Engineering topics

Lecacheux, A.

Publications and source records attributed to Lecacheux, A..

At least 19 records

NASA NTRS2005

Radio and Plasma Wave Observations at Saturn from Cassini's Approach and First Orbit

We report data from the Cassini radio and plasma wave instrument during the approach and first orbit at Saturn. During the approach, radio emissions from Saturn showed that the radio rotation period is now 10 hours 45 minutes 45 k 36 seconds, about 6 minutes longer than measured by Voyager in 1980 to 1981. In addition, many intense impulsive radio signals were detected from Saturn lightning during the approach and first orbit. Some of these have been linked to storm systems observed by the Cassini imaging instrument. Within the magnetosphere, whistler-mode auroral hiss emissions were observed near the rings, suggesting that a strong electrodynamic interaction is occurring in or near the rings.

Gurnett, D. A.↗

NASA NTRS2005

Cassini Radio and Plasma Wave Observations at Saturn

Results are presented from the Cassini radio and plasma wave instrument during the approach and first few orbits around Saturn. During the approach the intensity modulation of Saturn Kilometric Radiation (SKR) showed that the radio rotation period of Saturn has increased to 10 hr 45 min plus or minus 36 sec, about 6 min longer than measured by Voyager in 1980-81. Also, many intense impulsive radio signals called Saturn Electrostatic Discharges (SEDs) were detected from saturnian lightning, starting as far as 1.08 AU from Saturn, much farther than terrestrial lightning can be detected from Earth. Some of the SED episodes have been linked to cloud systems observed in Saturn s atmosphere by the Cassini imaging system. Within the magnetosphere plasma wave emissions have been used to construct an electron density profile through the inner region of the magnetosphere. With decreasing radial distance the electron density increases gradually to a peak of about 100 per cubic centimeter near the outer edge of the A ring, and then drops precipitously to values as low as .03 per cubic centimeter over the rings. Numerous nearly monochromatic whistler-mode emissions were observed as the spacecraft passed over the rings that are believed to be produced by meteoroid impacts on the rings. Whistlermode emissions, similar to terrestrial auroral hiss were also observed over the rings, indicating that an electrodynamic interaction, similar to auroral particle acceleration, may be occurring in or near the rings. During the Titan flybys Langmuir probe and plasma wave measurements provided observations of the density and temperature in Titan's ionosphere.

Gurnett, D. A.↗

NASA NTRS2003

A Low Frequency Radio and Radar Instrument to Explore Jupiter's Icy Moons

The addition of a comprehensive wave investigation to the Jupiter Icy Moons Orbiter (JIMO) science payload will provide a broad range of information on the icy moons of Jupiter including the detection of subsurface liquid oceans; mapping of their ionospheres; their interaction with the magnetospheric environment; and on the Jovian magnetosphere. These measurements are obtained through the use of both passive and active (sounding) means over broad frequency ranges. The frequency range of interest extends from less than 1 Hz to 40 MHz for passive measurements, from approximately 1 kHz to a few MHz for magnetospheric and ionospheric sounding, and between 1 and approximately 10 MHz for subsurface radar sounding. An instrument to detect subsurface radar sounding, magnetospheric interactions, and ionospheric sounding is discussed.

Kurth, W. S.↗

NASA NTRS2002

Control of Jupiter's Radio Emission and Aurorae by the Solar Wind

Radio emissions from Jupiter provided the first evidence that this giant planet has a strong magnetic field and a large magnetosphere. Jupiter also has polar aurorae, which are similar in many respects to Earth's aurorae. The radio emissions are believed to be generated along the high-latitude magnetic field lines by the same electrons that produce the aurorae, and both the radio emission in the hectometric frequency range and the aurorae vary considerably. The origin of the variability, however, has been poorly understood. Here we report simultaneous observations using the Cassini and Galileo spacecraft of hectometric radio emissions and extreme ultraviolet auroral emissions from Jupiter. Our results show that both of these emissions are triggered by interplanetary shocks propagating outward from the Sun. When such a shock arrives at Jupiter, it seems to cause a major compression and reconfiguration of the magnetosphere, which produces strong electric fields and therefore electron acceleration along the auroral field lines, similar to the processes that occur during geomagnetic storms at the Earth.

Gurnett, D. A.↗

NASA NTRS1995

Outburst of Jupiter's synchrotron radiation after the impact of comet Shoemaker-Levy 9

Jupiter's nonthermal microwave emission, as measured by a global network of 11 radio telescopes, increased dramatically during the Shoemaker-Levy 9 impacts. The increase was wavelength-dependent, varying from approximately 10 percent at 70 to 90 centimeters to approximately 45 percent at 6 and 36 centimeters. The radio spectrum hardened (flattened toward shorter wavelengths) considerably during the week of impacts and continued to harden afterward. After the week of cometary impacts, the flux density began to subside at all wavelengths and was still declining 3 months later. Very Large Array and Australia Telescope images of the brightness distribution showed the enhancement to be localized in longitude and concentrated near the magnetic equator. The evidence therefore suggests that the increase in flux density was caused by a change in the resident particle population, for example, through an energization or spatial redistribution of the emitting particles.

