Pages 823-832


Ingrid Sandahl1, Rickard Lundin2, Masatoshi Yamauchi2, Ulrik Eklund2, Jana Safrankova3, Zdenek Nemecek3, Karel Kudela4, Ronald P. Lepping5, Robert P. Lin6, Volt N. Lutsenko7, and Jean-André Sauvaud8

1 Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229,    Japan (on leave from Swedish Institute of Space Physics), E-mail:
2 Swedish Institute of Space Physics, P.O. Box 812, S-981 28 Kiruna, Sweden
3 Faculty of Mathematics and Physics, Charles University, V Holesovickach 2, 180 00 Prague 8,    Czech Republic
4 Institute of Experimental Physics, Slovak Academy of Sciences, Kosice, Slovakia
5 NASA/GSFC, Laboratory for Extraterrestrial Physics, Code 695, Greenbelt, MD 20771, USA
6 Physics Department & Space Sciences Laboratory, University of California, Berkeley, CA 94720,    USA
7 Space Research Institute, Profsojuznaya 84/32, 117810, Moscow, Russia
8 CESR, BP 4346, 31029 Toulouse, France


Interball Tail Probe was launched on August 2, 1995 together with its subsatellite MAGION-4 into a highly elliptical orbit with apogee at 31 RE and inclination 63 degrees. During the course of one year all local times are visited. In this paper we will present initial results obtained from measurements in the cusp, mantle and eveningside plasma sheet in January, 1996. It is found that the cusp was well defined and persistent at altitudes of 4-10 RE. In one case both the main satellite and the subsatellite were in the cusp proper for two uninterrupted hours. We believe this to be the first ever multi-point satellite observation of the high-altitude cusp. The data indicate that the cusp was very stable with a wide entry area and that plasma entry took place at high latitudes rather than at the subsolar point. Pressure pulses, possibly due to Alfvén waves were found. Sunward and antisunward moving plasma was measured simultaneously during a case of northward IMF but no convection was discovered. Plenty of plasma of cusp/magnetosheath type was also found mixed with plasma sheet plasma both equatorward of the cusp and in the eveningside plasma sheet.


One of the major problems of magnetospheric physics is where and how plasma and energy from the solar wind enter into the magnetosphere. In spite of many years of study, this problem is still largely unsolved, simply because it is so complex. Above all the large-scale picture is still missing. When the solar wind plasma encounters the terrestrial magnetic field it is slowed down and flows around the Earth in the magnetosheath. Plasma entry can take place in the cusps and along both the low and high latitude boundary layer and different mechanisms, like impulsive penetration and different types of merging, have been studied. However, the relative importance of different regions and mechanisms is not known. In addition the situation depends on the solar wind conditions.

The study of the entry of mass and energy is one of the major objectives of the Interball project (Figure 1). Interball consists of two satellites, the Tail Probe and the Auroral Probe, and two subsatellites. The spacecraft used in this study are Interball Tail Probe and its subsatellite MAGION-4, which were launched on August 2, 1995. Interball Tail Probe has a 63 degree inclination orbit with apogee at 31 RE and perigee very close to Earth at 800 km. The period is about four days. The orbit of the subsatellite is very similar, the distance between the spacecraft varying between one and several thousand km. Descriptions of the Interball project and its instruments as well as of MAGION-4 are found in Interball Mission and Project (1995) and in the Interball home page

 Fig1. The interball project

In one year the two satellites visit all local times. At launch apogee was in the morningside, in November it was at midnight, and in February in the eveningside. Because of the high inclination, most outbound magnetopause crossings take place at ZGSM-values of about 10-12 RE, while inbound crossings occur close to the equatorial plane.

In this paper we will report some initial results from ongoing studies of the boundary layer and the entry of magnetosheath plasma.


January 17

The cusp is certainly one of the main ports of entry for magnetosheath plasma. In January 1996 Interball Tail Probe and MAGION-4 were able to get an unusual perspective of the cusp between 4 and 13 RE, finding that the cusp was really like an open funnel with magnetosheath plasma pouring in all the time, probably entering the cusp at a high latitude. Figure 2 shows the orbit of Interball Tail Probe on January 17, 1996 in the GSM coordinate system. The interval between the major tickmarks is two hours. The orbit had its perigee in the prenoon sector and spent a long time in the cusp when outbound. Figure 3 is an overview of data from a number of Interball instruments on this day. The five top panels contain proton data from the ion mass spectrometer PROMICS-3 (Sandahl et al., 1996), the sixth panel data from the electron instrument ELECTRON (Sauvaud et al., 1996), panel number seven ion data from the solid state detector instrument DOK-2, and panel number eight data from the magnetic field experiment MIF-M. There is also a timeline showing when measurements were obtained with MAGION-4 (Safrankova et al., 1996).

