2.2 Heating of the lower ionosphere by the lightning electromagnetic pulse: elves

Remarkably, both sprites and elves were predicted theoretically before they were scientifically documented through observation. In the case of elves, which lend themselves to relatively straightforward modeling but are difficult to observe and identify without highly specialized equipment, the theoretical literature is considerably more extensive than the experimental. Many of the studies are similar and provide only incremental advances or parallel model development between different research groups. Nevertheless, the progression of the theoretical literature is briefly outlined below, followed by a discussion of experimental reports. The early idea [Inan et al., 1991] that optical flashes (now called elves) may result from molecular excitation due to a lightning electromagnetic pulse heating and increasing the electron population in the lower ionosphere has not varied among the majority of theoretical studies.

2.2.1 Theoretical studies of elves

Interest in the subject of the immediate effects of lightning electromagnetic pulses (EMPs) on the lower ionosphere arose in recent times in conjunction with VLF radio scattering studies rather than with optical phenomena. The first experimental evidence of detectable heating of ambient $D$ region electrons by VLF waves was realized serendipitously [Inan, 1990], leading to the suggestion that VLF energy produced in lightning may heat the $D$ region and produce optical emissions [Inan et al., 1991]. The heating of the ambient electron population was calculated by Inan et al. [1991] and Rodriguez et al. [1992] in one and two dimensions, respectively, in order to assess whether resultant electron density enhancements could be sufficient to cause subionospheric VLF signal perturbations similar in amplitude to those seen in lightning electron precipitation events. This model assumed a Maxwellian distribution function for electrons, considered only elastic collisions in calculating electron heating, and calculated the effects of EMP-induced heating only in terms of a modified wave attenuation rate as given by the imaginary part of the refractive index. An estimate of ionization enhancements was made based on the final electron temperature, and these were found to be sufficient to affect VLF propagation. The possibility of airglow enhancement was mentioned [Inan et al., 1991] and the shapes of affected ionospheric regions for horizontal and vertical lightning currents were assessed [Rodriguez et al., 1992].

Taranenko et al. [1992] used the same model (in one spatial dimension) as Inan et al. [1991] to predict optical emissions from the EMP-ionosphere interaction. This work was motivated by the observations of Boeck et al. [1992], discussed below. Taranenko et al. [1992] compiled optical line excitation rates for atmospheric species, and suggested that the optical bands ${\rm N}_2(1{\rm P})$ and ${\rm N}_2(2{\rm P})$ in elves could be seen from the ground and should be measured using an instrument with 50 $\mu$s or better resolution. This instrument could be triggered by large lightning sferics.

The calculations published before 1993 did not properly treat the modified shape of the distribution function under an applied electromagnetic wave (or electric field), nor did they propagate the EMP using self-consistent solutions to Maxwell's equations in the presence of a nonlinearly varying conductivity. Taranenko et al. [1993b]; Taranenko et al. [1993a] developed an essentially modern description of the interaction of lightning EMP with the lower ionosphere, including its optical emissions, though in only one spatial dimension. A simplified Boltzmann equation was solved, including inelastic collisions, for an initially thermal (Maxwellian) weakly ionized plasma in a constant electric field. It was found that the electron distribution function attained a steady-state shape in $\sim$10 $\mu$s. Because an EMP electric field at 10 kHz varies slowly compared to 10 $\mu$s, ``equilibrium'' values of relevant physical parameters (current density, ionization and attachment rates, and optical excitation rates) from this calculation could be tabulated for various altitudes and electric field magnitudes for use in the simulation. The lightning EMP was propagated using Maxwell's equations and self-consistent (i.e., modified throughout the calculation) conductivity, allowing reflection and thus constructive interference of the wave electric field. Results from this model differed quantitatively from past work, and also showed the effects of two-body attachment, which in some regions lead to a decrease in the electron density below its ambient value (see Section 2.5.1). Taranenko et al. [1993a] also calculated (in one dimension) the cumulative ionization for multiple lightning strokes, a concept which was not addressed again until results described in this dissertation (Section 2.5.3).

Rowland et al. [1995] presented a similar model in two spatial dimensions to calculate ionization changes but using experimental swarm data rather than the results of a solved Boltzmann equation. Optical emissions were not considered and attachment processes were neglected, resulting in an overestimate of ionization changes in the lower ionosphere.

The next, and arguably the latest, major advance in modeling elves came from Inan et al. [1996c] who investigated the dynamics of the ionospheric optical emissions due to a vertical lightning current using a two-dimensional code and correctly predicted the rapid lateral expansion of the optical luminosity at 80 to 90 km altitudes. This study used Maxwell's equations self-consistently to propagate the EMP and, like Rowland et al. [1995], used experimental data for conductivity, ionization, and attachment rates. The results were also used to interpret the first ground-based optical detection of elves by Fukunishi et al. [1996b]. Inan et al. [1996c] and all subsequent works refer to the optical emissions from lightning EMP heating of the lower ionosphere as ``elves.''^2.4 This work was flawed by a limitation in the radial extent of the model, and thus of the resultant emissions. Nevertheless, its primary conclusions and the underlying physical process of elves as described therein have not changed since this publication, which inspired the development of the Fly's Eye (Section 3.4).

Rowland et al. [1996] used the Rowland et al. [1995] model to investigate elves due to horizontal and vertical lightning. They also noted the effect on elves of neutral density fluctuations due to atmospheric gravity waves.

