Energetic electron microsignatures as tracers of radial flows and dynamics in Saturn s innermost magnetosphere

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1 Click Here for Full Article JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2009ja014808, 2010 Energetic electron microsignatures as tracers of radial flows and dynamics in Saturn s innermost magnetosphere E. Roussos, 1 N. Krupp, 1 C. P. Paranicas, 2 D. G. Mitchell, 2 A. L. Müller, 1,3 P. Kollmann, 1 Z. Bebesi, 1 S. M. Krimigis, 2,4 and A. J. Coates 5 Received 20 August 2009; revised 18 December 2009; accepted 4 January 2010; published 5 March [1] Signatures of energetic electron depletions from Saturn s inner moons (microsignatures) have been mainly utilized to infer properties of magnetospheric diffusion. The same data, however, indicate that microsignatures can be a much more powerful tool for magnetospheric studies. Here we introduce a new method where the energy structure of a microsignature can be used to trace the location, the formation date, the evolution, and the physical properties of dynamic events in the inner Saturnian magnetosphere. Application to a complex electron microsignature of Tethys, seen by Cassini s MIMI/LEMMS and CAPS/ELS detectors, reveals the location and the age of such an event. The development stages of this event, that can also be distinguished, indicate the presence of local, pulse like azimuthal electric field variations that can locally enhance radial transport by a factor of with respect to radial diffusion. We also extract several physical properties of this dynamical region and speculate about its possible nature. Citation: Roussos, E., N. Krupp, C. P. Paranicas, D. G. Mitchell, A. L. Müller, P. Kollmann, Z. Bebesi, S. M. Krimigis, and A. J. Coates (2010), Energetic electron microsignatures as tracers of radial flows and dynamics in Saturn s innermost magnetosphere, J. Geophys. Res., 115,, doi: /2009ja Introduction [2] Data collected from the Cassini spacecraft have revealed that the middle to inner magnetosphere of Saturn is highly dynamic, with interchange events or energetic particle injections commonly observed in every periapsis [Paranicas et al., 2007; Chen and Hill, 2008]. The standard picture of the circulation is dense, heavy, cold plasma flux tubes being transported outward to remove new plasma created from the neutral gas vented from Enceladus, while hot particles move inward. [3] The most typical signature of this circulation process is visible in the form of the energy time dispersed injections in energetic particle data [Mauk et al., 2005]. Paranicas et al. [2007] have shown that the spatial distribution of enhanced energetic electron or ion fluxes is consistent with a picture in which the inner magnetosphere is populated by injections and one type of these events likely occur over many L shells simultaneously but a narrow range of longitudes. 1 Max Planck Institute for Solar System Research, Katlenburg Lindau, Germany. 2 Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA. 3 Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany. 4 Office of Space Research and Technology, Academy of Athens, Athens, Greece. 5 Mullard Space Science Laboratory, University College London, Dorking, UK. Copyright 2010 by the American Geophysical Union /10/2009JA014808$09.00 [4] A statistical analysis by Chen and Hill [2008] has revealed that most injections occur between L = 6 and L = 10, with a peak close to L = 6 (where L is the dipole L shell). Around L 6 the flux tube content peaks [Sittler et al., 2008], meaning that inside that point plasma should be stable against the interchange instability. A break in electron phase space densities observed at the same location is also indicative of such a transition region [Rymer et al., 2007]. [5] The aforementioned observations suggest that the importance of interchange transport should reduce at L < 6, and other transport mechanisms, such as radial diffusion, should increasingly dominate at lower L. Indeed, Chen and Hill [2008] show that the number of detected injections drops significantly inside L = 6. [6] Still, radial diffusion appears to be a very slow process: diffusive infilling of energetic electron absorption signatures by Saturn s icy moons (microsignatures), with extents of only few hundred kilometers, lasts many hours [Paranicas et al., 2005; Roussos et al., 2005, 2007]. This means that the residence time of electrons in regions with important loss terms (dense neutral cloud and E ring) is rather high. In this case, it is interesting to investigate what sustains the kev electron foreground measured at the orbit of Enceladus ( 3.9 R s,1r s = km) [Jones et al., 2006]. Other means of radial transport, e.g., through wave particle interactions, have not been assessed so far. [7] In the present study we introduce a new method to identify and characterize dynamic processes in the inner magnetosphere, based on the structure and the location of an energetic electron microsignature by Tethys ( 4.9 R s ) that 1of9