  Fig2. Orbit of Interball Tail Probe on January 17, 1996 in the GSM coordinate system.

Fig. 3. Overview of Interball Tail Probe measurements on January 17, 1996. From top to bottom are five panels of proton data, one panel of electron data, one panel of high energy ion data, and one panel of key parameter magnetic field data. The designations of the regions are the same as in Figure 2. The boundary cusp and cusp proper were first traversed between about 04.35 and 07.00 UT and then the cusp/cleft was encountered once more between 08.30 and 09.20 UT. The lowermost panel shows the times when MAGION-4 particle data were obtained and region identifications from MAGION-4. A detailed description of the figure is given in the text.

The two top panels of PROMICS-3 data show tailward streaming protons in two energy ranges so that the panels together cover from 4 eV to 28 keV. The next two panels show protons moving approximately perpendicular to the solar direction. These detectors are looking perpendicular to the satellite spin axis and the data are therefore spin modulated, which is seen when they are plotted on a higher time resolution. The fifth PROMICS-3 panel shows sunward streaming protons in the energy range 4 eV to 1.5 keV.

The electron data were taken from a detector cell looking perpendicular to the spin axis. High resolution data between 7 and 8 UT are missing in the plot. Ions from the DOK-2 instrument are shown in three energy ranges, 22 to 28 keV, 46 to 60 keV, and 101 to 131 keV. The direction of these ions was sunward, and thus most of the ions measured were moving in a direction quasiperpendicular to the magnetic field and were trapped. The magnetic field data were taken from the Interball key parameter files. The total field and the Bx-component are shown.

The cusp proper was encountered between 05.03 and about 07.00 UT. It is seen as an intense flux of ions from all directions in the energy range 0.1 to 1 keV and at the same time a very low flux of ions at higher energies. There was also a strong flux of electrons below 200 eV. We believe this to be the longest cusp encounter ever reported in satellite measurements. Equatorward there was the plasma sheet with ions up to above 50 keV. The region where both the plasma sheet plasma and cusp plasma were found together, between about 04.35 and 05.03 UT, is the boundary cusp.

Poleward of the data gap the satellite found itself in the lobe, but between 08.30 and 09.20 UT the cusp, or here perhaps rather the cleft, was again encountered. The data look different from the previous cusp encounter, partly because of a different telemetry mode. Between 10.15 and 13.45 UT plasma of magnetosheath or cusp type was again measured, but now only moving tailward and transverse, not sunward. This is typical of the mantle.

At 14.38 UT Interball crossed a very sharp boundary, which we identify as the magnetopause. We will show that the particle increase was probably due to a solar wind induced temporal variation.

The various regions are marked in Figure 2. The cusp/cleft was entered at a geocentric distance of 7 RE and seen for the last time at a distance of 13 RE at a negative XGSM. It was because of the orbit that it was possible to remain in the cusp for such a long time.

The solar wind speed measured by WIND on this day was 500 km/s, the ion density 3-4 cm-3 and the interplanetary magnetic field between 3 and 7 nT. Bz kept flipping back and forth between positive and negative, but was mostly a few nT positive when Interball Tail Probe was in the cusp.

We will now look at the data in greater detail. Figure 4 shows the proton data in the energy range 4 eV to 1.5 keV. It is quite clear that the general character of the cusp was preserved all the way out to the data gap at beyond 9 RE. The data there do not look much different from what is seen in low-altitude satellites. It is unlikely that such measurements could have been obtained if the cusp had been very narrow. Comparing with the orbit in Figure 2 it is difficult to imagine that this is a population on its way from the sub-solar point down to the ionosphere. Rather it seems likely that this magnetosheath plasma has entered the cusp at a high latitude. However, more careful studies are needed to determine this.

Fig. 4. Proton measurements by the PROMICS-3 instrument on Interball Tail Probe on January 17, 1996 between 04.30 and 06.30 UT.

In the boundary cusp region and early part of the cusp proper there were multiple injection structures, most easily visible in the tailward and sunward moving protons.

The plasma flux was much more persistent than one would expect if it had entered through a series of isolated flux transfer events. But it is still pulsed, possibly due to standing Alfvén waves. These pressure pulses are more clearly seen in panel 4 of Figure 5 which shows more detailed data of the sunward streaming protons. The pulses are most easily seen in this detector because the pitch angle variation is not so big. The pulse period was of the order of four minutes. In the transverse detector, which measures almost all pitch angles in one satellite spin, the pressure pulses are somewhat masked by the V-shapes, which appear at the satellite spin period, about 120 seconds.

