Available online at www.sciencedirect.com Pergamon

www.elsevier,com/locate/asI doi: 10.1016/SO273-1177(03)00777-4

SCIENCE DIRECT.

EVIDENCE FOR SPIRAL MAGNETIC STRUCTURES AT THE MAGNETOPAUSE: A CASE FOR MULTIPLE

RECONNECTIONS

O.L. Vaisberg’, V.N. Smimov’, L.A. Avanov’.2, T.E. Moore3

’ Space Research Institute, 84/32 Profsoyuznaya, Moscow, I1 7810 Russia ’ National Space Science and Technology Center, 320 Sparkman Drive, Huntsville, AL 35805,

USA 3 NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA

ABSTRACT

We analyze plasma structures within the low latitude boundary layer (LLBL) observed by Interball Tail spacecraft under southward interplanetary magnetic field. There is a variety of ion velocity distributions observed in the LLBL under this condition: (a) D-shaped distributions, (b) ion velocity distributions consisting of two counter-streaming magnetosheath-type, and (c) distributions with three components where one of them has nearly zero velocity parallel to magnetic field (VJ, while the other two are counter-streaming components, D-shaped ion velocity distributions (a) correspond to magnetosheath plasma injections into reconnected flux tube, which is consistent with a spacecraft location relative to the reconnection site. Simultaneous counter-streaming injections suggest multiple reconnections. Three- component ion velocity distributions and their evolution with decreasing number density in the LLBL indicate that the significant part of it is located on long spiral flux tube islands at the magnetopause, as has been proposed and found to occur in magnetopause simulations. We interpret these distributions as a natural consequence of the formation of spiral magnetic flux tubes consisting of a mixture of alternating segments originating from the magnetosheath and magnetospheric plasmas. We suggest that multiple reconnections play an important role in the formation of the LLBL. 0 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

INTRODUCTlON

Magnetic reconnection between the terrestrial magnetic field and the interplanetary magnetic field is considered as a principal mechanism for the transfer of mass, energy and momentum from the shocked solar wind plasma across the magnetopause into the magnetosphere (Dungey, 1961). The southward magnetosheath magnetic field may reconnect with northward dayside magnetospheric fields forming the open magnetic flux tubes at the X-line (Petcheck, 1964). The open magnetic tubes are identified by their rotational discontinuity properties (Sonnerup et al., 1987). Magnetosheath plasma can enter the magnetosphere along the open field lines, forming the low latitude boundary layer (LLBL). The difference between magnetosheath plasma velocity and convection velocity of the rotational discontinuity leads to velocity cut-off of the ions entering the magnetosphere. As a result D-shaped velocity distributions on the magnetospheric field lines should be observed, which were theoretically predicted by Cowley (1982) and were first observed by Smith and Rogers, (1991), within the magnetopause current layer. Simultaneous observations of D-shaped distributions and open magnetopause confirm such a reconnection scenario (e.g. Phan et al., 2001).

However, Lee and Fu (1985) and Nishida ( 1989) proposed alternative multiple reconnection scenario in which magnetic field lines may reconnect simultaneously (or almost simultaneously) at more than one location. Simulation shows that the multiple reconnection leads to the formation of the spiral magnetic flux rope islands (Lee et al., 1993). The consequences of these two scenarios are very different because only multiple reconnections may provide a penetration of the magnetosheath plasma onto the closed magnetospheric magnetic field lines. Direct evidence for multiple reconnections has not been found yet.

Adv. Space Res. Vol. 32, No. 10, pp. 1989-1999,2003 0 2003 COSPAR. Published by Elsevier Ltd. All rights reserved Printed in Great Britain 0273- 1177/$30.00 + 0.00

1990 0. L. Vaisberg er al.

In this paper we present an experimental evidence for multiple reconnections on the dusk-side of the Earth’s magnetosphere using plasma and magnetic field data from the Interball Tail spacecraft on 15 February 1996.

