Measurement of field aligned electron density distribution, ducts, and Z mode cavities from ducted and nonducted fast Z mode echoes observed on the image satellite

Abstract

Z mode (ZM) sounding from the Radio Plasma Imager (RPI) onboard the Imager for Magnetopause to Aurora Global Exploration (IMAGE) satellite has provided a new method to measure the geomagnetic field aligned electron density (Ne) distribution, magnetospheric ducts, and Z mode cavities in the low- to mid-latitude region of the magnetosphere from the ducted and nonducted fast ZM echoes observed from radio sounding at 20-1000 kHz. Z mode is a trapped wave mode of the plasma confined in frequency between the ZM cutoff frequency, fz, and the upper hybrid resonance frequency, fuh. In the past, trapped ducted Z mode echoes in the ZM cavity were used to measure field aligned electron density thousands of kilometers above the satellite altitude. However, no attempt was made to study the properties of the ducts that can guide ZM waves. Magnetospheric ducts play an important role in the propagation of plasma waves, wave particle interactions, particle acceleration, and precipitation. In the past, ducts have been known since the discovery of the plasmasphere, but there is limited knowledge of the properties of ducts, their origins and occurrence patterns, and their distribution on a global scale in the magnetosphere especially for ducts extending to high altitudes (above 1000 km). Analysis of ducted echoes from the conjugate and local hemispheres has enabled nearly instantaneous magnetic field aligned electron density profiles. Field aligned density distribution plays an important role in magnetospheric dynamics. Despite its importance, there is a lack of accurate representation of the latitudinal dependence of density distribution along the field lines by the existing empirical models, due to limited measurement of off-equatorial electron density profiles and availability of relatively fewer measurements of Ne at higher altitudes. There is also a need to describe the transition of field aligned electron densities from the topside ionosphere into the plasmasphere properly. RPI on IMAGE, designed to sweep from 3 kHz to 3 MHz, has observed both ducted and nonducted ZM echoes in the frequency range of 40 - 800 kHz in an altitude range of ~600-10,000 km, invariant latitude of ~20°-90°, at all magnetic local times (MLTs) and for an fpe/fce range of 0.25-6, where fpe and fce are electron plasma frequency and electron gyro frequency respectively. About 72,000 plasmagrams have been surveyed, out of which ~1500 cases of ducted- and ~3500 cases of nonducted fast ZM echoes have been identified. Two cases of ducted, two cases of nonducted, and a set of seven successive cases of nonducted and ducted fast ZM echoes have been analyzed to study the propagation, reflection, and guidance of fast ZM waves in the magnetosphere and measure: (1) field aligned electron density distribution; (2) duct parameters (half-width ΔL and density variation ΔN/N); and (3) Z mode cavity parameters. In absence of ducts, four nonducted echoes are obtained, each reflecting from locations where fz ~f, where f is the wave frequency. Three of the echoes retrace their path after reflection and one forms a loop. When a Ne depletion duct is present inside a Z mode cavity, fast Z mode waves can be guided within the duct and propagate back and forth between their reflection points above and below the satellite altitude. Ray tracing calculations of ducted Z mode echoes show that: (1) in the presence of ducts, both nonducted and ducted echoes are obtained, and echoes are formed from both looping as well as retracing paths; (2) average time delays (tg) depend upon the electron density distribution along geomagnetic field lines and the shape of the Z mode cavity; (3) time delay spread (Δtg) and upper cutoff frequency (fuc) depend upon duct half-width (ΔL) and density perturbation (ΔN/N); (4) echoes are obtained from local as well as conjugate hemispheres when the duct extends to the conjugate hemisphere; and (5) the discovery of a new phenomenon of gap in frequency for echoes reflecting from below the satellite altitude. The gap frequency range depends upon the duct parameters and is a consequence of the peculiar shape of the refractive index surface for f~fpe. From ZM soundings of 11 case studies, it is found that the measured ducts have half-widths of ~160-500 km at the equator and density depletions of ~1%-6%, covering an altitude range of ~1000-10,000 km. The first quantitative measurements of ZM cavities have been presented in this thesis. The bandwidth and the length of ZM cavities lie in the range of ~10-50 kHz and ~3800-15,000 km, respectively. From the inversion of time delay versus frequency dispersion of the fast ZM echoes, field aligned electron density distributions both above and below the satellite from an altitude of ~1000-10,000 km (and ~90- 10,000 km if accompanied by Whistler mode echoes) are obtained. The electron density profiles obtained from ZM sounding are in good agreement with in-situ electron density measurements from the CHAMP (~350 km) and DMSP (~850 km) satellites, from RPI passive measurements, and give a better estimate of electron density as compared to empirical density models. Our results demonstrate that ZM sounding is a new and powerful method to measure field aligned electron density distribution, magnetospheric ducts, and ZM cavities in the altitude range of ~1000-10,000 km (and ~90-10,000 km if accompanied by Whistler mode echoes) at low- to mid-latitude region of the magnetosphere. These measurements can improve the existing empirical density models and provide a new understanding of the magnetospheric ducts as well as of waves propagating therein.