Meridional thermospheric winds over the Antarctic Peninsula longitude sector

Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 305 – 314

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Meridional thermospheric winds over the Antarctic Peninsula longitude sector A.J. Foppianoa;∗ , X.A. Torresb , M.A. Arriagadab , P.A. Floresb a Universidad

de Concepcion, Casilla 160-C, Concepcion, Chile del Bo-Bo, Casilla 5-C, Concepcion, Chile

b Universidad

Received 2 July 2001; received in revised form 19 June 2002; accepted 27 September 2002

Abstract Diurnal variations of the magnetic meridional component of the thermospheric neutral wind have been derived for Port Stanley (51:7◦ S; 57:8◦ W), King George Island (62:2◦ S; 58:8◦ W) and Argentine Islands (65:3◦ S; 64:3◦ W). A servo-model based algorithm and two semiempirical procedures are used with ionosonde data input. Derived winds correspond to conditions of low geomagnetic activity and both low and high solar activity for months representative of autumn, winter, spring and summer. The shapes of the diurnal variations determined using the three di7erent methods for a given location and month are fairly similar for all three locations and most months. Wind velocities at a given hour, however, may generally di7er by 100 m=s and by up to a few hundred m/s in a few cases. The amplitude of the diurnal variations is larger in winter than summer for all latitudes and at both low and high solar activity, and generally larger at low solar activity. Daily mean winds are poleward in winter and equatorward in summer at low solar activity for all latitudes but are poleward all the year round during high solar activity, and stronger during winter. There is almost no latitudinal dependence of the daily mean winds in summer and autumn, with velocities being less than about 25 m=s. By contrast, winter daily mean winds change with latitude, particularly during low solar activity. Moreover, there seems to be a latitude at which the mean wind velocity is minimal both at low and high solar activity. A Fourier decomposition of the diurnal variations for all cases shows that all variations can be reproduced using at the most the :rst three Fourier components. Results are discussed with reference to other published wind determinations for the Antarctic Peninsula area and for the New Zealand longitude sector, and with reference to winds derived using the well-known HWM93 empirical model. c 2003 Elsevier Science Ltd. All rights reserved.  Keywords: Thermosphere; Antarctic Peninsula sector; Neutral winds; Ionosonde data

1. Introduction During the past few years several studies have been published dealing with thermospheric winds in the Southern Hemisphere. These have been welcome since most other studies deal with Northern Hemisphere locations. The present paper studies the latitude dependence of the winds for three locations in the Southern Hemisphere near a

∗ Corresponding author. Tel.: +56-41-203083; fax: +56-41220104. E-mail address: [email protected] (A.J. Foppiano).

longitude sector where unusual e7ects may occur because of the presence of the South Atlantic Anomaly (SAA) in the geomagnetic :eld. The present paper is considered signi:cant for several reasons. First, it extends latitudinally, results reported by Arriagada et al. (1997) for King George Island, because it uses higher and lower latitude locations. Second, three di7erent methods of determining winds are compared. This makes it possible to compare observations at Argentine Islands published by Dudeney (1973, 1976) and Dudeney and Piggott (1978), and Port Stanley reported by Canziani et al. (1990). Finally, this paper contrasts the observations it reports with those corresponding to a

c 2003 Elsevier Science Ltd. All rights reserved. 1364-6826/03/$ - see front matter
PII: S 1 3 6 4 - 6 8 2 6 ( 0 2 ) 0 0 2 8 7 - 0

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A.J. Foppiano et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 305 – 314

Table 1 Locations Port Stanley (PS) Geographic, ◦ S; ◦ E Geomagnetica , ◦ S; ◦ E L-value DIP, ◦ Declination, ◦ a Corrected

51.7;

King George Island (KGI) 302.2

37.28;

Argentine Islands (AI)

62.2;

10.58

301.1

47.04;

65.2;

11.20

1.64 49.4 4.60

295.7

49.78;

8.70

2.24 56.6 11.19

2.49 59.0 16.32

geomagnetic co-ordinates at a height of 250 km.

