- Email: [email protected]
Application of alternating codes for EISCAT observations during the ERRRIS campaign for E-region plasma irregularities
./ournul
of Atmmphm
und Trrr<~.~trml Phwcs,
OO21-9169,'90$3.00+ .OO PcrgamonPress plc
Vol.52,No.6-8,pp.431438,1990.
Printed I"GreatBritam
Application of alternating codes for EISCAT observations during the ERRRIS campaign for E-region plasma irregularities I.
H~Gc;sTR~~M,* H. OPGENDDRTH,~ P. J. S. WILLIAMS,~ G. 0.
L. JONESI and K. SCHLEGELQ:
* Institutet f6r Rymdfysik, Box 812, S-981 28 Kiruna, Sweden ; t Institutet far Rymdfysik, Uppsala,
Sweden
; 1 University Institut
College of Wales, Aberystwyth SY23 3B2, U.K. fur Aeronomie. D-341 1 Katlenburg-Lindau, F.R.G.
(Recked
S-755 90
; 3Max-Planck
infinal,form I I April i 990)
Abstract-The first extensive use of alternating codes in incoherent scatter experiments took place during the ERRRIS campaign in March and April 1988. Alternating codes provide a very powerful technique which in theory offers time resolution a factor of 5 shorter than is possible with the commonly used multipulse codes for low signal-to-noise ratios. For quiet conditions during the ERRRIS campaign this was confirmed by the remarkable consistency of measured parameters which were adjacent in time or height, indicating a low level of random error even for integration times as short as 45 s. This gave confidence in the value of the parameters measured during disturbed periods when the electron temperature at 110km reached values of several thousand kelvin for short periods. with simultaneous increases in ion temperature at greater heights.
INTRODUCTION ERRRIS (E-Region Rocket/Radar Instability Study) is an experiment which combines the use of sounding rockets, coherent and incoherent radars and various other instruments to study the generation of E-region irregularities by electric fields in the aurora1 zone. Wave turbulence driven by large electric fields leads to field-aligned irregularities in the electron concentration and a sharp increase in electron temperature over the height range IO&l20 km. Rockets can make in-situ measurements of the electric field, the turbulence, the irregularities themselves and the associated electron heating. The coherent radars, such as CUPRI (Cornell University Portable Radar Interferometer). can detect the field-aligned irregularities while EISCAT makes continuous measurements of the electric field and height profiles of electron concentration and electron and ion temperature. The first ERRRIS campaign was carried out in the spring of 1988 and this paper describes the EISCAT contribution to the experiment and in particular the use of alternating codes (LEHTINEN and H&GSTR~~M, 1987) to combine good height, time and spectral resolution. Three other papers (WILLIAMS et cd., 1990a ; OPGEN~DRTH et ul., 1990; JONES et al., 1990) describe the scientific results achieved using EISCAT to date.
EISCAT CONTRIBUTION TO ERRRIS In 1986 joint measurements using CUPRI and EISCAT demonstrated the existence of short-lived
events where large electric fields were associated with characteristic echoes from the E-region, received at CUPRI, and enhanced electron temperatures over a height range of about IO&120 km, measured by EISCAT (PROVIDAKESet u1., 1988). The challenge to EISCAT was to measure the electric field with a time resolution better than 1 min and E-region electron temperatures with a similar time resolution and a height resolution of 5 km or better. Moreover, the events usually occurred when the electron concentration and, hence, signal-to-noise ratio were low, thus making the challenge even more daunting. Previous measurements of this kind had been made by splitting the signal from EISCAT into a long pulse to measure the electric field, and a multipulse group to measure the E-region profile of electron temperature with adequate height and spectral resolution. The most advanced pulse codes using this approach were developed by TURUNEN (1986) in the suite of GEN programmes, where a series of multipulse groups and longer pulses, all transmitted at different frequencies, were interwoven to make optimum use of the radar duty cycle. These represented a major advance on previous codes used by incoherent scatter radars, but were still not adequate to study the electron heating events with time resolution better than 1 min. To achieve such time resolution the 1986 CUPRII EISCAT observations combined eight different Spulse codes, but unfortunately this did not allow simultaneous measurements of the electric field (PROVIDAKESet al., 1988). The answer was to use the recently developed alter-
431
432
et ai
I. HXGGSTR~M
nating codes (LEHTINEN and HXGCSTR~M, 1987). In this technique a relatively long pulse is divided into a number of subpulses, with the phase of each subpulse determined by a special code. A sequence of coded pulses is transmitted, with each pulse coded in a different way. The echoes received by the radar can thon be decoded so that at every height each lag in the autocorrelation function can be uniquely determined without contamination. The advantage of alternating codes is ihat the whole duty cycle can be dedicated to one frequency, and in the received signal every possible cross-product for lags up to the pulse length can contribute valid information, while any clutter from other lags or other heights cancels exactly. This can be illustrated by comparing alternating codes with the optimum multipulse codes contained within the same overall pulse length and achieving the same height resolution. This has been done both for the comparison of an s-bit alternating code with a 3pulse group and a l&bit alternating code with a 4puise group. The results are s~~mmarized in Figs 1 and 2. In each case the overall length of the codes are about the same and the individual subpulses are the same as the bit-length of the alternating code. If we assume identical receiver settings we can compare the number of independent estimates of each lag in the alitocorrelation function made within a single pulse repetition period. In Fig. la the layout of two S-pulse groups is compared with one of the g-bit alternating codes, and in Fig. 1b the number of independent estimates per interpulse period is shown as a function of lag in both cases. Figure 2 shows a similar comparison
a)
mfiTm
WIIll
+--+**--+ b) Number of estimates
lternatinq
codes
Fig. I. Comparison of a 3-pulse multipulse with &bit alternating codes. The sequences are illustrated in (a), where each pattern in the multipulse corresponds to one frequency and the signs in the alternating code example represent the phase of each bit. The number of independent estimates per interpuke period for different lags is shown in (b).
a)
W l
EZI -++--*-+-----+++
b1 Number of estimates
lternating
codes
lag number Fig. 2. As Fig.
