Plans for a new rio-imager experiment in Northern Scandinavia

Journal ofdmospheric and Solar-Terrestrrul Phwics, Vol. 59, No. 8, pp. 939-949. I997 C, 1997 Elsewrr Science Ltd

Pergamon PLI: SOO21-9169(96)00065--7

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Plans for a new rio-imager experiment in Northern Scandinavia

E. Nielsen and T. Hagfors

Max Planck Institut fur Aeronomie, D-37189, Katlenburg, Lindau, Germany

(Received infinal,form 4 March 1996; accepted 21 March 1996)

Abstract-To observe the spatial variations and dynamics of charged particle precipitation in the high latitude ionosphere, a riometer experiment is planned, which from the ground will image the precipitation regions over an area of 300 x 300 km with a spatial resolution of 6 km in the zenith, increasing to 12 km at 60’ zenith angle. The time resolution is one second. The spatial resolution represents a considerable improvement over existing imaging systems. The experiment employs a Mill’s Cross technique not used before in riometer work: two 32 element rows of antennas form the antenna array, two 32 element Butler Matrices achieve directionality, and cross-correlation yield the directional intensities, 0 1997 Elsevier Science Ltd. All rights reserved

1. INTRODUCTION

In studying the Earth’s plasma environment it is clear that in situ measurements are of importance in pro- viding direct information on the local conditions. In situ measurements are even relevant to ‘local’ physics. No matter how detailed measurements are made on board a spacecraft or rocket, there are still essential parameters which these measurements cannot determine: the spatial extent over which the conditions are as in the local region, i.e. spatial variations and gradients are not determined, nor can the time variations of these spatial patterns be measured. These are areas where ground based experiments can comp- lement and extend space observations. Spacecraft or rockets above the ionosphere can measure the characteristics of energetic particle fluxes precipitating into the ionosphere in great detail. The spatial pattern and the dynamics of the precipitation can, however, only be determined by an instrument fixed on the ground such as an imaging riometer.

Riometers using antennas with narrow antenna beams have been widely used to study the acceleration, propagation, and precipitation of energetic charged particles (for example Nielsen et cd., 1991). Charged particles are believed to be accelerated in the magnetosphere by the merging of magnetic field lines, at the magnetopause or in the magnetotail. Other acceleration mechanisms proposed involve electric fields parallel to the geomagnetic field lines above the ionosphere, or shocks propagating in the tail. Study- ing the shape of these precipitation regions, their

motion relative to the cold plasma, and relative to the Sun-Earth line, their location with respect to other ionospheric features, such as visual aurorae. yield evi- dence on the acceleration mechanism, and therefore clues as to how particles are accelerated in a magnetospheric plasma. Once energetic particles are gen- erated in the magnetosphere, different mechanisms cause particles to precipitate, and these mechanisms can be studied with an imaging riometer. For example geomagnetic pulsations will interact with the particle fluxes to cause distinct patterns of precipitation in the ionosphere, or an interplanetary shock may cause a sudden reconfiguration of the magnetosphere leading to particle precipitation. Solar protons and electrons accelerated in solar flares precipitate into the polar cap and high latitude ionosphere in patterns determined by their propagation characteristics in the magnetosphere. Such events belong to the most spectacular ionospheric absorption events. Detailed measurements of vertical variations of electron density and electron collision frequency can, together with riometer measurements, be used to study the radio wave absorption process.

A new riometer imager experiment, with a spatial resolution exceeding that of current systems by an order of magnitude, is planned to be built in Northern Scandinavia near Tromso. This site in the aurora1 zone is chosen because it allows joint observations with many other experiments: STARE ( = Scan- dinavian Twin Aurora1 Radar Experiment) measures horizontal electron flows in the ionospheric E region over a large area (400 x 400 km) with good spatial

939

940 E. Nielsen and T. Hagfors

resolution (20 x 20 km); EISCAT provides detailed plasma physical parameters in an area over Tromso (for example vertical variations of electron density and electron collision frequency). Also optical measurements, neutral wind observations, and HF radar experiments are available over northern Scan- dinavia. Rockets launched from the Andoya - and Kiruna Rocket Ranges have trajectories passing inside the field of view of the riometer imager. Several spacecraft missions will provide further opportunities for joint research: POLAR, WIND, INTERBALL, GEOTAIL, CLUSTER, SOHO.

