Electron density variations in the southern hemisphere topside ionosphere during a magnetic storm

Journal of Atmospheric and Terrestrial Physics, 1969,Vol. 31, pp. 463-468. Pergamon Press. Printed in Northern Ireland

SHORT PAPER

Electron density variations in the southern hemisphere topside ionosphere during a magnetic storm ALAN H. KATZ Avco Space Systems Division, Lowell, Massachusetts 01851 (Received 15 &lay 1968; in revised form. 30 August 1968) Ab&&--Alouette I topside electron density profiles at southern latitudes are compared for magnetically quiet and disturbed periods surrounding the Sudden Commencement magnetic storm of 1’7December 1962. Of particular note was the fact that the first post-commencement pass, which was recorded only 18 min after the Storm Sudden Commencement (SSC) at dip angles between 60’ and 70’S, showed a large increase in electron density between 67’ and 69% dip, with the percentage increase being greater at higher altitudes. It is suggested that, because of the short time interval between the SSC and the observed ionospheric effects, and because the high altitudes were affected more strongly than lower altitudes, the increase at these dip angles can be attributed to additional ionization resulting from precipitating particles in the low energy range (i.e., several eV and above) caused by the contraction of the magnetosphere. OBSERVATIONAL

RESULTS

true height profiles derived from Alouette I ionograms taken at southern hemisphere stations along the 75”W meridian (Canadian Defense Research Board, 1965), the effects of an M-region Sudden Commencement magnetic storm on the topside ionosphere were studied. The storm began at 1649 UT, 17 December 1962, and the observations in the southern hemisphere during this storm occurred between 1200 and 1600 LMT and 2200 and 0000 LMT. The local time intervals correspond to the satellite traveling over a longitude range from 80” to 3O”W. Individual profiles were compared when they were at the same dip angle, and the longitudes of all profiles were within a 15” interval for the daytime passes and within 10” for the nighttime passes. Synoptic mapping of foF2 over the southern polar region (Penndorf, 1968) has shown that south of 30”s geographic latitude during December, foF2 is independent of longitude between 75” and 3O”W for the hours 15-18 UT. Meaningful comparisons can thus be made for the local noon observations, but care must be taken when comparing nighttime profiles. This is due to the position of the sunrise line during the summer night at high latitudes. In order to present the results in a concise form, the percentage deviations of the storm time electron densities from the quiet electron densities are shown at selected heights as a function of dip angle for both the day and night observations. The 17 December 1962 electron density profiles were obtained shortly after the SSC: 30”s dip angle at 1654 UT (5 min after SSC), and 67”s dip angle at 1707 UT (18 min after SSC). Figures 1 (a) and 1 (b) show the percentage deviations for the midday observation for 17 and 18 December, respectively, where 16 December 1962 is considered

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Fig. 2. Ionograms on 16 December 1962 (quiet day) and 1’7 December 1962 (disturbed day) for t,hemidday pass at latitudes where the peak in electron density is observed. (Note the difference in frequency scale of each set of ionograms.)

464

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ALAN

H. KATZ

the quiet reference day. At dip angles (all dip angles refer to the satellite height) between 66’ and 69‘3, a large increase in electron density is observed at all altitudes. (No data are available south of 69”s dip angle.) The increase on 17 December 1962 is approximately 500 per cent at 1000 km, as compared to electron densities on 16 December, and the apparent enhancement decreases with decreasing altitude,

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Fig. 1 (a). Percentage deviation of disturbed electron density profile (1’7 December 1962) from quiet day electron density profile (16 December 1962) for midday observations.

being approximately 250 per cent at 400 km. The increase at 400 km implies an additional 23 x lo4 cl/cc, whereas a 500 per cent increase at 1000 km implies only an additional 4.9 x lo4 cl/cc. True height profiles were not available for the 65°-700S dip angle range after 18 December for the noontime observations, but plasma frequencies at the height of satellite were available from Alosyn data (Defense Research Board, 1965). Table 1 shows the maximum percentage increase in electron density at the satellite height, observed between 67’ and 69”s dip (where the peak occurs), along with the daily sum of K,, for 16-23 December 1962. During disturbed periods, the Alouette I ionograms can become quite difficult to scale. Features such as spreading of the extraordinary trace and the occurrence of multiple echoes renders interpretation of ionograms recorded uncertain at this time. The ionograms recorded at Solant were inspected in order to determine whether the results discussed in this paper are at least qualitatively correct. Figure 2 shows several ionograms recorded on 16 December 1962 and 17 December 1962 during midday passes at the latitudes where the increase in electron density

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Fig. 1 (b). Percentage deviation 1962) from quiet day electron

in the southern

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of disturbed electron density profile (18 December density profile (16 December 1962) for midday observations.

was observed (see Fig. l(a)). There is only a one degree change in dip between Fig. 2 (a) (68”s) and Fig. 2 (c) (69’S). The ionograms corresponding to 1’7 December display spreading where the increase in electron density is observed. It would, at first glance, seem difficult to scale these ionograms and compare the resulting electron densities with those determined from the 16 December ionograms. The accuracy of the disturbed ionograms is obviously not as good as that of the quiet Table 1. The percentage deviation of disturbed electron densities from quiet electron density for the midday observations at the height of satellite, and daily sum of the planetary 3-hr range indices (I&,), for 16-23 December 1962 Date 16 17 18 19 20 21 22 23

Dec. Dec. Dec. Dec. Dec. Dec. Dec. Dec.