I, but for 4-pulse and I&bit codes.
between one of the I&bit alternating codes and the most compressed form of a 4-frequency 4-pulse group. In both cases it can be seen that the alternating codes have many more estimates than the corresponding multipulse codes, especially at short lags. If WC assume low signal-to-noise ratios (S 1) and define the speed of the codes to be the total number of estimates per unit time, then the alternating codes are about five times faster. In other words, when electron concentrations are low, integration times can be about five times shorter for alternating codes to still achieve similar accuracy. Due to the limitations of the existing EISCAT correlator, the longest alternating codes possible for high time resolution experiments were the g-bit codes. These are listed in Table 1 in the order in which they were transmitted. On examining the codes it can be seen that they are ordered in pairs, where each pair shows the same decoding scheme for even lags but with opposite sign for the odd lags. This means that the even lag-profiles could be added br each pair, whereas the odd lag-profiles were kept separate. Repeating this scheme on four frequencies it is possible to include one single pulse with its length equal to the bit-length of the codes to provide an estimate of the zero-lag, The zero-lag profile for the alternating codes can be summed for all codes with the result that it appears as the zero-lag from a ‘multipulse’ where eight subpulses are transmitted without an intervening gap. The zero-lag profile can then be used to improve the power profile as described by LEHTINENand HuusKONEN(1986).
433
EISCAT alternating codes during the ERRRIS campaign Table I. The sequence of the g-bit alternating codes, in the order they were transmitted in the ERRRIS campaign 1988. With a bit-length of 3011s the total length of the pulse is 240 ps Number
Code _...-
1 2
++++f+++ -t”-+-+-+-
3 4
+-t-++---t-----f+-+
5 6
+++-i”+-+-i++--+
7 8
++--+-++ +--++++-
9
-
++++-++-
10
+_-_+---++
II I?
++-+---+ +_---+--
13 14
+++--+-•+ +_++___-
15 I6
++___-f+---+-•+ff
With 1 I profiles for each code pair, the single-pulse power profile and a zero-lag profile from the codes, we have 90 profiles in all, and with a result ~nemory of 4095 complex values it was possible to obtain 41 range gates. Using a 30~s bit-length, and pointing the Tromss antenna at an elevation of 77.6’ where it lies parallel to the magnetic field line, the gates cover the altitude range from 93 to 270 km with a gate separation of 4.4 km and height resolution of about 4 km. The coded pulse can also be used to provide a scattered signal at the remote stations. The bit-length corresponds to a pulse which comfortably tills the intersection volume of the beams of the transmitting and receiving antennae. The scattered signal is sampled twice per subpulse, and by decoding the scattered signal in a similar way as for the monostatic measurements, and weighting the estimates to allow for the effect of the subpulse boundaries passing through the scattering volume, the autocorrelation function of the F-region ionospheric plasma is obtained. Alternating codes do not offer such great advantages for multistatic observations as they do for monostatic ones, but the use of a single frequency transmitting a single coded pulse allows the full duty cycle to be received at Kiruna and Sodankyla. As a result, a basic integration time of 5 s is available at the remote stations as well as Tromss.
OBSERVATIONS
AND RESULTS
DURING
ERRRlS 1988
The full ERKRIS experiment was run for a total of 110 h on 19 different nights between I5 March and 10 April 1988. In the standby mode, the Tromslir antenna was set pointing along the field line, and the remote stations were set pointing at a common scattering volume at a height of 260 or 170 km depending on where the scattered signal was strongest for the remote antennae. Two other modes were also allowed, one pointed at the volume where the ERRRIS rocket would pass through the E-region, while the other pointed at a scattering volume on the same field line but in the F-region rather than the E-region. However, most of the time was spent in the field-aligned mode where the electric field measured in the F-region ‘maps’ down the field line into the volume of E-region observed in the experiment. The results presented in the present series of papers were all obtained in this mode. As an introduction to these results, measured parameters are given to represent quiet conditions and magnetically disturbed conditions, on the grounds that the high consistency of the results obtained during quiet conditions gives strong support to the validity of the results when an amoral disturbance occurs. Figure 3 shows the results of analyzing autocorrelation functions in the form of altitude profiles of electron density (iv,.) and electron and ion temperature (Tc,and 7;, respectively) at 12 different heights between 93 and 141 km at 2011 UT on 23 March after integrating the data for 45s. This day was quiet on the whole with an .4~ index of only 5, and the measured electric field at 2011 UT was 36mVm- ‘. At greater heights, as the autocorrelation functions become
UT 2011:37 23 March 1988 ‘“I
0
1000 2000 3000 4000 50000.0
02
Temperature (K)
Density (IO ’ ‘mm3)
OA
0.6
0.8
1.0
12
Fig. 3. Altitude profiles of E-region electron density, electron and ion temperatures on 23 March at 2011 UT. The scales are 0-1.2. 10” rn..-’ for the density and t&6000 K for the temperatures. The integration time is 45 s.