Table 2 Time intervals Low solar activity (1986)

Autumn Winter Spring Summer

High solar activity (1989)

Autumn Winter Spring Summer

March May June September October November December

9, 14, 5, 1, 10, 8, 3,

10, 15, 6, 3, 11, 9, 5,

11, 22, 12, 4, 12, 19, 6,

20 28 15, 6, 26 21, 8,

16, 7,

19, 8,

20, 9,

23, 10,

25, 16,

26 22,

22 12,

15,

17,

28,

29,

30

April May June August September October November December

12, 8, 5, 1, 3, 4, 15, 6,

19, 9, 17, 2, 11, 5, 16, 8,

21, 10, 18, 3, 14, 13, 22, 9,

22, 11, 21, 5, 17, 14, 23, 10,

23, 22 22, 24, 20, 15 25 11,

23, 25, 23,

25, 26 24,

26,

27,

28

25,

27,

28

12,

13,

18,

19,

20

di7erent longitude sector reported by Titheridge (1993, 1995). 2. Data analysis and sources of error Diurnal variations of the magnetic meridional component of thermospheric neutral winds have been derived for Port Stanley (PS), King George Island (KGI) and Argentine Islands (AI) corresponding to geomagnetically quiet days (daily Ap ¡ 10) during Southern Hemisphere autumn, winter, spring and summer for low and high solar activity. Geophysical information and speci:c times used are given in Tables 1 and 2. F-region peak heights (hmF2) were :rst determined for each day and location using a well-known empirical equation (Bradley and Dudeney, 1973; Eyfrig, 1974) applied using foF2, M(3000)F2 and foE values scaled from ionograms. When foE was not observed, it was calculated from a modi:ed version of the CCIR formula (Buonsanto and Titheridge, 1987). Diurnal variations of foF2 for September 1986 and

30

24

1989 were carefully examined to con:rm all diurnal variations were similar and corresponded to the days prior to the well-known winter-to-summer transition (Dudeney and Piggott, 1978). The accuracy of hmF2 values derived from empirical formulae have been extensively discussed previously (e.g. Dudeney, 1974, 1976, 1983; Berkey and Stonehocker, 1989). In any case, hmF2 errors are considered to be a few tens of kilometers. No attempt to derive hmF2 from ionograms using the inversion technique was made, as it has been done in other studies when quality digisonde ionograms were available, and for which the error in hmF2 is about 15 km (e.g. Dyson et al., 1997). Secondly, mean values of hmF2 corresponding to geomagnetically quiet days for each hour were determined. Finally, for each month and location hourly values of the magnetic meridional component of the thermospheric neutral wind were derived from the hmF2 values using three di7erent algorithms: a servo model based algorithm (Rishbeth, 1967; Rishbeth et al., 1978; Buonsanto, 1986; Buonsanto et al., 1989), the method of Miller et al. (1986, 1993) and an improved version of this method developed

A.J. Foppiano et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 305 – 314 June

Equatorward wind, m/s

200

September

307

December

100

0

-100

-200

0

6

12

18

0

6

12

18

0

6

12

18

24

Local time (60°W), hours

Fig. 1. Diurnal variations of the magnetic meridional component of the thermospheric neutral wind velocity (equatorward positive) corresponding to 10 geomagnetically quiet days (daily Ap ¡ 10) for low solar activity, 1986 (F10:7 Iux ¡ 72:1), using three di7erent techniques. June (winter) at Argentine Islands (AI, 65:2◦ S; 295:7◦ E geographic, −49:78◦ ; 8:07◦ corrected geomagnetic). September (spring) at Port Stanley (PS, 51:7◦ S; 302:2◦ E geographic, −37:28◦ ; 10:58◦ corrected geomagnetic). December (summer), at King George Island (KGI, 62:2◦ S; 301:1◦ E geographic, −47:04◦ ; 11:20◦ corrected geomagnetic). (Full line) Servo model Buonsanto et al. (1989). (Dotted line) Miller et al. (1986). (Dashed line) Richards (1991).

