phases and amplitudes of tids in the high latitude f-region observed by eiscat

~ Pergamon Journal of Atmospheric and Terrestrial Physics, Vol. 58, No. 1~, pp. 245-255, 1996

Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved

0021~ 169/96 $15.00 + 0.00 0021-9169(95)00033--X

Phases :and amplitudes of TIDs in the high latitude F-region observed by EISCAT

K. Hocke,* K. Schlegel* and G. Kirchengastt

* Max-Planck-Institut ffir Aeronomie, Katlenburg-Lindau, Germany t lnstitut ftir Meteorologie und Geophysik, Universit~it Graz, Graz, Austria

(Received 27 October 1994; accepted 12 December 1994)

Abstract--Travelling Ionospheric Disturbances (TIDs) are the ionospheric response to gravity waves passing through the neutral atmosphere. They can be measured simultaneously as wavelike perturbations of the ionospheric electron density (Are), field-aligned ion velocity (V~), and ion and electron temperature (T~ and Te) by the EISCAT radar at Tromso. In order to derive the average amplitude and phase relationships between Are, Vi, T~ and T~ we have selected 45 TIDs with periods in the range 30-150 rain from EISCAT data of a quiet daytime F-region from November 1987 to December 1991. It is shown that there exist characteristic phase differences between the TID parameters which can be used to identify gravity waves; in incoherent scatter data. Furthermore, we present height profiles of the average phases and amplitudes o1' the various TID parameters. Finally, the spectral behaviour of the TID amplitudes is studied using the average power spectra of Are, V~, T~ and To. We conclude that the use of the incoherent scatter information of all TID parameters allows a sure identification of gravity waves in the thermosphere.

1. I N T R O D U C T I O N

Gravity waves produce wavelike perturbations, so- called Travelling Ionospheric Disturbances (TIDs), in the ionospheric electron density, ion velocity, ion and electron temperature. These quantities can be measured simultaneously over a wide height range with the incoherent scatter technique.

The phase and amplitude relationships between N~, Vi, T~ and To are not well known because only a few observations have been published so far describing the response of these ionospheric parameters to gravity waves. Testud and Vasseur (1969) analysed a TID which was measured in all parameters on 13 September 1967 and explained approximately the observed amplitude and phase behaviour with the gravity wave theory of Hines (1960). A similar analysis of a large nighttime gravity wave at Arecibo was carried out by Harper (1972). Later, Bertin et al. (1983) observed one TID in Ne, V~, T~ and Te, and established Vi as best tracer for gravity waves since the other parameters might be disturbed by electron precipitation and Joule heating.

The aim of this investigation is to derive the typical phase and amplitude behaviour of gravity wave induced TIDs in the high latitude F-region using incoherent scatter d~.ta of 45 TIDs discernible in Are, V~, T~, and T~. The phases and amplitudes of the different TID parameters, their relationships and height vari-

ations are analysed in order to find average values characterizing the response of the ionosphere to gravity waves.

The data of this statistical study were carefully selected from the EISCAT data base to ensure that the investigated fluctuations are induced by gravity waves (Section 2). The observed TIDs have in general amplitudes of perturbation magnitude (less than 10 per cent) and occurred during a quiet daytime ionosphere. The data reduction will be described by selecting one TID event (110 min-TID on 7 September 1988) as an example and showing its maximum entropy spectra, the relative fluctuations, the bandpass filtered data sets and the phase profiles of No, Vi, T~ and Te.

In Section 3 of the paper the average phase differences and the phase profiles are presented and dis- cussed for the ensemble of 45 TIDs. Of special interest is the Te-TID since there are almost no reports in the literature about its phase and amplitude behaviour. We show that the To fluctuations can be explained as a consequence of the T~ and Are fluctuations. The average amplitude profiles and the amplitude spectra of the ionospheric variables are presented in Section 4 and conclusions in Section 5.

2. DATA SELECTION AND ANALYSIS

In this study we used EISCAT CP-1 and CP-2 data from November 1987 to December 1991, containing

245

246 K. Hocke et al.

simultaneous measurements of electron density N~, field-aligned ion velocity Vi, ion and electron temperature Tj and To. The data cover the height range from 147 to 587 km with a resolution of 22.5 km in height and 5 min (CP-1) or 6 min (CP-2) in time. The radar beam at the station Tromso (geographic latitude 69.59°N) was directed along the geomagnetic field line (dip angle I = 77.59 ° at 300 kin). The EISCAT system was described in detail by Rishbeth and Williams (1985).

