high-resolution incoherent scatter radar measurements during electron precipitation events

High-resolution incoherent scatter radar measurements during electron precipitation events

C. J. BURNS? W. G. HOWARTH and J. K. HARGREAVES

Ionosphere Group, Environmental Science Division, University of Lancaster, Lancaster, LA1 4YQ, U.K.

(Receiwd in,final,furm 2 Jammy 1990)

Abstract-The EISCAT UHF radar was used to record high-resolution measurements of D-region electron density profiles during periods of energetic electron precipitation. Two sudden onsets in precipitation are studied in detail. The events occurred on 23 August 1985 in the post-midnight and evening sectors. The electron spectra deduced from the measurements indicate that each onset is associated with a complex influx of soft and hard energy particles. Examples are given of periods when the EISCAT radar measured significant variations in electron density even when the riometer system recorded no activity.

The EISCAT radar facility is now a well-established tool for studying the high-latitude D-region ionosphere. A major asset of the system is its capability to make high-resolution electron density measurements in both time and space. With a knowledge of the

electron density profile and use of a suitable model atmosphere it is possible to infer the electron spectrum

associated with the profile (DEVLIN et al., 1986). As in-situ particle data are rather difficult to obtain on a

regular basis, this is a useful technique for studying electron precipitation into the D-region, as during periods of aurora1 radio absorption (AA). COLLIS et al. (1984) and COLLIS and KORTH (1985)

investigated tho relationship between GEOS-2 satellite observations of energetic electrons and AA measured by riometers. The energy range 60-70 keV was found to be the most effective in contributing to the observed AA. However, valid measurements were

possible only when magnetic conjugacy between satellite and riometer station was maintained. As a consequence of this, only slowly varying absorption events were suitable for study. The mechanism of production and acceleration of particles has been studied from balloon measurements (KANGAS et

a/., 1984) and sounding rocket measurements (e.g.

KODAMA rr al., 1981), but these only provide “snap- shot’ details. A comprehensive study of AA events requires both long-term measurements (over several hours) and high-resolution measurements (seconds) to suit the differing phenomena that can occur.

In the previous paper (HARGREAVES and DEVLIN,

1990), EISCAT observations of morning sector electron precipitation events were discussed. To suit the

type of event under study (slowly varying absorption), data were post-integrated to one minute or more. In this paper we study events in the post-midnight and evening sectors. In particular, we concentrate on the

temporal development of two sudden onset features and in contrast we also examine two ‘quiet’ periods

which occurred in the morning sector. The data were taken at 10 s resolution, which was the basic inte-

gration time of the particular experiment being used. Several workers have already presented results on

the study of 10 s EJSCAT data. RANTA et al. (1985) concluded that EISCAT measurements at high-resolution were consistent with riometer measurements. DEVLIN et al. (1986) found that spike events, although intense, are not characterised by hard precipitation. COLLIS et al. (1986) saw similar spike event characteristics and found that during pre-onset and onset phases of an AA substorm, the overall flux intensity increased whilst the spectral shape remained constant.

2. EXPERIMENT DETAILS

The EISCAT experiment used for our D-region studies has been described in detail by DEVLIN et cd. (1986). It is basically made up of two parts--one for power profile measurements and one for spectral measurements. Switching between the two parts can be made at pre-determined time intervals. This paper deals with the power profile measurements from an experiment that was run on 22/23 August 1985. The UHF antenna remained stationary, pointing parallel to the local geomagnetic field. throughout the cx-

periment. Measurements were recorded every IO s between 53.25 and 145.65 km range at 1.05 km inter-

205

206 C. J. BURNS et nl.

vals. The experiment was programmed to record

power profiles for a period of 50 min followed by a 10 min period of spectral measurements, the cycle repeating on the hour in UT. On this occasion the

experiment yielded over 23 h of data. Some of the features seen, including a slowly varying absorption event starting at 0240 UT and strong absorption pul- sations starting just after 0300 UT, were discussed by HARGREAVES and DEVLIN (1990). Of the features reported in this paper, the sharp onsets occurred at 00 14 and 2 110 UT whilst the ‘quiet’ periods occurred during the two hours preceding the slowly varying

absorption event at 0240 UT. The term ‘quiet’ is used here to describe the level of activity measured by the

riometer system.

