Classification of large-scale and meso-scale ion dispersion patterns observed by VIKING over the cusp-mantle region M. YAMAUCHI AND R. LUNDIN Swedish Institute of Space Physics Box 812, S-98128 Kiruna Sweden ABSTRACT. Large-scale and meso-scale ion injection patterns over the cusp are studied using Viking particle data. Spatial structures and temporal effects are separated by comparing statistically poleward-bound passes and equatorward-bound passes. Statistics of each pattern are obtained for three different IMF conditions (southward, weak, northward), respectively. The large-scale patterns are found to be of spatial origin while the meso-scale patterns are mostly of temporal origin. The result suggests that subsolar reconnection is not the direct source of the cusp particles. For meso-scale patterns, multiple injections (independent injections on the same field line) are much more frequently observed than Òstair cases,Ó indicating that flux transfer events (FTEs) are not causing the main part of the cusp. On basis of these observations, we propose an alternative scenario for the cusp formation which is consistent with high-latitude merging. -------------------------------------------- Yamauchi, M., and R. Lundin (1994), Classification of large-scale and meso-scale ion dispersion patterns observed by Viking over the cusp-mantle region, in "Physical Signatures of Magnetospheric Boundary Layer Process", edited by J. A. Holtet, and A. Egeland, 99-109, Kluwer Acad. Pub., Dordrecht, Netherlands. doi:10.1007/978-94-011-1052-5_7 (accepted manuscript) Copyright: Kluwer Academic Publishers 1994. 1. Introduction The cusp morphology has been extensively studied by Newell and his co-workers using the classification of cusp, mantle, boundary-layer, polar cap, etc. [Newell and Meng, 1992, and references therein] A similar classification was introduced by Kremser and Lundin [1990]. However, the overall large-scale pattern, e.g., characterized by ion energy-time dispersion, yet lacks a good classification. The main established pattern is the decreasing characteristic energy of injected ions toward higher latitude for southward interplanetary magnetic field (IMF) conditions. Other types of overall ion dispersion patterns remain poorly investigated. Here we attempt to do this using Viking particle data. We also investigate meso-scale structures. They can be more important than the large-scale patterns in identifying the actual injection process at high-altitude, but previous satellite measurements have not resolved these meso-scale structures. One difficulty for these studies is the separation of spatial structures and temporal effects. In general a single pass of a satellite cannot determine weather the observed dispersion pattern is an energy-time pattern or an energy-latitude pattern. However, a distinction is possible if we compare statistically the equatorward-bound passes and the poleward-bound passes (see Figure 1). In this paper we show typical examples of ion energy-time (energy-latitude) patterns and their occurrence frequency for different IMF conditions. 2. Southward IMF Figures 2, 3, and 5 to 12 show Viking ion and electron energy-time spectrogram for 0.05 - 1 keV electrons and 0.05 - 40 keV ions. Details of the Viking particle instrument can be found in Geophys. Res. Lett. Special Issue [1988]. 2.1. ORBIT 983 (FIGURE 2: POLEWARD-BOUND PASS) This is a typical southward IMF case. Except for the meso-scale structure, the ion characteristic energy decreases toward the pole [Burch et al., 1982; Reiff et al., 1977]. We very often observe this type of injection for southward IMF cases as summarized below. For meso-scale ion injection there are two types of step-like structures, a discontinuous step (1350:40 UT; type I) and a sequential continuous one (1352:40-1355:30 UT: type II). The characteristic energy Òsteps upÓ across the type II (weak) discontinuity while it Òsteps downÓ across the type I (stair case) discontinuity. The type I could be an FTE signature [Escoubet et al., 1992] if the Òstep downÓ feature is a spatial structure, but this is not clear according to the table below. A distinguishing feature of the type II is the overlap of two independent injections on the same field lines. According to the composition data, both injections constitute the same ion species. From the overlapping feature, it is evident that the magnetosheath plasma may access the same magnetic flux tube in different time, generating two independent plasma ÒcloudsÓ injected toward the ionosphere. The overlapping feature is, however, not always clear in the Viking data because the spatial-energy resolution may not be high enough to resolve it. Recently, the Freja satellite with higher spatial resolution data has indicated that the type II discontinuities may be always overlapping ones [Norberg et al., 1993]. Even with