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Planets and Solar System Science at Low Frequencies Philippe Zarka LESIA, CNRS-Observatoire de Paris France [email protected] Towards a European Infrastructure for Lunar Observatories Bremen, 22-23/3/2005 - EADS / ASTRON / Radionet • Limitations of ground-based LF radioastronomy : RFI (man-made, lightning spherics) Ionospheric cutoff (~10 MHz) + propagation effects (≤30 MHz) Sky background (fluctuations) IP, IS scintillations (Solar radio emissions) • Limitations of LF radioastronomy in Earth orbit : RFI (man-made, lightning spherics) Auroral Kilometric Radiation Sky background (fluctuations) IP, IS scintillations (Solar radio emissions) • LF Earth environment : AKR day/night (at 60 RE) Thermal noise (≠flux) Galactic background Ionospheric LF cutoff Solar wind LF cutoff Solar emission/ burst/storm Spherics • Galactic background for a short dipole antenna, i.e. with =8/3, A=32/8 • Antenna effective area : A = k2 with k = 3/8 ~1/8 for a short dipole, k ~N/8 for N dipoles A ~ 2 ~1/k ~ 8/N LOFAR ~ 104 dipoles • Jovian radio emissions (near opposition) : Solar wind / magnetosphere interaction (auroral emissions) Io/magnetosphere interaction Io torus + Synchrotron from radiation belts (HF) Radiosources in Jupiter's environment Io-Jupiter plasma interaction • + Saturn, Uranus, Neptune auroral emissions : Saturn Uranus Neptune • + Saturn, Uranus atmospheric lightning : LF cutoff dayside peak ionospheric density Saturn Uranus • Detectability from the ground (Earth) : In absence of solar bursts & spherics In absence of RFI / after successful mitigation ≥10-20 MHz Jovian DAM with C=(dipole/)(b)1/2~N( b)1/2 ≥100 C (ex : N=1, 10 kHz 1 sec) 102 Saturn’s lightning with C ≥105 (N=200, 10 104 MHz 25 msec), without access to LF cutoff 106 • Moon : Shielding of RFI, spherics, AKR, Solar emissions Only limitation to sensitivity = sky background fluctuations Ionospheric LF cutoff <<500 kHz • Detectability from the Moon : all Jovian emissions + Saturn auroral emissions with C ≥ 1001000 (N=1-10, 10 kHz 1 sec) C 102 + Uranus & Neptune auroral emissions + Saturn & Uranus lightning (including LF cutoff) with C ≥ 104 (N=10- 104 100, 200 kHz 50-500 msec) 106 Long-term magnetospheric radio observations (+ multi- correlations) Variabilities/periodicities magnetospheric dynamics (role of SW, planetary rotation, satellite interactions, Io volcanism, short-lived bursts, substorms ?…) planetary rotation period B anomalies + secular variations Io torus probing (nKOM+Faraday effect) SW monitoring from 1 to 30 AU Saturn/Titan interaction (+other satellites ?) SW influence, substorms ? Uranus & Neptune auroral emissions observed only once by Voyager 2 ! Lightning : long-term monitoring, correlation with optical observations, planetary comparative meteorology • Extrasolar Jupiter-like radio emissions at 10 pc range : Flux up to 105 Jupiter’s strength for magnetized hot Jupiters with solar-like stellar wind input, or unmagnetized hot Jupiters in interaction with strongly magnetized star C 103 + possible stronger stellar wind, focussing events, … 105 105 107 103 10 1 109 • Magnetic Radio Bode's Law • • Hot Jupiters ? • Detectability from the ground (Earth) : No solar bursts /spherics , RFI mitigation C ≥10-20 MHz 103 105 107 requires C≥107 (N=1000-10000, 1-10 MHz 1-10 sec) 109 • Detectability from the Moon : ≥1 order of magnitude better C 103 (C ≥ 105-6 : N=100, 1-10 MHz 1-10 sec) 105 + access to less energetic sources (C ≥ 106-7 : N>>100) 107 + access to VLF (weakly magnetized bodies) 109 • NB : Angular resolution required ~1°-10° D = 6-60 (18-180 km @ 100 kHz ; 1.8-18 km @ 1 MHz) if detectability of exo-planetary radio emissions same for solar-like stellar radio emissions complementarity to ground-based LOFAR difficult from space weak scattering/broadening effects at sources distances <a few 10’s pc possible active sounding of Terrestrial magnetosphere (~IMAGE)