The selected data are representative for a quiet daytime F-region with low ion drift perpendicular to the geomagnetic field (E x B drift : V < 300 m/s) and low geomagnetic activity (Kp < 3-). Therefore, the gravity wave induced TIDs analysed in this paper are nearly free of contamination by electrodynamical effects like Joule heating or particle precipitation.

In the data analysis the background ionosphere (N~o, V~o, Ti0, T~0) has been estimated by using a But- terworth lowpass filter which removes all short time variations with periods less then 3 h from the data. The relative fluctuations of the measured parameters Are, Ti, T~ are defined as ( Ne - Neo) / Neo, ( Ti - Tio) / Tio, (T , - T~o)/T~o and the absolute fluctuations of V~ as (V~ - V~0). In order to find the dominant periods of the relative fluctuations the maximum entropy method (MEM) is applied, which gives reasonable spectral estimates when dealing with short data records.

The main criteria for the selection of the dominant TID periods are :

• occurrence of a peak at the same wave period in the MEM spectra of all four parameters

• occurrence of the spectral peak over a wide height range (e.g. 200-400 km) and during the same time interval.

For example, a dominant period of 110 min is found in the MEM spectra of all TID parameters during the time interval 11:00-17:00 UT on 7 September 1988 as illustrated in Fig. 1. The spectral density is sketched as a function of wave period (not frequency) because by working with time series it is more intuitive to consider the period as a parameter. The periods of the observed TIDs are nearly constant with height as it is shown in Fig. 1. The observation of an almost constant wave period with height was also documented by Lanchester et al. (1993); Tedd e t al. (1984) and Hearn and Yeh (1977), so that TIDs can be well approximated as monochromatic waves.

After the spectral analysis the relative fluctuations are filtered with a (6th order) Butterworth bandpass filter at the dominant periods. In Fig. 1 the cutoff periods of the used filter of the 110 min-TID on 7 September 1988 are drawn as dotted vertical lines. The cutoff periods of the bandpass are equal for all

TID parameters and heights. The bandwidths are chosen between 20 and 40 min, increasing with period, and cover the major dominant period band. The attenuation rate of each Butterworth filter was calculated from the corresponding filter transfer function (e.g. Hamming, 1977), and the data were corrected accordingly in order to obtain reliable TID amplitudes. In Fig. 2 the relative fluctuations (dotted lines) are plotted together with the corresponding 110 min- bandpass filtered time series (solid lines) for several height steps. The comparison demonstrates that the 110 min-TID is the most dominant wave during the time interval 11:00-18:00 UT. During the time interval 15:00-18:00 UT in addition a 35 min-TID is present, especially in Ne and Te (Fig. 2) and which is also included in our study.

The height dependence of the 110 min-TID is illustrated as a contour plot in Fig. 3. The solid contours denote positive values and the dotted contours denote negative values. A positive V~ stands for an upward ion motion. In the figure the contours are drawn for the height range 200-550 km because the TID appears most clearly in this height range.

The described data analysis has been performed for 45 TIDs. Their periods are distributed over the range 30-150 min as illustrated in Fig. 4. Waves with periods greater or equal to 120 min are rare in our study because they require a long time interval (5-7 h) of a quiet ionosphere. Waves with periods less than 30 min are not considered because the time resolution of the CP- 1 or CP-2 data (5 or 6 min) is not sufficient for an exact phase determination by such short periods.

3. PHASE RELATIONSHIPS BETWEEN TID

PARAMETERS

In order to derive the phase profile of each TID parameter the bandpass filtered time serie at the height h = 322 km is taken as a reference. The phase shifts of the time series of the other heights are then calculated by cross-correlation with respect to the time series of the reference height. The phase profiles of Ne, Vi, T~ and Te are illustrated for the 110 min-TID in Fig. 5. All four parameters have tilted wave phase fronts, typical for a gravity wave with downward phase progression.

