Median ionospheric height variations over a sunspot cycle in the Australian-Japanese longitudinal sector

P&WI. SpoecScr..Vol. 39. No. 12. pi. 16074615. 1991

00324633/9l 53.00+0.00 0 1991Pqamon Prem pk

mated 111 crmt antin.

MEDIAN IONOSPHERIC HEIGHT VARIATIONS OVER A SUNSPOT CYCLE IN THE AUSTRALIAN-JAPANESE LONGITUDINAL SECTOR L. A. HAJKOWICZ

Department of Physics, University of Queensland, Queensland 4072, Australia (Received infinalform 9 August 1991) Abetract-The monthly median virtual height (h’F) of the F-region was studied for a period of 6 years (19W1985) from sunspot maximum to minimum, using data from I 1ionosonde stations in the Japanese Australian longitudinal sector, in an invariant latitude range : 37”N to 54”s. The night-time maximum in the median height progressively decreases equatorwards, particularly in the local winter and spring, while a reverse weak tendency is observed in summer. The median height reaches peak in both hemispheres from 1to 2 years after sunspot maximum then decmases towards sunspot minimum. A second diurnal maximum in h’F, preceded by a well-defined minimum, was consistently observed over the solar cycle close to the sunrise time at the F-region, mainly at low invariant latitudes (9-20”). The second maximum has a distinct seasonal variation, being most pronounced in winter and diminishing in summer. It is envisaged that the second peak in h’F is associated with the wave disturbance generated by the supersonic motion of the sunrise terminator. Possible effects of the background height variations on the propagation of the magnetic storm-induced travelhng ionospheric disturbances are discussed.

1. INI’RODUCITON

height of the F-region, either the ionization peak height (hmF2) or virtual height (h’fl, is a sensitive

The

indicator of the state of the thermosphere. The average height of the F-region has been associated with the fixed pressure levels in the neutral atmosphere (Rishbeth and Edwards, 1990; Buonsanto, 1990) as well as with the neutral wind variations (Buonsanto, 1990; Titheridge and Buonsanto, 1983 ; Miller er al., 1986; Yagi and Dyson, 1985). In addition, the ionospheric height variations, or deviation from the mean height, are affected by large-scale gravity waves (AGWs) originating in the aurora1 oval and propagating equatorwards (Bowman, 1977; Walker et al., 1988; Hajkowicz, 1990, 1991a, b; Hajkowicz and Hunsucker, 1987; Reddy et al., 1990). Recently it became evident that a high degree of correlation (with an average correlation coefficient exceeding 0.7 and occasionally reaching 0.9) exists between the individual surges of the aurora1 electrojet, as represented by the variability of the aurora1 electrojet index (AE), and the following enhancements in h’F (Hajkowicz, 1991b). The average delay between the onset of the aurora1 disturbances and the following variations of h’F in mid-latitudes is 1.6 h which indicates that the height rises are associated with the equatorward propagation of large-scale travelling ionospheric disturbances (LSTIDs), originating in the southern and northern aurora1 ovals. The sequential height variations of the F-region are

invariably associated with LSTIDs which are ionospheric manifestations of the presence of aurorally generated AGWs. The recent studies of the global propagation of LSTIDs (Hajkowicz, 1990, 1991a, b) clearly indicate that the propagation characteristics of LSTIDs depend strongly on the solar illumination conditions; LSTIDs tend to occur at the night-time ionosphere whereas their attenuation in the day-time ionosphere is rapid as they move away from the auroral source of their generation. It has been postulated that the ionospheric drag, due to the increased ionization density in the daytime ionosphere, is responsible for the diurnal characteristics in the occurrence of LSTIDs. It appears that another parameter affecting the propagation characteristics of LSTIDs should be considered, namely the background (median) height of the F-region. Such background height variations may have a large effect on the occurrence of LSTIDs since it is well known that the amplitude of AGWs grows exponentially with altitude, i.e. with the height of the F-region. There appears to be little information on the global ionospheric height variations which might be useful for the TIDs study. The morphology of the height variations was studied over a solar cycle for one station only in Australia-Canberra (Scan and Dyson, 1990). Elsewhere studies were concentrated on a few stations mainly in one hemisphere (Ring et al., 1967; Buonsdanto, 1990) or for a few localized stations in both hemispheres (Titheridge and Buonsanto, 1983 ; Rishbeth and Edwards, 1990). The pre1607