3. STUDY OF TWO ONSETS

3.1. Obseroations

During the EISCAT experiment the 30 MHz cosmic radio noise was monitored at several stations in the

Lancaster riometer chain. Absorption plots for the

stations Andoya, Abisko, Kiruna and Vidsel are given in Figs 1 a-b. The riometers recorded onsets of absorp-

tion at approximately 0010 and 2109 UT. Visible time shifts of absorption between the four stations fall within the timing and digitising accuracy limits. It is thus not possible to infer any poleward or equator- ward motion of a precipitation region in either case. The geomagnetic Kp indices for the onset periods were 4-t and 3+, respectively, indicating moderate disturbance levels. The onsets observed by EISCAT were characterised by sharp peaks in electron density. Simultaneous peaks in absorption were not observed by the riometers, suggesting a spatially confined absorption band as was observed during December 1984 (DEVLIN et al., 1986). Figures 2a and 3a show contour maps of electron density over 5 min periods encompassing the onsets. The range coverage extends from 77.5 to 113.5 km. In both cases EISCAT measured virtually no activity prior to the onset. Figures 2b and 3b show electron densities for four selected

UNIVERSITY OF LANCASTER - DIGITISEO RIOIIETER DATA START : 1985-AUG-22 2300 UT

END : 1985-AUG-23 0200 UT

, . . . . . . . . . ..I....................... ,

ANOOYA 3_ -3

2_ -2

I_ -1

0 .,.,,..,.,,,.,,..,,,.., -0

ABISKO 3_ -3

2 2_ -2

z l_ _I

3 ;: 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

b a 3-

KIWNA -3

s 2_ -2

I_ _I

o_ . . . , . . . . . . -Lo

VIDSEL 3_ -3

2_ -2

I_ _I

o_- , . ./y?. . , . 0

(a) 2300 0000 0100 0200

UNIVERSAL TItlE (UT)

Fig. l(a).

High-resolution incoherent scatter radar measurements

UNIVERSITY OF LANCASTER - DIGITISED RIOMETER DATA START : 198%AUG-23 1930 UT

EN0 : 19G5-AUG-23 2230 UT

S_ . . . . . . . . . ..I...........I........... 5

4_ ANDOYA -4

3_ -3

2_ -2

1_ _I

o-yy-y--:.f-7-y, I ” ” ” .

4_ ABISKO -4

3_ -3

-2

-1

(b)

-3

2_ -2

-1

o-,,,,:,,,,,, , . . . . . . . ., . Q

t- VIOSEL -4

3_ -3

2_ -2

I_ _I

o_,--__o

1930 2030 2130 2230

UNIVERSAL TItlE (UT)

Fig. 1. The onset events on 23 August 1985 as observed by the University of Lancaster riometer chain : (a) 2300-0200 UT and (b) 1930-2230 UT.

207

ranges and these serve to demonstrate the quite large changes that occur over several km of altitude.

To study the onsets in detail, nine 10 s time slices were selected from each onset period. The arrows in Figs 2b and 3b indicate the time slices selected. In a similar treatment to that used by DEVLIN et al. (1986), electron density profiles at each time slice were used as input to a computer program (ZABMOD). To remove sudden discontinuities the profiles were fil- tered in height before modelling was attempted. The computer program determines the energetic electron fluxes that would be required to produce the observed electron density profile. Initial estimates of the fluxes are input to the program and an iterative procedure adjusts the fluxes until a fitted electron density profile

agrees with the observed profile to within a specified tolerance (I % in the present case). The program also determines the incremental 30 MHz absorption profile associated with the electron density profile.

