AE52A-01 INVITED 10:20h
Successes and Problems of Conventional Breakdown Theory of Sprites
Sprite phenomenon is one of the most frequently observed forms of transient luminous events occurring at mesospheric/lower ionospheric altitudes, which is directly related to the lightning activity in underlying thunderstorms [Sentman et al., GRL, 22, 1205, 1995]. In this talk we will provide overview of conventional breakdown theory of sprites, which is build on original ideas advanced by C.T.R. Wilson [Wilson, Proc. Phys. Soc. Lond., 37, 32D, 1925]. We will discuss similarity properties of electrical discharges as a function of gas pressure and a selected set of results of recent laboratory studies of filamentary channels of ionization (termed streamers) [e.g., van Veldhuizen et al., IEEE Trans. Plasma Sci., 30, 162, 2002; Yi and Williams, J. Phys. D. Appl. Phys., 35, 205, 2002], which are directly applicable for understanding of high spatial resolution imagery of sprites revealing many internal filamentary features with transverse spatial scales ranging from tens to a few hundreds of meters [Gerken and Inan, JASTP, 65, 567, 2003]. The specific set of features, which can be successfully explained by existing conventional theory of sprites and which we will discuss in this talk, include: (1) sprite halos [Barrington-Leigh et al., 106, 1741, 2001]; (2) the observed diffuse and streamer regions of sprites [Pasko and Stenbaek-Nielsen, GRL, 29, 1440, doi:10.1029/2001GL014241, 2002]; (3) the observed ELF radiation from sprites [Cummer et al., GRL, 25, 1281, 1998; Pasko et al., GRL, 25, 3493, 1998]; (4) the observed spatial transverse scales of streamers in sprites; (5) the observed high-speed vertical development of sprites; and (6) the detections of short bursts of blue emissions from sprites (see [Liu and Pasko, JGR, 109, A04301, doi:10.1029/2003JA010064, 2004] for experimental references and recent modeling results pertaining to items (4), (5) and (6)). The talk will be concluded with a discussion of a set of unsolved problems in exiting sprite theory, which include: (1) the initiation of sprite streamers in low applied electric fields; (2) the minimum fields required for propagation of sprite streamers; (3) the branching mechanisms of sprite streamers; (4) the neutral gas heating in sprites; (5) the thermal runaway electrons associated with sprite streamers.
http://www.bath.ac.uk/~eesmf/SUMMER/MS/pasko-lecture.pdf
AE52A-02 INVITED 10:40h
A Review of Runaway Air Breakdown and its Role in High-Altitude Discharges
The fundamental physical processes inherent to the relativistic breakdown mechanism known as runaway breakdown are reviewed. The kinetic theory is described and the most recent calculations for the electron distribution function and the associated avalanche rates are presented. Given the kinetic results it is possible to develop a set of fluid equations which when coupled to Maxwell's equations governs the evolution of a discharge driven by external electric fields such as the quasi-electrostatic fields generated in a high-altitude discharge. Simple analytic expressions that describe the basic spatial and temporal scales characteristic of a high-altitude runaway discharge are presented along with the results of detailed 2-D simulations. Precisely how runaway breakdown may manifest itself in the class of discharges known as sprites is noted.
AE52A-03 INVITED 11:00h
Observations of Sprites Using High Spatial and Temporal Resolution Instruments
In the 15 years since the accidental discovery of optical flashes above thunderstorms [{\it Franz et al,} 1990], now commonly termed sprites, the wide variety of spatial and temporal characteristics exhibited by these phenomena has challenged modeling efforts. The highly dynamic and variable structures observed in sprites can be classified either in terms of basic geometric descriptions (``cylinders," ``tendrils," ``branches," etc.) or by their apparent relation to features previously observed in laboratory gas discharges (i.e. ``streamers" and ``diffuse glow"). High-resolution imagery shows that the lower portion of a sprite typically consists of downward branching filaments, or positive streamers, while the middle to upper portions are made up of upward branching filaments, or negative streamers. The topmost part of a sprite is generally amorphous, or diffuse glow. Filamentary structures (streamers) have been observed to range in width from $\sim$20 - 200 m with the radius having some altitude dependence. Much of the observed streamer and diffuse glow structure can be explained in terms of conventional electrical gas discharge physics. Some features such as beads and low-altitude diffuse glows, however, are currently unexplained by any sprite model. This presentation describes the spatio-temporal characteristics of several sprites with various features as observed by 1) a high-spatial-resolution telescope, previously described in {\it Gerken et al.} [2000], with spatial resolution down to tens of meters, and $\sim$17 ms temporal resolution; and 2) similar events observed by a high-temporal-resolution ``high speed imager" with 1 ms temporal and approximately 300 m spatial resolution, previously described in {\it Stenbaek-Nielsen et al.} [2000]. Franz. R. C., R. J. Nemzek, and J. R. Winckler, Television image of a large upward electrical discharge above a thunderstorm, {\it Science, 249,} 48, 1990. Gerken, E. A., U. S. Inan, and C. P. Barrington-Leigh, Telescopic imaging of sprites, {\it Geophys. Res. Lett., 27,} 2637, 2000. Stenbaek-Nielsen, H. C., D. R. Moudry, E. M. Wescott, D. D. Sentman, and F. T. Sao Sabbas, Sprites and possible mesospheric effects, {\it Geophys. Res. Lett., 27,} 3829, 2000.
