The Experts below are selected from a list of 8634 Experts worldwide ranked by ideXlab platform
Gregory Provan - One of the best experts on this subject based on the ideXlab platform.
-
planetary period oscillations in saturn s magnetosphere comparison of magnetic oscillations and skr modulations in the postEquinox interval
Journal of Geophysical Research, 2014Co-Authors: Gregory Provan, S W H Cowley, L Lamy, M K DoughertyAbstract:We compare the properties of planetary period oscillations observed in Saturn kilometric radiation (SKR) and magnetospheric magnetic field data from Saturn Equinox in August 2009 to July 2013. As shown previously, the southern and northern oscillation periods converged across Equinox from ~10.8 h and ~10.6 h, respectively, during southern summer, to closely common values ~10.7 h approximately 1 year after Equinox. Near coalescence is judged to have occurred approximately 3 months earlier in the SKR data, centered in late June 2010, than in the magnetic data, in late September, though SKR periods were particularly difficult to determine during this interval due to less clearly modulated emissions. Both data sets agree, however, that by early November 2010 the two periods had separated again but remained closely spaced with a difference in period of ~3 min about a mean of ~10.67 h, with the southern period remaining longer than the northern. Thus, no enduring reversal of the northern and southern periods took place following near coalescence in mid-2010, the periods remaining uncrossed to the end of the interval studied here. The SKR modulations also show effects related to the sharp amplitude changes observed in the magnetic oscillation data at ~100–200 day intervals since February 2011, though the correspondences are not exact, indicating that other factors such as “seeing” effects on the variable Cassini orbit are also involved. PostEquinox variations in the relative phase between the magnetic and SKR oscillations are also shown to be related to changes in orbit apoapsis orientation.
-
planetary period magnetic field oscillations in saturn s magnetosphere postEquinox abrupt nonmonotonic transitions to northern system dominance
Journal of Geophysical Research, 2013Co-Authors: Gregory Provan, D J Andrews, S W H Cowley, J K Sandhu, M K DoughertyAbstract:[1] We examine the “planetary period” magnetic field oscillations observed in the “core” region of Saturn's magnetosphere (dipole L ≤ 12), on 56 near-equatorial Cassini periapsis passes that took place between vernal Equinox in August 2009 and November 2012. Previous studies have shown that these consist of the sum of two oscillations related to the northern and southern polar regions having differing amplitudes and periods that had reached near-equal amplitudes and near-converged periods ~10.68 h in the interval to ~1 year after Equinox. The present analysis shows that an interval of strongly differing behavior then began ~1.5 years after Equinox, in which abrupt changes in properties took place at ~6- to 8-month intervals, with three clear transitions occurring in February 2011, August 2011, and April 2012, respectively. These are characterized by large simultaneous changes in the amplitudes of the two systems, together with small changes in period about otherwise near-constant values of ~10.63 h for the northern system and ~10.69 h for the southern (thus, not reversed postEquinox) and on occasion jumps in phase. The first transition produced a resumption of strong southern system dominance unexpected under northern spring conditions, while the second introduced comparably strong northern system dominance for the first time in these data. The third resulted in suppression of all core oscillations followed by re-emergence of both systems on a time scale of ~85 days, with the northern system remaining dominant but not as strongly as before. This behavior poses interesting questions for presently proposed theoretical scenarios.
-
Saturn's magnetospheric planetary period oscillations, neutral atmosphere circulation, and thunderstorm activity: Implications, or otherwise, for physical links
Journal of Geophysical Research: Space Physics, 2013Co-Authors: Stanley W. H. Cowley, Gregory ProvanAbstract:Suggestions that the planetary period oscillations (PPOs) observed in Saturn's magnetosphere may be driven or influenced by neutral atmospheric perturbations motivate an exploratory comparison of PPO rotation periods with available tropospheric and stratospheric determinations. Nonpolar atmospheric rotation periods occupy the range ~10.2–10.7 h associated with the latitudinal jet structure, are similar north and south, and are independent of season, while PPO periods lie in a narrower partly overlapping range ~10.6–10.8 h, are persistently shorter north than south, and undergo a seasonal cycle. In this cycle, widely separated north-south PPO periods during southern summer converge across Equinox to values lying within the atmospheric west jet band, remaining well-separated from east jet periods. Closest convergence occurred 1 year post Equinox, contemporaneously with the switch in seasonal thunderstorm activity from Southern to Northern Hemispheres. Since most large-scale atmospheric phenomena are related to the west jets, rotating with closely similar periods, they also rotate with periods close to the PPOs under post equinoctial conditions but not otherwise. Specifically, post Equinox northern PPOs rotate with a period close to the southern thunderstorms, as well as the north polar spot and hexagon features, while the post Equinox southern PPOs rotate with a period close to the pre-Equinox northern “string of pearls” and the first colocated post Equinox northern thunderstorm, the Great White Spot event. However, even under these conditions, no consistent correspondences in period are found at a detailed level, which taken together with the lack of correspondence at other times does not suggest a direct physical link exists between these phenomena.
