1 Introduction

At Saturn it is unclear that a plasmasphere similar to that at Earth exists. Gurnett et al. (2010) have reported a rotating “plasmasphere-like” boundary with increased density, and with distinctive, intense, low-frequency whistler mode emission (some with “funnel” signatures) and often with approximately 1-hr periodicity. Whereas at Earth the plasmaspheric boundary is typically 3 < L < 4, at Saturn this boundary lies at larger L values (8 < L < 15 as reported by Gurnett et al. (2010)), consistent with a fast rotating magnetosphere (Brice & Ioannidis, 1970).

Auroral hiss at Earth is distinguished on frequency-time spectrograms by funnel-shaped outer boundaries of the emission. These boundaries are a result of a frequency-dependent source mechanism and propagation/emission limits determined by a resonance cone angle relative to the magnetic field (cf. Gurnett et al., 1983; Gurnett & Bhattacharjee, 2005). Terrestrial auroral hiss has been shown to be generated by electron beams in a Landau interaction (cf. Maggs, 1976). The electron beams generate whistler mode auroral hiss propagating in the same direction as the parallel component of the electron beams. Electron and ion dynamics of these processes observed by the FAST satellite have been reported by Ergun et al. (1998) and Carlson et al. (1998). Hiss observed at Saturn is observed with the same funnel-shaped frequency limits (Kopf et al., 2010; Sulaiman et al., 2018; Xin et al., 2006), as well as less descriptive signatures (cf. Gurnett et al., 2010). These emissions have also been associated with energetic electron beams (Mitchell et al., 2009, 2016). Mitchell et al. (2016) have reported Saturn hiss observations correlated with ion conics and electron beams during a high-latitude orbit of the Cassini spacecraft at a distance of approximately 15 Rs (Rs = 60,330 km). These observations revealed the presence of upward electron beams associated with quasiperiodic hiss episodes. In addition, UV observations of the auroral oval at this time revealed an expansion of the oval into the polar cap and a repeated intensification of the oval with a period of about 1 hr. Mitchell et al. (2009) indicated that these events appeared to occur in the region ~9 < L < ~70, but may occur at L < 9 where more isotropic energetic ion and electron populations can occur. These regions occur equatorward of the visible aurora in the so-called “black aurora,” a region of downward currents. Observations of periodic pulsations within the radiation zone have not been reported in the past. This paper reports observations of periodic hiss during the proximal orbits of Cassini and provides a unique opportunity to observe auroral hiss near the source region, as well as the region L < 9.

In an extensive survey of low-frequency plasma waves at Saturn, Carbary et al. (2016) reported oscillations with an approximate 1-hr period. These electromagnetic waves, centered near 100 Hz and identified as whistler mode auroral hiss, were observed throughout the magnetosphere, but predominately for local times between dusk and midnight. Yates et al. (2016) have presented evidence that these 1-hr oscillations are due to low-frequency magnetic field line oscillations, which are known to occur at Earth. These observations have a parallel with terrestrial electron beam-generated auroral hiss seen on downward current field lines near upward current magnetic field lines supporting auroral kilometric radiation source regions (Gurnett et al., 1983). Alternatively, recent observations of Cassini Ultraviolet Imaging Spectrograph (UVIS) auroral images (Bader et al., 2019) indicate the presence of ~1-hr periodic auroral “flashes.” These authors suggest that they are related to electron injections and auroral hiss observations, and are likely due to “recurrent small-scale magnetodisk reconnection on closed field lines.” In addition, Roussos et al. (2016) and Palmaerts et al. (2016) demonstrate that the ~1-hr periodicities of Cassini LEMMS injection signatures lie mainly outside of Titan's orbit (20 Rs), often close to the magnetopause, and predominantly on the duskside; however, Palmaerts et al. (2016) identified ~1-hr electron pulsations in the high-latitude magnetosphere as well, some of them simultaneous with auroral hiss quasiperiodic oscillations. As suggested by Palmaerts et al. (2016) observations of 1-hr periodicities in the LEMMS and auroral hiss data during the Cassini high-latitude proximal orbits may provide more insight into the source mechanism of these phenomena.

