Now showing items 1-11 of 11

• #### A Computer Simulation Of Auroral Arc Formation

Recent satellite measurements have revealed two intriguing features associated with the formation of auroral arcs. The first is that an auroral arc is produced by a sheet of electrons accelerated along a geomagnetic field-aligned potential drop, and the second is that these electrons carry a field-aligned, upward directed electric current. In order to explain these measurements, a self-consistent, time dependent, computer simulation of auroral arc formation has been developed. The simulation demonstrates for the first time that a stable V-shaped potential structure, called an auroral double layer, develops spontaneously as a result of an ion shielded electron current sheet interacting with a conducting ionosphere. The double layer accelerates current-carrying electrons into the upper atmosphere at auroral energies. The double layer potential depends critically on the drift speed of the current-carrying electrons and on the temperature of the ambient shielding ions. Localized double layers occur near the ionosphere when the geomagnetic field is assumed to be uniform, but when a converging magnetic field is introduced, the double layer becomes extended due to the presence of an additional population of electrons trapped between the magnetic mirror and the double layer potential. The simulated auroral current sheet is subject to auroral curl and fold type deformations due to unstable Kelvin-Helmholtz waves. The previous incompletely understood auroral fold producing mechanism is described.
• #### An investigation of small-scale relationships between optical and HF radar aurora

An investigation is undertaken of the relationship between visual aurora and the occurrence of radar-detectable irregularities in the nightside ionosphere. Understanding how auroral signatures appear in HF radar backscatter could combine the advantages of detailed information about auroral fluxes in optical measurements with extended coverage of HF radars. Auroral particle precipitation ionizes the ionosphere and creates localized plasma density enhancements. Irregularities with various smaller sizes are generated from larger density structures through instability-induced cascading. HF waves are coherently scattered by decameter structures within the ionospheric plasma. Hence aurorally induced irregularities can be seen by the radar in the form of "HF radar aurora." A statistical treatment of the occurrence of optical and HF radar aurora reveals a high degree of variability in backscatter patterns even under seemingly similar auroral displays. The small-scale correspondence between visual aurora and HF backscatter thus represents a more differentiated picture than the spatially and temporally averaged data of earlier studies. The relationship between the occurrence or the characteristics of aurora and the occurrence of HF echoes can therefore not be quantified. An analysis of single events isolates processes that lead to the observed variety of backscatter patterns in the presence of aurorally induced irregularities. They involve the ambient ionospheric density and localized enhanced densities at different altitude regimes and locations in the path of the radar signal. Conditions for HF wave propagation are partly determined by the aurora itself, partly they are imposed by ambient ionospheric density levels. It is found that low or high ambient densities have a dominating effect on the success of ionospheric probing. Low densities hamper the return of radar signals despite the presence of irregularities. High ambient densities can overcome some of the adverse effects on HF wave propagation associated with sporadic E. The information contained in the diversity of the relationships between optical and HF backscatter improves thus our knowledge about the nighttime ionosphere. A more detailed specification of ionospheric parameters is necessary to gain better insight into these relationships.
• #### Analysis of optical observations and three-dimensional hybrid code simulation of the CRRES plasma injection experiments in space

The Combined Release and Radiation Effects Satellite (CRRES) was a NASA funded campaign designed to study a variety of plasma processes in the Earth's space environment. An analysis of optical data from three CRRES plasma injection experiments, in conjunction with results from a three-dimensional hybrid code simulation, have provided new insights into small-scale coupling processes in the ionosphere. The results have direct application to auroral processes, comets, and other similar geophysical/astrophysical systems.
• #### Influence Of The Kelvin -Helmholtz Instability On The Plasma Transport At The Magnetospheric Boundary