The phase differences of the 110 min-TID can also he read from Fig. 5. ¢P(Ne) - ¢p(Te) is equal to 150- 180 ° at heights 250-500 km. To is nearly in phase with Vi, while T~ has a height dependent phase advance with respect to V~, T~ of 20 to 90 ° in the height range 300-500 kin.

In order to show the average phase behaviour of

o

O

z" ~d

O 3

50

- 5 0

v

c O ° ~

O

O 3

Phases and amplitudes of TIDs 247

Ne 7 Sept. 1988 1 lh00m- 17h00m UT

4 0 80 120 160 2 0 0 P e r i o d ( r a i n . )

587 ~" 565 543 ¢

o w.J

477 4 5 5 *~ 4 3 3 = 411 3 8 9 >- 36'7 3 4 5 323 =,., 301 - ' 2 7 9 2 5 7 2 3 5 2 1 3 169 -~,

0

147 c~ O 3

I00

50

Vi 7 Sept. 1988 lh00m- 17h00m UT

I I I I

:

, ,

I I I I

0 4 0 80 120 160 2 0 0 P e r i o d ( r a i n . )

5 6 5

543

521

4 9 9

477 455

433 411 389 367 345 323

50

Ti 7 Sept. 1988 llh00m- 17h00m UT

5 4 3 521

477

411 3 8 9 36'7

Te 7 Sept. 1988 1 l hOOm- 17hOOm UT

-50 I I I I

0 4 0 80 120 160 2 0 0 0 4 0 80 120 160 2 0 0 P e r i o d ( r a i n . ) P e r i o d ( r a i n . )

' ' i

587

521 4 9 9 4 7 7 455 433

~, 411 389

'~ 367 345

~'~ 51 323 301 2 7 9 2 5 7 2 3 5

169 ~- 147 O3

-50[

Fig. 1. Maximum entropy spectra of a TID event observed on 7 September 1988. Spectral density vs wave period of Are, Vi, T~ and To fluctuations are plotted at 21 discrete heights. To avoid overlap, the curves of the different heights are shifted equidistantly along the spectral-density axis. The two vertical dotted lines

indicate the cut off periods of the bandpass filter for the dominant 110 min-TID.

248 K. Hocke et al.

10 0

- t O

P e r i o d = 110 rain. I I I , 1 I 1

. , ~

• .. ~ . . • • . ' . . z

• . ~ - - - . -

~ - . . . . . . . . . . " ...~._ ..". ~ ....~"~.. . :".,

i I i i i [

11 12 13 14 15 16 17 8 U n i v e r s a l T i m e [ h ] , 7 S e p t . 1 9 8 8

Height [kin]

464.700

432.700

410.600

388.600

368.600

344.600

322.600

300.500

278.500

256.500

234.500

212.500

E

>-

I0 0

- I O

Period= t 10 rain. 1 I I I ] I

.,. : , . • .; ' . ; t. ..

~ ~ " ' . . ' . . ; , :.

- , - " ~ .. -., :,, ;, • . .

,.'., . .",.", :',, ~',. ,":: ,", :'.. ,:v,

i I i i i I

Height [krn]

454.700

432.700

410.600

388.600

366.600

344.600

322.800

300.500

278.500

256.500

234.500

212.500

11 12 13 14 15 16 17 18 Universal Time [h], 7 Sept• 1988

Period= 110 min. Height [km] I ! ' ' ' I

~ " ~ : ~ - - ~ "" '"" "'"'" " "";I 484.v0o

~ . " " " -/".": ":i." ...... .1432.v00

" "'.'.. "-- . '. .~','; ~.....'....'.',. 410.600 '.-" ?

,"- ' , : ' ,.~ , ."" . ~ .. " '..~/"'388.800

~ ' - ' : ' T - , ~ ,.y_..~',. , " ,,',..~,,, 368.800

~ . . ". ..~ - ' . .'.. ,:.. 344,600

. " . " . . . . " ' " . . . . / " . . . . ' . . " " .: , . . . . , ' 3 2 2 . 6 0 0

",' . : \ ' . , " ' - " ,. "" " ' . . . . . . 300.500

~ . ' - . . . ":' . . - - ~ " . " ~. 2'78.500

• "" . . . . ":. . . " " " . ". ~ . " " .. '~ ~ !256.560

- a I - "", '"": ' , '=2 , ~"

i1 12 13 14 15 16 17 18 Universal Time [h], 7 Sept. 1988

5 0

- -5

Period=l I0 rain. Height [kin]

