J. K. Hargreaves, 1 D. L. Detrick, and T. J. Rosenberg

Transcription

1 Radio Science, Volume 26, Number 4, Pages , July-August 1991 Space-time structure of auroral radio absorption events observed with the imaging riometer at south pole J. K. Hargreaves, 1 D. L. Detrick, and T. J. Rosenberg Institute for Physical Science and Technology, University of Maryland, College Park (Received July 9, 1990; accepted December 17, 1990) An imaging riometer system comprising 49 independent beams has been operathag at South Pole station since January A study of intense, short-duration events from the premidnight sector has defined their typical shape as elliptical, with axial ratio 2.3 oriented along the local L shell. The space-time evolution shows rapid intensifications of the moving absorption patches. INTRODUCTION ray of 64 crossed dipoles, 49 independent beams are formed simultaneously by means of a network built up from 15 Butler matrices. The beam width For more than 25 years the riometer has been a depends on the elevation and is 13' between 3-dB standard instrument for monitoring the incidence points for the overhead beams. Adjacent beams of abnormal radio absorption in the D region [Hargreaves, 1969], and it has been well established that in touch between the 3- and 6-dB levels. Projected to auroral regions most of the sporadic absorption the lower ionosphere at 90 km altitude (Figure 1), the inner beams are about 20 km across and the events are due to energetic (> 20 kev) electrons outer are wider and more elongated. The whole penetrating below 100 km. For practical reasons, array covers 200 to 300 km; for comparison, the most riometer installations use a relatively small dotted circle in Figure 1 shows the area viewed by a antenna with the consequence that the spatial resolution is about 100 km and smaller structures in the typical wide-beam riometer. absorption are not resolved. In order to achieve fin- The signals received from the 49 beams are recorder resolution, some narrow-beam systems, having ed by rapid switching into seven riometers. The dwell larger antennas, have been constructed lnsar/, 1964; time for each step is about 1/8 S, and the whole array Berkey, 1968; Theander, 1972; Nielsen and Axford, is scanned every second. The data are recorded digi- 1977; Nielsen, 1980; Kikuchi et al., 1988]. Whereas tally, and are averaged to 10 s for the present analysis. the earlier systems formed a single beam, Nielsen and collaborators employed the Butler [1966] matrix to The instrument operates at a radio frequency of form several beams from a single array Ml-Iz. Following standard riometer technique, it At the south pole the "imaging riometer for ionois necessary to derive 49 quiet-day curves instead of spheric studies," IRIS, has taken the concept of the only one. (See Ktishnaswarny et al. [1985] and Detn'ck multiple beam riometer to a further level of sophis- and Rosenberg [1990] for further discussion of the tication [Detn'ck and Rosenberg, 1990]. From an ar- calculation of quiet-day curves and examples derived from the IRIS data.) However, there are several outstanding advantages. Compared with most standard riometers, the IRIS covers a wider area, has better spatial resolution, achieves excellentemporal resolasso at Environmental Science Division, Univer- lution, can measure the spatial size of absorption sity of Lancaster, Lancaster, England. patches up to 200 km across, and can observe their motion. With only a single beam, one cannot say Copyright 1991 by the American Geophysical Union. whether a change of absorption is due to a movement of the absorbing region or to a true change of Paper number 91 RS intensity. The IRIS enables us to examine this ques /91 / 91 RS tion for different kinds of events. 925

2 926 HARGREAVES ET AL.: AURORAL RADIO ABSORPTION EVENTS STUDY OF INTENSE, SHORT-DURATION EVENTS OCCURRING BEFORE MIDNIGHT Quicklook Statistically, the absorption events observed at the south pole fall into two groups, one before midnight IRIS // Ionospheric // / if..,. Projection / / Ok N Fig. 1. IRIS beams projected to 90 km altitude as viewed from above the ionosphere. The dotted cir- cle is the field of view of a conventional broadbeam riometer. The indicated directions are referenced to a magnetic compass. and the other before noon [Hargreaves et al., 1964]. Quick look plots show that the premidnight group contains some short but intense events, often with spiky appearance, and three have been selected for study (Table 1). Two points immediately stand out. First, in each case the greatest absorption seen in any of the narrow beams is considerably larger than that in the wide beam, and this means that the precipitation was not uniform over the field of view of the wide-beam riometer. Second, the first event is remarkable for its intensity; 8.9 db at 38.2 MHz is equivalento 14.4 db at 30 MHz, making this possibly the most intense event of the auroral kind that has yet been reported. (Note that the absorption levels quoted in this paper have not been corrected for nonlinearity in the response of the riometer receivers or for the influence of the D region temperature. Corrected values would be 10 to 20% higher for each effect in the case of a 10-dB absorption measured from a riometer quiet level corresponding to a radio sky temperature of 10,000 K and D region temperature of 250 K.) Earlier studies of large nightside auroral absorption events at the south pole have shown that the absorption can be accounted for by intense fluxes of electrons in the 10- to 100-keV energy range [Imhof et al., 1984; Rosenberg et al., 1987]. Figure 2 compares the event as seen by the widebeam riometer and by one of the narrow beams, and this comparison emphasizes that the event contains fine structure that is not visible to a wide-beam riometer. The importance of the location and scale size of the region of auroral precipitation on broadbeam measurements has been discussed previously by Hargreaves et al. [1979]. This work showed the effects of the antenna reception pattern, ½onvolved with the geometry of the electron precipitation region, on measured auroral absorption. A recent extension of this work was presented by Van Bavel et al. [1989], demonstrating that the imaging riometer can be used TABLE 1. Selected Events Event Date Time, UT Peak Absorption Peak Absorption Wide Beam, db IRIS, db Ratio I July 22, May 18, Feb 24,