434
I. H~~GGSTR~M rl al.
narrower, the electron and ion temperatures increase steadily. The smooth variation of temperature with height in both cases indicates a small random error in the measurements, even after such a short integration time. Figure 4 shows a similar set of altitude profiles at 2021 UT on the same day during a brief period of aurora1 disturbance when the measured electric field had increased to 126 mV mm ‘. We see that the electron temperature between about 100 and 125 km is greatly enhanced, reaching a maximum of about 4000K at 110 km, whereas the ion temperature is enhanced at all heights above 115 km, also reaching values of 4000 K and over. In both cases the variation of temperature with height is relatively smooth, indicating that the random errors are still small. A closer look at the data in Fig. 4 can be carried out by examination of the autocorrelation functions in Fig. 5, showing that they are unusually narrow for this height range. There are dramatic changes of shapes with altitude, being almost harmonic at 110 km but going to Gaussian from above 125 km. A similar conclusion of small random errors can be reached by plotting measured parameters as a function of time. Figure 6 presents the variation of electric field strength between 1800 and 2400 UT on 23 March. Until 2006 UT, and after 2132 UT, the field strength was always less than 50 mV m _ ‘. However, between 2006 and 2132UT the field strength is characterized by a series of short-lived enhancements reaching peak values at 2009,2022,2044 and 2127 UT. Figure 7 shows electron temperature for the same interval, plotted for a number of different heights. One of the most remarkable features of these results is the consistency of the measured electron temperature with time during the quiet periods, where it can be assumed that the small fluctuations represent an upper value to the random noise. It is the consistency of the measured values of T, with height and time during
UT 2022:07 23 March 1988 150
907 0
1000 2000 3000 4000500001)
Temperature
(K)
02
Op.
0.6
0.8
1.0
Density (10 ’ ‘me3)
Fig. 4. As Fig. 3, but for 2022 UT.
12
.
23 March 1988
202145-202230 UT
I
If
-
W’
. lmaglnary
Real
Imaginary
Fig. 5. Autocorrelation functions from 23 March at 2022 UT. The real part is on the left and imaginary part on the right (same scales) in each panel. The measured ACFs are shown as dots and the lines are the best fits of theoretical ACFs giving the parameters in Fig. 4. The lag separation is 30~s and the radar frequency is 933 MHz. The altitudes in kilometres are given at the bottom of each panel.
quiet periods that allows us to be confident that the dramatic increases in T, between 2006 and 2 132 UT are genuine and not seriously affected by random errors. However, the first range gate at 93 km shows for the first hour large random errors. This is a combined effect of very low signal-to-noise ratios in that time and height interval, and that the measured ACF is so short that a zero-crossing is normally missing, the latter due to the system limitations not allowing longer codes. The ion temperature measurements lead to a similar conclusion, as displayed in Fig. 8. In this case the increase in temperature is at greater heights, where the ion population moves at a high velocity with respect to the neutral atmosphere and frictional heating occurs (e.g. ST-MAURICE and HANSON, 1982). At 110 km the ion-neutral collision frequency is so high that the ion population is constrained to remain at the same temperature as the neutral gas, and with the exception of a single measurement (corresponding to the peak
EISCAT
alternating
codes during
UT Fig. 6. Perpendicular
electric field between
the ERRRIS
Mar 23
1988
18 and 24 UT on 23 March
electric field of 138 mV m- ‘) the results obtained confirm this. This is welcome confirmation that the analysis of the incoherent scatter spectrum remains valid although the electron temperature has increased so dramatically. Once again it is the consistency of the measured parameters at most times and for most heights that gives us confidence in the validity of the large and rapid variations in the temperature measurements, when these are observed.
GI,._:A,J!111_.~.~~~_.~
435
campaign
SCIENTIFIC
1988 with integration
RESULTS
times of 45 s.
FROM THE EISCAT ERRRIS
EXPERIMENT
The quality of the data taken by EISCAT during the ERRRIS experiment was so high that we anticipate a series of valuable scientific results, some of which appear in this volume. Here we will present a brief account of some of the most important results.
93
&.A,*..~~.~*.
~1800
1900
MOO
2100
UT Fig. 7. Electron
temperature
between
Mar 23
I
I
2200
I
I
I
2300
I
t
0000
1988
18 and 24 UT on 23 March 1988 at 12 heights from 93 up to 141 km with integration times of 45 s.
I. HKGGSTRBM et al.
436
119
@ -CT
115 1 110 2 106
1600
WI0
2000
2100 UT
Mar 23
2200
2300
ODOO
1988
Fig. 8. Ion temperature between 18 and 24 UT on 23 March 1988 at 12 heights from 93 up to 141 km with integration times of 45 s.
Electron heating in the E-region The prime aim of the experiment was to study the effect of large electric fields on the E-region plasma. Large increases in electron temperature are observed in association with wave turbulence (SCHLEGEL and ST-MAURICE, 1981; FWEK er al., 1986 ; IGARASHIand SCHLEGEL, 1987 ; PRCNIDAKESef al., 1988 : WILLIAMS et ul., 1988). Various theories have been proposed to explain the electron heating (e.g. ST-MAUKICE and LAHER, 1985; PKIMUAHL and BAHNSEN, 1986; ROBINSON, 1986 ; ST-MAURICE, 1987). The theory by Robinson is self-consistent and predicts a unique relationship between the effective electric field applied to the E-region and the ~llhancement in electron temperature. In comparing this theory with others it is crucial to discover whether large electric fields can ever occur without causing E-region electron heating (STMAURICE, private communication). This test requires measurements to be made with high accuracy and a short time resolution to prevent the true relationship between electric field and 7;. being blurred by random errors or by variations in the parameters during the integration period. The results from the 1988 ERRRIS experiment show that on every occasion when the effective electric field was greater than 50 mV m ‘, there was an increase in T, at 110 km of at least 200 K (WILLIAMS et cd., 1990a). A full set of measurements are discussed at length in a subsequent paper (JONESet al., I990), and it is
clear that the data are consistent with Robinson’s theory producing approximately the expected Eregion electron temperature for the measured electric fields, albeit with a small uncertainty due to the missing neutral atmospheric parameters.
