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Phase velocity studies of 34-cm E-region irregularities observed at Millstone Hill
Journal of Atmospheric and Terresrrial Printed inGreat Britain.
Physics, Vol.
54, No. 6, pp. 759-768,
OOZI-9169/92 %5.00+ .oO
1992.
Pergamon Press Ltd
Phase velocity studies of 34-cm E-region irregularities observed at Millstone Hill J. C. Massachusetts
Institute
FOSTER
of Technology,
and D. TETENBAUM
Haystack
(Received injinalform
Observatory,
Westford,
MA 01886, U.S.A.
6 May 1991)
Abstract--Phase velocity observations at E-region heights made with the Millstone Hill 440 MHz radar find no evidence of an ion acoustic limiting speed for phase speeds observed near 0” magnetic aspect angle. Under most circumstances the phase speed increases steadily with increasing backscattered power amplitude. For a 34cm volume backscatter cross-section, o,, less than -5 x lO~“m~‘, the phase speed is at or below the usual ion acoustic speed in the E-region (350m/s), and increases only slowly with the observed backscattered power amplitude (- 50 m/s per 1OdB). At higher power levels, the phase speed exceeds 350 m/s, reaching values in excess of 750 m/s at times, and increases more rapidly with backscattered power (-200 m/s per 1OdB). Phase velocity/time maps observed over a 3” span of latitude suggest that many features of the phase speeds observed are directly related to changes in the ambient convection electric field in the E-region due to changing activity conditions or the effects of superimposed mag-
netospheric pulsations.
2. EXPERIMENTAL
1. INTRODUCTION
TECHNIQUE
Previous studies have shown that the most intense E-region echoes are observed from the Millstone site when the radar is pointed approximately perpendicular to the magnetic field at E-region heights (110 km). Using the lowest operational elevation angle (4”) in order to observe as far poleward as possible, the combined altitude and aspect angle criteria are met at two distinct azimuths to the northeast and northwest of the site (-20 and 310”). Since there is a 70” azimuth difference between these two positions, under most circumstances the magnitude as well as the sign of the flow angle (the angle between the electron drift vector and the radar beam) differs between the northeast and northwest high-sensitivity positions. The northeast position is at somewhat higher magnetic latitude and thus reaches threshold electric field conditions, appropriate for the onset of the Farley-Buneman instability and irregularity growth, more often. Unless noted otherwise, all observations reported here were taken with the 46m steerable antenna held fixed, directed at this northeast position (20” azimuth, 4” elevation). For the phase velocity studies reported here, a 10 s integration period was used with a long-pulse (640~s) spectral mode which provided 96 km line-of-sight range resolution. During alternate integration periods, a 40~s pulse provided power profile observations with 6 km resolution. The spectra from the long-pulse spectral mode are transformed into autocorrelation functions (ACFs) in post-analysis. This analysis consists only
The Millstone Hill 440MHz UHF radar receives intense backscatter from 34 cm irregularities when viewing E-region heights to the north at aspect angles near perpendicular to the magnetic field (DEL Pozo, 1988; ST-MAURICE et al., 1989). Although the Millstone Hill site is situated at a subauroral latitude (288S”E longitude, 42.6”N latitude, 55”A), the appropriate aspect angle for coherent backscatter is attained some 5-8” poleward of this using low antenna elevation angles (4-10”). When the electric field in the E-region exceeds a threshold value of the order of 20mV/m, electron drifts greater than the local ion acoustic speed lead to the onset of the two-stream instability (FARLEY, 1963; BUNEMAN, 1963) and the generation of a spectrum of irregularities which produce radar aurora1 backscatter. A review of recent results in the study of E-region irregularities and radio aurora has been provided by HALDOUPIS (1989). Most recently, the use of large incoherent scatter radars has opened a new window onto E-region irregularity studies. Experiments at EISCAT (SCHLEGEL and MOORCROFT, 1989) and Millstone Hill (FOSTER and TETENBAUM, 1991) are providing a wealth of new, high-sensitivity, high-resolution observations. In this paper we use the capability of the Millstone Hill UHF radar to observe and map irregularity phase velocities over a 3” span of latitude to investigate the spatial structure and temporal variability of the irregularity phase speed and to determine its relationship to the strength of the coherent backscatter observed. 759
760
J. C. FOSTERand D. TETENBAUM
of a quick-fit velocity estimator, using the slope of the ACF phase. For the strong coherent backscatter reported here, the phase is very linear, allowing us to apply a different slope estimator than used in incoherent scatter quick-fits. The northeast fixed radar beam at 4” elevation angle intercepts the 110 km altitude layer approximately perpendicular to the magnetic field (0’ aspect angle) at 49.2‘ geodetic latitude (6l”A) at a range of 825 km. The antenna beam pattern has been carefully calibrated with a fixed source providing 0.1’ resolution of the shape of the main beam and the near side lobes. The latitude extent covered by the fixed beam at 110 km altitude at the -20 dB beam pattern sensitivity point is 1” at this elevation angle. The 1” antenna beam width at the - 3 dB point translates into a 14km altitude extent at the range and elevation angles used in this study. The high sensitivity of the Millstone Hill radar system permits main-beam coherent backscatter returns to be observed over a 90 dB range of amplitudes which extends from an apparent saturation amplitude near IO 9 m I (volume backscatter cross-section) down to the incoherent scatter background level (FOSTERand TETENBAUM, 199 1). The irregularity layer responsible for the radar returns is largely confined to altitudes near 110 km. During strong events and for a fixed antenna position at low elevation angle, we observe appreciable backscatter power at ranges well beyond the E-region intersection point along the radar beam. These signals are due to the reception of off-main beam returns from irregularities at - 110 km altitude at the indicated range [model studies discussed by FOSTER and TETENBAUM (1991) have quantified this effect]. Under such conditions, the power level observed is strongly attenuated by the beam shape factor, but the Doppler velocity observed with the long-pulse spectral mode is unaffected. Since the Eregion irregularities can produce main-beam coherent returns which are 100 dB above the receiver threshold, the off-main beam returns, although greatly reduced in amplitude, are still strong by incoherent scatter standards. Irregularities at progressively higher latitude are sampled as the range of the observations increases. For experiments at 4” elevation angle and 20” azimuth, the Millstone Hill data provide accurate lineof-sight phase velocity observations over a 3” span of geodetic latitude (49-52”). The equatorward extent of this range is set by the rapidly increasing off-beam angle and magnetic aspect angle at the 110 km point and the (usual) decrease in irregularity intensity with decreasing latitude. The poleward extent of the phase velocity observations is observed exactly at the 0”
Table 1.Off-main beam observations at 1IOkm altitude
Latitude (degrees)
Range (km)
Actual elevation (degrees)
Aspect (degrees)
Beam factor (W
48.5 49.0 49.4 51.0 52.0
730 775 825 1040 1160
5.3 4.2 3.6 1.2 0.0
0.0 0.0 0.0 0.5 1.1
-40 -2 -5 -50 -60
elevation angle horizon for the 110 km altitude. Poleward of the main beam E-region intersection point, the magnetic aspect angle at the 110 km altitude point remains less than 1” over the latitude span of interest. Table 1 lists relevant parameters for Millstone Hill phase velocity observations with the antenna at 4” elevation and 20” azimuth.
