Lightning Generated Gamma Ray Bursts

The prime focus of this effort is to advance the state of understanding of correlation between lightning strokes and gamma-ray flashes. key issue addressed was the revision of the existing models of runaway breakdown in the stratosphere due to low altitude lightning, which are related to the source of gamma-ray flashes. The revision includes the assessment of the effect due to geomagnetic field on the development of runaway discharge

Low frequency waves in HF heating of the mid-latitude ionosphere

The heating of the ionsosphere by high frequency (HF) radio waves leads to plasma processes with a wide range of scales. In the high-latitude ionosphere, extensive studies using numerical simulations using a Hall-magnetohydrodynamic model and experiments with the HAARP facility have provided a comprehensive understanding of the generation of low frequency hydromagnetic waves, both in the presence and absence of the auroral electrojet. Modulated HF heating in the F-region produces a local modulation of the electron temperature and the resulting pressure gradient give rise to a diamagnetic current, which in turn excites magnetosonic waves that propagate away from the heating region. In the E-region, where the Hall conductivity is dominant, these waves lead to oscillating Hall currents that produce shear Alfvén waves. These waves propagate along the field lines to the ground, where they are detected by ground-based magnetometers and into the magnetosphere. The observations of the shear Alfven waves by DEMETER satellite when its trajectory is over the HAARP magnetic zenith have shown the wave propagation to higher altitudes. For the mid latitude ionosphere the simulations use the Earth’s dipole magnetic field and the heating region is located at L = 1.6 and altitude of 300 km. With HF waves modulated at 2 – 10 Hz the low frequency waves are generated by essentially the same processes as in the high-latitude case, with additional features arising from the magnetic geometry of the mid-latitude ionosphere. The shear Alfven waves propagating to the magnetosphere become electromagnetic ion cyclotron (EMIC) waves at higher altitudes but do not propagate beyond the ion cyclotron resonance layer. The heating of the ionosphere generates many plasma modes, viz., the Alfven, magnetosonic, helicon, whistler and electromagnetic ion cyclotron waves, and comparison of their properties with measurements during experiments (Arecibo and Sura) will be presented

Ionospheric Non-linear Effects Observed During Very-Long-Distance HF Propagation

A new super-long-range wave propagation technique was implemented at different High Frequency (HF) heating facilities. The HF waves radiated by a powerful heater were scattered into the ionospheric waveguide by the stimulated field aligned striations. This waveguide was formed in a valley region between the E- and F- layers of the ionosphere. The wave trapping and channeling provide super-long-range propagation of HF heater signals detected at the Ukrainian Antarctic Academik Vernadsky Station (UAS) which is many thousand kilometers away from the corresponding HF heating facility. This paper aims to study the excitation of the ionospheric waveguide due to the scattering of the HF heating wave by artificial field aligned irregularities. In addition, the probing of stimulated ionospheric irregularities can be obtained from analyses of the signals received at far distance from the HF heater. The paper uses a novel method of scattering of the HF radiation by the heating facility for diagnostics of non-linear effects at the super-long radio paths. Experiments were conducted at three different powerful HF facilities: EISCAT (Norway), HAARP (Alaska), and Arecibo (Puerto Rico) and by using different far spaced receiving sites. The key problems for super-long-range propagation regime is the feeding of ionospheric waveguide. Then the energy needs to exit from the waveguide at a specific location to be detected by the surface-based receiver. During our studies the waveguide feeding was provided by the scattering of HF waves by the artificial ionospheric turbulence (AIT) above the HF heater. An interesting opportunity for the channeling of the HF signals occurs due to the aspect scattering of radio waves by field aligned irregularities (FAI), when the scattering vector is parallel to the Earth surface. Such FAIs geometry takes place over the Arecibo facility. Here FAI are oriented along the geomagnetic field line inclined by 43 degrees. Since the Arecibo HF beam is vertical, the aspect scattered waves will be oriented almost horizontally toward the South. Such geometry provides unique opportunity to channel the radio wave energy into the ionospheric waveguide and excites the whispering gallery modes

Vlasov simulations of ionospheric heating near upper hybrid resonance

It is well-known that high-frequency (HF) heating of the ionosphere can excite field- aligned density striations (FAS) in the ionospheric plasma. Furthermore, in the neighborhood of various resonances, the pump wave can undergo parametric instabilities to produce a variety of electrostatic and electromagnetic waves. We have used a Vlasov simulation with 1-spatial dimension, 2-velocity dimensions, and 2-components of fields, to study the effects of ionospheric heating when the pump frequency is in the vicinity of the upper hybrid resonance, employing parameters currently available at ionospheric heaters such as HAARP. We have found that by seeding the plasma with a FAS of width ~20% of the simulation domain, ~10% depletion, and by applying a spatially uniform HF dipole pump electric field, the pump wave gives rise to a broad spectrum of density fluctuations as well as to upper hybrid and lower hybrid oscillating electric fields. We also observe collisionless bulk-heating of the electrons that varies non-linearly with the amplitude of the pump field

Geometric dependence of electric field swelling in simulation of HF ionospheric heating