3.2. The event onset at 0014 UT

Figure 2a shows the enhancement in electron density starting at 001400 UT between 84 and 110 km

range. From 001410 to 001440 UT the production rate is almost constant. Between 001440 and 001500

UT a second, larger enhancement (of the order of 3 x lo5 cm-j at 90 km range) is seen. During the

second minute the electron density decays to a level similar to that at 001440 UT.

Figure 2c shows the electron density profiles during the rapid development of the onset. At 001400 UT there is relatively little ionisation at D-region altitudes. The noise level is exceeded only above 90 km range, the peak in electron density reaching 7 x IO4 cm- ’ at 118 km. Thirty seconds later ionisation production

commences as low as 83 km range. This production rate continues to increase steadily until 001450 UT, and in the 10 s to 001500 UT the profile almost doubles in size (profile 5). The most prominent feature of profile 5 is the very sharp gradient in electron density amounting to 3.5 x lo4 cm- 3 km-’ between 80 and 95 km. The electron density peak occurs in the E- region at 111 km range.

Figure 2d shows the decay of the onset between 001500 and 001600 UT. The rate of decay is not as rapid as the rate of growth and the ionisation peak lifts from 111 km range to 120 km during the decay.

RANGE (KM)

1 13.5

89.5

Etectran oeos1 ty

EISCAT D-REGION ELECTRON DENSITY CONTOURS 22,‘23 AUGUST 1985 00:13 - 00:18 UT

I I I I I I

00114 00115 ooh o&7 ( 00118

TIME (UT) CONTOUR HEIGHT x103 CM-’

2.0 7

1.8 -

1.6 -

icm -3

1 x lo5 1.4 -

I.2 -

I.0 -

O-8 -

EISCkT TROtIS tMEG1ON EXPERIRENT SF102 22~23 AUGUST 1985

FROt? 23/08/85 00:13:00 TO 23/0865 00:18:00 Ranges (km) A 80 B 82 c 84 D 85

E

Times (UT) -----_--_

1 OO:14:00

"3 DO:14:30 00:14:40

_: OO:f4:50 00:15:00

6 00:15:10 7 00:15:20 _ 8 00:15:30 9 00:16:00 _

0 i 2 3 4 5

ELAPSED TItlE I(flINS)

Fig. 2(a) and fb).

(cl

EISCAT TRUtlSO O-REGION EXPERltlENT SPIOZ 2203 AUGUST 1985

PROFILES FOR 23fOW85 00:lt:OO 23/08/85 00:14:30

FROM RANGE 5 TO RANGE 70 23/08/85 oo:l4:4o 23/W/85 00 : 14 :50 23/08/85 00 : 15 :00

I I I I ! I I I

125.0

RANGE

(km)

97 .o

so .o

83.0

76.0

69.0

62 .o

55 .o

1

69.0

62.0

55.0

I I I I I I I I

0.0 I .o 2.0 3.0 4.0 5.0 6.0 7.0

Electron density (a~-~) x IO5

EISCAT TROllSO O-REGION EXPERIIIENT SP102 22~23 AUGUST 1985

PRGFILES FOR 23/08035 00:15:00 - 5 23/08&5 00:15:10 - 6

FROll RANGE 5 TO RANGE 70 23/08&5 00:15:20 - 7 23~08185 00:15:30 - 8 23/08/85 00:16:00 - 9

125.0

RANGE

(km)

97.0

90.0

83.0

76.0

69.0

62.0

Cd)

- 104.0

- 97.0

- 90.0

- 83.0

- 76.0

- 69.0

- 62.0

- 55.0

I I I I I I 1 I

0.0 I .o 2.0 3.0 4.0 5.0 6.0 7.0

-3 Electron density (cm ) x IO5

Fig. 2. The onset event between 0013 and 0018 UT as observed by EISCAT: (a) contours of Ne vs time and range showing the start at 0014 UT and the peak at 0015 UT; (b) time-series of Ne for four range gates showing selected time periods l-9 ; (c) Ne profiles for periods l-5 ; (d) Ne profiles for periods 5-9.