The Kelvin-Helmholtz (KH) instability has long been suggested as a mechanism for viscous interaction at the magnetospheric boundary but it was not expected to produce significant mass transport. Satellite observations show that the density, temperature, particle pressure and total pressure of the plasma sheet are strongly correlated with those of the solar wind on a time scale of ~2 hours. I present a systematic 2-D study of reconnection in KH flow vortices using MHD and Hall-MHD approximations depending on magnetosheath and magnetospheric plasma and field properties. The presented results show that the Kelvin-Helmholtz instability can be a major plasma transport mechanism during times of strongly northward IMF providing a source of plasma into the low latitude boundary layer and plasma sheet on a time scale of ~2 hours. I have also analyzed Equator-S and Cluster satellite data at the dawnside magnetospheric flank and compared these results with MHD simulations in order to distinguish signatures caused by Kelvin-Helmholtz instability. In addition I have discussed typical ionospheric signatures caused by KH instability.
• #### Investigation Of Auroral Hiss Observations On The Ground: Application To Remote Sensing Of Auroral Magnetosphere

Observed both on the ground at high latitudes and on spacecraft in the auroral zone, auroral hiss (AH) emissions (~1 kHz to ~1 MHz) are intense electromagnetic emissions emitted from the auroral region. Standard whistler mode propagation theory in a smooth magnetosphere predicts that AH generated at large wave-normal angles along the auroral field lines by Cerenkov resonance cannot penetrate to the ground. This thesis presents a new mechanism of AH propagation to the ground in which presence of density depletions along the field lines in the auroral zone and meter-scale density irregularities at altitudes <5000 km at high latitude permits the AH propagation to the ground. In the proposed mechanism AH generated at high altitudes (>5000--20,000 km) propagates to lower altitudes (<3000--5000 km) in two modes, the ducted mode and the non-ducted mode, with large wave-normal angles. At altitudes <5000 km meter-scale irregularities scatter the hiss into electrostatic waves with large wave-normal angles that are reflected into the magnetosphere and electromagnetic waves with small wave-normal angles that can penetrate to the ground. The AH propagation model proposed in this thesis also explains the spectral characteristics of AH including the upper and lower frequency cutoffs, the dispersion of AH, the location of ionospheric exit points of AH with respect to visible aurora, and the 2--5 orders of magnitude difference in the power spectral density ratio measured on satellites versus ground. The new understanding of AH permits the determination of AH source region, energetic electron parallel resonance energy, and cold plasma electron concentrations along field lines. Analysis of AH spectra, recorded at South Pole (July 09, 1996 0005 UT), using the model developed in this thesis shows that: (a) AH source region altitude for frequencies 7--9 kHz should be >16,000 km while for frequencies 12--20 kHz it should be <8000 km, (b) parallel resonance energy of the energetic electrons generating the frequencies should be <1 keV, and (c) cold plasma electron concentration along the field line Lambda = 79� should be ~100 el cm-3 at 12,740 km altitude.
• #### Optical observations of critical ionization velocity chemical releases in the ionosphere: The role of collisions

In recent years researchers have pointed out the importance of collisional processes in ionospheric chemical releases performed to study Alfven's critical ionization velocity effect (CIV). Ionizing collisions, including charge exchange with ambient O$\sp+$ and associative ionization, can not only help initiate CIV, but can also lead to 'contamination' of the ion cloud. Most of the proposed collisions have associated emissions which should be observable with sensitive detectors, but until now have not been attempted since atomic processes had not been considered important. The first four releases of the CRRES satellite were performed to study CIV. The releases were at local dusk over the south Pacific in September, 1990, and were observed from two aircraft with low light level cameras, both filtered and broadband. Ion inventories of the releases show ionization yields (number of ions per number of available neutrals) of 0.02% for Sr, 0.15% for the first Ba release, 0.27% for Ca and 1.48 for the second Ba release. The release clouds were seen to glow quite strongly, below the terminator. The measured light is found to be primarily from line emissions which indicates that it is due to collisional processes in the release cloud. Two measurements were made on the release cloud data; (1) the absolute intensity of the release clouds and (2) the ratio between a broadband intensified CCD (ICCD) and an imaging photon detector filtered for the Ba$\sp+$ 455.4 nm emission line. The measured ratio is compared to the expected ratio for charge exchange collisions, and to electron impact excitation of Ba. The measured ratio is consistent with emissions being from charge exchange collisions. However, when compared to the total intensity of emissions expected from charge exchange, the absolute intensity in the release cloud measured by the ICCD is five times greater. The two measurements are in conflict, and with this limited set of data cannot be fully resolved. The ratio measurement does indicate that any CIV discharge in the Ba releases was extremely weak, and that charge exchange is the dominant collisional process in Ba releases.
• #### Studies of compositional variations in the thermosphere and ionosphere using far-ultraviolet images from DE-1

