Chapter 1 Introduction




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Chapter 1




Introduction



Because it is relatively inaccessible, the ionospheric D region (<95 km) is one of the least studied regions of the earth’s atmosphere. The altitudes (~70-95 km) of this region are far too high for balloons to enter and too low for most satellites to be positioned, causing in situ measurements to be sporadic and the continuously monitoring of the ionospheric D region to be difficult. The fact that very low frequency (VLF, 3-30 kHz) waves are most completely reflected by the D region makes them a useful tool for measuring electron density in this altitude range. Lightning discharges radiate most of their electromagnetic energy in the very low frequency and extremely low frequency (ELF, 3-3000 Hz) bands [Uman, 1987, p. 119] and are, consequently, an effective tool for remotely sensing the ionospheric D region electron density [Cummer et al., 1998].

This dissertation focuses on detecting and measuring the ionospheric D region variations and perturbations, caused by extern sources such as high-energy precipitating electrons and intense lightning discharges, using broadband VLF radio emissions from lightning.

1.1 VLF Radio Atmospherics and Waveguide Mode Theory



Radio atmospherics, which are commonly called ‘sferics’ in short, are the electromagnetic signals that are launched into the Earth-ionosphere waveguide by individual lightning discharges [Budden, 1961; pp. 5,69]. Lightning radiates electromagnetic energy from a few Hz [Fukunishi et al., 1997] up to many tens of MHz [Weidman and Krider, 1986]. However, the bulk of the energy of a sferic lies in the ELF and VLF bands. Since the VLF sferics that reflect between the earth and ionosphere have low attenuation over long distance (~2-3 dB/1000 km [Jones, 1967]), they can be observed literally around the world from a single source lightning discharge.

Some of the early research work about VLF sferics has focused on understanding the propagation characteristics of the long-delayed sferic components that form the so-called ‘tweek’ within the Earth-ionosphere waveguide [Yamashita 1978; Hayakawa et al., 1994; Sukhorukov, 1996]. More efforts of VLF sferic research have been put into the work about lightning localization for either single site or multiple sites [Horner, 1954; Rafalsky et al., 1995; Dowden et al., 2002; Price et al., 2002; Wood and Inan, 2004].

Using the broadband VLF sferic from lightning, Cummer et al. [1998] developed a technique to remotely sense the nighttime ionospheric D region electron density profile, and this technique can also be applied on Mars [Cummer and Farrell, 1999]. In this work, we apply this broadband VLF technique to detect and measure the detailed ionospheric D region electron density profiles during different externally driven disturbances.

Among the three primary mathematical formulations for VLF propagation in the Earth-ionosphere waveguide, only Budden’s mode theory [Budden, 1962] makes allowance simultaneously for (a) the gradualness of the lowest part of the ionosphere, i.e. the electron density Ne(h) is a continuous function of the height h; (b) the curvature of the earth; (c) the effect of the earth’s magnetic field, i.e. the anisotropy property of the ionosphere; (d) electron-neutral collision frequency. For the remaining two formulations, one is developed by Galejs [1972] who treated the ionosphere as a sharply stratified medium instead of the real smoothly varying medium. The other is developed by Wait [1970] who neglected the anisotropy property of the ionosphere, which was treated as smoothly varying medium though.

According to Budden’s mode theory, VLF sferic propagating in the Earth-ionosphere waveguide can be represented by a superposition of different waveguide modes after multiple reflections between the earth and ionosphere. For a waveguide mode to exist in any waveguide, the uniform plane wave reflected once each other from the upper and lower boundaries must be in phase with the incident plane wave. For the Earth-ionosphere waveguide, this condition results in the mode equation,

det [I-RL(θ)RU(θ)] = 0, (1.1)

which is known as fundamental equation of waveguide mode theory. And I is the identity matrix, RL is the reflection matrix for lower boundary, i.e. the ground here, RU is the reflection matrix for the upper boundary, i.e. the ionosphere here, and θ is the angle of the plane wave front with respect to the boundary [Budden, 1961, p. 115-117]. The entries of reflection matrix for ground is complex, and both complex and anisotropic for the ionosphere. In general the eigenangle θ is complex so that Real(cos θ) and Imag(cos θ) determine, respectively, the phase velocity and attenuation of the mode.