by Richards (1991). The use of the servo model, as used by Buonsanto et al. (1989) [hereafter referred to as B] for previous wind determinations over King George Island was reported by Arriagada et al. (1997). The use of the Miller et al. (1986) [M] and Richards (1991) [R] methods for campaign observations over Southeast Australia were reported by Dyson et al. (1997). The accuracy of servo model velocities is considered to be between ±20 and ±40 m=s. Accuracy of the other two methods is considered to be similar. A 3-h running mean was used to smooth the wind velocity data sets to reduce oscillations of periods less than about 2 h. 3. Results Fig. 1 shows three out of the 21 calculated diurnal variations of the magnetic meridional component of the thermospheric neutral wind during low solar activity (1986) using the three di7erent techniques. The results presented are for June at AI, September at PS and December at KGI. It should be noted that wind velocities are plotted on a common local time axis, since the largest local time di7erence between the three locations is only about 26 min. For a given month, and all locations, the shapes of the diurnal variations derived using the three di7erent techniques are similar, except at PS during June. Moreover, no signi:cant phase di7erences are observed. Wind velocities at a given hour, however, often di7er by 100 m=s and by up to a few hundred m/s in a few cases. It should be noted that M and R equatorial wind velocities around mid-

night are systematically larger than those of B. Furthermore, this di7erence between the methods seems to increase with increasing latitude for all seasons, being largest during summer. Although the amplitudes of the diurnal variations given by the three methods for a given location di7er signi:cantly for each season, all three methods consistently give larger amplitudes for winter than summer. All three methods give mean wind velocities which systematically change from large poleward to equatorward as season changes from winter to summer at PS and KGI. Moreover, at AI a systematic change is also observed with equatorward mean-wind velocities increasing from winter to summer. These same three methods were used for a few cases out of the 24 corresponding to high solar activity (1989). The wind velocities given by the three methods are also similar to each other as were those for low solar activity. However, for the rest of the present study, only wind velocities determined using the B technique are considered. This is because the B technique is simpler to use, the results are more stable from one hour to the next and because it is hard to ascertain whether one of the techniques gives the more realistic winds. Figs. 2 and 3 show sample diurnal variations at PS, KGI and AI for June, September and December in 1986 and 1989, respectively. Fig. 4 shows the variation of the amplitude of the diurnal variation and the mean wind velocity for all months considered. Fig. 5 shows the latitudinal dependence of the mean wind velocity also for all months. The results are organised to highlight the seasonal and solar activity dependencies of the latitudinal variation.

308

A.J. Foppiano et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 305 – 314 June

100

September

December

0

-100

PS

-200

Equatorward wind, m/s

100

0

-100

KG I

-200

100

0

AI

-100

-200

0

6

12

18

0

6

12

18

0

6

12

18

24

Local time (60°W), hours

Fig. 2. Diurnal variations and Fourier components of the diurnal variations of the magnetic meridional thermospheric neutral wind velocity (equatorward positive) corresponding to 10 geomagnetically quiet days (daily Ap ¡ 10) of June, September and December for low solar activity, 1986 (F10:7 Iux ¡ 72:1) using the servo model technique of Buonsanto et al. (1989). Port Stanley (PS). King George Island (KGI). Argentine Islands (AI). (Full line) diurnal variation. (Short dashed line) diurnal mean wind velocity plus diurnal component. (Long dashed line) diurnal mean wind velocity plus diurnal and semidiurnal components. (Long short dashed line) diurnal mean wind velocity plus diurnal, semidiurnal and terdiurnal components.

It can be seen that the shapes of the diurnal variations for low solar activity (Fig. 2) are very similar at all three latitudes for all months, except for June at PS. Note that the June wind velocities between 6 and 14 h are slightly different to those reported by Arriagada et al. (1997) due to a slightly di7erent smoothing procedure. The wind velocity changes from maximum equatorward velocity to maximum poleward velocity in less time than for the corresponding change from poleward to equatorward. Thus, generally the shape seems to depend neither on latitude nor on season. However, the small secondary maximum observed in the afternoon occurs earlier with increasing latitude. For high solar activity (Fig. 3), there is again generally no latitudinal dependence of the overall shape of the diurnal variation. How-

ever, there is a clear seasonal dependence. During winter the maximum equatorward to maximum poleward change is slower than the change in the reverse direction, i.e. the opposite to that for low solar activity. Furthermore, the small secondary maximum observed around noon seems to occur later with increasing latitude. Also, the diurnal variation shape for spring is consistent with an almost constant wind velocity during daytime, while a nearly sinusoidal variation applies for summer. For low solar activity, there is a clear latitudinal dependence of the time interval over which a polarward wind prevails, the time interval being largest for the lowest latitude for all three months. However, no such clear latitudinal dependence is observed at high solar activity.

