neutral dynamics of the high latitude e region from eiscat measurements: a new approach

~ ) Pergamon Journal of Atmospheric and Terrestrial Physics, Vol. 58, No. 1~4, pp. 121 138, 1996

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

00214169/96 $15.00+0.00 0021--9169(95)00024-0

Neutral dynamics of the high latitude E region from EISCAT measurements: a new approach.

W. Kofman, C. Lathuillere and B. Pibaret

CEPHAG CNRS URA 346 BP 46, 38402 St Martin d'H~res, France

(Received 27 October 1994; accepted in revised form 12 December 1994)

Abstract We have analysed the EISCAT data from two long campaigns, in October 1992 and in January 1993, in orde: to derive the three components (N S, E-W, vertical) of the neutral wind in the E region. The winds are derived using the simultaneously measured ion-neutral collision frequency. We calculate the tidal parameters of the measured winds and compare them with models. In addition the CP2 experiment allows us to determine the vertical wind in two independent ways. During the disturbed day of the October campaign we derived a very large vertical wind. Such large values are not reproduced in any model. In this article we discuss the validity of our results.

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

Studies of the neutral atmosphere and neutral dynamics using the incoherent scatter technique star- ted in 1967 (Carru et al., 1967). At mid-latitude regions studies were conducted using the St Satin, Arecibo and Millstone Hill facilities (Vasseur, 1969; Salah and Holt, 1974; and Alcayd6 and Bauer, 1977). The first studies in the auroral E region were made by Brekke et al. (1973) using the Chatanika radar. Comfort et al. (1976) and Johnson (1987) continued the studies of the neutral wind using the same facility. Using the multipulse technique (Kofman and Lathu- illere, 1985; Kirkwood, 1986; Williams and Virdi, 1989 ; and Kunitake and Schlegel, 1991) EISCAT has furnished high quality data. Alternating codes (Leht- inen and Haggstrom, 1987; Sulzer, 1986) have been used recently for these experiments, providing better time resolution and generally better data.

There are two :~tandard experiments using the EISCAT facility which give access to the neutral dynamics: Common Program 1 (CP1) and Common Program 2 (CP2) (Virdi and Williams, 1991; Williams et al., 1994). In this paper we use data obtained by the CP2 program using alternating codes, a technique which gives very good data with about 2 km resolution in height. This program is in our opinion the best currently experimental procedure for observing the E region neutral dynamics. CP2 consists of 4 position measurements obtained by moving the Tromsa antenna with a cycle of about six minutes. The antenna points Jin four directions: vertical, parallel to the magnetic field (B), south-east and south.

The last two positions have an elevation angle of about 60 degrees. These positions give the possibility of combining measured line of sight velocities in order to build the velocity vector.

One builds the ion velocity vector (Vi) from the best trihedral, which gives the smallest error. This best trihedral does not include the pointing direction parallel to B. Therefore one has two independent measurements Vi and gi//. These measurements are obtained as functions of altitude with 2 km resolution in the E region. The remote station antennas in Kiruna and Sodankyla follow the Tromso antenna at 278 km altitude giving, without any assumptions, sim- ultaneous and instantaneous measurements of the electric field. There are two assumptions required for building the vector in the E region, time stationarity on the scale of a few minutes and space homogeneity on the scale of 100 km. These assumptions can be checked by comparing the ionospheric parameters measured in different directions (positions). One can also build the electric field from trihedral data and compare it with the tristatic measurements. As we will show, the neutral dynamics can be studied very well with these data.

In addition to velocity data, one obtains the scalar ionospheric parameters in the E and F regions. These data were analysed deriving the ion-neutral collision frequency below 110 km in addition to normal ionospheric parameters (Lathuillere et al., 1983 ; Kofman and Lathuillere, 1985; Fla et al., 1985; Kirkwood, 1986; Huuskonen et al., 1986; and Kofman, 1992). This, as we shall show, is an important feature giving

121

122 W. Kofman et al.

the possibility of building an actual diurnal model which is used in the derivation of neutral wind. In this paper we show the results obtained during two long campaigns, one in October 1992 and the other in Jan- uary 1993 from the MLTCS (Mesosphere Lower Theremosphere Coupling Study) international days.

