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Session 116 - The Solar System.
Oral session, Thursday, January 18
Salon del Rey Central, Hilton
It has long been known that the occurrence probability per apparition of the Jovian decametric radiation observed from Earth undergoes a cyclic variation having a period on the order of a decade. Although it had first been suggested that the occurrence probability was inversely correlated with the smoothed sunspot number, Carr et al. (Radio Science, 5, 495, 1970) demonstrated that the correlation with the Jovicentric declination of Earth (D_E) is much closer. (The period of the D_E variation is 12 years, while that of the sunspot number is about 11 years on the average.) In the 25 years since the Carr et al. paper, no further observational studies of the long-term periodicity in occurrence probability have been reported. In this paper we present preliminary results of a much more thorough investigation of the long-term occurrence probability variation based on the analysis of some 40 years of Jovian decametric observations from the University of Florida Radio Observatory and 10 years of observations from the Maipu Radio Astronomy Observatory in Chile. It appears that the occurrence probability per apparition is correlated jointly with D_E and sunspot number. The decametric activity almost disappears during apparitions for which either D_E is less than about -2.5 (its range is from approximately -3.3 to +3.4), or the sunspot number is greater than about 100. Our empirical model based on both D_E and sunspot number provides a good fit to the observed occurrence probability data. We suggest that 1) the D_E control is a geometrical effect resulting from the sharp lower edge of an emission beam that corotates with the source (Earth remaining just outside the beam for the most negative values of D_E but not for the more positive ones), and 2) the observed decrease in occurrence probability with increasing sunspot number results from the increased ionization in the source region (from solar UV) which increases the ratio f_p/f_c toward the limit of about 0.1 (this is the limit at which the cyclotron maser radio emission mechanism ceases to operate). There are other possible explanations for the latter effect.