L. A. HAIKOWICZ

1608

sent study gives a latitudinal profile of the ionospheric height variations for the years of sunspot maximum and minimum using 11 stations distributed over a large range of latitudes but confined to one longitudinal sector. The presence of the morning height rise, which was noted in the previous study (Hajkowicz, 1991b), became now quite evident for certain latitudinal ranges and seasons. It appears that this height enhancement has not been given sufficient attention in the previous studies of the F-region diurnal height variation. It might indicate the presence of a nonaurora1 source in the generation of LSTIDs.

2. METHOD

AND RESULTS

The behaviour of hmF2, or the F-region peak height can be inferred from the virtual height (h/F) of the layer ; it is known that h’F is a good indicator of the height of the region (Appleton, 1960 ; Lyon et al., 1961). It is also a more reliable parameter since under spread-F conditions only h’F can be obtained and not hmF2. This parameter (h’F) is readily available from the routine scaling of ionograms and is particularly suitable for the analysis of large amounts of ionosonde data where the derivation of the hmF2 parameter is impractical. The monthly median values of h’F were used to derive the diurnal, seasonal and solar cycle (19801985) variations in these parameters for the Japanese and Australian vertical-incidence ionosonde stations. The data were divided into four northern seasons (or to southern seasons in the brackets) : winter (summer) (November-January), spring (autumn) (FebruaryApril), summer (winter) (May-July) and autumn

(spring) (August-October). The mean height was found for each season (averaged over three months) which was then applied to the yearly and solar cycle analysis of h’F. Table 1 gives the station’s geomagnetic latitude and longitude (east), invariant latitude, magnetic dip angle, and the average night-time (in UT and for an altitude of 300 km) for various seasons. The Japanese stations are located in a relatively narrow longitudinal sector (Table I), encompassing midlatitude stations (Wakkanai, Akita and Kokobunji ; referred to as a latitudinal sector 3) and low latitude stations (Yamagawa and Okinawa ; referred to as a sector 2). They are particularly suitable for the analysis of latitudinal variations of the ionospheric parameters since they are approximately evenly spaced in latitude, from north to south. The Australian stations are less evenly spaced, consisting of high mid-latitude stations (Hobart-Canberra, section 4), mid-latitude stations (Brisbane and Townsville ; sector 3), low and equatorial latitude stations Darwin (sector 2) and Vanimo (sector l), respectively. It should be also noted that the night-time at the F-region differs relatively little from one station to another. Since the Fregion height strongly depends on the solar illumination conditions this aspect also simplifies the study of latitudinal variations in the ionospheric parameters for this longitudinal region. The diurnal variation of h’F, characterized by the day-time minimum and night-time maximum as predicted by the model of ionization production (King et al., 1967), is well known. This general trend is clearly seen in Fig. 1 for all seasons and for the entire half sunspot cycle for the Japanese stations (Fig. 1) and for the Japanese and Australian stations (Fig. 4).

TABLE 1. GJXMAGNETIC COORDINATES AND AVERAGE NIGHT-TIME FOR THE IONOSONDE STATIONS USED IN THE ANALYSIS

Stations

Geomagnetic long. (E) lat. (deg.) (deg.)

Invar. dip lat. (deg.) (deg.)

Winter

Seasons Autumn Summer (night-time, UT)