2. RIOMETER EXPERIMENTS

A riometer takes advantage of the fact that the electromagnetic cosmic noise intensity above the atmosphere in any given direction in space is constant in time, at least over time intervals of interest for a riometer experiment, that is of the order of one year. An increase of the signal level is caused either by interference from transmitters, or by scintillations of a radio star; it is in practice never a problem to identify these causes. A decrease in the intensity can con- fidently be taken to indicate an increase in the ionospheric electron density causing absorption of the cosmic noise signal. A density increase is normally caused by precipitations of energetic charged particles from the magnetosphere. Riometers are operated in the frequency interval from 30 to .50MHz, and for those frequencies the absorption, for a given density increase, maximizes in the height interval from about 70 to 100 km.

The development of riometer experiments is essen- tially an evolution of antenna arrays, to improve the spatial coverage and resolution of the absorption measurements, and of receivers to improve the time resolution. In the following this evolution is briefly outlined in order to place our new plans in the frame- work of the general developments. Figure 1 illustrates the evolution of the intersections of the riometer antenna lobes with the ionosphere to improve the spatial resolution and coverage.

Twenty years ago nearly all riometer antennas con- sisted of a vertical three element yagi antenna, or two parallel horizontal dipoles. Both antemra types has a wide beam width, which at the 3 dB points intersects the ionosphere at 90 km altitude in a two dimensional area with a linear scale of about 100 km (for example Hargreaves, 1969). (Fig. 1 (a)).

One of the first steps in this direction was the con- struction of an antenna array with a single narrow beam directed towards zenith (Nielsen and Axford,

1977). A square antenna array was constructed as a so-called co-co array (Balsley and Eklund, 1972): eight parallel strings of co-axial cable, where each cable had the inner and outer conductor connected every half wavelength (Handel and Pfister, 1935) and the feed points at the middle, tuned to 50Ohm, were with a suitable impedance transformation network connected together, and connected to a normal riometer. The opening angle of the antenna beam was 14” at the half power points. (Fig. 1 (b)).

The next step in the evolution was guided by a wish to determine the dynamics of the observed small scale absorption regions. This was accomplished by using an array with several narrow beams intersecting the ionosphere in a two dimensional pattern; the observations could therefore be used to measure the horizontal velocity of absorption regions as they sweep over the array. A co-co array was used consisting of 16 strings, covering an area on the ground a factor 4 larger than the previous array, and it had a beam width of 7” (Nielsen, 1980). The strings of dipoles (coaxial cables) were connected to a 16 element Butler matrix, a passive phasing device, which produce 16 antenna beams symmetrically distributed around zenith in the north-south vertical plane. In the same area on the ground at a right angle to the other array one more co-co array was constructed, and here the strings were connected together to one lobe in the east-west vertical plane. Only four lobes were used. (Fig. I(c)).

A similar combination of co-co arrays and Butler matrices (Kikuchi et al., 1990) formed 22 beams (11 in the north-south vertical plane, and 11 in the east- west vertical plane), which were connected to 16 riometers, to yield very detailed observations on lati- tudinal and longitudinal moving absorption regions. (Fig. l(d)).

Detrick and Rosenberg (1990) took a major step toward a real imaging system, i.e. a system which over a large field of view provides observations which allow a coherent spatial picture to be constructed. The authors used horizontal dipoles arranged in a square (8 x 8 dipoles). Each row of dipoles was connected to an eight element Butler Matrix. Each of the eight outputs of the Butler Matrix is associated with a dish- formed lobe, and the eight lobes form a fan symmetric about the zenith and with the dishes perpendicular to the direction of the row of dipoles. Corresponding outputs of these eight Butler Matrices were then connected to the inputs of a 9th Butler Matrix, which separate these lobes into eight narrow beams forming a fan perpendicular to the rows of antennas. In this way first 49 (only using seven of the fans), and later 64 beams were formed. The linear dimension of the intersection of a vertical beam with the 90 km height

0 2zn

C.

I+ 2oTm

DIsTAwcEwlsr-eAs.KM

a.

I J

100 km

b.

Fig. 1. lntersectlon patterns between the ionosphere and riometer antenna radiation patterns, as they have developed over the last 20yr. Panel (f) represents the spatial coverage and resolution of the proposed

experiment.


is about 20 km, increasing to 40 km for 60’ zenith angle. (Fig. l(e)).

In the fall of 1995 a 256 beam system using a square 16 x 16 element antenna array was put into operation at Poker Flat, Alaska (Murayama rt al., 1996). Thus, the evolution of these imaging riometers have developed from l-4, 16,49, 64256 antenna lobes.