-
planetary period oscillations in saturn s magnetosphere evolution of magnetic oscillation properties from southern summer to post Equinox
Journal of Geophysical Research, 2012Co-Authors: D J Andrews, Gregory Provan, S W H Cowley, M K Dougherty, L Lamy, D J SouthwoodAbstract:We investigate the evolution of the properties of planetary period magnetic field oscillations observed by the Cassini spacecraft in Saturn's magnetosphere over the interval from late 2004 to early 2011, spanning Equinox in mid-2009. Oscillations within the inner quasi-dipolar region (L <= 12) consist of two components of close but distinct periods, corresponding essentially to the periods of the northern and southern Saturn kilometric radiation (SKR) modulations. These give rise to modulations of the combined amplitude and phase at the beat period of the two oscillations, from which the individual oscillation amplitudes and phases (and hence periods) can be determined. Phases are also determined from northern and southern polar oscillation data when available. Results indicate that the southern-period amplitude declines modestly over this interval, while the northern-period amplitude approximately doubles to become comparable with the southern-period oscillations during the Equinox interval, producing clear effects in pass-to-pass oscillation properties. It is also shown that the periods of the two oscillations strongly converge over the Equinox interval, such that the beat period increases significantly from similar to 20 to more than 100 days, but that they do not coalesce or cross during the interval investigated, contrary to recent reports of the behavior of the SKR periods. Examination of polar oscillation data for similar beat phase effects yields a null result within a similar to 10% upper limit on the relative amplitude of northern-period oscillations in the south and vice versa. This result strongly suggests a polar origin for the two oscillation periods.
M K Dougherty - One of the best experts on this subject based on the ideXlab platform.
-
planetary period oscillations in saturn s magnetosphere comparison of magnetic oscillations and skr modulations in the postEquinox interval
Journal of Geophysical Research, 2014Co-Authors: Gregory Provan, S W H Cowley, L Lamy, M K DoughertyAbstract:We compare the properties of planetary period oscillations observed in Saturn kilometric radiation (SKR) and magnetospheric magnetic field data from Saturn Equinox in August 2009 to July 2013. As shown previously, the southern and northern oscillation periods converged across Equinox from ~10.8 h and ~10.6 h, respectively, during southern summer, to closely common values ~10.7 h approximately 1 year after Equinox. Near coalescence is judged to have occurred approximately 3 months earlier in the SKR data, centered in late June 2010, than in the magnetic data, in late September, though SKR periods were particularly difficult to determine during this interval due to less clearly modulated emissions. Both data sets agree, however, that by early November 2010 the two periods had separated again but remained closely spaced with a difference in period of ~3 min about a mean of ~10.67 h, with the southern period remaining longer than the northern. Thus, no enduring reversal of the northern and southern periods took place following near coalescence in mid-2010, the periods remaining uncrossed to the end of the interval studied here. The SKR modulations also show effects related to the sharp amplitude changes observed in the magnetic oscillation data at ~100–200 day intervals since February 2011, though the correspondences are not exact, indicating that other factors such as “seeing” effects on the variable Cassini orbit are also involved. PostEquinox variations in the relative phase between the magnetic and SKR oscillations are also shown to be related to changes in orbit apoapsis orientation.
