Saturn Kilometric Radiation Emissions Research Articles

Effect of an Interplanetary Coronal Mass Ejection on Saturn’s Radio Emission

The Saturn Kilometric Radiation (SKR) was observed for the first time during the flyby of Saturn by the Voyager spacecraft in 1980. These radio emissions, in the range of a few kHz to 1 MHz, are emitted by electrons travelling around auroral magnetic field lines. Their study is useful to understand the variability of a magnetosphere and its coupling with the solar wind. Previous studies have shown a strong correlation between the solar wind dynamic pressure and the SKR intensity. However, up to now, the effect of an Interplanetary Coronal Mass Ejection (ICME) has never been examined in detail, due to the lack of SKR observations at the time when an ICME can be tracked and its different parts be clearly identified. In this study, we take advantage of a large ICME that reached Saturn mid-November 2014 (Witasse et al., J. Geophys. Res. Space Physics, 2017, 122, 7865–7890). At that time, the Cassini spacecraft was fortunately travelling within the solar wind for a few days, and provided a very accurate timing of the ICME structure. A survey of the Cassini data for the same period indicated a significant increase in the SKR emissions, showing a good correlation after the passage of the ICME shock with a delay of ∼13 h and after the magnetic cloud passage with a delay of 25–42 h.

Planetary Period Oscillations in Saturn's Magnetosphere: Comparison of Magnetic and SKR Modulation Periods and Phases During Northern Summer to the End of the Cassini Mission

AbstractWe compare periods and phases of Saturn planetary period oscillations determined from Cassini magnetic field and Saturn kilometric radiation (SKR) data from the beginning of 2016 to the end of mission in mid‐September 2017, encompassing northern summer solstice in May 2017. Both data sets show that the periods are almost unchanging, varying by only ~ ±0.01 hr about 10.79 hr for the northern system and 10.68 hr for the southern system, close to values attained by mid‐2015 after period coalescence between mid‐2013 and mid‐2014. The mean absolute differences between the magnetic and SKR periods are ~0.0036 hr (~13 s), consistent with estimated magnetic measurement uncertainties, while the overall mean difference is less than 0.001 hr (~2–3 s), at the limit of resolution. The relative phasing between magnetic and SKR modulations is correspondingly near constant and such that the equatorial planetary period oscillation fields of the northern/southern systems point radially outward near‐oppositely at ~14.3/2.5 hr local time at corresponding SKR maxima, with upward planetary period oscillation currents located ~2 hr postdawn for both systems, consistent with previous intervals having dawnside spacecraft apoapsides. Southern SKR emissions are found to be significantly dual modulated at both southern and northern periods in data limited to lie well within the southern shadow zone of the northern sources. These northern period modulations are shown to be approximately in phase with those in the northern emissions, consistent with a recent suggestion that bidirectional auroral electron acceleration may generate in phase SKR emissions in both hemispheres.

Field‐aligned currents in Saturn's magnetosphere: Local time dependence of southern summer currents in the dawn sector between midnight and noon

AbstractWe examine and compare the magnetic field perturbations associated with field‐aligned ionosphere‐magnetosphere coupling currents at Saturn, observed by the Cassini spacecraft during two sequences of highly inclined orbits in 2006/2007 and 2008 under late southern summer conditions. These sequences explore the southern currents in the dawn‐noon and midnight sectors, respectively, thus allowing investigation of possible origins of the local time (LT) asymmetry in auroral Saturn kilometric radiation (SKR) emissions, which peak in power at ~8 h LT in the dawn‐noon sector. We first show that the dawn‐noon field data generally have the same four‐sheet current structure as found previously in the midnight data and that both are similarly modulated by “planetary period oscillation” (PPO) currents. We then separate the averaged PPO‐independent (e.g., subcorotation) and PPO‐related currents for both LT sectors by using the current system symmetry properties. Surprisingly, we find that the PPO‐independent currents are essentially identical within uncertainties in the dawn‐dusk and midnight sectors, thus providing no explanation for the LT dependence of the SKR emissions. The main PPO‐related currents are, however, found to be slightly stronger and narrower in latitudinal width at dawn‐noon than at midnight, leading to estimated precipitating electron powers, and hence emissions, that are on average a factor of ~1.3 larger at dawn‐noon than at midnight, inadequate to account for the observed LT asymmetry in SKR power by a factor of ~2.7. Some other factors must also be involved, such as a LT asymmetry in the hot magnetospheric auroral source electron population.