Fig. 5. Cusp measurements by MAGION-4 and Interball Tail Probe on January 17, 1996 between 05.32 and 05.52 UT. A detailed description of the figure is given in the text.

Interball Tail probe was not alone to measure the cusp on January 17. The particle measurements on the subsatellite MAGION-4 were very similar. The subsatellite time line in Figure 3 also shows the preliminary region identifications. In fact, the subsatellite entered the cusp well before the main satellite, but further out most boundaries were crossed almost simultaneously. At 04.00 UT the distance between the spacecraft was 12 000 km at 07.00 UT it had decreased to about 8 000 km.

Figure 5 shows ion and electron data from MAGION-4 in panels 1 and 2 together with proton data from Interball Tail Probe in panels 3 and 4 at a time when both spacecraft were in the cusp proper. The viewing direction of the MAGION-4 detectors is approximately perpendicular to the satellite spin axis. The subsatellite spin period was about 95 seconds. Figure 6 is a data comparison at the magnetopause. Interball Tail data of tailward streaming protons are in the top two panels and Interball Tail transverse electron data in the third panel. The three bottom panels show MAGION-4 ion and electron data. During this time period the distributions and boundaries seen by the two spacecraft were almost identical, and it is likely that the variations are due to temporal changes caused by variations in the solar wind conditions.

Fig. 6. Measurements at the magnetopause on January 17, 1996 by Interball Tail Probe and MAGION-4.

January 20, 24, and 28

We will now present three more examples of cusp crossings by Interball. Figure 7 shows the perigee parts of the three orbits following January 17. The positions of the northern cusp as measured by Interball are marked with lines and in two cases Interball also measured the southern cusp/cleft. The northern cusp encounters occurred at distances of four to eight RE geocentric distance and close to YGSM=0.

Fig 7. Interball Tail Probe orbits on January 20, 24 and 28, 1996.

Proton measurements in these cusps are presented in Figures 8, 9, and 10, here plotted in counts. On January 20 (Figure 8) the cusp proper was found between 23.38 and 23.54 UT and the boundary cusp between 23.18 and 23.38 UT. However, the cusp plasma seems to continue deep into the plasma sheet as seen by the weaker fluxes in detectors 3 and 5 (panels 4 and 5) starting already at 22.30 UT. At about 23.35 UT there were some multiple injection structures. When the cusp was encountered the IMF Bz had been clearly positive, about 5 nT, for more than six hours and the solar wind ion velocity was 450 km/s. The fluxes in both the tailward, transverse and sunward direction were very similar and with similar temporal histories. Thus there were no obvious signs of either a sunward or an antisunward convection. This indicates that the magnetosphere was stable, with no significant magnetic flux and particle transfer from the nightside to the dayside magnetosphere as a consequence of reconnection processes occurring tailward of the cusp boundary.

Fig. 8. Proton measurements for northward IMF Bz by the PROMICS-3 instrument on Interball Tail Probe on January 20, 1996 between 22.00 and 24.00 UT. The top two panels show tailward streaming, the next two transverse and the fifth panel sunward streaming protons.

Fig. 9. Proton measurements for weakly southward IMF BZ by the PROMICS-3 instrument on Interball Tail Probe on January 24, 1996 between 17.00 and 19.00 UT.

Fig. 10. Particle measurements for northward IMF Bz by the PROMICS-3 instrument on Interball Tail Probe on January 28, 1996 between 12.30 and 14.30 UT.

The cusp at 4-5 RE on January 24 (Figure 9) could almost be mistaken for a cusp measured by Viking at an altitude of 13 000 km. Interball Tail Probe was in the cusp proper between 17.46 and 18.16 UT. There were both the typical V-shaped ion signature measured by a spinning spacecraft because of the energy-pitch angle dependence of the particle distribution and field-aligned outstreaming ions at 10 to 100 eV energy. In the boundary cusp, between 17.40 and 17.46 UT, where also the poleward edge of the plasma sheet was detected, there was an injection structure. Since about 15.40 UT IMF Bz had been about -2 nT, thus weakly negative. The solar wind ion velocity was about 380 km/s. Although IMF Bz was negative the cusp was found very close to XGSM =0. This is probably not consistent with entry at the sub-solar point.

The cusp on January 28 (Figure 10) was a stagnant cusp, that is with little signs of convection. It was encountered between 13.12 and 13.26 UT. The solar wind was 3-5 nT northward and the velocity about 400 km/s. As on January 17, the cusp was again measured further out along the orbit, between 13.56 and 14.50 UT. The layout of Figure 10 is somewhat different from that of previous PROMICS-3 spectrograms, the top three panels showing protons in the energy range 4 eV to 1.5 keV only, and the fourth panel showing transverse electrons from 12 eV to 35 keV.


