TY - JOUR A1 - Buratti, Bonnie J. A1 - Thomas, P. C. A1 - Roussos, Elias A1 - Howett, Carly A1 - Seiss, Martin A1 - Hendrix, A. R. A1 - Helfenstein, Paul A1 - Brown, R. H. A1 - Clark, R. N. A1 - Denk, Tilmann A1 - Filacchione, Gianrico A1 - Hoffmann, Holger A1 - Jones, Geraint H. A1 - Khawaja, N. A1 - Kollmann, Peter A1 - Krupp, Norbert A1 - Lunine, Jonathan A1 - Momary, T. W. A1 - Paranicas, Christopher A1 - Postberg, Frank A1 - Sachse, Manuel A1 - Spahn, Frank A1 - Spencer, John A1 - Srama, Ralf A1 - Albin, T. A1 - Baines, K. H. A1 - Ciarniello, Mauro A1 - Economou, Thanasis A1 - Hsu, Hsiang-Wen A1 - Kempf, Sascha A1 - Krimigis, Stamatios M. A1 - Mitchell, Donald A1 - Moragas-Klostermeyer, Georg A1 - Nicholson, Philip D. A1 - Porco, C. C. A1 - Rosenberg, Heike A1 - Simolka, Jonas A1 - Soderblom, Laurence A. T1 - Close Cassini flybys of Saturn’s ring moons Pan, Daphnis, Atlas, Pandora, and Epimetheus JF - Science N2 - Saturn’s main ring system is associated with a set of small moons that either are embedded within it or interact with the rings to alter their shape and composition. Five close flybys of the moons Pan, Daphnis, Atlas, Pandora, and Epimetheus were performed between December 2016 and April 2017 during the ring-grazing orbits of the Cassini mission. Data on the moons’ morphology, structure, particle environment, and composition were returned, along with images in the ultraviolet and thermal infrared. We find that the optical properties of the moons’ surfaces are determined by two competing processes: contamination by a red material formed in Saturn’s main ring system and accretion of bright icy particles or water vapor from volcanic plumes originating on the moon Enceladus. Y1 - 2019 U6 - https://doi.org/10.1126/science.aat2349 SN - 0036-8075 SN - 1095-9203 VL - 364 IS - 6445 SP - 1053 PB - American Assoc. for the Advancement of Science CY - Washington ER - TY - JOUR A1 - Kollmann, Peter A1 - Roussos, Elias A1 - Clark, George A1 - Cooper, John F. A1 - Sturner, Steven J. A1 - Kotova, Anna A1 - Regoli, Leonardo A1 - Shprits, Yuri A1 - Aseev, Nikita A1 - Krupp, Norbert T1 - Spectra of Saturn's proton belts revealed JF - Icarus N2 - Saturn is permanently surrounded by 6 discrete proton radiation belts that are rigidly separated by the orbits of its inner moons and dense rings. These radiation belts are ideal environments to study the details of radial diffusion and the CRAND source process, yet progress has been hindered by the fact that the energy spectra are not known with certainty: Reanalysis of the response functions of the LEMMS instrument on-board the Cassini orbiter has shown that measurements of less than or similar to 10 MeV protons may be easily contaminated by greater than or similar to 10 MeV protons and that many available measurements characterize a very broad energy range, so that the calculation of an energy-resolved spectrum is not as straightforward as previously assumed. Here we use forward modeling of the measurements based on the instrument response and combine this technique where useful with numerical modeling of the proton belt physics in order to determine Saturn's spectra with higher certainty. We find significant proton intensities up to approximate to 1 GeV. While earlier studies reported on proton spectra roughly following a power law with exponent approximate to -2, our more advanced analysis shows harder spectra with exponent approximate to -1. The observed spectra provide independent confirmation that Saturn's proton belts are sourced by CRAND and are consistent with the provided protons being subsequently cooled in the tenuous gas originating from Saturn or Enceladus. The intensities at Saturn are found to be lower than at Jupiter and Earth, which is also consistent with the source of Saturn being exclusively CRAND, while the other planets can draw from additional processes. Our new spectra can be used in the future to further our understanding of Saturn's proton belts and the respective physical processes that occur at other magnetized planets in general. Also, the spectra have applications for several topics of planetary science, such as space weathering of Saturn's moons and rings, and can be useful to constrain properties of the main rings through their production of secondary particles. KW - Radiation belts KW - Saturn KW - CRAND KW - Proton Y1 - 2022 