We discuss here the ion velocity distributions within a highly structured LLBL in which isolated transients of LLBL plasma are observed on the background of magnetospheric plasma. This type of LLBL is usually associated with southward or variable IMF (Vaisberg et al., 2001). De Keyser et al. (2001) have shown that the observations from a single spacecraft do not allow distinguishing between the magnetopause surface waves that bring occasionally the LLBL to the spacecraft and LLBL plasma blobs passing the spacecraft. Vaisberg et al. (1998) argued that at least part of LLBL transients is magnetically disconnected from the magnetosheath, and dubbed them the Disconnected Magnetosheath Transfer Events (DMTEs). We will-concentrate on the velocity distributions rather than on structural properties of LLBL that will be discussed elsewhere.

INSTRUMENTATIONS AND OBSERVATIONS

In this study we use the data from ion spectrometer SCA-1 (Vaisberg et al., 1995) which provides three-dimensional ion distributions in the energy range 0.05-5.0 keV/q within - 10 sec. Narrow field of view (2”) and energy pass band (10%) provide differential velocity space measurements. The total amount of bins in the velocity space is 960. Magnetic field data taken from the three-axis fluxgate Multi- component Investigations of Fluctuations of Magnetic Field (MIF) magnetometer (Klimov et al., 1995) with a sampling rate of 4 Hz averaged to 1 sec.

We analyze here the ion velocity distributions within the LLBL for a southward magnetic field when the LLBL was classified as a highly structured (see Vaisberg et al., 2001). Interball-Tail spacecraft crossed the magnetopause current layer at -2250 UT on February 15, 1996 on its inbound trajectory at magnetic local time (MLT) - 18h30m, and geomagnetic latitude GM - 24.9’. Strong southward- duskward magnetosheath magnetic field was observed before magnetopause crossing, with the angle between magnetosheath and magnetospheric field lines of about - 140’. Magnetic field measurements made by Geotail spacecraft which was located at - 5 Rs upstream of Interball Tail, shows that B, component of the magnetic field remains southward-duskward during the time interval, when the Interball Tail crossed the magnetopause current layer and LLBL. The duration of observation of LLBL was about 1.5 hour. The use of ion velocity distributions gives a possibility to study the plasma structures and magnetic field topology of the LLBL.

The magnetopause current layer itself has been identified by the change of signs of three magnetic field components, by depression of the magnetic field magnitude, and by decreasing the number density from the typical magnetosheath values -15 cm-’ to 2-3 cme3 in the boundary layer (see Figure 1). Prior to the magnetopause crossing, Interball Tail observes sporadic magnetospheric ions leakage into the magnetosheath, bipolar magnetic field variations and velocity variations, which are indicative of ongoing reconnection at the magnetopause.

The de Hofhnann-Teller (dHT) frame was found for magnetopause current layer in which the convection electric field V x B vanishes on both sides of current layer. Measured velocity and magnetic field vectors in GSE coordinate system were used to define the dHT frame by minimization of the residual electric field assuming that HT frame is accelerating according to a procedure described in Sonnerup et al. (1987). As a result, good (with a correlation coefftcient 0.995) dHT frame with the quality measure D/DO = 0.015 (where D=<](V-VET) x B] 2> and DO=<]V x B12>) is found for the time interval 22:49:14-22:50:32 UT. The calculated de Hoffmann-Teller frame velocity Vuro = [-264.9, 146.6, -67.01 km/set and acceleration anr = [1.993, -1.767,2.944] km/sec2.

WalCn test for a rotational discontinuity was applied to the magnetopause current layer to see if the plasma flow is at a local Alfvenic speed in dHT frame, i.e. V-V HT=VA. This test found quite a good fit between measured and calculated Alfven velocity across the discontinuity with a correlation coefficient of - 0.87 and with a slope of - - 0.78. This indicates that the magnetopause current layer is a rotational discontinuity, and a negative slope of Walen relation implies that the reconnection site is located northward relative to the spacecraft position. This is in agreement with the negative deviation of Z component of plasma velocity for a significant part of LLBL compared to that of observed in the magnetosheath. This also agrees with the expected location of antiparallel reconnection (Crooker, 1979; Luhmann et al., 1984) at north-dusk sector for the IMF clock angle 100” - 120’ as measured by Geotail located in the magnetosheath. However, positive Vz values within part of current layer indicate more complex nature of this transition, possibly associated with deviation from planar geometry. Strong acceleration of de Hoffman-Teller frame during the time interval Interball crossed the current layer also indicate that it is locally non-steady and complicated.