3.PHASE VELOCITY OBSERVATIONS 3.1. Event characteristics A typical coherent backscatter event observed from Millstone Hill begins when the convection electric field in the E-region at 50” geodetic latitude exceeds the threshold for the onset of the Farley-Buneman instability. Since the region of enhanced convection electric field expands equatorward with increasing activity (FOSTER et al., 1986) the field often first exceeds threshold at a higher latitude and the initial coherent echoes seen in the fixed beam experiment appear from off-main beam intersection points with the irregularity layer poleward of the main beam intersection point. As the region of enhanced electric field strength expands equatorward, the amplitude of the coherent returns increases. [Irregularity intensity has been found to increase with increasing ionospheric electric field magnitude beyond threshold (MOORCROFT, 1979; HALDOUPIS et al., 1990).] Figure 1 presents a 20 min interval of Millstone Hill coherent backscatter power and phase velocity observations (near 15 MLT), taken with 20 s temporal resolution, which encompasses the onset of strong irregularities in the main beam at 49.5” latitude. Although simultaneous observations of the electric field were not taken, the flow angle is expected to be >45” at all latitudes shown at this local time. The time-latitude map of backscattered power shown at the top of the figure convolves the irregularity pattern in the ionosphere with the steeply tapered beam-shape factor which produces maximum sensitivity near 49.5” latitude. The phase velocity map at the bottom indicates the lineof-sight phase speed at each latitude (range) for which
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Phase velocity studies a
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LATITUDE
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----me----
20.00
20.10
20.00
20
10
20.20 UNIVERSAL
20.40
20.30
20.20
20
30
20
40
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Fig. I. Latitude-time maps of backscattered power (top) and phase velocity (bottom) observed during a 20 min interval from Millstone Hill near 15 MLT. Backscatter is from an irregularity layer at _ 110km altitude and the increasing altitude of the radar beam with range results in a strong beam-factor attenuation of the received power beyond the E-region penetration point. Phase speed, unaffected by the off-beam observation, increases regularly from north to south during the onset of strong irregularities in the main beam at 49.5’ latitude.
adequate backscattered signal strength is observed. The onset of the event is gradual and regular, and the observations strongly suggest that the conditions appropriate for intensifying irregularity growth proceed equatorward as the event progresses. Although an equatorward motion of the irregularity region is not apparent in the backscattered power observations at the top of the figure, a 0.1” min-’ equatorward progression of the region of elevated phase velocity across the 3’ span of latitude of the observations is clearly seen.
3.2. Phuse v&city
magnitude
The phase speed (phase velocity magnitude) over much of the region shown in Fig. 1 is considerably less than the expected E-region ion acoustic speed (_ 350 m/s) which suggests that much of the observed
backscatter was due to ‘secondary’, turbulenceinduced waves. In addition, the phase speed and the backscattered power increase in unison at a given latitude, also suggestive of secondary waves. Figure 2 shows the relationship observed at the beam-center latitude (49.5”) where the phase speed increased at a rate of 200 m/s per 10 dB power increase. The characteristic latitudinal progression and positive correlation of echo power and phase speed observed during strong events suggest that both the amplitude and phase speed of the 34cm E-region irregularities increase with increasing electric field strength. The phase velocity (line-of-sight Doppler velocity) from Type I E-region irregularities is expected to saturate near the E-region ion acoustic velocity, -350 m/s. During many strong events we observe phase speeds >700m/s, much in excess of an ion acoustic
J. C. FOSTERand D. TETENBAUM
762
APR 25 89
20:05
UT
-
20:25
UT 1
11.0
14.0
13.0
12.0 log
POWER
Fig. 2. Variation of phase speed with backscattered amplitude at the beam-center latitude (49.5’) for the observations shown in Fig. 1.Phase speed increased at a rate of 200m/s per 10dB.
‘saturation level’. Figure 3 shows the increase of phase speed magnitude with increasing power observed over a 60 dB dynamic range during a very disturbed event on 20 October 1989 fir’, = 8-f). The antenna, aspect angle and flow angle conditions during this experiment were the same as for the data shown in Figs 1 and 2. A five-pulse multipulse mode (TETENBAUMet al., 1989) with 60 ,USbaud length, 1320 ps total lag extent was used to determine the E-region line-ofsight phase velocity with 9 km range resolution during
0
OCT '0
”
20
89
22
this experiment. ST-MAURICE et al. (1989) present a similar observation of the increase in phase speed with backscattered power during an intense event, but an error in the analysis program used to compute the phase speeds included in that paper put an arti~cia~ upper limit of 460 m/s on the phase velocity magnitude which they reported. For Fig. 3, shown here, the phase speed is below the ion acoustic speed for power levels more than 30 dB below the maximum and well above the expected ion acoustic speed at high backscattered
”
23
UT 0
-
K:p =
7
00
r‘6.0
7:0
8:0
9:0 log
ts.O POWER
it
.c3
d.0
I 3. 0
Fig. 3. Phase speed magnitude increased to > 700 m/s as the backscattered power increased over a 60 dB dynamic range during the very disturbed event on 20 October 1989 (4 = 8 +).