A.J. Foppiano et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 305 – 314 June

September

100

309

December

0

-100

PS

-200

Equatorward wind, m/s

100

0

-100

KGI

-200

100

0

AI

-100

-200

0

6

12

18

0

6

12

18

0

6

12

18

24

Local time (60°W), hours

Fig. 3. As for Fig. 2, except high solar activity: 1989 (F10:7 Iux ¿ 163:2).

The amplitude of the diurnal variation (Fig. 4) is larger in winter than summer at all latitudes and for both solar activity levels. However, only during summer does the amplitude increase with latitude. Furthermore, the largest amplitude corresponds to the highest latitude for nearly all months during high solar activity. Furthermore, as already reported (Arriagada et al., 1997), the amplitude of the diurnal variation tends to be smaller during high solar activity, particularly for spring and summer. Mean wind velocities (Fig. 4) are poleward almost all-year-round for all latitudes at high solar activity. Velocities increase from autumn to winter much faster than they decrease from winter to summer. In summer they are almost zero. The same seasonal pattern is observed at low solar activity, except for AI, where most velocities are either very small or equatorward, particularly in summer. However, a signi:cant latitudinal dependence is observed for both high and low solar activity (Fig. 5). At low solar activity, from May to September, mean wind velocities

decrease for increasing latitude, the larger latitudinal dependence occurring in June. For all other months, almost no latitudinal dependence is observed. By contrast, at high solar activity, mean wind velocities decrease with latitude from PS to KGI and increase at a larger rate from KGI to AI from April to September. As with low solar activity, summer months do not show a signi:cant latitudinal dependence. Moreover, for low solar activity the latitude for which the mean wind velocity is zero varies systematically with season, being nearer the equator in summer and the pole in winter. For high solar activity, although mean wind velocities are mostly poleward for all three locations, the latitude at which mean wind velocity is minimal also tends to change with season, albeit in a less systematic manner. Fourier components of the diurnal variations were derived for all cases. Table 3 gives the amplitudes and phases of the diurnal, semidiurnal and terdiurnal components. Figs. 2 and 3 clearly show that the diurnal variations are well reproduced

310

A.J. Foppiano et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 305 – 314 500

500

1986

400

Amplitude, m/s

Amplitude, m/s

400

1989

300

200

100

300

200

100

0

0 0

8

4

12

0

4

month 150

150

1986

12

8

12

1989

100

Equatorial mean wind, m/s

100

Equatorial mean wind, m/s

8

month

50

0

-50

-100

50

0

-50

-100

-150

-150

0

8

4

month

12

0

4

month

Fig. 4. Seasonal dependence of amplitude and mean wind velocity of the diurnal variations of the magnetic meridional thermospheric neutral wind velocity (equatorward positive) derived using the servo model technique of Buonsanto et al. (1989), and for low (1986) and high (1989) solar activity as described in Figs. 2 and 3. (circles) Port Stanley. (triangles) King George Island. (squares) Argentine Islands.

using only the diurnal, semidiurnal and terdiurnal harmonic components. All three components are needed to describe the winter variations both at high and low solar activity. The spring and summer diurnal variations need only diurnal and semidiurnal components to describe them, except PS and KGI at high solar activity, where just the :rst Fourier component is good enough. Diurnal amplitudes are larger than semidiurnal amplitudes for all seasons and during both low and high solar activity. Results for winter can be explained with reference to the amplitudes and phases of diurnal and semidiurnal components. For low solar activity the phase of the maximum in amplitude of the diurnal component occurs at about midnight, while the phase of the maximum of the semidiur-

nal amplitude component occurs about 4 h later. This explains why the wind changes from maximum equatorward velocity to maximum poleward velocity in less time than for the corresponding change from poleward to equatorward. However, the phase of the maximum diurnal component for high solar activity occurs at about 03 LT and that of the semidiurnal component at about midnight. This explains why the wind changes from maximum equatorward velocity to maximum poleward velocity in more time than that for the corresponding change from poleward to equatorward. Results for spring can be explained also with reference to the amplitudes and phases of diurnal and semidiurnal components. For low solar activity the phase of the maximum of the diurnal component occurs at about midnight, while the

A.J. Foppiano et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 305 – 314 1986

