Contexts in source publication

Context 1
... dynamics of the D and E regions are complicated. On top of strong diurnal and seasonal changes, the electron density can change drastically and rapidly as a result of energetic particle precipitation or other magnetospheric activity, particularly at night. The right panel of Figure 1.1 shows more detail on the electron density in the lower portion of the ionosphere, and its variability from day to night, and winter to summer. The IRI typically does not define the electron density in the lowest parts of the D region, since the numbers become so small and variable. Below this altitude, the electron densities are generally extrapolated with an exponential decrease, in line with a two-parameter ionosphere described by Wait and Spites [1964], defined by is the lowest defined electron density in the IRI, and h is km below that altitude. The steepness parameter, β, is taken to be −0.15 km −1 in the daytime, and −0.35km −1 in the nighttime. Some typical values of a two-parameter ionosphere in the D and E regions can be found in Thomson et al. [2007] and Thomson ...
Context 2
... electromagnetic radiation from the sun and cosmic rays are chiefly respon- sible for the formation of the ionosphere, a larger region around the Earth, known as the magnetosphere, is shaped by the interaction of the so-called 'solar wind' with the Earth's magnetic field. The solar wind consists mostly of protons and electrons at 1 keV energy, originating from the sun, traveling at ∼400−500 km/s, and consisting typically of ∼5 protons or electrons per cm 3 [Tascione, 1994, ch.3]. The geomagnetic field of the Earth, however, acts to block this flow and deflect these particles. The re- sult is that Earth's magnetic field lines (which have roughly a magnetic dipole pattern within a few Earth radii of the surface) are squashed in on the day side of the Earth, and elongated into a tail on the night side. Figure 1.2 (available from NASA) shows the basic structure of the Sun-Earth system and formation of the magnetosphere (not to scale). The resulting current systems and dynamics of the magnetosphere driven by the solar wind interaction with the geomagnetic field are in general very compli- cated, and lead to a broad array of phenomena. For instance, some of the solar wind that penetrates into the magnetosphere becomes trapped by the geomagnetic field, forming two bands of energetic particles known as the Van Allen radiation ...
Context 3
... this dissertation, we are interested in one particular aspect through which the magnetosphere system couples with the ionosphere. Above a portion of the high lati- tude ionosphere known as the auroral zone, strong electric fields in the magnetosphere accelerate energetic electrons in the magnetosphere. Since electrical conductivity is much higher along the magnetic field in a magnetized plasma, the resulting current moves primarily along the geomagnetic field line. At high latitudes, the geomagnetic field lines are close to vertical, connecting the ionosphere to the magnetosphere, and driving a current upward into the magnetosphere (or, electrons accelerated downward into the atmosphere). The collision of these 'precipitating' electrons with the neutral atmosphere is primarily responsible for the aurora borealis (in the northern hemi- sphere) and the aurora australis (in the southern hemisphere), pictured in the right part of Figure 1.3, which arise from photons emitted as a result of these ...
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... auroral electrojet is highly variable. Its size and extent is largely a function of solar and geomagnetic activity. During geomagnetic storms, the auroral electrojet The arrows in Figure 1.3 indicate the electric field lines, which are typically pointed roughly in the north-south geomagnetic direction, but there are two diurnal discon- tinuities [Baumjohann, 1983]. In the few hours before midnight, the direction of the electric field changes from northward to southward, and this transition is known as the Harang discontinuity [Baumjohann, 1983]. At midday, the auroral electrojet fields typically are ...
Context 5
... ionospheric reflection tensor R g can be calculated in a manner first outlined by Budden [1955]. Using the complex permittivity at each altitude, the ionosphere can be divided into horizontal layers, with each layer causing some reflection and transmission of the wave. Two initial solutions satisfying in-plasma boundary condi- tions at high altitude are separately integrated downward through each of these layers An example of this calculation is shown in Figure 2.31, at a frequency of 2 kHz, for both the winter and summer nighttime ionospheres defined in Figure 1.1, and a realistic geomagnetic field for the ionosphere above HAARP. The four components R are plotted as a function of the angle of incidence. For a nearly vertically incident wave (θ=0), R and ⊥ R ⊥ merge to the same value, because for vertical incidence, the two polarizations are nearly indistinguishable. At the highest angle of incidence, R and ⊥ R ⊥ both approach unity, since the wave is just barely grazing the ionosphere, so that perfect, isotropic-like reflection takes place. A second consequence of the imperfect reflection from the anisotropic ionosphere is that some amount of ELF/VLF energy leaks through, that is to say the transmission coefficient is ...
Context 6
... most recent facility to begin operation is the High Frequency Active Auro- ral Research Program (HAARP). The HAARP facility was built in three stages, the latter two being completed in 2003 and then 2007. Again demonstrating the strong interest in ELF/VLF wave generation, Milikh et al. [1999] conduct experiments on modulated heating even at the very early construction stage known as the 'develop- mental prototype', which produced only 10 MW of ERP. Milikh et al. [1999] are also the first to utilize two nearby HF heating facilities, with simultaneous detection of ELF/VLF signals from HAARP and HIPAS HF heating. In this dissertation, we use experimental data obtained with the HAARP facility. A picture of the HF antenna array, known as the Ionospheric Research Instrument, is shown in Figure 1.6, as of August 2005. The HAARP facility utilizes 3.6 MW of power, but some power is reflected at the antenna array (due to imperfect impedance matching between the near-field coupled antenna elements), so actual radiated power is slightly less. Ap- pendix B discusses the actual transmitted powers and ERPs for a number of different HAARP beam ...
Context 7
... [2007] also utilized a full theoretical model consisting of the HF heating model similar to the one here, as well as a Finite Difference Time Domain (FDTD) solution to ELF/VLF wave radiation. The radiation pattern on the ground from a planar 'disk of current' is found to be similar to the results of the complete model (See Figure 4.13), indicating that at least for distances within 50 km of HAARP, a simple free-space model fairly accurately reproduces the radiation on the ...
Context 8
... the relatively small electron densities in the D region are sufficiently high for Debye shielding to take place. For instance, the plasma frequency is 3 kHz for an electron density of ∼1.1 × 10 5 m −3 , while electron densities in the D region (above 65 km) are substantially higher, as shown in Figure 1.1. So the plasma frequency is well above the wave frequency at all ionospheric altitudes, for ELF/VLF ...
Context 9
... left hand panel of Figure 1.1 shows the structure of the ionosphere's vertical profile, with electron density (N e ) in logarithmic scale on the horizontal axis, as a function of the altitude above the Earth's surface on the vertical axis. The daytime ionospheric profile is shown with a solid line, the nighttime ionospheric profile in dashed line. The data come from the International Reference Ionosphere (IRI) 2007 model, an empirical model of ionospheric parameters taking into account basic diur- nal, seasonal and geographic variations, for 01-Jan-2009, at the geographic location (0,0), at 00 UT (midnight) and 12 UT (high ...