In this paper we present results of ~1-hr quasiperiodic auroral hiss observations during a Cassini high-inclination proximal orbit, providing a unique proximity to the source region of auroral hiss. The data show a quasicorrelation with LEMMS electron bursts. We are able to map the centers of two auroral hiss bursts to Saturn ionospheric footprints quite near auroral features in each hemisphere, and the sequence of auroral quasiperiodic bursts appear to initiate near and at higher L-shells than magnetic field lines traversing SKR source regions.

2 Instrumentation

The Cassini Radio and Plasma Wave Science (RPWS) instrument directly measures oscillating electric fields (1 Hz to 16 MHz) and magnetic fields (1 Hz to 12 kHz; cf. Gurnett et al., 2004). The instrument uses three nearly orthogonal electric field antennas and three orthogonal magnetic search coil antennas and five receiver systems. In this study we focus on the high-, medium-, and low-frequency receivers (HFR, MFR, and LFR) covering respective frequency ranges of 3.5 kHz to 16 MHz, 24 Hz to 12 kHz, and 1 to 26 Hz.

The UVIS is an instrument that is part of the remote sensing payload on board Cassini (Esposito et al., 2004). The narrow low-resolution slit of the FUV channel (110.5–191.2 nm) allows 64 spatial pixels of 1 by 1.5 mrad along and across the slit. Each image displayed in this paper was scanned in approximately 13 min, and shows the emissions between 118 and 165 nm projected onto a polar plane at an altitude of 1,100 km above the 1-bar level (Grodent et al., 2011).

The MIMI/LEMMS detector on board Cassini measures charged particles (electrons and ions) using two telescopes (low energy, LET, and high energy, HET) pointing in opposite directions (Krimigis et al., 2004). LEMMS measures electrons from 18 keV to over 10 MeV. The HET electron channels, E0 to E6, cover energies from 110 keV to over 10 MeV (see Krupp et al., 2009, Table 1). The electron channels of the LET, C0 to C7, measure energies from 18 to 532 keV.

3 Observations

Figure 1 is a frequency-time spectrogram of quasiperiodic auroral hiss (electric field spectral density) observed by the Cassini RPWS during the perikrone pass of orbit 285 in 2017 extending from 03:00 of day 206 to 03:00 of day 207. The Cassini spacecraft at this time was in a high-inclination proximal orbit traveling from the northern to the southern hemisphere, crossing the ring plane near 19:00 where it traverses the ionosphere. White lines on this figure designate the cyclotron frequency (f_c) and the plasma frequency (f_p) which increases dramatically within the ionosphere. We also indicate intense periodic auroral hiss which extend up to frequencies near 3 kHz, particularly in the southern hemisphere. In addition we point out the Saturn kilometric radiation (SKR) emission near source regions in the north and south hemispheres. The auroral hiss is seen to be the most intense emission on the spectrogram, and is frequently so (cf. Menietti et al., 2019). Indicated by vertical arrows is the location of hiss at the highest latitude in each hemisphere, which in each case is near and at higher L-shells than a magnetic field line containing an SKR source region.