Spring

Japan

Wakkanai Akita Kokobunji Yamagawa Okinawa

35.3 29.5 25.5 20.4

13.6

206.6 205.9 205.8 198.3 193.8

36.7 30.2 25.6 19.7 8.9

59.0 53.2 48.6 43.9 34.2

-51.6 -43.9 -35.7 -28.4 -23.0 - 12.5

224.9 224.8 221.4 219.3 201.3 211.6

53.6 44.8 35.6 26.4 16.8 1.0

-72.7 -66.1 - 57.6 -48.4 -39.7 -21.1

09-U) 09-20 09-20 l&21 10-21

10-19 l&20 l&20 1 l-20 11-21

13-16 12-17 12-18 12-19 12-19

lo-19 lo-19 l&19 1 l-20 1t-20

Summer 12-17 11-17 l&17 lo-18 lo-20 IO-19

Spring

Winter 08-20 08-20 08-19 09-20 1 l-20 l&21

Autumn 09-19 w-19 09-19 09-19 1 I-20 w-19

Australia

Hobart Canberra Brisbane Townsville Darwin Vanimo

l&19 l&19 09-19 l&19 l&20 lo-20

Median ionospheric height variations over a sunspot cycle

FIG. 1.DIIJRNAL AND ~EASZNAL VAXIATIONSIN THE MEDLM V~RTIJALHEIGHT &‘FoF THE F-REGION FOR FWE JAPANESE ~A~NS(~~G~-~P~O~~~ IN TABLE!1).

However the less known aspects of the height variation become apparent for various seasons and at different latitudes, particularly for the Japanese and Australian low-latitude sectors (Figs 1 and 2). It can be seen that in addition to the main h’F maximum around the midnight (the night-time periods for various stations and seasons are given in Table 1) there is a consistent second peak in h’f: occurring in the morning hours, close to the sunrise at the F-region. This peak was noted from a study of the diurnal variations of h’F for the Japanese stations on individual days (Hajkowicz, 1991b). The second maximum clearly depends on the season, being most pronounced in winter and spring (Fig. la and b, and Fig. 4), becoming residual in summer and then increasing again in autumn. It is also most pronounced for lower latitude stations, Yamagawa and Okinawa, and becomes barely discernable at the northernmost station Wakkanai. It is of considerable interest that the peak occurs also in sonthem low latitudes, at approximately the same time, at Darwin (Fig. 2). The coincidence between the morning peak is evident for Yamagawa, Okinawa and Darwin for 1983-1985 (the Darwin data were available only for these years of the cycle). This coincidence, however, exists only if the

FKLZDIURNAL A~XSMONAL V~~~ONSOF h’F ~R~WLA~EAU~L~N~JAPA~~A~ONS FOR 1983-1985.

1609

THEMESAN

same seasons are considered, e.g. winter in both hemispheres. It is also evident that the peak is preceded by a well-defined minimum, again particularly evident for winter and spring. The peak occurs at 21 UT for Yamagawa and Okinawa, and 20 UT for Akita and Kokobunji, which are the average ionospheric sunrise times (cf. Table 1) for these two pairs of stations (winter and spring}. It also occurs at the same time (21 UT), as for a low-latitude Australian station (Darwin), since the latter station has almost the same local time and similar invariant latitude as Yamagawa and Okinawa. Figure 4 gives the mean dinmal variations of h’F for different northern and southern latitudinal sectors (as defined above) and seasons. It can be seen that the morning peak in h’F is most clearly defined for the Japanese and Australian low latitude sector (sector 2). The morning peak is considerably less pronounced for sectors 1,3 and 4 in both hemispheres. Thus, the lambent is a low latitude ~enomeno~ but not an equatorial one. This trend in h’F is clearly different from the well-known post-sunset height rise at the equator, which is clearly evident at Vanimo (sector 1) for all seasons except winter (Fig. 4e-h). Ionosonde

