PUBLICATIONS. Journal of Geophysical Research: Space Physics. Local time dependences of oxygen ENA periodicities at Saturn

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1 PUBLICATIONS RESEARCH ARTICLE Key Points: Periodicities of energetic oxygen atoms depend on local time Dual periods near midnight but mono periods near noon or midnight Periodicities may disappear entirely at any local time Correspondence to: J. F. Carbary, Citation: Carbary, J. F., D. G. Mitchell, and P. C. Brandt (2014), Local time dependences of oxygen ENA periodicities at Saturn, J. Geophys. Res. Space Physics, 119, , doi:. Received 22 MAY 2014 Accepted 3 AUG 2014 Accepted article online 6 AUG 2014 Published online 28 AUG 2014 Local time dependences of oxygen ENA periodicities at Saturn J. F. Carbary 1, D. G. Mitchell 1, and P. C. Brandt 1 1 Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA Abstract The periodicities of energetic neutral atoms ( kev oxygens) at Saturn are determined by applying Lomb-Scargle periodogram analyses to energetic neutral atom (ENA) fluxes observed in eight local time sectors of the equatorial plane between 5 and 15 R S (1 R S = 60,268 km). The analyses come from four long intervals (>180 days each) of high-latitude viewing from 2007 to 2013 and represent an essentially global view of Saturn s periodicities. The periodograms display rich and complex structures in local time. Sectors near midnight generally exhibit the strongest periodicities (in terms of highest signal-to-noise ratios) and often show the dual or single periods of the Saturn kilometric radiation (SKR). Sectors near noon display single or multiple periodicities or none. Furthermore, dayside periods may be much shorter (~10.3 h) than SKR periods. Sectors near dawn or dusk display periodicities intermediate between midnight and noon or may show no periodicities whatsoever. These patterns of local time dependence do not remain constant from interval to interval. 1. Introduction The magnetosphere of Saturn displays a variety of periodic phenomena, including those of radio emissions [Gurnett et al., 2005, 2009a, 2009b, 2010], magnetic fields [Giampieri et al., 2006; Andrews et al., 2008, 2010a, 2010b, 2012; Provan et al., 2012, 2013], energetic particles [Carbary et al., 2007, 2009, 2012], and energetic neutral atoms [Krimigis et al., 2005; Paranicas et al., 2005; Carbary et al., 2008a]. Similar periodicities can also be found in Saturn s aurora and plasma sheet motions [Nichols et al., 2008, 2010; Carbary et al., 2008b; Arridge et al., 2011; Szego et al., 2013]. Saturn s periodicities have two peculiar aspects to them. First, they vary by about 1% on timescales of Earth years [Galopeau and Lecacheux, 2000; Gurnett et al., 2010; Lamy, 2011]. This slow variation may result from seasonal or solar cyclic effects or both [Kimura et al., 2013; Cowley and Provan, 2013]. Second, Saturn periodicities display two distinct branches, one associated with the northern polar region (~10.6 h period) and one associated with the southern polar region (~10.8 h period) [Gurnett et al., 2009a, 2009b; Carbary et al., 2009; Nichols et al., 2010; Andrews et al., 2010b, 2012; Provan et al., 2012]. The two branches apparently merged to essentially the same period a few months after Saturn equinox in 2009 [Gurnett et al., 2010; Provan et al., 2013]. A review of Saturn s magnetospheric periodicities can be found in the paper by Carbary and Mitchell [2013]. An intriguing question arises: could these well-known periodicities have a local time dependence? There are several good reasons to expect this type of asymmetry. First, the intensity of Saturn kilometric radiation itself displays a local time asymmetry, peaking between 05 and 10 h local time [Lamy et al., 2009]. Second, the electrons that stimulate Saturn kilometric radiation (SKR) emission also show a local time asymmetry [DeJong et al., 2011; Schippers et al., 2012], albeit the electron asymmetry is not necessarily commensurate with the SKR asymmetry. Third, ion fluxes and pressures also exhibit local time asymmetries [Carbary et al., 2008c; Sergis et al., 2009]. These morphological asymmetries might logically extend to the dynamical asymmetries, namely, those of the periodicities. No study has yet addressed the possibility of local time variation in the periodicities, in large part, because of the difficulty of separating the periods in local time. Such a local time separation would be very difficult using in situ measurements, mostly because of the long revisit intervals between observations for different local times. A global view of the entire magnetosphere, continuously observed over several months, would also be necessary for segregating periodicities by local time. Such observations have actually been accomplished by the Ion Neutral Camera (INCA) of the magnetospheric imaging instrument (MIMI) instrument on Cassini. These INCA energetic neutral atom (ENA) observations can readily be separated into eight local time sectors (of 3 h each) to check for local time dependence of periodicities in Saturn s magnetosphere. CARBARY ET AL American Geophysical Union. All Rights Reserved. 6577

