The EISCAT mesospheric measurements during the CAMP campaign

JavwIofA~marphrric~nd Riled in Great Britain.

0021.9169/8453.00+ .OO PergamonPressLtd

Terresrriol Pltysics. Vol. 46. No. 617. pp 565-575. 1984.

The EISCAT mesospheric measurements during the CAMP campaign W. KOFMAN,’F. BERTIN,’ J. R6rrc~& ‘* A. CREMIEUX’and P. J. S. WILLUMS~ ’ CEPHAG, (LA 346). BP 46.38402 Saint-Martin-d’Hires Cedex, France, ’ CNET, 92131 Issy-les-Moulineaux, France, ’ EISCAT Scientific Association, 98127 Kiruna, Sweden and 4University College of Wales, Aberystwyth, U.K. (Receiwdfor publication 21 February 1984)

Abstract-A brief outline is given of the experimental technique used during the Cold Arctic Mesopause Project lo record the first D-region ion line spectra with the EISCAT incoherent scatter radar. The data analysis shows that echoes from mesospheric heights between about 70 km and 90 km can be detected during disturbed periods ofenhanced electron density during particle precipitation events. Electron density profiles were determined which show a fairly high density, up to 5 x 10”’ rns3 in the upper D-region. The measured meridional winds were lower than 10 m s-l. A fit of the measured height profile of spectral width to temperature and neutral density models yielded a measured temperature profile in good agreement with simultaneous rocket data. The mesopause temperature was determined lo be as low as 130 K. This detailed analysisofthespectral widthprofileindicates that below about 77-80km theratioofnegativeions toelectrons exceeded unity. Finally, some discussions are added on the limitations and significance of these first mesosphere observations. INTRODIJCITON

first significant observation of the ion component of the incoherent Scatter spectrum in the D-redon (7090 km) with the EISCAT UHF-radar was performed during the Cold Arctic Mesosphere Project in JulyThe

August 1982. Earlier mesosphericmeasurements by the incoherent scatter technique were mainly carried out with the UHF-radars at Chatanika (e.g. Warr, 1977) and at Arecibo (HARPER, 1978 ; MA’IIIEWSet al., 1981; GANGULYet al., 1981). The mesosphere observations with the VHF-radar at Jicamarca (50 MHz) were shown to be dominated by scatter from turbulence (e.g. Rtisnx;~ and WOODMAN,1974). CAMP was a European cooperative programme including ground based instruments, balloon-borne experiments and rocket techniques, to study physical and chemical processes in the cold arctic mesopause region, namely negative ion chemistry, and their influences on the formation of noctilucent clouds. The EISCAT UHFsystem was operating before and during the rocket launches, both in the monostatic and the bistatic modes. The main feature of the backscattered echoes obtained in the monostatic and bistatic configurations was their high variability. The signal-to-noise ratio was sufficiently strong to obtain measurements only during precipitation periods. The appearance of signals was mostly limited to short durations, up to about 20 min. Intense echoes were connected with high absorption

l On leave from Max-Planck-Institute Lindau, West Germany.

fiir Aeronomie,

events recorded with riometers. It is fairly evident, therefore, that the observed mesospheric echoes were due to enhanced D-region ionization, which was created by high-energy particle precipitation. The parameters obtained from the mesospheric ion spectra are :

(1) the power P,, which is proportional

to the electron

density;

(2) the Doppler shift, which is proportional

(in the mesosphere) to the neutral windmeasured along the direction of the k vectoralong k, in the monostatic configuration (Fig. l), along (k, - k2) in the bistatic configuration (Fig. 1); (3) the half-power spectral width AJ which depends on temperature, collision frequency and negatioe ion density.

From these parameters we obtained some information on the electron density profile, wind profile, temperatureprofileandthenegativeionconamtration.

