Radio wave propagation below the Jovian ionosphere

The lightning and radio emission detector (LRD) instrument on board the Galileo probe measured the spectral intensity and other characteristics of radio frequency (RF) signals during descent. The measured spectra show a frequency dependent height profile with a maximum at the beginning of descent around the 1 bar level and a minimum around the 3–6 bar level [Rinnert et al., 1998]. The RF propagation is modeled assuming a source region in the water cloud level between 0 and −25 km altitude, an ionospheric reflector above 300 km, and a primarily absorbing interior below about −2000 km altitude (zero altitude is at 1 bar). This paper shows that the LRD observations can be described by an electrical discharge source at a distance of about 15,000 km; we conclude that no sources existed within the equatorial clouds inside this approximate radius around the probe. We also conclude from the propagation modeling that the measured spectral frequency dependence can be interpreted in terms of a source spectrum that is dominated by low frequencies.