2 Table 1. High-Latitude Intervals of MIMI/INCA Observations Interval Start Date Stop Date Total Days No. of Samples day day , day day , day day , day day , Instrument and Data Processing The data used in this study are from the Ion Neutral Camera (INCA), which is part of the magnetospheric imaging instrument (MIMI) on the Cassini spacecraft. Complete details about MIMI and INCA have been published elsewhere [Krimigis et al., 2004], so only a cursory description of the instrument and its operation is provided here. INCA obtains images of energetic neutral atoms within a field of view at a nominal time resolution of ~6 min. INCA can discriminate in energy as well as species; this investigation concentrates on the kev energetic oxygen atoms. The oxygen ENAs are used because they exhibit strong morphological asymmetries in local time and because they tend to have stronger, more consistent periodicities than the hydrogen ENA [Carbary et al., 2008a, 2008c]. Standard MIMI data processing saves the ENA images in a pixel format, performs flat fielding, and converts the count rates to integral fluxes (over energy) with the units of atoms cm 2 s 1 sr 1. The ENA observations were not continuous but were interspersed with intervals when INCA operated in an ion mode to detect energetic H + and O + ions. Only ENA-mode data are used here. The ENAs originate from charge exchange between slow neutral atoms and energetic ions in the thin magnetodisk of Saturn, which is essentially the equatorial plane for the ranges 5 15 R S considered here. The pixel-by-pixel ENA fluxes were combined with Cassini/INCA pointing and projected onto the equatorial plane of Saturn, thus generating a map of the ENA emissions [e.g., Carbary et al., 2008c]. One map of ENA emissions is generated per image (i.e., one image per 6 min). If the spacecraft is at sufficiently high latitude (above ~20 latitude), the maps encompass the entire equatorial plane and make possible the monitoring of ENA emissions on a global scale. Table 1 summarizes the four intervals of high-latitude observation analyzed here. These intervals were chosen for their high-latitude observations, which allow global monitoring of Saturn s equatorial plane. The durations of each interval were chosen to maintain an approximately equal number of sample images in each, not to preserve a fixed number of days. Sustaining about the same numbers of samples ensures that the Lomb statistics will be approximately the same for each interval. 3. Overview of Oxygen ENA Observations Figure 1 summarizes the entire Cassini mission from orbit injection on day through the end of The top row indicates the range of distances of the Cassini spacecraft from Saturn. These ranges vary considerably, from ~3 R S to over 50 R S. The middle row shows the spacecraft latitudes. In this panel marked are the four intervals of high-latitude observations, as well as the latitude of the Sun as seen from Saturn. Note that the first interval occurred during southern summer, while the last interval occurred during northern spring, and the middle two intervals were prior to the vernal equinox (dotted line). The bottom row in Figure 1 displays the F 10.7 index as an indicator of solar activity. The first three intervals took place near solar minimum, while the fourth interval occurred near a more active part of the solar cycle. The reader should bear in mind the season and solar cycle in interpreting the following. Figure 2 shows composite maps of energetic neutral oxygen for the four intervals. Each map was formed by averaging the projected pixel intensities within 2 2 R S squares in the equatorial plane. Similar composite maps were made for a different interval in 2007 [Carbary et al., 2008c]. The Sun lies to the right, with dawn being down, dusk being up, and midnight to the left. The orbits of Saturn s principal moons are shown, and the eight pie slices (in interval 4) indicate particular sectors used in the periodograms. The composites were formed from filtered images obtained when Cassini was over 30 R S from Saturn at latitudes exceeding 30 (north or south); also, aberrant pixels (e.g., corner pixels) were masked out prior to forming the composites. CARBARY ET AL American Geophysical Union. All Rights Reserved. 6578

