Low-energy near-equatorial protons: the flux dependence on magnetic coordinates
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| Low-energy near-equatorial protons: the flux dependence on magnetic coordinates O.R. Grigoryan, M.I. Panasyuk, A.N. Petrov Scobeltsin Institute of Nuclear Physics, Moscow State Univercity, Russia. K. Kudela, J. Stetiarova Institute of Experimental Physics, Slovak Academy of Science, Košice, Slovakia. Abstract. This work is result of experimental study of low-energy (Ep from several tens of keV to several MeV) proton population below radiation belts (L-shell number less 1.15). The flux increasing in this region is well estimated for several experiments from 1969. In this work the results of three experiments are presented additionally to previous experiments – ACTIVE (Ep = 30-500 keV, 1990 year, altitude 500-2500 km, inclination 81.3), SPRUT-VI (Ep = 0.3-5.0 MeV, 1999 year, MIR space station, altitude 350 km, inclination 51.6) and SAMPEX (Ep>770 keV, 1992-1998 year, altitude 520-670 km, inclination 82). The results concerned to the flux dependence on geomagnetic coordinates (L, B) are presented. These results allow us to conclude that low-energy protons near the geomagnetic equator are quasitrapped. Introduction The existence of some analogue of the proton belt of low-energy particles near geomagnetic equator at low altitudes (<1000 km) was discovered by “Azur” satellite (^ , [1972]; Hovestadt et al., [1972]). This fact was confirmed by several satellite experiments later [Butenko et al., 1975] indicating the constant band of low-energy protons near the equator at low altitudes. Examples of protons registration are showed in Figure 1. These figures show the proton flux dependence on time during the low L-shells crossing. Other important parameters such as L,B or magnetic latitude are shown at the bottom axis. These examples are typical. They are shows that protons are registered at L<1.15 mainly. The first explanation of this phenomenon proposed by Moritz used a double charge-exchange mechanism. The protons of radiation belt and ring current can interact with geocoronal protons and produce energetic neutral atoms (ENA). These ENA can lose electrons at low altitudes. After that they become trapped in the Earth’s magnetic field. High atmosphere density (mainly oxygen) at low altitude trends to protons losses therefore these particles are not trapped permanently. However some features of proton flux distribution are not explained by the double charge-exchange model. The first sign of model imperfection was the dependence of proton flux on magnetic local time. The protons are registered mainly in night and evening hours [Butenko et al., 1975] as the experiment onboard Сosmos-484 satellite showed. Later this result was confirmed by other authors (Greenspan et al., [1999], Grigoryan et al., [2002], Grachev et al., [2002]) used the SAMPEX and ACTIVE satellite data. Another feature of the near-equatorial proton formation is the flux dependence on L-shell number. The protons appear mainly at L>1.04 [Grigoryan et al., 2002], there is the “slot” directly on geomagnetic equator. Observations and discussion The ACTIVE (Intercosmos-24) satellite was launched at the end of 1989 and the lifetime of this satellite was to 1993. In this work, the data of 1990 year are discussed. Altitude range of the orbit was 500 x 2500 km and inclination was 81.6. The particle detection SPE-1 device was installed onboard the ACTIVE satellite. The main part of the system was based on solid state silicon barrier detector. There were three detectors of protons and three detectors of electrons. They were oriented in different directions to the satellite axis. Proton detectors were protected by magnetic filters to prevent electrons with energy up to 800 keV penetrate to the detector (see details of detector set in Kudela et. al, [1992]). Energy of registered protons was from 30 keV to 550 keV. The SAMPEX satellite was launched in 1992 in a near-circular orbit with 550 x 675 km altitude and 82 inclination. To compare this satellite data with ACTIVE data we used 1992 year only. The LICA device onboard  Figure 1. Examples of near-equatorial proton registration by the SPE-1 (ACTIVE), LICA (SAMPEX), SPRUT-VI (MIR) experiments.   Figure 2. Proton flux (Ep>770 keV) dependence on L according to SAMPEX experiment at altitude ~600 km (black dots-experimental points, white line - approximation).  Figure 3. Average proton flux dependence on L according to ACTIVE experiment at altitudes 500-1300 km for different energies from ~30 keV to ~ 550 keV  Figure 4. The proton (Ep>770 keV) flux dependence on magnetic field strength (SAMPEX satellite data) at 600 km altitude (dots-experimental points, line - approximation) SAMPEX satellite allows register protons from 770 keV to 8 MeV. LICA is a time-of-flight mass spectrometer that identifies particles type, energy, and arrival direction by their time-of-flight over a ~0.5 m flight path, along with their residual energy (Mason et. al, [1993] and Greenspan et. al, [1999]). The SPRUT-VI device onboard MIR space station was installed in 1999. This experiment allows registering protons with energy from 0.5 to 5.0 MeV. The protons were registered by a solid state detector. Using the one-year data from ACTIVE and one-year data from SAMPEX satellite we investigated the proton flux dependence on L (McIlwein parameter). The result of this investigation is showed in Figure 2 and Figure 3. We used polynomial approximation  of the SAMPEX data at L= 0.99-1.11 interval (see coefficients in table).