~ .~\..:'" ~ 454.700

~ 432.700

~ . : . ' " - - . , . :L,: 388.600

~ ~ . ~ !-:; 388.800

!344.800

~ : " 322.800

• ~ " " ; . ' . . . . -:"3oo.5oo

. . . ~ - . . ~."-.:2,8.5oo

256.500

234.500

212.500 • . . . . . , . - , . . ' " .

i I I $ I i

1 1'2 13 14 15 16 17 18

Universal Time [hi, 7 Sept. 1988

Fig. 2. Electron density (No), ion velocity (V~), ion and electron temperature (T~, Te) at different heights vs UT on 7 September 1988. Dashed profiles : fluctuations of the original EISCAT CP-1 data. Solid profiles : bandpass (110 + 20 min) filtered data of the dominant 110 min-TID. The labelling for the time series at

height 212 km holds for all height steps depicted•

Phases and amplitudes of TIDs

Contours of N Vo]

450 E

.~ 400

350

=:: 3 0 0 -

25o-i21111 i.. 200

1 12 13 14 15 18 17 UT [h], 7 Sept . 1988

550

500

450

.=, 4oo

~ 3 5 0

,-r. 3O0

250

ZOO

. . . : ;

C o n t o u r s of T% [%] ,

ii 12 13 14 UT [h], 7 Sept.

249

15 16 17 1988

550

500 -

4 5 0 - E

400

350

,-.v 300

25O

ZOO

i i 12 13 14 15 18 17 UT [h], 7 Sept. 1988

Contours of T [%]

450 : : :!: " .;: i!"..: iil

~ 400 1 ":.:': i' i :' ."i'i" ~ '; : 4:

7o . . . . .

11 12 13 14 15 16 17 UT [h], 7 Sept . 1988

Fig. 3. Relative fluctuations of electron density No (%), ion temperature T~ (%), electron temperature To (%) and abselute fluctuations of field-aligned ion velocity V~ (m/s) on 7 September 1988, bandpass filtered

with a central period of 110 min. The solid (dotted) contours denote positive (negative) values.

I I I I I ! I 10

8

8

4

2

0

0

[IjlL z

I I I I I 1 I

2 0 40 6 0 8 0 100 1 2 0 t 4 0 1 8 0

P e r i o d (mln )

Fig. 4. Distribution of the wave periods of all 45 TIDs used in our study.

55O

500 •

450"

400-

~ 3 5 0 -

300.

250 "

20O

i , ,

, \

"\,

"\ ,. "\ ",,

\ , x

\ \

\ \

\ \

90 180 270 360 450 Relat ive Phase ~o [deg],

Fig. 5. Phase profiles of Are ( - - - ) , Vi ( ), Ti ( - . - ) and T e (.--) for the 110 min-TID on 7 September 1988.

250 K. Ho ck e et al.

18 I

16

, . , O O 6

o 4 ~ 2

0

- 3 0 3 0

1 A 1o

, , . , 8 O B

o 4 z 2

0

-ao 3'o

i I I I I f 1 8 I I I I I I I

1 6

1o

1 I I I I I I 90 t.50 210 270 330 300 4,50 - IOO-PO-OO 0 60 120 IaO 240 300

q(Vt)-~(No) [deg] @(Vt)-~(Tt) [dog] I I I I I I 1 6 1 I I I I I I

"t 14

10

~/H~///x///~ 4-1a-lZdO - I

0 ~0 I ;0 ' ' ' ' ' ' 30 ' gr~ll l ' 210 2"/0 330 390 450 - 3 0 90 150 210 270 330 390 450

~(N.)-w(T,) [deg] ~(N.)-~(T.) [deg]

1 8 I I I I I I

1 6 _ 1 4 .

1 2 .

I O .

8 . O . 4 .

2 .

I I I ~ I I I - 1 8 ~ 1 2 0 - 6 0 0 120 180240 300

~(VJ-~(T.) [deg] 1 8 I I I I I I I

16 .

1 4 . 1 2 .

10 .

8 .

2 .

