Paul A. Bernhardt

Preliminary study of the CRRES magnetospheric barium releases

Preliminary theoretical and computational analyses of the Combined Release and Radiation Effects Satellite (CRRES) magnetospheric barium releases are presented. The focus of the studies is on the evolution of the diamagnetic cavity which is formed by the barium ions as they expand outward, and on the structuring of the density and magnetic field during the expansion phase of the releases. Two sets of simulation studies are discussed. The first set is based upon a two-dimensional ideal MHD code and provides estimates of the time and length scales associated with the formation and collapse of the diamagnetic cavity. The second set uses a nonideal MHD code; specifically, the Hall term is included. This additional term is critical to the dynamics of sub-Alfvenic plasma expansions, such as the CRRES barium releases, because it leads to instability of the expanding plasma. We performed detailed simulations of the G4 and G10 releases. In both cases the expanding plasma rapidly structured: the G4 release structured at time t ≲ 3 s and developed scale sizes ∼ 1-2 km, while the G10 release structured at time t ≲ 22 s and developed scale sizes ∼ 10-15 km. We also find that the diamagnetic cavity size is reduced from those obtained from the ideal MHD results because of the structure. On the other hand, the structuring allows the formation of plasma blobs which appear to free stream across the magnetic field; thus, the barium plasma can propagate to larger distances transverse to the magnetic field than the case where no structuring occurs. Finally, we also discovered a new normal mode of the system which may be excited at the leading edge of the expanding barium plasma. This mode is a magnetic drift wave which propagates azimuthally around the barium cloud in the frequency range Ωi ≪ ω ≪ Ωe.

Artificial Airglow Excited by High-Power Radio Waves

High-power electromagnetic waves beamed into the ionosphere from ground-based transmitters illuminate the night sky with enhanced airglow. The recent development of a new intensified, charge coupled-device imager made it possible to record optical emissions during ionospheric heating. Clouds of enhanced airglow are associated with large-scale plasma density cavities that are generated by the heater beam. Trapping and focusing of electromagnetic waves in these cavities produces accelerated electrons that collisionally excite oxygen atoms, which emit light at visible wavelengths. Convection of plasma across magnetic field lines is the primary source for horizontal motion of the cavities and the airglow enhancements. During ionospheric heating experiments, quasi-cyclic formation, convection, dissipation and reappearance of the cavites comprise a major source of long-term variability in plasma densities during ionospheric heating experiments.

The modulation of sporadic-E layers by Kelvin-Helmholtz billows in the neutral atmosphere

Abstract Modulation of electron densities in ion layers between 90 and 150 km altitude has been observed using a number of ionospheric diagnostic measurements including scatter of VHF radar waves, artificially pumped optical emissions, scintillations of satellite beacon transmissions. Kelvin–Helmholtz (K–H) turbulence driven by a sheared wind profile is a strong candidate for the source of these modulations. A two-dimensional numerical model is used to calculate the nonlinear evolution of ion layers in ionosphere near 100 and 120 km altitude in response to neutral turbulence driven by a wind shear. The amplitude of a K–H billow is allowed to grow as a linear perturbation on the neutral atmosphere to a level that is 10% of the wind shear. The time dependent model of the ionosphere responds to neutral wind perturbation initially by imposing a quasi-sinusoidal modulation near the altitude of the ion layer. This is followed by compression of the initially stratified layer into structures with the wavelength of the K–H instability. These structures are uniform strips in the meridian perpendicular to the direction of the zonal wind. Near 120 km , where the ion gyro frequency ( ω i ) is about equal to the ion collision frequency ( ν i ), the equilibrium solutions are clumps at the altitude of the shear. Near 100 km , two stable, rippled layers are produced with a separation of about 1 km . The amplitudes of the density modulations in the ion layers vary by as much as 500% throughout the simulation. The simulations illustrate the complex evolution of the ion layer structures from small-amplitude, K–H wind turbulence.