(4

lb)

RANGE EISCAT D-REGION ELECTRON DENSITY CONTOURS 22/23 AUGUST 1985 21:09 - 21 :14 UT

(KM) ; I I I I I I

95.5 -

89.5 -

83.5 -

77.5 f I

I I I I I

21:09 21:lO 21:ll 21:12 21:13 21:14

5.0 -J

4.5

Electron 4.0

Dens L ty

hl-3) x lo5 3.5

3.0 i

2.5

2.0

I I .5

1 .o

0.5

0.0 1

TIME (UT) CONTOUR HEIGHT ~10’ CM-’

EISCAT TROllSO D-REGION EXPERItlENT SPlO2 22/23 AUGUST 1985

FROn 23/08/85 21:09:00 TO 23/08/85 21:14:00

Ronges (km) A 81

B 85

C 86

r

2 3 4 5

ELAPSE0 TIME <fiINS)

D 88

Times (UT) ------__-

1 21:lO:OO : 21:10:20 21:?0:10

4 21:10:30 5 21:10:40 6 21:10:50 7 21:t1:00 8 21:ll:ZO 9 21:12:00

Fig. 3(a) and (b).

(c)

EISCAT TROtlSO O-REGION EXPERItlENT SPIOZ 2203 AUGUST 1985

PROFILES FOR 2WOW85 21 :lO:OO 23/O&85 21 :lO:lO

FROll RANGE 5 TO RANGE 70 23/08/85 21 :10:20 23~08185 21 :10:30 23/08/85 21 :10:40

RANGE

(km)

118.0

ill.0

104.0

37 .o

90.0

83.0

76.0

69.0

62.0

55.0

69.0

62.0

55 .o

0.0 I .o 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Electron density <‘CM-~) Y IO5

EISCAT TROtlSO O-REGION EXPERINENT SPlO2 22/23 AUGUST 1985

PROFILES FOR 23/08/85 21 :lO:tO - 5 23/08/85 21 :lO:SO - 6

FROtl RANGE 5 TO RANGE 70 23/08/85 21 : 11 :00 - 7 23/08/85 21 : 11 :20 - 8 23/08/85 21 :12:00 - 9

125.0

RANGE

(km)

118.0

111 .o

104.0

97.0

90.0

83.0

76.0

- 125.0

- 118.0

- 111.0

- 104.0

- 97.0

- 90.0

- 83.0

- 76.0

69.0

62.0

t 69.0

62.0

(d) 55.0 55.0

I I I I I I I I I I I

0.0 I .O 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

-3 Electron density (cm ) x lo5

Fig. 3. The onset event between 2109 and 2114 UT as observed by EISCAT: (a) contours of Ne vs time and range showing the start at 2110 UT and the peak at 211030 and 211040 UT; (b) time-series of Ne for four range gates showing selected time periods l-9; (c) Ne profiles for periods l-5; (d) Ne profiles for

periods 5-9.

212 C. J. BAIRNS ri al.

Between I I5 and 100 km range the decay occurs at a

gradual rate but between 100 and 95 km the profiles

merge for about 30 s. Below 95 km the decay is actu-

ally reversed for a short time. This is why we see the onset spike splitting into two (visible in Fig. 2b). At 001600 UT the electron density profile has returned to a level similar to that at 001440 UT--a trough in a period of continuing activity.