2. WIND DERIVATION TECHNIQUE IN THE E REGION.

To derive the neutral wind from the ion velocity vector, one solves the equation of ion motion. This equation, under assumptions of stationarity, is

1 Un = V i - B (v~i) (E +Vi x B) (1)

where Un is the neutral velocity vector, V~ is the ion velocity vector, E is the electric field, B is the magnetic field and ~'~i is the gyro frequency.

Usually the ion-neutral collision frequency v,, used in this equation is derived from an empirical neutral model. In this paper we derive it from the actual measurements. The collision frequency derived from the incoherent scatter spectra was averaged over 3- hour periods for each altitude between 92 and 108 kin. This procedure gives us a set of data ordered as functions of time over a 24-hour period for each day during the observations. Every averaged profile is fitted by an exponential:

Vin(t ) = Vin(100,t) exp (-- (z-- 100)/H(t)) (2)

where t is the time, z the altitude in km and H the scale height.

The ion temperature T is averaged in the same way and fitted for altitudes between 110 and 120 km by a quadratic form:

T(t) = T(110,t)+c~(t)(z- l l0)+/~(t)(z- 110) 2 (3)

In Fig. 1 we show an example of fitted collision frequencies and temperatures. The obtained parameters v~n(100, t), H(t), Y(110,t), ~(t), fl(t) permit us to build a time-dependent model in each case by fitting each of them with the function

A sin (2~/24(t- q~)) + b (4)

which implies only a diurnal variation. In Table 1 we summarise the parameters (A, ¢p,

b) of the model which is displayed for the October measurements in Fig. 2.

Finally our model of collision frequency is syn- thesised for altitudes between 92 and 120 km for each campaign. In the region between 92 and 110 km the exponential form (2) is used and between 110 and 120 km the hydrostatic extrapolation is used assuming

110

100

D

< 9O

CP2 921028 f rom 15ut to 18ut

10 a 104 105 Collisions Frequency (Hz)

8o02

140 I t I I I ( I 3 1 " ) ' ' ' ' : . . . . . ' ~ l I I -

TllO=332. . 130 slope=27 (1.4) .... '- ,, i~i :

Curve=l .2 (.13) . . . . . . . . E ~ "

120

g

= 110

100

9 0 , , ~ T ~ , m " ~ , , , , , , , , , , , i , , ,

0 200 400 600 800 1000 Ion Temperature (K)

Fig. 1. Examples of the collision frequencies and temperature measurements averaged over the 3-hour period between 15 and 18 UT and their fits. The fit parameters are written in

the figure.

that the neutral temperature is equal to the ion temperature described by equation 3.

During the campaign in January the magnetic conditions were quiet except for short periods of weak (> 10 mV/m) electric field which we took out in mod- elling. This means that our assumption of thermal equilibrium is correct because we eliminated the periods of Joule heating and anomalous electron temperature events (St Maurice et al., 1990, and references therein). In the October data, the extrapolation used to obtain the collision frequencies, assuming the ion temperature is equal to the neutral temperature, is perhaps more questionable due to the presence of stronger electric fields but we think that the differences are small, if they exist. To check this assumption we also derived the neutral wind assuming MSIS90 model (Hedin, 1991). Results of this comparison will be discussed in the conclusions. The use of the measured ion neutral collision frequency is a new important advance in the studies of the neutral dynamics because in the past models like MSIS have normally been used. As has been shown, uncertainty in this parameter can induce a large error in the neutral wind determination (Johnson et al., 1987, Kunitake and Schegel, 1990). In October the comparison between the measured v~n and that obtained from the MSIS90 model (see Lathu-

Neutral dynamics of the high latitude E region from EISCAT measurements

Table 1. Parameters of the model A sin (2~/24(t - q~)) + b

123

Collisions Temperature Coll,00 Slope T, L0 c~ fl

A 795_+244 0.4_+2 -22.3_+9 11.2_+4 q~ 9 . 3 _ + 1 . 1 10.3_+1.6 5.1_+1.4 9.2_+1.5 b 5241_+173 4.9+1.5 312.8_+6 8.4+3 A 997.4_+255 0.9_+0.5 0.0___5 0.0_+4.4 ~0 10.1_+0.9 9.2_+0.8 4_+5 10.3_+5.8 b 5987_+ 187 6.9+0.2 207.8_+5 14.4_+3

-1 _+ 0.4 10.6_+ 1.5 in October

1.2_+0.3 0.91 _+0.54 11.3 _+ 2.3 in January 0.1 _+0.4

illere et al., 1983) shows that the mean values are close but different by about a factor of two in January at 114 km (at 100 km the values for the two periods are

CP2 27 -29 Oc tober 1992

E 80oo I 6000 T

u .