Based on Budden’s waveguide mode theory, a two-dimensional, single-frequency ELF-VLF propagation model was developed at Naval Research and Development Laboratory [Ferguson et al., 1989, and references therein], which is known as Long Wave Propagation Capability (LWPC) codes. Throughout this work, we use the sferic propagation model described by Cummer et al. [1998], which is based on the LWPC codes. In this model, the transverse horizontal magnetic field By at a distance x along the ground from a vertical electric dipole source transmitting at an angular frequency of ω is [Pappert and Ferguson, 1986]

(1.2).

The wavenumber k = ω/c, Me(ω) is the vertical electric dipole moment of the source (Me(ω)=I(ω)/iω, where I is the source current amplitude and is the length of the current channel), and the term [] accounts for the spreading of the fields over a spherical surface of radius R. The summations include the significant waveguide modes, excluding the strongly attenuated and evanescent modes. Each mode has an index of refraction given by the sine of the corresponding eigenangle θn, solved from the mode equation 1.1. The excitation and receiver factors and contain the altitude dependence of the fields and also depend on the observed field component.

To calculate the fields from a broadband source, one simple needs to calculate over the range of frequencies significant to the problem at hand. With an inverse Fourier transform operation, the time-domain waveform can be reconstructed from the sferic spectrum. However, this reconstruction is not necessary for our measurements, for we evaluate the sferic spectra rather than the waveforms.

1.2 The Ionosphere and D Region Ionospheric Sounding



Dr. Robert Waterson-Watt first referred the term ionosphere to the part of the atmosphere that has sufficient free ions to affect the propagation of radio waves. Thus, the ionosphere can be regarded as lying between ~50 km and approximately a few earth radii [Kenneth, 1965]. Usually the ionosphere is classified into three regions including D, E, and F (includes F1 and F2) (See Figure 1.1). The lowest region is called the D region that extends in height from about 50 km to 90 km. Its electron density is up to about 2.5×103/cm3 by day and decreases to < 103/cm-3 at night [Hargreaves, 1992, p. 81]. The middle region is called the E region. It is the region of the ionosphere between about 90 km and 160 km altitude. The electron density in this region behaves regularly so far as its dependence on the solar zenith angle and the solar activity are concerned. The density can be up to 2×105/cm3 in the daytime and is more than one order of magnitude lower at night. Above the E region is the F region. The F region behavior is fairly irregular and is usually classified into a number of anomalies such as equatorial anomaly and seasonal anomaly. The electron density at the peak has an average value 2×106/cm3 by day and 2×105/cm3 by night. Extreme ultraviolet (EUV) and X-ray lights from the sun are the main sources of the free electrons in the ionosphere during the daytime [McNamara, 1991, p. 17], while non-solar ionizing source including precipitating energetic electrons, meteoric ionization and cosmic rays maintain the free electron concentrations at night [Hargreaves, 1992, p. 231]



Figure 1. 1: The structure of the ionosphere


To obtain D region electron density profiles by rockets, different probing techniques [Holt and Lerfald, 1967; Smith, 1969; Danilov and Vanina, 2001], the exploitation of ionospheric radio wave propagation (differential absorption, Faraday rotation [Jacobsen and Friedrich, 1979; Friedrich and Torkar, 2001; Mechtly et al., 1967] and the coherent frequency technique [Seddon, 1953]) have been applied. Although these in situ measurements are no doubt relatively precise, the rocket techniques can be used only episodically and at a limited number of locales. The ground-based measurements make it possible to monitor the state of the D region ionosphere. Among these techniques, the cross-modulation [Fejer, 1970], partial reflection [Newman and Ferraro, 1976] and incoherent scatter [Mathews et al., 1982] methods have been discussed in the literature but with relatively low accuracy and electron density limits for good SNR echoes [Rapp et al., 2002], causing them difficult to remotely sense the nighttime D region where the electron density is typically < 103 cm-3 [Hargreaves, 1992, p. 81].