The Dynamics Explorer mission returned a wealth of information from its two orbiting platforms. Of interest here are the three scanning photometers aboard the high-altitude platform DE-1, which obtained hundreds of thousands of global images of Earth, beginning in September of 1981, while using broad- and narrow-band filters to isolate particular terrestrial emissions. The far-ultraviolet (FUV) emissions include the line emissions of OI (130.4 and 135.6 nm) and the band emissions of $\rm N\sb2$ LBH, the brightness of each yielding information on the composition of the upper atmosphere. The OI emissions are related to the column density of atomic oxygen in the upper-atmosphere as well as the abundance of thermospheric $\rm N\sb2,$ both of which are affected by geomagnetic processes. This thesis presents a model of the DE-1 response to the OI emissions during periods of low geomagnetic activity and uses this model for studies of thermospheric response to geomagnetic storms and substorms. Variations in brightness observed after geomagnetic events are most often seen as decreases corresponding to reduced thermospheric O column densities. The relation between compositional variations in the morning sector at middle latitudes and the orientation of the magnetic field embedded in the solar wind is investigated. The orientation, which strongly affects the circulation of the thermosphere at high latitudes where these variations originate, is shown to be a significant parameter. Variations in brightness within the southern polar cap are investigated in the first study of its kind, demonstrating 20-30% decreases in brightness with the onset of magnetic activity and revealing structure in composition over distances on the order of ${\sim}300$ km. Compositional disturbances are observed immediately after heating takes place, demonstrating for the first time that an FUV instrument can detect changes in thermospheric composition on time scales under one hour. During these events, mid-latitude composition often remains relatively unperturbed. The first survey of FUV images to include ground-based measurements of ionospheric properties demonstrates that decreases in OI brightness correspond to decreases in peak F2 electron densities, known to be related to the ratio of the densities of O and $\rm N\sb2.$

• #### Three-Dimensional Structure Of The Heliosphere: Quiet-Time And Disturbed Periods (Kinematic, Flare Propagation, Solar Wind, Interplanetary Magnetic Field)

An improved kinematic method is used to perform simulation studies of temporal and spatial variations of solar wind speed and interplanetary magnetic field during periods when the sun is quiet and when it is active. The procedure of Hakamada and Akasofu (1982) is improved and calibrated with a one-dimensional magnetohydrodynamic solution of solar wind flow. The solar-cycle evolution of solar wind velocity is studied using the data sets of King (1979, 1983) and Hoeksema et al. (1982, 1983) for the period 1976 to 1982. It is found that the gradient of the quiet-time solar wind speed as a function of magnetic latitude is steepest near solar minimum and most broad at solar maximum. The background solar wind velocity and magnetic field are simulated and compared to observations near the earth. Three-dimensional representations of the heliospheric current sheet (HCS) are displayed out to 5 AU for idealized dipole and quadrupole cases, and for observed source field configurations. The high latitude IMF and surfaces of constant magnetic latitude are also presented. The propagation of solar wind disturbances in the solar equatorial plane to 30 AU is simulated. Two major disturbance event periods are simulated, and it is seen how a series of solar flares can greatly disrupt both the inner and outer heliosphere. Visual representations of the distorted HCS due to a series of hypothetical solar flares are presented. A method of generating the polar component of the IMF vector, B(,z), is also developed. It is shown that field-line motion at the source surface provides a mechanism for the propagation of B(,z) into interplanetary space. This study shows that an improved kinematic method can be used to quantitatively model the three-dimensional heliospheric structure. Such a modelling scheme, which takes the stream-stream interaction into account, is necessary for the accurate prediction of near-earth solar wind parameters during quiet times and active periods.