Viking's lower resolution, the type II discontinuity is much more frequently observed than the type I. The observation frequencies of these discontinuities are listed in Table 1 for traversals when the IMF data is available. Note that there is another type of discontinuity at the interface between the boundary cusp and the cusp proper [Woch and Lundin, 1992], but that will be excluded from the present survey. Table 1: Meso-scale features discontinuityBz < -1Bz=0~2Bz>3Type I (stair case)*1,*24 (1)*3 / 353 (3)*3 / 290 (0)*3 / 24Type II (multiple injection)17 (10)*4 / 2919 (13)*4 / 2518 (13)*4 / 21no discontinuity9 / 294 / 253 / 21*1 including both step ÒupÓ and ÒdownÓ toward higher latitude across the discontinuity. *2 excluding clear discontinuities at the acceleration region [Woch and Lundin, 1992]. *3 only clear cases (energy steps ÒdownÓ across the discontinuity toward higher latitude) *4 clear overlapping cases 2.2. ORBIT 271 (FIGURE 3: EQUATORWARD-BOUND PASS) The same large-scale spatial dispersion pattern is again recognized for the equatorward-bound pass, indicating that it is spatial rather than temporal. However, the direction of the meso-scale dispersion is opposite to the previous examples if considered an energy-latitude signature (decreasing energy toward the equator). It thus depends on the direction of the satellite traversal. This tendency is confirmed with all 15 equatorward-bound passes and all 18 poleward-bound passes which showed meso-scale injections when the large-scale pattern is of southward IMF conditions. Therefore, it should be considered as an energy-time signature. The overlapping feature, suggesting multiple injections/access on the same flux tube, is also recognized. The typical ion dispersion patterns for southward IMF conditions are summarized in Figure 4. Solid lines show the meso-scale injection patterns and the dashed lines show the large-scale patterns and the envelope of the meso-scale injections. It is very difficult to explained this result by the velocity filter effect alone. Fig. 4. Summary of the large- and meso-scale ion patterns for southward IMF. 3. Northward IMF 3.1. ORBIT 1044 (FIGURE 5: POLEWARD-BOUND PASS) In this example during northward IMF conditions, the ion characteristic energy shows a poleward increase. This is often referred to as the typical pattern for northward IMF conditions with sunward convection. However, the meso-scale dispersion is in the opposite direction. Multiple injections are more frequently observed during northward IMF conditions than during southward IMF conditions. 3.2. ORBIT 119 (FIGURE 6: EQUATORWARD-BOUND PASS) The large-scale ion dispersion is similar to that of the previous example; i.e., the characteristic energy increases in the poleward direction. However, the direction of the meso-scale dispersion is again opposite to the previous case indicating a temporal effect. The main part of the cusp is less dispersive than the poleward part with strong dispersion, and they are well separated from each other by an overlapping feature in between. Again, the velocity filter effect cannot alone explain this feature. 4. Weak IMF 4.1. ORBIT 1049 (FIGURE 7: POLEWARD-BOUND PASS) This example is for weakly northward IMF conditions. The equatorward part is similar to that for southward IMF, but the poleward part is different: the mantle ions suddenly disappear (Òcut offÓ in the figure) without a long tail of dispersion. Thus, this type is clearly distinguished from the other types described above. 4.2. ORBIT 218 (FIGURE 8: EQUATORWARD-BOUND PASS) The magnetosheathlike ion population suddenly disappears at the poleward end in this example too. Again by comparing to the previous case, we identify the large-scale feature as a spatial structure and the meso-scale features as temporal plasma injections. 4.3. ORBIT 967: STAGNANT CUSP (FIGURE 9) Sometimes, we do not recognize any well-defined ion dispersion. This is only observed when the IMF is very weak (see also Reiff et al. [1977]). Table 2 summarizes the observation frequency of the above three types. It shows a clear separation between classes based on the IMF Bz, thus supporting our classification scheme. Table 2: Large-scale classification large-scale dispersionBz<-1Bz=0~2Bz>3typical ÒsouthÓ type2961typical ÒweakÓ type413 (17)*15typical ÒnorthÓ type0520*1 including ÒstagnantÓ type. 5. Variation of the basic patterns 5.1. ORBIT 1003: TRANSIENT CUSP (FIGURE 10) The meso-scale feature sometimes dominates the entire cusp. This example clearly shows that the cusp is consist of multiple plasma injection structures superposed on each other. 