Since VLF waves are almost completely reflected by the D region, they contain useful information of D region; and, consequently, they are a useful tool for measuring electron density in this altitude range. Steep and oblique incidence VLF and LF radio wave reflection data have been inverted to derive D region electron density profiles [Deeks, 1966; Thomas and Harrison, 1970]. Single-frequency VLF propagation measurements have been used to estimate the D region electron density parameters along a given propagation path [Bickel et al., 1970; Thomson, 1993]. Phase-coherent narrowband VLF data recorded at multiple sites have also been used to determine the nighttime lower ionospheric electron density profiles [Bainbridge and Inan, 2003].

Cummer et al. [1998] developed a different nighttime D region measurement technique based on wideband, long-distance VLF propagation effects observed in sferics which are the electromagnetic signals launched by individual lightning discharges. This technique is significantly different from most of those mentioned above in that it is not a point measurement; rather, it is sensitive to the average electron density profile across the entire path, and is, therefore, uniquely capable of measuring electron density of large regions. This method has few spatial and temporal limitations. As long as there are lightning strokes at target locations, we are able to monitor the state of the nighttime D region ionosphere over the propagation path between the lighting source and the receiver. This dissertation focuses on applying this broadband VLF technique to detect and measure the ionospheric D region variations and disturbances caused by different external sources, which are discussed in next section.

1.3 Sources of Ionospheric D Region Variations and Disturbances



In general, the ionosphere not only varies with the height, but also might have large-scale variations or localized disturbances caused by extern sources such as solar flares [Burgess and Jones, 1967; Thomason et al., 2005], high-energy particle precipitations [Kikuchi and Evans 1983; Baker et al., 1993a; Cummer et al., 1997], earthquakes [Molchanov and Hayakawa, 1998; Rodger et al., 1999], nuclear explosions [Peterson, 1959; Crook et al., 1963], lightning-induced electron precipitations [Helliwell et al., 1973; Strangeways, 1996] and direct heating and ionization of lightning [Armstrong, 1983; Inan et al., 1993] . In this work, we focus on the ionospheric D region large-scale variations caused by high-energy electron precipitations and the small-scale disturbances caused directly by intense lightning discharges.

1.3.1 Large-Scale Variations Caused by High-Energy Electron Precipitations



High-energy protons, electrons and alpha particles ejected from the surface of the sun can precipitate into the Earth’s atmosphere and cause significant changes in the chemistry of the lower ionosphere [Callis et al., 1991; Baker et al., 1993a]. High-energy (>50 keV, although it depends somewhat on the ambient density of the atmosphere) electrons can penetrate below 90 km, enhancing the D region electron density, and, thereby, decreasing the ionospheric height h’, while lower-energy electrons do not penetrate to these altitudes [Rees, 1989]. Potemra and Rosenberg [1973] found the nighttime ionospheric D region perturbations at middle latitudes occurred simultaneously with the onset of a magnetospheric substorm. Kikuchi and Evans [1983] found that the occurrence of ionospheric D region disturbances is well correlated with an increase in the precipitation of > 300 keV electrons at high latitudes. Cummer et al. [1997] found that precipitating energetic (>100 keV) particles account for the variations of the nighttime ionospheric D region electron densities at the auroral area.

In this work, we examine the correlation between the energetic electron flux over the propagation path and the corresponding nighttime ionospheric height over that region to investigate the source of the nighttime ionospheric D region electron density variations in the middle latitude region.

1.3.2 Small-Scale Disturbances Caused Directly by Intense Lightning Discharges



Strong lightning discharges, with their maximum intensity in the VLF range, can cause direct heating and ionization of the lower ionosphere. The first experimental evidence of the impulsive direct coupling of energy released by lightning discharge to the lower ionosphere was reported by [Armstrong, 1983] in the form of early/fast perturbations of narrowband subionospherically propagating VLF signals, which exhibit rapid changes that begin within 20 ms of the causative lightning; followed by a relatively slow recovery (typically 10-100 s) [Inan et al., 1993]. The physical mechanism underlying these events is still not quantitatively understood and remains somewhat controversial.