~(Tt)-~(T.) [ d e g ]

Fig. 6. H i s t o g r a m s conta in ing the phase differences of 45 gravi ty wave induced T I D s in the height range 301-345 km.

gravity wave induced TIDs, histograms of the phase differences of the observed TIDs have been prepared. We found that the phase differences do not depend on the wave period. Therefore, the phase difference histograms in Fig. 6 contain all 45 TIDs with periods between 30 and 150 min. The phase differences are averaged for the height gates 301,323 and 345 km. A positive phase difference ~o(1) - ~o(2) means that the wave of parameter 1 follows the wave of parameter 2 with a phase lag. It is obvious from this figure that there are characteristic phase differences which are well defined for the height interval 301-345 km. The average phase differences and their standard deviations for different height intervals are listed in Table 1.

A remarkable result is that Ti has a clear phase advance with respect to V~. The average phase difference ~o(VJ-~o(T 0 is 60-70 ° at heights 235-411 km. Since the phases of Ti and V~ are very close to the phases of neutral temperature (T) and neutral velocity (V) (Testud and Vasseur, 1969; Testud and Francois, 1971 ; Kirchengast, 1992), the measured phase difference q ) ( V i ) - - ~ 0 ( T i ) indicates the phase difference between the corresponding neutral gas parameters. Testud and Vasseur (1969) calculated the phase difference between the neutral temperature and velocity using polarization relations (Hines, 1960) and predicted a phase advance of the temperature wave :

Table 1. Ave ra ge phase differences a nd s t anda rd deviat ions o f 45 gravi ty w a v e induced T I D s with per iods in the range 30-150 min

Phase difference h = 235,257,279 k m h = 301,323,345 k m h = 367,389,411 k m

~o(Vj - ~o(Nc) 2 0 0 + 5 5 ° 210-1-35 ° 2 1 5 + 5 0 ° ~o(V,) - ~o(T 0 7 0 + 5 0 ° 7 0 + 5 0 ° 6 0 + 6 0 ° ~P(V0 - ~o(Te) 55_+55 ° 2 0 + 3 5 ° 10_+40 ° ~o(N~) - q~(T 0 245 ± 55 ° 230 _ 50 ° 215 _+ 55 ° ~o(Ne) - ~o(To) 225 + 4 5 ° 175 ___ 40 ° 170+__45 ° q~(TJ - q~(Te) - 1 5 + 5 5 ° - 5 5 + 6 5 ° - 4 0 + 8 0 °


~ = x/7 -- 1 exp (iq~)~ -~,

with ~0 = arctan ( - 2kzH) (1)

where

T~/To relative neutral temperature perturbation due to gravity wave

Vh neutral horizontal velocity perturbation due to gravity wave

q) phase difference between T~ and Vh C sound speed 7 ratio ofspecffic heats

H scale height k~ vertical wavenumber.

For typical values kz = - 2 " 10 5 m-I and H = 50 km, the phase difference between the neutral temperature and velocity becomes 63 ° . This phase advance of the temperature wave could not be verified experimentally from the TID observations of Testud and Vasseur (1969), Harper (1972) and Bertin et al. (1983) probably due to an insufficient time resolution (10-15 min) or to a limited accuracy of the data. Therefore ; the average phase difference ( P ( V i ) - - ( f l (T i )

which was derived from EISCAT data (Table 1) is a late confirmation of the prediction by Testud and Vasseur (1969).

The average phase difference q~(Vi)-~0(Ne) is 210 ° and constant with height while the phase difference between V~ and Te varies with height. The phase behaviour of the TJD parameters can be illustrated more clearly when instead of the phase differences the phase profiles are considered as in Fig. 5 for the TID on 7 September 1988. In Fig. 7 average phase profiles of the whole ensemble of 45 TIDs are presented. The statistical spread of the average phases is of the same order as the standard deviations of the phase differences in Table 1. Il is obvious that the wave phase fronts of Vi, N e and T~ have almost the same inclination. The typical forward inclination of the phase fronts is suitable to distinguish gravity wave induced TIDs from other quasi-periodic ionospheric disturbances which have normally a small phase vari- ation with height, if any at all. On the other hand, the identification of gravity waves using To is difficult because the phase profiles of T~ are nearly constant with height and do not show the typical inclination of a gravity wave phase front. The phase behaviour of To is very interesting because at heights below 250 km T~ is in phase with it] while at heights above 300 km To is almost in antiphase with N~ (t0(No) -(p(Te) = 175°).