Turning to the inferred energetic electron spectra in Figs 4a--b, the growth phase of the onset can be characterised by an overall increase in flux over the energy range shown. The spectral shape, however, does not appear constant during the growth of the onset. At 001400 UT only the soft part ofthe spectrum is evident. At 001430 and 001440 UT the spectrum hardens with an increase in the flux of particles with energies between 70 and 1 IO keV. This corresponds with the ionisation occurring down to 83 km seen in

Fig. 2c. The prominent profile of electron density (number 5. Fig. 2~) would indicate an increase in flux

over the entire energy range up to 001500 UT. The decay of the onset is characterised by an overall reduction in flux at all energies. Between 001510 and

7- -\ TIME (UT)

6-

Table 1. Characteristic electron energies for onset I

Profile UT & (keV)

001400 001430 001440 001450 001500 001510 001520 001530 001600

001530 UT an injection of harder particles could explain the reversal of the decay seen at ranges less than 95 km (profiles 6, 7 and 8, Fig. 2d). It is difficult to attribute a single characteristic energy (E,) to these spectra since they clearly do not follow an exponential form over the entire energy range. Instead, a value for E,, has been taken from each spectrum, in the energy

band below 25 keV where the exponential relationship is applicable. Table 1 shows E,, to vary between 2 and 6 keV. By comparison, DEVLIN e’t nl. (1986) deduced E. to be 7.5 keV at the peak of a spike event.

INFERRED ENERGETIC ELECTRON SPECTRUM Z/23 August 1985 00:14:00 - 00:15:00 UT

3 0 10 20 30 40 50 60 70 80 90 100 110 120

Em-g,, (keV>

Fig. 4(a).

High-resolution incoherent scatter radar measurements 213

(b)

6-

INFERRED

ENERGETIC ELECTRON SPECTRUM Z/23 August 19.35 00:15:00 - 00:16:00 UT

TIME (UT)

5 00:15:00 6 O&15:10

7 00:15:20 8 00: 15:30

9 00: 16:OO

3O lo 20 30 40 50 60 70 80 90 100 110 120

Energy (keV )

INFERRED ENERGETIC ELECTRON SPECTRUII 22/23 August 1985 21:lo:OO - 21:10:40 UT

TME (UT)

1 21:lo:OO 2 21:lo:lO

3 21:10:20

4 21:10:30

5 21:10:40

0 10 20 30 40 50 60 70 80 90 100 110 120

Energy ( keV)

Fig. 4(b) and (c).

214 C. J. BURNS et 01.

INFERRED ENERGETIC ELECTRON SPECTRUII Z/23 August 1965 21:10:40 - 21:12:00 UT

7- TIME (UT) . 5

5 21:10:40 . 0 6 21:lO:SO

- 6 7 21:ll:OO 6 21:11:20

7 9 21:12:00

_ 9

6-

4-p ’ ’ - 0 10 20 30 40 50 60 70 80 90 100 110 120

Energy (keV )

Fig. 4. Inferred energetic electron spectra for the two onsets: (a) onset 1, periods l-5 ; (b) onset 1, periods 5-9 ; (c) onset 2, periods t-5 ; (d) onset 2, periods 5-9.

The inferred incremental absorption determined by

the computer model indicated the height of maximum absorption to be constant at 85 km during both growth and decay phases of the onset. The width of the absorption region at the 50% levels remained stable between 13 and 14 km.

3.3. The event onset at 2110 UT

This onset starts at 210940 UT (see Fig. 3a). Ion- isation reaches a peak in the E-region at 211030 UT and in the D-region at 211040 UT. The enhancement in electron density is of the same order of magnitude

as in the first onset but the decay gradient is less severe, especially in the D-region where the onset spike is not as visibly obvious (Fig. 3b). The profiles of electron density show the steady increase in ionisation in the E-region up to 211030 UT followed by the decay at 211040 UT (Fig. 3~). Below 97 km range the profiles merge and we see the maximum electron density occurring at 211040 UT. Profile 5 has a gradient of 3 x lo4 cm-’ km- ’ which is very similar to the equi- valent profile in the first onset. After the peak of the onset the electron density decays slightly, but further

ionisation causes the profiles to merge at 97 km range (Fig. 3d).