2 Figure 1. (top) Energy dispersion of injections and (bottom) microsignatures. The energy dispersion of the injections is directly observed, while electron microsignatures are observed at a common point with each energy structure corresponding to a different age. A common point of detection is expected assuming circular drift shells. Noncircular drift shells and elliptical or inclined moon orbits can sometimes lead to a very small dispersion in the detection position of the microsignature as a function of energy (see also section 3 and Figure 2). That is, however, not illustrated in this sketch for simplicity. Time, in the horizontal axis of all plots, can be converted to any spatial coordinate and the opposite. happened to form in the vicinity of a dynamic event. This method can provide additional insight on radial transport mechanisms at low L shells, and their relative importance to radial diffusion. The electron energies considered are kev, where most of the microsignatures are detected with the MIMI/LEMMS detector [Krimigis et al., 2004] (channels C0 C3). CAPS/ELS data in the energy interval of 1 ev to 28 kev are also used to support the analysis [Young et al., 2004]. 2. Microsignatures as Event Timers [8] Energetic electron microsignatures are unambiguously detected around the L shells of Tethys and Dione, even at very large longitudinal separations from these moons [Roussos et al., 2007]. Their signal at the different energy channels of LEMMS is detected at about the same time, unlike that of an injection which arrives at the detector with a significant time dispersion. The explanation for such a difference is not obvious, but understanding it is essential before we introduce our proposed analysis method. Comparing injections and microsignatures would help to understand how the energy structure of the latter can be a useful source of information. Such a comparison is shown in Figure 1. [9] The Figure 1 (top left) shows that injections occur at all energies at a specific location or time. For an electron injection, energy dependent magnetic drifts cause lower energies to drift faster than the higher energies. After some time the injection signal will disperse, with low energies leading and high energies trailing. Eventually, low energies will reach at Cassini earlier than higher energies and be detected first (although this profile can also appear reversed depending on Cassini s velocity or radial gradients in the corotation flow) [Mauk et al., 2005]. The energy dispersion can be resolved because injections occur at a large L shell range that Cassini requires a considerable time to cross, sampling in this way the injections energy structure (Figure 1, top right). [10] Electron microsignatures, like injections, form simultaneously at all energies. However, since their L shell width is less than 1% of one planetary radius, their dispersed signal develops along a thin line that Cassini crosses quickly, sampling only a very narrow energy range of a depletion with a certain age (Figure 1, bottom right). However, unlike injections, electron microsignatures form continuously, meaning that Cassini will always detect them at all energies and at the same time, but with each energy corresponding to a different age. Backward tracing of the observed signal at the different energy levels then leads to an energy dispersion of their time of origin (Figure 1, bottom left). [11] If D is Cassini s longitudinal separation from the absorbing moon at the time of the microsignature observation, the age of the microsignature at a given energy E is t rk = D/w rk, where w rk (E) is the relative drift frequency between electrons and an icy moon in a circular Keplerian orbit. 2of9

