Prediction of background levels for the Wind/WAVES instrument and implications for the galactic background radiation

Details Prediction of background levels for the Wind/WAVES instrument and implications for the galactic background radiation Prediction of background levels for the Wind/WAVES instrument and implications for the galactic background radiation

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsh13b1536h&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #SH13B-1536

Astronomy and Astrophysics

Astrophysics

[0609] Electromagnetics / Antennas, [2159] Interplanetary Physics / Plasma Waves And Turbulence, [6994] Radio Science / Instruments And Techniques, [7534] Solar Physics, Astrophysics, And Astronomy / Radio Emissions

Scientific paper

We investigate and predict the observed background levels for the WAVES instrument onboard the spacecraft Wind. Estimates for the receiver noise and capacitive transfer functions for the three receivers (TNR, RAD1 and RAD2) and three antennas (X, Y, and Z) are obtained, allowing detailed theory/data comparisons. With the TNR receiver connected separately to a single antenna (SEP mode), the predicted backgrounds agree to within 20% or better across all frequencies when modelled using a two component model for the quasithermal noise (QTN) at low frequencies. Calibrating the RAD1 observations against TNR allows us to calculate the relative (RAD1 versus TNR) receiver gain of GR1=1.43 ± 0.18 using the SEP mode data. Using the SUM mode data (connected to X and Z antennas) the ratio of antenna gains is found to be R=6.5 in agreement with pre-flight measurements. Using these values, the predicted backgrounds for the QTN and galactic background model of Novaco and Brown [1978] are also found to agree to within 20% or better for the RAD1 receiver in both SUM and SEP modes. Furthermore the observed differences in the background levels at low frequencies between the SEP and SUM modes of operation are explained for the first time by combining the expected QTN from two antennas of different lengths. The RAD2 data are calibrated against RAD1 data near 1 MHz, itself calibrated against TNR data, yielding a receiver gain of GR2=2.5 ± 0.3 relative to TNR. The differences between the predicted and observed galactic background spectra are used to estimate the effective antenna lengths, which are found to be between the physical antenna lengths and the Hansen [1981] prediction of √(2/3) times the physical lengths for the X and Y antennas. Our analyses are also consistent with the Novaco and Brown [1978] fit to the galactic background, which decreases much faster than the Cane [1979] spectrum. We describe a method for converting a radio signal of interest into absolute flux units using our combined model for QTN and galactic background radiation. This model will be useful for extracting type II and III bursts for theory/data comparisons.

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