5.2. ORBIT 1032: HYBRID CUSP (FIGURE 11) The cusp may contain more complicated patterns, but most of them are just a mixture of the basic patterns mentioned above. This is one of the clear cases when IMF switched from -5 nT to +5 nT instantaneously (1130 UT at nearly 35 Re dayside). Correspondingly, both types of cusp are seen consecutively in the form of multiple injection. The response of the cusp morphology to the change in IMF direction is very pronounced and rapid (less than 10 minutes) even for a northward IMF. A close inspection reveals two types of double peaks in intensity at the same time. The first one (1146-1147 UT) is due to the difference of masses of the injected particles, while the second one (1147-1149 UT) indicates two independent injections on the same field line. Thus the multiple injection is well separated from the different mass effect. 5.3. ORBIT 965: NARROW CUSP (FIGURE 12) The cusp width is not constant. It depends on IMF Bz and the solar wind dynamic pressure [Newell and Meng, 1987, 1993], but apparently on other parameters as well. In this southward IMF type case, the solar wind dynamic pressure was above normal, yet the cusp is very narrow. Thus, great caution is evidently required when predicting the instantaneous cusp based on existing knowledge of the cusp response versus IMF and solar wind dynamic pressure. Fig. 12. Same as Figure 2 but for orbit number 965 (August 16, 1986). 6. Summary and a Possible Cusp Model The large-scale cusp ion injection patterns are classified into several basic patterns as summarized in Figure 12a. The observation frequencies of each pattern for different IMF conditions are summarized in Table 2, which indicates a clear one-to-one correspondence between the three basic cusp morphologies and IMF conditions: one for southward, one for northward, and one for weak IMF conditions, respectively. The large-scale feature is persistent regardless of the existence of meso-scale injection structures. In other words, it is a spatial energy-latitude dispersion. Therefore, the large-scale cusp ion injection pattern can be used as a monitor of the IMF direction. Fig. 13. Summery of the magnetosheath ion injection pattern inside the cusp for (a) the large-scale and (b) the meso-scale. The meso-scale ion dispersion patterns are also classified as shown in Figures 13b. Multiple injections are well distinguished from the Òstair casesÓ of Escoubet et al. [1992]. Their observation frequencies are found in Table 3, an improvement from Table 1 by using the large-scale ion injection patterns as monitors of the IMF directions. According to the table, it is not clear that the stair case is a spatial structure (snap shot), i.e., a signature of the FTE. Even if it is so, the FTE is less likely the primary process producing the cusp particles because multiple injections, clearly temporal ones, are rather common while the Òstair casesÓ are very rarely observed. Thus, poleward propagating auroral transients observed on ground [e.g., Sandholt et al., 1986] may correspond to multiple injections rather than to the stair cases. Table 3: Meso-scale features discontinuityÒBz < -1Ó typeÒBz=0~2Ó typeÒBz>3Ó typeType I (stair case)*1,*213 (8)*3 / 836 (3)*3 / 644 (2)*3 / 53Type II (multiple injection)38 (22)*4 / 7144 (30)*4 / 5347 (25)*4 / 49no discontinuity22 / 714 / 532 / 49*1 including both step ÒupÓ and ÒdownÓ toward higher latitude across the discontinuity. *2 excluding clear discontinuities at the acceleration region [Woch and Lundin, 1992]. *3 only clear cases (energy steps ÒdownÓ across the discontinuity toward higher latitude) *4 clear overlapping cases From these large-scale and meso-scale particle observations as well as the other cusp observational characteristics found in this book, we expect the existence of a standing large-amplitude wave-like structure above the cusp relatively close to the Earth as shown in Figure 14. Such a structure can explain the stable spatial large-scale structure of the cusp and the midday gap of the cusp aurora, while a train of wakes inherently generated behind the large-amplitude wave-like structure can explain the meso-scale multiple injections and the poleward propagating auroral transients. The time scale for the formation of such transients (about two minutes) is determined by the AlfvŽn wave transit time between the standing structure and the ionosphere. Here we assume that any event taking place at the magnetopause is mapped to the boundary cusp, not the main part of the cusp. The theoretical feasibility for such a standing structure is studied by Yamauchi and Lundin [1992, 1993] where the geometrical effect is taken into account for the flow passage from the magnetosheath to the exterior cusp when high-latitude merging takes place (Figure 15). Using the mass-, momentum-, and energy-conservation laws in a magnetized plasma, we find three main solutions for the flow unless the magnetic stress force drastically modifies the conservation laws [Yamauchi and Lundin, 1993]. All these solutions are more dependent on the solar wind dynamic pressure than the strength