At the same time as the applied electric field causes an increase in r,. in the height range IOO--120 km, r, increases at all heights above 125 km. This increase in ion temperature is caused by frictional heating as the electric field drives the ions at high speed through the neutral atmosphere. In principle. careful measurement of the electric field allows us to compare the measured profile of r, with the profile predicted for assumed models of the neutral atmosphere and on ion composition, and this should indicate whether such models are valid. A preliminary comparison between the predicted and observed profile of ion temperature shows good agreement below 160 and above 220 km, but in the intervening height-range the observed values of T, are consistently lower than predicted (see WILLIAMSet al., 1990a). A careful analysis of the data is planned using different assumed profiles of neutral mass, ion composition and ion--neutral collision frequencies and allowing for any anisotropy in r, to discover whether these factors explain the discrepancy. As so many factors are involved this will not be a simple comparison, but the improved time resolution possible with
EISCAT alternating codes during the ERRRIS campaign the alternating codes should at least eliminate the large errors which arise on time-averaging the electric field strength and the ion temperature-two rapidly varying parameters which bear a non-linear relationship with each other. &ctric,field
cariations
The relationships between the measurements of electric field strength, T,, and Tj, can also be used to validate the measurements of electric field. The improved time resolution available with the alternating codes has allowed the electric field to be measured after averaging over an intertial of only 45s. When this is done, it appears that the field is varying on a time-scale of a minute or so-a far shorter timescale than has been generally assumed in the past. The consistency with which the measured variations in electric field strength, r, and T,, remain in step is the strongest vindication of the reliability of the data, especially when we bear in mind that these parameters are derived from echoes scattered at different heights in the ionosphere. This is discussed at greater length by WILLIAMS et al. (1990a), where the ERRRIS data are used to confirm the indication from previous EISCAT data that the aurora1 electric field does indeed show large variations over a short time-scale. The one question that has to be resolved in each case is whether the variation occurs silnultaneously over an extensive part of the convection pattern or is associated with a localized structure moving rapidly through the TromsB beam. There exist. in fact, strong indications that most of the very short-lived spikes of electric field intensities, particularly during stationary field-aligned EISCAT experiments, arc associated with aurora1 structures moving through the EISCAT beam. OPGENOORTHet crl. (1990) present evidence that strong enhancements of the horizontal electric field can occur in a narrow region just outside aurora1 arcs. Also, CUPRI data from the co-ordinated ERRRIS campaign show that apparently short-lived spikes in the EISCAT electric field data coincide with the drift of latitudinally confined regions of strong backscatter through the EISCAT beam. Since the electric field enhancements are observed to be very pronounced only at the leading
edge of moving aurora1 structures, this new observation must have implications on our understanding of the electrodynamics within aurora1 structures and the mechanisms for their formation. More tong-lived enhancements of the electric field associated with a general spatially extended increase of the magnetospheric convection have also been observed, but they reach far less frequently values high enough to cause E-region plasma waves and associated electron temperature elevations (WILLIAMS et al., 1990b).
This paper describes the first operations of an alternating code experiment for high time resolution measurements with the analysis following the procedure given in I%GGSTR~M et al. (1989). It has been shown that the improvements of incoherent scatter radar measurements which the alternating code technique provides are substantial, making measurements possible with higher time and/or space resolution than used before. Confirmation of this is most clearly seen in the consistency of the measured parameters during quiet periods both in time and height, even for low signal levels and rather short integration times. During disturbed conditions, large electric fields cause turbulent heating of the electrons in the height range 100&120 km and frictional heating of the ions above 125 km. The close similarity between the measured variations in these three parameters is a further vindication of the technique, The ERRRIS experiment was performed with the existing hardware of EISCAT, and was necessarily a compromise of scientific demands and system limitations. Ongoing projects at EISCAT will make it possible to take full advantage of the alternating code technique in future, making almost any experiment design possible. Acknoil’ieirqenzmts--The EISCAT Scientific Association is supported by the Centre National de la Recherche Scientifique of France, Suomen Akatemia of Finland, Max-Planck C;esellschaft of West Germany, Norges Almenvitenskaplige Forskningsrad of Norway, Naturvetenskapliga Forskningsridet of Sweden and the Science and Engineering Council of the United Kingdom.
UEFERENCES FEJEKB. G., PKOVIUAKES J. F.. FARLEYD. T. and SWARTZW. E. H~GGSTR~MI.. LEHTINEN M., TCRUNENT. and VALL~NKOSKI M. I~ARASH~ K. and SCHL~GELK. JONES9.. ROHIUSON T. R., SCXLEGEL K. and HXGGSTKKM I.
437
1989
Radio Sci. (submitted).
I987 1990
J. utmos. fur. Phys. 49, 213
Ann. Ge@lys. (submitted).
438 LEHTINENM. and H;~GGSTR~M I. LEHTINENM. and HUUSKONEN A. OPGENOORTH H. J., HXGGSTRBMI., WILLIAMSP. J. S., JONESB. and PROVIDAKES J. PKIMDAHL F. and BAHNSENA. PROVIDAKES J., FARLEYD. T., FEJER B. G., SAHRJ., SWARTZW E., H~GGSTR~MI., HEDBERGA. and NORDLING J. A ROBINSON T. R. SCHLEGELK. and ST-MAURICE J.-P. ST-MAURICE J.-P. ST-MAURICE J.-P. and HANSON W. B. ST-MAURICE J.-P. and LAHER R. TURUNEN T. WILLIAMS P. J. S., JONESB., USPENSKYM. V. and STARKOV G. WILLIAMS P. J. S., JONESG. 0. L., JONESB., OPG~NOORTH H. and HAGGSTR~~MI. WILLIAMS P. J. S., VIRDI T. S., COWLEY S. W. H. and LESTER M.
I. HKGGSTRBMct ul. 1987 1986 1990
Radio Sci. 22, 625. J. utmos. terr. Phys. 48,787. J. atmos. terr. Phys. (submitted).
1986 198X
J. geophys. Res. 91, 1734. J. almos. terr. Phys. 50, 339.
1986 1981 1987 1982 1985 1986 1988
J. J. J. J. J. J. J.
l990a
J. atmos. terr. Phys. 52, 439.
1990b
J. atmos. terr. Phys. 52, 42 1
atmos. terr. Phys. 48,417. geophys. Rex 86, 1447. geophys. Res. 92,4533. geophys. Res. 87, 7580. yeophys. Res. 90,2843. atmos. terr. Phys. 48, 777. atmos. terr. Phys. 50, 3 15.
of Atmmphm
und Trrr<~.~trml Phwcs,
OO21-9169,'90$3.00+ .OO PcrgamonPress plc
Vol.52,No.6-8,pp.431438,1990.
