planets and solar system science at low frequencies philippe zarka lesia, cnrs-observatoire de paris...

Planets and Solar System Scienceat Low Frequencies

Philippe ZarkaLESIA, CNRS-Observatoire de Paris

Francephilippe.zarka@obspm.fr

Towards a European Infrastructure for Lunar ObservatoriesBremen, 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 inJupiter's environment

Io-Jupiter plasma interaction

• + Saturn, Uranus, Neptune auroral emissions :

Saturn

Uranus

Neptune

• + Saturn, Uranus atmospheric lightning :

Saturn

Uranus

LF cutoff dayside peak ionospheric density

• 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(ex : N=1, 10 kHz 1 sec)

Saturn’s lightning with C ≥105 (N=200, 10 MHz 25 msec),

without access to LF cutoff

C

102

104

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 ≥ 100-1000(N=1-10, 10 kHz 1 sec)

+ Uranus & Neptune auroral emissions + Saturn & Uranus lightning (including LF cutoff) with C ≥ 104 (N=10-100, 200 kHz 50-500 msec)

C

102

104

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

+ possible stronger stellar wind, focussing events, …

C

103

105

107

109

105

103

10

1

Magnetic Radio Bode's Law

••

Hot Jupiters ?

•

• Detectability from the ground (Earth) :

C

103

105

107

109

No solar bursts /spherics , RFI mitigation

≥10-20 MHz

requires C≥107

(N=1000-10000, 1-10 MHz 1-10 sec)

• Detectability from the Moon :

C

103

105

107

109

≥1 order of magnitude better(C ≥ 105-6 : N=100, 1-10 MHz 1-10 sec)

+ access to less energetic sources(C ≥ 106-7 : N>>100)

+ access to VLF (weakly magnetized bodies)

• 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)