Glukhov and Inan [1996] used a (Monte Carlo) particle code to assess the validity of the quasi-stationary distribution function used by Taranenko et al. [1993b]; Taranenko et al. [1993a] and the quasi-stationary assumption of those employing experimental swarm data. They confirmed that the electron distribution function relaxes rapidly enough in response to electric field changes to justify the assumptions in models using lightning current input with spectral content up to 50 kHz.

Sukhorukov et al. [1996] presented a similar Monte Carlo particle model, and similar conclusions. Fernsler and Rowland [1996] extended the Rowland et al. [1995] model to include the full Maxwell's equations and thus the effects of the static electric field from model lightning currents. Inan et al. [1997], an experimental paper, was the first to show modeled optical emissions as they would be seen from the ground. Model video and photometric array signatures were given and compared with observations. Taranenko et al. [1997] and Roussel-Dupre et al. [1998] both suggested that elves may be caused by sprites; in particular, Roussel-Dupre et al. [1998] suggested that a relativistic runaway mechanism may cause sprites, and the resultant forward-focused VLF radiation may cause elves. More recent calculations indicate that, on the contrary, radiation electric fields from the relativistic runaway process are small as compared with the thundercloud quasielectrostatic field [Nikolai Lehtinen, private communication, 2000]. Rowland [1998] reviewed the theoretical literature on elves to date.

Cho and Rycroft [1998] gave a two-dimensional fully electromagnetic model, like Fernsler and Rowland [1996], but included optical emissions. They presented extensive plots of brightness and power profiles. Veronis et al. [1999] provided a fully electromagnetic version of the model used by Inan et al. [1996c], including a larger grid extent and simulated views of optical emissions from the ground.

2.2.2 Experimental optical studies of elves

The first photometric recordings of elves may have occurred well before they were recognized or adequately discriminated. A number of studies have sought evidence of broad nighttime atmospheric light pulses caused by fluorescence from supernovae gamma-rays [e.g., Ogelman, 1973; Charman and Jelley, 1972] or other particle precipitation [Li et al., 1991] and found more than one class of anomalous flash, some of which remain unexplained. Nemzek and Winckler [1989] reviewed the studies to date and explained those flashes of $\sim$1 ms duration as scattered light from lightning, based on their association with low frequency sferics in a new study. Winckler et al. [1993] further described the equipment used in this study and discussed the possibility that upper-atmospheric discharges may contribute to anomalous optical events.

The first unambiguous recording of elves is shown in Figure 2.3. Boeck et al. [1992] described this phenomenon as an ``airglow brightening'' and suggested the mechanism of Inan et al. [1991] as an explanation. This event was, however, rather unique among the Space Shuttle data.

Figure 2.3: The first observed elve. Video image of ``airglow brightening'' taken from the Space Shuttle and reported by Boeck et al. [1992].

In 1996 Fukunishi et al., using just three photometers, reported the first ground-based evidence of elves which convincingly differentiated their flash from that of a Rayleigh-scattered lightning flash or that due to sprites. The authors proposed the name of ``elves'' for the flash, and this name has since become standard. Figure 3 of Fukunishi et al. [1996b] shows an event which would now likely be recognized as the halo region of a sprite, rather than an elve [Barrington-Leigh et al., 2000]. Fukunishi et al. [1996b] reported that elves occur in response almost exclusively to large positive cloud-to-ground lightning discharges, while it is now known [Barrington-Leigh and Inan, 1999] that elves are produced by both positive and negative discharges. Fukunishi et al. [1996b] interpreted their data in terms of possible downward motion of the optical emissions while Inan et al. [1996c] showed that this apparent downward motion would result from the predicted rapid lateral expansion of the optical luminosity.

The definitive photometric determination of the rapid lateral expansion was realized using an instrument especially designed for this purpose, namely the photometric array referred to as the Fly's Eye. Measurements with the Fly's Eye constitute the bulk of this dissertation, the first results of which were reported by Inan et al. [1997].

Fukunishi et al. [1997] used, but did not show, data from a vertically-arrayed sixteen-anode photomultiplier tube as well as a low-light-level CCD camera and a framing/streak camera with exposures at 0.5 or 1 ms intervals. These optical measurements were used to identify and discriminate between elves and sprites for a study of ultra-low frequency magnetic field transients. It was concluded that elves are associated almost exclusively with clustered sprites with bead-like fine structure and never with ``carrot-like'' sprites, and that ULF signatures differed for these two classes of event.

Armstrong et al. [1998a] used photometers with two blue filters having different degrees of response to emissions from neutral and ionized N$_2$, in an attempt to differentiate between elves, sprites, and lightning based on the ratio of photometer responses. No explanation is given for how elves were identified, but for the ``very few'' cases observed, their data suggest little ionization in comparison with sprites.

[Barrington-Leigh and Inan, 1999] described explicit criteria for the identification of elves using the Fly's Eye and studied 39 elves from one storm to determine their relationship to causative lightning. The spatial extent of each optical event was assessed and possible effects of large thunderstorm systems on $D$ region ionization were discussed. This work is detailed in Sections 4.2 and 4.4.

Watanabe [1999], in his Master's thesis, presented data from studies using the optical equipment mentioned by Fukunishi et al. [1997]; these results had previously only appeared at conferences. The analysis concerning elves consists largely of the conclusion that ``column-sprites'' are always preceded by elves.