3 Figure 2. MIMI/LEMMS observations of a Dione electron microsignature on day 191/2009 in the energy range of kev. On average, the microsignature is displaced inward of Dione s L shell, but this displacement shows an additional, well organized energy dispersion around that average. The signatures of energy dispersed injections are also visible in the background. [12] For the energy range considered here, w rk is always less for the higher energy electrons, meaning that the high energy component of an observed microsignature was created earlier than the lower energy one. For any observed microsignature we can assign a date of origin to each of its components that are measured with LEMMS s different energy channels. This date will not be unique, but will have a range, as LEMMS channels have a certain energy width. [13] The aforementioned technique can be especially interesting to apply for microsignatures that have peculiar, energydependent displacements from the expected L shell of observation [Roussos et al., 2005]. It is therefore essential to present beforehand several aspects of the microsignature displacements. 3. Microsignature Displacements [14] Electron microsignatures are usually detected displaced from the expected L shell [Paranicas et al., 2005; Roussos et al., 2005, 2007]. This expected L value is estimated under the assumption of circular drift shells (dipolar magnetosphere) and circular and equatorial orbits for most of Saturn s moons. [15] Roussos et al. [2007] found that displacements can be both positive (outward) or negative (inward): most are organized by local time, with positive displacements at the dayside and negative in the nightside. Observations indicate that the displacements have two components. [16] 1. The first component is an average displacement of a given microsignature (D 1 ). This is probably associated with the global drift pattern in Saturn s magnetosphere, that seems to be organized by local time, as mentioned previously. The local time dependence indicates that the absolute value of D 1 does not increase monotonically with the microsignature age, but can also increase or decrease as the microsignature drifts across different local times. The D 1 displacements probably result from global, nondipolar drifts (and noncircular drift shells), the origin of which is, however, not yet understood. [17] 2. The second component is an additional, energydependent displacement (D 2 (E)), around the average displacement, D 1. The D 2 (E) displacements show most of the times an organized energy dispersion. Such organized energy dispersion could result from drifts across noncircular drift shells: as a moon moves on its circular orbit, it crosses different (noncircular) drift shells, since drift shells have a local time dependent shape. Microsignatures created at each one of these shells will acquire a slightly different radial displacement with respect to a reference circular moon orbit. This will lead to a very small dispersion in the detection position of the microsignature as a function of energy. [18] A relevant analogy is that of the various geostationary satellites, located at different longitudes around the Earth: although they all orbit at the same radial distance, they belong to a different L shell, which is commonly termed as L* [Roederer, 1970]. Part of the difference is due to the inclination of the dipole moment vector with respect to the Earth s rotation axis (a case not applicable to Saturn) and part because of deviations of the magnetospheric field configuration from that of a dipole. [19] This microsignature energy dispersion driven by the aforementioned effects will be organized, if the drift shells are not disturbed by any local event. An example of such a case is shown in Figure 2, when LEMMS detected a microsignature by Dione on day 191 of Such organized D 2 (E) displacements should be a permanent feature of all microsignatures, since noncircular drift shells are also permanent. However, D 2 (E) is not always so organized, as in Figure 2. [20] LEMMS sometimes observes microsignatures with complex D 2 (E) profiles. The anomalous displacements in such disrupted microsignatures are sometimes larger than 3of9

4 the organized D 2 (E) displacements. Such cases can be attributed to local, dynamic events in Saturn s magnetosphere, that happened to occur in the formation region of a microsignature [Roussos et al., 2005]. Depending on the electron energies at which this disruption appears and the overall energy structure of the absorption feature, we can, based on the principles discussed in section 2, extract information about the time of origin and the duration of the disturbing event, as well as additional properties, if the nature of this event can be identified. [21] Our purpose in sections 4 and 5 is to apply this method to a complex microsignature observed with LEMMS and CAPS. The main input for such an analysis is contained only in the structure of D 2 (E). For this reason, in what follows we mainly focus on the analysis of the D 2 (E) profile. Whether the value of the average microsignature displacement, D 1, depends on the dynamic event is a subject that is also researched here. On the other hand, the identification of the global source of D 1 type of displacements, some aspects have been documented by Roussos et al. [2007], will be a topic of future research. 