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GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L22104, doi:10.1029/2009GL041086, 2009
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Solar wind periodicity in energetic electrons at Saturn
J. F. Carbary,1 E. C. Roelof,1 D. G. Mitchell,1 S. M. Krimigis,1 and N. Krupp2
Received 22 September 2009; revised 19 October 2009; accepted 22 October 2009; published 21 November 2009.
[1] Fluxes of energetic electrons (110– 365 keV) have
been subjected to a long-term Lomb periodogram analysis
from the middle of 2004 to the middle of 2009. Fluxes
within 20 RS and outside the magnetopause were excluded,
so only fluxes within the magnetosphere were included in
the analysis. The periodicity ‘‘box’’ was expanded from
5 hours to 50 days. In addition to the familiar rotational
period near 10.8 hours, the electron fluxes exhibited a
strong periodicity near 26 days (the solar wind period) and
also a weaker periodicity near 13 days (half the solar wind
period). A simulated periodogram using a ‘‘rotating
anomaly’’ as it would be seen from the Cassini orbit does
not display 26-day and 13-day periods, so the solar wind
periodicity cannot result from a possible orbital resonance.
The long-term electron periodogram does not show any
periods associated with the periods of Saturn’s moons.
Citation: Carbary, J. F., E. C. Roelof, D. G. Mitchell, S. M.
Krimigis, and N. Krupp (2009), Solar wind periodicity in
energetic electrons at Saturn, Geophys. Res. Lett., 36, L22104,
doi:10.1029/2009GL041086.
1. Introduction
[2] The Cassini spacecraft has orbited the planet Saturn
from July 2004 to the present time and has returned a wealth
of scientific information. Among the most intriguing of
Cassini’s observations are those regarding the 10.8 hour
periodicities associated with the rotation of the planet.
Strong periodicities have been observed in the magnetic
field [Espinosa and Dougherty, 2000; Giampieri et al.,
2006], energetic charged particles [Carbary et al., 2007],
energetic neutral atoms [Paranicas et al., 2005; Carbary et
al., 2008a], radio emissions [Gurnett et al., 2005; Kurth et
al., 2008], and low energy plasma [Gurnett et al., 2007;
Burch et al., 2008, 2009], and the movements of the
magnetopause, plasma sheet, and auroral oval [Clarke et
al., 2006; Carbary et al., 2008b; Nichols et al., 2008].
[3] However, this rotational periodicity has been known
since the Voyager fly-bys of Saturn in the 1980’s [e.g.,
Desch and Kaiser, 1981; Carbary and Krimigis, 1982].
Until recently, most measurements of Saturn periodicities
were necessarily confined to shorter periodicities of less
than 24 hours because of the short observing intervals.
Now extending to over five years, the Cassini mission offers
much longer observations and makes possible a search for
periodicities considerably longer than the well-known 10.8
periodicity.
1
Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA.
2
Max Planck Institute for Solar System Research, Katlenburg-Lindau,
Germany.
Copyright 2009 by the American Geophysical Union.
0094-8276/09/2009GL041086$05.00
[4] A natural period to investigate with this extended
database is the rotation period of the Sun, a periodicity
known for over a century [e.g., Bartels, 1934]. The solar
period is 27 days relative to an Earth observer who is
moving at Earth’s orbital speed, while the ‘‘true’’ inertial
period of the Sun is 26 days, which would be seen by an
observer at Saturn because of its much slower orbital speed.
The 26-day rotation period will be referred to as the solar
period or the Sun’s period. Manifest in the solar wind and
geomagnetic activity, the solar periodicity is caused by the
interaction of high-speed and low-speed solar wind flows
that form co-rotating interaction regions (CIRs) [e.g.,
Crooker and Cliver, 1994]. Solar periodicities in solar wind
speed, density, and magnetic field are known to exist
throughout the solar system [e.g., Gosling and Bame,
1972; Fenimore et al., 1978; Mursula and Zeiger, 1996],
and 27-day periodicity in various geomagnetic activity is
also well documented [e.g., Clúa de Gonzalez et al., 1993,
and references therein]. Several investigators have suggested a strong coupling between the solar wind and
Saturn’s magnetosphere must exist [e.g., Cowley et al.,
2005; Bunce et al., 2008], so solar periodicities might be
expected to appear within Saturn’s magnetosphere, as they
do at Earth.
