Pages 779-788

GEOTAIL OBSERVATION AT THE DAYSIDE MAGNETOPAUSE - CONFIRMATION OF RECONNECTION EVENTS

M. Nakamura1, M. Fujimoto2, H. Kawano1, T. Mukai3, Y. Saito3, T. Yamamoto3, K. Tsuruda3 ,
T. Terasawa1, and S. Kokubun4

1 Earth and Planetary Science, University of Tokyo, Tokyo 113, Japan, E-mail:   mnakamur@grl.s.u-tokyo.ac.jp
2 Tokyo Institute of Technology, Tokyo 152, Japan
3 Institute of Space and Astronautical Science, Sagamihara 229, Japan
4 STE Lab., Nagoya University, Toyokawa 442, Japan

ABSTRACT

Geotail has surveyed the dayside magnetopause in the equatorial plane and studied the dayside reconnection of the interplanetary magnetic field (IMF) with the magnetospheric field lines. When the IMF is directed southward leakage of ions from the low latitude boundary layer (LLBL) to the magnetosheath boundary layer (MSBL) are observed. Reconnection is shown to be responsible for the leakage. A variation in lower cut-off levels occurring in the velocity distribution function of the leakage ions is explained by the "velocity filter effect" model with a finite source region. In addition to such conditions, reconnection occurs also when the IMF has low-inclination angles, i.e., the condition between the southward and northward IMF conditions. Between the magnetosphere and the magnetosheath, two types of boundary region develop, i.e., the inner-LLBL and the outer-LLBL. The inner-LLBL is characterized by the bi-directional cold electrons and trapped cold ions coexisting with hot magnetospheric plasma. The field lines are regarded to be closed. The outer-LLBL is, on the other hand, open to the magnetosheath. It is characterized by the uni-directional cold electrons escaping to the magnetosheath. Newly penetrating solar wind ions are overtaking the trapped cold ions. The formation of the outer-LLBL is explained by the high-latitude reconnection at the equatorward regions of the cusp.

INTRODUCTION

The idea of reconnection of the interplanetary magnetic field with the magnetospheric field lines was proposed by Dungey (1961). Observational studies using the moment data from satellite plasma instruments was used to validate this particular reconnection theory, with Paschmann et al. (1979) using data from ISEE satellites to show that the plasma flow velocity is in good agreement with theoretically determined values. Later the distribution of ion velocity was studied from a kinetics standpoint and was found to support dayside reconnection (e.g., Cowley, 1982; Gosling et al, 1990; Fuselier et al., 1991; Nakamura et al., 1996).

Under southward IMF condition reconnection at the dayside magnetopause is associated with the low latitude boundary layer (LLBL) and the magnetosheath boundary layer (MSBL) and making kinetic considerations are of key importance in clarifying the formation of the LLBL and MSBL. Our attention here is focused on studying the observed signatures of ion velocity distributions appearing near the reconnection region, especially that in the MSBL where the velocity filter effect influences the hot ion component leaking from the LLBL. This novel analysis is based on data obtained by the Geotail spacecraft: magnetic field data (MGF), electric field data (EFD), and low-energy particle data (LEP). For respective details see Kokubun et al. (1994), Tsuruda et al. (1994), and Mukai et al. (1994).

Fig.1. Geotail Orbit in SM coordinate from 0000UT, 25 January 1994 to 2400UT, 26 January 1994.

Fuselier et al (1991, 1995) studied the particle signatures at the magnetopause and concluded that the particle signatures resulted from reconnection are commonly observed independent of the IMF orientation. In a later section of this paper we present the particle signature at the dayside magnetopause when the IMF inclination is low. In such a condition the magnetic shear is low at the equator region, but high in the high latitude region near the cusp. We introduce the idea of high-latitude reconnection at the equatorward region of the cusp. We define two different regions in the LLBL, i.e., the inner-LLBL where the field lines are closed and the outer-LLBL where the field lines are open. We use the electron distribution signature to identify the difference of these regions, i.e., the bi-directional electron beam is used to identify the inner-LLBL and the uni-directional electron beam is used to identify the outer-LLBL. In the magnetic field topology we propose in this paper, MSBL exists only in the high latitude region when the IMF has low inclination, thus it is not observed by the in situ measurement near the magnetic equator by Geotail.