4. Tethys Microsignature, Day 196/2005 [22] On day 196/2005 at 0114 UT Cassini crossed Tethys dipole L shell moving outbound and observed a complex combination of microsignatures several minutes later. At that point of time, Cassini was 199 downstream of Tethys, at a local time of 22:45 and a latitude of Data are shown in Figure 3. We note that this event violates the overall local time displacement direction dependence reported by Roussos et al. [2007]. [23] Figure 3 (top panel) shows LEMMS rate channel data (low energy resolution channels C0 C3). C0 and C1 depletions are in the same location. The C3 microsignature is equally sharp, but displaced away from Saturn with respect to the C0 and C1 features. Interestingly, the signal in the intermediate energy channel, C2, appears broader and shallower, as if it is connecting the C3 and the C0 C1 microsignature group. [24] Higher energy signatures are older, meaning that the C3 signature was created first, followed by that of C2, C1 and finally C0. Tracing each microsignature backward on circular drift shells, we find that ages are (C0), (C1), (C2) and (C3) h. [25] Value ranges are due to the energy width of each LEMMS channel. For our calculations, we used drift equations by Thomsen and Van Allen [1980] and corotation fractions by Wilson et al. [2008], under the assumption of circular drift shells and a circular and equatorial orbit for Tethys. With such assumptions, backward tracing of microsignatures cannot constrain the corotation velocity of the cold plasma. Assumption of a different corotation velocity would simply change the age of the microsignature and its location of origin. [26] LEMMS/PHA channel (high energy resolution) data of the same event are shown in Figure 3 (second panel). Here the C2 energy range is decomposed into six shorter energy intervals. While C2 shows the integrated effects of absorption over a time period of 20 min, each PHA channel shows that for 3 min intervals depletion widths are similar as those of depletions in the C0, C1, and C3 energy channels. This means that at each point of time during the disturbed period, Tethys was exposed to different L shells. The fact that no disruption is visible in C3 (and the relevant PHA channels) means that the C3 absorption occurred before the initiation of the dynamic event. [27] The microsignature is also visible in the CAPS/ELS data set. Figure 3 (third panel) shows the count rate spectrogram of CAPS/ELS anode 5 (anode with the best signal tonoise ratio). The microsignature is visible down to about 100 ev. In the range of kev the electron depletion shows a slight outward displacement with respect to the position of the C0 and C1 signatures. This displacement reverses again at lower energies. The ages of the CAPS depletions range from 8.2 h (10 kev) to 8.07 h (100 ev). Overall, age differences are smaller below 1 kev where magnetic drifts become insignificant: the 1 kev microsignature from Tethys is only 40 s older than a 100 ev one. [28] Moving to even lower energies in the CAPS/ELS spectrogram, we see a significant increase in the count rates of cold electrons (<20 ev). This count rate enhancement in low energies can be described also as an increase in electron density (by a factor of 2 3) and a decrease in electron temperature (Figure 3, fourth panel). Although electron densities are probably underestimated due to the negative spacecraft potential and temperatures are overestimated due to penetrating MeV electrons at that distance [Lewis et al., 2008], it is the sharp change in these parameters that may be important. Whether the structure identified by these changes is associated to the event that led to the peculiar microsignature formation, is discussed at section Parameterization of the Dynamic Region [29] Microsignature displacements are tracers of radial electron drifts around Saturn, since the radial distance of the microsignature formation is accurately known. In our case, however, we choose the C3 microsignature location for the reference radial distance, since the initiation of the dynamic event took place after the formation