1989

0

Equatorial mean wind, m/s

311

MAR APR

0

0

MAY

0

0

JUN

0

AUG

0

0

SEP

0

0

OCT

0

0

NOV

0

0

DEC

0

50 0

50

60

50

70

60

70

latitude, °S

latitude, °S

Fig. 5. Latitudinal dependence of mean wind velocity of the diurnal variations of the magnetic meridional thermospheric neutral wind velocity (equatorward positive—open circles) derived using the servo model technique of Buonsanto et al. (1989), and for low (1986) and high (1989) solar activity as described in Figs. 2 and 3. Table 3 Amplitude (m/s) and phase (local time, hour) of the diurnal (D), semidiurnal (S) and terdiurnal (T) Fourier components of the mean diurnal variations of wind velocities derived using the servo model, as used by Buonsanto et al. (1989) Location

Low solar activity (1986)

High solar activity (1989)

Amplitude

PS KGI AI

Phase

PS KGI AI

Amplitude

PS KGI AI

Phase

PS KGI AI

Winter

Spring

Summer

D

S

T

D

S

T

D

S

T

209 133 119

43.7 41.3 55.8

25.2 25.8 31.5

102 82.3 155

40.6 26.8 34.4

9.30 9.50 6.90

40.2 67.0 77.5

19.6 27.0 26.2

11.4 2.80 4.60

4.39 3.13 1.42

0.16 23.9 0.22

2.38 3.82 3.27

1.91 2.02 7.07

4.10 5.60 7.10

30.1 30.9 50.0

6.00 3.30 10.9

1.60 1.20 4.60

2.90 5.67 5.36

6.57 0.43 1.10

0.58 23.3 23.7 115 94.8 134 3.84 2.88 3.14

5.12 3.69 2.88 67.6 38.9 54.5 0.17 23.8 0.34

phase of the maximum of the semidiurnal component occurs about 2 h later. This explain why the wind changes from maximum equatorward velocity to maximum poleward ve-

3.33 5.87 5.34 43.9 30.9 50.9 5.63 5.26 6.05

23.1 23.5 23.7 33.2 52.7 73.5 1.32 0.83 0.74

2.03 1.99 3.31 15.5 17.5 20.8 0.83 1.39 1.65

5.74 5.52 5.05

2.64 1.79 1.84

locity in less time than that for the corresponding change from poleward to equatorward. However, the phases of the maxima of the diurnal and semidiurnal components are

312

A.J. Foppiano et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 305 – 314

Table 4 Amplitude (m/s) and phase (local time, hour) of the diurnal (D) and semidiurnal (S) Fourier components for PS derived by Canziani et al. (1990) and those corresponding to the present result using this time the method of Miller et al. (1986). For present results, equinox means spring Winter

Equinox

Summer

D

S

D

S

D

S

Canziani

Amplitude Phase

70 05

25 05

55 04

30 04

30 06

10 06

Present results

Amplitude Phase

67.5 3.44

28.4 23.5

98.0 0.51

42.5 0.86

64.7 1.03

24.3 1.71

almost the same for high solar activity. This leads to a sort of “W like” diurnal wind variation. Summer results for both low and high solar activity are explained in the same way as at spring during low solar activity. The e7ect of the terdiurnal component is clearly seen in winter only, as already mentioned. This corresponds to the small secondary maxima seen at about 16 LT for low solar activity and at about noon for high solar activity. Moreover, the terdiurnal amplitude is similar to that of the semidiurnal amplitude only during high solar activity. As regards to the latitudinal dependence of the diurnal and semidiurnal amplitude components, it can be seen from Table 3 that these increase with latitude from KGI to AI during all seasons and solar activity levels. The dependence from PS to KGI is not so clear. Values for PS are more or less the same as for KGI during spring and summer. Winter amplitudes seem to be minima for KGI in all cases except for the diurnal amplitude during low solar activity. Finally, the ratio between the diurnal amplitude and the semidiurnal amplitude components becomes larger as the latitude increases during spring and summer for low solar activity, and during spring and winter at high solar activity. 4. Discussion Results published by Canziani et al. (1990) for PS are directly comparable with those presented here. They use the same technique of Miller et al. (1986) for deriving diurnal variations. Ionospheric hourly data for 20 days of March, June, October and December 1984 were used. Table 4 gives the amplitudes and phases of the diurnal and semidiurnal Fourier components for PS derived by Canziani et al. (1990) and those corresponding to the present results using this time the M method. Canziani et al. (1990) :nd that there is a well de:ned seasonal trend, with the diurnal amplitude reaching a maximum during winter and the semidiurnal amplitude reaching a maximum during the equinoxes. However, the diurnal component winter maximum is slightly