of the IMF Bz. Approximate profiles of total (thermal plus magnetic) pressure for southward IMF are drawn in Figure 15. Here, case a is eliminated because this solution requires very low pressure at the outflow boundary (plasma mantle), but in reality upflowing ionospheric plasma in the plasma mantle and an increased magnetic field strength provides a higher pressure. Thus, cases b and c, both with plasma compression in the exterior cusp, are the realistic solutions in the present situation, and we predict a standing compressional structure there. This compression causes substantial deceleration of the inflow, converting kinetic energy to thermal and electromagnetic energy. Theoretical details of the energy conversion to the field-aligned current system inside such a compressional standing structure is found in Yamauchi et al. [1993]. Acknowledgements. The Viking project is sponsored by the Swedish National Space Board. The IMP 8 data were provided by R. P. Lepping and A. J. J. Lazarus through WDC-A. References Burch, J. L., P. H. Reiff, and R. A. Heelis, J. D. Winningham, W. B. Hanson., C. Gurgiolo, J. D. Menietti, R. A. Hoffman, and J. N. Barfield, Geophys. Res. Lett, 9, 921-924, 1982. Escoubet, C. P., M. F. Smith, S. F. Fung, P. C. Anderson, R. A. Hoffman, E. M. Basinska, and J. M. Bosqued, Geophys. Res. Lett., 19, 1735-1738, 1992. Kremser, G. and R. Lundin, J. Geophys. Res., 95, 5753-5766, 1990. Newell, P. T., and C.-I. Meng, J. Geophys. Res., 92, 13673-13678, 1987. Newell, P. T., and C.-I. Meng, Geophys. Res. Lett., 19, 609-612, 1992. Newell, P. T., and C.-I. Meng, J. Geophys. Res., 98, in press, 1993. Norberg, O., L. Eliason, M. Yamauchi, and R. Lundin, paper presented at AGU spring meeting, 1993. Reiff, P. H., T. W. Hill, and J. L. Burch, J. Geophys. Res., 82, 479-491, 1977. Sandholt, P. E., C. S. Deehr, A. Egeland, B. Lybekk, R. Viereck, and G. J. Romick, J. Geophys. Res., 91, 10063-10079, 1986. Viking Special Issue, Geophys. Res. Lett., 14, 379-478, 1988. Woch, J., and R. Lundin, J. Geophys. Res., 97, 1421-1430, 1992. Yamauchi, M. and R. Lundin, 3rd Japanese STEP Report, 782pp, edited by H. Oya, 666-672, Sendai, Japan, 1992. Yamauchi, M., and R. Lundin, A model of plasma flow near the exterior cusp, submitted to J. Geophys. Res. 1993. Yamauchi, M., R. Lundin, and A. T. Y. Lui, J. Geophys. Res., 98, 13523-13528, 1993. Fig. 1. Illustration of Viking cusp traversals. Equatorward-bound passes and poleward-bound passes are statistically compared. Fig. 2. Viking energy-time spectrograms for electrons (50 eV - 1 keV) and ions (50 eV - 40 keV) of orbit number 983 (August 19, 1986). IMF Bz was negative during this pass. The lower panel shows a blow up of part of the upper panel. Fig. 3. Same as Figure 2 but for orbit number 271 (April 12, 1986; IMF Bz < 0). Fig. 4. Summery of the large- and meso-scale ion patterns when IMF points southward. Fig. 5. Same as Figure 2 but for orbit number 1044 (August 30, 1986), and for a northward IMF. Fig. 6. Same as Figure 2 but for orbit number 119 (March 15, 1986), and for a northward IMF. Fig. 7. Same as Figure 2 but for orbit number 1049 (August 31, 1986), and for a weak IMF. Fig. 8. Same as Figure 2 but for orbit number 218 (April 2, 1986), and for a weak IMF. Fig. 9. Same as Figure 2 but for orbit number 967 (August 16, 1986), and for a weak IMF. Fig. 10. Same as Figure 2 but for orbit number 1003 (August 23, 1986). Fig. 11. Same as Figure 2 but for orbit number 1032 (August 28, 1986). Fig. 12. Same as Figure 2 but for orbit number 965 (August 16, 1986). Fig. 13. Summery of the magnetosheath ion injection pattern inside the cusp for (a) the large-scale and (b) the meso-scale. The meso-scale ion dispersion patterns are also classified as shown in Figures 13b. Multiple injections are well distinguished from the Òstair casesÓ of Escoubet et al. [1992]. Their observation frequencies are found in Table 3, an improvement from Table 1 by using the large-scale ion injection patterns as monitors of the IMF directions. According to the table, it is not clear that the stair case is a spatial structure (snap shot), i.e., a signature of the FTE. Even if it is so, the FTE is less likely the primary process producing the cusp particles because multiple injections, clearly temporal ones, are rather common while the Òstair casesÓ are very rarely observed. Thus, poleward propagating auroral transients observed on ground [e.g., Sandholt et al., 1986] may correspond to multiple injections rather than to the stair cases. Fig. 14. Proposed wave-like standing structure right above the cusp (8-10 Re) may explain many cusp features. Fig. 15. Due to the special geometry of the flow passage from the magnetosheath to the exterior cusp, we may have unusual pressure profiles along the flow as solutions in magneto-gasdynamics. See Yamauchi and Lundin [1993] for more details.