Lightning driven EMP in the upper atmosphere

Large lightning discharges can drive electromagnetic pulses that cause breakdown of the neutral atmosphere between 80 and 95 km leading to order of magnitude increases in the plasma density. The increase in the plasma density leads to increased reflection and absorption, and limits the pulse strength that propagates higher into the ionosphere.

Two-dimensional mapping of the plasma density in the upper atmosphere with computerized ionospheric tomography (CIT)

Tomographic imaging of the ionosphere is a recently developed technique that uses integrated measurements and computer reconstructions to determine electron densities. The integral of electron density along vertical or oblique paths is obtained with radio transmissions from low-earth-orbiting (LEO) satellite transmitters to a chain of receivers on the earth’s surface. Similar measurements along horizontal paths can be made using transmissions from Global Position System (GPS) navigation satellites to GPS receivers on LEO spacecraft. Also, the intensities of extreme ultraviolet (EUV) emissions can be measured with orbiting spectrometers. These intensities are directly related to the integral of the oxygen ion and electron densities along the instrument line of sight. Two-dimensional maps of the ionospheric plasma are produced by analyzing the combined radio and EUV data using computerized ionospheric tomography (CIT). Difficulties associated with CIT arise from the nonuniqueness of the reconstructions, owi...

Excitation of artificial airglow by high power radio waves from the “SURA” Ionospheric Heating Facility

The SURA facility for generation of high power radio waves, located near the village of Vasilsursk USSR, operates between 4.5 and 9.0 MHz and has a maximum effective radiated power (ERP) of 300 MW. Nonlinear interactions between the HF radio waves and F-layer plasma occur near the electromagnetic wave reflection point. Energetic electrons are accelerated out of the interaction regions by the electrostatic waves. Ambient oxygen atoms collisionally excited by these suprathermal electrons yield enhanced airglow. Low-light-level, optical measurements were made at SURA during September 1990. Images of enhanced red-line (630 nm) emissions were recorded during radio wave transmissions at 4.786, 5.455, and 5.828 MHz. The antenna radiation pattern, ionospheric irregularities, and the magnetic field orientation affected the shape of the observed airglow structures. The airglow clouds drifted across the night sky, disappeared, and reformed at the zenith of the antenna array. This has been interpreted in terms of radio beam refraction in drifting plasma irregularities and bifurcation when the beam is split between two density cavities. Subject to clear skies, the authors experience indicates that the low-light-level-imaging technique is a reliable method to study large scale irregularities and electron acceleration with high-power HF transmitting facilities.

Breakdown of the neutral atmosphere in the D region due to lightning driven electromagnetic pulses

Electromagnetic pulses (EMP) driven by lightning can cause breakdown of the neutral atmosphere in the lower D-region. Using a computer simulation model, we study the dependence of the breakdown on the pulse strength, the orientation of the lightning discharge, the ambient plasma density, the ionization model, and the neutral density. For a discharge along a straight line the EMP is strongest in the plane perpendicular to the current so that for a given current, horizontal discharges will radiate the D-region more strongly than a vertical discharge. For horizontal currents, breakdown occurs for E100 > 20 V/m (I > 55 kA) in a low-density, nighttime ionosphere, where E100 is the amplitude of the pulse normalized to 100 km from the discharge and I is the discharge current. Vertical strokes require E100 > 50 V/m (I > 140 kA). Discharges with higher currents and fields form ionization patches which are larger in volume, larger in degree of ionization, and lower in altitude. The ionization is most sensitive to the pulse strength, pulse orientation, ambient plasma density, and neutral gas density at breakdown threshold. Higher ambient plasma densities reduce the ionization, but for large EMP, breakdown can occur even with high daytime densities. The breakdown increases the plasma density which acts to limit the EMP and ionization. This feedback reduces the sensitivity of the breakdown to the ionization model. Neutral density variations, such as caused by atmospheric gravity waves, can cause spatial variations in the ionization density.