From January 25 to 26, 1994, Geotail flew along the magnetopause near the equatorial plane (Figure 1). Two outstanding magnetopause crossing events were observed. One case (0620 - 0700UT, January 26) under southward IMF condition is a good example of the ion leakage from the LLBL to the MSBL as mentioned above. In another case (0325 - 0335UT, January 26), the IMF orientation was monitored by IMP 8 which was located in the upstream of the solar wind. The IMF inclination was northward at the beginning of this event and changed to southward. We report these two magnetopause crossing events in this paper.

RECONNECTION EVENT UNDER SOUTHWARD IMF CONDITION

Observation

While flying along the magnetopause on 26 January 1994 from 0632 to 0650UT, Geotail crossed from the magnetosheath into the magnetosphere and then back again; a unique event occurring near the sub-solar point, i.e., X = 8.1 RE, Y = 5.7 RE, and Z =-2.6 RE in SM coordinates.

Fig. 2.

Figure 2 shows the corresponding magnetic field (B), electric field (E), ion density (N), and plasma bulk velocity (V) records, where a high plasma density from 0620 to 0632UT confirms the spacecraft was in the magnetosheath. Passing across the magnetopause occurs at 0632UT as indicated by the sharp transition in the magnetic field orientation and plasma density, as well as by the change in the direction of bulk velocity. A fluctuating magnetic field can still be seen only in the LLBL from 0632 to 0633UT. Also, the entrance into the magnetosphere takes place at 0633UT as shown by a stable magnetic field, low ion density, and small plasma bulk velocity. At 0639UT the spacecraft again enters the LLBL where it stays until 0650UT. It finally crosses the magnetopause into the MSBL as shown by the sharp transitions in ion density, plasma bulk flow direction, and magnetic field orientation. Crossing the MSBL into the magnetosheath is not distinct.

Figure 3 shows the LEP ion velocity distribution signatures in the LLBL (0649-0650UT) and MSBL (0650-0652UT). The left panels present a two-dimensional (2D) slice of the respective distribution function in the equatorial plane as represented by log-scaled phase space density. The right panels present the corresponding one-dimensional (1D) slice of the distribution function along the magnetic field line. Observations taken in the LLBL ( Figure 3a) indicate an isotropic velocity distribution of hot magnetospheric ions. As the magnetopause is approached (Figure 3b), the hot ion component is no longer isotropic, instead flowing along the magnetopause. This data is strong evidence indicating that these adjacent boundary layer plasmas are being transported along the magnetopause due to the presence of reconnection. While in the MSBL and moving away from the magnetopause (Figure 3c-3e), the 1D distribution functions show that cold solar wind ions are flowing along the magnetic field (Figure 3c, 100 eV/q). In addition, they respectively provide the lower cut-offs of the hot ion components above 1 keV/q, i.e., 500, 700, and 1,000 km/s. Below these lower cut-off levels no magnetospheric hot ion components are observed. These hot components are believed to originate from the LLBL, since trends of the MSBL 1D distributions (Figure 3d, solid line) are identical to those of the hot component occurring in the LLBL (Figure 3b; superposed on Figure 3d as a dashed line).

Fig. 3 Observed ion velocity distribution functions. (Left panels) 2D slice of ion velocity distribution in the equatorial plane, (Right panels) corresponding 1D slice of the velocity distribution along the magnetic field line.

Fig. 4 Proposed "velocity filter effect" model assuming a finite source region.