Common for all these systems is that antenna signals are added together, either all at the same phase to form one lobe, or, in a Butler Matrix added together with appropriate phase tapering to form many lobes. In principle one could continue using these techniques for ever increasing arrays to achieve the desired spatial resolution. However, using, say, a 32 x 32 array and 33 Butler Matrices is becoming cumbersome, and thus a search for a more practical solution seems called for.

One possibility considered was a phased random square array. Measurement of the signal amplitude and phase at the antennas allows the intensity in any direction to be calculated. One potential problem with such a system is its sensitivity to interference. The random array can be considered a square grid with only some grid positions occupied with antennas. Pro- vided that the antennas are placed randomly in the grid positions, we find that even if only 25% of the total number of grid positions are occupied, the antenna lobes are still narrow, but the gain-level outside the main lobes is considerably increased. In an active radar experiment where the antenna gain is required this may not prohibit its use, but in a passive riometer experiment it implies poor directivity. Such an antenna is therefore not suitable for an imaging riometer experiment.

It is concluded that, to improve the spatial resolution of a riometer experiment much above current capabilities, ‘additive’ riometer techniques are either cumbersome or difficult to implement. In Section 3 a new and practical technique is outlined, which will improve the spatial resolution by more than an order of magnitude.

3. THE EXPERIMENT

The new riometer imager experiment is based on the principles of a Mill’s Cross (Thompson et al., 199 1). A schematic drawing of the measurement technique is shown in Fig. 2. The antenna array consist of two perpendicular rows of antennas, each row containing N antennas. Each row is connected to an N- element Butler Matrix. In this way N dish-like fan shaped antenna lobes are formed located symmetrically around zenith and with the fans perpendicular to the vertical plane containing the row of

antennas. The fans associated with the two rows are therefore perpendicular to each other. The signal is received in each of the outputs from the two Butler Matrices. Considering a fan from each Butler Matrix (fan-‘? and fan-‘j’), the common solid angle of these define a direction, a narrow beam, and the received signals have in common the signal from this direction. Cross correlating the signals from the two dish-like beams yields a measure, the cross-correlation coefficient (I,), of the signal intensity in the common direction. In this way directional intensities can be obtained in a number of directions that equals the square of the number of antennas in one arm of the Mill’s Cross (N’). Since the solid angle is inversely proportional to the length of an arm, it is relatively simple to decrease the beam width to a target value for the desired spatial resolution

3.1. Theoretical considerations

We now compare the antenna radiation pattern of the cross correlation coefficients with the antenna pattern of an additive array. In the following the gain functions for summations and cross-multiplications of antenna signals are derived.

The gain function of a horizontal antenna array at z = 0 with a current aperture distribution f(x,r), is

y(B,@) = ~~f(x,~) ee’2n(r’~+‘s~)dxdy

where x and y are horizontal spatial coordinates in units of wavelength, and

S, = sin (Q) cos (4)

S,. = sin (0) sin (4)

For the general case of aperture over an area, of length L along the x-axis and width W along the y-axis, one finds

s.JO, 4) = LW sin (7&J) sin (nS, w)

7CS,L 74 w)

If it is assumed that the antenna array is square with side length L, then the summation power polar gain diagram is

F sin (nS,L) sin (71&L) * s,(O,4) = L4 7-lS,L 6, L I

In contrast, multiplying the gain functions for two rows of antennas (for W << L) together, one obtains what corresponds to a power polar diagram

g,.(O, 4) = L2 sin (n&L) sin (nS,.L)

7lS,L n&L

The square of the sine-functions in the expression

Plans for a high resolution installation in Northern Scandinavia

RIO-IMAGER

943

X

Mill’s Cross x Antenna

X

(32) I (32) ;

i

X

L PA

(32) ;

BM (EW Fans) I

DPU

[‘I,]

Fig. 2. Block diagram of the experiment, with the two perpendicular rows of antennas. pre-amplifiers, Butler Matrices. receivers, and Data Processing Unit forming the output matrix of directional intensities

(IL,).

for the square array imply narrower antenna lobes and lower side lobes than for the cross-multiplication array. However, by choosing a larger length of the rows of antennas the cross-multiplied output may be sufficiently directive to be used in a riometer experiment.