Printed I"GreatBritam
Application of alternating codes for EISCAT observations during the ERRRIS campaign for E-region plasma irregularities I.
H~Gc;sTR~~M,* H. OPGENDDRTH,~ P. J. S. WILLIAMS,~ G. 0.
L. JONESI and K. SCHLEGELQ:
* Institutet f6r Rymdfysik, Box 812, S-981 28 Kiruna, Sweden ; t Institutet far Rymdfysik, Uppsala,
Sweden
; 1 University Institut
College of Wales, Aberystwyth SY23 3B2, U.K. fur Aeronomie. D-341 1 Katlenburg-Lindau, F.R.G.
(Recked
S-755 90
; 3Max-Planck
infinal,form I I April i 990)
Abstract-The first extensive use of alternating codes in incoherent scatter experiments took place during the ERRRIS campaign in March and April 1988. Alternating codes provide a very powerful technique which in theory offers time resolution a factor of 5 shorter than is possible with the commonly used multipulse codes for low signal-to-noise ratios. For quiet conditions during the ERRRIS campaign this was confirmed by the remarkable consistency of measured parameters which were adjacent in time or height, indicating a low level of random error even for integration times as short as 45 s. This gave confidence in the value of the parameters measured during disturbed periods when the electron temperature at 110km reached values of several thousand kelvin for short periods. with simultaneous increases in ion temperature at greater heights.
INTRODUCTION ERRRIS (E-Region Rocket/Radar Instability Study) is an experiment which combines the use of sounding rockets, coherent and incoherent radars and various other instruments to study the generation of E-region irregularities by electric fields in the aurora1 zone. Wave turbulence driven by large electric fields leads to field-aligned irregularities in the electron concentration and a sharp increase in electron temperature over the height range IO&l20 km. Rockets can make in-situ measurements of the electric field, the turbulence, the irregularities themselves and the associated electron heating. The coherent radars, such as CUPRI (Cornell University Portable Radar Interferometer). can detect the field-aligned irregularities while EISCAT makes continuous measurements of the electric field and height profiles of electron concentration and electron and ion temperature. The first ERRRIS campaign was carried out in the spring of 1988 and this paper describes the EISCAT contribution to the experiment and in particular the use of alternating codes (LEHTINEN and H&GSTR~~M, 1987) to combine good height, time and spectral resolution. Three other papers (WILLIAMS et cd., 1990a ; OPGEN~DRTH et ul., 1990; JONES et al., 1990) describe the scientific results achieved using EISCAT to date.
EISCAT CONTRIBUTION TO ERRRIS In 1986 joint measurements using CUPRI and EISCAT demonstrated the existence of short-lived
events where large electric fields were associated with characteristic echoes from the E-region, received at CUPRI, and enhanced electron temperatures over a height range of about IO&120 km, measured by EISCAT (PROVIDAKESet u1., 1988). The challenge to EISCAT was to measure the electric field with a time resolution better than 1 min and E-region electron temperatures with a similar time resolution and a height resolution of 5 km or better. Moreover, the events usually occurred when the electron concentration and, hence, signal-to-noise ratio were low, thus making the challenge even more daunting. Previous measurements of this kind had been made by splitting the signal from EISCAT into a long pulse to measure the electric field, and a multipulse group to measure the E-region profile of electron temperature with adequate height and spectral resolution. The most advanced pulse codes using this approach were developed by TURUNEN (1986) in the suite of GEN programmes, where a series of multipulse groups and longer pulses, all transmitted at different frequencies, were interwoven to make optimum use of the radar duty cycle. These represented a major advance on previous codes used by incoherent scatter radars, but were still not adequate to study the electron heating events with time resolution better than 1 min. To achieve such time resolution the 1986 CUPRII EISCAT observations combined eight different Spulse codes, but unfortunately this did not allow simultaneous measurements of the electric field (PROVIDAKESet al., 1988). The answer was to use the recently developed alter-
431
432
et ai
I. HXGGSTR~M
nating codes (LEHTINEN and HXGCSTR~M, 1987). In this technique a relatively long pulse is divided into a number of subpulses, with the phase of each subpulse determined by a special code. A sequence of coded pulses is transmitted, with each pulse coded in a different way. The echoes received by the radar can thon be decoded so that at every height each lag in the autocorrelation function can be uniquely determined without contamination. The advantage of alternating codes is ihat the whole duty cycle can be dedicated to one frequency, and in the received signal every possible cross-product for lags up to the pulse length can contribute valid information, while any clutter from other lags or other heights cancels exactly. This can be illustrated by comparing alternating codes with the optimum multipulse codes contained within the same overall pulse length and achieving the same height resolution. This has been done both for the comparison of an s-bit alternating code with a 3pulse group and a l&bit alternating code with a 4puise group. The results are s~~mmarized in Figs 1 and 2. In each case the overall length of the codes are about the same and the individual subpulses are the same as the bit-length of the alternating code. If we assume identical receiver settings we can compare the number of independent estimates of each lag in the alitocorrelation function made within a single pulse repetition period. In Fig. la the layout of two S-pulse groups is compared with one of the g-bit alternating codes, and in Fig. 1b the number of independent estimates per interpulse period is shown as a function of lag in both cases. Figure 2 shows a similar comparison
a)
mfiTm
WIIll
+--+**--+ b) Number of estimates
lternatinq
codes
Fig. I. Comparison of a 3-pulse multipulse with &bit alternating codes. The sequences are illustrated in (a), where each pattern in the multipulse corresponds to one frequency and the signs in the alternating code example represent the phase of each bit. The number of independent estimates per interpuke period for different lags is shown in (b).
a)
W l
EZI -++--*-+-----+++
b1 Number of estimates
lternating
codes
lag number Fig. 2. As Fig.