This can be exp]lained if we assume that at low heights the electron temperature fluctuation is caused

500

450

4 0 0 -

..,-, 3 5 0 -

• ~ 300 =::

250

200

N e

i

X x x

x x

T t T, V i i

o ~-

o .,4-

o 114-

o e4-

o e+

o o4-

o e+

o • 4-

o • +

o • +

o • 4-

n = 4 5 r i

x x

x

x x

x oD + X

oo 4- X

90 180 270 360 450 Relative Phase ~0 [deg]

251

The TID amplitudes are determined by using the bandpass filtered time series. As an example, the absolute value of filtered No fluctuations is depicted in Fig. 8 (solid curve). The time dependent amplitude

4. A M P L I T U D E S O F T H E T I D P A R A M E T E R S

by the neutral or ion temperature fluctuation, and at heights above 300 km the electron temperature fluctuation is mainly induced by the electron density fluctuation. The anti-correlation between Ne and Tc has often been recognized for long time variations of Are and T e (day-to-day variations, seasons, periods with changing solar activity) in the F-region (e.g. Mahajan, 1967a; Mahajan., 1967b; Lejeune and Waldteufel, 1970 ; Brace and Theis, 1978 ; McDonald and Williams, 1986). With increasing electron density the electrons are more cooled by collisions with the ions or neutrals, and the electron temperature decreases. Since the temperature difference between electrons and ions increases with height, the electron- ion cooling rate ( ~ Ne2(T~ - T0) increases in turn and the anti-correlation between N, and To becomes therefore more significant at higher altitudes. This behaviour of Are and To is also present in the No and Te fluctuations of our study which show a well defined anti-correlation at heights above 300 km.

At heights below 250 km the electron temperature fluctuations are induced by the neutral temperature fluctuations of the gravity wave. Because of the higher neutral density at these heights and more frequent ion-neutral collisions the thermal coupling between neutral gas and the plasma becomes stronger, and the ion and electron temperatures are close to the neutral temperature. Therefore, Testud and Francois (1971) have assumed that To and T~ follow the variations of neutral temperature (A To -- A Ti = A T). This assump- tion seems to be verified by the observed phase equality of To and T~ at heights below 250 kin.

Fig. 7. Phase profiles of the TID parameters averaged for 45 gravity wave induced TIDs.

252

T, T~ T~ T~ ! 0.05

0.04

0.03

0.02

0.01

0

8

K. Hocke et al.

minimum average maximum

F . . . . . . . . . . . t • 4' . . . . . . . . . . . . . . . . . -t

"v/

<

I I

10 12 14 16 18

Time [UT]

Fig. 8. The solid curve represents the absolute value of No fluctuations after the filtering. The dots denote the time dependent amplitude of the wave train calculated as described in the text. The single amplitude value used for the amplitude profiles in Fig. 10 is defined as average over the middle third T.-Tb of the time interval 7,-1"2 in which the

TID occurs.

(dots in Fig. 8) has been calculated by integration of the solid curve over time intervals of a half wave period at discrete time points. In the following, the single amplitude value of a wave train is defined as average over the middle third (Ta-Tb, Fig. 8) of the time interval (T~- Tz) in which the TID occurs. The

L J I upper I quartile quartile

mmplitude

Fig. 9. Scheme to explain the boxplot diagram used in Fig. 10.

amplitude behaviour of the 45 TIDs is illustrated in boxplot diagrams. A scheme to explain the boxplot is shown in Fig. 9, the diagrams themselves are depicted in Fig. 10. We chose this presentation because the observed amplitudes are spread over a wide range of values. The box contains the middle half of the measured amplitude values while the point inside the box indicates the arithmetic average of all amplitude values.

At first it is remarkable that there exist distinct average height profiles of the TID amplitudes since the boxes are relatively small compared to the difference between the maximal and minimal value. Further- more, the TID parameters show quite different height

"5

500

450

400 350

300

250

200

150

500

450

400

350

300

250

200

150

, I !

- - [ : : 3 . . . . . . . . . . . . . . . I

t. - c : : : a D . . . . . . . . . . . . . . i

~- < : : : ~ 3 0 . . . . . . . . . . . . . . i

L - - C::::3K3 . . . . . . . . . . . . . t

~ - - c : : : : : l& . . . . . . . . . . ,4

~-, t, .......................

