E. Kirsch1, B. Wilken1, G. Gloeckler2, A.B. Galvin2, U. Mall1, and D. Hovestadt3
für Aeronomie, D-37191 Katlenburg-Lindau, Germany,
2 University of Maryland, College Park, USA,
3 MPI für Physik und Astrophysik, D-85747 Garching, Germany
The WIND spacecraft, launched on November 1, 1994, undergoes prior to its halo orbit insertion several lunar swing-by maneuvers. The GEOTAIL spacecraft, skimming around the dayside magnetopause (Dec. 1994), passes also several times behind the moon. The HEP-LD spectrometer onboard of GEOTAIL and the Solar Wind and Suprathermal Ion Composition (SMS) Experiment onboard of the WIND spacecraft are well positioned for observing ions in the mass range between H and Fe in the direct vicinity of the moon. We report first observations of lunar pickup-ions in the magnetotail at a distance of 80 Re .
Due to ionization and charge exchange processes, secondary ions are produced in the interplanetary space which are not part of the primary solar wind flow. These ions are then ``picked-up'' by the solar wind and are convected outwards as suprathermal ions.
In recent years pickup-ions were used as a tool to investigate a wide range of topics. Mikhailov and Barnes (1981) used them to study the composition of planetary ionospheres. Möbius et al. (1985) and Gloeckler and Geiss (1996) infered parameters of the local interstellar medium based on pickup-ion measurements.
In an attempt to use pick-up ions as a remote sensing method information on the surface composition of comets (Ipavich et al., 1986; McKenna-Lawlor et al., 1986) and of the lunar surface (Hilchenbach et al., 1991, 1993; Kirsch et al., 1996) were gained. Pickup-ions are singly charged and reach 4 times the energy of solar wind ions. They can therefore be clearly distinguished from solar wind ions.
The release of atoms and ions from the lunar surface is caused by three major processes: Solar wind sputtering, photon sputtering and impact of micrometeroids. Solar wind sputtering has been studied by Elphic et al. (1991) (on lunar soil brought back from the moon by the Apollo missions). The most important compounds are SiO2, TiO2, Al2O3, FeO, MnO, MgO, CaO, Na2O, K2O, Cr2O3 and P2O5 and the most important ions: O, Si, Al, Ca, Fe, Mg, Na, K. Of special interest for the present study are therefore singly charged ions in the mass range 16-60 amu.
Smyth and Marconi (1995) compiled the results of model calculations and demonstrated differences by 2-3 orders of magnitude. Improved model calculations are therefore necessary.
Lunar pickup ions have been measured for the first time by Hilchenbach et al. (1991, 1993) using the time-of-flight spectrometer SULEICA on the AMPTE/IRM satellite. SULEICA observed the lunar pickup-ions when the moon was in front of the Earth (``new moon''), mainly in the mass range 16-22 (O to Na), 28-34 (Si, Al). Minor count rates were observed in the range 36-54 amu.
It is the purpose of the present paper to show lunar pickup ion measurements made during the WIND-moon flybys and to detect with GEOTAIL the entry of such ions on the day side magnetopause of the Earth. Furthermore we will discuss lunar pickup ions in the tail of the magnetosphere.
SPACECRAFT ORBIT AND INSTRUMENTATION
The WIND spacecraft completed 7 flybys around the moon in the interval from November 1994 to March 1996. GEOTAIL performed 14 flybys in the years 1992 to 1994. Among those the flybys on December 27, 1994 and January 16, 1996 were characterised by a high solar wind velocity (vsw 550 km/s). Higher solar wind speeds are in general favourable for our study because they increase the detection efficiency. However, the observation of singly charged heavy ion species like Fe with a mass of 56 amu and with a maximum energy of 423 keV/q will be limited because part of their velocity distribution is above the energy window of the STICS and HEP-LD sensors. Close to the moon, however, lunar pickup ions will have not reached their full pickup energy. due to their large gyroradii, so that their identification should be possible (Kirsch et al., 1996).
The HEP-LD spectrometer (with the sensors S1, S2, S3 from south to north) and the SMS Experiment apply both time of flight technology and measurement of the rest energy of the particles by solid state detectors. The HEP-LD detector (Doke et al., 1994) onboard the GEOTAIL measures omnidirectional particle fluxes and allows the identification of mass groups (H, He CNO, Si-Fe). Protons and CNO ions can be measured with the HEP-LD in the energy range 30-1500 keV and 160-1500 keV, respectively, the field-of-view is 12o by 180o. The Suprathermal Ion Composition Sensor on WIND measures the mass/charge ratio between 8-226 keV/q, the field-of-view is 4.5o by 156o (Gloeckler et al., 1995).
We studied four lunar flybys: 1) WIND flyby around the moon while GEOTAIL was skimming the morning side of the magnetosphere (December 27, 1994). 2) GEOTAIL passed the moon on the morning side of the bow shock, far outside of the magnetopshere (June 28, 1994). 3) GEOTAIL was in front of the bow shock and observed lunar ions under new moon condition (May 30, 1995). 4) One GEOTAIL-lunar flyby was observed in the distant tail (November 8, 1992) of the Earth.
Fig. 1. Total sector count rates of the WIND-STICS sensor (for m > 1.3 amu) are shown on the left side. The main maximum results from solar wind ions, the secondary maximum (sectors 4-7) is produced by lunar pickup ions under the assumption that the average magnetic field vector is inclined on 45o westward to the S/C-sun line. The total count rate as function of the mass/charge (0-60 amu/e) is shown on the right side (for m > 1.3 amu).
