Engineering topics

Gallagher, D. L.

Publications and source records attributed to Gallagher, D. L..

131 records · Page 8

NASA NTRS1985

Short-wavelength electrostatic waves in the earth's magnetosheath

Recent observations with the ISEE-1 spacecraft have found electric field emissions in the dayside magnetosheath whose frequency spectrum is modulated at twice the spacecraft spin period. The upper frequency cut-off in the frequency-time spectrum of the emissions has a characteristic parabola shape or festoon shape. The low frequency cutoff ranges from 100 Hz to 400 Hz, while the high frequency limit ranges from about 1 kHz to 4 kHz. The bandwidth is found to minimize for antenna orientations parallel to the wave vectors. The wave vector does not appear to be related to either the local magnetic field direction of the plasma flow velocity. The spacecraft observed frequency spectrum results from the spacecraft antenna response to the Doppler shifted wave vector spectrum which exists in the plasma. Imposed constraints on the plasma restframe wave vectors and frequencies indicate that the emissions occur within the frequency range from about 150 Hz to 1 kHz, with wavelengths between about 30 meters and 600 meters. These constraints strongly suggest that the festoon-shaped emissions are ion-acoustic waves. The small group velocity and k direction of the ion-acoustic mode are consistent with wave generation upstream at the bow shock and convection downstream to locations within the outer dayside magnetosheath.

Gallagher, D. L.↗

NASA NTRS1982

Chorus-related electrostatic bursts in the earth's outer magnetosphere

Studies of wideband plasma wave data from the ISEE 1 and ISEE 2 spacecraft are reported which reveal that whistler-mode chorus emissions in the earth's outer magnetosphere are often accompanied by high-frequency bursts of electrostatic noise. The chorus features that correlate to the electrostatic bursts have a hook-like frequency-time variation in the frequency range 200-400 Hz, and the associated electrostatic bursts have a frequency range of about 4-8 kHz. In some cases these electrostatic bursts have a harmonic frequency structure with a frequency spacing corresponding to the chorus frequency. This harmonic structure apparently results from the fact that the burst intensity is modulated at the chorus frequency.

Reinleitner, L. A.↗

NASA NTRS1981

Correlations between solar wind parameters and auroral kilometric radiation intensity

The relationship between solar wind properties and the influx of energy into the nightside auroral region as indicated by the intensity of auroral kilometric radiation is investigated. Smoothed Hawkeye satellite observations of auroral radiation at 178, 100 and 56.2 kHz for days 160 through 365 of 1974 are compared with solar wind data from the composite Solar Wind Plasma Data Set, most of which was supplied by the IMP-8 spacecraft. Correlations are made between smoothed daily averages of solar wind ion density, bulk flow speed, total IMF strength, electric field, solar wind speed in the southward direction, solar wind speed multiplied by total IMF strength, the substorm parameter epsilon and the Kp index. The greatest correlation is found between solar wind bulk flow speed and auroral radiation intensity, with a linear correlation coefficient of 0.78 for the 203 daily averages examined. A possible mechanism for the relationship may be related to the propagation into the nightside magnetosphere of low-frequency long-wavelength electrostatic waves produced in the magnetosheath by the solar wind.

Gallagher, D. L.↗

NASA NTRS1981

Parametric interaction and spatial collapse of beam-driven Langmuir waves in the solar wind

Observations are presented of the parametric decay and spatial collapse of Langmuir waves driven by an electron beam streaming into the solar wind from the Jovian bow shock. Long wavelength Langmuir waves upstream of the bow shock are effectively converted into short wavelength waves no longer in resonance with the beam. The conversion is shown to be the result of a nonlinear interaction involving the beam-driven pump, a sideband emission, and a low level of ion-acoustic turbulence. The beam-driven Langmuir wave emission breaks up into a complex sideband structure with both positive and negative Doppler shifts. In some cases, the sideband emission consists of isolated wave packets with very short duration bursts, which are very intense and are thought to consist of envelope solitons which have collapsed to spatial scales of only a few Debye lengths.

Gurnett, D. A.↗

NASA NTRS1979

Auroral kilometric radiation - Time-averaged source location

In the present paper, the location of the average generation region of auroral kilometric radiation is determined from an analysis of average electric field strengths as a function of spacecraft position in narrow frequency bands centered at 178, 100, and 56.2 kHz. Intense sources of power radiating at kilometric wavelengths are found near 65 degrees invariant latitude in the Northern and Southern Hemispheres, from 22 to 24 hours MLT, and between 2 and 4 earth radii. These dominant sources of power produce the electromagnetic radiation described by Kurth et al. (1975) as auroral kilometric radiation. Each Northern and Southern Hemisphere auroral kilometric radiation source is well confined and emits radiation into solid angles of about 4.1 sr at 178 kHz, 2.2 sr at 100 kHz, and 1.5 sr at 56.2 kHz.

Gallagher, D. L.↗