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ClickHereforFullArticl eAsymmetries in Saturn’s radiation beltsC. Paranicas,1D. G. Mitchell,1S. M. Krimigis,1J. F. Carbary,1P. C. Brandt,1F. S. Turner,1E. Roussos,2N. Krupp,2M. G. Kivelson,3K. K. Khurana,3J. F. Cooper,4T. P. Armstrong,5and M. Burton6Received 7 October 2009; revised 6 January 2010; accepted 24 February 2010; published 17 July 2010.[1] We present both energetic proton and electron data from the main radiation belts ofSaturn. When organized by L shell and equatorial pitch angle, data from Cassini’sMagnetospheric Imaging Instrument reveal proton radiation belts that are highlysymmetric in local time. The energetic electron radiation belts are asymmetric in twoprincipal ways. Using data from two close passes of the planet, we find for energies near afew MeV, electron intensity levels are different between noon and midnight. Furthermore,when Cassini was inbound to Saturn, electron fluxes dropped precipitously before thespacecraft reached the main rings. Outbound, electron fluxes returned to high levels whenthe spacecraft moved outward of the main rings. In this paper, we suggest that electron fluxlevel asymmetries are due in part to the presence of local time stationary particles inSaturn’s inner magnetosphere. We also consider possible mechanisms to account the dropoff of electrons near the ring edge on the dayside.Citation: Paranicas, C., et al. (2010), Asymmetries in Saturn’s radiation belts, J. Geophys. Res., 115, A07216,doi:10.1029/2009JA014971.1. Introduction[2] Each of the magnetospheres of the outer planets con-tains intense regions of stably trapped energetic chargedparticles. At Saturn, these radiation belts are most intensebetween the outer edge of the main rings (R ∼ 2.27 RS;1RS=60,268 km) and Enceladus’ s orbit (R ∼ 3.95 RS). Modelingof these belts has reproduced many of their salient features[e.g., Santos‐Costa et al., 2003]. Prior to Cassini, in situmeasurements of Saturn’s radiation belts were made duringthe flybys of three spacecraft with closest approach distancesof 1.35 RS(Pioneer 11), 3.07 RS(Voyager 1) and 2.67 RS(Voyager 2). Cassini had a closest approach distance of1.33 RSduring Saturn Orbit Insertion (SOI) on July 1, 2004.Cassini crossed over the main rings at about noon local time(LT) and exited the rings just before midnight (LT). Severalsubsequent Cassini periapses were within the radiation belts[Burton et al., 2009] but to date have not revisited theregion immediately adjacent to the main rings. While theradiation belts are thought of as fairly stable in time, Cassinidata have revealed transient components. Roussos et al.[2008] discovered a radiation belt in early 2005 thatextended approximately from the orbit of Tethys (R ∼4.89 RS)to8RS. This transient radiation belt was observedto decay in intensity on later orbits. In this paper, we willreport on the innermost portions of the electron and protonradiation belts and their observed structure in local time.[3] During SOI, the proton radiation belts of Saturn wereobserved to be symmetric in the noon‐midnight meridian[Paranicas et al., 2008], using data from the Magneto-spheric Imaging Instrument (MIMI). We found the energeticproton fluxes dropped to background levels close to theplanet (inward of about 2.37 RS). Later Cassini orbits alsoshowed highly organized MeV proton belts [Krupp et al.,2009]. But, as we will illustrate in this paper, the electronbelts are observed to be asymmetric in a number of ways. Theelectron belts (both in the MeV and down to the hundreds ofkeV energy range) do not extend to the edge of the main ringson the inbound leg of the Cassini pass on SOI, but do on theoutbound leg. Furthermore, even when the data are organizedby L shell and equatorial pitch angle, the inbound/outboundintensities are very different. A similar flux asymmetry in thefew MeV electrons was also observed during the Pioneer 11flyby [e.g., Simpson et al., 1980]. The Pioneer 11 trajectorycame close to the outer edge of the main rings between noonand dusk inbound to the planet and between midnight anddawn outbound.[4] In this paper, we will present Cassini data from the innerradiation belts. We will also look at the motion of trappedparticles in that region. We will focus specifically on twoissues. The first is the level of inbound/outbound flux levelsymmetry in different energy bands. The second is the reason1Johns Hopkins University Applied Physics Laboratory, Laurel,Maryland, USA.2Max‐Planck‐Institut für Sonnensystemforschung, Katlenburg‐Lindau,Germany.3IGPP, UCLA, Los Angeles, California, USA.4NASA GSFC, Greenbelt, Maryland, USA.5Fundamental Technologies, LLC, Lawrence, Kansas, USA.6JPL, California Institute of Technology, Pasadena, California, USA.Copyright 2010 by the American Geophysical Union.0148‐0227/10/2009JA014971JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, A07216, doi:10.1029/2009JA014971, 2010A07216 1of7for the absence of electrons on the dayside in the region ofspace just beyond the outer edge of the main rings.2. Data[5] In Figure 1, we show the intensities (counts percm2‐s‐sr‐keV) of 2.28–4.49 MeV protons from MIMI’slow energy magnetospheric measurements system (LEMMS)sensor [Krimigis et al., 2004]. These data were obtainedduring SOI on days 2004–182 and 2004–183. We have useda dipole model of the magnetic field with a northward offsetof 2,230 km along the spin axis of the planet [Doughertyet al., 2005] to compute the L shell and equatorial pitchangle at each point. For Figure 1, data were binned by theirequatorial pitch angle, aeq. Figure 1a shows the most field‐aligned particles with Figures 1b–1d showing more equato-rial particles. Line colors are as follows: black (spacecraftinbound to Saturn, particle aeq< 90°), red (inbound, aeq>90°), gray (outbound, aeq< 90°), and blue (outbound, aeq>90°). The range of L shells encountered by the moons Janusand Mimas are indicated on the plot. The satellite positionswere determined using the information in SPICE (see http://naif.jpl.nasa.gov/naif/) for the period around SOI and thenthese positions were used to approximate each satellite’s fullexcursion in L shell (for more details, see Paranicas et al.[2008]). Moons of all sizes can efficiently remove trappedenergetic protons from the inner magnetosphere of Saturn.Therefore, we do not focus on the causes of the structure ofthe proton belts in this paper.[6] It is clear from Figure 1 that the points obtained on theinbound/outbound legs of the


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