~-i , . . . . . . . . . . . . . . . . . . . . . . . .

g ~ 3 1 . . . . . . . . . . . . . . . . . . . . . t

0 5 i0 15 2 0

Amplitude of N, [%]

I I I

i - - , , . . . . . . . . . . . . . . . . . . . . . .

e - - ~ m - ........

I- -- C:]CD---I

I* * -C3c] .....

5 0 0 , p I , [ 4 5 0 " ' = = . . . . . . . . . . . . . . . . . . . . . . . . ~--

r - - - : - - C 3 C : ~ . . . . . . . t

400 . . . . . . . . . . . . . . I-- '-=:~ . . . . . . . . . ,~ |

3 5 0 ~ = = . . . . . . . . .

r I - < C : : 3 : 3 . . . . . . . . . I

3 0 0 ' = ' =

.~ 2 5 0 . . . . . . . . I - - C ~ . . . . . . I

~= 2 0 0 - - - , :=~ ....... I .... , ! r ............ 4

t 5 0 . . . . , • , . . . . . . ; . . . . . . ; . . . . . .

0 i 2 3 4 5

A m p l i t u d e of T i [%]

5 0 0 ~ I i E l 450 .....

400 ......

,---.- 3 5 0 I . . . . i • ~ . . . . . . . . . . . . . . . . . . . "1

,.~ 3 0 0 . . . . . . . . - . . . . . . . . . . . . . . . . . . . I - - - , • . . . . . . . . . . . . . . . . . . . . . . .

i ~ O ~ - - , ~ , . . . . . . . . . . . . . . . . . . . . . . . 4

• ~ 2 5 0 , - - = = - . . . . . . . . . . . . . . . . . . . [ 2 0 0 . . . . . . . . . . . . . . . . . . . . . . . .

i . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 ,-,=o . . . . . 1 5 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0 5 10 15 2 0 0 1 2 3 4 5

Amplitude of V, [ m / s ] Amplitude of T, [%]

Fig. 10. Amplitude profiles of the TID parameters Are, ~, T~ and T~ (box plot explained in Fig. 9).

P-,..a

0.1

I z"

10

0.1

Ne: r - 0 . 9 7 m- ,0 .93

h , , 2 5 7 - 3 6 7 k m

I I I I

f


Tl: rmO.99 m - 0 . 4 7

h = 2 5 7 - 3 6 7 k m 1 0

- 0 . 1 I I I I I f t I 2 0 3 1 . 7 5 0 . 2 7 9 . 6 1 2 6 2 0 0 2 0 3 1 . 7 5 0 . 2 7 9 . 6 1 2 6 2 0 0

P e r i o d [rain] Per iod [min]

253

P~

q,1

I .,.,.,>-

10

Vi: r- ,0 .85 m , , 0 . 2 5 Te: r , ,0 .95 m - 0 . 6 3

h - 2 5 7 - 3 6 7 k m h - 2 5 ? - 3 8 7 k m

I I t I I I I I 10

i 1 J I

0 . 1 I I I ~ [ -" 0 . 1 I I 1 I ;)0 3 1 . 7 5 0 . 2 7 9 . 6 1 2 6 2 0 0 2 0 3 1 . 7 5 0 . 2 7 9 . 6 1 2 6 2 0 0

P e r i o d [ l ~ n ] Per iod [mia]

Fig. 11. Average amplitude spectra of the TID parameters No, Ti, V~ and To for the height range 257-367 km. The amplitude spectra (solid lines) are approximated by the straight (dotted) lines of regression, where

m is the gradient of the regression line and r the correlation coefficient.

variations of the amplitude. The average No amplitude has a maximum of about 5% at the height 210 km and decreases to 2.5% at 400 km, while the Vi amplitude increases slightly from about 3 m/s to 5 m/s in the same height range. At heights above 300 km the Te amplitude is almos~L height-constant and greater than the Ti amplitude which is around 1%.