Spiral Magnetic Structures at the Magnetopause 1991

22 59 Lx?:55 23:on 23.05 &E - 3 5.; --x57 --356 -3 56 YOSE 17 a- 17.23 17.15 17 07 “- -L’.*s* -224 -2.37 -2 40 MLT !8 18 la aa 18..m 16 48 GML‘AT 2 $35 2.i G H ‘ii 5R 2451

Fig. I. An overview of Interball Tail ion and magnetic field data for the magnetopause and LLBL crossing observed on February 15, 1996. Five typical ion velocity distributions in VII-Vi coordinate system, observed in the LLBL, are shown on the top (see text for details). Next panels represent from top to bottom: two velocity-time spectrograms showing the phase space density integrated over parallel velocity (upper spectrogram) and the phase space density integrated over perpendicular velocity (lower spectrogram). The transverse plasma velocity component is superimposed on the lower spectrogram (black line). Phase space density is coded according to the color table shown on the right. Next panel is ion number density profile; three components of the magnetic field in GSE and the last panel is the magnitude of the magnetic field. Two vertical lines indicate the magnetopause current layer. Letters on each velocity distribution correspond to their locations on the spectrograms.

Figure 1 shoals velocity distributions observed in the LLBL near the magnetopause current layer. They are shown as velocity-time spectrograms. Thcsi: spectrograms were obtained as time sequence of the individual ion velocity distribution in VII-VI coordinate system. Each measured three-dimensional velocity distribution was transformed from the laboratory coordinate system to the magnetic field coordinate system with one axis directed parallel io the magnetic field and another one transverse to it. In order to do this transformation the vclocily vector have been calculated as a first moment of each measured ion distribution, assuming that all eons are protons. An averaged magnetic field vector over the time interval corresponding to the measurement of individual il> distribution (-10 s) and calculated velocity vector were used to construct the magnetic coordinate system. Then all 960 measured points in the velocity space of individual distribution have been transformed to the new coordinate system. Note that this coordinate system is not the de Hol‘finan-Teller coordmatc system but a local magnetic coordinate system obtained by subtracting the velocity component perpendicular to the averaged magnetic field. This velocity component is shown on the second panel of Figure 1 as a black solid line. It

1992 0. L. Vaisberg er al.

is seen from second panel of Figure 1 that the boundary layer plasma moves quite steadily transverse to the magnetic field. Note that significant ion number density is most frequently observed at zero velocity and at positive velocities respective to the magnetic field direction (upper spectrogram). Two vertical lines mark the magnetopause current layer that separates magnetosheath from the LLBL. Five typical examples of ion velocity distributions observed within LLBL are shown on the top of the Figure 1: (a) a distribution with plasma injection antiparallel to the magnetic field direction, (b) a distribution with denser component antiparallel to the magnetic field and less denser component parallel to magnetic field, (c) distribution with properties which are opposite to distribution c, (d) distribution with two nearly identical ion components relative to the magnetic field, and (e) two oppositely moving ion components and the third component at nearly zero velocity. It is worth noting that only one ion distribution of type (a) must be expected for the reconnected flux tube originated from the reconnection site located northward relative to the spacecraft position.