of that depletion, as discussed earlier. [30] We manually extracted the L shell profile of the electron depletion from the LEMMS/PHA and the CAPS/ELS signal. For each energy, E i, there is a corresponding microsignature age, t i, and an L shell displacement, L i, with respect to Tethys s L shell. If L o is the observed L shell displacement of a microsignature that formed before the dynamic event was initiated, then the relative displacement for each microsignature energy component is DL=L i L o. Practically, both Figure 3. MIMI/LEMMS and CAPS/ELS observations on day 196/2005 between 0100 and 0200 UT. (top) LEMMS C0 C3 low energy resolution electron channels; (second panel) LEMMS PHA electron spectrogram (high energy resolution) of the same event, with the corresponding ranges of the C2 absorption indicated; (third panel) CAPS/ELS anode 5 count rate energy spectrogram; (fourth panel) CAPS/ELS electron number densities; and (fifth panel) the L shell of Cassini, with the L shell of Tethys and the location of the microsignatures indicated. 4of9

5 Figure 3 5of9

6 [32] The upper limit of the integral, t f, indicates the time that the microsignature exits the region of influence of E,i (t), or the time that the intensity of this electric field disturbance has decayed substantially. The fitting of the theoretical DL i of equation (1) to the observed one requires that a function for E,i (t) has to be predefined. This can be directly estimated by the derivative of the observed DL i (multiplied by B), as we can also see from equation (1). The observed DL i profile and the extracted E,i (t) are shown in Figure 4 (top and bottom), respectively. [33] We can see that E,i (t) shows a periodic variation. A Lomb Scargle periodicity analysis revealed periodicities peak in the range of h, with a peak around 0.7 h. Periods shorter than 0.16 h could not be resolved due to the time resolution of our profile. This time resolution depends on the energy resolution of the detector and the average age of a microsignature. For instance, for microsignatures that are ten times younger than the one analyzed here, the time resolution is of the order of 2 25 sec. This means that depending on the age of a disrupted microsignature, a wide range of frequencies can be probed. [34] Moving back to the Tethys microsignature, our analysis provides a good starting point for defining a theoretical electric field function. If, for example, we assume a single pulse with a period T, a total duration S and an electric field function of the form E,i (t) = A sin( 2 T (t t o)), where t o defines how much time before the Cassini observations the pulse was initiated and t f = t o S, application to equation (1) gives Figure 4. (top) The relative displacement of each microsignature energy component, as a function of microsignature age. The location of microsignatures in the energy range of 5 8 kev was not well resolved, which is why a data point gap exists around 8.1 h. (bottom) The extracted electric field time series based on the relative displacement profile and a reference polynomial fit showing the periodic signal variation. L i and L o are the integrated displacements due to the varying radial velocity perturbations, u r,i (t), experienced by the microsignatures during their residence time within the dynamic region. DL is the relative displacement in the reference system of microsignatures that have final displacement equal to L o. [31] Since at Tethys s distance plasma beta values are low [Sittler et al., 2006], we can also assume that any changes in the magnetic field magnitude within the dynamic region are small with respect to the background dipole field magnitude. Then, the associated azimuthal electric field can be written as E,i =Bu r,i, where B is the equatorial magnetic field magnitude. Using the magnetic field magnitude measured during the microsignature of Figure 3 at high latitude, we estimate an equatorial field of 167 nt assuming a dipolar magnetosphere. This value will be used for calculations that are described later. We then have L i ¼ 1 B Z tf t i E ;i ðþdt t L o : ð1þ L i ¼ 0; ðt i t o Þ L i ¼ AT 2 B cos 2 T S cos 2 ð T t o t i Þ L o ; ðt o S < t i < t o Þ L i ¼ L o ; ðt i t o SÞ L o ¼ AT 2 B cos 2 T S 1 : [35] Here, t i is the age of each microsignature and A is the amplitude of the pulse. We found that the use of a single pulse with a period around 0.7 h was sufficient to simulate only part of the relative displacement profile, and we therefore performed several tests using the sum of several electric field pulses as an