The Cassini Radio and Plasma Wave Science Instrument

Transcription

1 The Cassini Radio and Plasma Wave Science Instrument Roger Karlsson Space Research Institute of the Austrian Academy of Sciences, Graz Graz in Space, September 7, 2006

2 The Cassini Radio and Plasma Wave Science Instrument Roger Karlsson Space Research Institute, Austrian Academy of Sciences, Graz

3 Magnetosheath Bow Shock Solar Wind Magnetopause Plasma Sheet Magnetotail

4 The Radio and Plasma Wave Science (RPWS) Investigations Radio emissions Plasma waves Lightning Thermal plasma Dust

5 The Radio and Plasma Wave Science (RPWS) Investigations (II) Radio emissions Important because of the involved plasma processes. Provides remote sensing tools, such as determination of the internal rotation period by the modulation of the radio emissions. Type of emissions: Saturn kilometric radiation (SKR) Generated over the dayside auroral zone. Believed to be generated by the cyclotron maser mechanism. Frequency 3 khz 1.2 MHz Power W Synchrotron radiation Mode conversion from electrostatic upper hybrid waves Mode conversion from Langmuir waves

6 The Radio and Plasma Wave Science (RPWS) Investigations (III) Plasma waves Transfer energy and momentum between particles in a plasma. Processes involving these wave-particle interactions: Loss of particles trapped in the radiation belts Heating of plasma at the bow shock and magnetopause Thermalisation of plasma escaping from moons and rings Acceleration of plasma in regions of strong field-aligned currents Observable plasma waves: Electron-plasma oscillations Upper hybrid resonance emissions Electron cyclotron waves Whistler mode chorus and hiss Electrostatic turbulence at the bow shock Low frequency magnetohydrodynamic turbulence Broadband electrostatic noise

7 The Radio and Plasma Wave Science (RPWS) Investigations (IIII) Lightning Give rise to intense impulsive radio bursts, Saturn Electrostatic Discharges (SED). High temperatures are reached along the discharge path and organic molecules can form. The organic molecules in Titan s atmosphere are believed to be caused by the solar ultraviolett radiation. Search for lightning in Titan s atmosphere. The SED low frequency cutoff should give an estimate of the ionospheric plasma frequency.

8 The Radio and Plasma Wave Science (RPWS) Investigations (V) Thermal plasma Key parameters: Electron density Electron temperature Magnetic field Control dispersion and damping of waves. Important also in other plasma processes. Titan is of particular interest and the following will be studied in Titan s ionosphere: Spatial and temporal distribution of the electron density and temperature Escape of thermal plasma from Titan s ionosphere

9 The Radio and Plasma Wave Science (RPWS) Investigations (VI) Dust Micron-sized particles concentrated near the ring plane near and inside the G ring. Probably produced by micro-meteoroid impacts on the rings and icy moons. The particles evaporates quickly and is a source of magnetospheric plasma. Absorbs energetic radiation belt particles. Due to photoelectron emission, they are charged and affected by electrostatic and electromagnetic forces. Example of dust impact on the spacecraft:

10 Saturnian Radio and Plasma Wave Phenomena

11 Saturnian Radio and Plasma Wave Phenomena Plasma frequency ω 2 pe = n ee 2 ε 0 m e f pe = ω pe 2π Gyro frequency Ω ce = eb m f ce = Ω ce 2π

12 RPWS Sensors Search coil magnetometers Langmuir probe E antenna u E antenna v Electric antenna assembly E antenna w

13 Electric Antenna Assembly Includes Antenna deployment mechanisms 3 electric antennas, each 10 m long and with a diameter of 2.86 cm. High frequency receiver The antennas measure the electric field. They can also be connected to a sounder to stimulate plasma resonances and thereby obtain the electron density and the electron temperature.

14 View from +x View from +y v u v u w y x z w z

15 Tri-axial Magnetic Search Coil Assembly Magnetic antennas Measure 3 orthogonal components (x,y,z) of the magnetic field. Preamplifiers at the base of the mounting boom. Thermal blankets minimizes heat loss.

16 Langmuir Probe A 5 cm diameter titanium sphere coated with titanium nitride. Measures the electron density and plasma temperature. Mounted on a 0.8 m boom at the magnetic search coil assembly. Preamplifier near the base of the boom.

17 Main Electronic Box

18 Saturnian Radio and Plasma Wave Phenomena

19 Receivers Frequency Hz 1 Hz 26 Hz Low Frequency Receiver 24 Hz Medium Frequency Receiver 12 khz 1 Hz 26 Hz 5 Channel Waveform Receiver 3 Hz 2.5 khz 60 Hz 10.5 khz Wideband High Frequency Receiver 3.5 khz 16 MHz Receiver 800 Hz 75 khz 16 MHz

20 The RPWS Instrument

21 Mass and Power Element Mass (kg) Peak Power (W) Main electronics Antenna bracket assembly Magnetic search coils Magnetic search coil preamps Langmuir probe Langmuir probe preamp Total

22 Main Responsibilities Data processing unit Medium frequency receiver Wideband receiver Waveform receiver Electric antenna motor driver Electric antenna procurement Antenna modeling Direction finding calibration Data compression software High frequency receiver Sounder Power supply Electric preamplifiers Electric antenna procurement Magnetic antennas Institutions responsible for data analysis Langmuir probe Langmuir probe preamplifiers Langmuir probe digital processor

23 Antenna Calibration Due to interactions with the conducting spacecraft body, the electrical antenna directions deviate from their physical directions. Antenna calibration = determination of the antenna effective length vectors (directions and lengths). Before launch: Rheometry Antenna modeling by measuring the antenna response of a Cassini 1:30 scale model immersed in a water tank. A time-varying electric field is applied to two parallel conducting plates at opposite sides of the tank and the voltage response on the antennas are measured. Wire-grid modeling Numerical calculations of the antenna effective length vectors using a wire-grid model of the spacecraft. In flight: Calibration using jovian radio emissions of known origin as sources. Calibration after the Huygens probe release.

24 RPWS Results Saturn s rotation period has changed. Voyager measured 10 h 39 m 24 ±7 s, while Cassini has measured 10 h 45 m 45 ±36 s. Saturn s aurora has been found to differ morphologically from the aurora at Earth and Jupiter. Enceladus is a plasma source to Saturn s magnetosphere. The outflow from Titan s atmosphere appears to be about 10 times larger than previously predicted. SKR observed also on the nightside region of Saturn. The potential of the rings change with the season. Radio signal from Saturnian lightning (SED) has been linked to storm systems observed by imaging instruments.

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26 Frequency [khz] Cassini-RPWS (Dipole) 30/6/2004-1/7/2004 (DOY ) autox [V 2 Hz -1 db] Frequency [khz] autoz [V 2 Hz -1 db] Frequency [khz] crossr Frequency [khz] crossi 20:00:00 00:00:00 04:00:00 08:00:00 SCET [hh:mm:ss]

27 Frequency [khz] Cassini-RPWS (Dipole) 30/6/2004-1/7/2004 (DOY ) autox [V 2 Hz -1 db] Frequency [khz] :00:00 00:00:00 04:00:00 08:00: autoz [V 2 Hz -1 db]