In Figure 2 we show a two-panel plot of RPWS hiss magnetic intensity (top panel) and LEMMS differential intensity (normalized units). The hiss intensities were calculated by integrating the spectral density over the frequency range 400 Hz < f < 4 kHz for the magnetic field receiver of RPWS. The intense pulses of hiss are sometimes coincident with LEMMS electron fluxes in the time range 03:00 < t < ~16:00 of day 206, and later intervals as we will show subsequently. The “C” and “E” LEMMS detectors measure electrons in opposite directions along the field line at this time. The pitch angle of the pulsed E electrons during day 206 is 110° before 7:30 and ranging from 10° to 70° (upward away from Saturn) after 7:30. Palmaerts et al. (2016) have shown that electron fluxes can be bidirectional during pulsations. Focusing on the strongest emission episode of periodic hiss that begins very near 20:00 on day 206 to about 03:00 on day 207, in Figure 3 we plot low-rate hiss intensity (top) and LEMMS detrended electron flux (middle), and electron pitch angle data (bottom). The LEMMS data have been detrended using a fourth-degree polynomial fit (different for each channel) in order to highlight the fluctuations which are not visible in Figure 2. The plot of pitch angle indicates bidirectional electrons with a modulation due to a rolling of the spacecraft with a period of about 56 min. However, the hiss emissions are noted to begin before the spacecraft roll begins and continue after it ends (207/00:22), indicating that the hiss pulsations are not in phase with the roll period.

The Cassini UVIS instrument was imaging the aurora during a portion of this perikrone pass. In Figure 4a we display a UVIS auroral image for the northern hemisphere, taken at 06:38 of day 2017/206, which is near the beginning time of the northern hemisphere episode of periodic auroral hiss shown in Figures 1 and 2 for auroral hiss observed near LT ~1 hr. Auroral intensities are displayed with a linear color scale saturated at 12 kiloRayleighs (kR). The lines of latitude in Figure 4 are shown at 10° intervals. The image is scanned over a period of about 13 min, and it is one that is closest in time to the data of Figure 2. The footprint of the magnetic field line traversed by Cassini at 06:47, centered on a pulse of auroral hiss, is shown by an orange dot on the plot near 1 hr LT and a latitude of 76.6°, near the northern edge of rather intense auroral features centered near 75° latitude. The field line was traced using the Cassini 11 magnetic field model for Saturn (Dougherty et al., 2018). We have also included the ring current model as described in Bunce et al. (2007), which have very little impact on our results. All footprints are mapped to an ionospheric altitude of 1,100 km above the 1-bar level (r = 55,464 km). The orange crosses mark the footprint of Cassini from 206/06:47 to 11:17 (see Table 1 for times and footprint locations).

In Figure 4b we display a southern hemisphere auroral image taken on day 207 of 2017 at 02:38, and it is one that is closest in time to the data of Figures 1 and 3. The color scale is saturated at 18 kR. On this image are auroral signatures within the latitude range λ < −70°. The orange dot indicates the footprint of the magnetic field line on which Cassini lies at 01:42, coincident with a burst of hiss near the end of the hiss periodic emission shown in Figure 1, on the nightside. This point is near 22.5 hr LT and at latitude of about −71.3°, which is within the intense auroral activity seen above −75° latitude. The orange crosses track the Cassini footprint from 206/21:21 to 207/01:42 (Table 1). We have also mapped the field line of the SKR source regions to the ionosphere, and these are shown in Figure 4 (Table 1) as red asterisks in the north and south hemisphere plots. The auroral image is not taken at the same time as the SKR source regions encounters, but it does indicate that in each hemisphere the SKR field line footprint is equatorward of the lowest latitude of the hiss footprints.

4 Discussion

An interesting result of our studies of periodic hiss along a proximal orbit is that the sequence of ~1-hr episodes begins in each hemisphere near the magnetic field line that traverses the observed SKR source region. SKR is widely believed to be generated by the cyclotron maser instability (Lamy et al., 2008; Menietti et al., 2011; Mutel et al., 2010; Wu & Lee, 1979). The cyclotron maser instability is proposed as the source mechanism of auroral kilometric radiation at Earth along auroral field lines associated with precipitating field-aligned electrons (upward currents). At times, the free energy source of SKR is likely injected electrons from dayside or nightside reconnection sites. Bader et al. (2019) reported observations of periodic auroral flashes due to injections believed to occur on reconnected magnetodisc flux tubes. Delamere et al. (2015) analyzed Cassini magnetometer data from current sheet crossings, concluding that reconnection sites occurred predominately in the dusk flank of Saturn. Note in Figure 4 that the SKR source region ionospheric footprints occur at lower latitude than the periodic auroral hiss episodes in each hemisphere. Roussos et al. (2016) and Palmaerts et al. (2016) demonstrate that the ~1-hr periodicities of LEMMS injection signatures lie mainly outside of Titan's orbit (20 Rs), often close to the magnetopause, and predominantly on the duskside. SKR source regions are predominately observed on the dawnside of Saturn (Lamy et al., 2008), where we see the strongest intensity of SKR and auroral hiss, but not exclusively (Lamy et al., 2008).