Planetary period oscillations in Saturn's magnetosphere: Further comments on the relationship between post-equinox properties deduced from magnetic field and Saturn kilometric radiation measurements

We discuss the planetary period oscillations (PPOs) observed by the Cassini spacecraft in Saturn's magnetosphere, in particular the relationship between the properties of the PPOs in the post-equinox interval as observed in magnetic field data by Andrews et al. (2012) and Provan et al. (2013, 2014) and in Saturn kilometric radiation (SKR) emissions by Fischer et al. (2014, 2015), whose results are somewhat discrepant. We show that differences in the reported PPO periods, a fundamental property which should be essentially identical in the two data sets, can largely be accounted for by the phenomenon of dual modulation of the SKR emissions in polarization-separated data, in which the modulation associated with one hemisphere is also present in the other. Misidentification of the modulations results in a reported reversal in the SKR periods in the initial post-equinox interval, south for north and vice versa, relative to the magnetic oscillations whose hemispheric origin is more securely identified through the field component phase relations. Dual modulation also results in the apparent occurrence of phase-locked common periods in the northern and southern SKR data during later intervals during which two separate periods are clearly discerned in the magnetic data through beat modulations in both phase and amplitude. We further show that the argument of Fischer et al. (2015) concerning the phase relation between the magnetic field oscillations and the SKR modulations is erroneous, the phase difference between them revealing the local time (LT) of the upward field-aligned current of the PPO current system at times of SKR modulation maxima. Furthermore, this LT is found to vary significantly over the Cassini mission from dawn, to dusk, and to noon, depending on the LT of apoapsis where the spacecraft spends most time. These variations are consistent with the view that the SKR modulation is fundamentally a rotating system like the magnetic perturbations, though complicated by the strong LT asymmetry in the strength of the sources, and rule out a mainly clock-like (strobe) modulation as argued by Fischer et al. (2015), for which no physical mechanism is suggested. We also elucidate the nature of the magnetic periods, criticized by Fischer et al. (2015), which have previously been derived in ∼100–200 day post-equinox intervals between abrupt changes in PPO properties, and further show that their argument that the magnetic phase data provide evidence for the occurrence of common phase-locked magnetic oscillations in some intervals is fallacious. The most important consequence of our results, however, is that they demonstrate the essential compatibility of the post-equinox magnetic field and SKR data, despite the contrary results published to date. They also show that due to the dual modulation effect in polarization-separated SKR data, analysis and interpretation may contain more subtleties than previously realized. Joint examination of the combined magnetic and SKR data clearly provides greater insight and enhanced confidence compared with analyses of these data sets individually.

Planetary period oscillations in Saturn's magnetosphere: comments on the relation between post-equinox periods determined from magnetic field and SKR emission data

Abstract. We discuss the properties of Saturn planetary period oscillations (PPOs) deduced from analysis of Saturn kilometric radiation (SKR) modulations by Fischer et al. (2014), and from prior analysis of magnetic field oscillations data by Andrews et al. (2012) and Provan et al. (2013), with emphasis on the post-equinox interval from early 2010 to early 2013. Fischer et al. (2014) characterize this interval as showing single phase-locked periods in the northern and southern SKR modulations observed in polarization-separated data, while the magnetic data generally show the presence of separated dual periods, northern remaining shorter than southern. We show that the single SKR period corresponds to the southern magnetic period early in 2010, segues into the northern period in late 2010, and returns to the southern period in mid-2012, approximately in line with changes in the dominant magnetic oscillation. An exception occurs in mid-February to late August 2011 when two periods are again discerned in SKR data, in good agreement with the ongoing dual periods in the magnetic data. Fischer et al. (2014) discuss this change in terms of a large jump in the southern SKR period related to the Great White Spot storm, which the magnetic data show is primarily due instead to a reappearance in the SKR data of the ongoing southern modulation in a transitory interval of resumed southern dominance. In the earlier interval from early April 2010 to mid-February 2011 when Fischer et al. (2014) deduce single phase-locked periods, we show unequivocal evidence in the magnetic data for the presence of separated dual oscillations of approximately equal amplitude. We suggest that the apparent single SKR periods result from a previously reported phenomenon in which modulations associated with one hemisphere appear in polarization-separated data associated with the other. In the following interval, mid-August 2011 to early April 2012, when Fischer et al. (2014) again report phase-locked northern and southern oscillations, no ongoing southern oscillation of separate period is discerned in the magnetic data. However, the magnetic amplitude data show that if a phase-locked southern oscillation is indeed present, its amplitude must be less than ~ 5–10 % of the northern oscillation.

