Radio observation of Venus at meter wavelengths using the GMRT

Abstract

The Venusian surface has been studied by measuring radar reflections and thermal radio emission over a wide spectral region of several centimeters to meter wavelengths from the Earth-based as well as orbiter platforms. The radiometric observations, in the decimeter (dcm) wavelength regime showed a decreasing trend in the observed brightness temperature (T b ) with increasing wavelength. The thermal emission models available at present have not been able to explain the radiometric observations at longer wavelength (dcm) to a satisfactory level. This paper reports the first interferometric imaging observations of Venus below 620 MHz. They were carried out at 606, 332.9 and 239.9 MHz using the Giant Meterwave Radio Telescope (GMRT). The T b values derived at the respective frequencies are 526 K, 409 K and < 426 K, with errors of ∼7% which are generally consistent with the reported T b values at 608 MHz and 430 MHz by previous investigators, but are much lower than those derived from high-frequency observations at 1.38-22.46 GHz using the VLA.

Venus Atmospheric Thermal Structure and Radiative Balance

“…Thermal infrared measurements of the Venus upper atmosphere from Earth based telescopes have also been obtained in recent years (Clancy et al 2012;Sato et al 2014;. Venus has been observed at meter wavelengths also recently (Mohan et al 2017) extending the radio frequency coverage.…”

Polarization and brightness temperature observations of Venus with the GMRT

“…The values of the brightness temperature, T b , increases from ∼505 K at 1.3 cm to 680 K at 6.2 cm and then decreases at longer wavelengths (Butler et al 2001). The recent radiometric observation at higher dcm/m wavelength observations using the GMRT reported T b values of 526±22 K, 409±33 K, and <426 K at 49 cm, 90 cm and 1.23 m, respectively, which indicate further decrease in the observed T b (Mohan et al 2017).…”

“…Full polarimetric data was recorded for the two higher frequencies, and only total intensity (Stokes I) data was recorded for 233.67 MHz as the polarised emission at this frequency is insignificant compared to the noise contributed by the sky background and the electronics system. The observation procedure described by Mohan et al (2017) was adopted for the current observations. Instead of tracking Venus, whose right ascension (RA) and declination (Dec) varied during the observations, the GMRT antennas tracked the mean position of its RA and Dec. For weak non-sidereal sources, this is the preferred strategy as it allows one to use the background celestial sources present in the field of view for self-calibration (Cornwell & Fomalont 1999).…”

“…Except for the polarisation calibration, the procedures involved and followed in the data analyses are detailed in Mohan et al (2017). Brief discussions of these procedures are given in this Section for completeness.…”

“…The uncertainty in the estimated flux density is contributed by -pointing errors, temporal variation of the gain of the system and the errors due to uncertainties in flux values of the calibrators due to flux density scale used. The detailed error estimation is presented in Mohan et al (2017). All of the above factors together can lead to systematic error of about 7% in the measured flux density of Venus.…”