1. MEASUREMENTTECHNIQUE

The use of the EISCAT radar facility for middle atmosphere research (mesosphere and stratosphere) has been outlined by KOFMAN(1982) and R~~;ER (1983). Because of the short wavelength of the EISCAT UHF-radar, it is evident that only incoherent scatter can be received from mesospheric heights (D-region). The technique of data acquisition, however, has to be different from the usual technique for incoherent scatter

565

566

W. KOFMAN et al

TROMSQI

KIRUNA a, P e (39 TO 43”) Abiltatic c (79 TO 90 km) A moonatic

(65

TO 90 km)

Fig. 1. Scatter geometry of monostatic and bistatic modes.

observations of the E- and F-regions, because the correlation time is longer than the usual interpulse period. The technique of incoherent scatter observations of the mesosphere is, therefore, similar to the data-taking technique of MST-radar, since the incoherent scatter spectra are about as narrow as the scatter spectra due to turbulence. Our observations were made bistatically at Kiruna and monostatically at Tromsra. The main instrumental characteristics are given in Table 1. The direction of observation was in the vertical plane, including the Tromse and Kiruna sites (azimuth = 166” from Tromse). The elevation range was taken between 39’

Table 1 Instrument characteristics Frequency : 932.5 MHz Pulse length : 167ys Interpulse period : 1.67ms Transmitter peak power: 1.2 MW Elevation angle between 39” and 42’ Monostatic mode: Altitude resolution: 16 km Altitude sampling steps: 8 km Bistatic mode : Altitude resolution: 1.8 km Altitude steps : 2 km Pulse-to-pulse correlation technique, N = 64 pulses, max. lag of ACF = 105 ms Analysed spectral bandwidth f 300 Hz Nonambiguous ion velocity *48 m s-l Data were analysed with 2 min integration time

and 42”, allowing a vertical scan over the altitudes 7990 km in the bistatic configuration half-way between Tromsra and Kiruna at geographic coordinates 68.7”N, 19.8”E (see Fig. 1). This geometry was chosen to match best the expected trajectories of the rockets launched from Esrange/Kiruna. The EISCAT/Sodankyl& site was also used, but the selected geometry was not suitable for receiving evaluable echoes. Only during a special operation, directly following the CAMP campaign, was the geometry changed and good echoes were also received at the SodankylP site. To obtain the ion component of the spectra in the Dregion we used the pulse-to-pulsecorrelation technique (WOODMAN and GUILLEN, 1974; KOFMAN,1987). The principle of this method is shown in Fig. 2. After each transmitted pulse the received signal is successively sampled, corresponding to different altitudes A,,A,. Each set of samples taken at each interpulse period corresponds to a given temporal sample of the ionospheric signal. After N pulses are transmitted, the autocorrelation function (ACF) for each altitude is calculated. In the present experiment the sampling period was equal to half the pulse length. The measured ACF is composed of three terms. respectively corresponding to the ion component, background noise and clutter received from higher altitudes ofthe ionosphere. The perturbation due to the background noise and clutter appears only at the zero lag of the ACF due to the fact that their bandwidths are much wider (correlation times much shorter) than that of the measured signal from the D-region. Also due to

Maospheric

FIRST PULSE

Fig. 2. Altitudctime

measurements

during the CAMP campaign

n PULSE

SECOND PULSE

diagram for pulse-to-pulse

this fact, the pulse-to-pulse correlation scheme allows us to measure the ion component only below 90 km, where the spectral width is smaller than the maximum analysed bandwidth. Every 2 min the antennas in Tromspl and Kirunachanged their elevation angles and by this method the observed altitudes of the common volume changed by 2 km. We have to point out, however, that the altitude resolution is only about 16 km for the monostatic observations in Tromsca, due to the long pulse width, but 1.8 km in Kiruna, defined by the intersection of the antenna beams. Wechose the long pulse width of 167 p and short interpulse period of 1.67 ms for several reasons. Estimates of D-region signal strengths achievable with the EISCAT UHF system indicated a marginal detectability for normal, and even fairly enhanced, Dregion ionization. There were no earlier measurements from the EISCAT system which we could use to verify our estimates. The original launch criteria for CAMP were to avoid conditions of aurora1 disturbances, thus yielding a low chance for detecting D-region echoes during launches of the CAMP rockets. We had good reasons, namely the above mentioned vicinity to rocket trajectories, to apply bistatic operation with an optimum geometry between Tromse and Kiruna. This bistatic operation yields an altitude resolution of 1.8 km, regardless of pulse length. The bistatic operation required an elevation scan to coverthealtituderange80-90km.Asasuitabletimefor the entire scan we selected 16 min, and kept the antenna at each elevation for 2 min to allow a sufficiently long integration time. For a signal-to-noise of a few percent