The Vi amplitudes of our study are smaller than the amplitudes of 25 Vi-TIDs which were found in the EISCAT data of November 1984-December 1985 by Natorf et al. (1992), having a median amplitude of 6 m/s at the height 207 km. The difference between our and their value,; may be due to the fact that Natorf et al. (1992) were mostly interested in V~-TIDs while we looked for TIDs which appear as well as possible in all measured parameters. The amplitudes of our N¢-TIDs agree with the Are amplitudes observed by Sharadze et al. (1986) who measured electron density fluctuations with amplitudes between 1% and 15% at the F2 peak.

While the TID amplitudes summarized in Fig. 10

are compiled from TIDs with various time periods, we have also studied the amplitudes as a function of TID period. These amplitude spectra have been obtained by averaging the power spectra of the relative fluctuations with periods less than 180 min. The spectra of the gravity wave induced TIDs are well rep- resented by the spectra of the relative fluctuations because the selected data sets belong to a quiet daytime ionosphere which is permeated by gravity waves with various spectral components. The average amplitude spectra of the different TID parameters are illustrated in Fig. 11 for the height range 257-367 km. Here the amplitude spectra (solid lines) are approximated by straight (dotted) lines of regression, where m denotes the gradient of the regression line and r the correlation coefficient. The amplitude of Are depends almost linearly on the period Tp (ANo ~ T~v, m = 0.9) as explained by Hooke (1970) on the basis of the electron continuity equation. The amplitude of the field-aligned ion velocity Vincreases only slightly with period (AVi ~ T°p25). Since this amplitude spectrum

254 K. Hocke et al.

of V~ can be affected by plasma diffusion, it is not suitable to represent the spectral behaviour of the neutral velocity amplitude. On the other hand, the amplitude spectrum of the ion temperature (ATe~ 05 Tp" ) is closely related to the amplitude spectrum of the neutral temperature gravity waves by ion- neutral coupling. Therefore, the average neutral temperature spectrum, which is difficult to determine in the thermosphere by direct measurements, will be rep- resented by the Ti spectrum of the TIDs. The physical relationship of the plasma parameters to the neutral parameters is described in detail by Kirchengast et al.

(1995). Finally, the dependence on period of the amplitude

of the electron temperature Te (ATe 06 T p ) lies between that of AT~ (oc AT of gravity wave) and ANt: this indicates again the influence of just these parameters on the generation of the T~-TID.

5. CONCLUSIONS

The average amplitude and phase behaviour of TIDs in the quiet daytime high latitude F-region was derived using EISCAT data of 45 gravity wave induced TIDs. The average phase differences between the fundamental TID parameters N¢, Vj, T~ and To, and their phase profiles are well defined and can be used to identify gravity wave induced TIDs in inco-

herent scatter data (Fig. 7). Especially the TID parameters Ne, V~ and T~ are good tracers for gravity waves because they have a similar phase slope with height and show the typical downward phase progression of gravity waves. The characteristics of the Te-TID suggest that the Te-TID is induced by the Ne- TID at heights above 300 km, while for heights below 250 km the Te-TID is caused by the temperature fluctuation of the neutrals and ions.

The average amplitude behaviour (Fig. 10) charac- terizes additionally the gravity wave induced TIDs. The amplitude values of the different TID parameters, the amplitude relations between the TID parameters and finally the height dependences of the TID amplitudes provide useful information in order to distinguish gravity wave induced TIDs from other quasi- periodic perturbations.

As a result of the undertaken statistical study the typical response of the fundamental ionospheric parameters (Ne, V~, T~, Te) to the passage of gravity waves is now well established. The use of the incoherent scatter information of all TID parameters allows a sure identification of gravity waves in the thermosphere. Acknowledgements--The authors wish to thank the Director and staff of EISCAT for operating the radar and supplying the data. The EISCAT Scientific Association is supported by national scientific agencies of Finland (SA), France (CNRS), Germany (MPG), Norway (NAVF), Sweden (NFR) and the United Kingdom (SERC).

Bertin F., Kofman W. and Lejeune G.

Brace L. H. and Theis R. F.

Hamming R. W.

Harper R. M.

Hearn A. L. and Yeh K. C.

Hines C. O.

Hooke W. H.

Kirchengast G.

Kirchengast G., Hocke K. and Schlegel K.

Lanchester B. S., Nygrrn T., Jarvis M. J. and Edwards R.

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phases and amplitudes of tids in the high latitude f-region observed by eiscat

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