POLARIZATION OF FIELD ALIGNED PROPAGATING ECHOES IN THE PLASPMASPHERE
USING THE RADIO PLASMA IMAGER[^1]

# [G. S. Sales^(1)^]{.underline}, B. W. Reinisch^(2)^, X. Huang^(3)^ I. A. Galkin^(4)^, and J.L. Green^(5)^

## ^(1)^Center for Atmospheric Research, University of Massachusetts Lowell, 600 Suffolk Street,

## Lowell, MA 01854, USA, <Gary_Sales@uml.edu>

***^(2)^As above, but E-mail: <Bodo_Reinisch@uml.edu>***

***^(3)^As above, but E-mail:
[Xueqin_Huang@uml.edu](mailto:Bodo_Reinisch@uml.edu)***

***^(4)^As above, but E-mail:
[Ivan_Galkin@uml.edu](mailto:Bodo_Reinisch@uml.edu)***

***^(5)^NASA Goddard Space Flight Center, Greenbelt, MD, USA,
green@mail630.gsfc.nasa.gov***

# ABSTRACT

Analysis of radio plasma imager (RP plasmagrams show that echo signals
often propagate along the earth's magnetic field to both hemispheres.
Using measured signals on RPI's orthogonal antenna system makes it
possible to determine not only the arrival direction but also the
polarization characteristics of the arriving wave. The measured
arrival-angles are nearly parallel or anti-parallel to the local
magnetic field. Based on plasmagram trace inversions, it has been
concluded that these echoes are X mode waves. This result is confirmed
here; all analyzed traces were X-mode, though the observed polarization
ellipses were often not circular.

**INTRODUCTION**

Magnetically field-aligned propagation dominates the radio signals from
RPI/IMAGE whenever the satellite enters or leaves the plasmasphere.
Analysis of 352 orbits of RPI over a nine month period has shown that
field aligned propagation (FAP) modes were present on approximately 1/3
of the passes through the plasmasphere. These signals appear to
propagate along the magnetic field passing through the satellite
position, often to both the northern and southern hemispheres. Depending
on the satellite location relative to the magnetic equator, the length
of these two paths differs. After reflection, the radio waves return to
the satellite along the same path. Fig. 1 is an example of a plasmagram
when the IMAGE is at L 3.

![](media/image1.png){width="4.7492125984251965in"
height="2.613779527559055in"}

Fig. 1. Typical RPI plasmagram (June 8, 2001; 2040:56 UT) as IMAGE
enters the plasmasphere. Indicated are the short path from the northern
hemisphere and the long path from the southern hemisphere.

The two traces, labeled short and long path, are from the northern
hemisphere and the southern hemisphere, respectively. The strong
resonance line, just above 300 kHz, corresponds to the local plasma
frequency.

Over the analysis period, these FAP modes were observed on L-shells from
L = 2 to L = 4. An important consideration in understanding the
structure of these propagation modes is that these traces appeared on
sequential plasmagrams, typically separated by four minutes, about 41 %
of the time. These FAP traces are, in general, not isolated events, but
are often strung together on two, three or more sequential plasmagrams.

Using the measured voltages received on the three orthogonal antennas on
board the RPI/IMAGE satellite, a technique was developed to uniquely
determine the arrival-angle (with a 180 ambiguity) and the polarization
of all echoes from both the northern and southern hemispheres. Fig. 2
illustrates the signal flow from transmission to reception and the
processing done to determine the echoes' arriving direction and
polarization.

![](media/image2.png)

Fig. 2. Flow chart for RPI signal acquisition and processing including
the determination of the polarization of the received echo.

The arriving wave is illustrated in Fig. 3 at an angle with respect to
the local magnetic field vector **B**. The technique developed here
begins with the received signals on the three orthogonal antennas
(X,Y,Z) and works back towards the electric field intensity vector of
the arriving wave in the polarization plane.

![](media/image3.png)

Fig. 3. Relationship between the arriving wave in the polarization plane
and the local magnetic field at the satellite.

The results of the analysis of a small test data sample showed that the
received signals arrived from directions clustered near the local
magnetic field direction. Fig. 4 compares the azimuth and zenith angles
of the arriving waves with the local magnetic field directions.

![](media/image4.png)

Fig. 4. Comparison of the arrival angles (azimuth and zenith) of the
field-aligned reflected waves and the local magnetic field direction.

Here parallel and anti-parallel refer to signals reflected from the
southern hemisphere and the northern hemisphere, respectively. The
spread in the measured arrival angles (Fig. 5) is likely caused by
uncertainties in the pre-launch calibration. This issue is being
addressed at this time.

Field aligned propagated (FAP) signals observed from both hemispheres
where exclusively X-mode polarized. This supports the observations by
Reinisch et al. \[1\] based on their electron density profile inversion
procedure.

![](media/image5.wmf)

Fig. 5. Distribution of the arrival angles for both northern and
southern hemisphere reflections.

The amplitude of FAP modes appear to decrease with a rate less than
*1/r*; the RPI system often records multiple hemisphere-to-hemisphere
echoes indicating low spreading losses. Ray tracing simulations at small
angles to the magnetic field direction were conducted to determine the
expected spreading losses. Using the 3-dimensional HASEL ray tracing
code a comparison is made between O-mode and X-mode propagation losses
as a function of the angle the rays make with the earth's magnetic
field. A systematic analysis of the amplitude of these FAP modes has
been carried out and compared to the propagation models proposed here.

### SUMMARY

In the plasmasphere/plasmapause region RPI field-aligned propagating
signals are observed on more than 30% of the orbits. These FAP modes are
often observed in a sequence of plasmagrams over a range of L-shells
that is indicative of the complex nature of the plasmasphere at certain
times.

All the processed traces analyzed to date have been identified as X-mode
polarization supporting the results of the Huang and Reinisch electron
density inversion analysis reported in \[1\]. These signals are
sometimes near-circularly polarized, though more near-linearly polarized
signals often appear at the higher frequencies along the traces.

These near-linearly polarized signals occur at a significant fraction of
time. We suspect that the observed axial ratio deviation from circular
and the sometimes-significant deviations of the arriving signal angles
with respect to the local magnetic field direction result from a
combination of weak signal-to-noise ratios and improper receiver
calibrations.

# REFERENCE

\[1\] Reinisch, B.W., X. Huang, P. Song, G.S. Sales, S.F. Fung, J.L.
Green, D.L. Gallagher and V.M. Vasyliunas, Plasma Density Distribution
Along the Magnetospheric Field: RPI Observations From IMAGE, Geophys.
Res. Lttrs, 28, 24, 4521-4524, 2001.

[^1]: UML was supported by NASA under subcontracts from Southwest
    Research Institute.