We analyzed ion velocity distributions in the LLBL with highdensity magnetosheath-type ion component moving along negative magnetic field direction that is consistent with magnetosheath plasma entering LLBL along open magnetic flux tube northward of the spacecraft. Figure 2d shows one such ion velocity distribution in the LLBL. In comparison with the magnetosheath velocity distribution observed just before the magnetopause crossing (see Figure 2b) the distribution of Figure 2c reveals an ion component moving along the negative magnetic field direction characterized by a high density and a high convection velocity. The de Hoffman-Teller frame velocity component parallel to the magnetosheath magnetic field is shown by the green vertical line on the magnetosheath VII-F(VII) distribution (Figure 2b). Projection of this dHT velocity onto the average magnetic field direction for LLBL velocity distribution is shown by the green vertical line on the VI~-F(VI~) distribution in the LLBL (Figure 2d). According to Cowley (1982) scheme we expect that magnetosheath particles having negative velocities relative to the green line on the Vl~-F(Vll) distribution will enter open field line. These particles should be observed at velocities to the lefl of the green line on the VI~-F(V~I) distribution in the LLBL (Figure 2d). The red curve shows this expected velocity distribution. Expected and observed velocity components at negative velocities are quite close in the locations of maxima and in the temperature. Figure 2 suggests that highdensity injections of magnetosheath plasma along negative magnetic field directions can be explained by magnetosheath plasma entry onto the open field lines reconnected to magnetosheath field lines to the north of the spacecraft.

Fig. 2. Magnetosheath velocity distribution observed before magnetopause crossing on February 15, 1996 (top) and LLBL velocity distribution with high-density high-speed ion component along negative magnetic field direction observed within (bottom). VII-VI velocities space cuts are on the left, VII-F(V,,) velocity distributions are on the right. The de Hoffman-Teller frame velocity cut is shown by the vertical green line on magnetosheath VII-F (Vi,) velocity distribution. The red curve on the VII-F(V,,) velocity distribution in the LLBL shows the magnetosheath plasma velocity distributions that should enter magnetospheric field lines if they are reconnected to magnetosheath field lines. The velocity cut location in VII-F(V,$ coordinates was determined as a projection of the dHT velocity onto the local magnetic field direction for both cases.

Spiral Magnetic Structures at the Magnetopause 1993

One subsequently observed LLBL transient, or DMTE at - 2355 UT is shown on Figure 3 in the same format as Figure 1. In the leading part of DMTE (see detailed analysis of this DMTE in Vaisberg et al. (1998)) the density is steadily decreasing onward from the maximum near the front edge. In the trailing part, the density remains nearly constant most of the time. There is a two-side injection observed at the front of DMTt (e.g. velocity distribution a). Weak mjections antiparallel and parallel to the magnetic field are observed occasionally within this DMTE (e.g. v&city distribution c). However, the main feature of the observed velocity distribution is a component at /em parallel velocity. It dominates in the iirst part ofthe DMTt: and suhsequcntly diminishes (velocity distributions b. d, and c).

a.52 a.54 8Y.!B 2xw UV.VV x, -3.46 -3 48 -848 -3.48 -3.48 YGMi 16.31 1628 16.24 16.21 P3.18

3 -2.88 ta.45 -3.69 1845 -2.70 16.45 -2.71 la.45 -2.72 si.45 Gh4L~t 23.S i?R 33 a.27 29 at 23 14

Fig. 3. Ion velocity distributions within DMTE observed at - 23:55 UT on February 15. 1996. Format of the Figure is the same as for Figure 1. Five typical ion velocity distributions (see text for explanation) are shown on the top.

The evolution of the velocity distribution cvith decreasing number density in this DMTE is shown on Figure 4. For this figure spectra with minnnum plasma injections along the magnetic field have been selected. We have chosen 6 ion velocity distributtons with progressively decreasing number density. In this subset of spectra (I;igure 4a) the llux at zero velocity is decreasing while the flux at the wings of the distributions remains nearly the same (the iltrx at ncgativjr: velocities drops to smaller density in comparison with the tlux at positive: velocities). It is clear that the velocity distributions evolve with the decreasing number density. when one normalizer the spectra to their maximum values by shifting them along ordinate (Figure 4b). This normalization indicates that the relativ;e contribution of wings in the velocity distribution is increasing as the dcnsitv decreases, and that they become less steep or more heated.

Reconnection at a single X-ltnc produces open magnetospheric flux tubes that connect to magmetosheath flux tubes through a rotational discontinuity. Magnetospheric and tnagnetosheath plasma

1994 0. L. Vaisberg et al.