AE52A-04 11:20h
Lightning-Driven Electric Fields in the Stratosphere: Comparisons Between In-Situ Measurements and a Quasi-Electrostatic Field Model
During the Sprite Balloon Campaign in southeastern Brazil (December 6-7, 2002), 38 electric field changes greater than $10$~V/m, correlated with cloud-to-ground and intra-cloud lightning, were measured above $30$~km in altitude in the stratosphere. These electric field signatures are compared directly to an axi-symmetric quasi-electrostatic field (QSF) model developed by the authors and based on the model of Pasko et al. '97. Both the amplitude and the relaxation time of the lightning-driven electric fields are compared to this QSF model at the exact location of the in-situ balloon-borne measurements. The Brazilian Integrated Network (BIN), a regional ground-based lightning detection network that covers the southeast of Brazil, provides lightning location and peak current values for cloud-to-ground strokes, while remote extremely low frequency (ELF) magnetic field measurements from Japan and Antarctica provide charge moment estimates. These network and remote measurements, along with in-situ measurements of the conductivity by the Sprite flight 1 balloon payload, provide realistic input parameters to this QSF model. The in-situ electric field measurements are compared to the output of the QSF model for various cloud charge distributions and atmospheric conductivity profiles. When the electric field measurements agree well with the QSF model, the model can predict the electric field signature everywhere in the stratosphere and mesosphere, including sprite initiation altitudes. Thus, by utilizing in-situ electric field and conductivity measurements, local lightning network data, remote ELF measurements, and a QSF model of the lightning discharge, we are able to predict the amplitude and relaxation time of the electric field at sprite altitudes.
http://www.ess.washington.edu/students/jnt/
AE52A-05 11:35h
Testing Sprite Initiation Theories Using Lightning Measurements and Modeled EM Fields
Previous research has shown that statistical measurements of charge moment changes in sprite-producing lightning are in general agreement with predictions based on conventional breakdown in the mesosphere. Measurements have progressed to the point where a detailed, event-level quantitative comparison between measurements and predictions could more rigorously test existing theories by estimating the electric field above the thunderstorm clouds responsible for sprite initiation. We selected for this analysis a set of sprite events from the summers of 2000 and 2004 whose initiation time is well-bounded from experimental data. Then we measured the current moments and charge moment changes from the recorded electromagnetic fields radiated by the causative lightning strokes. The measured current moments are then used as the input of our 2-D cylindrical full wave finite difference model for simulating lightning-generated EM fields. We compare the simulated mesospheric electric fields to the theoretical threshold electric field for conventional breakdown to see whether, according to theory, a sprite would be initiated by that electric field and at what altitude. By analyzing sprites that are both short and long delayed from the source lightning stroke, we are able compare measurements and theory across a wide range of time scales.
