Engineering topics

Macdowall, R. J.

Publications and source records attributed to Macdowall, R. J..

16 records

NASA NTRS1995

Unusual wave activity near the interplanetary shocks

It has been reported that quasi-parallel (Q(sub parallel)) shocks are generally characterized by hours of upstream ion-acoustic-like wave activity and that quasi-perpendicular (Q(sub perpendicular)) shocks show only minimal upstream wave activity. However, we report Ulysses observations of several exceptions to these trends. These include: (1) quasi-perpendicular shocks with long durations of high-frequency ion-acoustic-like waves and Langmuir waves in the upstream region, and (2) quasi-parallel shocks with minimal wave activity in the upstream. These observations suggest that there may not be any systematic difference in the wave phenomena associated with Q(sub perpendicular) and Q(sub parallel) shocks. The wave activity (.5-5) kHz near the interplanetary shocks poorly correlates with the electron to ion temperature ratio, T(sub e)/T(sub i). We provide arguments in favor of electron acoustic waves instead of ion-acoustic waves.

Thejappa, G.↗

NASA NTRS1995

Ulysses radio observations of the heliospheric activity in early June 1991

The radio receiver of the URAP (Unified Radio and Plasma Wave) experiment on Ulysses has recorded a heliospheric activity particularly intense between late May and early June 1991. Many solar radio emissions of types III and II were observed together with interplanetary (IP) shocks. In the same time, the radio spectrograph ARTEMIS at Nancay (France) observed several intense type II bursts. We investigate the association and/or interaction of these radio emissions, which are remotely observed, with some IP shocks detected in situ, in the context of a Coronal Mass Ejection (CME) induced scenario.

Hoang, S.↗

NASA NTRS1994

Possible radio wave precursors associated with the comet Shoemaker-Levy 9/Jupiter impacts

We suggest that prior to its impact with Jupiter, comet Shoemaker-Levy 9 will behave as an electrical generator in the Jovian magnetosphere, converting planetary rotational energy to electrical energy via a dust/plasma interaction. This electrical energy will then be deposited in the dayside auroral region where it may drive various auroral phenomena including cyclotron radio emission. Such emission could be detected by spacecraft like Ulysses and Galileo many hours prior to the actual comet impact with the upper atmosphere. We apply the theory originally developed to explain the spokes in Saturn's rings. This theory allows us to quantify the driving potential associated with the comet and, consequently, to determine the radio power created in the auroral region. We conclude that if enough fine dust is present in the cometary system, comet-induced auroral radio emissions will reach detectable levels. This emission should be observable in the dayside hemisphere about 12-24 hours prior to each fragment impact.

Farrell, W. M.↗

NASA NTRS1993

Sounder stimulated D(sub n) resonances in Jupiter's Io plasma torus

On February 8, 1992, the Ulysses spacecraft passed through Jupiter's Io plasma torus, where rich spectra of narrow-band resonances were stimulated by the relaxation sounder of the Ulysses unified radio and plasma wave (URAP) instrument. Since the gyrofrequency f(sub g) is comparable to the plasma frequency f(sub p) in the Io torus, it was predicted that the general classification of stimulated ionospheric D(sub n) resonances, developed for 1 is less than or equal to f(sub p)/f(sub g) is less than or equal to 8 in the Earth's topside ionosphere, should apply in the Io torus as well as the Earth's magnetosphere (Osherovich, 1989). The URAP plasmagrams (sounder spectra) in the portions of the Io torus satisfying these plasma conditions are dominated by the D(sub n) resonances for frequencies below f(sub p). On most of these plasmagrams the f(sub p) resonance is also present, but it is seldom the dominant resonance. Neither upper hybrid nor nf(sub g) resonances have been found on these plasmagrams. The identification of D(sub n) resonances has allowed both the electron density and the magnetic field amplitude to be calculated. The derived densities on the outbound pass agree well with a Voyager model of Bagenal (1992). The derived magnetic field values are close to the Goddard Space Flight Center O(sub 6) magnetic field model.

Osherovich, V. A.↗

NASA NTRS1993

Ulysses observations of auroral hiss at high Jovian latitudes

During the Ulysses flyby of Jupiter, a whistler-mode emission was periodically detected by the unified Radio and Plasma wave (URAP) experiment during intervals when the spacecraft extended to high magnetic latitudes. The signal was detected between the local electron plasma frequency and lower hybrid resonance and appears as a funnel-shaped structure on frequency-versus-time spectrograms; these characteristics are very reminiscent of whistler-mode auroral hiss observed at high latitudes at Earth. Ray tracing of the emission occurrences suggests the emission source is on magnetic field lines extending out to at least 65 R(sub J). This location associates the emission with the boundary between open and closed field lines -- not the Io torus. The emission radiates about 10(exp 7) W of power. Consequently, the auroral input power derived from the solar wind to drive the emission is believed to be 10(exp 10-12) W (or about 1% of the energy associated with Io torus electrical processes).

Farrell, W. M.↗

NASA NTRS1992

Plasma wave phenomena observed at interplanetary shocks by the Ulysses URAP experiment

Results of a study of 24 interplanetary shocks observed by the Unified Radio and Plasma Wave Experiment (URAP) on the Ulysses spacecraft are presented. These shocks, observed between approximately 1 and 4 AU, display a variety of wave phenomena similar to those detected in earlier studies of shocks near 1 AU. The correspondence of the observed low frequency magnetic and electric field waves with the parallel index of refraction for whistler waves was investigated. Observed B/E ratios are found to be typically about a factor of 0.7 times the computed index of refraction, supporting the whistler interpretation of these waves, but also implying a prevalent electrostatic wave component which may be due to whistlers propagating at an angle to the interplanetary magnetic field. A statistical correlation of the amplitudes of the various types of waves with shock and solar wind properties is presented.

