L.N. Makarova and A.V. Shirochkov Arctic and Antarctic Research Institute 38 Bering Str., Saint-Petersburg, 199397 Russia
Introduction
The polar ionosphere is basically a part of the Earth's magnetosphere and therefore it is influenced the standard ionospheric controlling factors and also by the level of magnetospheric disturbance which, in its turn, is mainly controlled by the interplanetary magnetic field (IMF) and solar wind dynamics.
Among the IMF and solar wind parameters which can be considered as polar ionosphere controlling factors, the solar wind dynamic pressure Psw = mnV2 (where m is a proton mass, n is the solar wind number density and V is the solar wind velocity) is a comparatively new candidate. Only recently Newell and Meng (1994) showed that enhanced solar wind dynamic pressure causes significant spatial changes to the particle precipitation regions of the magnetosphere. Contribution of other IMF and solar wind parameters to this process, including Bz, was rather insignificant.
Our straightforward attempts to find direct ionospheric signatures of the increased solar wind dynamic pressure give contradictory results. Sometimes notable drops in the daytime foF2 values at the high-latitude ionosondes, with a time lag of several hours, follow periods of enhanced Psw between the events. In these cases the diurnal variation of the F2 layer electron density actually disappears since the daytime and night-time foF2 values became equal. However, not every increase in Psw causes such dramatic ionospheric effects.
It is important to remember that the solar wind dynamic pressure is an external force determining the position of the Earth's magnetopause. The Earth's main internal magnetic field balances the solar wind dynamic pressure influence. The IMF is another force which influences the magnetopause position by means of geomagnetic field line erosion on the dayside of the magnetopause and by changing the net value of the Earth's magnetic field. Consequently it is necessary to take into account the influence of both the solar wind and the IMF to evaluate the magnetopause position. This has been done in the empirical model developed by Roelof and Sibeck [1993]. These considerations helped us solve the mysterious connection between the enhanced solar wind dynamic pressure and the previously mentioned anomalous foF2 diurnal variations. The former events occur only under a definite combination of IMF Bz-component and the solar wind dynamic pressure magnitudes, namely: when Bz is sufficiently negative and Psw is above about 2 hPa. Consequently it was quite naturally to try to find a connection between the daytime foF2 values in the polar ionosphere and the magnetopause position.
Results And Discussion
The ionospheric data used in this study were taken from the results of regular observations performed at several Russian ground-based vertical ionosondes: two Antarctic stations: Vostok (78.5 S;106.9 E; invariant latitude 83,2 S) and Mirny (66.6 S; 93.0 E; invariant latitude 76.9) as well as one Northern hemisphere station- Dixon (73,4 N; 80,6 E; invariant latitude 67.7 N). These stations were chosen for the study for two reasons: first of all, the very good quality of their data and second, to cover a wide range of invariant latitudes. Besides that, they represent both the Earth's hemispheres.
Noon (in magnetic local time - MLT) values of the F2 - region critical frequency - foF2 were taken, preferably during the winter season, in order to avoid periods when solar EUV radiation dominated the polar ionosphere. Statistically significant numbers of samples were taken to secure reliable results. Thus, data were taken for three winter months, day-by-day, apart from periods when IMF data were not available (King, 1977, 1986). The magnetopause position (in number of Earth radii - Re) was taken from the empirical model of Roelof and Sibeck (1993, their Figures 16 and 17) in accordance with the values of n, V and Bz, corresponding to the moment the foF2 recordings were made.
The results of this study are presented at Figure 1, where the foF2 noon dependence is given as a function of the subsolar magnetopause positions, expressed in the units of Re. The upper panel shows data for Vostok station, the middle one - for Mirny station while the bottom one - for Dixon. The results of a statistical analysis of the data are given on each panel. One can see that the correlation coefficients, R, between the two sets of data are impressively high for all three stations, with a very high confidence level (p).
The general conclusion is that the daytime ionisation level in the polar ionosphere reduces to about half when the magnetopause approaches the Earth to a distance as close as about 9Re. It was shown earlier that for undisturbed conditions, the magnetopause is located at a distance of about 14Re [Holzer and Slavin, 1978]. The trend discovered seems to be universal, i.e it is common for the stations located in the polar cap, and in the auroral oval, of the both hemispheres. Another intriguing effect is evident at Figure 1. The slope of the linear regression lines changes with the invariant latitude of the observational point. It seems to be a signature of magnetopause shape changes near the Earth's geomagnetic pole.
Right now, little can be said about the exact physical mechanisms responsible for these phenomena whose reality is beyond any doubt. We are playing with an idea of the universal version of the global electric circuit with the magnetopause as the external element and the Earth's surface the internal one. In this scheme, inward and outward displacements of the magnetopause can be considered as a measure of energy accumulated in the "magnetosphere-ionosphere-atmosphere" system. Such an approach seems to have a good chance of explaining some existing puzzles. Our opinion is that solving these problems will shed new light on the whole fundamental problem of energy transfer from interplanetary space to the Earth's magnetosphere and ionosphere. The high degree of correlation between the daytime foF2 values in the polar ionosphere and the magnetopause position suggests a rough diagnostic for the latter parameter using standard ground-based vertical ionosonde data, i.e. to expand the possibilities for using these ionospheric observations.
Conclusion
A rather unexpected connection between the magnetopause position and the daytime ionisation level in the polar ionosphere is found. It is premature to present exact physical mechanisms responsible for this phenomenon. It must be done during future studies, but the results presented here are the good evidence of the hidden possibilities for standard, routine ionospheric observations whose value has sometimes been attacked by advocates of more sophisticated techniques.
References
R.E. Holzer and J.A. Slavin, Magnetic flux transfer associated with expansions and contractions of the dayside magnetosphere, J. Geophys. Res., 83 (A8), 3831-3840, 1978.
J.H. King, Interplanetary Medium Data Book, NASA, 1977,1986.
P.T. Newell and C.-I. Meng, Ionospheric projection of magnetospheric regions under low and high solar wind pressure conditions, J. Geophys. Res., 99 (A1), 273-279, 1994.
E.C. Roelof and D.G. Sibeck, Magnetopause shape as a bivariate function of interplanetary magnetic field Bz and solar wind dynamic pressure, J.Geophys.Res.,98 (A12),21421-21450,1993.