larger than the equinoxes one. Our results indicate the same seasonal trend for both components, but with a clear maximum during spring. Moreover, their amplitudes are generally smaller than the ones presented here, and the phases they report are delayed relative to the ones presented. The amplitude di7erences found are consistent with the slightly di7erent level of solar activity (1984 instead of 1986), since amplitudes tend to decrease with increasing solar activity. Results reported by Dudeney (1973, 1976) for AI relate to a di7erent technique. He :rst :ts sinusoidal diurnal variations to hmF2 values derived from ionosonde observations, and then determines wind velocities using simple theory considerations. He shows that the amplitude of the diurnal variation is also larger in winter than summer, both for high and low solar activity. However, his results differ from those presented here in that his study shows the amplitude increasing with increasing solar activity during winter. Results reported by Titheridge (1993, 1995) for Auckland (A: 34:5◦ S; 179◦ E geographic, −37:9◦ geomagnetic latitude) and Invercargill (I: 42:9◦ S; 176◦ E geographic, −46:6◦ geomagnetic latitude) in New Zealand correspond to winds determined using the Interhemispheric Full Time Varying Model. They are representative of conditions during low levels of geomagnetic activity and both low and high solar activity. Although the geomagnetic latitudes of these locations are similar to those of PS and KGI, respectively, they are located at signi:cantly lower geographic latitudes. Magnetic dip and declination are also di7erent. However, these results are the only ones available for a longitude sector where the inIuence of the SAA should be negligible. The amplitude of the diurnal variation for A and I depends on season in the same way as PS and KGI at low solar activity. The same is true for mean wind velocity, although the absolute velocities are smaller for A and I. The main difference found between the Antarctic Peninsula sector and the New Zealand sector relates to the latitude dependence of mean wind velocity. Poleward mean velocities are larger (or equatorward ones smaller, according to season) at I than A for low and high solar activity, while they are smaller

A.J. Foppiano et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 305 – 314

313

Table 5 Amplitude (m/s) and phase (local time, hour) of the diurnal (D) and semidiurnal (S) Fourier components of the diurnal variations of wind velocities given by the HWM93 model (Hedin et al., 1996) Location

Low solar activity (1986)

High solar activity (1989)

Amplitude

PS KGI AI

Phase

PS KGI AI

Amplitude

PS KGI AI

Phase

PS KGI AI

(or approximately the same) at KGI relative to PS. Unfortunately, no clear conclusion can be drawn, since the difference in latitudinal dependence may be due to geographic latitude or dip/declination di7erences rather than longitude per se. Mean wind velocities determined using the HWM93 (Hedin et al., 1991, 1996) model for PS, KGI and AI for all months and solar activity levels signi:cantly differ from the results presented here. As already suggested for KGI (Arriagada et al., 1997), the HWM93 shapes of the diurnal variations are signi:cantly di7erent from those presented here. The amplitude of the diurnal variation changes slightly from winter to summer for all locations and solar activity levels when compared to the present results. As a consequence, largest di7erences (as much as 100 m=s) between present and HWM93 amplitudes are found in winter for low solar activity, and in summer at high solar activity. HWM93 mean wind velocities are equatorward for all seasons at low solar activity, showing almost no latitudinal dependence, while present results indicate a systematic latitude dependence which changes with season. Moreover, although there are HWM93 poleward mean velocities only in winter at high solar activity, there is little latitudinal dependence almost all year round. Comparisons have also been made between HWM93 winds and present B winds calculated with only diurnal and semidiurnal Fourier components. If model winds are to reproduce B winds, two components would be enough for most cases. However, signi:cant changes are needed for mean wind values and diurnal variation amplitude (e.g. mean winds have to be more polarward and amplitudes larger during low solar activity), and also in the phases of