-
planetary period magnetic field oscillations in saturn s magnetosphere postEquinox abrupt nonmonotonic transitions to northern system dominance
Journal of Geophysical Research, 2013Co-Authors: Gregory Provan, D J Andrews, S W H Cowley, J K Sandhu, M K DoughertyAbstract:[1] We examine the “planetary period” magnetic field oscillations observed in the “core” region of Saturn's magnetosphere (dipole L ≤ 12), on 56 near-equatorial Cassini periapsis passes that took place between vernal Equinox in August 2009 and November 2012. Previous studies have shown that these consist of the sum of two oscillations related to the northern and southern polar regions having differing amplitudes and periods that had reached near-equal amplitudes and near-converged periods ~10.68 h in the interval to ~1 year after Equinox. The present analysis shows that an interval of strongly differing behavior then began ~1.5 years after Equinox, in which abrupt changes in properties took place at ~6- to 8-month intervals, with three clear transitions occurring in February 2011, August 2011, and April 2012, respectively. These are characterized by large simultaneous changes in the amplitudes of the two systems, together with small changes in period about otherwise near-constant values of ~10.63 h for the northern system and ~10.69 h for the southern (thus, not reversed postEquinox) and on occasion jumps in phase. The first transition produced a resumption of strong southern system dominance unexpected under northern spring conditions, while the second introduced comparably strong northern system dominance for the first time in these data. The third resulted in suppression of all core oscillations followed by re-emergence of both systems on a time scale of ~85 days, with the northern system remaining dominant but not as strongly as before. This behavior poses interesting questions for presently proposed theoretical scenarios.
-
planetary period oscillations in saturn s magnetosphere evolution of magnetic oscillation properties from southern summer to post Equinox
Journal of Geophysical Research, 2012Co-Authors: D J Andrews, Gregory Provan, S W H Cowley, M K Dougherty, L Lamy, D J SouthwoodAbstract:We investigate the evolution of the properties of planetary period magnetic field oscillations observed by the Cassini spacecraft in Saturn's magnetosphere over the interval from late 2004 to early 2011, spanning Equinox in mid-2009. Oscillations within the inner quasi-dipolar region (L <= 12) consist of two components of close but distinct periods, corresponding essentially to the periods of the northern and southern Saturn kilometric radiation (SKR) modulations. These give rise to modulations of the combined amplitude and phase at the beat period of the two oscillations, from which the individual oscillation amplitudes and phases (and hence periods) can be determined. Phases are also determined from northern and southern polar oscillation data when available. Results indicate that the southern-period amplitude declines modestly over this interval, while the northern-period amplitude approximately doubles to become comparable with the southern-period oscillations during the Equinox interval, producing clear effects in pass-to-pass oscillation properties. It is also shown that the periods of the two oscillations strongly converge over the Equinox interval, such that the beat period increases significantly from similar to 20 to more than 100 days, but that they do not coalesce or cross during the interval investigated, contrary to recent reports of the behavior of the SKR periods. Examination of polar oscillation data for similar beat phase effects yields a null result within a similar to 10% upper limit on the relative amplitude of northern-period oscillations in the south and vice versa. This result strongly suggests a polar origin for the two oscillation periods.
S W H Cowley - One of the best experts on this subject based on the ideXlab platform.
-
planetary period oscillations in saturn s magnetosphere comparison of magnetic oscillations and skr modulations in the postEquinox interval
Journal of Geophysical Research, 2014Co-Authors: Gregory Provan, S W H Cowley, L Lamy, M K DoughertyAbstract:We compare the properties of planetary period oscillations observed in Saturn kilometric radiation (SKR) and magnetospheric magnetic field data from Saturn Equinox in August 2009 to July 2013. As shown previously, the southern and northern oscillation periods converged across Equinox from ~10.8 h and ~10.6 h, respectively, during southern summer, to closely common values ~10.7 h approximately 1 year after Equinox. Near coalescence is judged to have occurred approximately 3 months earlier in the SKR data, centered in late June 2010, than in the magnetic data, in late September, though SKR periods were particularly difficult to determine during this interval due to less clearly modulated emissions. Both data sets agree, however, that by early November 2010 the two periods had separated again but remained closely spaced with a difference in period of ~3 min about a mean of ~10.67 h, with the southern period remaining longer than the northern. Thus, no enduring reversal of the northern and southern periods took place following near coalescence in mid-2010, the periods remaining uncrossed to the end of the interval studied here. The SKR modulations also show effects related to the sharp amplitude changes observed in the magnetic oscillation data at ~100–200 day intervals since February 2011, though the correspondences are not exact, indicating that other factors such as “seeing” effects on the variable Cassini orbit are also involved. PostEquinox variations in the relative phase between the magnetic and SKR oscillations are also shown to be related to changes in orbit apoapsis orientation.