~ 4 0 0 0

o i i i i I I I I , I . . . . I , , , , I , , ,

400

o

o 3 0 0

200

5

o

-5

I O0

5O

q bq

!

tE i i i = i . . . . I , , , , I , , , , I , , ,

t

o

, , , , I , , , , I , , , , I ~ I

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

5 10 15 20 TIME UT

Fig. 2. The obtained time-dependent model derived for the ion-neutral collision frequency under l l0 km and the ion temperature between 1 l0 and 120 km. The stars correspond to the parameters obtained as shown in Fig. 1 and the continuous lines to fits. The ion-neutral collision frequency

model from 90 to 120 km was built from the fits.

close but the scale heights are different, see Table 1 and Fig. 3). The amplitudes of the variations are larger in the measurements. Differences between measurements and the MSIS model were reported previously (Kofman et al., 1986; Kirkwood, 1986) and these differences were partly due to the fact that the data base used in MSIS in the auroral region was very sparse.

The other important factor in equation 1 is the electric field E which is measured by the tristatic system. Therefore, one has the measurements of E for every position. Figure 4 shows the electric field measured for one day during the October campaign. The very similar behaviour of the electric field observed in the four positions shows that at the dis- tance between the measurement points up to about 300 km, the electric field is the same. This is an additional proof that the assumption of homogeneity is correct, at least in our case. This assumption is necessary in order to build the vector of ion velocity from 3 measurements of the line of sight velocity. Therefore, in the calculations of the neutral wind (equation 1) we used the electric field averaged over the 3 positions. We show neutral wind in geographic coordinates.

A different approach is used in the derivation of the meridional wind in the plane that contains B and the vertical direction. The study of the stationary equation of ion motion shows that one can derive this component of the wind only from the ion velocity parallel to the magnetic field Vi//(Wickwar, 1989 ; Lilensten et al., 1992; Kofman, 1992.).

U m e r = - V i . , / c o s I + V d i f f t a n I + U v t a n I (5)

where Va~ff is the ion diffusion velocity, I is the dip angle, Umer is the meridional velocity and Uv is the vertical velocity. This equation states simply that the observed ion velocity Vi,/is mainly a projection of the neutral meridional and vertical winds.

The diffusion velocity in the E region (100-120 km) for quiet conditions is much less than 1 m/s. During

124 W. K o f m a n et al.

120

~15

110

105

i00

1 2 0

115

110

105

100

l o d e l o f COLLISION OCTOBER 9 2

. . . . . . . . . . . . ~2-22 ..... _2 . . . . ~ - -

~ ~ ~ I ~ ~ r ~ l . . . . I . . . . I , ,

0 5 10 15 2 0

M o d e l of COLLISION MSIS

120

115

110

105

100

0

Mode l o f COLUSION JANUARY 93 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' '

5 10 15 2 0

M o d e l of COLLISION MSIS

_-7__20°0% . . . . . . . . 15001 . . . . 25001

- - --35001 - - - - - 45OOl - - - - 5 5 0 0 i 6 0 o o

, , , , , , , , , ' ' ' ' ' ' ' ' ' ' ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' '

115

110

105

- , , , , I . . . . L , , , 100 -=

0 5 10 15 2 0 0 5 10 15 2 0 TIME UT TIME UT

Fig. 3. C o m p a r i s o n o f the m e a s u r e d col l is ion f requencies a n d those o b t a i n e d f r o m M S I S 90 mode l .

E 50

_J ILl ,-r o f_)

t~ - 5 0 . J

b . I

~osition 1 • . ,,~ ,,~

k*l'oX~l~.l,=,,,l~'._ll I,~ ¥ lh , . !~ :

E

a ._1 Ld b -

n-"

IL l J W

50

0

- 5 0

, , l , t l , l , , , , l , , , I , , ,

0 5 10 15 20

position 3 I

"~i i , , I , i i i , , , , I , , , , I , ,

0 5 10 15 20 EIN TIME UT

. . . . . . . . . Ej. E

92 october 28 CP2

~ position 2 1

o -"---"~=* ,-~.r '

0 5 10 15 20

)osition 4

|

a~ ~]l~d.~.,.~ . . . . . .