3 HARGREAVES ET AL.: AURORAL RADIO ABSORPTION EVENTS MHz Riometer IRIS (b) 0 I I I I ß I UT, South Pole 22 July, 1988 Fig. 2. (a) Wide and (b) narrow beam observations of sharp event on July 22, The inset in Figure 2b identifies which IRIS beam (solid square) is plotted. to model, and verify, the absorption which should be expected from a broadbeam instrument. Propera'es at the maxima A contour plot of the absorption at the peak of the event of July 22 (Figure 3) shows a roughly elliptical 1.75 /, i, i ' I I I I ' km, LW absorption patch measuring about 150 x 60 km between absorption levels half that at the peak. The patch is dearly not circular, but neither is it a highly elongated strip. It has sometimes been supposed that events of this type (high intensity, short duration, premidnight) might be extended strips or bands that move rapidly across the observing beam. That is not so in this case, and the events of May 18 and February 24 present similar results. Table 2 gives the median values from 23 measurements taken during the growth and decay of these three events. To consider the possible significance of the axial ratio, we suppose that the energetic particle precipitation producing the absorption is due to an active region in the magnetosphere situated near the nose TABLE 2. Median Results Value Major dimension 205 km _1.75/, I I, I I, I, I, Distance in 100 km Fig. 3. Contour plot of absorption patch at peak of event on July 22, 1988, as viewed from above. Minor dimension 85 km Axial ratio 2.3 Orientation 65*

4 928 HARGREAVES ET AL.' AURORAL RADIO ABSORPTION EVENTS 0.25 "" ' ' I, I, I, I, I, I, Distance in 100 km Fig. 4. Contour plot of absorption patch on February 24, 1988, showing orientation along L shell as viewed from above. of the field line passing through the antenna. A circular region of diameter D, at the nose of the field line, maps in along the dipole field lines to an elliptical patch at the Earth's surface, having dimension y = D/L 3/ andx = D/(L3/ (4-3/L) /z) in the zonal and the meridional directions, respectively, where L is the usual L parameter. The axial ratio x/y = (4-3/L) /z). At the south pole, L - 13, giving x/y = This compares tolerably with the observed value of 2.3, though the comparison could obviously be refined by more realistic field line modeling. The circular active region in the magnetosphere would be 8000 to 9000 km in diameter. The orientation of the south pole IRIS was set by magneti compass, and the markings SWNE on Figure 1 are approximate directions. Computed L shells show that L = 13.2 (invariant latitude-74') passes virtually through the center of the beam pattern at an angle of 73' measured dockwise from the S-N line. This is almost the same as the measured orientation of the absorption patches (Table 2). Figure 4 illustrates a contour plot from the event of February 24 for which the orientation was about average. There 50- July 22 May 18 Feb 24 I I t I I I I I I I I I I I I i I ,043 9,9,30 9,9,31 I I I I I I 2235 UT UT UT Fig. 5. Relations between event magnitude and area.

