The effects of thermospheric winds on the ionosphere at low and middle latitudes during magnetic disturbances

Journalof At,mospherlc andTerreatrisl Physics,1973,Vol. 35, pp. 617-623.Pergamon Press. Printedin Northern Ireland

The effects of thermospheric winds on the ionosphere at low and middle latitudes during magnetic disturbances J. D. BURGE, D. ECCLES, J. W. KING and R. ROSTER* Radio and Space Research Station, Ditton Park, Slough SL3 9JX, England (Received

29 September

1972)

Abstract-Thermospheric temperature changes associated with the preferential heating of the atmosphere at high latitudes during magnetic disturbances are shown to produce an additional storm-time equatorward wind of about 50 m s-l at lower-middle latitudes. This is sufficiently large to substantially alter the normal daytime thermospheric winds at these latitudes; during equinox and winter conditions, for example, the normal poleward daytime wind is reversed. This storm-time wind opposes the poleward transport of ionization responsible for the equatorial anomaly and it will thus influence the latitudinal variation of electron concentration at lower-middle latitudes. It is suggested that the effect of the storm-time wind will be to produce negative storms during the day in summer; in winter positive storms may be expected during minor disturbances and negative storms during major disturbances. 1. INTRODUCTION MANY papers

have been published which demonstrate that thermospheric winds markedly influence the behaviour of the F-layer peak during magnetically quiet conditions and a comprehensive review of the subject has recently been published (RISHBETH, 1972). There is evidence (COLE, 1962; JACCHIA et al., 1967; ROEMER, 1971) which suggests that during magnetic disturbances the atmosphere at high latitudes is preferentially heated, i.e. the increase in exospheric temperature, AT, which occurs during magnetic storms is a function of latitude and increases towards high latitudes. The global distribution of exospheric temperature therefore differs appreciably from that existing on quiet days and, as pointed out by KOHL and KING (1967), it is reasonable to expect that the corresponding changes in the global wind pattern which occur may be partly responsible for the observed storm-time behaviour of the F-layer. Theoretical investigations of the effects of neutral-air winds on the F-layer during disturbed conditions have been hampered by the lack of a suitable model of the disIt is assumed in Jacchia’s model atmosphere (JACCHIA, 1965), turbed atmosphere. for example, that AT does not vary with latitude and the driving force (which produces the winds) calculated from such a model is independent of magnetic activity. JONES and RISHBETH (1971) have concluded, however, that if AT is assumed to increase towards high latitudes by 2 K/deg. lat., the corresponding change in the global wind system would result in increased critical frequencies, i.e. ‘positive’ storm effects, at, middle latitudes. Recently, on the basis of satellite drag data obtained during disturbed periods, ROEMER (197 1) has modified JACCHIA’S (1965) model by including a term to represent the preferential heating of the polar atmosphere. The aims of the present paper are, * Max-Planck-Institut

fiir Aeronomie, 3411 Lindau/Harz, 617

W. Germany.

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BDRQE,D. ECCLES, J. W. KING and R. ROSTER

firstly, to show how the thermospheric wind system calculated using Roemer’s modification to the Jacchia model differs from the normal wind system and, secondly, to suggest a mechanism by which the winds, through their interaction with the equatorial ionosphere, could produce the complicated P-layer storm effects observed at lower-middle latitudes. 2, LATITUDINALTHERMOSPHERIC PRESSUREGRADIENTS The broken curve in Fig. 1 shows the latitudinal variation of the maximum day time exospheric temperature, T,, and the corresponding night-time temperature,

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Fig. 1. Latitudinal variations of the maximum daytime exospheric temperature end the corresponding night-time temperature for equinox conditions at medium solar activity. Broken curve: quiet-day variation, calculated from Jacchia’s 1965 model atmosphere. Continuous curve: storm-time variation, calculated by including Roemer’s expression in the model.

TN, according to Jacchia’s model for equinox conditions and medium solar activity. This curve can be used to indicate the direction of the thermospheric pressure gradient along approximately the 1500-0300 LMT meridian during magnetically quiet conditions. The atmospheric pressure decreases between the sub-solar latitude on the day side of the Earth and the anti-solar latitude on the night side; the driving force responsible for the winds is thus directed towards the poles near midday and towards the Equator near midnight at all latitudes. Roemer’s expression for the change in exospheric temperature during magnetic disturbances is given by AT = (21-4 sin [y[ + 17*9)K, + 0.03 exp K, where AT is the increase in temperature (K) and v is the geographic latitude. The result of superimposing Roemer’s latitudinal variation of AT (for K, = ‘7) on the normal equinoctial temperature distribution is shown by the continuous curve

Tho effects of thermospheric winds on the ionosphere at low and middle latitudes

619

in Fig. 1. Instead of a single maximum centred on the Equator, two maxima (one in each hemisphere) are produced; these are located symmetrically about the Equator at latitudes of approximately 40’. On the night side of the Earth the latitu~al pressure gradients are increased, but on the day side they are altered in such a manner that at latitudes less than 40“ they are in the opposite direction to normal. 3. THE

THERMOSPHERIC

WINDS

In order to investigate theoretically how the storm-time winds differ from normal, diurnal vacations of wind velocity have been calculated in the manner described by KOHL et al. (1968) for quiet and disturbed conditions during summer, equinox and winter. Jacchia’s 1965 model atmosphere (with K, = 0) was used to represent the atmosphere under quiet conditions and was modified for storm conditions by including Roemer’s expression assuming K, = 7. too 0

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Fig. 2. Diurnal variations at 20cN, 160°W of the meridional wind velocity (positive towards the pole) at 300 km during summer, equinox and winter conditions at medium sotar activity. Broken curves: quiet-day variations, calculated using Jaochia’s 1965 model atmosphere. Continuous curves: storm-time variations, calculated by including Roemer’s expression in the model.