input. This function was extrapolated for the whole duration of the pulse, S, to explore displacement effects beyond the 1 h interval covered by the microsignature observations. [36] In Figure 5 (top), we show a theoretical profile using this equation set for the sum of three different pulses (thick black curve). All components have a similar duration of S = 3 h and t o = 8.92 h. The contribution of each individual pulse is shown with different color, while, the total pulse signal and its components are shown in Figure 5 (bottom). All microsignatures experience the same signal (no phase difference), as soon as they are formed. [37] The simulated displacement profile is in good agreement with the observed one between 10 and 100 kev, but deviates for lower energies. This indicates that the pulse ð2þ 6of9

7 with respect to Saturn. For instance, the choice of S = 3 h was done because it leads to an additional, average outward displacement of all microsignatures by about 0.1 R s. [40] The effect is illustrated in Figure 6, where the radial trajectories of microsignatures of three different energies are plotted. As long as two or more existing microsignatures (even of different energy) are exposed to the same perturbation, their relative displacement does not change any more. These microsignatures will be displaced collectively as a group. This is an important observation, since it reveals that a local dynamic region can also affect the average displacement of a microsignature (see section 3). This may also explain why the displacement direction of the microsignature discussed here deviates from the local time pattern discussed by Roussos et al. [2007]. [41] In order to produce the simulated displacement profile, we have assumed that all the energy components of the microsignature where exposed to the dynamic region simultaneously. This assumption can be also used to set constraints to the local time extent of the dynamic region. Figure 7 shows the local time trajectories of microsignatures of different Figure 5. (top) Relative displacement profile as a function of energy for the observed Tethys microsignature energy components (diamonds). The simulated profile based on the sum of three different pulses described by equation (1) is shown with black solid line. Each of the three pulse component contributions is with different color. (bottom) The electric field total pulse signal and its components, used as an input. function should contain more harmonics (and possibly a phase dependence), or that displacements at lower energies correspond to the organized type of energy dispersions, as in Figure 2, which are not modeled here. This can be further researched in future studies using similar microsignature events, which are not uncommon in the LEMMS and CAPS data sets. [38] Nevertheless, our analysis indicates that the total electric field magnitude is around 1.0 mv m 1 at its peak. This corresponds to u r,i (t) 6kms 1. Much shorter periods, that cannot be resolved from our previous analysis, lead to displacements that average out after many pulse periods and are probably relevant to the short scale diffusion of the electrons. Pulses with larger periods than we can infer, had also negligible contributions (as it is also visible in Figure 5 (blue curve). [39] We also found that only t o affects the final relative displacement between microsignatures of a different energy (age) and not the duration of the pulse. The value of S affects the total, net displacement of the whole microsignature group Figure 6. (top) Radial position for microsignatures of three different energies. Each trajectory calculation starts at the time that each microsignature formed. (bottom) The total electric field experienced by the microsignatures at each point in time. 7of9

8 Figure 7. Microsignature trajectories as a function of the time before the day 196/2005 microsignature observation. The first point of each track indicates the time and the local time of origin of each depletion at the C0 C3 channels. Crosses define the range of values for these initial points, due to width of the C0 C3 LEMMS channels. The central value for each channel is calculated to be the geometric mean of its upper and lower energy limits ((E high E low ) 1/2 ). energies, for few hours after they formed. For example, the C3 microsignature (see Figure 3) was about 1.2 h ahead in local time (or 18 in longitude), when the last observed microsignature formed (about 8.2 h before the microsignature detection by Cassini). If both microsignatures where exposed to the dynamic region, as we assumed, then the dynamic region should be at least 18 wide in longitude. [42] Cassini needs about 40 min to cross the cold electron enhancement region (Figure 3) with an azimuthal velocity relative to the plasma corotation speed of about 30 km s 1. This means that the longitudinal length of this enhancement is at least 14, in good agreement with the constraint set with the other method. This, however, may be coincidental, as there is no indication that definitely links the cold plasma structure with the disrupted Tethys microsignature. 