Exploring Jupiter with

Radio Waves

W. S. KurthThe University of Iowa

Iowa City, IA

Juno Science Objectives

OriginDetermine O/H ratio (water abundance) and constrain core mass to decide among alternative theories of origin.

InteriorUnderstand Jupiter's interior structure and dynamical properties by mapping its gravitational and magnetic fields

AtmosphereMap variations in atmospheric composition, temperature, cloud opacity and dynamics to depths greater than 100 bars at all latitudes.

MagnetosphereCharacterize and explore the three-dimensional structure of Jupiter's polar magnetosphere and auroras.

Vibrating string = Plasma

Sound Waves = Radio Waves

A Useful Analogy (hopefully)

History: Before Spacecraft Measurements

• Jovian decametric radiation (auroral) discovered from ground-based 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.

Janssen et al., Planetary Radio Emissions V, 2001.

Radio waves allow us to ‘see’ Jupiter’s radiation belts.

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.

4.64245.33-56.36

0.1615.12

4.75253.45-59.25

0.3718.15

4.85261.18-61.92

0.6121.92

4.97268.44-64.38

0.8726.57

5.08275.17-66.61

1.1732.25

5.20281.31-68.59

1.5239.05

07:40 08:00 08:20 08:40 09:00 09:20RS

LonLatLT

L

104

105

Fre

que

ncy

(Hz)

10-13

10-12

10-11

10-10

10-9

10-8

V2 m

-2H

z-1

Cassini Orbit 89October 17, Day 291, 2008

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Narrowband Z-mode Emissions

Saturn Kilometric Radi

Juno will allow us to fly through the source of Jupiter’s auroral radio emissions.

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 kmEarth x 11

Rotation Period 24 Hr ~ 10 HrEarth x 0.4

Magnetic Moment 0.3 4.3Earth x 14

Frequency Range 30 kHz to 800 kHzLike AM radio

10 kHz to 40 MHzLike Shortwave Radio

Total Power ~ 30 MegaWatts ~ 10 GigaWattsEarth x 300

Radio Emissions Most Influenced By:

Sun Io, Rapid Rotation

Scale of Magnetosphere 65,000 km 4,300,000 kmEarth x 65

Gurnett and Goertz, J. Geophys. Res., 1981.

Galopeau et al., Planetary Radio Emissions VI, 2005

Ladreiter & LeBlanc, Planetary Radio Emissions III, 1991.

de Feraudy et al., Planetary Radio Emissions V, 2001.

Earth

Both Earth and Jupiter produce intense radio emissions as part of the process of generating auroras.