Velocity Filter Effect

To explain the lower cut-offs of the hot ion components in the MSBL velocity distributions, we propose the "velocity filter effect" model with a finite source region, with Figure 4 providing a diagrammatic representation. For simple visualization, the magnetic field is horizontally oriented. The magnetopause and reconnection region are respectively represented by the finite source region situated on the right side and the adjacent upper boundary. Using a framework of steady-state reconnection, the electric field (E) is applied perpendicular (into the page) to the magnetic field (B) such that there is E x B drift starting from the source. Based on these features, let us consider two observation points (OP-1 and OP-2) and determine the conditions in which ions leaving the source region can reach them. To locate an observation we use its distance away from the source region to OP-1 (OP-2) along the magnetic field line L, and the distance of its magnetic field line from the upper boundary W. To reach OP-1 (OP-2) ions must have a parallel velocity greater than the lowest cut-off velocity VC1 (VC2). If OP-1 and OP-2 are reached at times T1 and T2, we can write

where VMP is the velocity of the magnetopause motion, i.e., the spacecraft's relative velocity with respect to the magnetopause, d is the distance between the OP-1 and OP-2, and finally t1 and t2 are the travel time of ions starting at T0 from the source region boundary to OP-1 and OP-2 at a parallel velocity of VC1 and VC2. Now, considering times T1 and T2 to correspond to actual observation times at 0650:35 and 0651:11UT, respectively, from the data we then obtain VMP = 29.5 km/s, E = 3 mV/m, B = 50 nT, VC1 = 500 km/s, and VC2 = 700 km/s, where VMP, B, and E are each determined by minimum variance analysis (Kawano et al., 1994). Substitution of values into Eqs. (1)|(3) and solving gives W = 1,062 km and L = 30,975 km  4.8 RE. Figure 5 represents a schematic diagram of the field line topology at the dayside magnetopause. As the spacecraft location was 2.6 RE south of the geomagnetic equatorial plane, and as this is the case that the hot leakage ions were flowing southward, then as calculated under the velocity filter effect model, the location of the reconnection region is at least 2.2RE north of the geomagnetic equatorial plane.

Fig. 5 Schematic diagram of the field line topology of the present study. The spacecraft was moving outward and at 0650:35UT it was located 1,062km from the magnetosheath-MSBL boundary and 4.8RE south of reconnection region.

RECONNECTION EVENT UNDER LOW INCLINATION IMF CONDITION

Observation

Geotail stayed inside the magnetopause until 0325UT, 26 January 1994, and the first magnetopause crossing event for this day occurred at X=7.6RE, Y=-0.8RE, and Z=-3.9RE in SM coordinates (Figure 1). The spacecraft moved from the magnetosphere out to the magnetosheath through the boundary layer and moved back to the magnetosphere at 0335UT.

IMF Observation by IMP 8. Bottom panel of Figure 6 shows the inclination angle of the IMF observed by IMP 8 which was located at X=32RE, Y=14RE, and Z=-3RE in GSM coordinates. The timing of the observation is shifted by the propagation time (7.3min) between IMP 8 and Geotail calculated from the solar wind speed (350km/s).

   Fig. 6 Geotail summary plot with the IMF elevation data observed by IMP 8. From the top, the magnetic field intensity, azimuth and elevation angles, plasma density, plasma bulk velocity, and ion temperatures are shown.

The IMF inclination turned positive at 0320UT and reached its maximum value (40 °) at 0323UT. It decreased to 0 ° (0329UT), and later it became negative. At high-latitude region, the high shear condition should have been satisfied when the IMF inclination was low, while low magnetic shear was expected at the equatorial region.

The solar wind velocity was stable (-350km/s) while a density enhancement was observed from 0320UT to 0329UT (not shown in this paper). The accompanied pressure increase/decrease of the IMF might have caused the inward/outward magnetopause motion and the Geotail magnetopause crossings.