Referring again to Figure 1, Cassini crosses many magnetic L-shells during a proximal orbit. While the periapsis is on the dayside, near the equator, Cassini approaches periapsis in the northern hemisphere on nightside, decreasing L-shells, passing to the morningside, still decreasing L-shells and nearly intercepting the northern SKR source field line (~17:50) at almost the highest latitude, followed by periapsis then nearly intercepting the southern hemisphere SKR source field line (~19:45) close to the highest latitude, and departing Saturn on slowly increasing, (but nearly constant, L~8) nightside L-shells until about 02:00 of day 207. Therefore, in the northern hemisphere Cassini RPWS intercepts convecting field lines until nearly traversing the SKR source field line at near the highest latitude. After periapsis, Cassini nearly intercepts the southern SKR source field line close to L~8. The magnetic field line that traverses the SKR source region is nearly the highest latitude at which the periodic hiss is observed in each hemisphere. This is consistent with the observations of Mitchell et al. (2016), Roussos et al. (2016), Palmaerts et al. (2016), and Bader et al. (2019).

In Figure 5 we show the Cassini trajectory in a ρ-z plot with hourly time tags for the northern and southern hemispheres. The hours of periodic hiss observations are indicated with blue dots in each hemisphere. The L-shells were determined using the Cassini 11 magnetic field model (Dougherty et al., 2018) including a ring current model (Bunce et al., 2007). The approximate time of the interception of the SKR source region field lines is indicated in each hemisphere (N, ~17:50 and S, ~19:50) by a red circle. We envision a region of return current (upward or bidirectional electrons and source of the hiss) on field lines adjacent to the SKR field line in each hemisphere, consistent with LEMMS data of Figures 2 and 3. Auroral hiss can be observed by the spacecraft only when it intercepts the hiss emission cone. As typically seen in the terrestrial auroral zone by a polar orbiting spacecraft, auroral hiss appears as a funnel-shaped emission due to the frequency-dependent nature of the whistler mode (Gurnett et al., 1983; Gurnett & Bhattacharjee, 2005). Funnel-shaped emission boundaries are not always seen on spectrograms of Saturn periodic auroral hiss because of the high velocity and trajectory of the spacecraft relative to the hiss emission cone, and the pulsating nature of the hiss. Note that in the northern hemisphere relatively narrow banded, quasiperiodic hiss is observed over a range of L-shells, ~11 < L < ~14, but from ~8:00 to ~17:00 on day 206 Cassini travels near to L~12. In contrast, in the southern hemisphere after Cassini encounters the SKR field line close to the highest latitude, the spacecraft travels along an almost constant L-shell (~7.5 < L < ~8) where RPWS observes intense hiss with a larger bandwidth and a more constant periodicity.

In the southern hemisphere the spacecraft observes periodic auroral hiss at different longitudes (as Saturn rotates) while maintaining an approximately constant L-shell (L~8). This would be possible if Cassini were observing hiss source regions resulting from electron injections at earlier local times on the dayside that periodically arrive at L~8 at postdusk to stimulate auroral hiss near the footprint of the spacecraft magnetic field line.