5 HARGREAVES ET AL.' AURORAL RADIO ABSORPTION EVENTS O O I [ I i [ [ ] I [ [ ' l Absorption (db) ABOVE o-6.o BELOW 0.5 Time (10 s) Fig. 6. Event of July 22, 1988, plotted against S-N distance and time. is to the top. South (poleward) is some variation in the orientation from event to event, but the data are not sufficient to indicate any systematic variation with season or time of day. Finally, it appears that for each of the events studied there is a relation between the magnitude of the event and its area (Figure 5), where the area is defined by the contour of absorption half that of the peak. In each example the event covers the smallest area while at its most intense, suggesting that the additional precipitation is confined to a more localized region. Dynamics Each of the events showed movement and, although the movements are not simple, it is dear that they were not in themselves responsible for the growth and decay of the major absorption as observed. In each case the peak absorption (Table 1) was due to an intensification lasting about 1 min, and the absorption patch did not move very far in that time. However, if we look over a longer period, then movements can be seen. In Figure 6 a cross section of the absorption taken from the seven beams along the S-N axis is plotted againstime. This plot, for the July 22 event, covers 193 km in distance and 15 min of time at 10-s resolution. The event moved into the field of view from the equatorward side (north), rapidly intensified and decayed (at point 47), and retreated toward the equator. The events of May 18 and February 24 were similar in so far that intensifications of short dura- tion occurred within a longer period showing movements, though each was different in detail. CONCLUSIONS The IRIS at the south pole is able to resolve spatial detail in ionospheric radio absorption regions down to 20 km and temporal detail to 1 s. A study of three intense short-duration events has shown that the precipitation region is elliptical, typically 205 x 85 km. The axial ratio of 2.3 and the orientation near the local L shell (on average) are consistent with a source region about 8000 km across in the magnetosphere near the nose of the local field line. The area covered by the event tends to shrink as the event intensifies. Movements of the absorbing patches can be ob- served but the intense events studied here are due to short-lived intensifications of absorption rather than to movement of steady absorption over the riometer beam. Acknowledgments. The IRIS project from design to installation was supported by NSF grant DPP Operation and maintenance of the instrument, as well as data processing and analysis, are being supported by NSF grants DPP and DPP One of us (J.K.H.) wishes to thank the Institute for Physical Science and Technology

6 930 HARGREAVES ET AL.: AURORAL RADIO ABSORPTION EVENTS for the support of several visits to the University of Maryland during various phases of the project. REFERENCES tense electron precipitation spike over the southern polar cap, J. Geophys. Res., 89, 10,837, Kikuchi, T., H. Yamagishi, and N. Sato, Eastward propagation of CNA pulsations of Pc 4-5 periods in the morning sector observed with scanning narrow beam riometer at L = 6.1, Geophys. Res. Lett., 15, 168, Krishnaswamy, S., D. L. Detrick, and T. J. Rosenberg, The inflection point method of determining riometer quiet-day curves, Radio Sci., 20, 123, Nielsen, E., Dynamics and spatial scale of auroral absorption spikes associated with the substorm expansion phase, J. Geophys. Res., 85, 2092, Nielsen, E., and W. I. Axford, Small scale auroral absorption events associated with substorms, Nature, 267, 502, Rosenberg, T. J., D. L. Detrick, P. F. Mizera, D. J. Gorney, F. T. Berkey, R. H. Eather, and L. J. Lanzerotti, Coordinated ground and space measurements of an auroral surge over South Pole, J. Geophys. Res., 92, 11,123, Theander, A., Studies of auroral absorption substorms, K/runa Geophys. Observ. Rep., 723, 1, Ansari, Z. A., The aurorally associated absorption of cosmic noise at College, Alaska, J. Geophys. Res., 69, 4493, Berkey, F. T., Coordinated measurements of auroral absorption and luminosity using the narrow beam technique, J. Geophys. Res., 73, 319, Butler, J. L., Digital, matrix and intermediate frequency scanning, Microwave Scanning Antennas, vol. 3, Array Systems, edited by R.C. Hansen, p. 262, Academic, San Diego, Calif., Detrick, D. L., and T. J. Rosenberg, A phased-array radiowave imager for studies of cosmic noise absorption, Radio Sci., 25, 325, Hargreaves, J. K., Auroral absorption of HF radio waves in the ionosphere -A review of results from the first decade of riometry, Proc. IEEE, 57, 1348, Hargreaves, J. K., H. J. A. Chivers, and J. D. Petlock, A study of auroral absorption events at the South Van Bavel, G., D. L. Detrick, T. J. Rosenberg, and Pole, 1, Characteristics of the events, J. Geophys. D. Venkatesan, The influence of spatial nonuni- Res., 69, 5001, formity and motion on the frequency dependence Hargreaves, J. K., H. J. A. Chivers, and E. Nielsen, of auroral absorption, Eos Trans. A GU, 70, 1250, Properties of spike events in auroral radio absorption, J. Geophys. Res., 84, 4245, Imhof, W. L., T. J. Rosenberg, L. J. Lanzerotti, J. B. Reagan, H. D. Voss, D. W. Daftowe, J. R. Kilner, D. L. Detrick, J. K. Hargreaves, and T. J. Rosen- E. E. Gaines, J. Mobilia, and R. G. Joiner, A coor- berg, Institute for Physical Science and Technology, dinated satellite and groundbased study of an in- University of Maryland, College Park, MD