - altering greenhouse gas concentrations in the stratosphere and mesosphere or modulating the atmospheric electric circuit.

EUROSPRITE CAMPAIGN 2006

References Barrington-Leigh, C.P., U.S. Inan, M. Stanley, and S.A. Cummer, Sprites directly triggered by negative lightning discharges, Geophys. Res. Lett., 26, , Boccippio, D., Williams, E., Heckman, S., Lyons, W., Baker, I., Boldi, R., Sprites, ELF transients, and positive ground strokes. Science 269, Cummer S., 2003: Current moment in sprite-producing lightning, J Atm.spheric and Solar-Terrestrial Physics 65,499 – 508 Cummer S and Walter A. Lyons,2004:Lightning charge moment changes in U.S. High Plains thunderstorms, GRL, v. 31, L05114, doi: /2003GL Cummer s. and al., 2005: Characteristics of Sprite-Producing Positive Cloud-to-Ground Lightning during the 19 July 2000 STEPS Mesoscale Convective Systems, GRL, VOL. 32, L08811, doi: /2005GL022778, 2005

Gerken A. and U.Inan, 2004: Comparison of photometric measurements and charge moment estimations in two sprite- producing storms, GRL, V. 31, L03107, doi: /2003GL Lyons et al., 2003: Characteristics of Sprite-Producing Positive Cloud-to-Ground Lightning during the19 July 2000 STEPS Mesoscale Convective Systems, Mon Wea Rev, v.131. p Wilson, C.T.R, The electric field of a thundercloud and some of its effects, Proc. Roy. Soc. London, 37, 32D, Williams, E., E. Downes, R. Boldi, W. Lyons and S. Heckman, The polarity asymmetry of sprite-producing lightning: A paradox, Preprint volume, 9-11, Conference on Atmospheric Electricity, Brazilian Society for Electrical Protection, Belo Horizonte, Brazil, November 7-11, 2004.