TheCassini Plasma Spectrometer (CAPS) is another instrument designed for the in-vestigation of the plasma environment around Saturn (Young et al. 2004), but at lower energies compared to the MIMI instrument. It includes three subsystems: theElectron Spectrometer (ELS), the Ion Mass Spectrometer (IMS) and the Ion Beam Spectrometer (IBS). Since only measurements from ELS are presented in this thesis, a description of the two other sensors are not given here, but can be found in Young et al. (2004).
The ELS spectrometer detects electrons with energies from 0.6 eV to 28 keV. Hence, the CAPS/ELS together with the MIMI/LEMMS sensor provide a continuous coverage of all the energy spectrum of electrons between 0.6 eV and 21 MeV. The energy bandwidth of
Instrument Purpose
MIMI/LEMMS Detection and characterization of the quasi-periodic electron injections
RPWS Detection of pulsed signatures of the electron injections in the radio emissions
MAG Detection of pulsed signatures of the electron injections in the magnetic field
UVIS Detection and characterization of pulsating auroral emissions CAPS/ELS Identification of magnetospheric environments
Table 2.3: Cassini instruments used in this thesis.
CAPS/ELS is divided in 63 logarithmically spaced energy levels with an energy resolution of∆E/E ∼ 0.17 (Young et al. 2004, Lewis et al. 2010). The ELS detector is composed of a fan of eight adjacent anodes with an individual field-of-view of 5.2°×20°. The total FOV of CAPS/ELS is then 5.2° in azimuth and 120° in elevation. The data provided by the central anodes (anodes 4 and 5) have a lower background count rate and suffer less from obscuration by parts of the spacecraft (Arridge et al. 2009). The CAPS/ELS data presented in this thesis have all been acquired with the anode 4. To extend the azimuthal FOV of CAPS, it is mounted on an oscillating platform which sweeps 208° in azimuth in about 4 min. In addition, from the CAPS/ELS measurements, different calculation methods allow to derive the electron moments, i.e., the electron temperature and density (Lewis et al. 2008, Arridge et al. 2009).
The CAPS instrument was switched offin June 2012 after it caused a series of elec-trical shorts to the spacecraft. It has been decided afterwards to not switch it on anymore.
3 Quasi-periodic hourly pulsations in the energetic electron fluxes
The magnetosphere of Saturn contains many periodic phenomena, some of them have been described in Section 1.4. Short periodicities of around one hour have been detected by several instruments on board Cassini. In particular, quasi-periodic energetic electron injections are observed in Saturn’s outer magnetosphere. These pulsed electron events are striking since they involve high intensities of MeV electrons in a region where MeV electron fluxes are generally at the instrument background level (Section 1.2.3.3). Their remarkable quasi-periodicity implies the existence of an impulsive trigger which is still unknown.
In order to increase our knowledge of this periodic phenomenon, this chapter1presents a survey of the quasi-periodic 1-hour injections of energetic electrons and a statistical study of their properties and their spatial distribution. In the first part of the chapter, an example of a quasi-periodic pulsed electron event is depicted.
3.1 Example of a quasi-periodic electron pulsed event
A typical example of a quasi-periodic injection of energetic electrons is given in Fig-ure 3.1. This event was observed on days 129 and 130 of 2009 (May 9 and 10), when the Cassini spacecraft was in the post-dawn local time sector,∼15 RS away from Saturn and at mid-latitude in the southern hemisphere. The two right subpanels in Figure 3.1 indicate Cassini’s position (blue thick line) projected in the equatorial (x-y) plane (Figure 3.1g) and in the north-south (x-z) plane (Figure 3.1h), using the Saturn Equatorial coordinate system (SZS). In the SZS coordinate system, thez-axis is aligned with the planet’s rota-tion axis, thex-axis and they-axis are in the equatorial plane towards the Sun and dusk, respectively. The orbit of Cassini, during the time interval covering half an orbit before and after the event, is drawn in green in Figure 3.1g and 3.1h. The orange curves represent two simulated magnetopause locations for two values of the solar wind dynamic pressure (2×10−3and 4×10−2nPa) using the model of Kanani et al. (2010). A red arrow indicates the subsolar latitude. In Figure 3.1g, Titan’s and Rhea’s orbit are indicated with the black circles (at 20.3 and 8.7 RS).
1. The main results presented in this chapter have been published in
Palmaerts, B., Roussos, E., Krupp, N., Kurth, W. S., Mitchell, D. G., Yates, J. N., 2016a, Statistical analysis and multi-instrument overview of the quasi-periodic 1-hour pulsations in Saturn’s outer magnetosphere, Icarus, 271, 1-18.
Left panel in Figure 3.1 is a stack of time series plots of Cassini in-situ measure-ments. The differential intensities (in counts/(cm2sr s keV)) of electrons measured in the LEMMS HET channels E0 to E4 and E6 are transcribed in Figure 3.1a. The energy pass-bands of the electron channels are reminded in the bottom right corner. A frequency-time electric field spectrogram acquired with the RPWS instrument is given in Figure 3.1b.