3 Figure 1. Summary of Cassini spacecraft (top row) trajectory range and (middle row) latitude, along with solar activity as evidenced by the (bottom row) F 10.7 index. The middle row indicates the four intervals of study and also shows the latitude of the Sun (dashed line) at Saturn. The vertical dotted line denotes the vernal equinox of Saturn. The four composites illustrate the evolution of the oxygen ENA source through several years, where source refers to the region from which the highest ENA fluxes emanate. The source generally lies between 5 and 15 R S but appears at different local times for different epochs. From late 2006 to mid-2007 (interval 1), the ENA fluxes appeared to come largely from the dusk sector, while during the first half of 2008 (interval 2) they originated during the prenoon region. Just prior to equinox (interval 3), the main oxygen ENA source shifted back to the dusk sector, with most of the emission coming from between dusk and midnight. In early 2013 (interval 4), the ENA intensities had decreased considerably compared to the previous intervals and had originated in the regions near noon. Thus, the ENA source apparently changes its local time location and its strength over intervals of months to years. The variations may be due to seasonal and/or solar cyclic effects, which are not the primary topic of this paper (but see, for example, Kimura et al. [2013] and Cowley and Provan [2013] for discussions of such effects). The ENA source locations do not seem to be correlated with the orbital phase of Enceladus. 4. ENA Periodograms The primary topic of discussion is the periodicities within the eight pie slice areas marked in the interval 4 map of Figure 2. Pixels within each slice were averaged at the time resolution of the ENA images (~6 min) for each interval of Table 1. Then the time series of these averages were subjected to a Lomb-Scargle analysis to reveal the periodicities [e.g., Press et al., 1992]. The same periodogram analysis has been applied before to reveal charged particle and ENA periodicities [Carbary et al., 2007, 2008a, 2009, 2011]. As an example of an ENA time series, Figure 3 shows a short interval taken from the midnight sector of interval 1. Although noise and gaps are apparent, the periodic nature of the signal can clearly be apprehended. The Lomb-Scargle analysis can readily overcome both the noise and gaps inherent in the signal. Simulations of periodic signals with the same time marks as the observations (i.e., with gaps) show that even with noise levels of ~50% of the signal, the Lomb analysis can readily distinguish the periodicities. Finally, the amplitudes of the Lomb-Scargle periodogram in the frequency domain are not directly related to the amplitudes of the periodic signal in the time domain. Thus, a low-amplitude signal only weakly perceived as periodic in the time domain can have a very strong peak in the Lomb periodogram, if the periodicity is truly present [see Press et al., 1992]. CARBARY ET AL American Geophysical Union. All Rights Reserved. 6579

4 Figure 2. Composite oxygen ENA images for the four intervals of study. The ENA intensities during each interval were projected onto Saturn s equatorial plane and averaged into 2 R S 2R S boxes. The Sun is to the right (+x axis), midnight to the left; dawn is down and dusk is up (+y axis). The orbits of Saturn s principal satellites are indicated as dotted circles. The thick lines in the fourth panel show the eight pie slice regions from which samples were taken to conduct the Lomb periodogram analyses. Figures 4 7 present the Lomb periodograms by interval and local time sector. The amplitudes of the periodograms have been normalized to the maximum within each interval, which facilitates comparison of the local time sectors. The SKR periodicities, both north and south, are shown as dotted/dashed lines [Lamy, 2011]. Each panel also gives the signal-to-noise ratio (SNR, here defined as the ratio of peak signal to standard deviation of the other peaks) as a measure of the fidelity of the periods. Finally, a dotted horizontal line indicates a level of ~0.1, which simulations indicate is an approximate level above which false alarms in the periodogram are negligible. First, consider