Radio Sounding in the Magnetosphere and Topside Ionosphere

.
BW Reinisch1, DM Haines1, RF Benson2, K Bibl1, G Cheney1, SF Fung2, IA Galkin1,
 JM Grebowsky2, JL Green2, X Huang1, R Manning3, and WWL Taylor4

1	University of Massachusetts Lowell, Center for Atmospheric Research, Lowell, MA
2	Goddard Space Flight Center, NASA, Greenbelt, MD
3	Observatoire Paris-Meudon
4	Raytheon STX, GSFC, Greenbelt, MD

Abstract. Two space borne instruments are being developed to remotely sense plasma distributions in the magnetosphere and the topside ionosphere. The first instrument is the Radio Plasma Imager (RPI); a low frequency sounder designed to sweep from 3 kHz to 3 MHz, which will be part of NASA’s IMAGE mission to be launched in January 2000. While in the magnetospheric cavity (7 RE altitude), RPI will receive echoes from the magnetopause and the plasmasphere and will measure the direct response of the magnetosphere’s configuration to changes in the solar wind. With three orthogonal dipole antennas, the angle of arrival of returning echoes will be determined with high accuracy. Similar to modern groundbased ionosondes, RPI will operate like a low-frequency radar system measuring range, location, plasma density, and motion of the reflecting plasma structures. 
The second instrument is the TOPside Automated Doppler Sounder (TOPADS) which is being developed for the Ukrainian WARNING mission scheduled for launch in 2001. At a high inclination circular orbit, TOPADS will measure the topside vertical electron density profiles every 75-km along its orbit. Three orthogonal 20 m tip-to-tip dipole antennas will be used for reception, one of them for transmission. Operating as a high-frequency radar, TOPADS will for the first time provide topside plasma velocities by remotely tracking the motions of plasma irregularities. Automatic processing of the ionogram data will provide real time electron density profiles that will be made freely available in real-time as a space weather diagnostics tool. 

1   Introduction

For the first time, modern sounding techniques will be applied to the remote sensing of plasma structures above the peak of the F2 layer. The Radio Plasma Imager (RPI) will be flying on NASA’s magnetospheric Explorer, the IMAGE satellite, to be launched in January 2000, and the Topside Automated Doppler Sounder (TOPADS) is being developed for the Ukrainian WARNING mission. Radio sounding is a well-established technique that was first deployed in the 1920’s for ionospheric sounding from the ground (Breit and Tuve, 1926). In recent years, advanced digital sounders were developed for groundbased observations that provide detailed information on the structure and dynamics of the bottomside ionosphere (Reinisch, 1996). Radio sounding relies on total reflection of radio waves from plasma structures that have plasma frequencies fN equal to the radio frequencies f. It is, therefore, not possible on the ground to receive echoes reflected from the topside ionosphere or the magnetosphere (i.e., from plasma less dense than the F-layer peak). Topside ionospheric sounders, first developed by Franklin and Maclean (1969), recorded the amplitudes and echo delay times of ionospheric echoes as function of frequency in the same way it was done by the groundbased sounders of the time. Data from the highly successful Alouette/ISIS topside sounders (Jackson et al., 1980; Jackson, 1986) were the basis for nearly 1,000 scientific publications (Benson et al., 1998) The typical frequency range of these analog sounders was 0.1 to 20 MHz corresponding to the plasma frequencies in the ionosphere (Pulinets, 1989). For magnetospheric sounding the required frequency range is 3 kHz to 3 MHz, corresponding to electron densities Ne  105 to 1011 m-3, since 

The potential use of IMAGE data for space weather analysis and forecasting is summarized in Table 1.

