2.3 Lightning quasielectrostatic fields and mesospheric discharges: sprites

While elves are produced by electromagnetic radiation, the intensity of which is proportional to the time deriative of a lightning current moment, sprites occur in response to the time integral of the current moment, where the integral is taken over a period of approximately the local relaxation time $\ensuremath{\epsilon_{}}/\sigma$.

The theoretical and experimental literature concerning sprites is large and continues to develop significantly. Recent reviews exist of theoretical [Rowland, 1998] and experimental [Heavner, 2000; Rodger, 1999; Boeck et al., 1998] studies. Several subjects of particular interest for this work are briefly addressed below.

Significant topics not treated below include the role of relativistic runaway breakdown in sprites and lightning-magnetosphere coupling [Lehtinen, 2000; Roussel-Dupre et al., 1998; Taranenko and Roussel-Dupre, 1996; Roussel-Dupre and Gurevich, 1996; Lehtinen et al., 1997; Fishman et al., 1994; Lehtinen et al., 1999; Inan et al., 1996b], the discovery and explanation of blue jets [Yukhimuk et al., 1998; Wescott et al., 1998; Pasko et al., 1996a; Wescott et al., 1995; Pasko et al., 1997b], VLF scattering effects of electron temperature and density changes in the ionosphere caused ``directly'' by lightning [Inan et al., 1995; Inan et al., 1996a; Inan et al., 1996d; Inan et al., 1993; Sampath et al., 2000; Dowden et al., 1998; Inan et al., 1988; Dowden et al., 1996; Johnson et al., 1999], and HF radar detection of transient ionization changes in the mesosphere [Roussel-Dupre and Blanc, 1997; Tsunoda et al., 1998].

History and nomenclature

Sprites properly entered the scientific arena in 1989 [Franz et al., 1990] but were not given their modern name until 1994 [Sentman et al., 1995]. Indeed, reports of exotic upward lightning and high-altitude discharges date to more than 100 years ago [see Heavner, 2000; Rodger, 1999, p. 10], and efforts by Vaughan and Vonnegut in the 1980's to consolidate eyewitness accounts revealed that aircraft pilots also had knowledge of such phenomena [e.g., Vaughan and Vonnegut, 1989]. Chalmers [1967, p. 377] includes a section entitled ``Discharges from Cloud to Electrosphere'' which discusses many studies giving possible visual, electrical, and even radar evidence for what might today be called sprites.

The theoretical foundations for the modern theories of sprites were laid by Wilson [1925], who predicted that thundercloud static electric fields would exceed the breakdown threshold of air at some altitude, that electrons could run away to highly relativistic energies above thunderstorms, and that these high altitude discharges would ``doubtless give rise to atmospherics.'' These insights correspond to modern quasielectrostatic field (QE or QSF) conventional breakdown theories [Pasko et al., 1997a; Pasko et al., 1995; Fernsler and Rowland, 1996; Pasko et al., 1998a; Pasko et al., 1996b; Pasko et al., 1997b; Winckler et al., 1996], relativistic runaway theories [Lehtinen, 2000; Lehtinen et al., 1997; Fishman et al., 1994; Lehtinen et al., 1999; Inan et al., 1996b; Roussel-Dupre and Gurevich, 1996], and the calculation and measurement of extremely low frequency (ELF) radiation from sprites [Stanley et al., 2000; Cummer and Stanley, 1999; Farrell and Desch, 1992; Reising et al., 1999; Pasko et al., 1998b; Pasko et al., 1999b; Cummer and Inan, 2000; Cummer et al., 1998b].

Through a number of studies in the 1990's from the ground, aircraft, and Space Shuttle, the term ``sprites'' has come to refer to a mostly cohesive set of observations, largely in relation to optical emissions between $\sim$40 km and $\sim$90 km altitude [Lyons, 1996; Inan et al., 1995; Sentman and Wescott, 1995; Boccippio et al., 1995; Lyons et al., 1998; Winckler, 1995; Lyons, 1994; Boeck et al., 1995; Sentman et al., 1995; Inan et al., 1996b; Sentman and Wescott, 1993; Winckler et al., 1996; Rairden and Mende, 1995]. Nomenclature has continued to play an important role in the understanding of sprites and the experimentally associated phenomena of blue jets (which propagate upward to $<$40 km) and elves. This is both because it serves to identify new phenomenology before it is understood theoretically (``hair,'' ``tendrils'') and because it may be used to group phenomena which are shown to have a common physical cause (sprites versus elves). Classifications such as ``carrot,'' ``column,'' and ``angel'' sprites are often not well-defined [Stanley et al., 1999; Wescott et al., 1998; Sentman et al., 1995] but may foster important insights to come. These terms are not used in the present work. Because of the rich diversity of sprite forms, there has been a tendency to invent new terms to describe phenomena which, if at all possible, ought to be understood and described as features or variations of already-named concepts.