Field dipolarization in Saturn's magnetotail with planetward ion flows and energetic particle flow bursts: Evidence of quasi-steady reconnection.

We present a case study of an event from 20 August (day 232) of 2006, when the Cassini spacecraft was sampling the region near 32 RS and 22 h LT in Saturn's magnetotail. Cassini observed a strong northward‐to‐southward turning of the magnetic field, which is interpreted as the signature of dipolarization of the field as seen by the spacecraft planetward of the reconnection X line. This event was accompanied by very rapid (up to ~1500 km s−1) thermal plasma flow toward the planet. At energies above 28 keV, energetic hydrogen and oxygen ion flow bursts were observed to stream planetward from a reconnection site downtail of the spacecraft. Meanwhile, a strong field‐aligned beam of energetic hydrogen was also observed to stream tailward, likely from an ionospheric source. Saturn kilometric radiation emissions were stimulated shortly after the observation of the dipolarization. We discuss the field, plasma, energetic particle, and radio observations in the context of the impact this reconnection event had on global magnetospheric dynamics.

Driving Saturn's magnetospheric periodicities from the upper atmosphere/ionosphere

Saturn's magnetospheric structure and the intensity of radio frequency emissions from its immediate surroundings are modulated at close to the planet's rotation period. Analogous rotation‐modulated variations at Jupiter are readily interpreted as effects of the non‐axisymmetric intrinsic magnetic field. At Saturn, to the contrary, the high level of axial symmetry in the intrinsic field suggests that the periodicity is not internally imposed. A number of mechanisms have been proposed to account for the observations. Each model explains a subset of the observations in a qualitative manner, but no quantitative models yet exist. Here, using a magnetohydrodynamic simulation, we investigate the magnetospheric perturbations that arise from a localized vortical flow structure in the ionosphere near 70° S‐latitude that rotates at roughly the rate of planetary rotation. The model reproduces nearly quantitatively a host of observed magnetospheric periodicities associated with the period of the dominant (southern) radio frequency emissions during the Cassini epoch including rotating, quasi‐uniform magnetic perturbations in the equatorial plane, rotating mass density perturbations, periodic plasmoid releases that we associate with observed bursts of energetic neutral atoms (ENAs), periodic oscillations of magnetospheric boundaries, current sheet flapping, and periodic modulation of the field‐aligned currents linked to Saturn's kilometric radiation (SKR). The model is not unique but is representative of a class of models in which asymmetric flows in the (as yet unmeasured) upper atmosphere couple to the ionosphere and generate currents that flow into the magnetosphere. It can be extended to include the second periodicity that has been associated with SKR emissions in the northern hemisphere.

Axial symmetry breaking of Saturn’s thermosphere

The source of the various planetary-period signals in Saturn’s magnetosphere is at present unknown. We investigate the possibility that the source of these signals is an axially asymmetric wind system in the thermosphere. We describe a feedback mechanism that has the potential to drive such axially asymmetric wind systems. The proposed mechanism relates thermospheric winds to heating from particle precipitation, via the generation of horizontal and field-aligned currents. The relevant physical processes are investigated using a highly simplified general circulation model of Saturn’s thermosphere and ionosphere. Our principal result is that the feedback mechanism is effective in permanently breaking the axial symmetry of the thermosphere, generating a drifting vortex-like structure at high latitudes. However, the precipitating electron energies required to power this structure are of the order of 5 MeV, 2–3 orders of magnitude greater than the observed auroral electron energies, and the highly axially asymmetric distribution of precipitation required across the polar regions of the planet is also inconsistent with observations. Despite these flaws, the model qualitatively explains several features of the observed variation in the pulsing of SKR emissions; in particular, the seasonal variation and the faster rotation rate in the winter hemisphere. We cannot reproduce the apparent 7 month lag in the response of the Saturn Kilometric Radiation (SKR) rotation rate to seasonal variation, but instead suggest the possibility that this effect may have its origin in long chemical time-scales in the upper atmosphere. We also predict the possible existence of secondary periodic features in the SKR emissions with periods of ∼15 planetary rotations, driven by complex wave behaviour in the thermosphere.