Lengyel-Frey, D.↗

NASA NTRS1992

Solar wind thermal electrons in the ecliptic plane between 1 and 4 AU - Preliminary results from the Ulysses radio receiver

The radio receiver of the Unified Radio and Plasma experiment aboard the Ulysses spacecraft records spectra of the quasi-thermal plasma noise. The interpretation of these spectra allows the determination of the total electron density Ne and of the cold (core) electron temperature Tc in the solar wind. A single power law does not fit the variations of Ne which result from the contribution from different solar wind structures. The distribution of the values of Tc suggests that, on the average, the solar wind is nearly isothermal.

Hoang, S.↗

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

Interplanetary fast shock diagnosis with the radio receiver on Ulysses

The radio receiver on Ulysses records the quasi-thermal noise which allows a determination of the density and temperature of the cold (core) electrons of the solar wind. Seven interplanetary fast forward or reverse shocks are identified from the density and temperature profiles, together with the magnetic field profile from the Magnetometer experiment. Upstream of the three strongest shocks, bursts of nonthermal waves are observed at the electron plasma frequency f(peu). The more perpendicular the shock, the longer the time interval during which these upstream bursts are observed. For one of the strongest shocks we also observe two kinds of upstream electromagnetic radiation: radiation at 2 f(peu), and radiation at the downstream electron plasma frequency, which propagates into the less dense upstream regions.

Hoang, S.↗

NASA NTRS1992

Plasma wave phenomena at interplanetary shocks observed by the Ulysses URAP experiment

We present Ulysses URAP observations of plasma waves at seven interplanetary shocks detected between approximately 1 and 3 AU. The URAP data allows ready correlation of wave phenomena from .1 Hz to 1 MHz. Wave phenomena observed in the shock vicinity include abrupt changes in the quasi-thermal noise continuum, Langmuir wave activity, ion acoustic noise, whistler waves and low frequency electrostatic waves. We focus on the forward/reverse shock pair of May 27, 1991 to demonstrate the characteristics of the URAP data.

Lengyel-Frey, D.↗

NASA NTRS1992

Remote radio observations of solar wind parameters upstream of planetary bow shocks

Radio emission is frequently produced at twice the electron plasma frequency 2fp in the foreshock region upstream of the terrestrial bow shock. Observations of this emission provide a remote diagnostic of solar wind parameters in the foreshock. Using ISEE-3 radio data, we present the first evidence that the radio intensity is proportional to the kinetic energy flux and to other parameters correlated with solar wind density. We provide a qualitative explanation of this intensity behavior and predict the detection of similar emission at Jupiter by the Ulysses spacecraft.

Macdowall, R. J.↗

NASA NTRS1990

Kilometric shock-associated events and microwave bursts

The peak times of impulsive microwaves bursts are compared with those of shock-associated (SA) kilometric radio events. The first peaks in these two frequency regimes are usually well-correlated in time, but the last peaks of the SA events observed at 1 MHz occur an average of 20 min after the last impulsive microwave peaks. In some cases, the SA events overlap in time with the post-burst increases of microwave bursts; sometimes there is general correspondence in their intensity time profiles. These observations suggest that the earlier components of the SA events are usually caused by electrons accelerated in or near the microwave source region. The possibility that the later components of some SA events could be associated with nonthermal electrons responsible for microwave post-burst increases, although they have traditionally been attributed to electrons accelerated at type II burst producing shocks in the upper corona is discussed.

Kundu, M. R.↗

NASA NTRS1989

Effects of interplanetary shocks on kilometric type III radio bursts

Interplanetary (IP) type-III bursts that undergo sudden intensity changes when their electron beams traverse the vicinity of an IP shock are examined. Three types of intensity changes are discussed: cutoffs in which the type-III intensity is abruptly reduced and remains at the reduced level for all lower frequencies, narrow-band intensifications that frequently occur on the high-frequency edge of a cutoff, and narrow-band intensity reductions. Pitch angle scattering of the beam electrons in the enhanced magnetic turbulence downstream of shocks is proposed as a principal cause of the intensity cutoffs and, possibly, the intensifications. These observations suggest that one type-III emission mode is frequently at least 10 times more intense than the other mode.

Macdowall, R. J.↗

NASA NTRS1987

Characteristics of shock-associated fast-drift kilometric radio bursts

The existence of a class of fast-drift, shock-associated (SA), kilometric radio bursts which occur at the time of metric type II emission and which are not entirely the kilometric continuation of metric type III bursts has been reported previously (Cane et al., 1981). In this paper unambiguous SA event criteria are established for the purpose of statistically comparing SA events with conventional kilometric type III bursts. Applying these criteria to all long-duration, fast-drift bursts observed by the ISEE-3 spacecraft during a 28-month interval, it is found that more than 70 percent of the events satisfying the criteria are associated with the radio signatures of coronal shocks. If a given event is associated with a metric type II or type IV burst, it is 13 times more likely to satisfy the SA criteria than an event associated only with metric type III activity.

Macdowall, R. J.↗

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.↗