-
planetary period magnetic field oscillations in saturn s magnetosphere postEquinox abrupt nonmonotonic transitions to northern system dominance
Journal of Geophysical Research, 2013Co-Authors: Gregory Provan, D J Andrews, S W H Cowley, J K Sandhu, M K DoughertyAbstract:[1] We examine the “planetary period” magnetic field oscillations observed in the “core” region of Saturn's magnetosphere (dipole L ≤ 12), on 56 near-equatorial Cassini periapsis passes that took place between vernal Equinox in August 2009 and November 2012. Previous studies have shown that these consist of the sum of two oscillations related to the northern and southern polar regions having differing amplitudes and periods that had reached near-equal amplitudes and near-converged periods ~10.68 h in the interval to ~1 year after Equinox. The present analysis shows that an interval of strongly differing behavior then began ~1.5 years after Equinox, in which abrupt changes in properties took place at ~6- to 8-month intervals, with three clear transitions occurring in February 2011, August 2011, and April 2012, respectively. These are characterized by large simultaneous changes in the amplitudes of the two systems, together with small changes in period about otherwise near-constant values of ~10.63 h for the northern system and ~10.69 h for the southern (thus, not reversed postEquinox) and on occasion jumps in phase. The first transition produced a resumption of strong southern system dominance unexpected under northern spring conditions, while the second introduced comparably strong northern system dominance for the first time in these data. The third resulted in suppression of all core oscillations followed by re-emergence of both systems on a time scale of ~85 days, with the northern system remaining dominant but not as strongly as before. This behavior poses interesting questions for presently proposed theoretical scenarios.
-
planetary period oscillations in saturn s magnetosphere evolution of magnetic oscillation properties from southern summer to post Equinox
Journal of Geophysical Research, 2012Co-Authors: D J Andrews, Gregory Provan, S W H Cowley, M K Dougherty, L Lamy, D J SouthwoodAbstract:We investigate the evolution of the properties of planetary period magnetic field oscillations observed by the Cassini spacecraft in Saturn's magnetosphere over the interval from late 2004 to early 2011, spanning Equinox in mid-2009. Oscillations within the inner quasi-dipolar region (L <= 12) consist of two components of close but distinct periods, corresponding essentially to the periods of the northern and southern Saturn kilometric radiation (SKR) modulations. These give rise to modulations of the combined amplitude and phase at the beat period of the two oscillations, from which the individual oscillation amplitudes and phases (and hence periods) can be determined. Phases are also determined from northern and southern polar oscillation data when available. Results indicate that the southern-period amplitude declines modestly over this interval, while the northern-period amplitude approximately doubles to become comparable with the southern-period oscillations during the Equinox interval, producing clear effects in pass-to-pass oscillation properties. It is also shown that the periods of the two oscillations strongly converge over the Equinox interval, such that the beat period increases significantly from similar to 20 to more than 100 days, but that they do not coalesce or cross during the interval investigated, contrary to recent reports of the behavior of the SKR periods. Examination of polar oscillation data for similar beat phase effects yields a null result within a similar to 10% upper limit on the relative amplitude of northern-period oscillations in the south and vice versa. This result strongly suggests a polar origin for the two oscillation periods.
Kathrin Altwegg - One of the best experts on this subject based on the ideXlab platform.
-
comparison of neutral outgassing of comet 67p churyumov gerasimenko inbound and outbound beyond 3 au from rosina dfms
Astronomy and Astrophysics, 2019Co-Authors: A Luspaykuti, Kathrin Altwegg, J J Berthelier, A Beth, Frederik Dhooghe, B Fiethe, S A Fuselier, T I Gombosi, K C Hansen, Myrtha HassigAbstract:Context. Pre-Equinox measurements of comet 67P/Churyumov-Gerasimenko with the mass spectrometer ROSINA/DFMS on board the Rosetta spacecraft revealed a strongly heterogeneous coma. The abundances of major and various minor volatile species were found to depend on the latitude and longitude of the nadir point of the spacecraft. The observed time variability of coma species remained consistent for about three months up to Equinox. The chemical variability could be generally interpreted in terms of surface temperature and seasonal effects superposed on some kind of chemical heterogeneity of the nucleus. Aims. We compare here pre-Equinox (inbound) ROSINA/DFMS measurements from 2014 to measurements taken after the outbound Equinox in 2016, both at heliocentric distances larger than 3 AU. For a direct comparison we limit our observations to the southern hemisphere. Methods. We report the similarities and differences in the concentrations and time variability of neutral species under similar insolation conditions (heliocentric distance and season) pre- and post-Equinox, and interpret them in light of the previously published observations. In addition, we extend both the pre- and post-Equinox analysis by comparing species concentrations with a mixture of CO₂ and H₂O. Results. Our results show significant changes in the abundances of neutral species in the coma from pre- to post-Equinox that are indicative of seasonally driven nucleus heterogeneity. Conclusions. The observed pre- and post-Equinox patterns can generally be explained by the strong erosion in the southern hemisphere that moves volatile-rich layers near the surface.