0 1000

Fig. 4. Six ion velocity distributions (times of measurements are shown to the right) within DMTE at - 2355 UT (top). Bottom: the same velocity distributions normalized to the maximum intensities.

can cross the open magnetopause freely and form D-shaped velocity distributions (Cowley, 1982). This type of ion velocity distributions has been observed previously (e.g. Smith and Rodgers, 1991; Smith and Owen, 1992; Phan et al., 2001). Interball measurements also provide examples of this type of the D- shaped distributions at the edges of LLBL and sometimes within boundary layer.

In addition to the one-side injection of the magnetosheath plasma along the magnetic field line, the Interball spacecraft also observed counter-streaming ion components in the LLBL, the third ion component at VII -0 in the local magnetic coordinate frame, the smaller number density within the LLBL parcel, the smaller relative contribution of the phase space density of the central (at Vi, -0) ion component compared to the phase space density of parallel and anti-parallel to the magnetic field velocity components. This evolution of ion velocity distributions suggests that the central (Vii -0) ion component is an additional source of ions to the plasma components moving parallel and anti-parallel to the magnetic field and that the ions are accelerated while the ion number density within LLBL decreases (Figure 4).

The ion component coming from the direction opposite to the injected plasma into magnetosphere along the open field line can be produced by reflection of earlier injected ions from parts of the same field line closer to the Earth, where magnetic field strength is much higher (this is frequently called reflection from the ionosphere).Fuselier et al. (1992) and Onsager and Fuselier (1994) analyzed counter-streaming plasma populations within the LLBL and explained the second component moving opposite to injected magnetosheath plasma due to the ionospheric reflection.

In order to better understand how the ions reflected from the ionosphere can be separated from the other possible sources for the second component we use a simple model for comparison of the motion of the possibly reflected ions along the magnetospheric field line and convection of this field line along the magnetopause. Travel time (T) for the ions injected at the equator to the mirror point and back is taken as:

T= 2% {1.38 - 0.32[Sin a + (Sin’%)]} /V (1)

Spiral Magnetic Structures at the Magnetopause 1995

Where T in set, Ro is equatorial distance in Earth Radii from the injection point to the mirror point (taken as 10 Ra); c1 is an ion pitch-angle, and V is ion velocity in kmsec-’ (Lyons and Williams, 1984). It is seen from Eq. (1) that it takes about two times longer for nearly field-aligned particles to bounce compared to the particles with nearly 90” pitch-angle.

1500 t

i

Fig. 5. The result of modeling of ionospheric reflection of ions injected into magnetospheric field line at the equator at 10 Rs. Abscissa is the ion velocity; ordinate is the travel time of ions from the injection point at equator at 10 RE to reflection point deeper in the magnetosphere and back. lsolines represent different ion pitch-angles (from 5’ for isoline farthest from zero coordinate point till 85’ for isoline closest to zero coordinate point). The darker shaded area between two horizontal solid lines defines the velocity-pitch- angle domain corresponding to ions injected in the LLBL from a region near the subsolar point (one to two Earth’s radii from it) and being convected at the magnetosheath speed to the 18:00 MLT sector. The lightly shaded area defines the same for injection at -1500 UT sector. The inset shows the same areas in VII-V1 coordinates.

We assumed that: (1) the ions are injected in the LLBL at the equator in the region at the distance from 1 RE to 2 Rn from the subsolar point; (2) the flux tube convects to the magnetospheric flank with a velocity of magnetosheath plasma according to Spreiter’s model (Spreiter et al., 1966) (this is supported by measured transverse velocity in the magnetosheath and in the LLBL, Figure 1). Figure 5 shows the result of this model calculation. lsolines on velocity-bounce time plane are for different pitch-angles of ions. Each horizontal line (constant time of motion to reflection point and back) defines velocity-pitch- angle cut corresponding to a chosen convection time. The area between two horizontal lines defines the velocity - pitch-angle box in the velocity space in which ions will arrive from the injection region to a specific location in the LLBL. Darkly shaded area between two horizontal solid lines indicates what should be observed at the LLBL location on February 15, 1995. Steady injection will provide a continuous supply of ions within this area while sporadic injection will provide a sporadic “illumination” of the same area. Inset in Figure 5 shows an area with oval-shaped boundaries on VII-V~ plane in which injected ions will be observed. The faster particles (larger oval) will be observed, when the local time difference between the injection and observation sites is smaller, while the slower particles (smaller oval) will be observed when the local time difference is larger. The lightly shaded areas in Figure 4 correspond to a location of ion injection close to the observation site. Ionospheric reflection should produce flattened oval velocity distributions with a limited velocity spread and with the parallel velocities about twice larger than the perpendicular velocities (and temperature ratio Ti{Tl - 4). There should be an oval-shaped void at smaller velocities where no ions will be observed. Ionospheric reflection will provide velocity distribution similar to one of magnetosheath ions entering the magnetosphere, only if the reconnected