[5] This investigation takes advantage of about five years
of Cassini observations of energetic electrons (110 –
365 keV) in Saturn’s magnetosphere to determine if solar
periodicities exist there. As used here, solar periodicities
refer to the approximate solar period and its ‘‘nearest’’
harmonics at 26/2 (13.0) and 26/3 (8.7) days as observed
at Saturn.
2. Instrument and Data Set
[6] This investigation will employ data from the Low
Energy Magnetospheric Measurement System (LEMMS),
which is a sub-system of the Magnetospheric IMaging
Instrument (MIMI) on the Cassini spacecraft. The entire
instrument, including LEMMS, is described by Krimigis et
al. [2004]. LEMMS consists of a ‘‘forward-looking’’ set of
charged particle detectors in the low energy range and a
‘‘backward-looking’’ set in the high energy range. Within
the high energy set, the ‘‘E’’ channels observe electrons
from 100 keV to 18 MeV. The lowest energy E channel,
called ‘‘E0,’’ measures mildly-relativistic electrons from
110 to 365 keV, and this study focuses on the E0 data. The
E0 channel was chosen because electron fluxes in its energy
range are abundant throughout the magnetosphere and
essentially absent in the solar wind. Also, E0 is well-shielded
from penetrating radiation and has a low background.
[7] The LEMMS instrument is mounted on a turntable.
By rotating, this turntable provides pitch angle information
on charged particles. However, the turntable was set to a
fixed position in early 2005, so pitch angle information is
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rotation. Two more peaks, less prominent, appear at 13
days, which probably indicate the second (‘‘half’’) harmonic
of solar rotation. For comparison, the periods of Saturn’s
largest moons are also shown. There are no spectral peaks
associated with these.
[11] Judged merely in terms of amplitude, the solar
period at 26 days contains approximately five times the
‘‘power’’ of the well-known period at 10.8 hours. Note that
the Lomb periodogram reports spectral power at particular
frequencies, and not power spectral density [Press et al.,
1992]. This suggests that, when considered on long time
scales, the solar modulation of energetic particles at Saturn
is stronger than that at the rotation period of the planet.
4. Simulation
Figure 1. Lomb periodogram for the 110 – 365 keV
electrons observed from day 150 2004 through day 150
2009. Fluxes inside 20 RS and outside the magnetopause
were excluded. Vertical arrows indicate periods of interest.
not available unless the Cassini spacecraft itself rolls. While
Cassini does occasionally execute rolls, most of the time the
spacecraft is three-axis stable. Therefore, most of the pitch
angle information about the electrons is unknown, although
the most of the samples come from pitch angles near 90°.
[8] Before analysis, some conditioning of the E0 data
was performed. Count rates were converted to differential
fluxes (number/cm2secsrkeV) using standard MIMI data
processing and calibration. The E channels are sampled
with a time resolution of several seconds, but this study
uses 15-minute averages in order to improve statistics. The
15-minute averages were surveyed from mid-2004 to mid2009, and a few noise spikes were manually removed. The
resulting time profile was then filtered by radial distance.
Data within 20 RS of Saturn were removed in order to
overcome satellite absorption effects as well as possible
Doppler shifts caused by spacecraft orbital motion in the
inner magnetosphere. Data outside of the model magnetopause of Arridge et al. [2006] were also removed. This
magnetopause model assumed a solar wind ram pressure of
0.001 nPa.
3. Results
[9] The E0 fluxes were subjected to a Lomb periodogram
analysis of the same type previously employed to determine
periodicities in other energetic particles [Carbary et al.,
2007, 2008a]. The analysis extended from day 150, 2004,
through day 150, 2009, and the ‘‘period’’ window was
enlarged from 5 hours to 50 days. The numerous gaps in
the E0 profile that result from extensive filtering in radial
distance do not have an effect on a Lomb analysis [e.g.,
Press et al., 1992].