50

0

- 5 0

0

, , I , , , , I , , , , I , , , , I ,

5 10 15 20 • ME UT

Fig. 4. Elect r ic field o n O c t o b e r m e a s u r e d f r o m t r is ta t ic d a t a for e ach a n t e n n a pos i t ion .


extremely disturbed conditions (characterized by Joule beating and precipitation) the diffusion velocity can increase at 120 km up to a maximum of 3 m/s. For this reason this term is neglected in equations 1 and 5. In addition, in equation 5 one usually supposes that the vertical velocity is zero. As we shall show the latter assumption can be wrong in the presence of a strong electric field, especially for altitudes higher than 110 km.

3. NEUTRAL WINDS RESULTS.

The campaigns from 27 to 29 October 1992 (Ap=31, 19, 27) and from 20 to 24 January 1993 (Ap= 14, 7, 7, 4, 113) were analysed. During October 27 and 28 a strong electric field (10 to 50 mV/m) lasting for about 10 hours each day was measured by EISCAT. In the data analysis we took into account all the data. During the January campaign the ionosphere was much quieter with only a few short periods of electric fields larger than 10 mV/m. As this experiment was of long duration and we were looking for quiet period measurements we took out the data corresponding to large electric field in our analysis (corresponding to calculations of collision frequency model and wind derivation). In this case the meridional wind obtained from V~// (equation 5 and Vda~= 0, Uv = 0), without any elimination of data, has to be close to the north-south wind obtained from the first trihedral. This is because the vertical wind under quiet conditions is surely very small and the ion velocity measured parallel to B is not perturbed by a perpendicular electric field. We will come back to this problem in the next section.

In the colour Fig. 5 and Fig. 6 we show the three components of the neutral wind derived respectively from the data of October (24 hours of data) and of January. In order to see better the tides, for the pur- poses of this figure only, we show the 24-hour average over the campaign in January; this is because the data in January are noisier than in October. In Fig. 5 one can see, in the top panel corresponding to the north- south (N S) wind, a clear indication of oscillations of about 12-hour period (time gap from blue to blue colour or yellow to yellow between 100 and 110 km). The visible change as a function of height of the maximum amplitude indicates that these oscillations are tides. For the altitudes between 112 km and 120 km and for the time between 9 UT and 20 UT there is an intensification of the wind. This period cor- responds to a strong electric field. The east west (E- W) component behaves in the same way (middle panel). The bottom panel shows the vertical wind.

125

One can see also a tidal behaviour, but it is much noisier. There is also an intensification of the vertical wind during the period of strong electric field.

In Fig. 6 one can find the same tidal motion as during the first experiment. The vertical component is different and shows essentially no winds or very small ones which can be hidden under the noise.

4. TIDAL ANALYSIS

For tidal studies we fitted the three components of the neutral wind and the meridional wind with a sinusoidal model derived with the assumption of vertical wind equal to zero:

A24 sin (2~/24(t- ¢P24)) + A~2 sin (2~z/12(t- ~0~2))

+As sin (2g/8(t-~o8)) (6)

In the fitting procedure we used the all days from October and January (unaveraged) and the data were weighted by the inverse square of the error.

To indicate the fit quality we show in Fig. 7 the fit of the data at 105 km altitude. In Figs 8-11 we plot the tidal parameters as a function of altitude. We plot, in addition to the three components of the neutral wind, the meridional wind derived using the equation 5 assuming vertical wind and diffusion velocity equal to zero.

The largest amplitude of horizontal winds cor- responds to a semi-diurnal tide. This amplitude maxi- mises (100 m/s) at about 110-115 km in October. In January, the semi-diurnal components maximise at about 105 km altitude with values between 50 and 70 m/s. For both campaigns the diurnal components are weaker than the semi-diurnal ones with a maximum at or above 115 km. The 8-hour components are weak- est. The vertical components are weak, but non zero, in January, and much stronger, essentially between 110 and 120 km, in October. The phase behaviour is complex but the semi-diurnal tide shows a decrease from a phase of 5 UT (6 LT) for N S wind and from 10 UT (11 LT) for E-W wind in January. In October the phase behaviour is similar.