Imager	Data Product (Level-1)	Derived Quantities	
RPI	•	Plasmagram

•	Echo Map

•	f(t) Spectogram (1/orbit)	•	Distance to Magnetopause, Plasmapause,
Polar Cusp (when observed)
•	Magnetospheric shape (with model), structure,
Gross irregularities
•	Storm conditions from a plasma/radio wave Perspective	
SI	•	FUV auroral image at (121.6 nm)
•	FUV auroral image (135.6 nm)	•	Structure and intensity of the Proton Aurora
•	Structure and intensity of the Electron Aurora	
WIC	•	Aurora Image (140-190 nm)	•	Size and shape of Auroral Oval from intensities
Of LBH bands	
GEO	•	Lyman alpha image (121.6 nm)	•	Integrated line-of-sight density map of the Earth’s geocorona	
EUV	•	Plasmasphere He+ image (30.4 nm)	•	Plasmaspheric density structure and plasmaspheric processes	
HENA
MENA	•	Neutral atom image (10-500 keV)

•	Neutral atom image (1-30 keV)	•	Neutral atom image of composition and energy of the Ring Current and near-Earth Plasma Sheet
•	Both instrument data combined to make a provisional Dst index
•	Plasma Sheet and Ring Current injection dynamics, structure, shape and local time extent	
LENA	•	Neutral Atom Image (10-500 eV)	•	Neutral atom composition and energy of the Auroral/Cleft ion fountain
•	Ionospheric outflow	

Table 1. IMAGE space weather measurements

2   The Radio Plasma Imager on the IMAGE Mission

NASA’s IMAGE mission is scheduled for launch from Vandenberg, CA, in January 2000. The 13.5 hour orbit will be highly elliptical (Figure 1) with 7 RE altitude at apogee and 1,000 km at perigee. The scientific objective of the mission is to determine the dynamic characteristics of the magnetosphere in response to changes in the solar wind. Remote sensing from a single satellite using imaging techniques can capture the changing characteristics of the magnetospheric plasma structures, from the magnetopause to the auroral ionosphere. IMAGE, which will be NASA’s first Medium-class Explorer (MIDEX) mission, will carry three different types of sensors: neutral atom imagers, UV imagers, and the radio plasma imager; J. L. Burch of Southwest Research Institute in San Antonio, TX, is the principal investigator of the IMAGE mission. The list of instruments and the instrument lead investigators is shown in Table 2. This ensemble of instruments will address the following questions: (1) What are the dominant mechanisms for injecting plasma into the magnetosphere on substorms and magnetic storm time scales? (2) What is the directly driven response of the magnetosphere to solar wind changes? (3) How and where are magnetospheric plasmas energized, transported, and subsequently lost during storms and substorms? The cartoon in Figure 2 illustrates the “field of views” of the different instruments, and it is immediately evident that RPI “sees” plasma structures in a different way than do the other instruments. The “plasmagram” shown on the left side of Figure 2 displays echo delay time versus frequency. RPI employs an active technique: short radio pulses are transmitted into all directions and echoes received from plasma structures with fN = f and gradient vectors parallel to the incident wave normal. 

Figure 1. Orbital characteristics of IMAGE

Image	Measurement	Critical Measurement Requirements	Principal Investigators	
NAI	Neutral atom composition and energy-resolved images over three energy ranges:
10-500   eV   (LENA)
  1-30   keV   (MENA)
10-500 keV   (HENA)	FOV: 90ºx90º (image ring current at apogee).
Angular Resolution: 8ºx8º (LENA), 4ºx8º (MENA), 4ºx4º (HENA),
Energy Resolution (ΔE/E): 0.8
Composition: distinguish H and O in magnetospheric and ionospheric sources, interstellar neutrals and solar wind.
Image Time: 4 minutes (resolve sub-storm development).
Sensitivity: effective area 1 cm2 for each sensor.	