To illustrate this the calculated polar diagram for a 32 x 32 square array has been compared to the polar diagram of a cross multiplication system using two rows of 32 antennas; the results for a single antenna lobe are shown in Fig. 3(a) and Fig. 3(b), respectively. It is clear that the side lobe performance for the filled array is better than for the Mill’s Cross, as predicted. The diagram for the multiplying system is also good:

it shows a narrow antenna lobe, and the side lobe levels are depressed more than 15 dB, except in a cross centred on the narrow lobe, where the gain level increases to - 8 dB. Thus, if the absorption is homo- geneous over the sky, the measured intensity in the lobe is somewhat smaller than it should be for an ideal lobe. However, because we have measurements in all parts of the ‘cross’, using other antenna lobes, it is possible in an iterative procedure to recover the real intensity associated with each beam with much reduced side lobes. Side lobes are further reduced by the use of tapering. Figure l(f) illustrates the dramatic improvement in spatial resolution in the ionosphere achieved by a cross multiplying system.


Table 1. Parameters of proposed rio-imager system, with those of a current imaging riometer system in parentheses

Frequency [MHz] Number of antennas (2 x 32) Half power beam width [degrees] Resolution in 90 km height [km] Number of beams Field of view [km*] Time resolution [s]

38.2 (38.2) 64 (64) 3.6 (15.0) 6612 (2448) 800 (64) 300 x 300 (300 x 300) 1 (1)

3.2. Design goals

A system is desired that increases the area resolution in the ionosphere by more than an order of magnitude, and that requires the linear scale of resolution to be increased at least by a factor 3.3. This implies that the length of the antenna array must be larger than current systems at least by the same factor. Since a Butler Matrix requires a number of inputs equal to 2”, a minimum number of 32 antennas, covering a distance of 15.5 wavelengths, is required.

The sky temperature (for an experiment location near Tromso, Norway, and zenith angles less than 70”) varies between 16000 K and 4000 K, and we expect absorption values up to 14dB; thus, the ADC must have a 20 dB dynamic range.

In this range we wish to measure the absorption with an accuracy of 0.1 dB, which leads to at least 200 points resolution in the dynamic range. However, to achieve this accuracy everywhere in the dynamic range, an ADC voltage resolution must correspond to 5 K (needed for 0.1 dB accuracy at large absorption events for a sky temperature of 4000K; at higher temperatures coarser resolution is sufficient); thus, less than or equal to 3200 point resolution in the dynamic range is required, corresponding to 12 bits resolution of the ADC voltage.

For a time resolution of one second. an accuracy of 0.1 dB on the absorption measurements may be achieved with a system bandwidth of 100 kHz.

3.3. Technical considerations

Table 1 lists the essential parameters describing the proposed system; the numbers in parentheses are typi- cal for current riometer systems. The frequency selec- ted is 38.2 MHz, a frequency in a quiet band reserved for scientific investigations. The frequency is low enough to cause clear intensity variations for ionospheric events, and it is high enough that the antennas from a mechanical point of view are relatively easy to handle. The number of antennas is 64. The nominal half power beam width is 3.6”. The number of antenna lobes, with low enough side lobes (the side lobe level

increases with increasing zenith angle) to ensure good directivity, that is for all lobes with zenith angle less than 60”, will be about 800. The area in the ionosphere covered by these lobes is 300 x 300 km*. The important aspect of this system is to improve the spatial resolution in the ionosphere.

3.4. The antenna urra_v

Figure 4 shows an individual antenna: two horizontal crossed half wavelength dipoles placed a quar- ter wavelength above the ground. Crossed half-wave dipoles are used to approximate equal gain as a function of azimuth. As a function of zenith angle the gain decreases by 10 dB from zero to 60” zenith angle. The dipoles have a nominal tip-to-tip length of 367cm, and are placed 196 cm above the ground.

A Mill’s Cross is made up of these antennas (Fig. 5); the distance between neighbouring antennas is a half wavelength to avoid grating lobes, and the length of one row of 32 antennas is therefore 121.5 m. A ground plane, extending half a wavelength to all sides of the antennas, ensure a good conducting ground. The total length of a row with the ground plane is 130m. Placing the ground plane on the ground requires that the ground be level over this distance, in order to preserve the phase relationship between all the antennas; the ground may have a slope but it must be a plane.

Figure 6 shows the directions of all the antenna lobes vs zenith angle and azimuth angle projected onto the celestial sphere.

3.5. Receiver system

Each antenna is connected by a coaxial cable, of length at least 70m, to a temperature stabilized box containing the receiver system. The antenna signals are first wide band (3 MHz) filtered and pre-amplified (13 dB) to counter losses in the subsequent units. Spe- cial care is taken to ensure that the group-delay time is less than 1 ns (corresponding to a phase error of 14”) over the band width of 1OOHz of the system. Provided that the phase error is not systematic for all antennas, but random with a maximum of 14”, this will have no effect on the directions of the antenna lobes, and will have negligible effects on the beam width.