Dual periodicities in planetary‐period magnetic field oscillations in Saturn's tail

We examine magnetic field data from 10 apoapsis passes of the Cassini spacecraft during 2006 when the spacecraft explored the midnight and dawn sectors of Saturn's magnetotail to down‐tail distances of ∼65 RS (Saturn radius, RS, is 60,268 km). Oscillations in the radial component of the field near the ∼11 hour planetary period associated with north‐south motions of the current sheet are ubiquitous in these data. Here, we examine and model the phase of these oscillations throughout the interval, taking account of both local time and radial propagation effects, and show that the oscillations exhibit dual periodicities. Those observed at distances exceeding ∼3 RS north of the modeled average center of the current sheet are found to oscillate near the modulation period of the northern Saturn kilometric radiation (SKR) emissions, while those observed south of this location oscillate near the modulation period of the southern SKR emissions. The phasing in both cases is consistent with the sense of the associated rotating quasi‐uniform perturbation fields within the quasi‐dipolar “core” region. We determine the structure of the current sheet as a function of the modeled phases, the results implying that the form of the modulation varies significantly over the beat cycle of the two oscillations. When the two field oscillations are in phase, the current sheet oscillates north‐south with a peak‐to‐peak amplitude of ∼3 RS. When they are in antiphase, however, the thickness of the current sheet is also strongly modulated during the oscillation by factors of ∼2.

Planetary period oscillations in Saturn's magnetosphere: Evidence in magnetic field phase data for rotational modulation of Saturn kilometric radiation emissions

Planetary period oscillations in Saturn's magnetosphere: Evidence in magnetic field phase data for rotational modulation of Saturn kilometric radiation emissions

Research Spotlight: North and south components of Saturn's radio emissions reversed

Saturn is known to emit intense radio emissions at kilometer wavelengths from its auroral regions. Observations in recent years found that the Saturn kilometric radiation (SKR) emission from the northern auroral region has a clocklike modulation with a period of about 10.6 hours, while the SKR emission from the southern auroral region has a period of about 10.8 hours. Analyzing more recent observations from the Cassini spacecraft, Gurnett et al. have now found that the rotational modulation rates of the southern and northern components reversed shortly after Saturn's equinox on 11 August 2009, so that the southern hemisphere SKR now has the shorter rotation period. They also analyzed data from the Ulysses spacecraft to show that a similar reversal occurred during the previous equinox, in November 1995. (Geophysical Research Letters, doi:10.1029/2010GL045796, 2010)

Dual periodicities in the rotational modulation of Saturn narrowband emissions

Using the Cassini Radio and Plasma Wave Science (RPWS) instrument it has recently been shown that the rotational modulation period of Saturn kilometric radiation (SKR) has two components, one with a period of 10.6 h and the other with a period of 10.8 h. The longer period is primarily observed in the southern hemisphere and the shorter period is primarily observed in the northern hemisphere. In this paper, the modulation period of 5 kHz Saturn narrowband radio emissions is examined, restricting the spacecraft location to either the northern or the southern hemisphere of Saturn. It is found that in both hemispheres, the modulation period of 5 kHz narrowband emissions has two components that are equal to the SKR periods. It is known that Saturn narrowband emissions are first generated in the auroral regions as Z‐mode and then mode convert to escaping L‐O modes at density gradients. These Z‐mode waves are trapped in a region close to the planet and can propagate from one hemisphere to another before they mode convert to L‐O modes, thereby leading to dual periods of Saturn narrowband emissions in each hemisphere. The phase shift between the SKR and narrowband emissions is around 90° in the longitude, which means that SKR leads narrowband emissions by 2–3 h.

On the character and distribution of lower‐frequency radio emissions at Saturn and their relationship to substorm‐like events

With the arrival of the Cassini spacecraft at Saturn in July 2004, there have been quasi‐continuous observations of Saturn kilometric radiation (SKR) emissions. Exploration of the nightside magnetosphere has revealed evidence of plasmoid‐like magnetic structures and other phenomena indicative of the Kronian equivalent of terrestrial substorms. In general, there is a good correlation between the timing of reconnection events and enhancements in the auroral SKR emission. Eight of nine reconnection events studied occur at SKR phases where the SKR power would be expected to be rising with time. Thus, while the recurrence rate of substorm‐like events at Saturn is likely much longer than the planetary rotation timescale, the events are favored to occur at a particular phase of the rotation. We show three examples in each of which the SKR spectrum extends to lower frequencies than usual. This can be interpreted as an expansion of the auroral particle acceleration region to higher altitudes along magnetic field lines as a direct consequence of an increase in the magnetosphere‐ionosphere current density driven by substorm‐like events. We then conduct a survey of such low‐frequency extensions during the equatorial orbits of 2005–2006 and place some constraints on visibility of these radio emissions.