the periodogram from interval 1, late 2006 to mid-2007, in Figure 4. The nighttime sectors from 18 to 03 h local time display dual periodicities commensurate with those of the Saturn kilometric radiation, with the southern (longer) period somewhat more dominant in terms of signal amplitude than the northern period.thisisthefirst time a dual period has been identified in ion-related observations. However, for other local times, especially those nearest noon, only the northern period can be observed. In these dayside sectors the northern period dominates, sometimes to the complete exclusion of the southern period. Apparently, dual periodicities tend not to occur on the dayside during this observing interval, and a local time dependence of the periodicities has appeared in the record. The periodograms also exhibit a wealth of tertiary structures with periods both longer and shorter than the primary SKR dual periods. The source of these is unknown. CARBARY ET AL American Geophysical Union. All Rights Reserved. 6580

5 Figure 3. Example of periodic flux variations in kev neutral oxygens observed in the midnight sector during a few days in A suggestive scale is overplotted showing fiducials at the SKR period of ~10.7 h. When these flux profiles are extended over intervals exceeding 180 days, a Lomb-Scargle periodogram analysis can be made. Figure 4. Lomb periodograms for the eight local time sectors during interval 1. Each periodogram has been normalized to the maximum value within the eight periodograms. SNR is the signal-to-noise ratio (ratio of highest peak value to the average of the other peaks). For comparison, the observed SKR-south (red) and SKR-north (blue) periods are also shown [Lamy, 2011]. CARBARY ET AL American Geophysical Union. All Rights Reserved. 6581

6 Figure 5. Lomb periodograms for the eight local time sectors during interval 2, in the same format as Figure 1. Figure 5 displays the periodograms for the second interval from days 1 to 280 of The periodicities from this interval exhibit an almost total absence of SKR periods, in spite of their being prominent in the radio record [Lamy, 2011]. Only one SKR-like signal appears at midnight at the southern period. Peculiarly, the most prominent ENA periodicity during this interval exists near noon and has a period of ~10.3 h. The 10.3 h signal Figure 6. Lomb periodograms for the eight local time sectors during interval 3, in the same format as Figure 1. CARBARY ET AL American Geophysical Union. All Rights Reserved. 6582

7 Figure 7. Lomb periodograms for the eight local time sectors during interval 4, in the same format as Figure 1. maintains a healthy SNR of ~17, the strongest of this interval. This period has never been observed heretofore among those of Saturn s magnetosphere, although a period of ~9.95 h has been reported in energetic electrons [Carbary et al., 2012]. This interval differs from the others because it contained more tail ENA events, which are thought to be externally driven [e.g., Mitchell et al., 2009]. By tail events are meant ENA intensifications known to be related to injections from the magnetotail. Possibly, such tail-driven events disrupt ENA periodicities. Figure 6 shows the ENA periodograms from interval 3, which extended from late 2008 to the middle of 2009 near equinox. Dual periodicities at the SKR period again appear but are prominent only in the nighttime hours near midnight. Again, the southern branch has a larger amplitude than the northern. Note that unlike the dayside periodograms of interval 1, the dayside periodograms of this interval show no evidence of the SKR-north period, and only a weak SKR-south period. Finally, Figure 7 displays the periodograms from the fourth interval from late 2012 to the end of 2013, the longest interval in the group. In this interval north and south SKR periods have not been published, and indeed may not exist, although the extant SKR data set reveals a single period at ~10.68 h. This period can be obtained by performing a periodogram analysis of SKR measurements available from the Planetary Plasma Interactions node of the NASA Planetary Data System [Kurth et al., 2004] (website: ppi.pds.nasa.gov). The strongest ENA periods do cluster near this period, but only in the nightside sectors. The nightside spectra also suggest a secondary period near 11.1 h. Many of these periodograms also display weak signals at periods in the h range; some of these periods are close to the drift periods of the energetic particles generating these ENA [Brandt et al., 2008; Carbary et al., 2010]. Because these drift periods do not appear prominently in the periodograms, and because they are generally much longer than even a 14 h period, one must conclude that drift phenomena are not causing the ENA periodicities observed here. The subject of ion drifts in relation to ENA movement has been addressed in a previous publication [Carbary and Mitchell, 2014]. 5. Discussion The predilection for ENA periodicities to arise in the nightside sectors, at least those of the SKR-like variety, suggests that the standard model of dual periodicities at Saturn is incomplete. Usually, the dual SKR CARBARY ET AL American Geophysical Union. All Rights Reserved. 6583