[10] Figure 1 displays the Lomb periodogram for the
entire time span of five years. The spectrum has been
normalized so that the maximum value is unity. Note that
the x-axis is logarithmic. The usual rotational period
appears near 10.8 hours, but a very interesting spectrum
appears in the 10– 30 day range. Two prominent peaks
occur near 26 days. These signify the first harmonic of solar
[12] For periods longer than a few days, the Lomb
spectrum of Figure 1 exhibits considerable noise in the
form of secondary spectral peaks. The noise could be
caused by gaps in the data record, which result from the
radial distance conditioning discussed above. But if orbital
motion of the Cassini spacecraft could cause these spikes, it
might also cause the main peaks near 26 days. This might
be expected because the Cassini orbits have a period in the
range of 20 – 35 days. Furthermore, some ‘‘resonances’’
between the orbital period and the 10.8 hour period might
also cause spikes.
[13] To demonstrate that the Cassini orbit is not, in fact,
causing the solar periodicities, a simple rotating anomaly
model was employed. A similar anomaly is often invoked
to explain Saturn’s 10.8-hour period [e.g., Carbary and
Krimigis, 1982; Burch et al., 2008, 2009]. In this case, a
longitudinal enhancement in the E0 fluxes is assumed to
rigidly rotate with a period of 10.8 hours. The fluxes
decrease exponentially with radial distance and distance
from the equatorial plane. A complete exposition of the
model is given by Carbary et al. [2009], where it was used
to model ENA periodicities.
[14] Figure 2 exhibits the simulated Lomb periodogram
resulting from the rotating anomaly model. To make the
simulation realistic, a time signal was first simulated using
the model and the actual Cassini trajectory. The resulting
time profile was then punctuated with exactly the same gaps
as appear in the dataset. Clearly, no spectral peaks appear at
the solar periods of 13 or 26 days. However, noise does
arise in the same region where noise is seen in the observed
spectrum. In particular, there is a noise enhancement near
15 days that appears in both the simulated and observed
spectra. The simulation suggests, then, that the ‘‘noise’’ in
the observation may be coming from orbital or gap effects.
5. Discussion
[15] The 110– 365 keV electrons discussed here do not
originate in the solar wind: they must originate within
Saturn’s magnetosphere. Corotating Interaction Regions
(CIRs) in the solar wind reflect the solar rotation as 26-day
periodicities and are most commonly observed during
declining and near-minimum solar activity [Gosling et
al., 1976]. Such periodic structures clearly existed in the
solar wind at Saturn before and during the Cassini mission
[e.g., Jackman et al., 2004, 2008a]. CIRs can and do
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97271 between NASA Goddard Space flight Center and the Johns Hopkins
University, and in part by NASA grant NNX08AQ77G from the Cassini
Data Analysis Program.
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Figure 2. Simulated Lomb periodogram using a rotating
anomaly model over the same time span and trajectory as
the observations. The model flux profile was interrupted
with the same gaps as the data.
impart energy to the terrestrial magnetosphere, so the
existence of a solar period in the energetic particles at
Saturn suggests that the solar wind either directly accelerates magnetospheric particles at Saturn or acts as a
trigger for the acceleration of such particles. Substorm-like
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2005; Jackman et al., 2007, 2008b], and these may be the
mechanism whereby particles are accelerated. The observation of a solar periodicity in Saturn’s energetic particles
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Saturn. This paper does not address whether the coupling
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(or 13-day) periodicity, which might explain the paucity of
reconnection observations.
6. Conclusions
[17] A Lomb periodogram of appropriately-conditioned
fluxes of energetic electrons (110– 365 keV) observed over
five years show not only the familiar period near 10.8 hours,
but also a strong periodicity near 26 days (the solar period)
and also a weaker periodicity near 13 days (half the solar
period). A simulated periodogram using a ‘‘rotating anomaly’’ model does not display 26-day and 13-day periods, so
the solar periodicity does result from effects of the Cassini
orbit or the signal conditioning. No periodicities associated
with Saturn’s moons appear in the periodogram.
[18] Acknowledgments. This research was supported in part by the
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J. F. Carbary, S. M. Krimigis, D. G. Mitchell, and E. C. Roelof, Johns
Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd.,
Laurel, MD 20723, USA. ([email protected])
N. Krupp, Max Planck Institute for Solar System Research, Max-PlanckStrasse 2, D-37191 Katlenburg-Lindau, Germany.
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