Injection of Intense Storm Ring Current Ions

INJECTION OF INTENSE STORM RING CURRENT IONS L. Xiel, Z. Y. Pu 1'2 , B. Yu 1, S. Y. Ful, Q. G. Zong 3 , J. N. Tu4,

1Department of Geophysics, Peking University, l O0871,Beijing, China 2Space Weather lab, CSSAR, Chinese Academy of Science, Beo'ing 100080, China 3Center for Space Physics, Boston University, MA 02215, USA 4Center for Space Plasma and Aeronomic Research, University of Alabama in Huntsville, A135899, USA

ABSTRACT Injection of storm-time ring current ions is investigated with 3-D test particle trajectory calculations (TPTCs). Two major and important new results are presented in this paper. It is found that a new seperatrix exits between open and trajectories of drifting particles and that the shielding electric field plays an important role for the formation of the closed symmetric ring current. INTRODUCTION Great and intense storms are the most severe space weather phenomena in geospace. Storm mechanism has long been a topic center to solar-terrestrial physics and space weather research. Most intense magnetic storms are the response of magnetosphere to the passage of a coronal mass ejection from the Sun. They are initiated when enhanced energy transfer from the solar wind into the magnetosphere leads to intensification of the ring current. However, it is not well understood yet where the main ion population of storm-time ring current originates and by what processes these ions are injected into low-L regions to form the ring current. In this paper we investigate these fundamental problems through 3-D test particle trajectory calculations (TPTCs). Two major and important new results are presented. D TPTCS AND MODEL Magnetic field models The T96 model (Tsyganenko, 1996) is used in the calculation. The model requires the following input parameters: solar wind pressure, Dst-index, By and Bz-components of the interplanetary magnetic field. Besides the T96 model, we also use the dipole magnetic field model in a part of calculations for the processes developing in the early main phase. Electric field models There are two main sources of magnetospheric electric fields: The solar wind related dawn-to-dusk convection field (Ec) and the co-rotation electric field (Eco) related to the rotation of the Earth along its spin axis. The Er fields have a strong effect on the drift paths of the magnetospheric plasma. The low energy particles move primarily under the ExB drift, and are less affected by the magnetic gradient and curvature drifts. The energetic particles follow the magnetic drifts more readily, while their trajectories are also affected by the ExB drift. Two types of Er fields are used in the present study: the Volland field (Volland, 1978) and uniform field. For particles mirroring at higher latitudes, one has to trace the field lines from the equatorial to off-equatorial points to get the corresponding potential and electric fields. Therefore our model is able to admit any magnetic field configuration and reflect field line stretching during storm and substorm activities. Our calculations indicate that the results given by the Volland field and the uniform field are qualitatively identical with each other when adequate parameter are adopted.

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RESULTS Suppose that at t=-00 § ions with E=5-20 keV are located at X=-8 and Y=-2--8 and that the Ec is 10-15 KV/Rz in great storms. The following are our new findings that are of importance for the storm-time ring current formation: A new type separatrix of an inversed" O" "shape is presented between open and closed drift trajectories. Fig. l a shows the conventional separatrix in the equatorial plane (Kamide et.al, 1997). Fig. lb shows a new type separatrix in the equator. It is seen that particle trajectories outside and inside the separatrix are open and closed respectively. Fig.2a plots time vs the drifting distance for O § corresponding to the separatrix trajectory in Fig. lb. It is seen that O § spends about 1 hour to move from the magnetotail (X=-8) to the ring current region and about 2 Pitchangle:90~ ConvectionElectic:10KV/Re MagneticField:T96

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hours to form a closed trajectory. This indicates that intense Ec can quickly (within 2 hours) drive particles in the near-Earth tail to low altitudes building up the ring current. Fig.2b plots the corresponding energization process of these O § ions. The TPTCs thus affirm that the intense Ec can effectively inject and energize charged particles to form the storm-time ring current as they drift closer to the Earth. 2. A new mechanism responsible for the formation of closed symmetric ring current exists.

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Injection of Intense Storm Ring Current Ions It was commonly accepted that the formation of the closed symmetric ring current critically relies on the presence of storm-associated fluctuations in the Ec field as these -5PitchAngle:90~ can lead to a diffusion transport of the particles from MagneticField:Dipolar open trajectories to close ones (Kamide et.al, 1997). However, our TPTCs indicate that the shielding electric -3field created along with the injection of charged particles from the near-earth tail to the inner -2magnetosphere can change a open trajectory into a closed one. This may play an important role in the tv" " ~-0building up of the symmetric closed ring current, which t,O is consistant with the fact that the symmetric ring >current was found to be mostly created during the late 2 storm main phase (Kamide et.al, 1997). Trajectories A and B in Fig.3 illustrate two drift paths of same particles in the case with or without a shielding electric field respectively. In our calculations, the shielding electric 12 ''01 . . 8. . . 6 4' . . 2. . . 0 -2' . .-4. . . -6. . -8 -; 0 field is assumed to banlance the Ec at X>7RE. We will X(GSM,Re) present more detailed calculations and dicussions in this aspect in the next paper. Figure 3 ACKNOWLEDGMENT This work is supported by the chineses project G20000784 and CNSF grants 49984002,49834040. REFERENCE Tsyganenko, N.A., Effects of the solar wind conditions on the global magnetospheric configuration as deduced from data-based field models, in: Proc.of 3rd International Conference on Substorms(ICS-3), Versailles, France, 12-17 May 1996, ESA SP-389, p. 181-185,1996. Volland, H., A model of the magnetospheric electric convection field, J. Geophysics.Res, 83, 2695, 1978. Kamide, Y., R.L.Mcpherron, W.D.Gonzalez, D.C.Hamilton, H.S.Hudson, J.A.Joselyn, S.W.Kahler, L.R.Lyons, H.Lundstedt, and E. Szuszczewicz, Magnetic storm: current understanding and outstanding questions, Magnetic Storm,Geophysics Monograp 98, pp. 1, AGU, Washington, D.C., 1997.

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