Comparison of O+H+ and O+(H++He+) transition levels

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A& SpaceRes.Vol. 22. No. 6, pp. 895-898, 1998 0 1998 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/98 $19.00 + 0.00 PII: SO273-I 177(98)00119-7

COMPARISON OF 0+/H+ AND O+/(H++He+) TRANSITION LEVELS L. Tiiskova’, V. Truhlik’, J. Smilauer’, and Yu. A. Shultchishin2

‘Institute ofAtmospheric Physics of Acad. Sci. Czech Rep., BOW II, 14131 Prague 4, Czech Republic 2Space Research Institute, Russian Acad. Sci., Profsoyuznqa 84, I1 7810 Moscow, Russia

ABSTRACT Ion composition data from the Bennett ion mass spectrometer onboard the Intercosmos 24 satellite (perigee -550 km, apogee -2500 km) show that including He+ ions results in a lowering of the upper transition height (from O+ to light ions). Previous results of 0+/H+ transition are compared with the O+/(H++He’) transition for equinox and solar maximum conditions. The O’/(H+ +He+) transition surface is shifted to higher latitudes by several degrees and to lower heights by about one hundred km in comparison with the 0+/H+ transition. 01998 COSPAR. Published by Elsevier Science Ltd.

INTRODUCTION The upper transition height (Hr) is a helpful tool in describing the distribution of major ions and may be used as an anchor point for empirical models of the ionospheric ion composition profile (Bilitza, 1991). Truhlik et al. (1997) dealt with the 0+/H+ transition during 1989-l 992 (maximum of the 22nd solar cycle) in the height range of 500 to 2500 km for invariant latitudes up to 83 ‘. As in other papers (e.g. Kutiev et al., 1994) Hr was then defined considering only O+ and H+ ions because He+ ions represent usually one of the minor components. In some cases for high solar activity and a certain contiguration of neutral winds, however, considerable concentration and even dominance of He+ is reported (Heelis et al., 1990), cf. Figure 1. Shultchisbin et al. (1996) observed for the first time a steady He+ dominance at low and middle latitudes of both hemispheres in a wide range of altitudes (700 - 2500 km). The data concerned equinox in maximum period of the solar cycle 22. Using the same mass spectrometer data base, 0+/H+ and O’/(H++He+) transitions will be compared. DATA We used in this study data from the ACTIVE mission (perigee 500 km, apogee 2500 km, inclination 83 “) obtained with two instruments aboard; three stage Bennett mass spectrometer: sensitivity -1 ion/cm, resolution MMM -30 (at 10% level), two working modes, a) full spectrum up to 65 a.m.u. (in 6 s), b) selected peaks (often 8) (0.1 s/l peak); Retarding Potential Analyzer: planar ion traps in the x and z direction, x parallel to the velocity vector, z towards the zenith, period: November 1989 - May 1991. Detailed description of the instruments and calibration of measured data to absolute concentration measured by RPA is given in (Afonin et al., 1994). About 1500 transitions of each type, 0+/H+ and O+/(H++He+) were found and analyzed. 895

56s 109 127 5.62 53.7 t4:20

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1474 117 413 122 -3.0

1983 108 454 1.77 -31.2

2351 96.1 557 4.06 54.5

2507 84.2 a44 10.4 -682

"2428 73.3 1312 8.38 -51.1

2124 64.7 1457 2Bs -43.9

IS49 61.0 M46 1.62 -23.3

1109 65.6 1627 1.18 5.1

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118 327

674 79.5 1733 2.18 i&w

51S 98.6 2232 18.1 l&WI

712 114 252 2.18

1165 ALTfkw 119 WI ml) 352 MLT(h:mf 1.20 l.

1R?o 1&M UT(h:n7}

Fig. 1. Example of ion composition with three major ions (0’, Ht, and He’ ) measured by ion mass speedometer onboard the Intercosmos 24 satellite. Dominance of He+ marked by arrows. (ALT - altitude, CHI - solar zenith angle, MLT - magnetic local time, L - McIlwain parameter, INL - invariant latitude, UT - Universal Time - 14.3.1991). -

RESULTS H, values depend strongly on the Iatitude. Plotted around Earth they form a kind of shell the shape of which is far from being spherical. Data of an elliptically orbiting satellite cannot yield any possibility of a s~~~eo~s i~fo~ation on the WIH’ and Q*/(H+ Me’) tuition; it is only possible to compare them as two separate sets. For ~dIati~d~ and equinox cond~tjons (March 21 f 20 days and September 23 it 20 days) we have grouped both sets in Figures 2 and 3. For Figure 2 we applied least squares fitting to obtain the transition height as a function of invariant latitude and Solar Local Time. Three characteristic shapes of its latitudinal dependence (night, morning, and noon) are shown for linvh 5 40”. For /invl/ > 40” H, rapidIy 2500 increases, This can be seen in Figure 3b that illustrates nighttime shape of I-L,in the &II range of latitudes for 2iXu the I991 spring equinox. The mpid increase of H-rin midlatitudes can be caused by a very low density of H* ions in the light ion trough md can be governed by the position of the equatorial edge of this trough.

g.QW %

Approximations in Figure 2 are: for day and down B, = AcoshfBinvL) + C and by night H, = AcosfBinvl)

+ C.

