Richard Woo

Ammonia abundance in Jupiter's atmosphere derived from the attenuation of the Galileo probe's radio signal

The radio signal from the Galileo probe to the orbiter experienced attenuation due to ammonia in Jupiters atmosphere during the probe descent. A profile of the ammonia content as a function of depth in the atmosphere has been derived from the measurements of the attenuation. The derived ammonia abundance rises to a molar fraction of 700±100 parts per million for pressures greater than 7 bars, about 4 times that expected based on the solar abundance of nitrogen.

RADIO SCIENCE INVESTIGATIONS WITH MARS OBSERVER

Mars Observer radio science investigations focus on two major areas of study: the gravity field and the atmosphere of Mars. Measurement accuracies expressed as an equivalent spacecraft velocity are expected to be of the order of 100 μm/s (for both types of investigations) from use of an improved radio transponder for two-way spacecraft tracking and a highly stable on-board oscillator for atmospheric occultation measurements. Planned gravity investigations include a combination of classical and modern elements. A spherical harmonic (or equivalent) field model of degree and order in the range 30–50 will be obtained, while interpretation will be in terms of internal stress and density models for the planet, using the topography to be obtained from the Mars Observer laser altimeter. Atmospheric investigations will emphasize precision measurement of the thermal structure and dynamics in the polar regions, which are regularly accessible as a result of the highly inclined orbit. Studies based on the measurements will include polar processes, cycling of the atmosphere between the poles, traveling baroclinic disturbances, small-scale waves and turbulence, the planetary boundary layer, and (possibly) the variability and altitude of the ionosphere. As the radio occultation is insensitive to dust in the atmosphere per se and measures only the resulting change in thermal structure, it is expected that the radio technique can contribute to understanding of dust storm phenomena. Mutual observations of the atmosphere by means of radio occultation and by the pressure modulator infrared radiometer and the thermal emission spectrometer are expected to strengthen the reliability and accuracy of all three investigations.

Heliospheric plasma sheet and coronal streamers

Helios 2 measurements of solar wind plasma and magnetic field are used to investigate the structure of the heliospheric plasma sheet between 0.3 and 1 AU. In agreement with previous observations at 1 AU, the plasma sheet thickness is much larger than that of the embedded current sheet. The plasma sheet appears surrounded by a density halo, a region of slightly raised density. High-time resolution data show that decreases in relative helium abundance coincide with the plasma sheet boundaries, reinforcing the notion that the solar wind within the plasma sheet is of a different nature (with different solar origins) than that outside it. Although radio occultation measurements of the corona were not available at the time of the Helios data, a synthesis of recent results on coronal streamers shows that there is a remarkable similarity between their major features and those of plasma sheets, demonstrating that the coronal counterpart of the plasma sheet is the stalk of the coronal streamer. These measurements also suggest that the density halo seen in the Helios data is associated with the radial extension of the boundaries of the streamer observed in the extended corona before the streamer narrows to a stalk.

The Depiction of Coronal Structure in White-Light Images

The very steep decrease in density with heliocentric distance makes imaging of coronal density structures out to a few solar radii challenging. The radial gradient in brightness can be reduced using numerous image processing techniques, thus quantitative data are manipulated to provide qualitative images. We introduce a new normalizing-radial-graded filter (NRGF): a simple filter for removing the radial gradient to reveal coronal structure. Applied to polarized brightness observations of the corona, the NRGF produces images which are striking in their detail. Total-brightness, white-light images include contributions from the F corona, stray light, and other instrumental contributions which need to be removed as effectively as possible to properly reveal the electron corona structure. A new procedure for subtracting this background from LASCO C2 white-light, total-brightness images is introduced. The background is created from the unpolarized component of total-brightness images and is found to be remarkably time-invariant, remaining virtually unchanged over the solar cycle. By direct comparison with polarized-brightness data, we show that the new background-subtracting procedure is superior in depicting coronal structure accurately, particularly when used in conjunction with the NRGF. The effectiveness of the procedures is demonstrated on a series of LASCO C2 observations of a coronal mass ejection (CME).

Extension of coronal structure into interplanetary space

We investigate the extension and evolution of the solar corona into interplanetary space by comparing 1995 Ulysses radio occultation measurements of path-integrated electron density and density fluctuations measured between 21 and 32 Ro, with simultaneous white-light measurements made by the HAO Mauna Loa K-coronameter below 2.5 Ro. The surprising picture of the extended corona to emerge from this comparison is one in which stalks of streamers, occupying a small fraction of volume in interplanetary space, are superimposed on a background corona distinguished by a plethora of ray like structures, often referred to as plumes in polar coronal holes. The radial preservation of the boundary between polar coronal holes and the base of streamers implies that the solar wind from polar coronal holes expands radially rather than undergoing any significant divergence as previously thought. Combining this picture of the extended corona with in situ velocity measurements made by Ulysses throughout its two polar passages, we conclude that the raylike structures, except for the stalks of streamers, seem to be the source of the fast wind. The existence of the fast wind at low latitudes can be attributed to these raylike structures, rather than the expansion of the boundaries of polar coronal holes to low latitudes.