567

correlation

technique.

this would yield an uncertainty in the power estimate of some 10%. With the present UHF-system sensitivity an increase of signal-to-noise ratio is only possible by reducing the receiver bandwidth, i.e. the noise bandwidth.This meant using as long a pulse as possible, and we selected a pulse width of 167 ps, corresponding to a postdetection-filter bandwidth of 3.3 kHz. This was used in Kiruna only, but in Tromser we had to stick to an existing filter with a bandwidth of 12.5 kHz. The price we obviously had to pay for the improvement of signalto-noise ratio at Kiruna was adeterioration in altitude resolution at Tromse. The consequences of these instrumental limitations we discuss in the following section. II. DATA AND INTERPRETATION The incoherent scatter theory for a collision dominated plasma in the presence of negative ions was developed by MATHEWS(1978) and FUKUYAMAand KOFMAN(1980). We applied this theory to interpret our measurements. Figure 3 shows the measured spectra for two altitudes with about 30min interval between the two measurements. One can see the high variability of the width of the spectra. We can interpret this behaviour in terms of negative ions. Figure 4 shows the measurements of the power of the signal received in Sodankyli, compared to the riometer data. The comparison of the riometer reading and the average signal power received from a scattering volume at 88 km altitude half-way between Tromse and SodankylH, clearly indicates the correlation between these two measurements.

568

W. KOFMAN er al

ALTITUDE 78 km 2038 UT

ALTITUDE 78 km 2008 UT

-200

“loo

0

loo

FREQUENCY

-

200

-200

Hz

-100

0

FREQUENCY

loo -

200

Hz

Fig. 3. Sample spectra measured in the monostatic mode with 2 min postintegration time. The altitude corresponds to the centre of the sampling range gate (Tromse on 28.7.82).

IIa. Electron density projile The

total

power

Pi of an ion spectrum

can be

expressed as : Pi K

N&+&l+21) (2(1 +L)+a-*)(l

+r-‘f

(I)

where N, is the electron density, rl theclassical electron radius and L the negative ion to electron density ratio : I = n-/N, a-2 =

4%%, * t A

( >

A is the wave length and An the Debye length. With the assumption I = Oand a-* CCI, the power Pi gives, after normalization, the electron density. Figure 5 shows theelectron density profiles obtained during the periods of enhanced echo power by assuming I = 0. We plotted the profiles in Fig. 5 by joining the measurements closest in time, which on average were obtained with 2 min time separation. The different profiles in Fig. 5 obtained every 16 min show an important variability of the ionization in the altitude range 75-80 km. During these short periods, very high

values ofionization were observed. These values were almost three orders of magnitude greater than those given by the classical quiet models. We estimate that our electron density measurements are precise within a factor of about 2 for the measurements at Tromso. This is due to two facts ; the assumption I = 0 is certainly not true for this altitude range and, principally, we do not know if the signal return is from the whole scatter volume [in data reduction we use half of the scatter volume (see Section Ml.

IIb. Wind profile In the bistatic configuration, the Doppler shift measured the vertical ion velocity (Fig. 1). For the observed altitudes it foliowed closely the neutral velocity V,, and it was observed to be in the range of + 5 m s- I. The monostatic configuration measured the component of the neutral wind close to the N-S direction. The sampling frequency ( =pulse repetition frequency of 600 Hz) allowed a nonambiguous measurement of the velocity V, in the -48 ms-’ < V, c +48 m s-‘.

Mesospheric measurements during the CAMP campaign

569

EISCAT- UHF TROMW-SOWNKYLA Z=BBKm

RIOMETER

KIRUNA

t

I

I

I

I

18X

1900

n90

a00

1

TIME (UT)

Fig. 4. Comparison ofincoherent scatter Power received from a common volume at altitude 88 km, half-way between Tromss and Sodankylk and riometer recordings at Kiruna.

