Transcript Exploring Jupiter with Radio Waves

Exploring Jupiter with
Radio Waves
W. S. Kurth
The University of Iowa
Iowa City, IA
Juno Science Objectives
Origin
Determine O/H ratio (water abundance) and constrain
core mass to decide among alternative theories of origin.
Interior
Understand Jupiter's interior structure and dynamical
properties by mapping its gravitational and magnetic
fields
Atmosphere
Map variations in atmospheric composition, temperature,
cloud opacity and dynamics to depths greater than 100
bars at all latitudes.
Magnetosphere
Characterize and explore the three-dimensional structure
of Jupiter's polar magnetosphere and auroras.
A Useful Analogy (hopefully)
Vibrating string =
Plasma
Sound Waves =
Radio Waves
History: Before Spacecraft Measurements
• Jovian decametric radiation (auroral) discovered from groundbased measurements in 1955 (Burke and Franklin)
• Period (IAU) 9h 55m 29.37 +/- 0.04s
• Maximum frequency of decametric radiation (39.5 MHz)
yields estimate of magnetic field strength (14.1 Gauss)
• Study of decimetric radiation (from the radiation belts)
provided information on the tilt of the magnetic dipole (about
10 degrees) from the rotation axis and with a small offset
from the center.
• Io influence on Jupiter’s decametric radiation (Bigg, 1964)
Jupiter’s Magnetosphere
Solar Wind
Periodicities in radio emissions allow us to
determine the rotation period of a planet.
Bigg’s discovery of an Io influence
on the visibility of Jovian radio
emissions foreshadowed the
Voyager discovery of volcanoes
on Io in 1979.
Radio waves allow us to ‘see’ Jupiter’s
radiation belts.
Janssen et al., Planetary Radio Emissions V, 2001.
Auroras are a visible
manifestation of a
magnetized planet’s
interaction with it’s space
environment.
Jupiter has the brightest auroras in
the solar system.
Juno will provide an opportunity to determine how
Jupiter’s auroras are generated.
Jupiter produces a veritable ‘zoo’ of
radio emissions.
Auroral radio emissions are composed of a
plethora of fine structure.
Juno will allow us to fly through the source of
Jupiter’s auroral radio emissions.
Cassini Orbit 89
October 17, Day 291, 2008
105
10-8
Saturn Kilometric Radiation
V2m-2Hz-1
Frequency (Hz)
-9
10
10-10
10-11
104
fce
-12
10
Narrowband Z-mode Emissions
10-13
RS
Lon
Lat
LT
07:40
4.64
245.33
-56.36
0.16
08:00
4.75
253.45
-59.25
0.37
08:20
4.85
261.18
-61.92
0.61
08:40
4.97
268.44
-64.38
0.87
09:00
5.08
275.17
-66.61
1.17
09:20
5.20
281.31
-68.59
1.52
Radio waves allow us
to look for lightning in
Jupiter’s atmosphere.
Radio Planets: Comparing Earth & Jupiter
Earth
Jupiter
Planetary Radius
6378 km
71,400 km
Earth x 11
Rotation Period
24 Hr
~ 10 Hr
Earth x 0.4
Magnetic Moment
0.3
4.3
Earth x 14
Frequency Range
30 kHz to 800 kHz
Like AM radio
10 kHz to 40 MHz
Like Shortwave Radio
Total Power
~ 30 MegaWatts
~ 10 GigaWatts
Earth x 300
Radio Emissions Most
Influenced By:
Sun
Io, Rapid Rotation
Scale of Magnetosphere
65,000 km
4,300,000 km
Earth x 65
Galopeau et al., Planetary Radio
Emissions VI, 2005
Gurnett and Goertz, J. Geophys. Res., 1981.
Both Earth and Jupiter produce intense radio emissions
as part of the process of generating auroras.
Earth
de Feraudy et al., Planetary
Radio Emissions V, 2001.
Ladreiter & LeBlanc, Planetary Radio
Emissions III, 1991.