Download The galactic background detected by a short dipole antenna can be

Jupiter and Saturn science with LRX
P. Zarka & H.O. Rucker
The galactic background flux density (Wm-2Hz-1) detected by a short dipole antenna can be
written :
Sgal = 2kTgal/Aeff = 2kTgal 2/
(Aeff
~ 2)
with =8 /3, Aeff=3 2/8 , Tgal~103-106 K at low frequencies
The sensitivity of an observation using N dipoles, with bandwidth b and integration time
written:
Smin = Sgal/(b )1/2
with Aeff ~ N Aeff(1 dipole), thus
Smin ~ Sgal(1 dipole) / C
and C = N(b )1/2
can be
This is the ultimate sensitivity achievable after RFI mitigation and in the absence of
superimposed solar bursts, sferics or Terrestrial AKR emission.
Comparison of spectra for Sgal(1 dipole) and planetary radio emissions at Earth orbit show that
detection of all Jovian magnetospheric radio emissions, as well as of Saturn’s auroral radio
emissions in the kilometer wavelength range (so-called SKR = Saturnian Kilometric Radiation),
from the kilometer to the decameter range, requires C≥102-3. This is easily achievable with N=12, b=a few to a few tens kHz, =1-10 sec.
Detection of Uranus’ and Neptune’s auroral radio emissions below 500-700 kHz requires C≥1034
. This may be achievable with N=1-2, b=100-500 kHz, =10-100 sec.
Interest of (solar system) planetary radio emission studies includes :
- Long-term magnetospheric radio observations from a ~fixed vantage point ;
- Possibility for multi- correlations (e.g. with UV images from Hubble Space Telescope) ;
- Variabilities/periodicities of radio emissions, from short pulses to planetary rotation
period, solar wind and satellite modulation ;
- This will allow us to address magnetospheric dynamics, solar wind / magnetosphere
coupling (substorms ?), electrodynamic coupling of the magnetosphere with satellites
(e.g. Io, Ganymede, Titan…);
- Possibility for solar wind monitoring from 1 to 30 AU (by correlation with radio emission
intensity) ;
- Monitoring of Io volcanism, plasma torus probing (via propagation effects on radio
emissions), possible presence of magnetic anomalies (and their secular variations) ;
- Finally, Uranus & Neptune radio emissions were observed only once each by Voyager 2,
and Moon observations would constitute the only opportunity in decades to study them
further.
Planetary lightning, which require short integration times ( << 1 sec) are not detectable with
N≤10-100 dipoles.
Single dipole measurements will give access to total flux (and polarization). Interferometric
measurements will additionally provide instantaneous 1D radiosource sizes. Interest of such solar
system studies is that measurements at low frequencies (<0.1-1 MHz) will not be affected by
propagation effects (temporal broadening, dispersion, etc.) due to interstellar scintillation, but
only in a limited way by interplanetary propagation.
Such observations will be very complementary from higher frequency (≥10-30 MHz) groundbased observations with large instruments such as LOFAR, and will prepare for future Moonbased LF radio search for exoplanets with a full-scale dipole array (N>100).
Figure 1: Radio emissions from planetary magnetospheres (adapted from Zarka, 1998; Zarka et
al., 2004). Coefficient C is explained in the text. C=1 corresponds to the galactic background
spectrum detected with a single dipole. Numbers 1 to 3 refer to Jovian radio components
described in Figure 2.
References :
• Zarka, P., Auroral radio emissions at the outer planets : observations and theories, J. Geophys.
Res., 103, 20159-20194, 1998.
• Zarka, P., B. Cecconi, and W. S. Kurth, Jupiter's low frequency radio spectrum from
Cassini/RPWS absolute flux density measurements, J. Geophys. Res., 109, A09S15, 2004.
Figure 2: Sketch of Jupiter’s magnetosphere and sources of low-frequency radio emission: 1)
auroral kilometric to decametric ; 2) induced by Io-Jupiter interaction, decametric ; 3) from Io’s
torus, kilometric.