Phase velocity studies power. It is noteworthy that the increase of phase speed with power (slope in Fig. 3) is steeper in the region where the phase speed exceeds the expected ion acoustic value (- 350m/s). In that region the slope is -2OOm/s per IOdB while in the low backscatter power region, where the phase speed is less than 350 m/s, the slope is - 50 m/s per 10 dB. FOSTER and TETENBAUM(1991) suggested that weak irregularities grow linearly with the ambient electric reaching a saturation amplitude field strength, to 10.5 log power (0, - 5 x lo- “mm’, corresponding units in Fig. 3) beyond which their intensity increases considerably more slowly with increasing electric field. That paper also provides evidence for irregularity growth beyond the linear saturation level to a backscatter cross-section of - 1 x 10.. lo m- ’ and finds that electric fields in excess of 50 mV/m are associated with such intense events. The data presented here indicate that the weak, linearly growing irregularities have phase speeds at or below the ion acoustic speed and irregularities beyond the saturation amplitude, presumably where nonlinear phenomena (SUDAN, 1983) act to suppress further wave growth, are associated with phase speeds in excess of the ion acoustic speed. The change in slope of the phase speed vs power plot presented in Fig. 3 can be interpreted as indicating that the phase speed continues to increase with increasing electric field strength, perhaps linearly, as the amplitude of the 34cm irregularities reaches a saturation level beyond which the irregularity amplitude increases more slowly with electric field. The occurrence of phase speeds in excess of the expected ion acoustic speed could indicate an increased E-region electron temperature, and associated increase in the ion sound speed, during strongly driven events (e.g. SCHLECELand ST-MAURICE. 1981), or could be a further indication that strong secondary waves were being observed. FEJER et al. (1986) suggest that electron temperatures > 2000 K and ion acoustic velocities of - 900 m/s can occur at mid-latitudes as evidenced by the appearance at small magnetic aspect angle (< 1”) of two-stream waves with similarly large phase speeds. HALDOUPIS and NEILSEN (1989), however, present evidence that the ion sound speed may not be greatly elevated at the time when such large phase velocities are observed.
163
1979), those authors found that the radar backscatter is often modulated by -30 dB in amplitude at Pc5 frequencies (150-500 s) by waves with spatial wavelength 50-100 km. The ionospheric electric field is the vector sum of the background convection electric field and that associated with the Pc5 micropulsation. Spatially localized growth of irregularities in the oscillating electric field of the PC pulsation results in a periodic variation in amplitude at a given latitude and a regular progression of the amplitude maxima in range along the radar beam with time. The data of Fig. 4 were taken during such an event and show a quasi-periodic variation with -300s periodicity in the main beam at 49.5” latitude. In this figure we have emphasised the latitude variation of the irregularity amplitude by deconvolving the observed power-range profiles by dividing the backscatter amplitude at each range by the appropriate beamshape attenuation factor. This technique gives a good approximation of the relative irregularity amplitude at each latitude and the resultant time-latitude map, shown at the top of Fig. 4, reveals a wave-like variation which spans the 3” experimental field-of-view. The phase velocity map, however, directly shows the poleward progression of the associated regions of high phase speed. As was the case in Fig. 1, phase speeds are higher at the higher latitudes and, as seen in Figs 2 and 3, the higher phase speeds are associated with greater irregularity amplitude at each latitude. We believe that the time and latitude periodicities of the irregularity amplitude and phase speed are those of the oscillating ionospheric electric field, modulated by the PC wave field. 3.4. Dependence of phase speed on power
The 25 April 1989 coherent backscatter event, whose gradual onset as a function of latitude was described in Fig. 1, continued to expand equatorward, eventually producing strong echoes in the northwest antenna beam position (305” azimuth, 4” elevation, -59’11). Extended data sets were taken in both the northeast and the northwest positions near 00 UT on 26 April (- 19 MLT). Figure 5 presents a latitudetime map of backscattered power and phase velocity observed in the northeast position. The dramatic variations in both power and phase speed provide a striking contrast to the conditions shown in Fig. 1. 3.3. Spatial/temporal variations Observed phase velocities at this point in the event Power/phase speed maps, as shown in Fig. 1, are exceed the nominal ion acoustic speed of 350m/s at useful in the interpretation of the periodically varying nearly all latitudes and at each latitude the intensity coherent returns discussed by FOSTER and TETENBAUM of the backscattered power and the phase velocity (1991). In accord with previous observations of magnitude vary in unison. Although the phase speeds periodically varying radio aurora (e.g. WALKER et al., are significantly higher than at the onset of the event,
J. C. FOSTER and D. TETENBAUM
JANUARY
1
20
1
30
1 .40
12
20:30
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1
MLT P 0 WE R
1
50
60
1 .70
1 .8
c3 Vph
Cmis~ -550 -500. -450. -400. -350. -300. -250.
1.20
1
30
1 .40
I UNIVERSAL
1 .60
50
1
70
1.80
TIME
Fig. 4. Latitude-time maps in the format of Fig. 1 showing a quasi-periodic variation of both backscattered power and phase speed with - 300 s periodicity in the main beam at 49.5” latitude. The latitude variation of the irregularity amplitude has been emphasized (top) by deconvolving the observed power-range profiles by dividing the bdckscatter amplitude at each range by the appropriate beam-shape attenuation factor.
observed backscattered intensity is 10~15 dB lower than those shown in Fig. 1. Figure 6 (top) superimposes the temporal variations of both the power and phase velocity observed at 49.5” latitude. A scatter plot of data from range gates spanning latitudes from 49.5 to 50.5” latitude is shown at the top of Fig. 7 and indicates that an increase of lOOm/s in phase velocity magnitude accompanied each IOdB increase in the backscattered power for phase speeds in excess of 300m/s. As was seen for the multi-pulse data from the northeast position shown in Fig. 3, the phase speed decreases more slowly with decreasing power for weak backscatter (log power between 6 and 10). The data observed in the northwest antenna position immediately thereafter exhibited a different relationship between phase speed and power. The lower portions of Figs 6 and 7 present the northwest sector data. Phase speeds to the northwest at this time the peak
were positive, corresponding to a velocity component away from the radar, in contrast to the negative velocities seen to the northeast. A longitudinally uniform westward convection velocity (northward convection electric field) is expected in this local time sector (FosTER et al., 1986) and is consistent with these observations. At the time when the antenna position was changed from northeast to northwest (_ 0.15 UT) the phase speed magnitude was near 600 m/s and the log power near 12 in each of the positions. Two significant differences are apparent when comparing the upper and lower portions of these figures. however. For strong backscatter (log power > IO), the phase speed varies more rapidly with power in the northwest position ( -200 m/s per 10 dB). Also, for weak backscatter (log power < lo), the phase speed is remarkably constant near 3OOm~s for the northwest position data. The sharp break in slope seen in the lower portion of Fig. 7 occurs at a power level IO-20 dB above
765
Phase velocity studies a
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LATITUDE
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3 tH ;m
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ii nQ
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IQ
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s ov
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Cm/s> -650. -600. -550 -500 -450. -400. -350,
23
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75 23.85 UNIVERSAL
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95
0
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0.15
TIME
Fig. 5. Latitude-time map of backscattered power and phase velocity observed at 20” azimuth near 19 MLT during the event whose onset is shown in Fig. 1. Phase velocities exceed the nominal ion acoustic speed of 350m/s at nearly all latitudes and at each latitude the intensity of the backscattered power and the phase velocity magnitude vary in unison.
that of the break data
in the upper
point portion
seen for the northeast
position
of the figure.
