Simulations of Radio Imaging in the Earth’s Magnetosphere

J. L. Green, S. Boardsen, W. W. L. Taylor, S. F. Fung, R. F. Benson,

B. Reinisch, and D. L. Gallagher

Chapman Conference on Space Based Radio Observations At Long Wavelengths, Paris France

October 21, 1998

ABSTRACTThe Radio Plasma Imager (RPI) will be a first-of-its-kind instrument designed to use radio wave sounding techniques to perform repetitive remote sensing measurements of electron number density (Ne ) structures and the dynamics of the magnetosphere and plasmasphere. RPI will fly on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission to be launched in the year 2000. The design of the RPI is based on the recent advances in radio transmitter and receiver design, and modern digital processing techniques perfected for ionospheric sounding over the last two decades. Free-space electromagnetic waves transmitted by the RPI located in the low density magnetospheric cavity will be reflected at distant plasma cutoffs. The location and characteristics of the plasma at those remote reflection points can then be derived from measurements of the delay time, frequency, Doppler shift, and direction of an echo. The 500 m tip-to-tip X and Y (spin plane) antennas and 20 m tip-to-tip Z axis antenna on RPI will be used to measures echoes coming from perhaps as great as 10 RE. RPI will operate at frequencies between 3 kHz to 3 MHz and will provide quantitative Ne values from 0.1 to 105 cm-3.

Using ray tracing calculations, combined with specific radio imager instrument characteristics, enables simulations of what RPI will measure. These simulations have been performed throughout an IMAGE orbit and under different model magnetospheric conditions and dramatically show that radio sounding can be used quite successfully to measure a wealth of magnetospheric phenomena. The radio imaging technique will provide a truly exciting opportunity to study global magnetospheric dynamics in a way which was never before possible.

Outline

• Radio Sounding Background

• What is the IMAGE mission

• Radio Plasma Imager (RPI)

• Instrument Characteristics

• Simulations and modeling

• Summary and Conclusions

Reflection Principles• 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 =

• Echo from reflections perpendicular to density contours

gf2

+ gf2

⎛

⎝ ⎜

⎞

⎠ ⎟

2

+p

2f

Echo

Refracted rays

Refracted rays

n=0n>0

n<0

Sample Ground-based Ionogram

Topside and Bottomside Ionograms

Overview of Radio Plasma Imaging (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– Transmitter can utilize either x or y or both x and y axis antennas– Echo reception on all three axes

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

• Insitu density and resonances measurements

IMAGE Spacecraft

Orbital Characteristics

Magnetospheric Density Structure

Representative RPI Targets

• Magnetopause boundary layer 5 -20 20 - 40

• Polar Cusp 7 - 30 25 - 50

• Magnetopause 10 - 60 30 - 70

• Plasmapause 30 - 800 50 - 250

• Plasmasphere 800 - 8x103 250 - 800

• Ionosphere 8x103 - 1x105 800 - 3000

Optimal range to reflection point 1 to 5 RE

RPI will detect these targets sufficiently often to define their structures and motions

Electron Density Plasma Freq.Target Range (cm-3) Range (kHz)

Ray Tracing Calculations• Ray Tracing Code

– Developed by Shawhan [1967] & modified by Green [1976], has supported over 30 studies during the last 15 years

– Uses the Haselgrove [1955] formalism and cold plasma dispersion relations [Stix, 1992]

• Modeled receiver noise level

• Three-dimensional magnetospheric plasma model– Diffusive equilibrium plasmasphere [Angerami & Thomas, 1964]

– Plasmapause model [Aikyo & Ondoh, 1971]

– Magnetopause boundary [Roelof & Sibeck, 1993]– With Gaussian or multi-Gaussian density profile

• Magnetopause density characteristic of MHD models– 4 times the solar wind density at subsolar point

– 1.1 times the solar density at the dawn-dusk meridian

– Solar Wind electron density = 10 /cc

Magnetopause Echoes and Density Structure

Summary of Science Benefits• Plasma density profiles vs. distance

and insitu

• Nearly simultaneous configuration of several magnetospheric boundaries

• Monitoring of magnetospheric structure & dynamics

• 2-D orbital plane density images (scanned by s/c motion)

• 3-D images (under certain magnetospheric conditions and at certain locations in orbit)

Summary• IMAGE will be launched in January, 2000

• RPI data modeled by ray tracing calculations

- Echo times are extremely sensitive to density variations

• RPI measurements will provide:

- Plasmapause, cusp, and magnetopause positions

- Line-of-sight electron density profiles and insitu density

- Density distributions

• RPI measurements permit quantitative studies of magnetospheric

- Plasma structures and their temporal variations

- Evolution with substorm phase and solar wind pressure

• RPI measurements would significantly enhance our understanding of global magnetosphere configuration and dynamics

• IMAGE Web site: http://image.gsfc.nasa.gov/