Geotail Observation. Upper panels of Figure 6 shows a summary plot of the magnetic field and the plasma moment from 0320 until 0340UT. Hot (3keV) and low density (1cm-3) plasma was observed with the northward magnetic field (85nT) until 0325:48UT. No large plasma bulk flow was observed. These are the typical magnetospheric plasma signatures. The same plasma signature was observed after 0333:48UT. Figure 7 shows the electron E-t diagram and pitch angle distribution from 0325 until 0335UT. In the magnetosphere, the electrons were hot and had positive anisotropy (defined by T / T|| - 1).

On the other hand, at 0326:00-0326:48UT, 0328:36-0329:36UT, and 0331:30-0332:30UT, the electron signature was different from that in the magnetosphere. There was no hot component, and a cold component appeared in low energy channels whose anisotropy was positive. The magnetic field was weaker than that in the magnetosphere, and the ion density and temperature were higher (>10cm-3) and cooler ( 200eV), respectively. Earthward plasma flow (350 - 400km/s) was observed which was consistent with the solar wind velocity. From all these signatures, we conclude that the spacecraft was in the magnetosheath during these time intervals.

Between the magnetosphere and the magnetosheath, 2 types of boundary regions are identified. One region, e.g., 0326:48-0328:00UT, is characterized by the coexistence of hot magnetospheric electrons with positive anisotropy and cold bi-directional (counter streaming) electrons. The magnetic field strength was high (80-100nT) and the plasma density gave medium values between the magnetosphere and the magnetosheath. We interpret that the field lines in this layer are closed because the trapped solar wind electrons flowing along the field line are bouncing between the northern and the southern hemispheres. These closed field lines are regarded to be inside the boundary and is called the inner-LLBL.

Another boundary region, e.g., 0328:00-0328:36UT, is characterized by the existence of uni-directional cold electron flows. Sometimes hot magnetospheric electrons with positive anisotropy appeared. The magnetic field signature is the same as in the inner-LLBL and significant anti-parallel bulk flow (in this case it is in the negative Z direction) was observed. This boundary region is called the outer-LLBL as it is adjacent to the magnetosheath.

Fig. 7 Electron energy-time diagram (top panel), and pitch angle plots (0-180°) in 93eV, 248eV, 384eV, 654eV, and 1.1keV energy channels from 0325T to 0335UT, 1994 Jan. 26. The regions of the spacecraft are shown at the bottom.

Careful study of the electron signature in low energy channels from 0327:48 to 0328:36UT (inner-LLBL to outer-LLBL) tells that the anti-parallel flow (pitch angle 180 °) does not disappear completely at 0328:00UT yet, but its intensity was much less than that of the parallel flow. The anti-parallel component disappeared immediately after 0328:00UT. The intensity of the parallel flow (pitch angle  0 °) at 0328:00UT was the same as that of the parallel flow in the inner-LLBL at 0327:48UT in all low energy channels. The parallel component slowly disappeared until 0328:36UT and it happened faster in higher energy channels. We interpret that this uni-directional parallel flow component observed in the outer-LLBL is a component which survived from the bi-directional flows in the inner-LLBL.

Fig. 8 Ion velocity distribution function change from inner-LLBL to the outer-LLBL.

Figure 8 shows the 2-dimentional slice of ion distribution (phase space density) from 0327:49UT to 0328:37UT (left panels). Also presented in the right panels are the 1-dimensional slice of the ion distribution along the dashed line in the 2-dimensional plot. In the inner-LLBL at 0327:49UT, a cold and pancake shaped component (hereafter It component) was seen at the center of the plot. Hot magnetospheric ions are superposed on the It component. In the outer-LLBL, at 0328:01 - 0328:25UT, another cold component appeared. This new component (hereafter Is component) was dense and cold as the It component and flowing anti-parallel to the magnetic field. The Is component is crescent in shape which indicates that this component originated from the magnetosheath where the magnetic field was weak and is pitch angle scattered in the outer-LLBL where the magnetic field was strong due to the conservation of the 1st invariant. From 0328:13 to 0328:25UT the It component was disappearing while the Is component increased its density. It is noted that the temperature in perpendicular direction of the It component is higher than that of the Is component (not shown in this paper).

