LEMMS

General description

TheLow-Energy Magnetospheric Measurement System(LEMMS) is made up of two detectors pointing in opposite direction and measuring ions and electrons: the Low-Energy Telescope (LET) and the High-Energy Telescope (HET). The double heads of the detector are visible on the picture in Figure 2.4. The LET has an aperture of 15° and detects ions with energy above 30 keV and electrons in the energy range 18 keV-1 MeV.

The high-energy end has an opening angle of 30° and covers the energy range from 1.4 to several hundreds of MeV per nucleon for the ions and from 110 keV to energies greater than 10 MeV for the electrons.

In order to measure particles coming from all directions, the MIMI/LEMMS detector is mounted on a turntable platform which nominally rotates at a period of 86 sec. The data

2.3 The Magnetosphere Imaging Instrument (MIMI)

Figure 2.4: Flight model of the MIMI/LEMMS sensor. The double heads of the detector head surrounds the central electronic box which is mounted on the turntable platform, while the calibration shield is used to perform background measurements. From Krimigis et al. (2004).

were separated into 16 angular subsectors, providing an angular resolution of 22.5° and a time resolution of 86 sec (Krimigis et al. 2004). Unfortunately, on February 2, 2005, the scan platform malfunctioned, resulting in a fixed pointing of LEMMS after this date.

Since then, only two directions are sampled at a given time (one per telescope). A better pitch-angle coverage can be achieved only during Cassini spins. However, the LEMMS rotation stop allowed the time resolution at a fixed pointing to be improved by a factor 16, at 5.3 sec. Then, to increase the signal-to-noise ratio, a longer averaging can be applied on the LEMMS data. In the following chapters, LEMMS data are averaged on 86 sec, namely the nominal rotation rate of LEMMS.

Measurement technique

The detection method of charged particles differs between the two telescopes of the LEMMS sensor. In the low-energy end, a magnetic deflection system separates electron and ions (Figure 2.5): the electrons are deflected towards the detectors E and F while the ions, because of their larger mass, are not significantly deviated by the magnet and con-tinue straight until they strike the detectors A. The electron detectors E and F consists of three components: E1, F1 and a shared component E2/F2, as represented in the sketch of LEMMS in Figure 2.6. The specific components reached by a given electron depend on its incident energy. From the logic coincidences of detection of particles by the different components, the species and the energy of the incoming particle is derived. The coinci-dence method allows also to identify and reject the counts due to particles reaching the detectors under a non-nominal path. In the case of the ions, if their energy is high enough,

Figure 2.5: Simulated trajectories of electrons and ions in the MIMI/LEMMS Low-Energy Telescope. From Krimigis et al. (2004).

they are not absorbed by the detector A and continue their path until the B detector located behind. Electrons with high energy are not deflected enough so that they do not hit the detectors E and F but, if their energy exceeds 884 keV, reach the A detector like the ions.

A gold absorber (marked by a C in Figure 2.6) prevents the particles penetrating the B detector to enter the High-Energy Telescope.

The HET is made up of a stack of five detectors D1, D2, D3a, D3b and D4 preceded by a thin aluminum foil which suppresses the incoming light and the flux of low-energy particles. As is the case for the detectors A and B and the coupled detectors E and F, the species and the energy of the incoming particles are derived from the logic coincidences of detection of particles by the five detectors. This coincidence technique enables to derive the so-called rate channels of MIMI/LEMMS. Each rate channel is defined by an energy passband and the combination of detection and no-detection by the different LEMMS detectors of the particles with energy included in this energy passband. In total, 57 rate channels are defined for LEMMS. The main ion channels are named from A0 to A8 (for the LET telescope) and from P1 to P9 (for the HET telescope). The main electron channels are C0 to C7 (LET) and E0 to E7 (HET). The energy passbands of the LEMMS channels are summarized in Krupp et al. (2009). A list of the rate channels used in this thesis and their corresponding energy passband is given in Table 2.2.

The E3 and the E4 channels have a similar energy bandwidth but their response with energy is very different. The E5 channel is an integral channel with no upper bound and has the same sensitivity as E4 for the lowest energy of the channel. Since most of the particles are detected in the lowest part of the energy bandwidth, there is not a big difference between the E4 and the E5 count rates.

For six rate channels, each of the 16 angular subsectors are subdivided into 8 micro-sectors improving the angular resolution (from 22.5° to 2.81°) and, after the failure of the LEMMS rotating platform, the time resolution from 5.3 to 0.67 sec. These six priority channels have been modified a few times throughout the mission. From the end of 2016 onwards, the priority channels are A3, C2, C5, E0, E4 and P7.

Every rate channel listed in Table 2.2 has a relatively large energy bandwidth. To increase the energy resolution, another type of data is available: the Pulse Height Analysis

2.3 The Magnetosphere Imaging Instrument (MIMI)

Figure 2.6: Side view of the MIMI/LEMMS double-ended detector, with the Low-Energy Telescope on the left and the High-Energy Telescope on the right. From Armstrong et al.

(2009).

Telescope Channel Species Energy (keV)

LET C0 Electrons 18-40

LET C1 Electrons 27-48

LET C2 Electrons 41-60

LET C3 Electrons 56-100

LET C4 Electrons 92-183

LET C5 Electrons 175-300

LET C6 Electrons 265-550

LET C7 Electrons 510-832

HET E0 Electrons 110-365

HET E1 Electrons 220-485

HET E2 Electrons 340-1350

HET E3 Electrons 600-4950

HET E4 Electrons 790-4750

HET E5 Electrons >800

HET E6 Electrons 1600-21000

HET E7 Electrons 7000-20000

HET P2 Ions 2280-4492

Table 2.2: Energy passbands for the ion and electron channels of MIMI/LEMMS used in this thesis. From Krupp et al. (2009).

Figure 2.7: Optical design of the UVIS FUV telescope-spectrograph. From Esposito et al.

(2004).

(PHA) data. The PHA data provide 64 energy channels for the A detector, 64 channels for the E detector and 64 others for the F detector.