A frequently observed feature in the Interball data is the presence of plasma of cusp or magnetosheath type in regions belonging to the plasma sheet. Such observations are of importance when plasma entry mechanisms are studied.

One site of such observations is the dayside plasma sheet equatorward of the cusp proper. Two examples have already been mentioned above, January 17 and January 20. The region of co-location is also marked in the orbit plot in Figure 2, where it is identical to the boundary cusp. These data show that cusp plasma is able to enter onto closed plasma sheet field lines and that thus the frozen-in condition is violated.

Figure 11 is a 24-hour plot of protons measured on January 20, showing an example of another such mixing region, the eveningside plasma sheet in the tail. During most of the time the fluxes were mainly in the energy range 1 to 30 keV, typical of the plasma sheet. But between 02.15 and 07.40 UT there were in addition plenty of protons in the 0.1 to 1 keV energy range appearing in both the sunward, transverse and tailward looking detector, which we interpret as magnetosheath plasma. Similar observations were made on January 24 and 27.

Fig. 11. Protons measured on January 20, 1996. Between 02.15 and 07.40 UT a mixing region was encountered in the tail.

The locations of this mixing region are shown in Figure 7. On January 20 and 28 plasma sheet plasma without the magnetosheath component was measured at even larger absolute YGSM-values so it is not obvious that the mixing region is just the low latitude boundary layer. It is not yet known if this plasma may have entered through the eveningside low latitude magnetopause, but at least that seems to be the closest alternative.


Interball Tail Probe and its subsatellite MAGION-4 have made very interesting measurements in the magnetospheric boundary layers at different local times, in the mantle and in the cusps. It has been found that the cusp was well defined and persistent at altitudes of 4-13 RE. In one case both the main and the subsatellite were in the cusp proper for two uninterrupted hours. We believe this to be the first ever multipoint satellite observation of this part of the cusp. The long duration of the cusp encounter suggest that the cusp was both wide and stable. The location of the measurements indicates that plasma entry took place at high latitudes rather than at the subsolar point. Fluxes of simultaneously measured antisunward and sunward streaming plasma were very similar, and did not indicate any strong sunward or antisunward convection. Pressure pulses, possibly due to Alfvén waves were found.

The energy and pitch angle distribution of the particles will provide much information about the source location and mechanism, as is demonstrated by low- and mid-altitude satellite data (e.g., Woch and Lundin, 1992, and references therein), but this will be done in the future.

Plenty of plasma of cusp/magnetosheath type was also found mixed with plasma sheet plasma both equatorward of cusp and in the eveningside plasma sheet.


The Interball mission was designed by the Space Research Institute of the Russian Academy of Sciences under the auspices of the Russian Space Agency. The spacecraft was developed by the Lavochkin association. The subsatellite MAGION-4 was built by an international team headed by the Institute of Atmospheric Physics in the Czech Republic. The scientific payloads were provided by Russia together with Austria, Bulgaria, Canada, Cuba, Czech Republic, ESA, Finland, France, Germany, Great Britain, Greece, Hungary, Italy, Kirgizia, Poland, Rumania, Slovakia, Sweden, Ukraine, and Uzbekistan. We thank S. Klimov, S. Romanov, and A. Petrukovich for providing MIF-M key parameter data. The paper was finalized during a visit by the first author to ISAS and the support of ISAS is gratefully acknowledged.


Interball mission and payload, CNES-IKI-RSA, (May 1995).

Nemecek, Z., A. Fedorov, J. Safrankova, and G. Zastenker, Structure of the low-latitude magnetopause: MAGION-4 observations, accepted for publication in Ann. Geophys., 1996.

Sandahl, I., S. Barabash, H. Borg, E. Yu. Budnik, E. M. Dubinin, U. Eklund, H . Johansson, H. Koskinen, K. Lundin, R. Lundin, A. Moström, R. Pellinen, N. F. Pissarenko, T. Pulkkinen, P. Toivanen, and A. V. Zakharov, First results from the plasma composition spectrometer PROMICS-3 in the Interball project, accepted for publication in Ann. Geophys. , 1996.

Sauvaud, J.A, P. Koperski, T. Beutier, H. Barthe, C. Aoustin, J.J. Thocaven, J. Rouzaud, The Electron experiment onboard Interball-Tail: Initial results on the low latitude boundary layer of the dawn magnetosphere, submitted to Ann. Geophys., 1996.

Woch, J., and R. Lundin, Magnetosheath plasma precipitation in the polar cusp and its control by the interplanetary magnetic field, J. Geophys. Res., 97, 1421-1430, 1992.