The average wind shows clearly an altitude depen- dence with sign and gradient changing below 110 km. In October this mean wind (N-S and E-W) shows very large values (50-100 m/s) above 110 km. The values below 110 km are smaller and close to the values during the quiet days of January; in October the mean wind is eastward while in January it is in the south-east direction. The October vertical wind exhibits also a very large value of about 15 m/s in contrast with the January vertical wind.

Now we will compare the tidal behaviour of the


400

200

- 2 0 0

921027-28 a l t= 104.9

x4=81.32 err4=7.22 xl =-5.68 err1 =4.57 × ×

x5=-0.59 errS=0.13 x2=-20.86 err2=6.83.

x6=18.92 err6=6.40 x x3=15.96 err3=1.19 x x × × ×

x7--3.24 err7=0.37 x x × x x

X X X ~<X X ~ vX~ X X

x × ~ - - ~ × x x x × x ~ x Xx x × J " x x x x x" x ~ ' ~ x x x X x x j x x ~ & , ~ z - , ' ~ x x x x/f.. . . . . . . ~ ' ~ x X ~x x x ,,x "4

" ~ ~'~^" v~'~ .~ ~ x ~<,~ ~ v "~,7 x x x x x~x .~I

X X X × XX X xx ~ × x~ x -

×X X X ~)( XxX

X X X

x

i i I I I i i i i I I i i i I i t i i I i i i

0 5 10 15 20 TIME UT

- 4 0 0

Fig. 7. An example of the meridional wind at 105 km, fitted by a tidal model. The uncertainty on each point is about 25 50 m/s.

meridional wind and the N-S component of the neutral wind. As was outlined previously they should in fact be equal (to a very good approximation). One can see that the tide's amplitudes and phases are very close in January. In October there are large differences between the semi-diurnal amplitude for the altitudes higher than 112 km. The diurnal component is also different but in this case the altitudes range between 105 and 115 km. The explanation of the different behaviour in October and in January is that the vertical wind was probably induced by the electric field energy input. An estimate of the meridional wind Umer was derived from field aligned velocity assuming that both diffusion and the vertical wind was negligible. The fact that Umer was derived from measurements of Vi//offers the possibility of measuring the vertical wind in a new and independent way. From the trihedral measurements of ion velocity the true value of the meridional wind UN-s was derived using the measured collision frequency. The vertical wind ignored in equation 5 can then be equated to (UN s -UmJ ctg I. This derivation is independent of the previous one in the sense that V~//is independent of the data used to calculate UN-s. In fact, the two vertical winds are not completely independent because the UN s wind was

obtained using the measured collision frequencies and, if they are in error, this will propagate in the determination of (Us ~Umer).

In Fig. 12 we show the two derivations of the vertical wind. In this figure we plot the winds in October built from tidal parameters. The fitted image in Fig. 12a compares very well with the raw image in Fig. 5c. One can see the intensification of the wind between 112 and 120 km. The comparison between Fig. 12a and b shows clearly that the two derivations of the vertical wind are very close. The values of this wind are large (20-30 m/s). We will discuss this point in the conclusion.

5. C O M P A R I S O N W I T H M O D E L S A N D P U B L I S H E D

D A T A

The first comparison which we will make is with the semi-empirical HWM93 model (see Figs 8-11) (Hedin et al. , 1994). In January, the mean E-W wind of the model and that derived from the measurements are close above 106 km. However, the N-S mean winds show significant differences, with the model showing smaller velocities, close to zero, and only small variations with altitude. In October the situation is slightly

Neutral dynamics of the high latitude E region from EISCAT measurements 127

120

110

100 400

CP2 28 O c t o b e r 1992

200

0 0

120 5 I0 15 20

i

0

~>

110

100

-200

0 °~,,I

120

ii0

100

0 5 i0 15 20

50

0

0 5 i0 15 20

TIME UT Fig. 5. The neutral wind measured on 28 October.

09

.a

o

~>

120

09

09

;>

u]

V

aJ 0

o r ' - I .,.z

> ;>

CP2 2 0 - 2 4 J a n u a r y 1993

110

100

120

110

100

120

110

100

400


0 5 10 15 20

0 5 10 15 20

0 5 10 15 20 TIME UT

Fig. 6. The neutral wind averaged over 24 hours for the whole January campaign.