8 periodicities at ~10.6 and ~10.8 h are explained as differentially rotating sources in the northern and southern polar regions of the planet [Gurnett et al., 2009a, 2009b; Andrews et al., 2010b, 2012; Provan et al., 2012; Jia et al., 2012; Southwood and Cowley, 2014]. These are generally referred to as m = 1 periodicities. None of these explanations have suggested that the periodicities should vanish on the dayside while appearing only on the nightside, although there have been intimations that Saturn s ionospheric conductivities may vary and somehow alter the SKR periods on a longer-term seasonal basis. If ionospheric conductivity at Saturn also changes diurnally, as it does on the Earth, then it could enhance the periodicities in certain local time sectors. The mechanism causing a day-night periodic asymmetry might be the following. During the night, ionospheric conductivities decrease (and resistivities increase), owing to a decrease of ionization caused by extreme and far ultraviolet sunlight on the day. Because the periodicities are thought to be associated with Birkeland (field-aligned) currents (see references in previous paragraph), the currents on the nightside would encounter an increased resistive load. The increased resistance would generate increased I 2 R power, thus enhancing the periodic signal. Resistive power loss has recently been implemented in explaining the north-south dual periodicities [Southwood and Cowley, 2014]. A local time asymmetry in the periodicities would be a logical extension of the resistive loss concept. Whether the day-night asymmetry in I 2 Rpowerissufficient to completely extinguish dayside periodicities is a problem that remains to be resolved. A north-south asymmetry in ionospheric conductivity might account for the different amplitudes of the dayside periods. In fact, the periodicity of the most illuminated polar hemisphere is less pronounced on the dayside and far more pronounced in the nightside. Alternately, Birkeland currents can be affected by a number of factors such as plasma flowandparticlespeciesintheplasma sheet or vortices in the ionosphere, and many of these phenomena have known local time dependences. Thus, local time dependence need not be tied to ionospheric conductivity. During the 2008 interval 2, all nightside periodicities vanished, which suggests the ionospheric conductivity scenario may not always operate. ENA periodograms from that interval show an unusually short period of ~10.3 h, apparent only on the dayside. This feature does not appear in the SKR, charged particle, or magnetic field periodicities of this interval [Lamy, 2011; Carbary et al., 2009; Andrews et al., 2010b]. The origin of this 10.3 h period is not known. If related to some rotating ionospheric anomaly, then it must be imbedded within an unusually rapid zonal flow at that time. ENA imagery from high latitudes has been used to estimate Saturn s magnetospheric periodicities from a global perspective, which in turn allows determination of periodicitic behavior as a function of local time. Several caveats attend this new way of examining periodicities. First, the analyses have been confined to the radial range between 5 and 15 R S. This region generally defines the ring current where the ENA emissions are highest, but it also maps to latitudes equatorward of the auroral region where SKR emissions originate. Second, only energetic oxygen atoms ( kev) were used. These display stronger periodicities than their hydrogen counterparts, but possibly, different periods might be realized with other species of neutrals. Indeed, the periodicities themselves may be species or even energy dependent [e.g., Carbary et al., 2012]. Third, the generation of any of the energetic neutrals depends on the presence of cold neutral atoms such as are released by Enceladus. The ENA emissions might be strongly biased, therefore, by