-
direct simulation monte carlo modelling of the major species in the coma of comet 67p churyumov gerasimenko
Monthly Notices of the Royal Astronomical Society, 2016Co-Authors: N Fougere, Kathrin Altwegg, Jeanjacques Berthelier, Andre Bieler, Dominique Bockeleemorvan, Ursina Calmonte, F Capaccioni, Michael R Combi, J De Keyser, V DeboutAbstract:We analyze the ROSINA-DFMS data between August 2014 and February 2016 to examine the effect of seasonal variations on the four major species within the coma of 67P/Churyumov-Gerasimenko (H2O, CO2, CO, and O2), resulting from the tilt in the orientation of the comet's spin axis. Using a numerical data inversion, we derive the nonuniform activity distribution at the surface of the nucleus for these species, suggesting that the activity distribution at the surface of the nucleus has not significantly changed and that the differences observed in the coma are solely due to the variations in illumination conditions. A 3D DSMC model is applied where the boundary conditions are computed with a coupling of the surface activity distributions and the local illumination. The model is able to reproduce the evolution of the densities observed by ROSINA including the changes happening at Equinox. While O2 stays correlated with H2O as it was before Equinox, CO2 and CO, which had a poor correlation with respect to H2O pre-Equinox, also became well correlated with H2O post-Equinox. The integration of the densities from the model along the line of sight results in column densities directly comparable to the VIRTIS-H observations. Also, the evolution of the volatiles’ production rates is derived from the coma model showing a steepening in the production rate curves after Equinox. The model/data comparison suggests that the seasonal effects result in the northern hemisphere of 67P's nucleus being more processed with a layered structure while the southern hemisphere constantly exposes new material.
Myrtha Hassig - One of the best experts on this subject based on the ideXlab platform.
-
comparison of neutral outgassing of comet 67p churyumov gerasimenko inbound and outbound beyond 3 au from rosina dfms
Astronomy and Astrophysics, 2019Co-Authors: A Luspaykuti, Kathrin Altwegg, J J Berthelier, A Beth, Frederik Dhooghe, B Fiethe, S A Fuselier, T I Gombosi, K C Hansen, Myrtha HassigAbstract:Context. Pre-Equinox measurements of comet 67P/Churyumov-Gerasimenko with the mass spectrometer ROSINA/DFMS on board the Rosetta spacecraft revealed a strongly heterogeneous coma. The abundances of major and various minor volatile species were found to depend on the latitude and longitude of the nadir point of the spacecraft. The observed time variability of coma species remained consistent for about three months up to Equinox. The chemical variability could be generally interpreted in terms of surface temperature and seasonal effects superposed on some kind of chemical heterogeneity of the nucleus. Aims. We compare here pre-Equinox (inbound) ROSINA/DFMS measurements from 2014 to measurements taken after the outbound Equinox in 2016, both at heliocentric distances larger than 3 AU. For a direct comparison we limit our observations to the southern hemisphere. Methods. We report the similarities and differences in the concentrations and time variability of neutral species under similar insolation conditions (heliocentric distance and season) pre- and post-Equinox, and interpret them in light of the previously published observations. In addition, we extend both the pre- and post-Equinox analysis by comparing species concentrations with a mixture of CO₂ and H₂O. Results. Our results show significant changes in the abundances of neutral species in the coma from pre- to post-Equinox that are indicative of seasonally driven nucleus heterogeneity. Conclusions. The observed pre- and post-Equinox patterns can generally be explained by the strong erosion in the southern hemisphere that moves volatile-rich layers near the surface.