I, but for 4-pulse and I&bit codes.
between one of the I&bit alternating codes and the most compressed form of a 4-frequency 4-pulse group. In both cases it can be seen that the alternating codes have many more estimates than the corresponding multipulse codes, especially at short lags. If WC assume low signal-to-noise ratios (S 1) and define the speed of the codes to be the total number of estimates per unit time, then the alternating codes are about five times faster. In other words, when electron concentrations are low, integration times can be about five times shorter for alternating codes to still achieve similar accuracy. Due to the limitations of the existing EISCAT correlator, the longest alternating codes possible for high time resolution experiments were the g-bit codes. These are listed in Table 1 in the order in which they were transmitted. On examining the codes it can be seen that they are ordered in pairs, where each pair shows the same decoding scheme for even lags but with opposite sign for the odd lags. This means that the even lag-profiles could be added br each pair, whereas the odd lag-profiles were kept separate. Repeating this scheme on four frequencies it is possible to include one single pulse with its length equal to the bit-length of the codes to provide an estimate of the zero-lag, The zero-lag profile for the alternating codes can be summed for all codes with the result that it appears as the zero-lag from a ‘multipulse’ where eight subpulses are transmitted without an intervening gap. The zero-lag profile can then be used to improve the power profile as described by LEHTINENand HuusKONEN(1986).
433
EISCAT alternating codes during the ERRRIS campaign Table I. The sequence of the g-bit alternating codes, in the order they were transmitted in the ERRRIS campaign 1988. With a bit-length of 3011s the total length of the pulse is 240 ps Number
Code _...-
1 2
++++f+++ -t”-+-+-+-
3 4
+-t-++---t-----f+-+
5 6
+++-i”+-+-i++--+
7 8
++--+-++ +--++++-
9
-
++++-++-
10
+_-_+---++
II I?
++-+---+ +_---+--
13 14
+++--+-•+ +_++___-
15 I6
++___-f+---+-•+ff
With 1 I profiles for each code pair, the single-pulse power profile and a zero-lag profile from the codes, we have 90 profiles in all, and with a result ~nemory of 4095 complex values it was possible to obtain 41 range gates. Using a 30~s bit-length, and pointing the Tromss antenna at an elevation of 77.6’ where it lies parallel to the magnetic field line, the gates cover the altitude range from 93 to 270 km with a gate separation of 4.4 km and height resolution of about 4 km. The coded pulse can also be used to provide a scattered signal at the remote stations. The bit-length corresponds to a pulse which comfortably tills the intersection volume of the beams of the transmitting and receiving antennae. The scattered signal is sampled twice per subpulse, and by decoding the scattered signal in a similar way as for the monostatic measurements, and weighting the estimates to allow for the effect of the subpulse boundaries passing through the scattering volume, the autocorrelation function of the F-region ionospheric plasma is obtained. Alternating codes do not offer such great advantages for multistatic observations as they do for monostatic ones, but the use of a single frequency transmitting a single coded pulse allows the full duty cycle to be received at Kiruna and Sodankyla. As a result, a basic integration time of 5 s is available at the remote stations as well as Tromss.
OBSERVATIONS
AND RESULTS
DURING
ERRRlS 1988
The full ERKRIS experiment was run for a total of 110 h on 19 different nights between I5 March and 10 April 1988. In the standby mode, the Tromslir antenna was set pointing along the field line, and the remote stations were set pointing at a common scattering volume at a height of 260 or 170 km depending on where the scattered signal was strongest for the remote antennae. Two other modes were also allowed, one pointed at the volume where the ERRRIS rocket would pass through the E-region, while the other pointed at a scattering volume on the same field line but in the F-region rather than the E-region. However, most of the time was spent in the field-aligned mode where the electric field measured in the F-region ‘maps’ down the field line into the volume of E-region observed in the experiment. The results presented in the present series of papers were all obtained in this mode. As an introduction to these results, measured parameters are given to represent quiet conditions and magnetically disturbed conditions, on the grounds that the high consistency of the results obtained during quiet conditions gives strong support to the validity of the results when an amoral disturbance occurs. Figure 3 shows the results of analyzing autocorrelation functions in the form of altitude profiles of electron density (iv,.) and electron and ion temperature (Tc,and 7;, respectively) at 12 different heights between 93 and 141 km at 2011 UT on 23 March after integrating the data for 45s. This day was quiet on the whole with an .4~ index of only 5, and the measured electric field at 2011 UT was 36mVm- ‘. At greater heights, as the autocorrelation functions become
UT 2011:37 23 March 1988 ‘“I
0
1000 2000 3000 4000 50000.0
02
Temperature (K)
Density (IO ’ ‘mm3)
OA
0.6
0.8
1.0
12
Fig. 3. Altitude profiles of E-region electron density, electron and ion temperatures on 23 March at 2011 UT. The scales are 0-1.2. 10” rn..-’ for the density and t&6000 K for the temperatures. The integration time is 45 s.