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ELF/VLF radio waves (300 Hz to 30 kHz) have been successfully generated via modulated HF (3-10 MHz) heating of the lower ionosphere in the presence of natural currents, most recently with the HAARP facility in Alaska. Generation is possible via amplitude modulation or via two techniques involving motion of the HF beam during the ELF/VLF cycle, know...

Citations

... The ON-OFF modulation of the HF wave at ELF/VLF rates, allows HF energy to be converted into ELF/VLF radiation (see Sheerin and Cohen 2015 and references therein). Various configurations of the HF heating have been experimented to determine the best wave generation (Cohen 2009, 2012. To study the induced ionospheric effects many ground-based experiments are associated to HAARP, and in particular, ELF/VLF data are registered with broadband high-sensitivity receivers which consist of two orthogonal air-core loop antennae, measuring the two horizontal components of the magnetic field between 300 Hz and 40 kHz. ...
... W, 704 km SE of HAARP) are used. Their data are available from a web server named WALDO which is presented in Cohen (2020). ...
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Thesis
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... The beam pattern radiated by the HAARP array in different modes.Figure andtable adapted from Cohen[7] ...
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... However, the choice of HF heating parameters is also quite important. For instance, proper utilization of motion of the HF beam can yield 7–11 dB more ELF/VLF power in the Earth-ionosphere waveguide [Cohen et al., 2008b, 2010b], and 5–7 dB more power radiated into the magnetosphere [Cohen et al., 2011]. [8] We consider the effect of HF frequency, beam width, and ERP, on generated ELF/VLF amplitudes, both near the heated region, at longer distances in the Earth-ionosphere waveguide, and in the magnetosphere. ...
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