The polar ionosphere of venus near the terminator from early pioneer venus orbiter radio occultations.

Fourteen profiles of electron density in the ionosphere of Venus were obtainecd by the dual-frequency radio occulation method with the Pioneer Venus orbiter between 5 and 30 December 1978. The solar zenith angles for these measurements were between about 85� and 92�, and the latitudes ranged from about 81� to 88� (ecliptic north). In addition to the expected decreasein peak electron density from about 1.5 x 103 to 0.5 x 103 per cubic centimeter with increasing solar zenith angle, a region of almost constant electron density above about 250 kilometers was observed. The ionopause height varies from about 300 to 700 kilometers and seems to be influenced by diurnal changes in solar wind conditions. The structures of the profiles are consistent with models in which O2+ dominates near the ionization peak and is replaced by O+ at higher altitudes.

Multipacting Discharges between Coaxial Electrodes

Experimental evidence is presented to demonstrate the scaling relations of the similarity principle for multipacting as applied to coaxial electrodes with b/a (b and a are the radii of the outer and inner electrodes, respectively) varying between 2.3 and 18.26. The two field configurations considered are: (1) rf voltage applied to the electrodes and (2) rf and dc voltages applied to the electrodes. The modes of multipacting are also studied and identified.

Source regions of the slow solar wind

While the fast solar wind is generally thought to originate from coronal holes, the source of the slow solar wind is only known to be associated with the highly structured and highly variable streamer belt. There is growing evidence from multiple-station intensity scintillation measurements of solar wind velocity—often referred to as IPS for interplanetary scintillation—that the slow wind originates in localized regions of the solar corona overlying the streamer belt. In this paper, velocity structure in the corona is investigated based on estimates obtained from the power spectra of single-station intensity scintillation measurements. These measurements were conducted with the S- and X-band (13 and 3.6 cm wavelengths, respectively) radio signals of Voyager 2 during its superior conjunctions in 1979 and 1982. Unlike previous studies, simultaneous estimates of density fluctuation from the same intensity scintillation measurements were available to provide the coronal context for the velocity measurements. Solar eclipse pictures show that with increasing heliocentric distance, coronal streamers taper to narrow extensions or stalks of angular size 1–2° by a few solar radii. Prominent enhancements in density fluctuation characterizing such streamer stalks are found to coincide with the low speed wind measured by Voyager 2, leading to the first observational evidence for streamer stalks as the long sought coronal sources of the slow solar wind.

White-light and radio sounding observations of coronal transients

A concerted search for coronal transients was conducted with the ‘Solwind’ coronagraph during the solar occultations of the two Helios spacecraft in October/November 1979. The polarization angle and bandwidth of the linearly polarized S-band downlink signal were monitored at the three 64-m tracking stations of the NASA Deep Space Network to determine coronal Faraday rotation and spectral broadening. A one-to-one correspondence could be established between abrupt disturbances in the two signal parameters and the passage of a white-light transient through the signal ray path from spacecraft to Earth. The white-light morphology and the additional information provided by the radio sounding coverage are presented for each of the five distinct events recorded. Although no specific example could be observed in sufficient detail in both white light and Faraday rotation to derive the small-scale magnetic structure, some qualitative descriptions of the orientation and rough estimates of the magnitude of the transient magnetic field could be made.

Variation of fractional electron density fluctuations inside 40 Ro observed by Ulysses ranging measurements

The first measurements of fractional electron density fluctuations δne/ne, where δne is rms electron density fluctuation and ne is the mean electron density, have been carried out inside 40 Ro using 1991 Ulysses dual-frequency S- and X-band (13 and 3.6 cm) ranging (time delay) measurements. In the frequency band of ∼ 6 × 10−5 - 8 × 10−4 Hz (periods of 20 min to 5 hr), δne/ne varies from a high near 20% in the slow wind close to the neutral line to a low of 1% in the fast wind far from the neutral line. For spatial wavenumber K ∼ 1.4 × 10−6 km−1 (period of 5 hr at 250 km/s), δne/ne is essentially independent of heliocentric distance over 0.03–1.0 AU in the slow wind; it is a factor of 30 lower in the fast wind than in the slow wind inside 0.1 AU, but exhibits dramatic growth with heliocentric distance inside 0.3 AU. This latter result reinforces current views of the evolution of MHD turbulence and the association of Alfven waves with high speed streams based on in situ fields and particles measurements beyond 0.3 AU. That regions of enhanced density fluctuations near or above the neutral line coincide with regions of enhanced density confirms previous conclusions that they are the interplanetary manifestation of the heliospheric current sheet and extensions of coronal streamers. While the regions of enhanced density fluctuations lie within those of enhanced density, they have boundaries that are distinctly more abrupt, suggesting the separation of plasma of different nature and origin.