434
I. H~~GGSTR~M rl al.
narrower, the electron and ion temperatures increase steadily. The smooth variation of temperature with height in both cases indicates a small random error in the measurements, even after such a short integration time. Figure 4 shows a similar set of altitude profiles at 2021 UT on the same day during a brief period of aurora1 disturbance when the measured electric field had increased to 126 mV mm ‘. We see that the electron temperature between about 100 and 125 km is greatly enhanced, reaching a maximum of about 4000K at 110 km, whereas the ion temperature is enhanced at all heights above 115 km, also reaching values of 4000 K and over. In both cases the variation of temperature with height is relatively smooth, indicating that the random errors are still small. A closer look at the data in Fig. 4 can be carried out by examination of the autocorrelation functions in Fig. 5, showing that they are unusually narrow for this height range. There are dramatic changes of shapes with altitude, being almost harmonic at 110 km but going to Gaussian from above 125 km. A similar conclusion of small random errors can be reached by plotting measured parameters as a function of time. Figure 6 presents the variation of electric field strength between 1800 and 2400 UT on 23 March. Until 2006 UT, and after 2132 UT, the field strength was always less than 50 mV m _ ‘. However, between 2006 and 2132UT the field strength is characterized by a series of short-lived enhancements reaching peak values at 2009,2022,2044 and 2127 UT. Figure 7 shows electron temperature for the same interval, plotted for a number of different heights. One of the most remarkable features of these results is the consistency of the measured electron temperature with time during the quiet periods, where it can be assumed that the small fluctuations represent an upper value to the random noise. It is the consistency of the measured values of T, with height and time during
UT 2022:07 23 March 1988 150
907 0
1000 2000 3000 4000500001)
Temperature
(K)
02
Op.
0.6
0.8
1.0
Density (10 ’ ‘me3)
Fig. 4. As Fig. 3, but for 2022 UT.
12
.
23 March 1988
202145-202230 UT
I
If
-
W’
. lmaglnary
Real
Imaginary
Fig. 5. Autocorrelation functions from 23 March at 2022 UT. The real part is on the left and imaginary part on the right (same scales) in each panel. The measured ACFs are shown as dots and the lines are the best fits of theoretical ACFs giving the parameters in Fig. 4. The lag separation is 30~s and the radar frequency is 933 MHz. The altitudes in kilometres are given at the bottom of each panel.
quiet periods that allows us to be confident that the dramatic increases in T, between 2006 and 2 132 UT are genuine and not seriously affected by random errors. However, the first range gate at 93 km shows for the first hour large random errors. This is a combined effect of very low signal-to-noise ratios in that time and height interval, and that the measured ACF is so short that a zero-crossing is normally missing, the latter due to the system limitations not allowing longer codes. The ion temperature measurements lead to a similar conclusion, as displayed in Fig. 8. In this case the increase in temperature is at greater heights, where the ion population moves at a high velocity with respect to the neutral atmosphere and frictional heating occurs (e.g. ST-MAURICE and HANSON, 1982). At 110 km the ion-neutral collision frequency is so high that the ion population is constrained to remain at the same temperature as the neutral gas, and with the exception of a single measurement (corresponding to the peak
EISCAT
alternating
codes during
UT Fig. 6. Perpendicular
electric field between
the ERRRIS
Mar 23
1988
18 and 24 UT on 23 March
electric field of 138 mV m- ‘) the results obtained confirm this. This is welcome confirmation that the analysis of the incoherent scatter spectrum remains valid although the electron temperature has increased so dramatically. Once again it is the consistency of the measured parameters at most times and for most heights that gives us confidence in the validity of the large and rapid variations in the temperature measurements, when these are observed.
GI,._:A,J!111_.~.~~~_.~
435
campaign
SCIENTIFIC
1988 with integration
RESULTS
times of 45 s.
FROM THE EISCAT ERRRIS
EXPERIMENT
The quality of the data taken by EISCAT during the ERRRIS experiment was so high that we anticipate a series of valuable scientific results, some of which appear in this volume. Here we will present a brief account of some of the most important results.
93
&.A,*..~~.~*.
~1800
1900
MOO
2100
UT Fig. 7. Electron
temperature
between
Mar 23
I
I
2200
I
I
I
2300
I
t
0000
1988
18 and 24 UT on 23 March 1988 at 12 heights from 93 up to 141 km with integration times of 45 s.
I. HKGGSTRBM et al.
436
119
@ -CT
115 1 110 2 106
1600
WI0
2000
2100 UT
Mar 23
2200
2300
ODOO
1988
Fig. 8. Ion temperature between 18 and 24 UT on 23 March 1988 at 12 heights from 93 up to 141 km with integration times of 45 s.
Electron heating in the E-region The prime aim of the experiment was to study the effect of large electric fields on the E-region plasma. Large increases in electron temperature are observed in association with wave turbulence (SCHLEGEL and ST-MAURICE, 1981; FWEK er al., 1986 ; IGARASHIand SCHLEGEL, 1987 ; PRCNIDAKESef al., 1988 : WILLIAMS et ul., 1988). Various theories have been proposed to explain the electron heating (e.g. ST-MAUKICE and LAHER, 1985; PKIMUAHL and BAHNSEN, 1986; ROBINSON, 1986 ; ST-MAURICE, 1987). The theory by Robinson is self-consistent and predicts a unique relationship between the effective electric field applied to the E-region and the ~llhancement in electron temperature. In comparing this theory with others it is crucial to discover whether large electric fields can ever occur without causing E-region electron heating (STMAURICE, private communication). This test requires measurements to be made with high accuracy and a short time resolution to prevent the true relationship between electric field and 7;. being blurred by random errors or by variations in the parameters during the integration period. The results from the 1988 ERRRIS experiment show that on every occasion when the effective electric field was greater than 50 mV m ‘, there was an increase in T, at 110 km of at least 200 K (WILLIAMS et cd., 1990a). A full set of measurements are discussed at length in a subsequent paper (JONESet al., I990), and it is
clear that the data are consistent with Robinson’s theory producing approximately the expected Eregion electron temperature for the measured electric fields, albeit with a small uncertainty due to the missing neutral atmospheric parameters.