Jupiter↗

NASA NTRS1994

High sensitivity dynamic spectral search for flare star radio

We observed ten well-known flare stars with the Arcibo radio telescope at 1.4 GHz and 5 GHz, using a special observing technique to discriminate between real flares and radio freqeuncy interference. With a high sensitivity of 5.5 K/Jy at 1.4 GHz when averaged over a 50 MHz band, we are able to recognize flux enhancements as weak as approximately 6 mJy above the sky background variations. In about 85 hours of observation, about a dozen bursts were detected, only from AD Leo. All had flux densities lower than 70 mJy, which probably explains their lack of fine structures (except for the strongest one), such as were reported in the literature for stronger flares. Half of the bursts that we recorded are 100% circularly polarized, and half are not circularly polarized. Our results are a first attempt of reliable statistics on dMe flare rates at 1.4 GHz. The high brightness temperatures we infer for the observed bursts are interpreted in terms of coherent emission processes, either the cyclotron maser instability or plasma radiation. Efficiencies are comparable to those of solar or planetary radio emissions in the case of the cyclotron maser, and higher than the solar efficiency in the case of plasma radiation, with the caveat that there are great uncertainties in the coronal model and the source size.

Abada-Simon, M.↗

NASA NTRS1994

The complete polarization state of Io-related radio storms from Jupiter: A statistical study

We report on the complete polarization state (four Stokes parameters) as a function of both frequency and time, of Io-related radiation from Jupiter as observed with the spectropolarimeter at Nancay, France. Our observations include 37 radio storms, with events of both lefthand (LH) and righthand (RH) elliptical polarization and from all Io-related sources (A,B,C and D), plus two non-Io events. The high degree of circular polarization often observed, particularly for LH emission, is difficult to explain in terms of the standard theory of cyclotron maser radiation. The interpretation of the observations in terms of the locations of the sources in Jupiter's magnetosphere is the subject of a companion paper.

Dulk, G. A.↗

NASA NTRS1993

The sidereal rotation period of Neptune

The two main, low frequency radio components discovered at Neptune by the Planetary Radio Astronomy experiment carried aboard Voyager 2 have a well defined periodicity at about 16.1 hour. By analyzing all the available data, i.e. about 60 days around the closest approach for the 'Burst' component and 15 days for the 'Smooth' component, we determine, for each component, the best estimate of the radio period. We conclude that the two estimates are not statistically different. While the two kinds of radio emissions have very different characteristics (in particular their frequency ranges and beaming properties), and probably correspond to different emission processes, their modulation is very likely due to the rotation of the planetary magnetic field tied to the core of the planet, as it has already been assumed for the other giant planets. The deduced estimate of the sidereal rotation period of Neptune is 16.108 +/- 0.006 (or 16h06.5m +/- 0.04 m).

Lecacheux, A.↗

NASA NTRS1992

Phenomenology of Neptune's radio emissions observed by the Voyager planetary radio astronomy experiment

The Neptune flyby in 1989 added a new planet to the known number of magnetized planets generating nonthermal radio emissions. We review the Neptunian radio emission morphology as observed by the planetary radio astronomy experiment on board Voyager 2 during a few weeks before and after closest approach. We present the characteristics of the two observed recurrent main components of the Neptunian kilometric radiation, i.e., the 'smooth' and the 'bursty' emissions, and we describe the many specific features of the radio spectrum during closest approach.

Pedersen, B. M.↗

NASA NTRS1992

In ecliptic observations of Jovian radio emissions by Ulysses - Comparison with Voyager results

During the Ulysses inbound cruise to Jupiter the Unified Radio and Plasma Wave (URAP) experiment observed a variety of the planet's radio components in the frequency range below 1 MHz. Most of these emissions were already detected by the Voyager Radio Astronomy and Plasma Wave experiments, however, with much less sensitivity and different spectral coverage. These different radio components within the URAP dynamic spectra are identified, and their appearance with the previous Voyager observations are compared.

Lecacheux, A.↗

NASA NTRS1992

Ulysses observations of escaping VLF emissions from Jupiter

The Ulysses URAP experiment has detected Jovian radio emissions in the VLF range at distances from Jupiter in excess of 1.5 AU. The URAP observations represent the first synoptic observations of Jupiter in the VLF band, from 3 to 30 kHz. In this band lie the low-frequency extent of the bKOM emission, the escaping continuum emission, and the Jovian type IIIs. Initial results indicate that the continuum varies in frequency with the solar wind ram pressure at Jupiter, whereas, the Jovian type IIIs appear to be controlled to some extent by the planetary rotation, often appearing when system III longitude 100 deg faces the spacecraft.