An example of a wind profile is shown in Fig. 6. It corresponds to the qu~i-meridion~ component measured on 28 July between 20.30 and 20.56 UT. The amplitude of the observed meridional wind is rather weak, which is fairly consistent with mean circulation models. The time evolution of the wind profile exhibited an oscillating feature with a period of about 35 min.The observation period, however, was too short to allow any final conclusion as to whether this event is consistent with a wave signature. IIc. The spectral width profile and its interpretation terms of negative ion to electron density ratio

in

One of the parameters which is directly obtained from the analysis is the width of the spectrum. The theoretical spectral width at half power can be expressed as (FUKUYAMAand KOFMAN,1980) : AJ=a-

T mivitt

where

T is the

2(1 +l)+av2 1 +a-’ (

temperature,

>

assuming

(2) thermal

equilibrium of ah species, mi the ion mass, vin the ionneutral coflision frequency and (I is a constant. The expression mivin is given by RANKS and KWXARTS (1973) : mivin

=

2.6 x 10-gn(a,~Ue)1/2

where n is the neutral ~lari2abiIity and

density, r.

1 1 -_=-+& mi

(3)

is the neutral

1 m,

where m, is the neutral mass. From (2) and (3), Afcan be expressed (with a, = constant) as

In this formula we neglect the influence of z-‘, which leads to an overestimate of the theoretical width of about loo/,, which is negligible in comparison with the precision of our measurements. To calculate the theoretical width of the spectra one

W. KOFMAN et al.

570 90

l-IT

I 0 .

85

For a given temperature and density model, we can deduce a theoretical profile of AJmin and AJm,, as functions of 1. It must be emphasized that due to the inequality (4a), the spectral width is not allowed to be smaller than L\f,, obtained for 1 = 0. This condition has been used to adjust a temperature and density model to the observed spectral width. To make this adjustment we used the data obtained from the bistatic observations for the altitudes between 80 and 90 km, where the presence of negative ions is improbable (I = 0). The best agreement was found for a rather cold mesosphere, namely : T,=130K;

Z,=90km;

& = 290 K ;

Z, = 55 km.

Figures 7 and 8 show the temperature and density model so selected. For comparison, we include the CIRA 72 model profiles for 70” northern latitude in August. Figure 7 also shows the measurements of __

70

I

1

I

I

)

I

2044

?

ii

65 109

10'0

ELECTRON

10” DENSITY

-

80

10’2 m-3

Fig. 5. Electron density profile-s obtained from the monostatic (Tromss on 28.7.83, 20-21 UT) and the bistatic operations (Kiruna on 3.8.83, 22-24 UT). I

needs a model of the neutral atmosphere. We used the model of ALCAYD~ (198 1). It is an analytical model in which the temperature profile is entirely determined by a set of parameters which cover a large variety of conditions in the stratosphere, mesosphere and thermosphere. In our case, four parameters were used : the mesopause temperature TMand its altitude 2, ; the stratopause temperature Ts and its altitude Z,. From our measurements we determined these parameters in the following way: for a given temperature, neutral density and negative ion concentration, two theoretical limits for Af can be estimated : AJmi, by assuming m, >>m,;

(4a)

A&

(4b)

by assuming mi = mnr

which is reasonable because the mass of the ions on average cannot be much less than the mass of the neutrals.

1056 70

2064

2052 i

2050)

65 -10

-5

0

VELOCITY

-

5

-Ii 10

m/s

Fig.6.Windprofile(l66’azimutheasrofnorth)measured with the monostatic mode in Tromse on 28 July 1982 (time in UT).

Mcsospheric measurements during the CAMP campaign 100

I

I

I

I

I

I

571

X

60

I

I -

I

D. A.

-

E 7

60 -

: 2 F <

40 -

20 -

100

180

140

220

TEMPERATURE

-

300

260

OK

Fig. 7. Model of temperature prohle (ALCAY&(D.A), 1981),comparison with CIRA 1972and measured data points obtained from Philbrick (CAMP) (1983, private communication).

100

I

I 111111~

I

I

I

111111~

I 111111~

I

I

Illlll1~ 0

I

I ll1ll

D. A. CIRA 1972

60-

20

I

10-6

I

1111111

I I

lIlrlll

10-5

I 10-4

DENSITY

Fig. 8. Model

I 1llllll

of density

I 10-S

-

11

111111 10-2

kg/m3

profile from ALCAYDE (1981) and CIRA 1972.

I

I

I 11111. 10-l

572

W.

