Magnetosphere Imaging Instrument (MIMI) onboard the CASSINI spacecraft

Magnetosphere Imaging Instrument (MIMI) onboard the CASSINI spacecraft

The Magnetosphere Imaging Instrument (MIMI) is a neutral and charged particle detection system on the CASSINI orbiter spacecraft designed to study the overall configuration and dynamics of Saturn's magnetosphere, and its interaction with the solar wind, Saturn's atmosphere, Titan, and the icy satellites.

MIMI consists of three different sensors which were built at different institutions in Europe and the United States. The principal Investigator is S.M. Krimigis from the Applied Physics Laboratory of the Johns Hopkins University, Laurel, Maryland, USA.

The different subsystems are:

  • INCA (Ion Neutral Camera) built at the Applied Physics Laboratory JHU/APL Laurel, Maryland, USA
  • CHEMS (Charge Energy Mass Spectrometer) built at the University of Maryland, College Park, USA
  • LEMMS (Low Energy Magnetospheric Measurement System) built at Max Planck Institute for Solar System Research
  • MEU (Main Electronic Unit) built at CESR, Toulouse, France and JHU/APL Laurel, Maryland, USA

Science objectives


  • Determine the global configuration and dynamics of hot plasma in the magnetosphere of Saturn through energetic neutral particle imaging of ring current, radiation belts, and neutral clouds.
  • Study the sources of plasmas and energetic ions through in situ measurements of energetic ion composition, spectra, charge state, and angular distributions.
  • Search for, monitor, and analyze magnetospheric substorm-like activity at Saturn.
  • Determine through the imaging and composition studies the magnetosphere- satellite interactions at Saturn and understand the formation of clouds of neutral hydrogen, nitrogen, and water products.
  • Investigate the modification of satellite surfaces and atmospheres through plasma and radiation bombardment.
  • Study Titan’s cometary interaction with Saturn’s magnetosphere (and the solar wind) via high-resolution imaging and in situ ion and electron measurements.
  • Measure the high energy (Ee > 1 MeV, Ep 15 MeV) particle component in the inner (L < 5 RS) magnetosphere to assess cosmic ray albedo neutron decay (CRAND) source characteristics.
  • Investigate the absorption of energetic ions and electrons by the satellites and rings in order to determine particle losses and diffusion processes within the magnetosphere.
  • Study magnetosphere-ionosphere coupling through remote sensing of aurora and in situ measurements of precipitating energetic ions and electrons.


  • Study ring current(s), plasma sheet, and neutral clouds in the magnetosphere and magnetotail of Jupiter during Cassini flyby, using global imaging and in situ measurements.


  • Determine elemental and isotopic composition of local interstellar medium through measurements of interstellar pickup ions.
  • Study the compositional evolution at low energies of shock-accelerated ions in the interplanetary medium.
  • Monitor cosmic ray intensity variations (Ep > 70 MeV/nuc) and anomalous cosmic rays, and compare with similar measurement in both the inner heliosphere (Advanced Composition Explorer, ACE) and the outer heliosphere (Voyager 1, 2).

Detailed instrument description of MIMI-Lemms

LEMMS is designed to measure the energy and spatial distribution of energetic particles (electrons and ions separately) in the interplanetary medium and in the magnetosphere of Saturn.
The LEMMS sensor is sponsored by the DLR (Deutsche Luft- und Raumfahrtagentur). DLR is the German counterpart of NASA.

LEMMS is based on a similar type of particle spectrometer which is currently flying on the NASA spacecraft GALILEO orbiting the planet Jupiter.

LEMMS consists of the following subsystems:

  • low energy detector head with collimator
  • high energy detector head with collimator
  • programmable turntable (manufactured in Finland by VTT)
  • electronics

The measurements of energetic particles are based on the loss of energy in semiconductor detectors. LEMMS has 11 different detectors E1, E2, F1, F2, A, B, D1, D2, D3a, D3b, D4

MIMI-LEMMS - Low Energy End

The low energy end of LEMMS is designed to measure low energy ions and electrons separately. An internal permanent magnet separates ions and electrons magnetically. The particles are detected with different detector systems inside the low energy end. The figure is the result of a simulation for the response of the low energy end of LEMMS (for details of the simulation contact Andreas Lagg ( Energetic particles enter the low energy end aperture from the left. Electrons are deflected by the magnet and counted by detectors E1, E2 (shown in green) and F1, F2 (blue) dependend on their energy. Ions are detcted by detectors A and B (shown in yellow).