U6 - https://doi.org/10.1016/j.icarus.2021.114795 SN - 0019-1035 SN - 1090-2643 VL - 376 PB - Elsevier CY - San Diego ER - TY - JOUR A1 - Long, Minyi A1 - Ni, Binbin A1 - Cao, Xing A1 - Gu, Xudong A1 - Kollmann, Peter A1 - Luo, Qiong A1 - Zhou, Ruoxian A1 - Guo, Yingjie A1 - Guo, Deyu A1 - Shprits, Yuri T1 - Losses of radiation belt energetic particles by encounters with four of the inner Moons of Jupiter JF - Journal of geophysical research, Planets N2 - Based on an improved model of the moon absorption of Jovian radiation belt particles, we investigate quantitatively and comprehensively the absorption probabilities and particle lifetimes due to encounters with four of the inner moons of Jupiter (Amalthea, Thebe, Io, and Europa) inside L < 10. Our results demonstrate that the resultant average lifetimes of energetic protons and electrons vary dramatically between similar to 0.1 days and well above 1,000 days, showing a strong dependence on the particle equatorial pitch angle, kinetic energy and moon orbit. The average lifetimes of energetic protons and electrons against moon absorption are shortest for Io (i.e., similar to 0.1-10 days) and longest for Thebe (i.e., up to thousands of days), with the lifetimes in between for Europa and Amalthea. Due to the diploe tilt angle absorption effect, the average lifetimes of energetic protons and electrons vary markedly below and above alpha eq ${\alpha }_{\mathrm{e}\mathrm{q}}$ = 67 degrees. Overall, the average electron lifetimes exhibit weak pitch angle dependence, but the average proton lifetimes are strongly dependent on equatorial pitch angle. The average lifetimes of energetic protons decrease monotonically and substantially with the kinetic energy, but the average lifetimes of energetic electrons are roughly constant at energies 5.6. During the main phase, butterfly PADs have stronger 90 degrees-minima and can be observed at lower L-shells (down to L = 5), then transitioning into flat-top PADs at L similar to 4.5 - 5 and pancake PADs at L < 4.5. The resulting PAD coefficients for different energies, locations and storm epochs can be used to test the wave models and physics-based radiation belt codes in terms of pitch angle distributions. KW - pitch angle KW - pitch angle distributions KW - electrons KW - radiation belts KW - magnetosphere KW - van alien probes Y1 - 2022 U6 - https://doi.org/10.3389/fspas.2022.836811 SN - 2296-987X VL - 9 PB - Frontiers Media CY - Lausanne ER - TY - JOUR A1 - Smirnov, Artem A1 - Shprits, Yuri A1 - Allison, Hayley A1 - Aseev, Nikita A1 - Drozdov, Alexander A1 - Kollmann, Peter A1 - Wang, Dedong A1 - Saikin, Anthony T1 - An empirical model of the equatorial electron pitch angle distributions in earth's outer radiation belt JF - Space Weather: the International Journal of Research and Applications N2 - In this study, we present an empirical model of the equatorial electron pitch angle distributions (PADs) in the outer radiation belt based on the full data set collected by the Magnetic Electron Ion Spectrometer (MagEIS) instrument onboard the Van Allen Probes in 2012-2019. The PADs are fitted with a combination of the first, third and fifth sine harmonics. The resulting equation resolves all PAD types found in the outer radiation belt (pancake, flat-top, butterfly and cap PADs) and can be analytically integrated to derive omnidirectional flux. We introduce a two-step modeling procedure that for the first time ensures a continuous dependence on L, magnetic local time and activity, parametrized by the solar wind dynamic pressure. We propose two methods to reconstruct equatorial electron flux using the model. The first approach requires two uni-directional flux observations and is applicable to low-PA data. The second method can be used to reconstruct the full equatorial PADs from a single uni- or omnidirectional measurement at off-equatorial latitudes. The model can be used for converting the long-term data sets of electron fluxes to phase space density in terms of adiabatic invariants, for physics-based modeling in the form of boundary conditions, and for data assimilation purposes. KW - pitch angle KW - radiation belt KW - model KW - magnetosphere KW - van allen probes; KW - electrons Y1 - 2022 U6 - https://doi.org/10.1029/2022SW003053 SN - 1542-7390 VL - 20 IS - 9 PB - American Geophysical Union CY - Washington, DC ER -