KOFMAN

neutral temperature obtained by the drag accelerometer flown during CAMP (PHILBRICK, 1983).A very good agreement can be observed between the temperature measured in siru and the temperature profile fitted from the spectral width by the method described above. In theoretical calculations the model has been used only between 69 and 90 km. Figures 9 and 10 show the theoretical profiles of Afmi. and Af,., for different values of 1 and two different temperature models. In these figures we also plotted the measured spectral widths. The sensitivity of our method in deriving neutral model parameters is demonstrated by comparing Figs. 9 and 10, in which the temperature at 90 km has been successively taken as &, = 140 K (Fig. 9) and TM = 130 K (Fig. 10). A good temperature choice is that which allows the measured widths to be inside the two limits, AfmiDand Af,,, for 1 = 0, which corresponds to Fig. 10. We have to emphasize that data obtained at Kiruna, which we used to determine the temperature model, have been recorded at the same time as the data recorded by the accelerometer. The discussion of Fig. 10 can be summarized as : (1) between 77 and 80 km a transition region appears which is characterized by a strong variability of the

er oi.

measured spectral widths that can vary by a factor of two in a time lag of a few tens of minutes ; (2) between 71 and 77 km a region of constant 1 = 1 can be observed ; (3) finally, below 71 km, a 1 = 2 region appears.

III.DISCUSSION

ON THE VALIDITY

OF THE ABOVE RESULTS

Three possible mechanisms can affect the spectral width measurement : the influence of electron density, temperature and wind gradients in the scatter volume. These mechanisms affect only the monostatic result (Tromser site), for which the altitude resolution is relatively poor and the wind gradient, although weak, is not negligible. The bistatic configuration, having a good altitude resolution and measuring a quasi-zero vertical wind, is not biased by these effects. This means that we have a great degree of confidence in the data observed with the bistatic operation. The interpretation of the monostatic results is much more complicated and the confidence which we have is more limited. Themost important problemis the real altitude extension of the scatter volume. In fact with a 167 ps pulse and about 40”elevation angle, the corresponding

100 1

0

KIRUNA 920003 2210 TO

2359

UT

820728 2002 TO 2032 TO

2024 2102

UT UT

90 E ; g

-

THEORETI

80

iF 2 a

REGION

OF k = 1

60

WIDTH

Fig. 9. Spectral widths computed

-

Hz

from model (assumingmesopause temperature with measured data points.

TM = 140 K)and comparison

Mesospheric measurements during the CAMP campaign I

I 0

I

I

I

I”‘~

KIRUNA

820003

573

I I I Illl’ T,,=13O'K Ts =290"K Z,,=BOKm Z,=SSKm

2210 TO 2359 UT 0

g

80

-

TRANSITION

TROMSji 820729 2002 TO 2924 UT 2932 TO 2102 UT THEORETICAL

WIDTH

REGION

REGION OF A = 1 70

-Afmi”

(mi >>

m,.,)~

80 10’

102 WIDTH -

103 Hz

Fig. 10. As Fig. 9, but TV = 1~ K.

altitude extent is about 16 km. However, the characteristics of the spatial distribution of the recorded power in successive range gates leads to the assumption that the scattering volume was generally concentrated in an altitude range smaller than 16 km. Mainly we observed signals in two adjacent ranges gates, but never the first and the last gate, which implies that the observed signal came essentially from a volume of about 10 km vertical extent, as is demonstrated in Fig. 11. We have to admit that the coarse monostatic height resolution adds some uncertainty to our conclusion that the number density of negative ions is equal to or larger than the electron density below 80 km. However, our observation of 1> 1 for Z < 80 km is consistent with expectations and other observations.

volume. Simulations have been performed by using the following theoretical expression of the spectra, which is a good approximation for ab2 << 1 and 1= 0 (FUKIJYAMA and KOFMAN, 1980). 1

NAZI sz’cf) = nAf(ZJ /

f

\’

_.

