Remote Radio Sounding Science For JIMO

J. L. Green, B. W. Reinisch, P. Song, S. F. Fung, R. F. Benson, W. W. L. Taylor, J. F. Cooper, L. Garcia, D. Gallagher, and T.

Markus

Jupiter Icy Moons Orbiter Community Science WorkshopJune 13, 2003, LPI Houston, TX

Sounding Overview from IMAGE

• Two successive Radio Plasma Imager (RPI) plasmagrams

– Virtual range vs frequency

• RPI simultaneously probes the plasmasphere and field-aligned paths in the local hemisphere

• RPI measurements can be used to investigate Ne distributions through echo inversion techniques

Reinisch et al., 2001Carpenter et al., 2002

Goal 3: Radiation Environment

• Radio sounding provides vertical ionospheric electron density (Ne) profiles

• Field-aligned echoes provide Ne profiles along magnetic field lines

– Ganymede - indication of closed or open field lines and magnetospheric extent

• Resonance and passive observations provide accurate determination of in situ measurements of Ne and magnetic field strength

• Passive observations of Jovian radio emissions (1 kHz to 40 MHz) provides provides an index to magnetosphere state or activity level

• Dynamics and variability of Jovian magnetosphere-moon interactions can be studied on various time scales

Ionospheres of Icy Moons: Existing Observations

• In addition, Ganymede has a magnetosphere

Possible Ionospheric Profiles• Vertical Ne profiles (in orbit plane) images aids in production analysis

Future Observations with Sounding

Jovian Emission Background

Precise In situ plasma densities from array of resonances

Kurth et al., 1997

In Situ Plasma Measurements• Two near apogee passes of

IMAGE/RPI on successive quiet and disturbed days

• Can determine the accuracy of:– fp to ~ 1%– fg to ~ 0.1%

• March 30, 2001 - top panel– Clear nfg resonances (n=2-14)– fg kHz– No fp resonance implies fp < 6

kHz

• March 31, 2001 - bottom panel– Increase of B with nfg only to

n = 6– Resonances also at D1, D1, D2+,

fp, fuhr, Q3 and Q4

Benson et al., 2002

fD1-

fD1

Goal #1 &2: Potential for Life/Origins

• Radio sounding at frequencies above the peak ionospheric fp will reach and penetrate moon surface

– Low frequency subsurface penetrating radar (LF-SPR)

• Can determine thicknesses of various layers to a certain depth

– Provides information on ice, impurities, water layers, and vertical temperature profile

– Lower frequencies penetrate deeper

– Ionosphere measurements will be necessary to correct subsurface penetrating radar measurements

Earth Example: Ground Echoes

• Ionosphere measurements will be necessary to correct ground penetrating radar measurements

Summary Assessment for JIMO• A radio sounding instrument provides significant

capability– Ionospheric & magnetospheric sounding

– Subsurface sounding

– In situ resonance measurements

– Passive plasma wave measurements

• Supports 3 major JIMO mission goals

• Complements other JIMO instruments (ie: VHF radars)

• Very low developmental risk: Proven instruments

• High power needed for:– Remote sensing at greater distances/depths

– Higher data rates

• Provides a high science value for the JIMO mission

Backup Material

Principles of Radio Sounding

• Radio waves are reflected at wave cutoffs (n = 0)

• In a cold, magnetized plasma– Ordinary (O-mode): Wave frequency = fp

– Extraordinary (X-mode): Wave frequency =

• Echoes from reflections perpendicular to density contours (direct) or at cutoffs along field lines

gf2

+ gf2

⎛

⎝ ⎜

⎞

⎠ ⎟

2

+p

2f

Echo

Refracted rays

Refracted rays

n=0n>0

n<0

Types of Sounding Echoes

Overview of Radio Plasma Imager (RPI)• RPI transmits coded EM waves and receives resulting echoes at 3 kHz to 3

MHz

– Uses advanced digital processing techniques (pulse compression & spectral integration)

• RPI uses a tri-axial orthogonal antenna system– 500 meter tip-to-tip X and Y axis dipole antennas– 20 meters tip-to-tip Z axis dipole antenna– X axis antennas are used for all transmissions– Echo reception is accomplished on all three

• Basic RPI measurements of an echo at a selected frequency – Amplitude– Time delay (distance or range from target)– Direction of arrival– Wave polarization (ordinary or extra-ordinary)– Doppler shift and frequency dispersion

• In situ density and resonances measurements

IMAGE Orbit

Earth’s Ionospheric Measurements

• Ionograms are echo amplitudes plotted as a function of apparent range versus frequency

– Note: Apparent range plotted as decreasing values

• Two wave polarizations are found (extraordinary - X and ordinary - O)

• Observations contain: direct echoes, resonances, and ground reflections

• Standard inversion technique used to determine the vertical Ne profile

IMAGE/RPI Field-Aligned Echoes

Variations in Orbital Configuration

• Study ionospheric production processes over orbital time scales

• Measure dynamics and temporal variability of Jovian magnetosphere-moon interactions

Plasmagram with Echoes and Resonances

• What does RPI see?

• Clear X mode echo and cutoff frequency identified

fx =

• Clear resonances at: – nfg (n = 1-3)

– fqn (n = 2, 3)

• Determine fp/fg = 0.99 in a self consistent manner Reinisch et al., 2001

Field-Aligned Profile Inversion

• Ne is obtained from an inversion technique

• Two traces are used to obtain Ne in both hemispheres

IMAGE Location

Continued Refilling of the Plasmasphere

Measured

Quiet Day Model

Passive Plasma Wave Measurements

• Passive observations provides a measure of “activity” in the Jovian magnetosphere (solar wind and non-solar wind influences)

Carr et al., 1983