Figures will be shown only for cases 1 and 4. Figure 1 presents total counts as a function of the sector number and counts as function of m/q (both for m > 1.3 amu) obtained by the WIND-STICS sensor during the interval Day 361 to 365, (Dec. 27-31) 1994 (Vsolar 550 km/s). The position of the moon and the trajectory of the WIND at the morning side of the Earth bow shock during the WIND flyby (Dec. 27, 1994) can be seen in the upper part of Figure 2.
The trajectory of the GEOTAIL-S/C is just inside the magnetosphere (Dec. 25-26). The sectored measurements on Figure 1, left side, show a main maximum in sectors 9/10 which results from solar wind ions and a side maximum in sectors 4-7 which we attribute to lunar pickup ions under the assumption that the nominal interplanetary magnetic field formed an angle of 45o with the Earth-Sun-Line. The counts as functions of m/q (Figure 1, right side) show beside solar wind ions (m/q < 16) also singly charged ions (m/q > 16) which should be lunar pickup ions such as oxygen, sodium, aluminium, silicon and some heavier ions.
The HEP-LD measurements (Figure 2, lower curve) show electron and ion fluxes from December 25 to 31, 1994. On December 25 and 26 an enhanced flux can be recognized before the S/C passed the radiation belt on December 27 to 28.
We attribute the enhanced fluxes (Dec. 25-26) to lunar pickup ions which may have been further accelerated at the Earth bow shock. Composition measurements from HEP-LD are not available for this time interval.
Lunar pickup ions mixed with ions from the magnetotail were observed from November 9 to 11, 1992. A lunar flyby of the GEOTAIL took place on November 8, 1992 (Figure 3, upper part). Enhanced omnidirectional fluxes were observed by HEP-LD especially around November 9 to 11, 1992 (bracket 1) and lower fluxes around November 12 to 15 when the moon was aready outside of the tail (bracket 2).
For this event mass groups have been measured by HEP-LD. The upper part of Figure 4 shows this for November 11, 1992. Protons, He, CNO and Si-Fe ions (curves from left to right) have been measured altogether 13.840 ions which result from the moon as well as from the magnetotail. The lower part in Figure 4 presents a similar distribution with altogether 6000 ions measured on November 15, 1992 when the moon had already left the tail.
Fig. 2. The upper panel shows the position of the moon on December 27, 1994 and the trajectory of WIND during the first flyby period as well as the trajectory of GEOTAIL inside the magnetosphere. The lower panel presents omnidirectional measurements of electrons and ions. The bracket indicates an interval with lunar pickup ions detected at the magnetopause.
3. GEOTAIL lunar flyby on November 8, 1992 in the tail region. Bracket 1
(lower panel) indicates lunar pickup ions mixed with tail ions, bracket 2
shows only tail ions measured by HEP-LD.
The WIND and GEOTAIL observations of lunar pickup ions confirmed that such ions reach the magnetosphere of the Earth under new moon conditions as was observed earlier by Hilchenbach et al. (1991). Model calculations on the propagation of such ions towards the Earth were published by Cladis et al. (1994). Lunar ions can also be detected in the distant tail as could be observed around a ``full moon'' time interval (Figure 3). Not yet confirmed is whether the lunar ions can also propagate along the tail field lines towards the Earth.
Magnetospheric ions have been observed with the GEOTAIL S/C (experiment STICS) in the distant tail by Lui et al. (1996). The ions H+, He++, He+, N+ and O+ could be identified and their acceleration and entry from the magnetosheath in the tail was observed in situ.
Fig. 4. HEP-LD scatter plots of mass groups obtained on November 11 and 15, 1992. Curves from left to right: H, He, CNO, Si-Fe. Note that the CNO group and also the Si-Fe group are enhanced on November 11, 1992 (upper panel) during the lunar flyby. Smaller fluxes of heavy ions were observed on November 15, 1992 because the moon had already left the tail (lower panel).
In Figure 3 lunar and magnetospheric ions were also measured by HEP-LD on GEOTAIL. However, Figure 4 shows that during the lunar flyby enhanced fluxes of the CNO group and also Si-Fe ions which should have their origin at the lunar surface were present. During full moon conditions the lunar surface is also bombarded by magnetospheric particles. One example for lunar pickup ions observed at the dawn side of the magnetosphere is shown in Figures 1 and 2. The WIND-STICS measurements (Figure 1) revealed the composition of the lunar pickup ions. Only small fluxes of singly charged heavy ions were observed because the full pickup energy is not yet reached. It can be calculated that the most important ions (C to Fe) have gyroradii of 20-40 lunar radii for a total energy of ~ 100 keV (B = 5 nT is assumed). Thus nearer to the moon even smaller ion energies should have been reached. The lunar ions measured simultaneously by HEP-LD (Figure 2, Dec. 25-26, 1994) confirm that such ions enter on the dayside bowshock and magnetopause perhaps via the dayside cusp region. They could have been further accelerated near the bowshock by a quasi-perpendicular shock or by the Fermi mechanism. Magnetic field measurements which could reveal whether a quasi-parallel or a quasi-perpendicular shock was operating during the measurements (Figures 1, 2) were not available but shall be considered later in a more detailed study.
Lunar pickup ions are a permanent particle source for the Earth magnetosphere. The entry of such particles in the magnetosphere has been observed on the day- and dawnside of the magnetosphere as well as in the tail. Further composition and sector measurements, energy spectra as well as magnetic field measurements are required for a more detailed study. The detection of pickup ions can also be used as a method of remote sensing. The sputtering yield and the ionisation potential for the various ions must then be considered.
We thank J. Cain, R. Lundgren, S. Lasely and Ed. Tums from the University
of Maryland for their work during the development of the SMS experiment
and S. Chotoo for the analysis of the STICS calibration data. The German
contribution to the SMS-Experiment was funded by DARA-Contract FKZ 50 OC
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