The white line represents the electron cyclotron frequency (see Section 1.2.4.1). The next three panels (c to e) show the Cassini magnetometer measurements of the three compo-nents of the magnetic field in spherical coordinates (azimuthal Bφ, radial Br and polar Bθ). The azimuthal component is positive in the direction of corotation, the radial com-ponent is positive outward from Saturn and the polar comcom-ponent is positive southward.
The strength of the radial component of the magnetic field being high in that region, small fluctuations are not distinguishable. Therefore, a third degree polynomial fit has been re-moved from the measurement and the resulting detrended radial component is given by the red line. Finally, the last plot (Figure 3.1f) represents the local pitch angle pointing of the LEMMS HET.
The electron pulsed event given as an example in Figure 3.1 can be divided in two parts: from 22 to 02 UT and from 02 to 06 UT. Cassini was rotating during the first part so that the LEMMS detector could measure electrons with a nearly entire pitch angle dis-tribution (Figure 3.1e). In the second part of the event, Cassini’s pointing was fixed and LEMMS detected energetic electrons with a pitch angle of∼25°. From 00 UT onwards, four consecutive pulsed enhancements are observed in the E3 and E4 channels (green and orange lines in Figure 3.1a). Each pulsation is separated by around 60 min. Pulsed variations are also observed in phase in the lowest energy channels E0 to E2. At these energies, periodic narrow peaks are superimposed when Cassini is rolling. These peaks are correlated with the extreme values of the pitch angle and they reveal bidirectional electron beams aligned with the closed magnetic field lines (e.g. Saur et al. (2006)). Flux peaks in the E3 and E4 channels are also higher in the field-aligned direction (peaks at
∼0030 and ∼0130 UT). Hence, two different electron populations are mixed: the field-aligned electron beams and the∼1-h pulsed electrons (see also the discussion in Section 3.5.4). After 02 UT, LEMMS points in a fixed direction. Two consecutive pulses sepa-rated by around 60 min are clearly visible in all the electron channels except E6 and two additional pulses are also distinguishable at∼04 and at∼05 UT. The electron intensity in the highest energy channel E6 peaks only once in the field-aligned direction, revealing an upward electron beam with an energy above 1.6 MeV. It is noteworthy that at these energies (above 500 keV), the electrons are relativistic. Other pulses in the E6 channel are likely below the detection limit.
The RPWS electric field spectrogram reveals periodic enhancements in the broadband plasma wave emissions. These enhancements are concomitant with the LEMMS electron pulsations during the main part of the time interval. Below the electron cyclotron fre-quency (fce), the radio emissions are electromagnetic waves propagating in the whistler mode. In this frequency range, the radio emissions are called auroral hiss (see Section 1.3.5). However, the two strongest broadband bursts, at∼0030 and∼0130 UT, extend to frequencies above fce, indicating an electrostatic mode, as discussed in Section 4.1.2.
Regarding the magnetic field, the azimuthal component (Figure 3.1c) exhibits strong fluctuations anti-correlated with the LEMMS electron pulsations: an enhancement of the electron flux coincides with a drop in the intensity of theBφ component. The detrended
3.1 Example of a quasi-periodic electron pulsed event
LocalXpitchXangle [deg]B¡BrB¢ Br
lff)bhl)Xll:ff:ffXzXlff)bhpfXfI:ff:ff
Figure 3.1: Quasi-periodic pulsed event observed on days 129 and 130 of 2009. The left panel is a stack of time series plots of (a) the differential intensities of energetic electrons from the LEMMS channels E0 to E4 and E6, (b) the RPWS electric field spectrogram, (c to e) the magnetic field components in spherical coordinates (Bφ, Br and Bθ) mea-sured by MAG and (f) the local pitch angle of the LEMMS HET. The white line on the RPWS spectrogram indicates the electron cyclotron frequency. The red line on the fourth panel is the detrended radial component of the magnetic field. Two subpanels on the right (g and h) indicate the location of Cassini during the event (blue line) along its orbit (green line), projected in the equatorial (x-y) plane and in the north-south (x-z) plane , using the Saturn Equatorial coordinate System (SZS). Titan and Rhea’s orbits (at 20.3 RS and 8.7 RS respectively) are represented in the equatorial plane by black circles. Two simulated magnetopause locations are drawn in orange for two values of the solar wind dynamic pressure. The red arrow indicates the direction to the Sun. The energy pass-bands of the considered E-channels of LEMMS are given in the lower right corner. From Palmaerts et al. (2016a).
radial component (red line in Figure 3.1d) varies also in an anti-correlated way with the electron fluxes. Finally, some pulsations are also distinguishable in the Bθ component while Cassini was rolling but they are not correlated to the other pulsations. The con-current hourly pulsations in the plasma wave and in the magnetic field will be tackled in Chapter 4.