## HF RADIO SOUNDING OF GEOSPACE PLASMAS

**Bodo W. Reinisch**

***Environmental, Earth, and Atmospheric Sciences Department, Center for
Atmospheric Research, University of Massachusetts Lowell, 600 Suffolk
Street, Lowell, MA 0185, USA, bodo_reinisch@uml.edu***

## ABSTRACT

Interferometric Doppler imaging has become an integral part in radio
sounding of geospace plasmas. Doppler and angle of arrival measurements
have become important tools of ionospheric HF radio science. The state
of the art in HF sounding and automated ionogram interpretation for
ground-based and space applications is reviewed in this paper.

## 

## INTRODUCTION

Radio sounding is a well-established technique that was first deployed
in the 1920's for ionospheric sounding from the ground \[1\]. In recent
years, advanced digital sounders were developed for ground-based
observations that provide detailed information about the structure and
dynamics of the bottomside ionosphere \[2\]. These modern sounders
measure more than just the time of flight and amplitude of the echoes,
they also determine the arrival angle, wave polarization, and Doppler
frequency. Radio sounding relies on total reflection of radio waves from
plasma structures that have plasma frequencies f~N~ equal to the radio
frequencies f for the ordinary wave mode. It is, therefore, not possible
on the ground to receive echoes reflected from the topside ionosphere
--unlike the incoherent scatter radar echoes- or the magnetosphere since
the ionospheric F2 layer prevents all transmitted waves with frequencies
f \< foF2 (the maximum plasma frequency) from propagating beyond the
height of the F2 layer peak. The first topside ionospheric sounders
\[3\], Alouette and ISIS \[4, 5\] recorded the amplitudes and echo delay
times of ionospheric echoes as function of frequency in the same way it
was done by the ground-based sounders of the time. The typical frequency
range of the analog sounders was 0.1 to 20 MHz \[6\] corresponding to
the plasma frequencies in the ionosphere. Recently, modern radio
sounding techniques were applied to magnetospheric sounding with
frequencies from 3 kHz to 3 MHz \[7, 8\], corresponding to electron
densities of N~e~ 10^5^ to 10^11^ m^-3^ since f~N~/1Hz 9 N~e~/1m^-3^.
Modern topside ionospheric sounders that provide automatic ionograms
scaling and electron density profiles are being prototyped but have not
been flown yet.

# 

# PRINCIPLES OF RADIO SOUNDING

Remote sensing of space plasmas with radio waves was originally
developed for ground-based ionospheric observations using different
techniques: partial reflection (2-6 MHz), total reflection or radio
sounding (0.1 --30 MHz), coherent scatter (10-100 MHz), and incoherent
scatter (50-900 MHz). Hunsucker \[9\] gives a brief description of these
different techniques. The discussion in this paper is limited to HF
radio sounding of space plasmas. Electromagnetic waves propagate through
a magnetoionic medium in the so-called free-space mode as long as n^2^
\> 0 where n is the index of refraction. Reflection occurs where n = 0
and the plasma density gradient is parallel to the wave normal \[10\].
Only waves with the so-called characteristic wave polarizations are
solutions of Maxwell's equations \[11\]. The characteristic
polarizations are generally right and left hand elliptical \[12\] with
regard to the magnetic field direction. In ionospheric radio science,
the left and right hand waves are usually referred to as ordinary and
extraordinary waves. Solving dispersion releation for n = 0 gives the
following three reflection conditions (cutoff frequencies):

![](media/image1.pct) (1)

N, e, and m are number density, charge, and mass of the electrons, ~0~
is the free space permittivity, and B~0~ is the earth's magnetic field.
The O and Z modes have left hand, and the X mode right hand polarization
with respect to the magnetic field direction. Except at high latitudes,
the Z mode is rarely observed from the ground and the two prevailing
modes, O and X, can be identified by the sense of rotation of the **E**
field vector. Ionosondes typically scan through frequencies from \~0.5
to 30 MHz transmitting narrow pulses into the vertical direction and
measuring the echo delay time. The data can be displayed in form of an
ionogram showing the echo delay as function of the sounding frequency.
The echo delay t~d~ is usually expressed in terms of the virtual height
h = 0.5 c t~d~ where c is the free space speed of light. The quiet
daytime ionogram in Fig. 1 was recorded at Millstone Hill, Massachusetts
(42N) and shows echoes from the E, F1 and F2 layers. This ionogram is
relatively easy to interpret since the ionosphere showed almost no
irregularities --there is a traveling disturbance in the F1 region - and
all the echoes were returned from overhead reflection points.

![](media/image2.png){width="4.079861111111111in"
height="3.048611111111111in"}

Fig. 1. Midday ionogram at Millstone Hill, MA displaying the O (red) and
X (green) echo traces. Automatic scaling of the traces (black dashes at
the leading edge of the traces) provides h'(f), from which the electron
density profile N(h) is calculated. The corresponding plasma frequency
profile is superimposed on the ionogram. A subset of the autoscaled
characteristics is displayed on the left.