The inferred energetic electron spectra in Fig. 4c

help to explain the differences observed in the above profiles. The flux intensities are generally higher than those seen for the first onset. The characteristic electron energies measured over O-25 keV vary from 5 to 12 keV (Table 2). The peak electron density in the E-

region at 211030 UT indicates an increase in the flux

Table 2. Characteristic electron energies for onset 2

Profile UT E. CkeV

I 211000 5 211010 8 211020 IO 211030 Ii 211040 12 211050 8 211100 5 211120 7 211200 6

High-resolution incoherent scatter radar measurements 215

of particles in the energy range from 10 to 40 keV. A

hardening of the spectrum (particle energies > 40 keV) occurring at 211040 UT would explain the increased

ionisation in the D-region. After the peak of the onset the flux of particles at energies greater than 80 keV appears fairly constant because of the very shallow decay gradient observed (Fig. 4d).

The inferred incremental absorption profiles showed the peak of the absorbing layer to be at 90 km from 211000 to 211030 UT, decreasing down to 86 km at 211040 UT. The width of the absorption

region at the 50% levels did not remain constant during the growth of the onset. The ionisation in the

E-region between 211020 and 211030 UT increased this width from 14 to 20 km. After 211040 UT, the absorbing layer remained at a constant width of 12 km and the peak remained at 85 km.

4. PRECIPITATION DURING ‘QUIET’ PERIODS

During some periods, significant variations in D- and E-region electron density are measured by the

EISCAT UHF radar when little activity is apparent

on the riometer charts. As an example we have chosen

two periods in the morning sector from 0100 to 0125 UT and 0200 to 0225 UT. Figures 5a-b show EISCAT 10 s resolution measurements for selected ranges between 77 and 108 km. In both cases the variations observed are clearly correlated over several ranges, which would not be expected if we were looking at noise. The true noise level has a standard deviation of about 5 x 10’ cm ~’ (at 78 km range). Marked on both

figures are trough and peak periods which were post- integrated to produce the profiles in Fig. 5c. Both sets of profiles shows a factor of two or three enhancement below 110 km range. If we deduce the spectra for the two periods, a significant energisation is seen to occur between trough and peak (Fig. 5d). The peak of electron density moves lower during the enhancement and the spectrum hardens. The absorption calculated from the electron density profiles is about 1.5 dB during the enhancement compared with observed absorption levels of about 0.1 dB. This difference would indicate that the absorption regions were spatially confined.

EISCAT TROtlSO O-REGION EXPERWlENT SPIOZ 22~23 AUGUST i985

FROtl 23/0845 0l:OO:lO TO 23/0845 01:25:00 ix;

-3 cm

A B

RANGE< km)

108.3

103.2

98.1

93 .o

87.9

82.8

77.7

0 5 IO 15 20 25

ELAPSED TIflE <tlINS)

Fig. 5(a).

103.2

93.0

67.9

62.6

(b) 77.7

0 5 IO I5 20 25

ELAPSE0 TIME (illNS)

216 C. J. BUKNS et al.

EISCAT TRWlSO O-REGION EXPERIflENT SPlO2 22123 AUGUST 1965 Electron

FROll 23/06/65 02:OO:lO TO 23/08/85 02:25:00 OenYy cn

c D

RANGE< km)

EISCAT TROtlSCl O-REGION EXPERItlENT SPlO2 22123 AUGUST 1965

PROFILES FOR 23/06/85 01 :11:30- 01:12:20 UT A 23/08/85 01:14:30- o,:,s:,r~ UT 6

FROM RANGE 5 TO RANGE 70 23JOW65 02:20:00 - 02:20:50 UT C 23/09/85 02:21:20 - 02:22:20 UT 0

(cl

125.0

RANGE 111.0

(km) \

104.0

97.0

90.0

63.0

76.0

69.0

62.0

55 .o

125.0

t 69.0

t

62.0

55.0

O'.O I .o I

2’.0 3’.0 4.0

Electron density (c.-~) x IO5

Fig. S(b) and (c).