any clumpiness in the cold neutral atoms. FUV observations suggest that the neutral hydrogen cloud around Saturn is, in fact, clumpy and may have a local time asymmetry [Shemansky and Hall, 1992; Shemansky et al., 2009]. How or if asymmetries in neutral hydrogen reflect asymmetries in neutral oxygen (from which most of the oxygen ENA at these energies derive) remains unclear (see the discussion in the next paragraph). Finally, each of Saturn s periodicities radio, charged particle, magnetic field, auroral, and ENA originates from different mechanisms that may or may not have a common driver. Therefore, asymmetries in one type of periodicity may not be common to another type, so care must be taken before assuming that all of Saturn s periodicities exhibit similar local time asymmetries. The issue of changing ENA periodicities warrants a final comment. The periodograms clearly indicate that the ENA periodicities evolve from interval to interval: intervals 1 and 3 show dual periods and interval 4 has a monoperiod that agrees with the SKR periods in the same time frame, while interval 2 has an unusually short period bearing no resemblance to any other Saturn period. These changes appear without regard to the positional arrangement of the observer. That is, the ENA periodicities are not subject to the trajectory bias that may affect in situ measurement of periodicities, which subsumes the usual methods of period measurement. The following observations can then be made regarding seasonal or solar cycle effects, which can be considered external generative agents. Dual periods were observed only during the decline or CARBARY ET AL American Geophysical Union. All Rights Reserved. 6584

9 minimum part of the solar cycle, while the most unusual period of ~10.3 h was seen just before solar minimum. Monoperiods were observed not only just before solar minimum but also during an active solar period. Dual periods were observed during Saturn s southern summer and near the equinox, but not during the northern spring. Apparently, neither the solar cycle nor the seasonal cycle offers a consistent pattern to the changing periodicities, a conclusion corroborated by recent reviews of non-ena periodicities [Kimura et al., 2013; Cowley and Provan, 2013]. If the evolution of periodicities cannot be attributed to an external generative agent such as the Sun, then an internal agent is recommended as the cause of the evolution in Saturn s periodicities. The ENA periodicities, with their close association with neutral clouds, imply that the internal agent may be the principal source of neutrals in the magnetosphere, namely, the moon Enceladus. Sporadic outbursts of neutrals from this moon [e.g., Smith et al., 2010] affect the neutral populations in the magnetosphere, thus painting the ENA periodicities strong during one epoch but nonexistent at another. This is especially true for an Enceladan source, which produces oxygen as well as hydrogen. An Enceladus agent would explain why the ENA periodicities might change from interval to interval but would not explain why the periods are stronger on the nightside than the dayside unless, of course, Enceladus cloud is preferentially clumped on the nightside! 6. Conclusions Lomb-Scargle analyses of four intervals of high-latitude viewing from 2007 to 2013 were conducted on the basis of local time. The midnight sector generally exhibits the strongest periodicities, which tend to follow the ~10.6 and ~10.8 h dual or ~10.7 monoperiodicities of the Saturn kilometric radiation. However, during one interval in 2008, all nightside periodicities vanished and the noon sector displayed a strong period of ~10.3 h. Periodicities in the dawn and dusk sectors are generally weaker than those closer to midnight, but periodic behavior may disappear altogether at these local times. Finally, the patterns in the periodicities do not remain constant from interval to interval. Assuming these periodicities are related to Birkeland (field-aligned) currents, the local time asymmetries noted here may be related to asymmetries in such currents that can arise from conductivity or other asymmetries in the ionosphere and in the plasma sheet. In particular, outbursts from the moon Enceladus may produce clumps of neutrals that are reflected in the ENA asymmetries noted here. Acknowledgments This research was supported by the NASA Office of Space Science under Task Order 003 of contract NAS between NASA Goddard Space Flight Center and Johns Hopkins University. 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