Optical remote sensing of the thermosphere with HF pumped artificial airglow

Optical emissions excited by high-power radio waves in the ionosphere can be used to measure a wide variety of parameters in the thermosphere. Powerful high-frequency (HF) radio waves produce energetic electrons in the region where the waves reflect in the F region. These hot or suprathermal electrons collide with atomic oxygen atoms to produce localized regions of metastable O(1D) and O(1S) atoms. These metastables subsequently radiate 630.0 and 557.7 nm, respectively, to produce clouds of HF pumped artificial airglow (HPAA). The shapes of the HPAA clouds are determined by the structure of large-scale (≈10 km) plasma irregularities that occur naturally or that develop during ionospheric heating. When the HF wave is operated continuously, the motion of the airglow clouds follows the E × B drift of the plasma. When the HF wave is turned off, the airglow clouds decay by collisional quenching and radiation, expand by neutral diffusion, and drift in response to neutral winds. Images of HPAA clouds, obtained using both continuous and stepped radio wave transmissions, are processed to yield the electric fields, neutral wind vectors, and diffusion coefficients in the upper atmosphere. This technique is illustrated using data that were obtained in March 1993 and 1995 at the ionospheric modification facility near Nizhny Novgorod, Russia. Analysis of HPAA clouds yields zonal plasma drifts of 70 m s−1 eastward at night. On the basis of artificial airglow from energetic electrons generated at 260 km the zonal neutral wind speed was estimated to be 96 m s−1 and the O(1D) diffusion coefficient was determined to be between 0.8 and 1.4 × 1011 cm2 s−1. The quenched lifetime of the O(1D) was determined to be 29.4 s. The diffusion and quenching rates are directly related to the atomic and molecular concentrations in the thermosphere. Improvements in the remote-sensing technique may be obtained if the intensity of the artificial airglow emissions is increased. High-power radio transmissions employing pulse sequences and tuning near electron cyclotron harmonics were attempted to increase the optical emissions. Both of these, however, produced reduced intensity, and consequently, continuous transmission at frequencies away from electron gyro harmonics is the preferred heating regime.

Self-consistent modeling of equatorial dawn density depletions with SAMI3

[1] Large-scale, dawn density depletions in the equatorial ionosphere have been observed by instruments on the STPSat1 and CHAMP satellites. The Naval Research Laboratory (NRL) ionosphere model SAMI3 (Sami3 is Also a Model of the Ionosphere) is used to study this new phenomenon using a self-consistent electric field. Two empirical Horizontal Wind Models (HWM) are used in the simulation study: HWM93 and HWM07. Dawn density depletions are found using HWM07 but not with HWM93. The cause of the depletions is a post-midnight enhancement of the eastward electric field that generates an upward plasma drift. This drift lifts low density plasma to high altitudes (i.e., ~600 km). We compare our model results to remote sensing data and to in situ satellite data.

Enhanced radar backscatter from space shuttle exhaust in the ionosphere

Enhancements in the backscatter from the 430-MHz radar at Arecibo were recorded during the Spacelab 2 mission when the space shuttle orbital maneuver system (OMS) engines were fired in the ionosphere. The modifications in the backscatter could have been the result of (1) compression of the electrons to produce higher densities, (2) generation of ion acoustic waves, (3) variations in the electron to ion temperature ratio, (4) enhanced scatter cross section by charging of ice particles in the exhaust, or (5) excitation of dust acoustic waves. Rapid cooling and condensation of the exhaust are important in determining the scattering properties of the modified ionosphere. A dusty plasma is formed when electrons are attached to ice particles in the exhaust plume. The calculated neutral temperature inside the exhaust plume is 120 K. Charge exchange between ambient O+ and the cold exhaust molecules yields low-temperature ion beams that excite weakly damped, ion acoustic waves. The enhanced radar echoes are probably the result of scatter from these waves, but the effects of the dusty plasma may be important. During future experiments, the space shuttle will fire the OMS engines over radars located at Arecibo, Puerto Rico; Jicarmarca, Peru; or Kwajalein, Marshall Islands. Measurements of the spectra from these radars will provide the means to distinguish between the various backscatter processes.