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J. D. BT_JROE, D. ECCLES, J. W. KINGand R. R~STEH

The results obtained for a location ZO”N, 16O”W are illustrated in Fig. 2 which contains curves showing diurnal variation of the meridional wind velocity (positive towards the pole). By comparing the continuous curves (storm conditions) with the broken curves (quiet conditions) it may be seen that the effects of the enhanced highlatitude heating is the production of an equatorward wind component of sufficient magnitude to reverse the poleward wind normally found at midday at this latitude during equinox and winter. On average the equatorward storm component is of the order of 60 m 8-l at 1500 LMT. It is important to note from Fig. 2 that the effect of the storm on the meridional wind during the midday and afternoon periods is, in summer, to change the wind from approximately zero to 50 m s-l towards the Equator and, in winter, to change it from about 60 m s-i towards the pole to zero. Calculations were made for a longitude of 16O’W because the magnetic and geographic equators coincide there and the results thus apply to a latitude close to that at which the crests of the equatorial anomaly occur during the afternoon. 4. THE EFFECTOF THE STO~M-~~~EWINDS ON THE IONOSPHERE NEAR MIDDAY Thermospheric winds affect f,,F2 by producing field-aligned drifts of the ionization which either raise or lower the layer thus changing the ionization loss rate. In this way equato~ard winds enhance, and poleward winds reduce, f,E”2. It would be expected at first sight, therefore, that the storm component of the wind will lead to increases in f,FZ, i.e. positive storm effects, as found by JONESand RISH~ETH (1971). The behaviour of the ionosphere at lower-middle latitudes, however, depends on the mechanisms (electric fields and d~usion) responsible for the equatorial anomaly in f0P2 and the motions produced by these mechanisms are also influenced by thermospheric winds; it is thus possible that storm-time thermospheric winds may not always produce positive storm effects. Various authors (HANSONand MOFFETT,1966; BRAMLEYand YOUNU, 1968; ABUT-~0~3, 1969) have studied theoretically the effects of thermospheric winds on the equatorial ionosphere; Fig. 3 contains results obtained by Bramley and Young. The curves show the latitudinal variations of N,F2 obtained when horizontal thermospheric winds of different velocities blow from the summer hemisphere (north) to the winter hemisphere (south). It was established in Section 3 that during disturbed conditions in summer the daytime meridional wind changes from zero to 50 m s-l towards the Equator; it may be inferred from Fig. 3 that such a change will result in decreased critical frequencies in the summer hemisphere. On the other hand, it was shown that during disturbed conditions in winter the wind changes from 50 m s-l towards the pole to zero ; Fig. 3 indicates that such a change will result in increased critical frequencies in winter. It should be stressed that the calculations made by Bramley and Young involved approximately uniform meridional winds blowing from one hemisphere to the other; their results (Fig. 3) cannot, therefore,be used to provide any indication of the exact changes which the storm-time winds will produce, but they can nevertheless be invoked to demonstrate that negative storms should be expected in summer and positive storms in winter. It is interesting to gain some physical insight into the various processes involved in the interaction between the storm-time winds and the mechanisms responsible

The effects of thermospheric winds on the ionosphere at low and middle latitudes WIND +------DIRECTION

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Fig. 3. Latitudinal variations of N,P2 calculated for three different arsumed velocities of north-to-south meridionttl winds. The northern hemisphere is the summer hemisphere (after BRAMLEY and Your% 1968).

MAGNETIC LATITUDE(DEGREES)

Fig. 4. Plot showing the directions in which ionization moves, at different heights and latitudes, as a result of the combined effects of the E-W electric field and diffusion down the field lines when the equatorial anomaly is being formed (after HANSONand MOFFE:TT, 1966).

for the equatorial anomaly. Figure 4 (after HANSONand MOFFETT,1966) shows the directions in which ionization moves, at different heights and latitudes, as a result of the combined effects of the E-W electric field and downward diffusion which result in the equatorial anomaly. A net almost horizontal poleward transport of ionization occurs, over quite a wide range of latitude, away from the Equator towards middle latitudes. The equatorward storm-time component of the wind will obviously oppose such transport of ionization and it seems likely that the storm-time thermosphere winds will thus exert some influence on the development of the equatorial anomaly. It is interesting that the equatorial anomaly is usually less developed on storm days than on quiet days (KING et al., 1967) ; the critical frequencies in the trough above the Equator are greater than normal and the anomaly crests are less pronounced. A possible explanation of the various daytime storm effects observed at lowermiddle latitudes may be as follows: On quiet days in the summer hemisphere the thermospheric winds are small (because the diurnal atmospheric ‘bulge’ is located