6. Nature of the Dynamic Event and its Effectiveness for Radial Transport [43] It is not certain what process can lead to the appearance of azimuthal electric fields like those extracted from our analysis. Inspection of the LEMMS data 8 9 h before the microsignature event do not reveal any peculiar or sharp change in the measured fluxes that would indicate that the electric fields originate from a global scale change in the magnetosphere (e.g., magnetospheric compression or breathing leading to large scale inductive electric fields). Although we are most likely dealing with a local disturbance within the inner magnetosphere, the scenario of solar wind variability may be tested further using predictions of solar wind properties at Saturn about 8 9 h before the microsignature observation, as from Zieger and Hansen [2008]. [44] We will now consider the case where the cold plasma enhancement observation by CAPS is connected to the peculiar Tethys microsignature. We cannot conclusively exclude the possibility that this simultaneous enhancement is stochastic and coincidental, but we note that this feature is the only one for a period of about 3.5 h when the electron density and the temperature are stable. If there is indeed a link, then this structure may be a signature of a cold plasma outflow finger and part of the centrifugal interchange process. Simulations show that these outflow fingers are corotating structures: that would also be consistent with the LEMMS and CAPS observations [Kidder et al., 2009]. Simulations also show that the anticorellation between density and temperature, as that shown in Figure 3, may be a typical signature of such an outflow [see Kidder et al., 2009, Figure 9]. [45] The extracted peak radial velocities ( u r,i (t) 6kms 1 ) may be too high compared to what has been reported so far for low L shell values [Sittler et al., 2006]. On the other hand, observations by [Sittler et al., 2006] were away from the equatorial plane. Furthermore, Wilson et al. [2008], who reports even lower radial velocities, have excluded from their analysis intervals where there was evidence for flux tube interchange. [46] The pulse like structure of the electric field signal creates the biggest problem for connecting it with the interchange process. A flux tube that participates in such a process would be expected to monotonically move inward or outward and not change its direction of motion periodically. Unless this aspect represents an unidentified mode of the development and the evolution of the interchange process, other mechanisms have to be considered in order to explain the observations, like poloidal ULF waves, as observed at the Earth [Takahashi and Anderson, 1992]. The corotating ring current asymmetries, observed by Cassini s energetic neutral atom camera (INCA) [Krimigis et al., 2007], may also cause a periodic perturbation in the magnetic/electric field, as they sweep across the local time that a microsignature is forming. [47] Irrespective of the dynamic event s nature, it is interesting to compare its effectiveness for radial transport with respect to radial diffusion. We estimate the radial diffusion coefficient (D LL ) to be (0.75 ± 0.20) 10 9 R s 2 s 1, by simulating the profiles of the observed microsignatures, as from Roussos et al. [2007]. Approximating diffusion timescales by t d = DR 2 /(4 D LL )[Mogro Campero and Fillius, 1976], we find that t d sec, for a DR equal to the radius of Tethys. [48] The extracted values suggest that the electric fields within the dynamic region can locally enhance radial transport by a factor between 1000 and 3000, with respect to radial diffusion and across the same DR. Since radial diffusion occurs continuously at all local times and latitudes this difference is probably reduced, when moving to global scales. To what level this enhancement is reduced would also depend on the frequency, the duration and the spatial extent of such dynamic events. Such parameters can be evaluated with a more statistically oriented study, using the continuously growing microsignature data set by LEMMS and CAPS. [49] Acknowledgments. The authors would like to thank M. Kusterer (APL) for data reduction, A. Lagg (MPS) for analysis software support C. Tubiana (MPS) for helping creating illustrations for this paper. We 8of9