In addition to the -2” lower invariant latitude of the northwest observation position, a further difference between the conditions observed to the northeast and the northwest is that the magnetic aspect angle is within 0.1” of perpendicularity at the 110 km altitude in the northeast position while the minimum aspect angle attainable at 30.5” azimuth in the northwest is - 1.O” (using the IGRF 87 magnetic field model). The fact that the maximum backscattered power observed in the northwest is - 10 dB less than that seen during the event in the northeast is consistent with the - 10 dB/degree aspect angle sensitivity expected and observed in other studies with the Millstone Hill UHF radar (FOSTERet al., 1991). The flow angle, of great importance to the magnitude and sign of the observed phase velocity, is unknown for these observations. Differences in either aspect angle or flow angle could
lead to a different irregularity population being sampled in the northwest position and thus to the different phase speed/backscattered power relationship seen. Sufficient data do not exist for this event to address these possibilities. The Millstone Hill observations reported by FOSTER and TETENBAUM(1991), FOSTERet al. (1991) and in this paper suggest that there exists a discrete population of irregularities associated with waves whose phase velocity is closely confined (< 1Oaspect) to the plane perpendicular to the magnetic field. Backscatter from these irregularities exhibits large and rapidly varying intensity and is associated with phase speeds in excess of the ion sound speed. The data of FOSTER et al. (1991) indicate an aspect angle sensitivity of - 15 dB/degree for these irregularities and the data presented here indicate a characteristic increase in phase speed with increasing backscatter amplitude of - 200 m/s per 10 dB.
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,:s 0 .6 UNIVERSAL
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i9
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0 :Q
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Fig. 6. Backscatter power (solid curve) and phase speed (dashed curve) observed looking to the northeast (top) and northwest (bottom) during consecutive intervals. The northeast data correspond to the event of Fig. 5 as observed at 49.5” latitude where temporal variations in phase speed and power were very closely interrelated.
4. SUMMARY
Phase velocity obse~ations at E-region heights made with the millstone Hill 440 MHz radar find no evidence of an ion acoustic limiting speed for phase speeds observed near 0” magnetic aspect angle. Under most circumstances the phase speed increases steadily with increasing backscattered power amplitude. For 34 cm irregularity backscatter cross-sections, o,, less than - 5 x 10-‘3m-‘, the phase speed is at or below the usual ion acoustic speed in the E-region (350 m/s), and increases only slowly with the observed backscattered power amplitude (-50m/s per 1OdB). At higher power levels, the phase speed exceeds 350 m/s, reaching values in excess of 750m/s at times, and increases more rapidly with backscattered power (-200m/sper IOdB).
The characteristics of phase velocity/time maps observed over a 3” span of latitude suggest that many features of the phase speeds observed are directly related to changes in the ambient convection electric field in the E-region due to changing activity conditions or the effects of superimposed magnetospheric pulsations. A detailed identification of the primary or secondary waves and irregularity generation mechanisms responsible for the various observations reported here, as well as a definitive explanation of the power/ phase velocity relationships seen in Fig. 7, has not yet been achieved. The important relationships between E-region irregularity phase speed and amplitude and the electric field structure and flow angle await further investigation. magnetic aspect angles are near perpendicularity for the observations
Phase velocity 4/25/89
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6.0
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NORTHEAST
19
AZ
1 7.0
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8.0
9.0
10.0
9.0 log
10.0 POWER
II
Il.0
.0
12.0
12.0
13.0
1:
0
Fig. 7. Scatter plots of phase speed vs log backscattered power at the 110 km beam penetration point for the data sets shown in Fig. 6. In the northeast (top), an increase of lOOm/s in phase velocity magnitude accompanied each IOdB increase in backscattered power for phase speeds in excess of 300m/s. In the northwest (bottom), the observed phase speed was constant at - 300 m/s for log power < 11.
reported and the flow angle is believed to be at an intermediate value between 30 and 60”. The extensive ability of the incoherent scatter radars to make coherent backscatter observations over a large dynamic intensity range, combined with their ability to map phase velocities and background ionospheric parameters in space and time, will make them strong contributors to this ongoing work.
Acknowledgements-Millstone Hill radar operations and analysis have been supported by the National Science Foundation Co-operative Agreement ATM-87-08137 with the Massachusetts Institute of Technology. The technical support of the Millstone Hill radar staff is gratefully acknowledged, as are helpful discussions with J.-P. St-Maurice, D. R. Moorcroft and the scientific staff of the Haystack Observatory Atmospheric Science group.
REFERENCES BUNEMAN 0.
1963
Excitation streams.
of field aligned sound waves Phys. Rev. Left. 10, 285.
by electron
768
J. C. FOSTERand D. TETENBAUM
XL. Pozo C. F.
1988
FARLEYD. T.
1963
FEJ~ B. G., PROVIDAKES J., FARLEYD. T. and SWARTZW. E. FOSTERJ. C., HOLT J. M., MUSGROVE R. G. and EVANSD. S. FOSTERJ. C. and TETENBAUM D.
1986
FOSTERJ. C., TETENBAUM D., DELPozo C. F., ST-MAURICEJ.-P. and MO~RCROFT D. R.
1991
HALLXXJ~~S C.
1989
HALDC~UPIS C. and NIELSENE.
1989
HAI.DOUPIS C., NIELSENE. and SCHLEGEL K.
1990
MOORCROFT D. R.
1979
SC~~I~EGEL K. and M~RC~OFT D. R.
1989
SCHLEGEL K. and ST-MAURICEJ.-P.
1981
ST-MAURICEJ.-P., FOSTERJ. C., HOLTJ. M. and DEL Pozo C.
1989
SUDANR. N.
1983
TETENBAU~~ D., HOLTJ. M. and SALP~IJ. E.
1989
WALKERA. D. M., GREENWALD R. A., STUARTW. F. and GREENC. A.