1996 0. L. V&berg et al.

magnetic flux tube will reside long enough close to the location where it became reconnected. Interball observations show that the LLBL plasma at smaller solar-zenith angle also has large transverse velocities indicating that one should not expect the reconnected flux tube reside long enough at the reconnection site.

Onsager and Fuselier (1994) noted that the deviation of the shape and magnetic field strength distribution along dayside magnetic field line would significantly change dependence (1) of the bounce time from the pitch-angle. However, as our estimation shows, in this case the bounce time will depend on the pitch-angle, though this dependence will be different from Eq. (1). In addition to that, the time-of- flight effects in the reflected ions should lead to velocity distributions with limited velocity spread, as shown in insert of Figure 5.

Within the LLBL observed on February 15th 1996 there were not ion components with the shape that were significantly modified by the pitch-angle dependence or had a limited velocity spread. There were a few cases of velocity dispersion that indicated injection of magnetosheath plasma quite close to the spacecraft location that we do not discuss in this paper. We often observe close similarity of the counter- streaming ion velocity distributions in the LLBL. The similarity of spectral properties of plasma components in the LLBL coming from opposite directions suggests their similar origin. Their similar shapes and simultaneous existence suggests nearly simultaneous injection of magnetosheath ions into the magnetospheric magnetic flux tube from two opposite locations. They strongly suggest that some magnetospheric flux tubes reconnect with magnetosheath magnetic field at two locations simultaneously or nearly simultaneously.

The ion component with maximum at nearly zero parallel velocity can be of ionospheric origin. Existence of ionospheric ions in the LLBL was first reported by Klumpar et al. (1990). However, ion component at small parallel velocity in LLBL observed by Interball Tail on February 15, 1996 usually has temperature close to that of magnetosheath plasma. It suggests that this component may originate from magnetosheath plasma during LLBL formation.

Several features of the ion velocity distributions observed in the LLBL on February 15, 1996: simultaneous existence of ion components moving in opposite directions respective the magnetic field direction, similarity of properties of these ion components, existence of the component at nearly zero parallel velocity and its close shape to the magnetosheath velocity distribution, the evolution of the ion velocity distribution with the number density in which the relative contribution of the component with nearly zero parallel velocity decreases compared to the two counter-streaming components as the number density decreases could be accounted for in a concept of multiple reconnections with the formation of a flux rope.

Multiple reconnections in which magnetosheath and magnetospheric field lines reconnect at more than one location lead to the injection of the magnetosheath plasma from both directions into magnetospheric field lines. Formation of the helical flux ropes initiated by multiple reconnections is possible at the magnetopause when IMF has significant By component (Le and Fu, 1985). That is exactly the case we have with the clock angle of magnetosheath magnetic field - 140” observed by Interball before magnetopause crossing, and loo’-120’ observed by Geotail. De Keyser et al. (2001) found evidence for the existence of flux tubes with helical field lines for the same LLBL crossing, The flux rope formed by the reconnection occurred at two locations consists of different parts that originate both from the magnetosheath and from the magnetosphere (see Lee et al., 1993). In the instant after its creation, a segregated mixture of both magnetosheath and magnetospheric plasmas tills the flux rope. In a simple picture of such a flux rope (Figure 6), the part of each magnetic spiral threads the former magnetosheath plasma, and another part of the same spiral threads the magnetospheric plasma. These subsequently begin to intermingle along the flux rope, according to their initial parallel velocity distributions. It can be readily seen that the process of intetminglement leads to regions of more or less magnetosheath-like, magnetosphere-like, and mixed plasmas, with inter-streaming of plasmas with different parallel velocity distributions. Two counter-streaming magnetosheath-type components should be observed in the parts of the flux tubes that originated from the magnetosphere (see spectra c) and d) in Figure 1). Simultaneous or nearly simultaneous reconnection of the magnetospheric flux tube at two locations with magnetosheath flux tubes leads to injection of the magnetosheath plasma from two directions parallel and antiparallel to the reconnected magnetospheric flux tube, where they form the bi-modal ion distribution of magnetosheath-type particles. Three-component velocity distributions should be formed in the parts of the flux tubes that originate from the magnetosheath. Ions with nearly zero velocity remain in these parts of the flux rope forming the component with nearly zero parallel velocity. Fast ions that traveled along the field lines to magnetospheric parts of the field lines return back to the magnetosheath part of the rope and