Winter

Spring

Summer

D

S

D

S

D

S

83.2 83.0 80.0

16.9 25.4 26.5

81.2 94.7 95.4

15.5 25.1 26.4

65.0 77.5 78.7

20.5 31.3 32.6

1.68 1.43 1.27 88.0 75.3 67.8

6.71 6.34 6.27 55.9 52.4 47.5

1.51 1.84 1.67

1.81 1.73 1.63 81.6 88.7 86.6

7.31 7.20 7.15

1.46 1.63 1.47

6.52 6.16 6.10 25.5 25.6 23.6 7.17 6.60 6.40

2.82 2.30 2.10 70.3 76.4 74.8 1.94 1.60 1.27

5.88 5.77 5.74 31.3 35.0 34.2 5.91 5.44 5.29

both diurnal and semidiurnal Fourier components (e.g. a 2 h lag for the diurnal component and 7 h advance for the semidiurnal component, during winter at high solar activity). Table 5 lists amplitudes and phases for all cases to be compared with those of Table 3. 5. Conclusions The shapes of the diurnal variations determined using the three di7erent methods for a given location and month are fairly similar for all locations and most months. The amplitude of the diurnal variations is larger in winter than summer for all latitudes and at both low and high solar activity, being generally larger at low solar activity. Daily mean wind velocities are poleward in winter and equatorward in summer at low solar activity for all latitudes but they are poleward all the year round for high solar activity, and are strongest during winter. The latitudinal dependence of daily mean wind velocities clearly depends on season, being largest in winter, particularly for low solar activity. Although this dependence differs from the one reported for a restricted latitude range in the New Zealand longitude sector, it cannot, as yet be de:nitely identi:ed as a particular e7ect of the so called South Atlantic Anomaly of the geomagnetic :eld. Present results signi:cantly di7er in many respects from those corresponding to the HWM93 model. Acknowledgements Ionospheric hourly values for Port Stanley and Argentine Islands were kindly made available by R. Stamper of

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WDC-C1, Appleton Rutherford Laboratory, Oxford, UK. The Instituto AntQartico Chileno made funds and partial logistic support to buy, install and operate the ionosonde at King George Island. The Fuerza AQerea de Chile also provided logistic support. The late M. Buonsanto kindly supplied the servo model code. The Miller et al. (1986) and Richards (1991) codes were supplied by P.G. Richards. The HWM93 code was kindly supplied by the National Space Science Center, Greenbelt, USA. Support for this study was provided by Fondo Nacional de Desarrollo CientQR:co y TecnolQogico under Proyecto No. 1990334. One of us (X.A.T.) is particularly indebted to P.G. Richards for providing all necessary means during a two months stay at Alabama University, Huntsville. Comments received from S. Zhang, and particularly from P.L. Dyson, which lead to a signi:cant revision of the text are greatly appreciated. References Arriagada, M.A., Foppiano, A.J., Buonsanto, M.J., 1997. Solar activity variations of meridional winds over King George Island, Antarctica. Journal of Atmospheric and Solar Terrestrial Physics 59, 1405–1410. Berkey, F.T., Stonehocker, G.H., 1989. A comparison of the height of the maximum electron density of the F2-layer from real height analysis and estimates based on M(3000)F2. Journal of Atmospheric and Solar Terrestrial Physics 51, 873–877. Bradley, P.A., Dudeney, J.R., 1973. A simple model of the vertical distribution of electron concentration in the ionosphere. Journal of Atmospheric and Terrestrial Physics 35, 2131–2146. Buonsanto, M.J., 1986. Seasonal variations of day-time ionisation Iows inferred from a comparison of calculated and observed NmF2. Journal of Atmospheric and Terrestrial Physics 48, 365 –373. Buonsanto, M.J., Titheridge, J.E., 1987. Diurnal variations in the Iux of ionisation above the F2 peak in the northern and southern hemispheres. Journal of Atmospheric and Terrestrial Physics 49, 1093–1105. Buonsanto, M.J., Salah, J.E., Miller, K.L., Oliver, W.L., Burnside, R.G., Richards, P.G., 1989. Observations of neutral circulation at mid-latitudes during the equinox transition study. Journal of Geophysical Research 94, 16987–16997. Canziani, P.O., Giraldez, A.E., Teitelbaum, H., 1990. Thermospheric meridional winds above Argentina during 1984. Annals of Geophysics 8, 549–558. Dudeney, J.R., 1973. Studies based on the antarctic ionospheric F-layer. Ph.D. Thesis, University of London, London, 337pp. Dudeney, J.R., 1974. Sample empirical methods for estimating the height and semithickness of the F2-layer at the Argentine Islands,

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