The output of the 32 pre-amplifiers are led to a Butler Matrix, where the signals are added using quadrature hybrids and fixed phase delay lines, to form 32 fan-beams symmetrical about the zenith. The Butler Matrix uses the phase of the signals to provide directionality of the antenna array. At the output the signal phases are no longer important and need not be kept constant through the remainder of the receiver system.

Plans foi a high resolution installation in Northern Scandinavia 945


Plans for a high resolution installation in Northern Scandinavia 941


Fig, 4. Each antenna in the Mill’s Cross consists of two crossed half wave dipoles, connected together over a 90” phase shift.

RIO-IMAGER: MILL'S CROSS 32+32

Fig. 5. Mill’s Cross antenna array with ground plane and box for receiver unit. Tapering of antenna current amplitudes is used to suppress side lobes.

The signals are low pass filtered to remove unwanted signals and to limit the signal to the system bandwidth of 1OOkHz. This is followed by ampli- fication to match the signal to what is needed for the ADCs.

Plans for a high resolution installation in Northern Scandinavia 949

is cross-correlated with the output (A,) from each of the 32 outputs from the other row of antennas. Thereby a matrix of cross-correlation coefficients is formed (I,,), each equal to the received power in the ‘particular’ direction common to the signals from the two outputs. This matrix is stored for later scientific analysis.

The phases of the antenna signals were used to determine the directions of observation. The cross correlation is carried out on the signal amplitudes, which are digitized. The amplitude (A) may be measured either by quadrature detection of the receiver voltage (A* = Re” (V) + Im* (V)), or by forming time averages of the real voltage (A = < L’>x~“~). It is the time series, A, from receiver ‘i’, and A, from receiver ‘j’, which are cross-correlated.

The aim is to carry out this cross-correlation analysis with a time resolution of 1 s.

4. SUMMARY

3.6. Corrrlation

In the Data Processing Unit (DPU) the output (A,) of each of the 32 channels from one row of antennas,

The planned experiment will ensure an order of magnitude better spatial resolution in the ionosphere than previously achieved. This is accomplished by using the Mill’s Cross technique, cross multiplying the output signals from two sub-array which each observe the same part of the sky.

REFERENCES

Balsley B. B. and Eklund W. L. 1972

Detrick D. L. and Rosenberg T. J. 1990

Handel, P. V. and W. Pfister 1935

Hargreaves J. K. 1969

Kikuchi T., Yamagishi H. and Lester M

Murayama, Y., Mori, H. Kainuma, S. Ishii, M. Nishimuta, I. Igarashi, K. Yamagishi, H and Nishimo, M.

Nielsen E. and Axford W. I.

Nielsen E.

Nielsen E.. Barrow C. H., Hargreaves J. K., Ranta A., Ranta H., Stauning P. and Zaitzev A. N.

Thompson, A. R., Moran, J. M. and Swenson, G. W. Jr.

1990

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1977

1980

1991

1991

A portable coaxial colinear antenna. IEEE Trans. Antennas Propug. (July), 513-516.

A phased-array radiowave imager for cosmic noise absorption. Radio Sci. 25(4), 325-328.

Ausbreitung der Mikrostrahlen, Z. tech. wiss. Bericht Luftfahrtforschung, Res. Rep. FB338.

Aurora1 absorption of HF radio waves in the ionosphere: a review of results from the first decade of riometry. Proc. IEEE 57(S), 1348.

Drift of aurora1 absorption due to the magnetospheric convection observed with the scanning narrow beam riometer during SUNDIAL-86. Ann. Geophysicar 8, 431.

Development of a high-resolution imaging riometer for the middle and upper atmosphere observation program at Poker Flat, Alaska J. a(mos. sol.-(err. Physics. 59, 9255937 (this issue).

Small scale aurora1 absorption events associated with substorms. Nature 267, 502.

Dynamics and spatial scale of aurora1 absorption spikes associated with substorm expansion phase. J. grophys. Res. 85, 2092.

European riometer observation system. Proc. 1Orh ESA Synp on Rocket and Balloon Progrclms ESA SP-317, 49.

Znferjtirometry and Synthesis in Radio Astronomy, Krieger.

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Plans for a new rio-imager experiment in Northern Scandinavia