Polarization of Impulsive Signals at Venus - Captions

Polarization of the Impulsive Signals Observed in the Nightside Ionosphere of Venus

R. J. Strangeway

Institute of Geophysics and Planetary Physics,
University of California at Los Angeles


J. Geophys. Res., 96, 22, 741-22, 752, 1991
(Received: June 11, 1991; accepted: October 1, 1991)
Copyright 1991 by the American Geophysical Union.
Paper Number 91JE02506.


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Figure Captions

   Fig. 1. Time series of the wave intensity as measured by the orbiter electric field detector. Six minutes of data acquired on orbit 526 are shown. Most of the impulsive signals occur in the 100-Hz channel only. The horizontal bars mark intervals for which polarization examples are given in Figure 2.
  Fig. 2. Three polarization plots for orbit 526. (a) and (b) Polarization of naturally occurring signals. (c) Polarization of an interference signal. Each plot is drawn using a loglo relative scale, as discussed in the text. The naturally occurring signals are polarized perpendicular to the average magnetic field.
  Fig. 3. Time series of wave intensity for orbit 501. Similar in format to Figure 1. On this orbit the impulsive signals are observed in all four channels.

  Fig. 4. Three polarization plots for orbit 501. Similar in format to Figure 2. (a) The maximum variance direction is more parallel to the magnetic field than perpendicular, while (b) the wave field is perpendicular. (c) The interference signal is greater than 45° to the magnetic field and would bias the statistics to perpendicular polarization if included in the analysis.

   Fig. 5. Scatterplot of the maximum variance direction as a function of orbit number for all the 30-s intervals in the season III nightside periapsis passes. There is a noticeable clustering of the phase angle as a function of orbit. Using a visual fit to the data, we have defined a phase angle filter as shown by the diagonal lines in the plot. The filter parameters are given at the bottom of the figure.
  Fig. 6. Phase angle histograms for all the season III data. The large histogram to the left shows the percent occurrence of different relative phases, while the two smaller histograms show the phase of the magnetic field and maximum variance directions. Auxiliary diagnostics are given at the right of the figure. The figure is described more fully in the text. The relative phase appears to be more perpendicular than parallel, although this is probably due to contamination by interference signals.
  Fig. 7. Phase statistics for the "cleaned" subset of the RvD&S data. The data are further restricted to those intervals for which the average magnetic field is sufficiently vertical to allow whistler mode propagation inside the resonance cone, assuming a subionospheric source. The 100-Hz waves that fall into this category are on the average polarized perpendicular to the ambient field as expected for whistler mode waves.
  Fig. 8. Phase statistics for the 30-kHz burst intervals in the RvD&S data set. In this case the relative phase is calculated using the instantaneous angle between the OEFD and antenna and the magnetic field direction in the spacecraft spin plane. Although there are relatively few intervals and the statistics are poor, the waves are mainly polarized parallel to the average field.
  Figure A1. Probability and angular error as a function of the test statistic. The probability is given by the single curve that approaches 100% for high values of the test statistic. The angular error depends on the degree of confidence desired, as indicated by the percentage labels. The horizontal dashed lines give the angular error for different confidence limits, assuming a test statistic that is 80% probable. In this paper we use 95% confidence limits when determining the error on the fit.


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