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Long term time variations of the suprathermal ions in Saturn's magnetosphere
Author(s) -
DiFabio R. D.,
Hamilton D. C.,
Krimigis S. M.,
Mitchell D. G.
Publication year - 2011
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2011gl048841
Subject(s) - magnetosphere , saturn , ion , physics , term (time) , astrobiology , astronomy , geophysics , planet , plasma , nuclear physics , quantum mechanics
We use the Charge‐Energy‐Mass Spectrometer (CHEMS) on Cassini to study long‐term time variations of the suprathermal (E/Q = 27–220 keV/e) ions in Saturn's equatorial ring current (Latitude = −10° to 10°, dipole L = 7–16) from the end of 2004 to the end of 2010. We identify five equatorial time periods, which vary in length from 124 to 282 days, and determine the average ion composition of the ring current for each period. The species examined are the water group W + (O + , OH + , H 2 O + , and H 3 O + ), H + , H 2 + , He ++ , He + , and O ++ . We find that the combined partial number density over this energy range varies less than a factor of two from a minimum of 1.3 × 10 −3 cm −3 to a maximum of 2.3 × 10 −3 cm −3 , implying that long‐term time variations in neutral source strength must be modest. The most abundant species (63% W + , 30% H + , 5% H 2 + ) show no long‐term trends. The H + /W + and H 2 + /W + ratios vary less than half as much as individual densities. The smaller variations in the ratios could be understood if all three species mostly originate from the Enceladus plumes either directly or via dissociation, but our results can accommodate differing neutral sources (currently thought to be Enceladus for W, Saturn's atmosphere for H, and Titan for H 2 ) if the suprathermal ion density variations are largely caused by changes in the overall acceleration rate of the thermal plasma. Both He ++ , originating from the solar wind, and He + , interplanetary pickup ions of interstellar origin, show a substantial decrease in 2009–2010 near solar minimum.