Similarly in the northern hemisphere the spacecraft observes periodic auroral hiss at different longitudes while maintaining an approximately constant L-shell (L~12). Likewise this would be possible if Cassini were observing source regions resulting from electron injections at earlier LT on the duskside. For the northern hemisphere the auroral hiss is somewhat irregular in period and is not as intense or broad-banded possibly because Cassini is nearer to the edge of the hiss emission beam pattern, the center of which is likely on lower L-shells.

Cowley and Bunce (2003) have presented a picture of overall field-aligned current at Saturn based on Voyager 1 and 2 data interpretation. The authors' Figure 1 displays an overall flow of current driven by partial corotation, with independent flows in northern and southern hemispheres. Cassini has allowed a much more detailed analysis of the field aligned current system at Saturn. Dougherty et al. (2018) summarize the fluxgate magnetometer instrument results from the Cassini Grand Finale mission phase including the F-ring and proximal orbits. Citing the work of Hunt et al. (2014, 2015, 2018), Dougherty et al. (2018) note that a “distributed downward current” region lies poleward of the main upward field-aligned current region in both the northern and southern hemispheres of Saturn. Bradley et al. (2018) clearly distinguished the polar regions of upward and downward current (cf. their Figure 5). The results we present are consistent with Cassini intercepting the polar region of distributed downward current where auroral hiss is encountered and briefly intercepting the lower latitude region of upward field-aligned current where SKR source regions occur. Hunt et al. (2018) and Bradley et al. (2018) suggest that the subcorotation current is directed downward over the polar cap and reverses to upward in the auroral region. LEMMS observed bidirectional electrons in the region of periodic hiss, which may correspond to the outermost layer of closed magnetic field lines extending a degree or two poleward of the region of closed field lines (auroral region).

5 Summary and Conclusions

We have investigated observations of ~1-hr periodicities in auroral hiss during the Cassini proximal orbits. Hiss emission is known to have a source along terrestrial auroral magnetic field lines containing upward electron beams, and a similar source region has been observed for Saturn auroral hiss (Gurnett et al., 2010). These orbits provide an excellent opportunity to observe the auroral region of Saturn at low altitude. Cassini approaches Saturn from the northern hemisphere high above the nightside, periapsis near the dayside equator, then exits high above the southern afternoon nightside. The Cassini RPWS data have been directly compared to LEMMS data, which show bidirectional electron beams with ~1-hr periodicity at times during the period of hiss emissions. We have mapped field lines containing auroral hiss emission to the auroral region, very near and just poleward in the intense UVIS northern hemisphere auroral emission data, and within the intense UVIS southern hemisphere auroral emission data. The SKR source region field lines map to ionospheric footprints that are lower in latitude than the auroral hiss ionospheric footprints.

Yates et al. (2016) have presented evidence that these 1-hr oscillations are due to low-frequency magnetic field line oscillations, which are known to occur at Earth. Subsequently, Cassini UV auroral images displaying ~1-hr periodic auroral flashes have been reported by Bader et al. (2019), who suggest they are likely due to “recurrent small-scale magnetodisk reconnection on closed field lines,” which occur preferentially on the duskside (Delamere et al., 2015) as do 1-hr periodic injections (Carbary et al., 2016; Palmaerts et al., 2016; Roussos et al., 2016).

Cassini observations of periodic hiss by RPWS occur on magnetic field lines that are adjacent to the field line containing the SKR source region. These observations are consistent with SKR occurring along a recently reconnected field line containing an upward current (downward electrons) while the auroral hiss occurs along return current (upward electrons). This picture would be consistent with the observations of Mitchell et al. (2016), Roussos et al. (2016), Palmaerts et al. (2016), and Bader et al. (2019), who discuss observations of ~1-hr periodicities of LEMMS and RPWS data. We have also discussed how the observations of this paper agree with the global current models presented in Cowley and Bunce (2003), Hunt et al. (2014, 2015, 2018), Bradley et al. (2018), and Dougherty et al. (2018). The relationship of all of these observations to those of Yates et al. (2016) should be further investigated.