Pulsed oscillation of the main auroral emission

The extended distribution of hourly electron injections throughout Saturn’s outer mag-netosphere and their pulsed signatures with no energy dispersion suggest that the hourly pulsations are produced at various places through a common global process. The auroral emissions provide a possible additional evidence that the∼1-h pulsations at Saturn result from global magnetospheric dynamics. In this section, the first observation of a ∼1-h stepped oscillation of the main auroral emission is reported.

The main auroral emission at Saturn is roughly located at the boundary between open and closed field lines (see the description in Section 1.3.1). Using Hubble Space Tele-scope (HST) observations of the UV southern aurora in 2007 and 2008, Nichols et al.

(2008) identified an oscillation motion of the center of the main auroral oval with a pe-riod of 10.75 h in 2007 and 10.79 h in 2008. This pepe-riod is close to the pepe-riodicity of the southern oscillations in the Saturn Kilometric Radiation (SKR) power (Section 1.4.1).

The oscillation of the oval center follows an eccentric ellipse of 1-2° semi-major axis, oriented in the prenoon-premidnight direction. During the 2009 equinox HST campaign, centers of both northern and southern main emissions oscillate by 1-2° in the dawn-dusk

7.2 Perspectives direction (Nichols et al. 2010b). Nichols et al. (2010b) pointed out that the observed periodic motion is caused by the displacement of the entire oval rather than a periodic asymmetric expansion of the oval. The authors suggest that these auroral oscillations are induced by a rotating external magnetospheric current system. This current system pro-duces rotating magnetic field perturbations associated with the observed planetary period oscillations (Provan et al. 2009, Andrews et al. 2010).

Bunce et al. (2014) reported throughout a 2h-long Cassini/UVIS imaging sequence a motion of the whole northern auroral main oval towards the premidnight sector at a rate of 1.1° in latitude per hour. Like in the previous studies, the latitudinal motion of the oval was consistent with the phase of the magnetosphere oscillations in the northern hemisphere at the time of the observations. Furthermore, using concurrent Cassini in-situ data, Bunce et al. (2014) argued that the field-aligned current system associated with the oscillating auroral oval is not static but also presents an oscillatory motion related to the magnetosphere oscillation phase.

During the Cassini/UVIS auroral imaging sequence on day 129 of 2008, a motion towards dayside of the main auroral oval is apparent. This motion does not seem to be continuous but jerky with increased displacement every hour. In order to analyze this motion, an elliptical fit has been applied to the main oval on the polar projections of all the UVIS pseudoimages of the sequence. This sequence is made up of 24 images and lasts for about 6 hours. The 24 fits of the main oval are displayed on the polar map on the left of Figure 7.1 with the Sun direction towards the bottom. The color scale enables the tracking of the main auroral emission position during the sequence. It is revealed that the auroral oval moves as a whole from the nightside towards the dayside. No significant change of the polar cap size is observed.

The crosses on the polar map indicate the center of the main oval inferred from the elliptical fits. A zoom on the center of the polar map is given on the right panel of Fig-ure 7.1 where the motion of the oval center can be seen. A superimposed ellipse corre-sponds to the general displacement of the oval center. This ellipse is oriented along the premidnight-prenoon direction, at 30° from the noon-midnight meridian. This orientation of the oval center motion is similar to the orientation of the ellipses fitting the oval center motion inferred by Nichols et al. (2008) for 2007 and 2008 from HST observations. The two ellipses of Nichols et al. (2008) are drawn in red dashes in Figure 7.1. During the 2-h UVIS sequence analysed by Bunce et al. (2014), the main emission moved along the same orientation too, towards 21 LT.

The latitudinal displacement of the main emission has been further analyzed at noon and midnight. The temporal evolution of the main oval latitude at 12 LT and 0 LT is given on the top panels of Figure 7.2. The oval latitude decreases at noon and increases at midnight. The latitude variation can be fitted by a sinusoid with a period of 8.6 h at midnight and 10.5 h at noon, which is close to the typical planetary period (10.6-10.8 h).

The latitudinal shift rate is close to 1° per hour, similar to the 1.1°/h inferred by Bunce et al. (2014) from their observations. Thus, the main motion of the main emission in this current observation follow the auroral oscillation described previously by Nichols et al.

(2008), Nichols et al. (2010b) and Bunce et al. (2014).

However, the latitude displacement of the oval exhibits fluctuations. Subtracting the sinusoidal fit to the latitude variation highlights these fluctuations which are quasi-periodic (middle panels in Figure 7.2). A Lomb-Scargle analysis revealed that the period

-30 -20 -10 0 10 20 30

Figure 7.1: Motion of the main auroral oval during the Cassini/UVIS imaging sequence on day 129 of 2008. Left panel: Position on a polar map of the main oval determined by an elliptical fit for each of the 24 images. The direction of the Sun is towards the bottom.

The parallels are drawn every 10° in latitude and the meridians every 2 h in local time.

The color indicates the time from the start of the sequence. The center of the elliptical fits are indicated by the crosses. Right panel: Position of the elliptical fit center. The red thick ellipse superimposed gives the general motion of the auroral oval center. The two dashed red ellipses indicate the fit of the motion of the auroral oval center determined by Nichols et al. (2008) for 2007 and 2008 using HST images.

of these fluctuations is close to one hour (bottom panels of Figure 7.2). Hence, a∼1-h stepped motion is superimposed to the oscillation of the main oval.

In the second part of another UVIS auroral sequence, on day 195 of 2008, the main auroral emission moves towards 10 LT with some fluctuations. However, during this ob-serving sequence, the main emission is well defined only in the morning, preventing an accurate fit of the main emission and thus making the analysis less reliable.

Further analysis needs to be carried out to validate this observation of this hourly pulsed oscillation of the main auroral oval. It is necessary to confirm that the fluctuations in the position of the oval are not due to instrumental or observational effects. If the ob-servation is confirmed, since the whole auroral oval displays the hourly stepped motion, it suggests that these pulsations are generated by a global magnetospheric process. Opening of magnetic flux in the dayside causes an equatorward motion of the dayside arc of the main emission. Hence, pulsed dayside reconnection at the magnetopause could explain the stepped equatorward displacement of the oval in the dayside. However, a poleward motion of the nightside arc is induced by closure of the flux in the magnetotail (following the Dungey cycle explained in Section 1.2.4.4). Although no observational evidence of pulsed tail reconnection has been reported, it is not inconceivable that it is the case. The hourly quasi-periodic pulsed electron events in the midnight-to-dawn local time sector (Figure 3.3) could be triggered by pulsed tail reconnection. However, the existence of a direct connection between the reconnection bursts at the magnetopause and the ones in the tail is not obvious.

7.2 Perspectives

Figure 7.2: Latitudinal displacement of the main emission at 12 LT (left) and 0 LT (right) during the Cassini/UVIS imaging sequence on day 129 of 2008. Top panels: Evolution of the latitude of the main emission and fit by a sinusoid function (red dashed line). Middle panels: evolution of the latitude of the main emission detrended from the sinusoidal fit.

Bottom panels: Lomb-Scargle periodogram of the detrended latitude evolution.

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Figure 7.3: Pulsed electron injection on day 129 of 2008 exhibiting energy dispersion.

Upper panel: differential intensities of energetic electrons from the MIMI/LEMMS chan-nels E0 to E4 and E6. Lower panel: energetic electron energy-time spectrogram from the LEMMS Pulsed Height Analysis data.