Discussion

The transition from the inner-LLBL to the outer-LLBL is regarded to be caused by the topological change of the field lines at the magnetopause when the IMF direction changed from northward to a low-inclination condition. It should be noted that due to the draping of the IMF along the magnetopause, the IMF and the magnetospheric field lines are in parallel condition at the equator and field line causes a low shear condition even if the IMF and the magnetospheric field lines are not necessarily parallel at high latitude region and cause a high shear condition there. In such cases, it is not good enough to use the local shear condition measured by a spacecraft near the magnetopause to discuss the magnetic field topology. In this study the IMF orientation is monitored by IMP 8 in the solar wind.

Fig. 9 Field line topological change during the IMF orientation from northward to low-inclination condition

From the data shown when the IMF inclination was changing, we may conclude that the inner-LLBL consists of closed field lines while the outer-LLBL consists of open field lines whose one end is still connected to the magnetosphere. The plasma signature in these regions are discussed by Fujimoto et al. (1997). Now let us discuss the formation process of the outer-LLBL with the observation. The IMF was directed northward around 0325UT and it is expected that the field lines of the dayside magnetopause do not merge with the IMF (Upper panel of Figure 9). Under such condition, the outer-LLBL where the field lines are open does not develop. Later the IMF inclination decreased from 0323UT to 0344UT. Thus the high shear condition is satisfied at the equatorward region near the northern cusp and reconnection starts (Lower panel of Figure 9). These field lines, once closed in the inner-LLBL, are now open to the IMF and fresh solar wind cold ions flow into the LLBL (Is component). As the inclination of the IMF is small, the solar wind speed is mainly converted to the anti-parallel velocity and does not increase the ion temperature. The IMP 8 observation tell us that the solar wind speed is about 350km/s which is consistent to the anti-parallel velocity of the Is component. This also explains why the perpendicular temperature of the Is component is lower than that of the It component. The It component is escaping from the outer-LLBL to the magnetosheath slowly due to its slow parallel velocity.

The electrons once trapped in the inner-LLBL are escaping to the magnetosheath, too. The parallel component (pitch angle  0 °) flowing from southern hemisphere disappears slowly as there is a reservoir of electrons in the southern hemisphere. On the other hand the anti-parallel component (pitch angle  180 °) has no source region any more except between the reconnection region and the observation point and disappears quickly. The electrons penetrating from the magnetosheath are, however, not observed. They may be too cold and fall below the lower energy channel of the detector (93eV).

In this picture the magnetopause is defined not as the boundary between the open and closed field lines, but as the current layer which separates the magnetosheath field lines from the magnetospheric field lines.

SUMMARY

We studied the particle signatures at the dayside magnetopause observed by Geotail under southward IMF and low inclination IMF conditions.

When IMF is directed southward leakage ions from LLBL to MSBL are observed. Reconnection is shown to be responsible for the leakage. A variation in lower cut-off levels occurring in the velocity distribution function of the leakage ions is explained by the "velocity filter effect" model. The calculated position of the reconnection site is close to the equatorial plane which confirms the idea that reconnection occurs near the sub-solar point under southward IMF condition.

Under the low inclination IMF condition, two boundary layers develop between the magnetosphere and the magnetosheath, i.e., the inner-LLBL and the outer-LLBL. The inner-LLBL is characterized by the bi-directional (counter streaming) electrons in low energy channels. We interpret this electron signature as represents the closed field line configuration. Trapped ions which are cold and regarded to be of solar wind origin are observed simultaneously with the hot magnetospheric component.