200

0

- 2 0 0

400

200

0

50

- 2 0 0

0

i il; ̧ ! ~

i - 5 0


1 3 0

~ 1 2 0

110

:~ lOO

CP2 2 7 - 2 8 O c t o b e r 1992

North

~,,I,,,, - 5 0 0 5 0

Mean Wind (m/s)

East

- 1 0 0 - 5 0 0 50 Mean Wind (m/s)

Vertical

, l,,i I,, , I,,

- 2 0 0 2 0 M e a n Wind ( m / s )

24h

,--'-'130~ 120 ~r'Z ~-~~ - ,,

11o

1 0 0 : , I

1 3 0 ~

~ 1 2 0

1 0 0 '

~ 130

v 120

110

100

- - - . 1 3 0

120

~ 110

100

I * l l

, I , , , , I , , , , I ,

T , I , i , , i . . . . I ,

-l' 0 50 100

A m p l i t u d e ( m / s )

12h

" I I I I

- = ~- ~ ~ 9 3

-?7 , I . . . . I ,

- ; I ~ a j ~ I I

, I ~ 1 0

I " il

- , I , , , a I , , , i l i

50 100 A m p l i t u d e ( m / s )

8 h

, I i

N o r t h (~)

N o r t h

(b)

-: S , I , , , , I , , , , I ,

- E a s t

, I , , I I I i , i i I ,

- Ver t i ca l

| i

, I , , , , I , , , , I ,

0 50 100 A m p l i t u d e ( m / s )

Fig. 8. Results of the tidal analysis for the October campaign. The top panel shows the mean wind in the N-S, E-W, and vertical directions. The second row of panels (North a) shows the three tidal components (24 h, 12 h, 8 h) of the meridional wind derived from V~//. The following panels (North b, East, Vertical) show the tidal components of the neutral wind derived from the trihedral. The difference between local time (LT) and universal time (UT) is LT UT = 1 hour. The dashed lines show the corresponding data

obtained from the HWM 93 model.

8 h

130

, .~ 1 2 0

110

lOO

1 3 0

- ~ 1 2 0 0

110

lO0

130

~ 1 2 0

~ 110

100

1 3 0

' ~ 1 2 0 ID

.~ 110

100

2 4 h

¢

i i i I i i I

- , , , I , , , , I , , , , I , , , , I , ,

- - I

- , , , I , , , , 1 , , , , I , , , , I , ,

-,,,l,,,,l,,,,l,,,,l,,

- I 0 0 I 0 2 0

P h a s e ( u t )

CP2 2 7 - 2 8 O c t o b e r 1992

12h

~'I I I I I , , , I

' - - X ~ ~ H W M 9 3 ' - -

i l l I I l l l l l

I I I l l l l l

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

0 10

P h a s e ( u t )

N o r t h

i i i I i i i


l,,,,l,

N o r t h

(b)

I I I l l l l

E a ~

~'1 , , , , I , , , , I , , ,

V e r t i c a l

-

l , , , , I , , , , I , , ,

0 5 10

P h a s e (u t )

Fig. 9. The phases corresponding to the maximum of each tidal component in October. The description of the panels is the same as for Fig. 8.

different. Below 110 k m the N S winds in b o t h the m o d e l and the m e a s u r e m e n t s are close. The E - W m e a n c o m p o n e n t o f the measu red wind is larger t h a n the mode l . A b o v e 110 k m the differences are large for b o t h c o m p o n e n t s and the variabi l i ty is also different .

This is p r o b a b l y due to the fact tha t in O c t o b e r the local electric field was s t rong for several hou r s and this is n o t r ep resen ted in the global H M W 9 3 mode l .