1979
1986 1991
440 MHz radar observations of plasma turbulence in the aurora1 lower ionosphere. Ph.D. dissertation, Massachusetts Institute of Technology, Cambridge, U.S.A. A plasma instability resufting in field aligned irregularities in the ionosphere. J. geop&s. Res. 68,60X3. Aurora1 E-region plasma waves and elevated electron temperature. J. ~~eophys.Res. 91, 13583. Ionospheric convection associated with discrete levels of particle precipitation. Geophys. Res. Lets. 13,656. High-resolution backscatter power observations of 440 MHz E-region coherent echoes at Millstone Hill. J. geophys. Rex 96, 1251. Aspect angle sensitivity studies of 440 MHz E-region coherent echoes at Millstone Hill. J. geophys. Res. (s~lbmitted). A review on radio studies of aurora1 E-region ionospheric irregularities. Ann. Geoph,ys. 27, 239. 140 MHz aurora1 backscatter: evidence for characteristic phase velocities below and above the ion acoustic speed. Geophys. Res. Lett. 7, 723. Dependence of radar aurora1 scattering cross-section on the ambient electron density and the destabilizing electric field. Ann. Geophys. 8, 195. Dependence of radio aurora at 398 MHz on electron density and electric field. Can. J. Whys. 57, 687. EISCAT as a tristatic amoral radar. J. geophys. Rm 94, 1430. Anomalous heating of the polar E-region by unstable plasma waves. 1. Observations. J. geophys. Res. 86, 1447. First results on the observation of 440 MHz high-latitude coherent echoes from the E-region with the Millstone Hill radar. J. aeouhvs. Res. 94. 6771. Nonlinear theory of &pi
Physics, Vol.
54, No. 6, pp. 759-768,
OOZI-9169/92 %5.00+ .oO
1992.
Pergamon Press Ltd
Phase velocity studies of 34-cm E-region irregularities observed at Millstone Hill J. C. Massachusetts
Institute
FOSTER
of Technology,
and D. TETENBAUM
Haystack
(Received injinalform
Observatory,
Westford,
MA 01886, U.S.A.
6 May 1991)
Abstract--Phase velocity observations at E-region heights made with the Millstone Hill 440 MHz radar find no evidence of an ion acoustic limiting speed for phase speeds observed near 0” magnetic aspect angle. Under most circumstances the phase speed increases steadily with increasing backscattered power amplitude. For a 34cm volume backscatter cross-section, o,, less than -5 x lO~“m~‘, the phase speed is at or below the usual ion acoustic speed in the E-region (350m/s), and increases only slowly with the observed backscattered power amplitude (- 50 m/s per 1OdB). At higher power levels, the phase speed exceeds 350 m/s, reaching values in excess of 750 m/s at times, and increases more rapidly with backscattered power (-200 m/s per 1OdB). Phase velocity/time maps observed over a 3” span of latitude suggest that many features of the phase speeds observed are directly related to changes in the ambient convection electric field in the E-region due to changing activity conditions or the effects of superimposed mag-
netospheric pulsations.
2. EXPERIMENTAL
1. INTRODUCTION
TECHNIQUE
Previous studies have shown that the most intense E-region echoes are observed from the Millstone site when the radar is pointed approximately perpendicular to the magnetic field at E-region heights (110 km). Using the lowest operational elevation angle (4”) in order to observe as far poleward as possible, the combined altitude and aspect angle criteria are met at two distinct azimuths to the northeast and northwest of the site (-20 and 310”). Since there is a 70” azimuth difference between these two positions, under most circumstances the magnitude as well as the sign of the flow angle (the angle between the electron drift vector and the radar beam) differs between the northeast and northwest high-sensitivity positions. The northeast position is at somewhat higher magnetic latitude and thus reaches threshold electric field conditions, appropriate for the onset of the Farley-Buneman instability and irregularity growth, more often. Unless noted otherwise, all observations reported here were taken with the 46m steerable antenna held fixed, directed at this northeast position (20” azimuth, 4” elevation). For the phase velocity studies reported here, a 10 s integration period was used with a long-pulse (640~s) spectral mode which provided 96 km line-of-sight range resolution. During alternate integration periods, a 40~s pulse provided power profile observations with 6 km resolution. The spectra from the long-pulse spectral mode are transformed into autocorrelation functions (ACFs) in post-analysis. This analysis consists only
The Millstone Hill 440MHz UHF radar receives intense backscatter from 34 cm irregularities when viewing E-region heights to the north at aspect angles near perpendicular to the magnetic field (DEL Pozo, 1988; ST-MAURICE et al., 1989). Although the Millstone Hill site is situated at a subauroral latitude (288S”E longitude, 42.6”N latitude, 55”A), the appropriate aspect angle for coherent backscatter is attained some 5-8” poleward of this using low antenna elevation angles (4-10”). When the electric field in the E-region exceeds a threshold value of the order of 20mV/m, electron drifts greater than the local ion acoustic speed lead to the onset of the two-stream instability (FARLEY, 1963; BUNEMAN, 1963) and the generation of a spectrum of irregularities which produce radar aurora1 backscatter. A review of recent results in the study of E-region irregularities and radio aurora has been provided by HALDOUPIS (1989). Most recently, the use of large incoherent scatter radars has opened a new window onto E-region irregularity studies. Experiments at EISCAT (SCHLEGEL and MOORCROFT, 1989) and Millstone Hill (FOSTER and TETENBAUM, 1991) are providing a wealth of new, high-sensitivity, high-resolution observations. In this paper we use the capability of the Millstone Hill UHF radar to observe and map irregularity phase velocities over a 3” span of latitude to investigate the spatial structure and temporal variability of the irregularity phase speed and to determine its relationship to the strength of the coherent backscatter observed. 759
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of a quick-fit velocity estimator, using the slope of the ACF phase. For the strong coherent backscatter reported here, the phase is very linear, allowing us to apply a different slope estimator than used in incoherent scatter quick-fits. The northeast fixed radar beam at 4” elevation angle intercepts the 110 km altitude layer approximately perpendicular to the magnetic field (0’ aspect angle) at 49.2‘ geodetic latitude (6l”A) at a range of 825 km. The antenna beam pattern has been carefully calibrated with a fixed source providing 0.1’ resolution of the shape of the main beam and the near side lobes. The latitude extent covered by the fixed beam at 110 km altitude at the -20 dB beam pattern sensitivity point is 1” at this elevation angle. The 1” antenna beam width at the - 3 dB point translates into a 14km altitude extent at the range and elevation angles used in this study. The high sensitivity of the Millstone Hill radar system permits main-beam coherent backscatter returns to be observed over a 90 dB range of amplitudes which extends from an apparent saturation amplitude near IO 9 m I (volume backscatter cross-section) down to the incoherent scatter background level (FOSTERand TETENBAUM, 199 1). The irregularity layer responsible for the radar returns is largely confined to altitudes near 110 km. During strong events and for a fixed antenna position at low elevation angle, we observe appreciable backscatter power at ranges well beyond the E-region intersection point along the radar beam. These signals are due to the reception of off-main beam returns from irregularities at - 110 km altitude at the indicated range [model studies discussed by FOSTER and TETENBAUM (1991) have quantified this effect]. Under such conditions, the power level observed is strongly attenuated by the beam shape factor, but the Doppler velocity observed with the long-pulse spectral mode is unaffected. Since the Eregion irregularities can produce main-beam coherent returns which are 100 dB above the receiver threshold, the off-main beam returns, although greatly reduced in amplitude, are still strong by incoherent scatter standards. Irregularities at progressively higher latitude are sampled as the range of the observations increases. For experiments at 4” elevation angle and 20” azimuth, the Millstone Hill data provide accurate lineof-sight phase velocity observations over a 3” span of geodetic latitude (49-52”). The equatorward extent of this range is set by the rapidly increasing off-beam angle and magnetic aspect angle at the 110 km point and the (usual) decrease in irregularity intensity with decreasing latitude. The poleward extent of the phase velocity observations is observed exactly at the 0”
Table 1.Off-main beam observations at 1IOkm altitude
Latitude (degrees)
Range (km)
Actual elevation (degrees)
Aspect (degrees)
Beam factor (W
48.5 49.0 49.4 51.0 52.0
730 775 825 1040 1160
5.3 4.2 3.6 1.2 0.0
0.0 0.0 0.0 0.5 1.1
-40 -2 -5 -50 -60
elevation angle horizon for the 110 km altitude. Poleward of the main beam E-region intersection point, the magnetic aspect angle at the 110 km altitude point remains less than 1” over the latitude span of interest. Table 1 lists relevant parameters for Millstone Hill phase velocity observations with the antenna at 4” elevation and 20” azimuth.