T.E. Moore, GSFC
D.T. Young, SwRI
D. Hamilton, Univ. of Maryland	
EUV	30.4 nm imaging of plasmasphere He+ column densities.	FOV: 90ºx90º (image plasmasphere from apogee),
Spatial Resolution: 0.1 Earth radius from apogee.
Image Time: several minutes to hours resolve plasmaspheric processes). 	B.R. Sandel, Univ. of
Arizona	
FUV	Far ultraviolet imaging of the geocorona at 121.6 nm (GEO) and the aurora at 140-190 nm (WIC) and 121.6 and 135.6 nm (SI)	FOV: 15ºx15º (SI) for aurora (image full Earth from apogee), 1ºx360º for geocorona, and 22.4ºx30º (WIC)
Spatial Resolution: 70 km (WIC), 90 km (SI)
Spectral Resolution: separate cold geocorona H from hot proton precipitation (λ~0.2 nm near 121.6 nm); reject 130.4 nm and select 135.6 nm electron aurora emissions.
Image Time: 2 minutes (resolve auroral activity).	S.B. Mende, Univ. of California, Berkeley	
RPI	Remote sensing of electron densities and magnetospheric boundary locations using
Radio sounding.	Density range: 105-1011 m-3 (determine electron density from inner plasmasphere to magnetopause).
Spatial resolution: 500 km (resolve density structures at the magnetopause and plasmapause).
Image Time: 1 minute (resolve changes in boundary locations.	B.W. Reinisch, Univ. of Massachusetts Lowell	

Table 2. IMAGE instruments

Figure 2. IMAGE observations

2.1   Radio Plasma Imaging

2.1.1   The RPI Instrument

The RPI functions as a radar transmitting radio waves simultaneously in all directions and measuring the angle-of-arrival, amplitude, Doppler frequency, and delay times of all echoes (Green et al., 1998). It uses three orthogonal dipole antennas for reception, two of which are 500-m tip-to-tip antennas in the spin plane; these two dipoles are also used for transmission. The third antenna along the spin axis is 20 m long and is used for reception only. Right and left-hand polarized signals can be transmitted by feeding the currents into the long-wire antennas 900 out of phase. This produces a fairly uniform radiation pattern with maxima in the direction of the spin axis and a 3 dB minimum in the spin plane. In a feasibility paper, Calvert et al. (1995) had shown that a 10 W transmitter together with low-noise receivers provide adequate signal to noise ratios to determine range and direction of echo signals up to a distance of at least 4 RE. . Unlike radio sounding on the ground, magnetospheric sounding is not limited by the interference from atmospheric and man-made sources, but by natural external noise, mainly solar noise bursts and storms (Bougeret et al., 1984), auroral kilometric radiation (Gurnett et al, 1979), and the non-thermal (escaping and trapped) continuum (Gurnett, 1975). In comparison, the receiver noise and cosmic noise are usually of less importance but may dominate at certain times and locations. Figure 3 shows a block diagram of RPI, and its specifications are listed in Table 3.

Figure 3.  RPI system configuration

ITEM	Specifications	Comments	
Radiated Power
	 40 W	10 W/monopole	
Antennas	3 orthogonal dipoles	Two electronically tuned 500m dipoles in spin plane
 one 20m dipole along spin axis	
Frequencies
	3 kHz–3 MHz	105-1011 m-3	
Bandwidth
	300 Hz	Matched to pulse width	
Pulse Width	3.2 ms	 480 km range resolution	
Pulse Repetition Rate	0.5,  1, 2, 4, 10, 20 pps		
Range
	5  RE	SNR depend.	