The semi-d iurna l c o m p o n e n t o f the H W M 9 3 m o d e l a n d tha t der ived f rom the m e a s u r e m e n t s are i mpor -


130

~ 1 2 0

~ 110

100

CP2 2 0 - 2 4 J a n u a r y 1993

N o r t h

- 2 0 0 20 Mean Wind ( m / s )

E a s t

- 2 0 0 20 K e e n Wind ( m / s )

Ver t i ca l

- 2 0 0 20 ~ e a n W'md ( m / s )

.-. 130

"" 120 0

-o ~ 110

100

,..., 130

~ 1 2 0

"o 110

100

,-. 130

"~ 120 "o ~ 110

100

.~ 130

1 "~ 120 ~o ~ 110

100

2 4 h 12h 8 h

! - > - -

: , l ~ b , , , I , , , , I , : , l ~ . . . ¢ r " T ' ~ , , I , , , , I , :

- ' - - : " ~ ~ 9 3 ' - -

~ N o r t . h

~~_ (a) N o r t h

, l I ,

/ I I I I I

0 , I , , , , I ,

50 100 k m p l i t u d e ( m / s )

I J | I t

I I I

I ,

I ,

, , , , I , , , , I

0 50 100 A m p l i t u d e ( m / s )

(b)

, I , I . . . . I ,

-- East -S , , I . . . . I ,

- Ve r t i c a l

0 50 100 A m p U t u d e ( m / s )

Fig. 10. Results of the tidal analysis of the January campaign. The description of the panels is the same as for Fig. 8.

132 W. Kofman etal.

130

~ - ' 1 2 0

110

lO0

1 3 0

- ~ 1 2 0

110

100

130 120

110

lO0

130

' ~ 1 2 0

~ 1 1 0

lO0

CP2 20-24 January1993

24h 12h

- - x x ~ H ' W ' M 9 3

~ , l , , , , i ,,,I,,

"lI,,,, ,I

I , , I

0

, I , , , , I , , , , I ,

10 2 0 3 0 - 1 0 - 5 0 5 10

P h a s e ( u t ) P h a s e (u t )

8 h

, I , , , , I , H , I ~ , ,

- - N o r t h (b)

i I i I I i

- East

I l I a I

- Ver t i ca l

,I,,,,I

-5 0 5

Phase (ut)

Fig. 11. The phases corresponding to the maximum of each tidal component in January. The description of the panels is the same as for Fig. 8.

tant . In January, the m ax i m um ampli tudes of the N - S and the E W componen t s are similar but the alt i tude of the max imum is higher in the model (maximum at abou t 118 km). The zonal and meridional winds maximise at the same heights in the model. In October

the alti tudes of the max imum (about 116 km) are close but the ampli tudes are much larger in the measurements (the model gives abou t 40 m/s for bo th components). The phases are close in January. The model in October indicates a shift of abou t one to

CP2


9 2 1 0 2 7 - 2 8 Vvert (trihedral) (a)

120

115

110

105

100

120

115

110

105

100

40

20

0 5 10 15 20 Vvert (trihedral-//) (b) 0

- 2 0

133

0 5 10 15 2 0 TIME UT

Fig. 12. Neutral vertical wind built from tidal parameters. In Fig. 1 la we show the wind obtained from the trihedral and in 1 lb the wind obtained from the difference between the meridional wind (derived from V~,,)

and the N-S component (see in the text).


t~

120

110

i00

120

110

I00

Northward

\~, model 10/92

----------~ ~'\ Forbes & Vial <..\,, ", , ,

, I , , , , I , ~ , ~ , ~ , I , 0 5 1 0

Phase (LT)

k

2 /

] I

0 5 0 t O 0 Amplitude

SEMI-DIURNAL TIDE

1 2 0

1 1 0

1 0 0

120

110

100

Eastward

, I . . . . I . . . . - "~-, ~, 0 5 10

Phase (LT)

\ /

/ \ / / /

/

, I ~ l ,

0 5 0 1 0 0 Amplitude

Fig. 13. The, comparison of the measured semi-diurnal tide and the Forbes and Vial (1989) model. The dashed lines correspond to October and the continuous lines to January. The model data are with stars

and measurements without.

two hours to earlier time, which is not seen in the data. The comparison with the Forbes and Vial (1989)

model results is shown in Fig. 13. This model was computed for the latitude of Tromso for October and January. As one can see the semi-diurnal component compares very well with the model. Modeled and measured amplitudes are very similar and peak at about the same heights in October and in January. The difference between the E-W and N-S phase components is constant at about 3 hours in the model but is more variable in the January data approaching 3 hours over the range 105 and 115 km. This difference is almost 3 hours in October. The difference in phases between October and January seen in the model are not observed in our data. We note that the seasonal variation in this model is in the opposite direction to the HWM93 empirical model.