3.PHASE VELOCITY OBSERVATIONS 3.1. Event characteristics A typical coherent backscatter event observed from Millstone Hill begins when the convection electric field in the E-region at 50” geodetic latitude exceeds the threshold for the onset of the Farley-Buneman instability. Since the region of enhanced convection electric field expands equatorward with increasing activity (FOSTER et al., 1986) the field often first exceeds threshold at a higher latitude and the initial coherent echoes seen in the fixed beam experiment appear from off-main beam intersection points with the irregularity layer poleward of the main beam intersection point. As the region of enhanced electric field strength expands equatorward, the amplitude of the coherent returns increases. [Irregularity intensity has been found to increase with increasing ionospheric electric field magnitude beyond threshold (MOORCROFT, 1979; HALDOUPIS et al., 1990).] Figure 1 presents a 20 min interval of Millstone Hill coherent backscatter power and phase velocity observations (near 15 MLT), taken with 20 s temporal resolution, which encompasses the onset of strong irregularities in the main beam at 49.5” latitude. Although simultaneous observations of the electric field were not taken, the flow angle is expected to be >45” at all latitudes shown at this local time. The time-latitude map of backscattered power shown at the top of the figure convolves the irregularity pattern in the ionosphere with the steeply tapered beam-shape factor which produces maximum sensitivity near 49.5” latitude. The phase velocity map at the bottom indicates the lineof-sight phase speed at each latitude (range) for which
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Fig. I. Latitude-time maps of backscattered power (top) and phase velocity (bottom) observed during a 20 min interval from Millstone Hill near 15 MLT. Backscatter is from an irregularity layer at _ 110km altitude and the increasing altitude of the radar beam with range results in a strong beam-factor attenuation of the received power beyond the E-region penetration point. Phase speed, unaffected by the off-beam observation, increases regularly from north to south during the onset of strong irregularities in the main beam at 49.5’ latitude.
adequate backscattered signal strength is observed. The onset of the event is gradual and regular, and the observations strongly suggest that the conditions appropriate for intensifying irregularity growth proceed equatorward as the event progresses. Although an equatorward motion of the irregularity region is not apparent in the backscattered power observations at the top of the figure, a 0.1” min-’ equatorward progression of the region of elevated phase velocity across the 3’ span of latitude of the observations is clearly seen.
3.2. Phuse v&city
magnitude
The phase speed (phase velocity magnitude) over much of the region shown in Fig. 1 is considerably less than the expected E-region ion acoustic speed (_ 350 m/s) which suggests that much of the observed
backscatter was due to ‘secondary’, turbulenceinduced waves. In addition, the phase speed and the backscattered power increase in unison at a given latitude, also suggestive of secondary waves. Figure 2 shows the relationship observed at the beam-center latitude (49.5”) where the phase speed increased at a rate of 200 m/s per 10 dB power increase. The characteristic latitudinal progression and positive correlation of echo power and phase speed observed during strong events suggest that both the amplitude and phase speed of the 34cm E-region irregularities increase with increasing electric field strength. The phase velocity (line-of-sight Doppler velocity) from Type I E-region irregularities is expected to saturate near the E-region ion acoustic velocity, -350 m/s. During many strong events we observe phase speeds >700m/s, much in excess of an ion acoustic
J. C. FOSTERand D. TETENBAUM
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Fig. 2. Variation of phase speed with backscattered amplitude at the beam-center latitude (49.5’) for the observations shown in Fig. 1.Phase speed increased at a rate of 200m/s per 10dB.
‘saturation level’. Figure 3 shows the increase of phase speed magnitude with increasing power observed over a 60 dB dynamic range during a very disturbed event on 20 October 1989 fir’, = 8-f). The antenna, aspect angle and flow angle conditions during this experiment were the same as for the data shown in Figs 1 and 2. A five-pulse multipulse mode (TETENBAUMet al., 1989) with 60 ,USbaud length, 1320 ps total lag extent was used to determine the E-region line-ofsight phase velocity with 9 km range resolution during
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22
this experiment. ST-MAURICE et al. (1989) present a similar observation of the increase in phase speed with backscattered power during an intense event, but an error in the analysis program used to compute the phase speeds included in that paper put an arti~cia~ upper limit of 460 m/s on the phase velocity magnitude which they reported. For Fig. 3, shown here, the phase speed is below the ion acoustic speed for power levels more than 30 dB below the maximum and well above the expected ion acoustic speed at high backscattered
”
23
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7
00
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it
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Fig. 3. Phase speed magnitude increased to > 700 m/s as the backscattered power increased over a 60 dB dynamic range during the very disturbed event on 20 October 1989 (4 = 8 +).