Table 3. RPI specifications
2.1.2   Imaging the Magnetospheric Structures

Electromagnetic wave propagation in a magnetoionic medium is anisotropic and only waves with the so-called characteristic wave polarizations can propagate. The two characteristic polarizations are generally right and left-hand elliptical, and the two characteristic waves propagate with different phase and group velocities, leading to Faraday rotation effects, different plasma cutoff frequencies (reflection levels), and different echo delay times for plasma sounder observations. Since the magnetospheric plasma is inhomogeneous the characteristic polarization slowly changes along the ray path without significant mode conversion, unless steep gradients are encountered, or when f  fp, where fp is the plasma frequency and f the wave frequency.
RPI uses a quadrature sampling technique to digitize the intermediate frequency available at the receiver output, the same way it is done in the groundbased Digisondes (Bibl and Reinisch, . This makes it easy to adopt the technique of finding the wave normal of the echo signal described by Shawhan (1970). The E field is measured with the three orthogonal antennas at two times separated by a quarter cycle (t=Each 3-antenna measurement determines an instantaneous field vector E(t), and the two quadrature samples give a pair of orthogonal E vectors, Ei = (Ix, Iy, Iz) and Eq = (Qx, Qy, Qz), measured on the x, y, and z antennas. The cross product EixEq determines the orientation of the wave normal n of the arriving wave (Figure 4):

Figure 4. RPI antenna system and the polarization ellipse.

Here it is assumed that that the E field component in the direction of the wave normal is negligible, which is a good approximation as long as fp << f at the satellite location. If the E vector rotates clockwise when looking in the direction of the wave normal, EixEq points in the direction of n, otherwise it points opposite to n. This ambiguity need be resolved with the help of the plasmagram and models of the magneto- and plasmapause. The sense of rotation of the E vector is determined by the direction of the EixEq vector. The accuracy of the angular measurement can be expressed in terms of the voltage signal-to-noise ratio SNR): 

For a signal to noise ratio of 100, or 40 dB, the angular accuracy is of the order of 1°.

2.2   RPI data displays

The distance to the reflection point is approximately given by the virtual range P’ = ½ ctd (c = free space speed of light, td = echo delay time) if fp << f for most of the propagation path between the spacecraft and the reflection point. Accurate target distances can be calculated from the plasmagrams giving P’ as function of frequency by “inverting” the P’(f) plasmagram traces. Simulated traces of plasmasphere and magnetopause echoes are shown in Figure 5 for an assumed plasma distribution. The figure shows the “ordinary wave” (Budden, 1985) echoes only; the extraordinary echo traces (not shown) only differ slightly. The satellite altitude is assumed as 6 RE where the model plasma frequency is about 3 kHz (Ne = 105 m-3). The maximum frequency reflected from the magnetopause is 41 kHz, corresponding to NE = 1.8x103 m-3. So-called profile inversion programs (Jackson, 1969; Huang and Reinisch, 1982) are available that calculate the Ne(R) profiles from the P’(f) traces (X. Huang, personal communication). 
It is difficult to present RPI’s multi-dimensional sounding data in one image, since for each sounding frequency, echoes can arrive from different directions with different delays (ranges). The most direct display is the plasmagram, which is similar to an ionogram, but it does not show the angles-of-arrival. A complimentary image will be the echo-map, a simulation of which is shown in Figure 6. Here the measured reflection points are projected into the orbital plane and a model plasmasphere, magnetopause and cusp is superimposed. The plasma frequency for each reflection point is indicated by the gray shading (color in the planned browse product displays). In this simulation, echoes were received from the plasmasphere/ionosphere, the magnetopause, and the cusp. Uniform gray shading is used for the “ghost” echo locations produced by the 180° ambiguity in the arrival angle.

Figure 5.  Ne distribution (dashed curve) as function of altitude (left scale), and the simulated plasmagram (right scale) with echoes from the plasmasphere and magnetopause.

Figure 6. Simulated RPI echo-map with superimposed model of the magnetosphere.