Spiral Magnetic Structures at the Magnetopause I991

form the wings of the ion distribution. Interball observations indicate that fast ions are accelerated in this process (see Figure 4).

Interball observations show that a process leading to decreasing of the number density in LLBL is accompanied by faster decreasing of the phase space density in the central component of the velocity distribution compared to its wings. This indicates that more ions leave the parts of the flux rope that came from the magnetosheath and contribute to wings of the velocity distribution. Therefore the concept of multiple reconnections is potentially able to explain the main properties of the ion velocity distribution within the LLBL and its evolution with decreasing number density (that appears to be the result of “aging” of an LLBL plasma). Two and three-component Ion velocity distributions within the LLBL suggest the existence of long spiral magnetic configurations.

In the case of February 15, 1996 LLBL crossing we occasionally observe D-shaped ion distribution that is an indicator of the open reconnected filed line. However, in the majority of cases quite different velocity distributions are observed. They consist either of two counter-streaming components or with identifiable, and sometimes dominating component at nearly zero velocity relative to the magnetic field. Observation of the component moving antiparallel to the direction of propagation from reconnected location of magnetospheric field line with no indication that it was reflected from the high-magnetic field region of the field line does not fit to the classic structure of reconnected field line. Ion component observed at nearly zero parallel velocity does not fit to the concept of the single reconnected field line. Simultaneous two-side injections directly indicate to multiple reconnections proposed by Nishida (1989). There are two possible scenario of the origin of observed ion velocity distributions within the LLBL. One is a formation of complex velocity distributions within the magnetopause current layer. Indeed, some ion velocity distributions that have been obtained with SCA-I measurements during the time interval when the Interball Tail was within the current layer, 22:49:30-22:51:00 UT have 2 or 3 component structure, reminding what is observed in the LLBL. However, the magnetic field within the current layer is quite variable within 10s duty cycle of SCA-1, that may lead to time aliased ion spectra. This precludes any substantial conclusions about possible origin of complex ion velocity distributions of LLBL within magnetopause current layer. Additional problem with magnetopause current layer as a source of LLBL plasma is the need to explain how the particles from the current layer region may access the LLBL region. It appears that all observations of different velocity distribution fit to the scenario of multiple reconnections. As simulation shows (Lee et al., 1993), the result of multiple reconnections is a formation of the flux ropes with their “ends” either connected to the magnetosheath or to the magnetosphere. As the flux rope forms at the magnetopause, one side of it comes from the magnetosheath, and another one comes from the magnetosphere (Figure 6). Initially only the side is filled with magnetosheath plasma. In these parts of the field lines the magnetosheath plasma was “frozen” as it was in the magnetosheath. Smooth magnetosheath velocity distribution has some ions moving along magnetic field line, some are moving opposite to the field line direction, and some arc stationary. The separatrix dividing the ions moving downward along the magnetic field line and those moving opposite to the field line is accidentally determined by the direction of the magnetic field relative to magnetosheath flow direction. Ions freely move along the magnetic field line, and velocity distribution “does not feel” the ends of the field line due to its large extension.