In fact, the comparison with the model (Forbes and Vial, 1989) is surprisingly good. The same heights of the maximum amplitude and about the same vari-

ations of the phase seem to indicate that the semi- diurnal component is well modelled, at least for our measurements.

A tidal analysis of the neutral wind in the auroral region has been made by other authors (Johnson et al. , 1987 ; Salah et al. , 1991 ; Johnson and Virdi, 1991 ; Virdi and Williams, 1991 ; and Brekke e t al. , 1994). The published data for different seasons and days show some differences which are to be expected due to different magnetic conditions during the measurements and often different locations. The amplitudes compare well for quiet days which is the case of the January campaign. For instance the data published by Salah et al, (1991) corresponding to the LTCS period in September 1987 show a behaviour similar to our January data except for the mean northward wind. The behaviour is different in October. The amplitudes are much larger, going up to 100 m/s for the diurnal and semi-diurnal components. We think that these are due to the large electric fields during

136 W. Kofinan et al.

this day. The good correlation between the increase of the meridional and zonal components of the wind and the electric field is seen in the data from Son- drestrom measurements during the LTCS campaign. Johnson (1991) suggests that the influence of the Joule heating may be important in the determination of the neutral wind. The same Joule heating may be important in the determination of the large vertical wind.

6. DISCUSSION AND CONCLUSION

The main advance in our analysis compared to those previously published consists in the use of the measured collision frequency. We built a diurnal model of the ion-neutral collision frequency and the ion temperature from measurements, and fit it with a sinusoidal function of 24-hour period. One could probably use a different function, but it will not essentially change the results. Neutral winds were derived from the measurements using our model of the ion neutral collision frequency and the ion temperature. To check the influence of the collision frequency model we derived winds with collisions obtained from the MSIS90 model. The winds obtained are of course slightly different but the average tidal behaviour is the same.

During the long campaign in January, the observed horizontal, meridional, and zonal winds show a typi- cal behaviour which we have correctly reproduced with our tidal model. The measured mean vertical wind is small, changing from about -2 m/s to 1 m/s as a function of height, with errors of about 1 m/s. The model reconstructed from the tides also shows a very small vertical wind.

The October campaign is different. The electric field is larger and the winds are larger especially during the periods of intense electric field. Strong horizontal winds have been observed previously by other

authors, but the very large vertical winds that we have observed ( > 20 m/s) are not reproduced in theoretical models. We checked to see if the spatial homogeneity assumption is met for these measurements. Figure 4 shows that the ionosphere is homogeneous in 4 directions as is the electric field. It is also possible that the assumption of equal electron and ion temperatures, which is required to derive the collision frequency from the incoherent scatter spectrum below 110 km, is not valid for these measurements. This equality in the presence of the strong electric field may not be met due to anomalous heating (T e > Ti). This heating is known to maximise at about 112 km and to occur mainly for the electric fields larger than about 50 mV/m. This means that this effect is negligible in our case. The winds calculated using the MSIS90 model collision frequencies are weaker but still larger (> 10 m/s) than those predicted by models. As we discussed previously, one could include in the derived vertical wind the diffusion velocity but this velocity is much less than 1 m/s for quiet days and less than 3 m/s for disturbed days.

We have, in some way, two independent measurements of the vertical wind showing large values during disturbed conditions. We checked the validity of the assumptions used in the wind determination and we believe that in the region between 110-120 km the vertical wind can be large, depending on energy input, larger than that predicted by models (empirical and theoretical). Additional measurements by incoherent scatter radar and independent observations by inter- ferometers are necessary to confirm such large vertical winds during disturbed conditions.

Acknowledgements The authors appreciate discussions with Dr F. Vial, and thank Drs Forbes and Vial for providing the data from the model. The EISCAT facility is supported by the research councils of Finland (SA), France (CNRS), Germany (MPG), Norway (NAVF), Sweden (NFR) and the United Kingdom (SERC). This work was supported by GDR PLASMAE.

Alcayd6 D. and Bauer P.

Brekke, A., Doupnik J.R. and Banks P.M.

Brekke, Nozawa A., S. and Sparr T.

Carru H., Petit M., Vasseur G. and Waldteufel P.

Comfort R.H, Wu S.T. and Swenson G.R.

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