Phase velocity studies power. It is noteworthy that the increase of phase speed with power (slope in Fig. 3) is steeper in the region where the phase speed exceeds the expected ion acoustic value (- 350m/s). In that region the slope is -2OOm/s per IOdB while in the low backscatter power region, where the phase speed is less than 350 m/s, the slope is - 50 m/s per 10 dB. FOSTER and TETENBAUM(1991) suggested that weak irregularities grow linearly with the ambient electric reaching a saturation amplitude field strength, to 10.5 log power (0, - 5 x lo- “mm’, corresponding units in Fig. 3) beyond which their intensity increases considerably more slowly with increasing electric field. That paper also provides evidence for irregularity growth beyond the linear saturation level to a backscatter cross-section of - 1 x 10.. lo m- ’ and finds that electric fields in excess of 50 mV/m are associated with such intense events. The data presented here indicate that the weak, linearly growing irregularities have phase speeds at or below the ion acoustic speed and irregularities beyond the saturation amplitude, presumably where nonlinear phenomena (SUDAN, 1983) act to suppress further wave growth, are associated with phase speeds in excess of the ion acoustic speed. The change in slope of the phase speed vs power plot presented in Fig. 3 can be interpreted as indicating that the phase speed continues to increase with increasing electric field strength, perhaps linearly, as the amplitude of the 34cm irregularities reaches a saturation level beyond which the irregularity amplitude increases more slowly with electric field. The occurrence of phase speeds in excess of the expected ion acoustic speed could indicate an increased E-region electron temperature, and associated increase in the ion sound speed, during strongly driven events (e.g. SCHLECELand ST-MAURICE. 1981), or could be a further indication that strong secondary waves were being observed. FEJER et al. (1986) suggest that electron temperatures > 2000 K and ion acoustic velocities of - 900 m/s can occur at mid-latitudes as evidenced by the appearance at small magnetic aspect angle (< 1”) of two-stream waves with similarly large phase speeds. HALDOUPIS and NEILSEN (1989), however, present evidence that the ion sound speed may not be greatly elevated at the time when such large phase velocities are observed.
163
1979), those authors found that the radar backscatter is often modulated by -30 dB in amplitude at Pc5 frequencies (150-500 s) by waves with spatial wavelength 50-100 km. The ionospheric electric field is the vector sum of the background convection electric field and that associated with the Pc5 micropulsation. Spatially localized growth of irregularities in the oscillating electric field of the PC pulsation results in a periodic variation in amplitude at a given latitude and a regular progression of the amplitude maxima in range along the radar beam with time. The data of Fig. 4 were taken during such an event and show a quasi-periodic variation with -300s periodicity in the main beam at 49.5” latitude. In this figure we have emphasised the latitude variation of the irregularity amplitude by deconvolving the observed power-range profiles by dividing the backscatter amplitude at each range by the appropriate beamshape attenuation factor. This technique gives a good approximation of the relative irregularity amplitude at each latitude and the resultant time-latitude map, shown at the top of Fig. 4, reveals a wave-like variation which spans the 3” experimental field-of-view. The phase velocity map, however, directly shows the poleward progression of the associated regions of high phase speed. As was the case in Fig. 1, phase speeds are higher at the higher latitudes and, as seen in Figs 2 and 3, the higher phase speeds are associated with greater irregularity amplitude at each latitude. We believe that the time and latitude periodicities of the irregularity amplitude and phase speed are those of the oscillating ionospheric electric field, modulated by the PC wave field. 3.4. Dependence of phase speed on power
The 25 April 1989 coherent backscatter event, whose gradual onset as a function of latitude was described in Fig. 1, continued to expand equatorward, eventually producing strong echoes in the northwest antenna beam position (305” azimuth, 4” elevation, -59’11). Extended data sets were taken in both the northeast and the northwest positions near 00 UT on 26 April (- 19 MLT). Figure 5 presents a latitudetime map of backscattered power and phase velocity observed in the northeast position. The dramatic variations in both power and phase speed provide a striking contrast to the conditions shown in Fig. 1. 3.3. Spatial/temporal variations Observed phase velocities at this point in the event Power/phase speed maps, as shown in Fig. 1, are exceed the nominal ion acoustic speed of 350m/s at useful in the interpretation of the periodically varying nearly all latitudes and at each latitude the intensity coherent returns discussed by FOSTER and TETENBAUM of the backscattered power and the phase velocity (1991). In accord with previous observations of magnitude vary in unison. Although the phase speeds periodically varying radio aurora (e.g. WALKER et al., are significantly higher than at the onset of the event,
J. C. FOSTER and D. TETENBAUM
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1.20
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Fig. 4. Latitude-time maps in the format of Fig. 1 showing a quasi-periodic variation of both backscattered power and phase speed with - 300 s periodicity in the main beam at 49.5” latitude. The latitude variation of the irregularity amplitude has been emphasized (top) by deconvolving the observed power-range profiles by dividing the bdckscatter amplitude at each range by the appropriate beam-shape attenuation factor.
observed backscattered intensity is 10~15 dB lower than those shown in Fig. 1. Figure 6 (top) superimposes the temporal variations of both the power and phase velocity observed at 49.5” latitude. A scatter plot of data from range gates spanning latitudes from 49.5 to 50.5” latitude is shown at the top of Fig. 7 and indicates that an increase of lOOm/s in phase velocity magnitude accompanied each IOdB increase in the backscattered power for phase speeds in excess of 300m/s. As was seen for the multi-pulse data from the northeast position shown in Fig. 3, the phase speed decreases more slowly with decreasing power for weak backscatter (log power between 6 and 10). The data observed in the northwest antenna position immediately thereafter exhibited a different relationship between phase speed and power. The lower portions of Figs 6 and 7 present the northwest sector data. Phase speeds to the northwest at this time the peak
were positive, corresponding to a velocity component away from the radar, in contrast to the negative velocities seen to the northeast. A longitudinally uniform westward convection velocity (northward convection electric field) is expected in this local time sector (FosTER et al., 1986) and is consistent with these observations. At the time when the antenna position was changed from northeast to northwest (_ 0.15 UT) the phase speed magnitude was near 600 m/s and the log power near 12 in each of the positions. Two significant differences are apparent when comparing the upper and lower portions of these figures. however. For strong backscatter (log power > IO), the phase speed varies more rapidly with power in the northwest position ( -200 m/s per 10 dB). Also, for weak backscatter (log power < lo), the phase speed is remarkably constant near 3OOm~s for the northwest position data. The sharp break in slope seen in the lower portion of Fig. 7 occurs at a power level IO-20 dB above
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Fig. 5. Latitude-time map of backscattered power and phase velocity observed at 20” azimuth near 19 MLT during the event whose onset is shown in Fig. 1. Phase velocities exceed the nominal ion acoustic speed of 350m/s at nearly all latitudes and at each latitude the intensity of the backscattered power and the phase velocity magnitude vary in unison.
that of the break data
in the upper
point portion
seen for the northeast
position
of the figure.