* No data available. 10

Deviation 0 560 250 NDA* 250 170 60 -30

( %)

=KP 10 30 37 38 35 29 20 7

ALAN H. KATZ

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ionograms, but a comparison between the two can be made, and it is seen that the effect shown in Fig. l(a) is real. The arrow at the top of each ionogram indicates the plasma frequency of the extraordinary wave at the height of the satellite (fxS), and the arrow at the bottom shows the extraordinary wave critical frequency of the F-layer (fxP2) as observed by the satellite. As can be seen, the ionograms on

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Fig. 3(a). Percentage deviation of disturbed electron density profile (18 Deoember 1962) from quiet day electron density pro& (16 December 1962) for night observations.

16 December do not change over this dip latitude range, but on 17 December fxS On the disturbed day fxF2 first increases, and fxF2 increase with dip latitude. then decreases, whereas fxS is always increasing, as can be seen from Figs. 2(a)2(c). This is reflected in Fig. l(a), where between 68’ and 69% dip the percentage change at lower altitudes is decreasing while the change at the higher altitudes is still increasing. Figures 3(a) and 3(b) show the percentage deviations for the observations obtained near local midnight (0400 UT). These observations are restricted to 40”-62”s dip angle range, with no data being available for the 17 and 19 of December. Figure 3(a) shows the percentage deviation of electron densities recorded on 18 December (11 hr after the start of the magnetic storm) from the values recorded There is an increase in the percentage deviation between 50” and on 16 December. 58”S, which then drops off rather sharply above 53%. The results pertaining to 400 km are not available at all latitudes, because the height of the FZ peak rose at times above 400 km. It appears from Fig. 2(a) that the 600 km level is more

Electron density variations in the southern hemisphere

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Fig. 3(b). Percentage deviation of disturbed electron density profile (20 December 1962) from quiet day electron density profile (16 December 1962) for night observations. strongly affected (as much as a 300 per cent increase) than higher and lower altitudes. By 20 December 1962 [Fig. 3(b)] the storm effects seem to have disappeared, and a slight decrease in percentage deviation is observed at all altitudes as the dip angle increases from 40’ to 60”s. DISCUSSION KING et al. (1967) have also reported on the effects of magnetic storms in the southern hemisphere. They observed a latitudinal effect which indicated a smaller percentage increase at greater geomagnetic latitude, and their results also showed that lower altitudes were more strongly affected than higher altitudes. These findings differ from those of the present investigation, particularly for the changes in Fig. l(a), which shows a large percentage increase between 67” and 69% dip, and where high altitudes are percentage-wise more strongly affected. King et al. suggested that some of the effects of magnetic storms could be due to an upward movement of the ionosphere. REDDY et al. (1967), using electron density at 640 km obtained from Tiros 7, showed that during three magnetic storms a maximum increase in electron density of approximately 700-800 per cent developed south of 50’S geomagnetic latitude. The maximum increase occurred on the average less than two hours after peak magnetic activity. Reddy et al. suggest that an expansion of the neutral atmosphere due to high latitude heating causes the increase in storm time electron densities at high latitudes.

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The large increase in electron density observed only 18 min after the beginning of the Sudden Commencement storm of 17 December 1962 suggests an additional mechanism which might influence the topside ionosphere at high latitudes during magnetic storms. Measurements by FRANK et al. (1964) of the equatorial flux of electrons with energies greater than 40 keV show an increase by a factor of 50 at L > 3 in the flux due to the magnetic storm of 17 December 1962. Although the peak production of electrons by 40 keV electrons typically occurs at a height of 100 km, an increase in flux at these energies indicates the changes in the magnetospheric particle densities that are occurring. It would appear that it is lower energy electrons, perhaps several eV and above caused by the contraction of the magnetosphere, which are important in producing the increase in electron density observed only 18 min after SSC. This is partially supported by a positive correlation between K, and electron densities at 2 eV on storm days, for the period 4 October-5 December 1964 (SERBU and MAIER, 1966). It must be noted that more electrons were added at the lower altitudes as a result of the magnetic storm, even though the electron density showed a smaller percentage increase than at the This could be due to higher energy electrons adding more higher altitudes. ionization at lower altitudes than was added by lower energy electrons at 1000 km. The peak observed on 18 December, and the increase in electron density at the height of the satellite as observed for the next 3-4 days, can either be attributed directly to precipitating particles, or to thermal expansion of the neutral atmosphere caused by high latitude heating mechanisms, as suggested by Reddy et al. The effects which were observed during the midnight observations are more in agreement with King et al., in that there is a sharp decrease in percentage increase above 55”s dip angle, although the largest percentage increases are seen at 600 km and not at lower altitudes as reported by King et al. The decrease seen below 50”s dip angle was not indicated by King et al., however. Acknowledgments-1

would like to thank Drs. R. Penndorf and C. Rush of Avco/SSD for helpful discussions in the preparation of this paper. This work was supported by the National Science Foundation under Contract NSF-C-515.

REFERENCES Defense Research Board

1965

Alouette I Ionoqhric

Data N(h), Vol. I(I), Defense Research Telecommunications Establishment, Ottawa, Canada.

Defense Research Board

1965

Alouette I Ionospheric Data ALOSYN, 16 December 1962 to 31 December 1962, Defense Research Telecommunications Establishment, Ottawa, Canada.

FRANK L. A., VAN ALLEN J. A. and HILLS H. K. KINC J. W., REED K. C., OLATUNJI E. 0. and LEGG A. J. PENNDORF R. REDDY B. M., BRACE L. H. and FINDLAY J. A. SERBU G. P. and MAIER E. J. R.

1964

J. geophys. Res. 69, 13.

1967

J. Atmosph. Terr. Phys. 29, 1355.

1968 1967

J. geophys. Res. 72, 2709.

1966

J. geophye. Res. 71, 3755.

Private communication.