The outer-LLBL is characterized by the uni-directional electrons and the existence of the penetrating cold ion component. This region develops well when the IMF inclination is low. The electron and ion signature can be explained by the reconnection between the IMF and the inner-LLBL field lines at the equator region near the cusp. The solar wind ions penetrating through the reconnection region have a parallel velocity which is the same as the solar wind velocity. This is because the inclination of the solar wind is low and the solar wind velocity is effectively converted to the parallel ion speed in the outer-LLBL. This ion component shows a crescent shape in the distribution function of the outer-LLBL due to the conservation of the 1st invariant, because they come from the magnetosheath where the magnetic field is weak to the outer-LLBL where the magnetic field is strong.

ACKNOWLEDGMENT

We appreciate R.Lepping, A.Lazarus and MIT Plasma group for providing us with the IMP 8 magnetic field and plasma data. M. N. thanks M. Hirahara for his help in plasma moment calculation.

REFERENCES

Cowley, S. W. H., The Causes of Convection in the Earth's Magnetosphere: A Review of Developments During the IMS, Rev. Geophys. Space Phys., 20, 531-565, (1982).

Dungey, J. W. Interplanetary Magnetic Field and the Auroral Zones, Phys. Rev. Lett., 6, 47-48, (1961).

Fujimoto, M., T. Mukai, H. Kawano, M. Nakamura, A. Nishida, Y. Saito, T. Yamamoto, and S. Kokubun, GEOTAIL observations of low-latitude boundary layer, Adv. Space. Res, this issue, 1997.

Fuselier, S. A., D. M. Klumpar, and E. G. Shelly, Ion Reflection and Transmission During Reconnection at the Earth's Subsolar Magnetopause, Geophys. Res. Letters, 18, 139-143, (1991).

Fuselier, S. A., B. J. Anderson, and T. G. Onsager, Particle Signatures of Magnetic Topology at the Magnetopause: AMPTE/CCE Observation, J. Geophys. Res., 100, 11,805 - 11,821, (1995).

Gosling, J. T., M. F. Thomsen, S. J. Bame, T. G. Onsager, and C. T. Russel, The Electron Edge of the Low Latitude Boundary Layer During Accelerated Flow Events, Geophys. Res. Letters, 17, 1833-1836, (1990).

Kawano, H., T. Yamamoto, T. Mukai, K. Tsuruda, and S. KOKUBUN, The Magnetopause Orientation and Motion: Method Development and Application to the GEOTAIL Data, EOS Suppl. AGU, 75 (44), 553, (1994).

Kokubun, S., T. Yamamoto, M. H. Acuña, K. Hayashi, K. Shiokawa, and H. Kawano, The GEOTAIL Magnetic Field Experiment, J. Geomag. Geoelectr., 46, 7-21, (1994).

Mukai, T., S. Machida, Y. Saito, M. Hirahara, T. Terasawa, N. Kaya, T. Obara, M. Ejiri, and A. Nishida, The Low Energy Particle (LEP) Experiment Onboard the GEOTAIL Satellite, J. Geomag. Geoelectr., 46, 669-692, (1994).

Nakamura, M., T. Terasawa, H. Kawano, M. Fujimoto, M. Hirahara, T. Mukai, S. Machida, Y. Saito, S. Kokubun, T. Yamamoto, and K. Tsuruda, Leakage Ions from the LLBL to MSBL: Confirmation of Reconnection Events at the Dayside Magnetopause, J. Geomag. Geoelectr., 47, 65-70, 1996.

Paschmann, G., B. U. Ö. Sonnerup, I. Papamastorakis, N. Sckopke, G. Haerendel, S. J. Bame, J. R. Asbridge, J. T. Gosling, C. T. Russell, and R. C. Elphic, Plasma Acceleration at the Earth's Magnetopause: Evidence for Reconnection, Nature, 282, 243-246, (1979).

Tsuruda, K., H. Hayakawa, M. Nakamura, T. Okada, A. Matsuoka, F. S. Mozer, and R. Schmidt, Electric Field Measurement on the GEOTAIL Satellite, J. Geomag. Geoelectr., 46, 693-712. (1994).