Kaiser, M. L.↗

NASA NTRS1992

The complete polarization state of a storm of millisecond bursts from Jupiter

Normal bursts with right-hand (RH) elliptical polarization, millisec S-bursts with RH elliptical polarization, and S-bursts with left-hand (LH) elliptical polarization, are presently observed in an extended storm of decametric radiation from Jupiter's Io-related source, 'Io-B'. This radiation was 100 percent elliptically polarized at all frequencies in the 14-31 MHz range measured. For an 80-min period, there was a mixture of RH and LH elliptically polarized S bursts in the 14-20 MHz range. An examination of the origin of a Faraday-rotation difference between RH and LH elliptical bursts suggests that some Faraday rotation occurs in the Io torus; this was traversed more centrally by the LH bursts from the southern auroral zone than the HH bursts from the northern auroral zone.

Dulk, G. A.↗

NASA NTRS1991

Water in dense molecular clouds

The G.P. Kuiper Airborne Observatory (KAO) was used to make initial observations of the half-millimeter ground-state transition of water in seven giant molecular clouds and in two late-type stars. No significant detections were made, and the resulting upper limits are significantly below those expected from other, indirect observations and from several theoretical models. The implied interstellar H2O/CO abundance is less than 0.003 in the cores of three giant molecular clouds. This value is less than expected from cloud chemistry models and also than estimates based on HDO and H3O(+) observations.

Wannier, P. G.↗

NASA NTRS1991

Uranus as a radio source

The complex nature of the Uranus radio emissions, both magnetospheric and atmospheric, is reviewed, with emphasis on the identification of distinct components and the determination of their source locations. Seven radii components were discovered in addition to the RF signature of lightning in the planet's atmosphere. Six of the seven magnetospheric components are freely propagating emissions; one component, the nonthermal continuum, is trapped in the density cavity between the magnetopause and the dense inner magnetosphere. The radio components are divided into two types according to their emission signature: bursty emission and smooth emission. The inferred source location for the dominant nightside emission is above the nightside magnetic pole, largely overlapping the UV auroral region and the magnetic polar cap. The N-burst component appears to be associated with solar-wind enhancements at Uranus, consistent with the idea that the solar wind was triggering magnetospheric substormlike activity during the encounter.

Desch, M. D.↗

NASA NTRS1989

Characteristics of type III bursts in the solar wind from simultaneous observations on board ISEE-3 and Voyager

The properties of solar type III bursts observed aboard ISEE-3 and Voyager that originate behind the sun are analyzed and compared to those originating on the near side. The measurement of the burst parameters and the correction for proximity effects are described. The diminution of flux densities, the increase of source sizes, the difference of source azimuths and elevations, the change of spectral properties, and the anomalous delays in burst arrival time at one spacecraft relative to another are determined. Many of the observations imply that the beaming of type III radiation is much more widespread at all frequencies than has been derived from statistical studies. The effective beam appears to consist of at least two components: a Gaussian core of half width about 60 deg, and a very broad halo that extends to 180 deg with an amplitude of a few percent.

Lecacheux, A.↗

NASA NTRS1989

Observations of H2(O-18) at 547 GHz with the Kuiper Airborne Observatory

The 1(10)-1(01) transition of H2(O-18) has been searched for in several sources without positive detection. However, in NGC 2264 and NGC 7538, possible emission features are suspected. Implications of these feataures, if confirmed, are discussed. The best upper limit obtained in Orion (0.9 K) indicates that in that cloud, the water abundance should not be more than 1 percent that of CO.

Wannier, P. G.↗

NASA NTRS1985

The visibility of type III radio bursts originating behind the sun

More than 95 percent of the solar type III bursts observed by the two Voyager spacecraft when they were in the solar hemisphere opposite the earth were also recorded by ISEE-3, and 10 of 21 type III bursts whose electron streams were directed toward the earth were recorded on the far side of the sun by Voyager. It is presently suggested that the reason for this widespread visibility is a high degree of radio wave scattering that originates at or near the local plasma frequency. Such other observed features of solar bursts as large apparent source heights may be explained by the strong scattering.

Dulk, G. A.↗

NASA NTRS1984

Type III radio bursts in the interplanetary medium - The role of propagation

Interplanetary type III radio burst observations are analyzed in order to ascertain the role played by propagation effects between the true source and the observer. Large source altitudes are noted, together with an increasing angular size of sources with increasing angular distance from the sun's center. These and other observations furnish strong evidence for the theory that propagation effects, group delays, ducting and/or scattering significantly affect the observed heights, sizes, and brightness temperatures of interplanetary type III bursts. This would be true irrespective of whether the bursts are due to plasma radiation at the fundamental or at the harmonic, and the effects would extend to the arrival times of the radiation to a greater or lesser extent, depending on the path from the source to the observer.

Steinberg, J. L.↗