The whole assembly of the low energy end including the detectors E1, E2, F1, F2, A, and B and the permanent magnet are caged by a platinum box and a mu-metal shielding.

Parameter of the low energy end detectors:

Detector  Thickness [µm]  Active area [mm2]  
E1 300±15 90
E2 300±15 90
F1 700±15 90
F2 300±15 90
A 150±15 35
B 700±25 35

Between detectors B and the stack of D1..D4 of the high energy end a Gold-Absorber (1000 µm, 75 mm2) is inserted to stop particles not stopped in A or B.

MIMI-LEMMS - High Energy End

The high energy end of LEMMS is designed to measure high energy ions and electrons with a stack of 4 detectors D1, D2, D3, D4. In front of detector D1 a 25 µm Al-foil suppresses incoming light on results.

Parameter of the high energy end detectors:

Detector  Thickness [µm]  Active area [mm2]  
D1 150±15 100
D2 700±25 150
D3a 700±25 150
D3b 700±25 150
D4 700±25 150

(Note: Detector D3 consists of two identical detectors D3a and D3b)

MIMI-LEMMS - Characteristics

order of 
specieslogiclow or high
energy end
1 A0 ions A1(A2)(B1) L
2 A1 ions A2(A3)(B1) L
3 A2 ions A3(A4)(B1) L
4 A3 ions A4(A5)(B1) L
5 A4 ions A5(A6)(B1) L
6 A5 ions A6(A7)(B1) L
7 A6 ions A7(A8)(B1) L
8 A7 ions A8(A9)(B1) L
9 A8 ions A9(B1) L
10 B0 ions A8(A9)B1(B5)(D41) L
11 B1 ions A6(A8)B4(B5)(D41) L
12 B2 ions A9B1(B5)(D41) L
13 B3 ions A9B5(D41) L
14 BE ions A2(A7)B1(B3)(D41) L
15 C0 electrons E11(E12)(E21) L
16 C1 electrons E12(E13)(E21) L
17 C2 electrons E13(E14)(E21) L
18 C3 electrons E14(E15)(E21) L
19 C4 electrons E15(E16)(E21)+F11(F12)(F21) L
20 C5 electrons F12(F13)(F21) L
21 C6 electrons F13(E14)(F21) L
22 C7 electrons F14(F15)(F21) L
23 AS singles A1 L
24 BS singles B1 L
25 E11 singles E11 L
26 E2F2 singles E21 L
27 F11 singles F11 L
28 E0 electrons D11(D12)(D21) H
29 E1 electrons D11(D12)D21(D31) H
30 E2 electrons (D11)D21(D22)(D31) H
31 E3 electrons (D12)D21(D22)(D31) H
32 E4 electrons (D12)D21(D22)D32(D33)(D41) H
33 E5 electrons D21(D22)D32(D33) H
34 E6 electrons D32(D33)D41(D42)(B1) H
35 E7 electrons D41(D42)B1(B2) H
36 G1 (gammas) (D21)D31(D32)(D41) H
37 P1 ions D13(D15)(D21) H
38 P2 ions D15(D16)(D21) H
39 P3 ions D15(D16)D21(D23)(D31) H
40 P4 ions D12(D16)D23(D24)(D31) H
41 P5 ions D12(D15)D24(D25)(D31) H
42 P6 ions D24(D25)D32(D34)(D41) H
43 P7 ions D21(D25)D33(D35) H
44 P8 ions D12(D15)D22(D23)D33 H
45 P9 ions D22(D23)D32(D33) H
46 H1 ions D16(D17)(D21) H
47 H2 ions D16(D17)D21(D26) H
48 H3 ions D25(D26)D32 H
49 H4 ions D23(D25)D35 H
50 H5 ions D22D34(D35)D45 H
51 Z1 ions D17(D21) H
52 Z2 ions D17D21(D31) H
53 Z3 ions D26D32 H
54 D1 singles D11 H
55 D2 singles D21 H
56 D3 singles D31 H
57 D41 singles D41 H

MIMI-LEMMS Calibration Campaigns

datelocationcalibration with
Dec 1995 Goddard Space Flight Center (GSFC)
Greenbelt, Maryland, USA
Mar 1996 Kernforschungszentrum (KFA)
Jülich, Germany
high energy protons and helium
1996-1997 Max Planck Institute for Solar System Research,
Katlenburg-Lindau, Germany
low energy protons, helium, oxygen
Feb 1997 Goddard Spaceflight Center (GSFC) 
Greenbelt, Maryland, USA
electrons, H-
Mar 1997 LBL, Berkeley, 
California, USA
high energy protons, helium, carbon, oxygen

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