Overview of Results from the Radio Plasma Imager (RPI)

on IMAGE

James L. GreenSpace Science Data Operations Office

Goddard Space Flight Center

LEP SeminarSeptember 6, 2002

Outline

• Overview of magnetospheric echoes

• Echo observations and results– Plasmapause and trough region

– Polar Cap

– Magnetopause

• Plasma Resonances (see Bob Benson et al.)

• Origin of kilometric continuum

• RPI & EUV comparisons

• Magnetospheric Tomography

• Summary of Results

http://image.gsfc.nasa.gov/

Propagation Modes

2

Types of Magnetospheric Echoes

Echoes Near the Plasmasphere

• RPI echoes received when IMAGE is outside the plasmapause can be used to:

– Locate the plasmapause to within ~ 0.1 - 0.2 Re

– Determine the approximate density level at the inner limit of the steep plasmapause density gradients

– Observe the density profile inside the plasmapause

• Range spreading may be caused by coherent backscattering due to small scale density irregularities

– Can yield information on both the scale and amplitude of the irregularities

Echoes in the Plasmasphere Refilling Region

Field-Aligned Profile Inversion

• Ne can be obtained from an inversion technique • Two traces are used to obtain Ne in both

hemispheres

Plasmasphere Refilling After the March 31, 2001 Storm

Reinisch et al., 2002Song et al., 2002

Kp Index

2 3

1

3 31

1

Pre-Storm Density Distributions

Empirical Plasmasphere Model Before March 31 Storm

Plasmagram and Profile After Storm

Quiet Day Model

Measured

Normalized Equatorial Ne After Storm

Storm Summary

• During the 31 March 2001 storm event • Enhanced cross tail E field reduces plasmapause to L 2.3

• Emptied the flux tubes between L=2.3 and 5

• Refilling process at L = 2.8 started at 1600 UT on 1 April, and is completed before 2000UT on 2 April

• Refilling at 2.8 is completed in less than 28 hours

• Inner plasmasphere L < 2.3 shows no depletion

• No evidence has been found for plasmaspheric filling from the “top-down” only from the ionosphere outward

Polar Cap Observations

Polar Cap Density Distributions• Using RPI echoes and the

density inversion technique an empirical model of electron density distribution over the polar cap can be obtained

• Combination of individual inversions show the variations of the polar cap density during each pass

July 18, 2000

Density Variations Over the Polar Cap

Comparison of Polar Cap Models

Magnetopause Echoes

Magnetopause Echoes and Density Structure

Putting the “M” in IMAGE

• Special measurement program designed for the magnetopause (~ 10 min/plasmagram)

• Magnetopause boundary layer echoes are diffuse suggesting a sharp but irregular reflecting surface

• Strong echoes observed over a 50 minutes period

Kilometric Continuum Observations

Kilometric Continuum from Geotail

• Geotail observations from Hashimoto et al., 1999– 100 to 800 kHz, many narrow bands observed at all local times

– Narrow latitude range of ~10o to 15o about magnetic equator

– May be generated inside the plasmasphere over a broad longitude range

Kilometric Continuum From CRRES

Carpenter et al., 2000• KC observed within

“plasmaspheric density cavities”

• KC frequency range extends from the local fp to well above the fp of the outer cavity wall

• Density cavities - a factor of 2 to 10 below nearby Ne levels

• Found at all local times but most common in the 18-24 LT range

• Suggest that the density cavity formed by earlier detached plasma associated with earlier periods of plasmaspheric erosion

KilometricContinuum

Kilometric Continuum From IMAGE

• Narrow banded kilometric continuum observed very near the magnetic equator associated with the fuhr at the plasmapause

• Observed during times of large density depletions in the plasmasphere (well below model fp -white line)

• Narrow beaming in latitude observed

Source of Kilometric Continuum

• RPI measurements within the bite-out show that Kilometric Continuum is:

– Generated deep inside the bite-out at the plasmapause

– Beamed along the magnetic equator from a confined source region

– Not generated over a broad source region as previously reported

– Also observe field-aligned echoes

• EUV observes distinct plasmaspheric bite-out structures; an unknown feature prior to IMAGE

Dynamics of the Bite-Out Region

• Position of bite-out changes from ~3 to ~8 hours LT

• Corotation and motion of IMAGE provide different perspectives of bite-out

• Plot of the plasmapause in magnetic longitude coordinates from each EUV observation

• Bite-out region corotes over entire ~5 hour time period

• Extent of bite-out ~10-15° in longitude

Ray Tracing Calculations

Geotail & EUV

• Geotail within 10o of magnetic equator over 01-11UT

• Enters KC beam at ~01 UT and then leaves at ~5 UT

Characteristics of KC

• Correlative Geotail observations confirm that:

– KC is generated in very narrow latitudinal beams (within ~10o of magnetic equator)

– Magnetic longitude extent of ~50o

– KC is also observed coming from inside of a plasma tail region

Observations Re-interpreted

• Previous KC observations maybe coming from a plasmaspheric bite-out region

• Ray tracing calculations show that KC sources in the bite-out are beamed and confined to the bite-out

• Narrow beaming can explain earlier observations

Are Bite-Outs Observed by CRRES?

• Typical CRRES orbit used with EUV bite-out structure observed by IMAGE

• CRRES observations of KC trapped in plasmaspheric cavities are consistent with plasmaspheric bite-outs structures

KC

KC

Generation of Kilometric Continuum

• Banded spectral characteristics of KC and its source region near the magnetic equator at the plasmapause is strong evidence for this emission to be generated by the same mechanism as the lower frequency non-thermal continuum (5-100 kHz)

• Favored mechanism is the linear or non-linear mode conversion theory (electrostatic Z mode to electromagnetic O mode) when fuhr = (n+1/2) fg

EUV & RPI Comparisons

RPI & EUV Comparisons

IMAGE/RPI Transmissions andWind/Waves Receptions

Experiment in Radio Tomography • RPI generated pulse were observed by the Wind/Waves instrument

during several perigee passes (Aug 3 & 15, 2000; Oct 23, Dec 2, 2001)

• Faraday rotation was measured and occurs when the received electric field is observed to rotate with time due to the changing density of plasma and magnetic field strength

• Many future multi-spacecraft missions propose to use Faraday rotation to obtain global density pictures of the magnetosphere

Cummer, et al., 2001; 2002

Data Analysis and Interpretation

• Signal modulation gives Faraday rotation

• Single-frequency FR gives relative path-integrated NeB product

• Recent experiments produced dual frequency Faraday measurements

RPI Transmissions Received by Cluster

Spectrograms of Received Signal at Each Spacecraft

04:41:00 04:41:30 04:42:00 04:42:30 04:43:00 04:43:30 04:44:00 04:44:30

Ez

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Summary of RPI Results1. Pervasiveness of the ducted echoes in the plasmasphere, plasmapause,

trough, and polar cap regions• Field-aligned density structures are prevalent throughout the magnetosphere

• Could this be a consequence of persistent ionospheric outflow?

2. Determine (nearly instantaneously) the density distribution along field lines in the plasmasphere refilling region • Refilling is faster than any models predicted by a factor of ~2

• No discontinuities in the density observed as part of the filling process

3. Observed diffuse echoes from plasmapause and magnetopause BL• Key magnetospheric boundaries have sharp densities but irregular surfaces

4. Determine polar cap density distributions below the s/c within one pass• Demonstrates the variable nature of the polar cap ionosphere as a source of

plasma for the tail

5. Measurement of fundamental plasma resonances• Like the ionosphere, the magnetosphere has clear D and Q resonances

6. KC emanating from plasmapheric bite-outs• Are bite-out structures a sufficient conditions for the generation of KC?

7. Reception of RPI pulses by Wind from distance of over 12 RE• Provides validity to future tomographic missions