Ion-induced aerosol variations in the atmospheres of the outer planets

K.L. Aplin and R.G. Harrison

Charge effects on aerosols in planetary atmospheres can arise through the effects of cosmic ray ionisation. Galactic cosmic rays (GCR) vary inversely with the 11year (Schwabe) cycle of solar activity, hence an 11year periodicity in aerosol optical properties sensed remotely in a planetary atmosphere may indicate a charged aerosol influence from cosmic rays. However, an 11year cycle also exists in the solar ultraviolet part of the solar spectrum, which could also cause the observed variation. A 1.68year periodicity in terrestrial galactic cosmic ray (GCR) data can discriminate between these two different (GCR or UV) solar influences on atmospheric aerosol. GCR data from the Voyager spacecraft show this periodicity in the outer heliosphere, during flybys of Saturn, Uranus and Neptune in the 1980s. Coincident but irregular photometric observations of Neptune, Titan and Uranus were made from Earth, revealing that Neptune’s magnitude varied with solar activity. Using the Lomb periodogram to identify a 1.68 year periodicity in planetary brightness, GCR or UV mechanisms of solar influence can be separated. Temporal variations of the 1.68 year periodicities retrieved in Voyager 2 GCR and Neptune magnitude’s are well correlated, suggesting the an electrical effect on aerosol in Neptune’s atmosphere.

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