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J. D. BURUE,D. ECCLES,J. W. KINU

and R. ROSTER

at the subsolar latitude and the pressure gradients at lower-middle latitudes are therefore small) and do not markedly affect the electron concentrations. On disturbed days, however, the equatorward storm-time meridional wind of about 50 m s-l is of sufficient magnitude to oppose significantly the poleward motion of ionization which leads to the equatorial anomaly ; the normal development of the anomaly crest is thus prevented and decreased foF2 values, i.e. negative storms, may result. On quiet days in winter, however, normal thermospheric winds of the order of 50 m s-1 are directed towards the pole; their main effect is to reduce the electron concentration by blowing ionization down the magnetic field lines into regions of rapid recombination. On disturbed da.ys the storm-time component opposes this process and larger-than-normal f,F2 values, i.e. positive storms, result. In particularly large storms in winter, however, the equatorward storm-time component will be larger than 50 m s-l and will thus not only overcome the normal poleward thermospheric wind, but will also significantly oppose the formation of the equatorial anomaly crest; negative storms may thus occur as in summer. 5. SUMMARY AND DISCUSSION It has been shown that Roemer’s modification to Jacchia’s 1965 model atmosphere to take into account the effects of storm-time preferential heating of the highlatitude atmosphere leads to significant changes in the calculated global thermospheric wind system. The heating produces an additional storm-time equatorward wind component of about 50 m s-i at 20“ latitude. This wind greatly enhances the very small equatorward wind which normally exists in the summer hemisphere by day and is of sufficient magnitude to reverse the daytime poleward wind which normally exists in winter. This storm-time wind may interact with the ionization movements responsible for the equatorial anomaly, as described in Section 4, producing negative storm effects at lower-middle latitudes in summer and positive storm effects in winter. It seems possible that during major disturbances, however, the storm-time winds will produce negative storm effects in both hemispheres. It is encouraging that the qualitative conclusions which relate to the lower-middle latitude ionosphere reached in this paper are consistent with the ionospheric behaviour observed by RAJARAMand RASTO~I(1970) during the main phases of magnetic storms in different seasons at, Maui (20“N, 156OW). It is interesting that the suggested consequences of the storm-time winds at lower-middle latitudes are also similar to the behaviour generally observed at middle latitudes, viz. negative storms in summer, but positive storms during minor disturbances and negative storms during major disturbances in winter (MAEDA and SATO, 1959). This agreement suggests the interesting possibility that middle latitude storm effects may also be explained in a similar manner. It is obviously desirable that a quantitative theoretical treatment of the problems discussed above should be undertaken. In order to do this, it will be necessary to obtain simultaneous latitude- and time-dependent solutions of the equations of motion of the neutral air and the ionospheric continuity equation taking into account the effects of electric fields and the diffusion of ionization along the Earth’s magnetic field lines. Such a treatment, although complicated, would enable the interesting and possibly very important interrelationships between the middle- and the lowerlatitude ionospheres to be properly investigated.

The effects of thermospheric winds on the ionosphere at low and middle latitudes

623

Acknowledgements-The work described in this paper forms part of a co-operative project undertaken by the Max-Planck-Institut fti Aeronomie, Lindau, and the Radio and Space Research Station, Slough, and is published by permission of the Directors of these laboratories. REFERENCES Planet. Space Sci. 17, 1269.

ABXJR-ROBBM. F. K. BRAMLEY E. N. and YOUNU M. COLE K. D. HANSON W. B. and MOFFETT R. J. JACCHIA L. G.

1969 1968 1962 1966 1965

JACCHIA L. G., SLOWEY J. W. and VERNIANI F. JONEZSK. L. and RISHBETH H. KINQ J. W., REED K. C., OLATTJNJI E. 0. and L~ao A. J. KOHL H. and KINQ J. W. KOHL H., KINU J. W. and ECCLES D. MAEDA K. I. and SATO T. RAJARAM G. and RASTOQI R. G. RISHBETH H. ROEMER M.

1967

J. atmoe. tern. Phys. 30, 99. Au&. J. Phys. 15, 223. J. geophys. Res. 71, 6559. Space Research V (Edited by D. G. KINQHELE, P. MULLER and G. RIQHINI), p. 1135. North-Holland, Amsterdam. J. geophys. Res. 72, 1423.

1971 1967

J. atmos. terr. Phys. J. atmos. terr. Phys.

1967 1968 1959 1970 1972 1971

J. atmos. tew. Phys. J. atmos. terr. Phys. Proc. IRE 47, 232. J. atmos. terr. Phys. J. atmos. ters. Phys. Space Research XI KONDRATYEV, M. SAUAN), p. 965. Berlin.

33, 391. 29, 1355. 29, 1045.

30, 1733. 32, 113. 34, 1. (Edited by K. YA, J. RYCROFT and C. Akademie-Verlag.