Fig. 2. Least squares fitting and ~o~spondin~ standard deviations of H.&m depending on invl/deg and SLT for equinoxes 1990 and )invl/II 40” (--0+/H+ , ----- 0+/f&” +He’)).

0+/H+ and O+/(H++He+) Transition Lcvrls

897

Figure 3a is a massplot of data used in the least squares fitting leading to Figure 2; only day and night values (for linvl( < 40”) are plotted. Night transition heights in Figure 3a which refer to a solar activity of F10.7 - 180 are lower than those of Figure 3b referring to an activity of F10.7 - 230.

Fig. 3. H$km depending on invariantlatitudefdega) measured at the 1990equinox; 0, 0 - day (OS- 16 h SLT); N, 0 - night (20 - 04 h SLT); l, m - Of/H+; 0, q - O+/(H++He+). b) ~i~~~e (20 - 04 h SLT) HT depending on invl for the 1991spring equinox (m 0+/H’, a O+/(H++He+)).

Comparing both sets in Figure 2 yields important height differences, up to 100 km, the general behavior of shells of both sets being alike. From Figure 2 we also see that the strongest changes of the transition height with SLT (day, night), about 1500 km, occur at invariant latitudes in the vicinity off 30 ‘, while at the equator changes of about 500 km appear.

0

-25

7 -30

6 -5 =4

-35 z .E

162

163

lb4

165 day

166

167

166

The changes of HT in middle latitudes during and after a geomagnetic storm are shown in Figure 4 demonstrating that with geomagnetic activity, both 0+/H* and 0+/@-P + He’) ~sitions move equatorward (at a constant altitude) as consequence of a shifting of the equatorial edge of the light ion trough. Vice versa, enhanced geomagnetic activity results in increasing of transition height at a given invariant latitude mainly due to the decrease of light ion density in the outer ionosphere. CONCLUSION

Fig. 4. Data measured during the ACTIVE mission over the Southern hemisphere (heights near apogee) in the period of 1l-1 6 June 1990 including the storm with SSC on 12 June 1990; 08.20 UT. Evolution of Kp (--) and of invariant latitudeldeg of the 0+/H+ (+) and O+/(H’ + He”) (0) transition.

A data base resulting from thermal ion composition me~~ements in the framework of the ACTIVE project allowed to construct height surfaces (shells) showing where n(0’) = n(H+) or ~(0’) = n(H+) + n(He”). Our analysis was made for a period of a very high solar activity (F10.7 - 200) in the altitude range up to 2500 km.

While the general behavior of both sets of HT is comparable, the transition with He+ inciuded appears at a lower height than that for 0+/H+ formerly defined.

898

L. Tifskovti er al

The transition height depends strongly on invariant latitude, solar local time, and geomagnetic and solar activity. At night, a local maximum occurs above the equator while a minimum appears there by day. The strongest changes of H, with SLT occurred in the vicinity off 30’ invariant latitude. For a given invariant latitude and Solar Local Time Hr increases with increasing geomagnetic activity, while for a given altitude both, the 0+&I+ and O+/(H+ +He’) transition are observed at a lower invariant latitude.

The authors thank F. Krupka for his help in data processing. This research was partially supported by grants No. A3042603/1996 and No. A30427030997 of the Grant Agency of Academy of Sciences of the Czech Republic. REFERENCES Afonin, V.V. , K.V. Grechnev, V.A. Erchova, 0.2. Roste, N.F. Smirnova, J. Smilauer and Yu.A. Shultchishin, Ion Composition and Temperature of Ionosphere at Maximum of 22nd Solar Activity Cycle from Satellite “Intercosmos-24” (Project Active), Cosmic Res., 32,82( 1994) (Russian). Bilitza D., The Use of Transition Heights for Representation of Ion Composition, A& Space Res., 10, (11)183 (1991). Heeiis, R.A., W.B. Hanson, and G.J. Bailey, Distributions of He+ at Middle and Equatorial Latitudes during Solar Maximum, J. Geop~ys. Res., 95, 10,313(1990). Kutiev, I., S. Stankov, and P. Marinov, Analytical Expression of O+ - H+ Ion Transition Surface for Use in IRI, Adv. Space Res., 14, (12)135-(12)138(1994), Shultchishin, Yu.A., V.V. Afonin, K.V. Grechnev, V.A. Ershova, V.A. Kochnev, 0. 2. Roste, N. F. Smimova, and J. Smilauer, Intercosmos-24: Helium Ion Predominance during Equinox at Low and Middle Latitudes in the 22nd Solar Activity Cycle, Adv. Space Res., 18,(3)15-(3)l S(1996). Truhlik, V., J. Smilauer, L. T%kova, Y.A. Sh~~~shin, and N.F. Smirnova, M~ifes~tion of Ionospheric Storms in the Upper Transition Height, 3 1st Scientific Assembly COSPAR, Bi~ingh~, July 1996. Symposium C4.2.