9 also thank the reviewers of the paper for helping improve the analysis and the interpretation of the results. The German contribution of MIMI/ LEMMS was in part financed by BMBF through DLR under contract 50 OH 0103 and by the Max Planck Gesellschaft. [50] Wolfgang Baumjohann thanks Michelle Thomsen and another reviewer for their assistance in evaluating this paper. References Chen, Y., and T. W. Hill (2008), Statistical analysis of injection/dispersion events in Saturn s inner magnetosphere, J. Geophys. Res., 113, A07215, doi: /2008ja Jones, G. H., E. Roussos, N. Krupp, C. Paranicas, J. Woch, A. Lagg, D. G. Mitchell, S. M. Krimigis, and M. K. Dougherty (2006), Enceladus varying imprint on the magnetosphere of Saturn, Science, 311, , doi: /science Kidder, A., R. M. Winglee, and E. M. Harnett (2009), Regulation of the centrifugal interchange cycle in Saturn s inner magnetosphere, J. Geophys. Res., 114, A02205, doi: /2008ja Krimigis, S. M., et al. (2004), Magnetosphere Imaging Instrument (MIMI) on the Cassini Mission to Saturn/Titan, Space Sci. Rev., 114, , doi: /s Krimigis, S. M., N. Sergis, D. G. Mitchell, D. C. Hamilton, and N. Krupp (2007), A dynamic, rotating ring current around Saturn, Nature, 450, , doi: /nature Lewis, G., N. Andre, C. Arridge, A. Coates, L. Gilbert, D. Linder, and A. Rymer (2008), Derivation of density and temperature from the cassini huygens caps electron spectrometer, Planet. Space Sci., 56(7), , doi: /j.pss Mauk, B. H., et al. (2005), Energetic particle injections in Saturn s magnetosphere, Geophys. Res. Lett., 32, L14S05, doi: /2005gl Mogro Campero, A., and W. Fillius (1976), The absorption of trapped particles by the inner satellites of Jupiter and the radial diffusion coefficient of particle transport, J. Geophys. Res., 81, , doi: / JA081i007p Paranicas, C., et al. (2005), Evidence of Enceladus and Tethys microsignatures, Geophys. Res. Lett., 32, L20101, doi: /2005gl Paranicas, C., et al. (2007), Energetic electrons injected into Saturn s neutral gas cloud, Geophys. Res. Lett., 34, L02109, doi: / 2006GL Roederer, J. G. (1970), Dynamics of Geomagnetically Trapped Radiation, Phys. Chem. Space, vol. 2, Springer, New York. Roussos, E., et al. (2005), Low energy electron microsignatures at the orbit of Tethys: Cassini MIMI/LEMMS observations, Geophys. Res. Lett., 32, L24107, doi: /2005gl Roussos, E., et al. (2007), Electron microdiffusion in the Saturnian radiation belts: Cassini MIMI/LEMMS observations of energetic electron absorption by the icy moons, J. Geophys. Res., 112, A06214, doi: / 2006JA Rymer, A. M., et al. (2007), Electron sources in Saturn s magnetosphere, J. Geophys. Res., 112, A02201, doi: /2006ja Sittler, E. C., et al. (2006), Cassini observations of saturn s inner plasmasphere: Saturn orbit insertion results, Planet. Space Sci., 54(12), , doi: /j.pss Sittler, E. C., et al. (2008), Ion and neutral sources and sinks within Saturn s inner magnetosphere: Cassini results, Planet. Space Sci., 56, 3 18, doi: /j.pss Takahashi, K., and B. J. Anderson (1992), Distribution of ULF energy (frequency < 80 mhz) in the inner magnetosphere: A statistical analysis of AMPTE CCE magnetic field data, J. Geophys. Res., 97, 10,751 10,773, doi: /92ja Thomsen, M. F., and J. A. Van Allen (1980), Motion of trapped electrons and protons in Saturn s inner magnetosphere, J. Geophys. Res., 85, , doi: /ja085ia11p Wilson, R. J., R. L. Tokar, M. G. Henderson, T. W. Hill, M. F. Thomsen, and D. H. Pontius (2008), Cassini plasma spectrometer thermal ion measurements in Saturn s inner magnetosphere, J. Geophys. Res., 113, A12218, doi: /2008ja Young, D. T., et al. (2004), Cassini Plasma Spectrometer Investigation, Space Sci. Rev., 114, 1 4, doi: /s Zieger, B., and K. C. Hansen (2008), Statistical validation of a solar wind propagation model from 1 to 10 AU, J. Geophys. Res., 113, A08107, doi: /2008ja Z. Bebesi, P. Kollmann, N. Krupp, A. L. Müller, and E. Roussos, Max Planck Institute for Solar System Research, D Katlenburg Lindau, Germany. (roussos@mps.mpg.de) A. J. Coates, Mullard Space Science Laboratory, University College London, Dorking RH5 6NT, UK. S. M. Krimigis, D. G. Mitchell, and C.P.Paranicas,JohnsHopkins University Applied Physics Laboratory, Laurel, MD 20723, USA. 9of9