# MODERN HF SOUNDERS

Modern sounders measure all echo parameters: echo delay time, amplitude,
phase, Doppler frequency, wave polarization, and angle of arrival.
Digital signal processing techniques have allowed reduced the radiated
pulse power requirements to a few hundred watts from a few kilowatts in
conventional ground-based ionosondes. These measurements make it
possible to determine the structure and dynamics of the reflecting
plasma, and they enable the automatic scaling of the ionograms and the
calculation of the vertical electron density profiles in real time. Use
of turnstile antennas determines the wave polarization, i.e., it
identifies the ordinary (O) and extraordinary (X) wave components, and a
properly spaced array of receiving antennas can determine the angle of
arrival for ground-based observations. The array dimensions are
comparable to the wavelength. Space-borne sounders require three
orthogonal antennas for direction finding, and amplitude and phase
measurements for each antenna signal \[13\]. Reinisch et al. \[14\] have
described a simple way of finding the arrival angle by using the
"quadrature vectors **E**~i~ and **E**~q~" obtained from the quadrature
samples of the three orthogonal signal components. The vector product
**E**~i~x**E**~q~ is parallel to the wave normal, at least as long as
the local plasma frequency is much smaller than the sounding frequency.
These techniques produce correct arrival angles for a single arriving
wave at frequency f. This is rarely the case, however, and echoes arrive
simultaneously from different directions depending on the existing
plasma structure. This creates complicated ionogram images with
overlapping vertical and oblique O and X echoes as illustrated for a
high altitude station in Fig.2.

To correctly identify and interpret overlapping echoes in terms of
plasma density and echo location it is necessary to measure the wave
polarization (O or X), the arrival angle, and the Doppler shifts of the
echoes together with the echo delay time, and not simply the echo
amplitude (see the insert in Fig. 2). Fourier analysis of the signals at
each range bin isolates the individual waves coming from different
directions because of their different Doppler shifts:

> ![](media/image3.pct) (2)

Here **k***~j~* is the wave vector, and **v** is the velocity of the
reflecting plasma. Applying interferometry to each spectral component
determines the angles of arrival for the different echoes \[15\].

![](media/image4.png){width="4.5055555555555555in"
height="2.9145833333333333in"}

Fig. 2. Spread F ionogram at Tromso, Norway. Traces formed by echoes
from the east, south-south-east, and overhead are clearly identified by
the modern sounder. The autoscaled vertical O-trace (black dots)
automatically generated the N(h) profile. The insert in the lower right
corner shows the ionogram that conventional ionosonde would produce. The
autoscaled characteristics on the left are reasonably accurate (Courtesy
P. Cannon).

Fig.3 shows a sequence of low latitude ionograms from Boa Vista in
Brazil just after sunset on 22 November 2002 during INPE's COPEX
campaign. The different colors give the directions from which the echoes
are arriving.

![](media/image5.png){width="6.697916666666667in"
height="1.6743055555555555in"}

Fig. 3. Sequence of 5-min directional ionograms on 22 Nov 2003 at the
Northern Equatorial Anomaly station in Boa Vista, Brazil. The red and
green traces are the vertical echoes.

Information on the F layer horizontal structure at a given time is
obtained from the echo skymap \[2\] that is similar to

optical sky images. The skymaps \[2\] show the locations of the radio
sources (reflection points), each source having its own line-of-site
(LOS) velocity. The example in Fig.4 illustrates the conditions at 2229
LT during a spread F night at Jicamarca, Peru on 25 Mar 2003. Blue and
red is used for positive and negative Doppler shifts, here clearly
indicating a west to east drift of the plasma. The three dimensional
velocity vector of the moving plasma is obtained from the set of LOS
velocities \[16\].The Digisonde at Jicamarca -- like the 4 Brazilian
Digisondes in Sao Luiz, Fortaleza, Cachoeira Paulista, and Sta. Maria,
and the Digisonde at Tucuman, Argentina - calculates the velocity
components in real time as illustrated in Fig. 5. The horizontal
velocity components are given in magnetic coordinates.

The radio plasma imager (RPI) on IMAGE \[17\] has for the first time
applied modern sounding techniques to space-borne sounding. The
magnetospheric measurements are recorded in the form of plasmagrams
\[7\], as illustrated in Fig. 6, where the virtual ranges are given in
units of Earth radii, R~E~.

![](media/image6.png){width="6.56875in" height="5.045138888888889in"}

Fig. 4. Digisonde skymap at 2229 LT in Jicamarca on 25 Mar 2003

![](media/image7.png){width="5.874409448818898in"
height="3.339763779527559in"}

Fig. 5. The three drift velocity components at Jicamarca on March 25
2003. The horizontal components are plotted in geomagnetic coordinates.

![](media/image8.pct)

Fig.6. RPI plasmagram scanning from 30 to 600 kHz with virtual ranges up
to 10 R~E~. An X trace from the polar cap region extends from 50 to 600
kHz. RPI transmits less than 10 W of pulse power.

**Acknowledgement**

This research was supported by AF contract F 19628-96-C-0159 and by NASA
under subcontract from Southwest Research Institute.

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