3.	Topside Sounder

A TOPside Automated Doppler Sounder (TOPADS) is being designed for flight on the Ukrainian WARNING mission, tentatively scheduled for launch in 2001. The design concepts for TOPADS are derived from UML’s highly successful groundbased ionosonde, the Digisonde Portable Sounder (DPS) (Reinisch et al., 1997). The same imaging technique described for RPI will be used by TOPADS. The frequency range of the topside sounder must be different, of course, to measure ionospheric densities from 108 to 1013 m-3, i. e., 0.5 to 30 MHz. A high inclination circular orbit at 600 to 800 km altitudes is planned. The specifications for TOPADS are summarized in Table 4. Three orthogonal 20-m dipole antennas will be used for reception, and one of them for transmission. The transmit antenna will be electronically tuned to optimize the transmission efficiency. Ten watts of radiated power will produce signal-to-noise ratios of 30 to 40 dB, making use of the digital processing gain obtained by applying the pulse compression and Fourier analysis techniques developed for the DPS. Use of a staggered sequence of 33.3 s pulses will expand the Doppler frequency range to 1 kHz, large enough to avoid aliasing caused by the satellite motion. 
By measuring the angle–of-arrival and the Doppler shift of the ionospheric echoes it will be possible to determine the vertical electron density in real time, and also to calculate the velocity of ionospheric irregularities. Figure 7 illustrates the different echoes that can be expected at the satellite. By identifying and disregarding the oblique echoes, the vertical echo trace can be automatically scaled and the vertical Ne profile calculated (Reinisch and Huang, 1982; Huang and Reinisch, 1982). The availability of topside profiles will be a valuable input to space weather forecasting. By analyzing the Doppler frequencies for the different echoes it will be possible to determine the drift velocity of the plasma irregularities similar to the method used by the groundbased Digisondes (Cannon et al., 1991; Smith et al., 1998). 
System Parameter	Nominal	Limits	Rationale	
Radiated Power	< 10 W, 5% or 10% duty cycle	10 W	Produces adequate SNR	
Frequency Range	2-18 MHz	0.5 - 30 MHz	Measure in-situ and F-layer plasma densities	
Frequency Steps	2.5% steps	 1 Hz to 500 kHz	Provides  5% in plasma density resolution	
Time/ionogram or echo-map	10 s	> 4 s	75 km along orbit path	
Maximum Range	3000 km	3000 km	vertical and ducted echoes	
Minimum Range	40 km	10 km using short pulses	No reception is possible during transmission	
Range Increments/iono
                            /echo-map	256 x 2.5 km
32 x 20 km	2.5, 5, 20 km	Sample period = range resolution	
Waveforms	Compl. phase code
Staggered pulses
Chirp			
Pulse Rep Rate	200 Hz or SPS	50, 100, 200 Hz or
SPS	Provides required unambiguous range and Doppler range	
Pulse Width/iono
                         /echo-map	33.3 s
266.7 s	1 or 8 x 33.3 s	5 km resolution /
40 km resolution	
Rec. Bandwidth/iono
                         /echo-map	30 kHz
4 kHz	30 kHz
4 kHz	Consistent with XTR pulse width	
Rec. Sensitivity/iono
                         /echo-map	500 nV
125 nV		Keeps receiver noise below cosmic noise	
Coherent Integr./iono
                      /echo-map	80 ms
10 s	40 ms - 10 s 	Provides processing 
gain & Doppler resolution	
Doppler Resolution/iono
                     /echo-map	12 Hz
0.1 Hz	0.1 Hz	Given by coherent integration time	
Doppler Range/iono
                  /echo-map	±1 kHz
±100 Hz	±1 kHz	SPS waveform eliminates Doppler ambiguities	
Rec. Saturation Recovery 	100 s		Specially designed monostatic radar receiver	
Amplitude Resolution	3 dB	3/8 dB	typical display is 3 dB	
Angle-of-Arrival Res.	2o	1o if SNR > 40 dB	Identifies echo direction	

Table 4. TOPADS specifications

Figure 7. Vertical and oblique echoes received by TOPADS.
4.   Summary

Two modern radio sounders are being developed for deployment in the next few years, one for magnetospheric the other for topside sounding. The angle-of-arrival and Doppler measurement capabilities will make it possible to “image” the positions of the plasma boundaries and irregularities, and give information about the velocities of the plasma structures.

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