Within newly formed flux rope the parts of flux tubes are not longer identical. The segments of the field lines originated in the magnetosheath are connected to the segments of the field lines originating from the magnetosphere. These plasma beads of two kinds: magnetosheath one and magnetospheric one are threaded by magnetic field lines of the flux rope. Particles suddenly start “to see” the magnetic field structure, and the fate of parallel moving ions, anti-parallel moving ions. and zero parallel velocity ions become different. The parallel velocity magnetosheath ions will stay in the magnetosheath part of the flux rope, and two other populations will cross the magnetospheric part of the flux rope and return back. The opposite is true for the magnetospheric ions. As the flux tube evolves, the number density of ions decreases. Observations indicate that the ions moving along and opposite to the field lines are accelerated, while a stationary component erodes. The ions coming to the opposite side and returning back are accelerated, as observations suggest, and three populations become distinguishable, parallal components form two shoulders. The relative magnitudes of two shoulders arc different, and are determined by the asymmetry ofmagnetosheath velocity distribution relative to the magnetic field line.

1998 0. L. V&berg et al.

Fig. 6. Flux rope cartoon modified from Lockwood and Hapgood (1998) with use of computer simulations results (Lee et al., 1993) and. Thick segments of the lines show magnetic field lines threading magneto- sheath plasma; thin segments are threading magnetospheric plasma. Gray plane is located at former magnetopause current sheet location. This cartoon does not show clearly the counterclockwise tilt of the X-line as follows from Le and Fu (1985) for observed IMF clock angle. Examples of two-component (B) and three-component (A) ion spectra measured within LLBL are tentatively identified with different parts of flux rope (see letters A and B).

CONCLUSION

Three different types of the ion velocity distributions within highly-structured LLBL were observed by Interball under southward magnetic field magnetosheath conditions: (a) one component moving along (or opposite to) local magnetic field, (b) two components moving in opposite directions relative to the magnetic field, and (c) three components one of which has nearly zero velocity along the magnetic field and two components moving in opposite directions relative to the magnetic field. Type (a) is the typical D-shape distribution observed in reconnected magnetospheric field lines which is consistent with predictions by (Cowley, 1982). Two oppositely moving ion components (b) usually have similar spectral characteristics but their relative densities vary. Their simultaneous existence and. similar spectral characteristics and comparison with ionospheric reflection model rule out the possibility to explain the second component by ionospheric reflection. These two oppositely moving components can be explained by the reconnection of particular magnetic flux tube in two sites. Type (c) is observed within LLBL structures; the existence of a component that is at rest in the magnetic coordinates is not compatible with reconnection of particular magnetospheric flux tube at one location. This type of ion velocity distribution changes in systematic way along with change of the number density. While the number density decreases the relative contribution of the components moving along the field line and in opposite direction increases compared to the component of the velocity distribution that is at rest in the magnetic coordinates. These observations could not be explained by the connection of magnetosheath and magnetospheric field lines at one location.

This could be explained by redistribution of ions along “closed” field lines from higher density parts of the flux rope formed by multiple reconnections with accompanying acceleration of these ions. Observed properties of LLBL ion velocity distributions indicate that multiple reconnections may play a role in the formation of LLBL structures. Fast and dense counter-streaming magnetosheath-type plasma components provide evidence of simultaneous or nearly simultaneous reconnection of magnetospheric

Spiral Magnetic Structures at the Magnetopause 1999

flux tube with magnetosheath flux tubes at two locations. The existence of three-component ion velocity distributions and their change with the number density indicate that the long spiral magnetic structures are formed at the magnetopause. We interpret these observations as evidence for multiple reconnections between magnetosheath and magnetospheric flux tubes with the formation of spiral magnetic flux tubes (Lee and Fu, 1985).

ACKNOWLEDGEMENTS

Authors gratefUlly acknowledge very useful discussions with A. Otto, V. Osherovich, A. Kropotkin, L. Zelenyi, and A. Petrukovich.

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E-mail address of 0. L. Vaisberg olepv@,iki.rssi.ru Manuscript received 16 January 2003; revised 17 May 2003; accepted 28 May 2003