1610

L. A. HAJKOWKZ

data from other longitudinal sectors in Europe, South Africa and Tahiti (South Pacific) were investigated (not shown) for the same period of time and the same trend in the morning height rise was found, clearly indicating that this is not a local phenomenon but a world-wide one. Another important feature of the seasonal and diurnal variations in h’F is a pronounced change in the spread of the night-time median ionospheric heights over a range of latitudes and seasons at night-time from winter to autumn, as it can be seen for the Japanese stations (Fig. 1). The average height tends to decrease substantially, in a sequential manner, from mid to low latitudes, particularly in winter and spring, and to a lesser extent in autumn. A much smaller spread in summer is associated with a reverse tendency : the heists tend to decrease from lower to higher latitudes. The latitudinal spread in h’F is large in winter, decreases slightly in spring and reaches a minimum in summer, starting to increase again in spring. It should be noted that the end of the evening sharp rise of h’F in winter terminates at 12 UT at midlatitude stations ~akkanai, Akita and Kokobunji), at 13 UT at Yamagawa, and at 14 UT at Okinawa. The sequential shift of this type might indicate the presence of the equatorward neutral winds which blow mainly equatorwards at night : when the wind blows equatorwards, the ionization will be dragged up along the magnetic field lines, causing the progressive F-layer rise at different latitudes. It should be noted that the morning peak has a reverse tendency to that of the evening one, being higher at low latitudes than at mid latitudes. The same trend in the night-time height levels, at different latitudes, can be seen for the Austraiian sectors (Fig. 4e-h) although not as clearly as for the Japanese stations. Thus in the austral summer h’F is decreasing from low to high latitudes (except for Darwin, 1983-1985) and it decreases from high to low latitudes in the austral winter. The average (over 6 years) h’F height spread, particularly evident for winter (Fig. I), is also evident on the yearly basis during the solar cycle for this season. Figure 3 shows that the average diurnal pattern in the ionospheric height variations in winter is present to a diminishing degree from the solar maximum year (1980) to the solar minimum (1985). In all these cases the evening sequential height rise is earlier and larger at mid-latitude stations than at low latitude stations. The morning peak in h’F is present throughout the cycle, being more prominent at low latitude stations than at mid-latitude stations or the equatorial station Vanimo. Figure 5 shows that for the Japanese stations night-

FIG.~.DIURNALV~IATIONS~~ MEDIAN VIRTUALHEIGHT (/I'@ OF THE F-REGION FOR FIVE JAPANESE STATIONS FOR VARlOUSYEARSOFTHESUNSPOTCYCLEIN WINTER.

time median h’F tends to decrease from the northernmost station Wakkanai (the starting point of all the yearly curves, from left to right) to the southernmost one-Okinawa. This trend is particularly strong in winter and spring. The latitudinal trend in h*F becomes much less pronounced in the period of the low sunspot activity, from 1984 to 1985, inclusive. It is of considerable interest that the latitudinal variations of h’F in summer (Fig. 5c) have an opposite trend : there is a smaller in magnitude but nevertheless quite evident increase in h’F from high to low latitudes. it can be also seen that the average median h’F for all stations and seasons tends to decrease towards sunspot minims. Similar latitudinal height variations, for different years of the solar cycle, were also recorded for the Australian stations, with the height decreasing from high mid-latitudes (sector 4) to low and equatorial latitudes in all the seasons except summer. This latitudinal trend in the height variations is shown in Fig. 6a, as an example, for all the years of the solar cycle for the austral winter. Figures 6b and c show yet another similarity in the seasonal variations of nighttime median h’F in the Australian and Japanese sectors, respectively. It can be seen that the average seasonal height reaches maximum during the local

Median ionospheric height variations over a sunspot cycle

AUTUMN

FIG.4.Tns A~AGE~~NAL~~TIONSOF~ F FORF~IJR LATM'IJDINALSECTORS IN AWTRALL~ AND TWO SECTORS IN JAPANOVWTHESUNSPOTCYCLE.

summer in both sectors, decreasmg in the years 19801981, 1982-1983 and 1984-1985, as the average sunspot number decreases from 148, through 92 to 32 in the same time intervals. It should be noted that in the real time the maximum summer height rise in Australia would coincide with the minimum winter heigbt in Japan and vice versa. Figure 7 summarizes the global variations in median h’F for the Japanes~Australian longitudinal sector. Figure 7a-d clearly indicate that the height variations vs years of the declining sunspot numbers are similar at the southern and northern locations, with a maximum in h’F shifted by l-2 years from the sunspot maximum year (1980). The invariant latitude profile of median h’F for all the seasons, except summer, indicates (Fig. 7e-h) that the height decreases rapidly in mid latitudes, towards low latitudes of 9-20” (north and south) and then increases to a maximum at the equator. The general trends in h’F might be affected by the disturbance level in the aurora1 region. This applies both to the neutral winds and to the presence of largescale travelling disturbances, enhancing ionospheric

FIG.~.~~~NALA~D~~~IN~~IG~v~~o~ RECORDED BY THE MERIDIONAL CHAIN OF THE JAPANBE STATIONS(THE STATION'SSyMBoLS ARE SPECIFIED), STARTING ~~WAK~NAI(AT~ST~TO~~~C~~,FORAG~ YEAROFTIIRS~NS~TCYCLE)ANDENDINGWITHOKINAWA.