In addition to the -2” lower invariant latitude of the northwest observation position, a further difference between the conditions observed to the northeast and the northwest is that the magnetic aspect angle is within 0.1” of perpendicularity at the 110 km altitude in the northeast position while the minimum aspect angle attainable at 30.5” azimuth in the northwest is - 1.O” (using the IGRF 87 magnetic field model). The fact that the maximum backscattered power observed in the northwest is - 10 dB less than that seen during the event in the northeast is consistent with the - 10 dB/degree aspect angle sensitivity expected and observed in other studies with the Millstone Hill UHF radar (FOSTERet al., 1991). The flow angle, of great importance to the magnitude and sign of the observed phase velocity, is unknown for these observations. Differences in either aspect angle or flow angle could
lead to a different irregularity population being sampled in the northwest position and thus to the different phase speed/backscattered power relationship seen. Sufficient data do not exist for this event to address these possibilities. The Millstone Hill observations reported by FOSTER and TETENBAUM(1991), FOSTERet al. (1991) and in this paper suggest that there exists a discrete population of irregularities associated with waves whose phase velocity is closely confined (< 1Oaspect) to the plane perpendicular to the magnetic field. Backscatter from these irregularities exhibits large and rapidly varying intensity and is associated with phase speeds in excess of the ion sound speed. The data of FOSTER et al. (1991) indicate an aspect angle sensitivity of - 15 dB/degree for these irregularities and the data presented here indicate a characteristic increase in phase speed with increasing backscatter amplitude of - 200 m/s per 10 dB.
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Fig. 6. Backscatter power (solid curve) and phase speed (dashed curve) observed looking to the northeast (top) and northwest (bottom) during consecutive intervals. The northeast data correspond to the event of Fig. 5 as observed at 49.5” latitude where temporal variations in phase speed and power were very closely interrelated.
4. SUMMARY
Phase velocity obse~ations at E-region heights made with the millstone Hill 440 MHz radar find no evidence of an ion acoustic limiting speed for phase speeds observed near 0” magnetic aspect angle. Under most circumstances the phase speed increases steadily with increasing backscattered power amplitude. For 34 cm irregularity backscatter cross-sections, o,, less than - 5 x 10-‘3m-‘, the phase speed is at or below the usual ion acoustic speed in the E-region (350 m/s), and increases only slowly with the observed backscattered power amplitude (-50m/s per 1OdB). At higher power levels, the phase speed exceeds 350 m/s, reaching values in excess of 750m/s at times, and increases more rapidly with backscattered power (-200m/sper IOdB).
The characteristics of phase velocity/time maps observed over a 3” span of latitude suggest that many features of the phase speeds observed are directly related to changes in the ambient convection electric field in the E-region due to changing activity conditions or the effects of superimposed magnetospheric pulsations. A detailed identification of the primary or secondary waves and irregularity generation mechanisms responsible for the various observations reported here, as well as a definitive explanation of the power/ phase velocity relationships seen in Fig. 7, has not yet been achieved. The important relationships between E-region irregularity phase speed and amplitude and the electric field structure and flow angle await further investigation. magnetic aspect angles are near perpendicularity for the observations
Phase velocity 4/25/89
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Fig. 7. Scatter plots of phase speed vs log backscattered power at the 110 km beam penetration point for the data sets shown in Fig. 6. In the northeast (top), an increase of lOOm/s in phase velocity magnitude accompanied each IOdB increase in backscattered power for phase speeds in excess of 300m/s. In the northwest (bottom), the observed phase speed was constant at - 300 m/s for log power < 11.
reported and the flow angle is believed to be at an intermediate value between 30 and 60”. The extensive ability of the incoherent scatter radars to make coherent backscatter observations over a large dynamic intensity range, combined with their ability to map phase velocities and background ionospheric parameters in space and time, will make them strong contributors to this ongoing work.
Acknowledgements-Millstone Hill radar operations and analysis have been supported by the National Science Foundation Co-operative Agreement ATM-87-08137 with the Massachusetts Institute of Technology. The technical support of the Millstone Hill radar staff is gratefully acknowledged, as are helpful discussions with J.-P. St-Maurice, D. R. Moorcroft and the scientific staff of the Haystack Observatory Atmospheric Science group.
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J. C. FOSTERand D. TETENBAUM
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FEJ~ B. G., PROVIDAKES J., FARLEYD. T. and SWARTZW. E. FOSTERJ. C., HOLT J. M., MUSGROVE R. G. and EVANSD. S. FOSTERJ. C. and TETENBAUM D.
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HALLXXJ~~S C.
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HALDC~UPIS C. and NIELSENE.
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HAI.DOUPIS C., NIELSENE. and SCHLEGEL K.
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SC~~I~EGEL K. and M~RC~OFT D. R.
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SCHLEGEL K. and ST-MAURICEJ.-P.
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ST-MAURICEJ.-P., FOSTERJ. C., HOLTJ. M. and DEL Pozo C.
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SUDANR. N.
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TETENBAU~~ D., HOLTJ. M. and SALP~IJ. E.
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WALKERA. D. M., GREENWALD R. A., STUARTW. F. and GREENC. A.
1979
1986 1991
440 MHz radar observations of plasma turbulence in the aurora1 lower ionosphere. Ph.D. dissertation, Massachusetts Institute of Technology, Cambridge, U.S.A. A plasma instability resufting in field aligned irregularities in the ionosphere. J. geop&s. Res. 68,60X3. Aurora1 E-region plasma waves and elevated electron temperature. J. ~~eophys.Res. 91, 13583. Ionospheric convection associated with discrete levels of particle precipitation. Geophys. Res. Lets. 13,656. High-resolution backscatter power observations of 440 MHz E-region coherent echoes at Millstone Hill. J. geophys. Rex 96, 1251. Aspect angle sensitivity studies of 440 MHz E-region coherent echoes at Millstone Hill. J. geophys. Res. (s~lbmitted). A review on radio studies of aurora1 E-region ionospheric irregularities. Ann. Geoph,ys. 27, 239. 140 MHz aurora1 backscatter: evidence for characteristic phase velocities below and above the ion acoustic speed. Geophys. Res. Lett. 7, 723. Dependence of radar aurora1 scattering cross-section on the ambient electron density and the destabilizing electric field. Ann. Geophys. 8, 195. Dependence of radio aurora at 398 MHz on electron density and electric field. Can. J. Whys. 57, 687. EISCAT as a tristatic amoral radar. J. geophys. Rm 94, 1430. Anomalous heating of the polar E-region by unstable plasma waves. 1. Observations. J. geophys. Res. 86, 1447. First results on the observation of 440 MHz high-latitude coherent echoes from the E-region with the Millstone Hill radar. J. aeouhvs. Res. 94. 6771. Nonlinear theory of &pi