Inan et al. [1993] suggested that fast VLF perturbations are produced by ionization changes in the D region over the thunderstorm due to the heating of ionospheric electrons by the electromagnetic impulse (EMP) from lightning (in a manner similar to that which produce fast optical emissions called elves [Fukunishi et al., 1996]) with the disturbance expanding to radial distances of up to 150 km [Inan et al., 1996a]. Taranenko et al., [1993] demonstrated that lightning-EMP could cause significant ionization especially above 85 km using a fully kinetic one-dimensional formulation of the EMP-ionosphere interaction.

On the other hand, fast VLF perturbations are believed to be caused by sustained quasi-electrostatic (QE) fields which quiescently heat the D region of ionosphere and modify the ambient conductivity over large region, with accompanying luminous glows observed as sprites in case of particularly intense discharge [Inan et al. 1996b]. Barrington-Leigh et al. [2001] suggested that fast VLF perturbations might be associated with sprite halos which result from transient intense quasi-electrostatic fields formed above thunderstorms and produce substantial ionization changes over altitude 70-85 km. This was supported by modeling the diffuse region of sprites with a more general fully electromagnetic model and more realistic viewing geometry. Later, Moore et al. [2003] found that at least some of fast VLF perturbations are related to transient QE heating and ionization (the process that creates sprite halos when it is sufficiently intense) by measuring the electron density changes using a full wave electromagnetic propagation model together with narrowband and geographically distributed measurements. Moreover, Moore et al. [2003] indicates that the scattering pattern of the sprite halo disturbance agrees well with that of the VLF perturbations observed by Inan et al. [1996c] and Johnson et al. [1999], which indicates that the lateral extent for transient QE type VLF perturbation should be around 100 km.

However, similar narrowband VLF measurements have also indicated that VLF perturbations might be produced by narrow plasma columns associated with sprites [Dowden et al., 1996]. Although Hobara et al., [2001] reported that small VLF perturbations could be produced by scattering from perturbations produced by lightning-EMP and elves, Roger and McCormick [2004] suggested that they might actually be due to transient QE fields instead of lightning-EMP.

The interpretation of these observations remains somewhat in dispute [Dowden et al., 1996; Inan et al., 1996b], and a consistent picture of the mechanism or mechanisms responsible for these D region perturbations remains elusive. More and repeatable measurements are clearly needed to better understand the nature of lightning-ionosphere coupling.

It has been shown that the D region electron density profiles can be measured reliably using the broadband VLF radiation from lightning discharges [Cummer et al., 1998]. Thus, in this work we apply this technique to measure the detailed height profiles of electron density changes inside D region perturbations and compare them to the theoretical predications to distinguish the source of the perturbations.

1.4 Contributions



The contributions of this dissertation are as follows:

  1. Cheng et al. [2005a], Chapter 2: First measured the mid-latitude nighttime D region electron density variability caused by high-energy electron precipitations, and found the relationship between our measured Ne profiles and the rocket-measured Ne profiles by comparing our measurement results to the results of the past rocket experiments.




  1. Cheng and Cummer [2005b, 2005c], Chapter 3 and 4: First detected and measured the ionospheric D region disturbances caused directly by intense lightning discharges using broadband VLF measurement. A clear broadband VLF perturbation produced by the elve, an optical emission created by strong lightning-EMP, simultaneously detected by the ISUAL instrument on the FORMOST-2 satellite was measured on September 1, 2004 over the United States east coast. This detection, which has been rarely reported, confirmed that we were measuring the ionospheric perturbations caused by lightning-EMP. By analyzing the characteristics of the perturbation-producing lighting strokes, the VLF sferic spectrum average over 3-25 kHz was determined to be the best indicator of the perturbation strength. In addition, by analyzing a set of high peak current lightning strokes that did and did not generate detectable VLF perturbations, we determined that at least some fast VLF perturbations in east coast U.S. were created by fast-discharging lightning-EMP.

  2. Chapter 5: First made the multi-path large-scale ionospheric D region measurement to investigate the potential of our measurement for tomographic reconstruction of the ionosphere parameters, which has not been made by other measurements.



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