heights at a range of latitudes (cf. Section 1). Figure 8 shows, as an example, the mean diurnal variations of h’F (averaged over 6 years) for two latitudinal sectors 2 and 3 in the northern hemisphere (Fig. 8a) and the corresponding average AE index (Fig. 8b) for the northern winter. An apparent coincidence between AE index and median h’F for the ad-latitude sector (sector 3) is evident. There is no indication that the second morning maximum in k’F (most pronounced for sector 2) is associated with a similar variation in AE. The coincidence between AE and h’F might be related to the universal time effect whereby the AE index reaches a maximum value between 10-22 UT (Berthelier, 1976), in the time which is also within the nighttime period for the Japanese and Australian stations. An inspection of the diurnal variations in the median height in other longitudinal sectors (Europe) indicates the presence of a similar night-time maximum in KF, shifted well away from the period of the enhanced aurora1 activity. Thus, the night-time height rise might be predominantly due to the diurnal height variation of the F-region and the presence of the enhanced

1612

L. A. HMKOWICZ MEDIAN H’F

: 1980-1985 SUMMW

H’Flkm)

WINTER

AUTUMN

AUTUMN k)

260 m SPRING

SPRING

WIN

SPU

SUM

Au1

FIG. 6. (a) AN EXAMPLEOF LATITUDINAL HEIGHT VARIATIONS IN WINTER FOR THE AUSTRALIAN STATIONS (STARTING WITH HOBART AND ENDING WITH VANIMO AS SPECIFIEDBY THE SYMBOLS) FOR EACH YEAR OF THE SUNSPOT CYCLE, (b) AND (C)-THE AVERAGE SEASONAL HEIGHT VARIATIONS IN TWO YEARINTERVALS(FROMSUNSPOTMAXlMUMTOMINIMUM)FOR

auroral activity is coin~dental. It should be noted, however, that the height increments, in relation to the background (median) height, show a strong correlation with the aurora1 electrojet index (Hajkowicz, 1991b). 3. DISCUSSION

AND CONCLUSIONS

The median virtual height (h’F) variations were investigated over a period of 5 years from the verticalincidence ionosonde data from 11 stations in the Japanese and Australian longitudinai sectors. This study is a part of the global investigation on the generation and propagation of large scale travelling disturbances (LSTIDs) whose presence is particularly evident in the height increments (Ah’F) following the onset of amoral substorms in both hemispheres. The following became evident : (a) The night-time median height increases, as predicted by the theory of diurnal plasma and neutral air motion, are present at approxima~ly the same time over the range of southern and northern Iatitudes. There is a sequential evening height increase which

80

81

82

83

YEAR

$4 85

-&tT

North -20 O 20 u) INVAR. LAT.

FIG. 7. (a-d) MEAN SEASONAL HEIGHT VARIATIONS,VS YEAR OF THE SUNSPOT CYCLE, FOR AUSTRALIA AND JAPAN, (e-h) LATITUDINAL CHANGES IN h’F FOR VARIOUS SEASONS AND FOR YEARS 1980-1982 AND 1983-1985; THE SOUTHERN LOW LATITUDEDATA AREMISSING FOR THEFIRST PERIOD.

starts earlier at mid-latitude stations and later at low latitude stations, with a progressive decrease in the maximum height towards equator, particularly evident in winter and for the Japanese sector. This trend is also evident during other seasons but an inverse trend occurs in summer, when the height decreases from low to high latitudes. This sequential trend in h’F becomes progressively less pronounced for the years of sunspot minimum. (b) In addition to the broad night-time peak in median h’F there is a pronounced morning peak, particularly evident for low latitude stations in winter and spring, consistently observed from the sunspot maximum to minimum. This peak is preceded by a well-defined minimum and occurs close to the Fregion sunrise time in southern and northern latitudes. Inspection of the data from other longitudinal sectors indicates that the peak is a world-wide phenomenon. There is no evidence that the morning peak in the median h’Fis of the aurora1 origin. (c) The seasonal height maximum occurs in summer, decreasing progressively from sunspot