AE52A-06 11:50h
Optical Emissions of Sprite Streamers in Weak Electric Fields
Sprites commonly consist of large numbers of needle-shaped filaments of ionization [e.g., Gerken and Inan, JASTP, 65, 567, 2003] and typically initiate at altitudes 70-75 km in a form of upward and downward propagating streamers [Stanley et al., GRL, 26, 3201, 1999; Stenbaek-Nielsen et al., GRL, 27, 3829, 2000; McHarg et al., JGR, 107, 1364, 2002; Moudry et al., JASTP, 65, 509, 2003]. The strong electric fields E exceeding the conventional breakdown threshold field Ek are needed for initiation of sprite streamers from single electron avalanches and recent modeling studies indicate that streamers propagating in fields E$>$Ek experience strong acceleration and expansion in good agreement with the above cited observations [Liu and Pasko, JGR, 109, A04301, 2004]. The initiated streamers are capable of propagating in fields substantially lower than Ek [Allen and Ghaffar, J. Phys. D: Appl. Phys., 28, 331, 1995] and it is expected that a significant part of sprite optical output comes from regions with E$<$Ek above and well below the initiation altitude. Principal approaches to remote sensing of electron energy distributions in sprites using absolute intensities and ratios of various emission bands arising from the excited electronic states of neutral and ionized molecular nitrogen have been extensively discussed in the existing literature [e.g., Armstrong et al., GRL, 27, 653, 2000; Takahashi et al., Adv. Space Res., 26, 1205, 2000; Morrill et al., GRL, 29, 100, 2002; Pasko and George, 107, 1458, 2002; Chern et al., JASTP, 65, 647, 2003; Miyasato et al., JASTP, 65, 573, 2003] and understanding of optical emissions produced by streamers propagating in weak electric fields represents an important component of related studies needed for correct interpretation of the existing experimental data. In this talk we will report results on application of time dependent optical emission model developed in [Liu and Pasko, 2004] to studies of sprite streamers in weak electric fields (E$<$Ek). The results indicate, in particular, substantially lower values (typically a factor of two) of the peak electric fields around tips of streamers in weak fields (E$<$Ek) in comparison with those in strong fields (E$>$Ek). Additionally, the values of electric fields inside of the streamer channel are always well below Ek and since the excitation coefficients for optical emissions are very sensitive to the driving electric field magnitude most of the optical luminosity of streamers in this case arises from streamer tips, indicating that observed streamer filaments in many cases may be produced by time averaging of optical luminosity coming from localized regions around streamer tips as streamers move through an instrument's field of view. We will discuss pressure dependent differences of optical emissions at different sprite altitudes, and important similarities between observed sprite streamers and recent time resolved ($<$1 ns) imaging of laboratory streamers in point-to-plane discharge geometry conducted at ground and near ground pressures [van Veldhuizen et al., IEEE Trans. Plasma Sci., 30, 162, 2002; Yi and Williams, J. Phys. D. Appl. Phys., 35, 205, 2002].
AE52A-07 12:05h
Simulation of Spectral Characteristics of Sprites From Runaway and Conventional Breakdown Processes and Comparison to Measurements
The breakdown processes that lead to sprite formation are still being debated. Are sprites formed through conventional breakdown, runaway breakdown, or a combination of both? What are the thunderstorm electrical conditions that produce one or the other or both processes? This study describes the improved two-dimensional fully electromagnetic Unified Maxwell (UNIMAX) model and an updated methodology to derive the associated optical spectra using the Physics-based Optical Emission Model (POEM). These models allow us to study the differences between conventional and runaway breakdown processes and obtain a better understanding of what information can be garnered from sprite spectral measurements. A detailed kinetic computational tool, Plume, has been developed to compute the self-consistent evolution of a seed electron population with energies from 0 to 50 MeV in the presence of a steady-state, spatially-uniform electric field (see Colman et al. abstract submitted to this meeting). This information is used to compute steady-state emission rates as a function of the applied electric field for 13 Nitrogen and 3 Oxygen band systems. These rates can be combined with the number and energy of electrons from the UNIMAX sprite model and quantum transition information from POEM to separate the band system into individual lines and generate simulated spectral images. Cascading between different bands is also included. Spectral measurements of sprites represent a temporal and spatial superposition of emissions. These emissions emanate from a wide variety of electrical and gas discharge conditions that exist along the body of the sprite and which change as a function of time. Interpretation of the spectra to obtain the properties of the gas is made difficult by this inherent averaging process and the fundamental discharge mechanism can be obscured. Thus, we fold in the properties of spectrometers (i.e. spatial and temporal averaging, slit size, atmospheric absorption, etc.) to compare with measurements and assess the effect of temporal and spatial averaging. These spectra can then be used to elucidate what would be theoretically observed separately from runaway and conventional breakdown and to suggest potential measurements that could differentiate between the different processes.