5’.0

High-resolution incoherent scatter radar measurements

INFERRED

ENERGETIC ELECTRON SPECTRUV 22/23 August 1985

A 01:11:30 - 01:12:20 UT

B 01:14:30 - 01:lS:lO UT

C 02:20:00 - 02:20:50 UT

0 02:21:20 - 02:22:2Q UT

217

Energy (ke’f )

Fig. 5. EISCAT measurements during ‘quiet’ periods: (a) time-series of Ne for selected ranges OlOOll& 012500 UT, showing trough (A) and peak (B) periods used for post-integration ; (b) time-series of Ne for selected ranges 02001&022500 UT, showing trough (C) and peak (D) periods used for post-integration ; (c) profiles of Ne for periods A, B, C and D; (d) inferred energetic electron spectra for periods A, B, C

and D.

Indeed, calculations based on the above values show

the horizontal extent of these regions to be less than 20 km at an altitude of 90 km (HARGREAVES et al.,

1979). Since the beam width of the wide-beam rio-

meters is about 120 km at 90 km altitude this would explain why the enhancements were not seen by the

riometers.

5.SUMMARYAND CONCLUSIONS

We have demonstrated two examples of sharp onsets in electron density measured by the EISCAT UHF radar at 10 s time resolution. In addition we have also examined periods during which the riometers recorded no activity. In summary :

1. EISCAT observed a local maximum at the onsets, not seen by the wide-beam riometers.

2. 10 s resolution measurements have revealed changes in electron density of an order of magnitude in less than 30 s.

The deduced energetic electron spectra associated with each onset exhibit a complex influx of soft

and hard energy particles. In the post-midnight onset, a simultaneous influx of soft and hard par-

ticles is seen, followed after 20 s by a further influx of hard particles. In the evening onset, an influx of soft particles is followed after 10 s by an influx of

hard particles. During the onsets the absorbing region generally remains at a constant height, being 12-14 km thick

at the 50 % level and peaking at 85 km. The EISCAT UHF radar is capable of measuring significant enhancements of electron density in the D-region during ‘quiet’ periods observed by the riometer. Calculations show the regions of enhancement to be less than 20 km in horizontal extent at an altitude of 90 km.

Gradient-curvature drift is a recognised mechanism for explaining the transport of electrons from an injection source at magnetic midnight. However, it is difficult to reconcile the characteristics of the post-

218 C. J. BURNS et al.

midnight onset with this type of motion. Drift velocity

is directly proportional to particle energy, so we would expect to see the high energy particles precipitating

earlier than the soft energy particles. We clearly do not see this in the inferred spectra. It is therefore concluded that the simultaneous peak in the flux seen at all energies is due to direct precipitation of particles from the plasma sheet.

High-resolution D-region measurements made by

EISCAT during aurora1 precipitation events are revealing interesting details. The computational technique, whilst having some uncertainties, allows infer- ences to be drawn about the electron spectra and their

variations during aurora1 absorption events. Future

experiments utilising both UHF and VHF radars or

the split-beam capability of the VHF radar should enable us to study the morphology of aurora1 precipitation regions in greater detail.

Acknowledgements-We appreciate the assistance of the EISCAT staff at the Rutherford Appleton Laboratory. The EISCAT Scientific Association is supported by the Science and Engineering Research Council (U.K.), Centre National de la Recherche Scientifique (France), Max-Planck Gesellschaft (F.R.G.), Suomen Akatemia (Finland), Norges Almenvitenskapelige Forskningsrad (Norway) and Natur- vetenskapliga Forskiningsradet (Sweden).

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high-resolution incoherent scatter radar measurements during electron precipitation events

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