The diffuse region of sprites

Theories of sprites based on the electromagnetic pulse mechanism [Milikh et al., 1995] and runaway electron breakdown [Taranenko and Roussel-Dupre, 1996] have not found much support in optical measurements. Other than these, theoretical studies accounting for the optical, electric, and plasma properties of sprites may be classified into two groups -- those which describe bulk gas breakdown and those which make use of a corona streamer model to account for the fluid behavior of electrons. The former group generally does not produce modeled optical emissions below $\sim$60 to 70 km [Pasko et al., 1997a; Pasko et al., 1995; Fernsler and Rowland, 1996; Pasko et al., 1998a; Pasko et al., 1996b; Pasko et al., 1997b; Winckler et al., 1996]. Pasko et al. [1998a] first placed this region into context and named it the diffuse upper region of sprites. As discussed in Section 5.1, virtually all studies in which elves were identified by means of ground-based video recordings have incorrectly referred to this upper diffuse region of sprites as `elves' and have presumed it to be due to the EMP mechanism, when it actually is manifested by quasielectrostatic fields which may or may not also lead to streamer formation [see Section 5.1 and Barrington-Leigh et al., 2000].

In 1999 the name of ``halos'' was agreed upon [Heavner, 2000; Barrington-Leigh et al., 1999b, p. 31] for the luminosity in this region, even though it had since 1996 been well modeled and understood by the name of ``sprites'' in the theoretical literature.

Corona streamers in sprites

The physical reason for the existence of upper diffuse and lower streamer regions in sprites was described on page  (Section 1.3). Streamers have been discussed and modeled in the context of upper atmospheric discharges by Pasko et al. [1998a]; Pasko et al. [1997b]; Pasko et al. [2000] and Raizer et al. [1998].

Pasko et al. [1998a] predicted a transverse scale for streamers of $\sim$10 m at 70 km altitude, and Raizer et al. [1998] predicted a maximum velocity of $\sim$ ${\ensuremath{1.2\!\times\!10^{7}}}$ m/s. These predictions have been assessed experimentally by Stanley et al. [1999] and Gerken et al. [2000].

ELF determination of current moments

As predicted by Wilson [1925] but not realized again until 1996 [Stanley et al., 2000; Cummer and Inan, 1997], the currents flowing in sprites may radiate as strongly as their associated lightning return strokes. This radiation occurs primarily in the ELF band (30 Hz to 3 kHz) in accordance with the several-to-tens of milliseconds lifetime of sprites.

In addition, ELF currents in cloud-to-ground lightning are thought to be most significant in setting up the large quasielectrostatic fields which initiate sprites [Huang et al., 1999; Williams, 1998; Bell et al., 1998; Cummer and Inan, 1997]. The method for determining source current moments from radiated electromagnetic fields was pioneered by Cummer and Inan [1997] and is described in a refined form by Cummer and Inan [2000]. This method uses Earth-ionosphere waveguide mode theory and can make use of a large number of lightning strokes to determine first the ionospheric propagation parameters [Cummer et al., 1998a]. Determination of lightning electrical input to the upper atmosphere is of prime importance in comparing models and observations of sprite breakdown and development.

Recently, multi-site ULF ($<$30 Hz) measurements were made in conjunction with sprite observations, and currents in sprites and lightning were determined in this frequency range too.^2.5

Spectral studies

The acquisition of optical spectra of sprites was among the prime objectives of early high-altitude ground-based and airborne sprite campaigns. Spectra of sprites were reported generally to be dominated by the neutral band ${\rm N}_2(1{\rm P})$ [Heavner, 2000; Mende et al., 1995; Hampton et al., 1996, p. 100]. Occasional contributions were detected from ionized species [Heavner, 2000, p. 87]. Multispectral photometric studies have concluded that ionized emissions likely occur during a brief pulse during the onset of sprite luminosity [Armstrong et al., 1998a; Armstrong et al., 2000; Suszcynsky et al., 1998] and that the degree of ionization in sprites varies greatly [Heavner, 2000; Armstrong et al., 2000, p. 98].

Spectroscopic studies may be compared with theoretical models [Morrill et al., 1998; Milikh et al., 1997; Pasko et al., 1995; Pasko et al., 1997b; Milikh et al., 1998]. It is found that the average electron energy which dominates observed emissions is near 1 eV [Green et al., 1996].