Median ionospheric height variations over a sunspot cycle

I

maximum to minimum in both hemispheres. On the yearly basis, the median height variations are similar in both hemispheres, peaking 1-2 years after the snnspot maximum. The median heights tend to decrease rapidly for all seasons (except in summer) and particularly at high sunspot activity, from high (invariant) mid-latitudes (44-525 and 35”N) to low latitudes (9-2O”N and S), with a consistent maximum observed at the equator. The above findings, in addition to providing some new i~o~tion on the global ionospheric height variations, are important in the study of LSTIDs as their amplitude depends strongly on the ionospheric height. A well-known characteristic of the acoustic gravity waves, based on the conse~atio~ of energy, indicates that the anmlitude ofthe wave increases ex~nen~ally with altitude. It follows that the F-region response to AGW is a function of time of day, season and sunspot cycle as was originally suggested by Hooke (1970) from theoretical considerations, The study of the stop-ind~ LSTIDs, over oneyear period ~ajk~i~ 199lb) and the case studies (Hajkowicz, 1990, 199la, b) clearly indicate critical dependence of the occurrences of LSTIDs on the time of day. For example, Hajkowicz (1990) studied LSTIDs in the Japanese and Australian morning

I613

se&or, launched on a globa scale after a step-like onset of an auroral s&storm. The propagation took place close to the ionospheric sunrise time (but still at night) in the Japanese sector, with the amplitnde enhancement (diB’F)reachingmaximumvahtesf.$2OO km at Yamagawa and Okinawa and over 100 km at other Japanese stations. At the same time there was little or no evidence of any TIDs in the Australian sector (the eastern coast) which was then in day-time, only 1 h ahead of the Japanese sector. The disturbance was already clearly visible for an Australian stat& on the west coast, which was then in the night-time sector. The arwent used to explain this phenomenon was based on the assumption that the ion drag, which becomes particularly effective in day-time, dissipated LSTIDs in the Australian sector, It appears, however from the present results, that the LSTID was diminished both by the ion drag and the rno~n8 decline of the average height at which the disturber propagated. For the particular event, recorded on 3 1 October 1972 during the sunspot minimum (with a mean yearly sunspot number of 69), the sequential height rises took place between 19-21 UT following a step-like onset of an aurora1 substorm at 19 UT. During this time Y~a~wa and Okinawa were close to the sunrise te~tor (on the night side), experiencing the morning height enhancement. This can be responsible for the above mentioned strong sequential height rise for these two low latitude stations. Note that the corresponding height rises at mid-latitude stations Wakkanai, Akita and Kokobunji were only half of that at Okinawa and Yamagawa, in avant with the general latitud~al trend of the morning height enhancement which is most pronounced for low latitude stations. The absence of any substantial height enhancement at the Australian stations can be associated with the latitudinal preference in the occurrence of the morning median h’F urns, as all the Australian stations were either in mid-latitude (Hobart, Canberra, Brisbane and Townsvilie) or near the equator (Vanimo). The data from Darwin were not then available. This strong morning enhancement for the storm-induced height rises is a typical feature of the Japanese stations as found from a yearly (1980) study of the global height increases (Hajkowicz, 1991b). It should be noted, however, that the storminduced height rises for the lapanese and Au&a&n stations were of the same magnitude when they occurred close to the local-midnight, away from the critical mo~ng period (Hajowicz, 1983; Hajkowicz and Hunsucker, 1987). The dechne in median height from high mid-latitudes to few latitudes, with an increase of h’F at the equator, is also a characteristic of the LSTIDs propagation. Their amplitude is quite