Influence of Saturnian moons on Saturn kilometric radiation

Similar to past studies at Jupiter, we conduct an investigation of possible associations of radio emission occurrence probability with the orbital phases of Saturnian moons. We use a new definition of the Saturn longitude system (SLS) based on the results of Kurth et al. (2007). This paper presents results of our findings to date, sampling a large portion of the Radio and Plasma Wave Science (RPWS) instrument data over the frequency range 12 kHz < f < 16 MHz. We also investigate the intensity of Saturn kilometric radiation (SKR) as a function of the local time and subsolar longitude. When Titan is near a local time of midnight, there is a significant increase in the occurrence probability of SKR and a diminution of SKR when Titan is near local noon and afternoon. This indicates Titan may play a role in the process of substorm generation at Saturn perhaps due to its large plasma wake. Rhea displays a marginal orbital phase “control” of a subset of the SKR. In the past there have been conflicting reports of the absence of SKR emission at particular local times of Dione. We find no long‐term, statistical influence of Dione on SKR occurrence probability. In addition, we find no significant statistical influence of Enceladus or Tethys on SKR occurrence probability.

Linear prediction studies for the solar wind and Saturn kilometric radiation

Abstract. The external control of Saturn kilometric radiation (SKR) by the solar wind has been investigated in the frame of the Linear Prediction Theory (LPT). The LPT establishes a linear filter function on the basis of correlations between input signals, i.e. time profiles for solar wind parameters, and output signals, i.e. time profiles for SKR intensity. Three different experiments onboard the Cassini spacecraft (RPWS, MAG and CAPS) yield appropriate data sets for compiling the various input and output signals. The time period investigated ranges from DOY 202 to 326, 2004 and is only limited due to limited availability of CAPS plasma data for the solar wind. During this time Cassini was positioned mainly on the morning side on its orbit around Saturn at low southern latitudes. Four basic solar wind quantities have been found to exert a clear influence on the SKR intensity profile. These quantities are: the solar wind bulk velocity, the solar wind ram pressure, the magnetic field strength of the interplanetary magnetic field (IMF) and the y-component of the IMF. All four inputs exhibit nearly the same level of efficiency for the linear prediction indicating that all four inputs are possible drivers for triggering SKR. Furthermore, they act at completely different lag times ranging from ~13 h for the ram pressure to ~52 h for the bulk velocity. The lag time for the magnetic field strength is usually beyond ~40 h and the lag time for the y-component of the magnetic field is located around 30 h. Considering that all four solar wind quantities are interrelated in a corotating interaction region, only the influence of the ram pressure seems to be of reasonable relevance. An increase in ram pressure causes a substantial compression of Saturn's magnetosphere leading to tail collapse, injection of hot plasma from the tail into the outer magnetosphere and finally to an intensification of auroral dynamics and SKR emission. So, after the onset of magnetospheric compression at least ~1.2 rotations of the planet elapse until intensified SKR emission is visible in a Cassini-RPWS dynamic spectrum.

In situ observations of a solar wind compression‐induced hot plasma injection in Saturn's tail

During the Saturn orbit insertion (SOI) fly‐through of the Cassini spacecraft, Saturn's magnetosphere underwent a significant corotating interaction region (CIR) related compression. Such compressions have recently been suggested to produce rapid bursts of tail reconnection, enhanced Saturn kilometric radiation (SKR), and consequent auroral dynamics. On the outbound pass the spacecraft became engulfed by hot plasma, associated with a reduction in field strength, and a change in orientation indicative of a dipolarisation. Concurrently, a substantial enhancement in SKR emissions took place, together with a disruption of the typical planetary modulation. We suggest this is the first in situ evidence of compression‐related tail collapse via magnetic reconnection and hot plasma acceleration in Saturn's magnetotail.

Saturn's radio emissions: rotational modulation

Observational data provided by Pioneer 11 and Voyager 1 have revealed that the Saturnian ring system has important effects on the structures of the planetary ionosphere as well as the magnetospheric environment, such as the unexpected rotational modulation of the Saturn kilometric radiation (SKR) and the Saturn electrostatic discharge (SED) as discovered by the Voyager planetary radio astronomy experiment. The challenge arises to devise a mechanism which can produce electrodynamic coupling between the ring system and the planetary ionosphere and/or magnetosphere in which the associated SKR and SED radio emissions are subject to longitudinal control. One candidate is the establishment of field-aligned current systems, connecting the rings and the ionosphere with longitudinal variation. Local acceleration processes in the vicinity of the rings are also needed. The interplay between the ring-current systems and the geometry of the dipole field and the rings is discussed, and a description is presented of some of the basic ingredients in the longitudinal modulations of SKR and SED.