(6)

The theoretical Af profile is given by equation (4), assuming 1. = 0 and using for N,(Zi) the measured mean electron density profile. The simulation shows that the measured spectral width S,(J) is the same as the width of the spectrum S&J at the altitude Z, corresponding to the middle of the scatter volume. If a wind gradient occurs in the scatter volume, the Doppler shift differs from one altitude to the other. The IIla. InJSdenceofelectron density, temperatureand wind widthofthemeasuredspectrumS,(f-I,)is widened by gradients in the scatter volume the contribution of the digerent Doppler shifts. The The measured spectrum S,,,m is in fact the integral of corresponding broadeningbfis approximately given by all the contributions at different altitudes in the scatter the maximum spread of the Doppler shift in the scatter volume limited by the two altitudes Zmin and Z,,, volume. The simulation was carried out by using the measured wind profile. The maximum broadening &I.. SCU-Jr,), ABr), J’J&)l dz. (5) effect which was found was ~5f,_ = 40 Hz. &U-_&J = I znli. The main result obtained is that all monostatic width The problem is to know how the spectral width of S,,,(J) measurements should be shifted towards smaller is affected by gradients of the parameters in the scatter values. This crude method of determining the influence

W. KOFMANet

574

al.

GATE 1

I

SIGNAL NO

ki a

2

TIME

Fig. 11. Range gate overlap, demonstrating the oversampling to obtain altitude steps of 8 km.

of gradients in the scatter volume is not entirely satisfactory, mainly because the variation of the parameters as function of altitude, which we used in the simulation, was smooth. However, in fact, as was mentioned above, an increase of density can occur only in part of the scatter volume. Consequently, the high variability of the width for altitudes around 80 km can be due as much to this effect as to the variability of i., This last interpretation can be supported by a few measurements obtained with the bistatic configuration at the same altitude range.

IV.

CONCLUSION

The first EISCAT measurements of incoherent scatter from the mesosphere show the capability ofthis system to obtain information on the electron density, wind, temperature and negative ion profiles during perturbed aurora1 conditions. Continuing this analysis, a unique occasion will be offered by further comparison between these EISCAT results and simultaneous in situ

data from rockets gown during the Cold Arctic Mesopause Project. In continuing experiments we will improve the experimental technique, mainly by diminishing the scatter volume in the monostatic mode.

~~k~wle~~-We are grateful for the support in preparing software and hardwas for these experim&s which we trained from R. GRAS. C. LAHOZand T. TURUNEN.The &.istance of H. DE&OM and H. RISHBE~ during the experiment operation is kindly acknowledged. We also appreciated the continuous interest of the CAMP project scientist L. BJ&N to keep EXSCATexperimenters involved in theCAMP~mp~~.~e~omet~data werekindly provided by the Kiruna Geophysical In~itute.Theassistan~o~a~ the cooperation with the EISCAT stsB during the preparation and operation of the experiments was very valuable and hdpful. The EISCAT incoherent scatter radar facility is funded and operated by Suomen Akatemia/Finland, Centre National de la ReehercheScientitiuue. France. Max-Planck Gesellschaft, West Germany, No&’ Almenvitenskapehge Forsknings~d, Norway, Naturvetenskapliga Forskningsr&det, Sweden, and the Science and Engineering Research Council. United Kingdom.

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ALCAY&D. FIJKUYAMA K. and KOFMAE~ W. GANGULYS. HARPERR. M. KOFMANW. MATHEWS J. D. MATHEWSJ. D. RASTOGIP.

K. and

WOODMAN R. F.

1981 1980 1981 1978 1982 1978 198i 1974

Annls GPophys. 37,515. J. Geomogn. Geoeleet., Kyo~o 32, 67, Geophys. Res. L.ett. 7, 369. Geophys. Res. Mt. 5, 784. EISCAT Technical Note

KirunaSweden. .I. yeophps. Res. 83, 50.5. .i. atmos. terr. Phys. 43, S49. J. atmos. rem. Phvs. 36, 1217.

82/35 EISCAT HQ

Mesospheric measurements during the CAMP campaign R&-~GERJ.

1983

WAl-r T. hf.

1977

Proceedings of the 6th ES.4 Symposium on European Rocket and Balloon Programmes, ESA SP-183,287. Proceedings of the ElSCAT summer school on radar probing o/the aurora1 plasma (Ed. A. BREKKE),385,

Universitetsfiirlaget, Tromw-Oslo-Bergen. WOODMAN R.

F. and GUILLENA.

1974

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