Engineering topics

Ho, C.-M.

Publications and source records attributed to Ho, C.-M..

5 records

NASA NTRS1995

Venus planetary lightning rate as deduced from VLF bursts

Statistical studies of the VLF bursts detected in the nightside ionosphere of Venus show that the bursts fall into two classes. The first consists of signals detected when vertical propagation within the whistler-mode resonance cone is allowed. The second consists of signals whose burst rate decreases rapidly as a function of increasing altitude, with a scale height of about 20 km. These non-whistler-mode signals also display a strong dependence on local time, with the burst rates being largest in the post-dusk local time sector. Since these signals are not propagating we assume that they correspond to a 'near-field' or prompt response to a lightning stroke. As such we can use these signals to estimate the planetary lightning rate, and we find that the rate at Venus is comparable to or greater than the terrestrial planetary rate of 100 flashes/sec.

Ho, C.-M.↗

NASA NTRS1994

Spatial distribution of plasma wave activity in the nightside ionosphere of Venus

In this study we use 14 years of Pioneer Venus Orbiter Electric Field Detector (OEFD) data to define the characteristics of (VLF) burst activity in the nightside ionosphere of Venus. Our statistical results show that there are essentially four types of VLF signals. The first type of signal is only observed in the 100 Hz channel and not in any of the higher frequency channels (730 Hz, 5.4 kHz or 30 kHz). Occurrence of these waves is controlled by the magnetic field with a weaker dependence on electron density. The occurrence of these waves is controlled by the magnetic field with a weaker dependence on electron density. The occurrence rate decreases with increasing altitude to a height of 600 km. For higher altitudes beyond 600 km the occurrence rate remains roughly constant. The statistics of these signals are what one would expect for whistler mode waves from a subionospheric source. The second type of signal is broadline wave activity appearing below 300 km in the low altitude ionosphere. These signals often occur in all four channels of the OEFD. These signals are also thought to come from a subionospheric source. The third type of signal appearing near the edge of the planetary optical shadow. They are probably ion acoustic waves generated by a current driven instability associated with plasma clouds in the wake. The fourth type of signal is a narrow band wave. It occurs in either of the two high frequency channels in the high altitude tail region, and is attributed to locally generated Langmuir waves. In addition, we also observe spacecraft interference noise in both the 100 and 730 Hz channels. These signals mainly occur near the edge of the planetary optical shadow and have an inbound and outbound asymmetry in activity.

Ho, C.-M.↗

NASA NTRS1992

Control of VLF burst activity in the nightside ionosphere of Venus by the magnetic field orientation

A burst identificaton method developed by Ho et al. (1991) is used to analyze data on the nightside ionosphere of Venus in order to determine whether VLF bursts observed there are better interpreted as whistler mode waves or ion acoustic waves. The correlation between burst occurrence and the angle between the magnetic field and the radial direction, as well as the spacecraft flight direction are examined. Narrow-band 100-Hz bursts are found to be more frequently associated with radial magnetic fields while wideband signals are more frequently associated with horizontal fields. Under the assumption of vertical propagation, the normalized 100-Hz burst rate inside the resonance cone is larger than that outside. The burst rate inside the resonance cone dominates the altitude distribution. By assuming vertical propagation, the 100-Hz signals clearly divide into two populations. One is whistler mode propagating inside the resonance cone. The other is a nonpropagating mode outside the resonance cone which decreases quickly with altitude with a scale height of about 20 km.

Ho, C.-M.↗

NASA NTRS1991

Occurrence characteristics of VLF bursts in the nightside ionosphere of Venus

Many of the impulsive VLF signals observed by the Pioneer Venus orbiter electric field detector (OEFD) in the nightside ionosphere have been interpreted as arising from lightning in the Venus atmosphere. In order to determine the characteristics of the source, and to compare with terrestrial lightning, the normalized occurrence rate for individual bursts at Venus are determined, as opposed to previous studies of activity in 30-s intervals. Burst identification criteria are determined which take into account the decay constant of the instrument to calculate the burst rate. Under the assumption that all OEFD observations in the nightside ionosphere are due to impulsive signals, it is found that the 100-Hz channel has the highest burst rate, about 0.20/s in the altitude range 150-180 km and in the postmidnight local time sector. The burst occurrence rates for all four frequency channels decrease with increasing altitude. Burst rates at frequencies above 100 Hz are greater in the premidnight hours while 100-Hz signals peak after midnight; however, these burst rates vary from orbit to orbit. The 100-Hz low-frequency burst rate has a stronger dependence on the magnetic field strength than the higher-frequency rates. Most 100-Hz bursts are closely spaced and occur independently of signals at higher frequencies. These dependences suggest that the low- and high-frequency signals have different propagation mechanisms. The properties of the bursts are generally consistent with a lightning source, and the planet-wide burst rate at Venus may be comparable to or larger than the terrestrial lightning rate.

Ho, C.-M.↗

NASA NTRS1985

Near field pressure fluctuations of a circular jet

The relationship between the evolution of vortices in a circular jet and the generation of fluctuating pressure was studied, using an apparatus consisting of a jet nozzle connected to a stagnation chamber with a fifth-order polynomial contraction. The data were digitized and computer-processed. The constant pressure contours near the jet shoow that the evolution of coherent structures is the source of pressure perturbations. The data obtained in the vicinity of the jet indicate that two length scales govern the decay of the fluctuating pressure. A thin hydrodynamic near-field extends about 1 instability wavelength from the jet edge, and pressure decays exponentially. A thick acoustic near-field takes about ten acoustic wavelengths to reach the far field. The pressure disturbances decay algebraically in this region.

Lafouasse, B.↗