At the same time as the applied electric field causes an increase in r,. in the height range IOO--120 km, r, increases at all heights above 125 km. This increase in ion temperature is caused by frictional heating as the electric field drives the ions at high speed through the neutral atmosphere. In principle. careful measurement of the electric field allows us to compare the measured profile of r, with the profile predicted for assumed models of the neutral atmosphere and on ion composition, and this should indicate whether such models are valid. A preliminary comparison between the predicted and observed profile of ion temperature shows good agreement below 160 and above 220 km, but in the intervening height-range the observed values of T, are consistently lower than predicted (see WILLIAMSet al., 1990a). A careful analysis of the data is planned using different assumed profiles of neutral mass, ion composition and ion--neutral collision frequencies and allowing for any anisotropy in r, to discover whether these factors explain the discrepancy. As so many factors are involved this will not be a simple comparison, but the improved time resolution possible with
EISCAT alternating codes during the ERRRIS campaign the alternating codes should at least eliminate the large errors which arise on time-averaging the electric field strength and the ion temperature-two rapidly varying parameters which bear a non-linear relationship with each other. &ctric,field
cariations
The relationships between the measurements of electric field strength, T,, and Tj, can also be used to validate the measurements of electric field. The improved time resolution available with the alternating codes has allowed the electric field to be measured after averaging over an intertial of only 45s. When this is done, it appears that the field is varying on a time-scale of a minute or so-a far shorter timescale than has been generally assumed in the past. The consistency with which the measured variations in electric field strength, r, and T,, remain in step is the strongest vindication of the reliability of the data, especially when we bear in mind that these parameters are derived from echoes scattered at different heights in the ionosphere. This is discussed at greater length by WILLIAMS et al. (1990a), where the ERRRIS data are used to confirm the indication from previous EISCAT data that the aurora1 electric field does indeed show large variations over a short time-scale. The one question that has to be resolved in each case is whether the variation occurs silnultaneously over an extensive part of the convection pattern or is associated with a localized structure moving rapidly through the TromsB beam. There exist. in fact, strong indications that most of the very short-lived spikes of electric field intensities, particularly during stationary field-aligned EISCAT experiments, arc associated with aurora1 structures moving through the EISCAT beam. OPGENOORTHet crl. (1990) present evidence that strong enhancements of the horizontal electric field can occur in a narrow region just outside aurora1 arcs. Also, CUPRI data from the co-ordinated ERRRIS campaign show that apparently short-lived spikes in the EISCAT electric field data coincide with the drift of latitudinally confined regions of strong backscatter through the EISCAT beam. Since the electric field enhancements are observed to be very pronounced only at the leading
edge of moving aurora1 structures, this new observation must have implications on our understanding of the electrodynamics within aurora1 structures and the mechanisms for their formation. More tong-lived enhancements of the electric field associated with a general spatially extended increase of the magnetospheric convection have also been observed, but they reach far less frequently values high enough to cause E-region plasma waves and associated electron temperature elevations (WILLIAMS et al., 1990b).
This paper describes the first operations of an alternating code experiment for high time resolution measurements with the analysis following the procedure given in I%GGSTR~M et al. (1989). It has been shown that the improvements of incoherent scatter radar measurements which the alternating code technique provides are substantial, making measurements possible with higher time and/or space resolution than used before. Confirmation of this is most clearly seen in the consistency of the measured parameters during quiet periods both in time and height, even for low signal levels and rather short integration times. During disturbed conditions, large electric fields cause turbulent heating of the electrons in the height range 100&120 km and frictional heating of the ions above 125 km. The close similarity between the measured variations in these three parameters is a further vindication of the technique, The ERRRIS experiment was performed with the existing hardware of EISCAT, and was necessarily a compromise of scientific demands and system limitations. Ongoing projects at EISCAT will make it possible to take full advantage of the alternating code technique in future, making almost any experiment design possible. Acknoil’ieirqenzmts--The EISCAT Scientific Association is supported by the Centre National de la Recherche Scientifique of France, Suomen Akatemia of Finland, Max-Planck C;esellschaft of West Germany, Norges Almenvitenskaplige Forskningsrad of Norway, Naturvetenskapliga Forskningsridet of Sweden and the Science and Engineering Council of the United Kingdom.
UEFERENCES FEJEKB. G., PKOVIUAKES J. F.. FARLEYD. T. and SWARTZW. E. H~GGSTR~MI.. LEHTINEN M., TCRUNENT. and VALL~NKOSKI M. I~ARASH~ K. and SCHL~GELK. JONES9.. ROHIUSON T. R., SCXLEGEL K. and HXGGSTKKM I.
437
1989
Radio Sci. (submitted).
I987 1990
J. utmos. fur. Phys. 49, 213
Ann. Ge@lys. (submitted).
438 LEHTINENM. and H;~GGSTR~M I. LEHTINENM. and HUUSKONEN A. OPGENOORTH H. J., HXGGSTRBMI., WILLIAMSP. J. S., JONESB. and PROVIDAKES J. PKIMDAHL F. and BAHNSENA. PROVIDAKES J., FARLEYD. T., FEJER B. G., SAHRJ., SWARTZW E., H~GGSTR~MI., HEDBERGA. and NORDLING J. A ROBINSON T. R. SCHLEGELK. and ST-MAURICE J.-P. ST-MAURICE J.-P. ST-MAURICE J.-P. and HANSON W. B. ST-MAURICE J.-P. and LAHER R. TURUNEN T. WILLIAMS P. J. S., JONESB., USPENSKYM. V. and STARKOV G. WILLIAMS P. J. S., JONESG. 0. L., JONESB., OPG~NOORTH H. and HAGGSTR~~MI. WILLIAMS P. J. S., VIRDI T. S., COWLEY S. W. H. and LESTER M.
I. HKGGSTRBMct ul. 1987 1986 1990
Radio Sci. 22, 625. J. utmos. terr. Phys. 48,787. J. atmos. terr. Phys. (submitted).
1986 198X
J. geophys. Res. 91, 1734. J. almos. terr. Phys. 50, 339.
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J. J. J. J. J. J. J.
l990a
J. atmos. terr. Phys. 52, 439.
1990b
J. atmos. terr. Phys. 52, 42 1
atmos. terr. Phys. 48,417. geophys. Rex 86, 1447. geophys. Res. 92,4533. geophys. Res. 87, 7580. yeophys. Res. 90,2843. atmos. terr. Phys. 48, 777. atmos. terr. Phys. 50, 3 15.