1614

L. A. HAJKOWICZ

large at sub-aurora1 latitudes, near the source region, but decreases towards mid-latitudes, followed by a slight increase at the equator (Hajkowicz, 1991b). A separate and an interesting effect is the morning median height rise, particularly evident at low latitudes. This effect is not well known although the equatorial evening height rise is well documented (e.g. Rao, 1966). The occurrence of this peak close to the sunrise at the F-region might indicate that it is generated in the wake of the supersonic motion of the sunrise terminator. Beer (1973) suggested that atmospheric gravity waves, of a horizontal wavelength of 500 km can be generated by this terminator which is supersonic at all altitudes below 100 km and between latitudes +45”. Thus, low latitudes will be particularly favourable for the generation of this disturbance. An experimental support of his theory was given by Raitt and Clark (1973) who detected wave-like disturbances in the dawn period, with a wavelength of 1000 km in the F-region, using the Langmuir probe on an orbiting satellite. The waves were consistently observed within 30” longitude of the sunrise terminator. There was little evidence of such disturbances being present in the dusk sector. An observation of the F-region in the dawn sector by Anderson and Lees (1988), using the Australian over-the-horizontal radar, indicates the onset of a sharp ionization gradient close to the sunrise terminator as solar production of free electrons begins and the height of the regions lowers initially. It follows that the onset of the solar heating of the Fregion will generate TIDs perhaps of a similar nature as those associated with the aurora1 heating (i.e. LSTIDs). The absence of sharp rise in h’F (cf. Figs l-3), characteristic of the sequential height increases due to LSTIDs (Hajkowicz, 1990) is due to the averaging effect used in the data analysis. An inspection of a number of the morning height rises on the daily basis, for Okinawa, indicates that the structure of these enhancements (in the real time) is very similar to that associated with LSTIDs. The morning height rise will be more pronounced at lower latitudes because of the supersonic speeds of the terminator in these regions. The seasonal difference in the enhancements could be associated with a large difference in the background ionization in winter and summer for the Japanese stations. The monthly median plots offoF (routinely published in the Ionospheric Data in Japan) indicate that the morningfoF2 for the Japanese stations in winter is typically below 4 MHz whereas in summer it is two times larger. Thus the attenuation of LSTIDs generated by the sunrise terminator will be considerably larger in summer than in winter due to the increased electron density in summer (as the ion drag is proportional to the electron

density). Finally, the change of the inclination of magnetic field from low to equatorial latitudes (Table 1) may diminish the height rise at the latter latitude range, as the morning height rise at the equatorial station Vanimo is much less pronounced than at low latitude stations Yamagawa, Okinawa or Darwin (Fig. 4). Acknowledgements-The ionosonde data were supplied by the Ionospheric Prediction Service (IPS), Sydney, Australia and the World Data Center C2, Communications Research Laboratories, Ministry of Posts and Telecommunications, Tokyo, Japan. I am grateful to Mrs D. J. Dearden for her assistance with the data analysis and to Dr G. G. Bowman for his discussion on some aspects of this work.

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Berthelier, A. (1976) Influence of the polarity of the interplanetary magnetic field on the annual and the diurnal variations of magnetic activity. J. geophys. Res. 81,4546. Bowman, G. G. (1977) Low-latitude ionospheric height changes associated with geomagnetic substorms. J. atmos. terr. Phys. 39, 1169. Buonsanto, M. J. (1990) Observed and calculated F2 peak heights and derived meridional winds at mid-latitudes over a full solar cycle. J. atmos. terr. Phys. 52, 223. Hajkowicz, L. A. (1983) Conjugate effects in the generation of travelling ionospheric disturbances (TIDs) in the Fregion. Planet. Space Sci. 31, 1409. Hajkowicz, L. A. (1990) A global study of large scale travelling ionospheric disturbances (TIDs) following a steplike onset of aurora1 substorms in both hemispheres. Planet. Space Sci. 38, 913.

Hajkowicz, L. A. (1991a) Global onset and propagation of large scale travelling ionospheric disturbances as a result of the great storm of 13 March 1989. Planet. Space Sci. 39, 583.

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Hooke, W. H. (1970) The ionospheric response to internal gravity waves ; 1, The F2 region response. J. geophys. Res. 15, 5535.

King, J. W., Kohl, H. and Pratt, R. (1967) The effect of atmospheric winds on the height of the FZ-layer peak in middle and high latitudes. J. atmos. terr. Phys. 29, 1529. Lyon, A. J., Skinner, N. J. and Wright, R. W. H. (1961) Equatorial spread-F at Ibadan, Nigeria. J. afmos. terr. Phys. 21, 100. Miller, K. L., Torr, D. G. and Richards, P. G. (1986) Mer-

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