Saturn suprathermal O₂⁺ and mass‐28⁺ molecular ions: Long‐term seasonal and solar variation

Abstract

Suprathermal singly charged molecular ions, O2+ (at ~32 Da/e) and the Mass‐28 ion group 28M+ (ions at ~28 Da/e, with possible contributions from C2H5+, HCNH+, N2+, and/or CO+), are present throughout Saturn's ~4–20 Rs (1 Saturn radius, Rs = 60,268 km) near‐equatorial magnetosphere from mid‐2004 until mid‐2012. These ~83–167 keV/e heavy ions measured by Cassini's CHarge‐Energy‐Mass Spectrometer have long‐term temporal profiles that differ from each other and differ relative to the dominant water group ions, W+ (O+, OH+, H2O+, and H3O+). O2+/W+, initially ~0.05, declined steadily until equinox in mid‐2009 by a factor of ~6, and 28M+/W+, initially ~0.007, declined similarly until early‐2007 by a factor of ~2. The O2+/W+ decline is consistent with Cassini's in situ ring‐ionosphere thermal ion measurements, and with proposed and modeled seasonal photolysis of Saturn's rings for thermal O2 and O2+. The water ice‐dominated main rings and Enceladus plume depositions thereon are the two most likely O2+ sources. Enceladus' dynamic plumes, though, have no known long‐term dependence. After declining, O2+/W+ and 28M+/W+ levels remained low until late‐2011 when O2+/W+ increased, but 28M+/W+ did not. The O2+/W+ increase was steady and became statistically significant by mid‐2012, indicating a clear increase after a decline, that is, a possibly delayed O2+ “seasonal” recovery. Ring insolation is driven by solar UV flux which itself varies with the sun's 11 year activity cycle. The O2+/W+ and 28M+/W+ declines are consistent with seasonal ring insolation. No O2+/W+ response to the late‐2008 solar‐cycle UV minimum and recovery is evident. However, the O2+/W+ recovery from the postequinox baseline levels in late‐2011 coincided with a strong solar UV enhancement. We suggest a scenario/framework in which the O2+ observations can be understood.

Ongoing hydrothermal activities within Enceladus

“…for pH 5 9 and 10, respectively). The mass resolution of Cassini instruments cannot distinguish between HCNH 1 , CO 1 , N 2 1 or Si 1 , and therefore no solid conclusion can be drawn for the origin of the mass 28 ions at the current stage 52 . The sputtering yield of Si-water ice mixture is unknown.…”

“…Nonetheless, the presence of nanosilica particles and ice grains forming from hydrothermal fluids surely will supply the magnetosphere with silicon ions. Future modelling efforts should focus on the ionization, ion lifetime and acceleration processes that may be responsible for the enhanced ratio of 28 M 1 to water-group ions, (3-7) 3 10 23 , at the 100 keV energy level 52 . Sample size.…”

Seasonal and radial trends in Saturn’s thermal plasma between the main rings and Enceladus

“…We also showed that there was a steep drop in O 2 + density, as well as in the total heavy ion thermal plasma, over that time period. Consistent with this, the Magnetospheric Imagining Instrument (MIMI) recently reported a decrease from SOI through 2010 in energetic O 2 + density as compared to the W + density, followed by a recovery in 2011-2012 (Christon et al, 2013, submitted for publication). More recently a seasonal variation was reported for equatorial electron densities measured by the Radio and Plasma Wave Instrument (RPWS) .…”

“…4. Although there will be no new data from the CAPS instrument to follow the growth to northern solstice, this interpretation appears to be supported by MIMI observations of energetic heavy ions (Christon et al, 2013, submitted for publication) and by the electron data . However, with the limited number of orbits in the region from 2.4 to 3.8R S that have good data, other interpretations are possible and the relative importance of the ring atmosphere and solar activity needs to be explored further.…”

Energetic Radiation From Galactic Cosmic Ray Interactions With Saturn's Main Rings

“…Johnson et al () computed a photolytic flux of 10 6 /cm 2 s for neutral O 2 emission from the rings from solar ultraviolet irradiation but found the radiolytic flux from GCR interaction products to be an order of magnitude lower. However, strong seasonal variability up to several orders of magnitude (Christon et al, , ; Elrod et al, , ) of the magnetospheric O 2 + and other heavy ion population beyond the rings argues against the GCR energy source that varies only slightly over decadal time scales at energies above 20 GeV. Roussos et al () report long‐term changes of CRAND proton fluxes by less than a factor of 2 during the Cassini mission; the GCR flux at the ring neutron source for CRAND in the radiation belts would vary comparably.…”