The interaction between a high frequency (HF) ordinary mode electromagnetic wave and the ionosphere induces electrostatic turbulence near the critical layer which results in the acceleration of electrons and ionization of the neutral gas by energetic electrons. Due to the artificial plasma created by this process, the reflection point of the electromagnetic wave is shifted downwards, leading to descending artificial ionospheric layers (DAILs). This work studies the dependence of DAIL formation on the injection angle of the HF wave and on the related ionospheric conditions. The model is based on a combination of ray-tracing techniques and numerical solutions of the Försterling equations. A model based on the Försterling equations has been developed to calculate the enhancement (swelling) of the electric field near the reflection point. As the swelling exceeds a certain threshold, it excites Langmuir turbulence, which in turn accelerates electrons to high energies, resulting in DAIL formation. Previous full-wave simulations of ionospheric turbulence have been able to capture some of the 2D nature of ionospheric heating but at great computational cost. This works presents an approach to performing rapid calculations of the electric field swelling of the ordinary mode, in order to facilitate a more computationally efficient 2D study of DAIL formation. Results show maximum swelling of the electric field near the magnetic zenith, with an amplitude on the order of several tens of volts per meter for a pump voltage of 1-2 V/m, which is in agreement with previous computational models as well as experiment. Preliminary work to incorporate a model for Langmuir turbulence induced by electric field swelling into the overall algorithm is also presented

Numerical modeling of artificial ionospheric layers driven by high-power HF heating

We present a multi-scale dynamic model for the creation and propagation of artificial plasma layers in the ionosphere observed during high-power high-frequency (HF) heating experiments at HAARP. Ordinary (O) mode electromagnetic (EM) waves excite parametric instabilities and strong Langmuir turbulence (SLT) near the reflection point. The coupling between high-frequency electromagnetic and Langmuir waves and low-frequency ion acoustic waves is numerically simulated using a generalized Zakharov equation. The acceleration of plasma electrons is described by a Fokker-Planck model with an effective diffusion coefficient constructed using the simulated Langmuir wave spectrum. The propagation of the accelerated electrons through the non-uniform ionosphere is simulated by a kinetic model accounting for elastic and inelastic collisions with neutrals. The resulting ionization of neutral gas increases the plasma density below the acceleration region, so that the pump wave is reflected at a lower altitude. This leads to a new turbulent layer at the lower altitude, resulting in a descending artificial ionized layer (DAIL), that moves from near 230 km to about 150 km. At the terminal altitude, ionization, recombination, and ambipolar diffusion reach equilibrium, so the descent stops. The modeling results reproduce artificial ionospheric layers produced for similar sets of parameters during the high-power HF experiments at HAARP

Attenuation of whistler waves through conversion to lower hybrid waves in the low-altitude ionosphere

VLF waves excited by powerful ground-based transmitters propagate in the Earth-ionosphere waveguide and leak through the ionosphere to the magnetosphere, where they are often recorded by satellites. Simulations of the propagation of whistler waves using coupled transionospheric VLF propagation and three-dimensional ray-tracing models have shown systematic overestimates of the VLF wavefield strength near 20 kHz in the magnetosphere by about 20 dB in the night and 10 dB during the day. The paper presents numerical simulations of the conversion between whistler and lower hybrid waves interactions in the presence of short-scale field-aligned density irregularities (striations) in Earth's lower ionosphere. The simulations, which incorporate a realistic ionospheric density profile, show that the mode conversion of whistler waves to lower hybrid waves leads to significant attenuation of whistler waves at altitudes between 90 and 150 km. The striation width plays an important role in the conversion efficiency between whistler and lower hybrid wave. Uniformly distributed striations with 8 m transverse size result in 15 dB attenuation in the 90–150 km propagation range, while a spectrum from 2 to 10 m results in 9 dB attenuation. It is argued that the attenuation of whistler waves in the presence of short-scale density striations in Earth's ionosphere can account for most of the observed ∼20 dB loss in VLF intensity. Furthermore, it predicts that VLF/ELF waves with frequencies below 5 kHz will not suffer similar attenuation

HF-Induced Modifications of the Electron Density Profile in the Earth’s Ionosphere Using the Pump Frequencies near the Fourth Electron Gyroharmonic

We discuss results on plasma density profile modifications in the F-region ionosphere that are caused by HF heating with the frequency f0 in the range [(−150 kHz)–(+75 kHz)] around the fourth electron gyroharmonic 4fc. The experiments were conducted at the HAARP facility in June 2014. A multi-frequency Doppler sounder (MDS), which measures the phase and amplitude of reflected sounding radio waves, complemented by the observations of the stimulated electromagnetic emission (SEE) were used for the diagnostics of the plasma perturbations. We detected noticeable plasma expulsion from the reflection region of the pumping wave and from the upper hybrid region, where the expulsion from the latter was strongly suppressed for f0 ≈ 4fc. The plasma expulsion from the upper hybrid region was accompanied by the sounding wave’s anomalous absorption (AA) slower development for f0 ≈ 4fc. Furthermore, slower development and weaker expulsion were detected for the height region between the pump wave reflection and upper hybrid altitudes. The combined MDS and SEE allowed for establishing an interconnection between different manifestations of the HF-induced ionospheric turbulence and determining the altitude of the most effective pump wave energy input to ionospheric plasma by using the dependence on the offset between f0 and 4fc