Main features of L-distribution of near-equatorial protons are:
We can try to explain these features using two sources – low-energy protons from ionosphere and high-energy protons from ring current/radiation belt. Maybe the main source of low-energy protons al small L is the ionosphere. And maybe for some reason the second stage of charge-exchange process is most effective at L~1.05. It is interesting to test if near-equatorial protons are trapped particles. Life-time of 100 keV proton at 500 km altitude is about 2.7101 s (due to charge-exchange on oxygen atoms with n~10-6 cm-3 (MSIS-E-90 atmosphere model, Hedin, 1991)), so we can suppose protons trapped in the Earth’s magnetic field. In this case the proton flux would have maximum in the pitch angle distribution near 90 and flux would have maximum at small B-magnetic field strength in the top of the magnetic field line. Moritz (1972) and Guzik (1989) showed that there is maximum in pitch-angle distribution of near-equatorial protons (Ep=0.25-1.65 MeV) detected onboard AZUR satellite and Ep>360 keV onboard S81-1. This maximum is near 90. Using the SAMPEX experimental data we found the proton flux dependence on magnetic field strength as it showed in Figure 4. We found linear approximation  of proton flux dependence on B. Flux increases while the magnetic field strength decreases. This fact confirms the suggestion of trapped particles. But the drift time around the Earth is 2.3104 s whereas the bounce motion time is 7.4 s at L~1.15. So the low-energy protons do not completely drift around the Earth, they are quasitrapped.If these protons are quasitrapped the proton flux should depend on longitude because of inhomogenity of the Earth’s magnetic field. There is the South Atlantic Anomaly (SAA) - the region where the magnetic field strength is less than in the other regions at the same L at the same altitude. Particles in SAA penetrate deeply into atmosphere and collide to neutral atoms and are scattered. As it was found in [Hovestadt et al., 1972] work the proton flux at longitudes [-80:-60] is less than in the other longitudes. The same character of longitude dependence was found in ACTIVE and SAMPEX experiments (see Figures 5, 6). Here is the proton flux dependence on geographic longitude based on ACTIVE (at altitude~500 km) and SAMPEX (at altitude ~ 600 km) plotted. We can see the proton flux minimum at longitudes [-90:-30] for SAMPEX and at longitudes [-90:0] for ACTIVE experiment. These three intervals intersect at longitudes [-80:-60]. This coincidence is wholly satisfactory. The proton flux dependence on energy is one of the most important features of this proton formation. As it is mentioned by Moritz, (1972) the proton spectrum at low latitudes near geomagnetic equator is similar to a ring current/radiation belt spectrum. The spectral index of proton spectrum is ~4 in energy range 0.5-2.7 MeV according to AZUR satellite. The spectrum constructed from our experimental data (ACTIVE, SAMPEX and SPRUT-VI) and OVI-17 data [Mizera et al., 1973] is shown in Figure 7. It is obviously that spectrum is not power-law in whole energy interval. The spectrum of protons in ring current (see review by Kovtyukh et al., 1992) can be approximated as   Figure 6. Proton (Ep>30 keV) flux dependence on geographic longitude at the 500 km altitude (ACTIVE data).  Figure 7. Energy spectrum of near-equatorial protons according to experiments onboard the OVI-17 (Mizera et al., 1973) ACTIVE, SAMPEX satellites and the MIR space station (SPRUT-VI). Line is approximation (see in text) of all experimental data.  Figure 8. Proton flux dependence on L, B of near-equatorial protons for altitudes 500-1300 km (ACTIVE experiment results).  (*)Pizzella et al., (1971) showed that E0=8 for L=4.5, E0=6 for L=5.0 and E0=3.5 for L=6.0. Using (*) approximation to near-equatorial proton spectrum we found E0=11 3 keV (line at Figure 7). This value is close to E0 at L=4.5 where is the maximum of ring current. So we can conclude that ring current contribution to near-equatorial proton flux is essential. If the particles in near-equatorial region are quasitrapped the proton flux dependence on L,B should not depend on altitude. Figure 8 shows the proton flux dependence on L, B based on the ACTIVE experiment data for 30-550 keV protons at altitudes 500-1300 km. Altitude ~1300 km is the maximal altitude where the satellite could pass through L<1.15. And we found the increasing of proton flux at L<1.15 at the altitudes up to 1300 km [Grigoryan et al., 2002, Grachev et al., 2002]. Main features of the L, B – distribution are:
The presentation of proton flux in L,B coordinates is useful and comprehensive in scientific purpose. But picture in usual Cartesian coordinates is simpler to understand. Here we will make first step to build 3D model of near-equatorial proton fluxes. Using the results of the SAMPEX experiment showed in Figure 2 and Figure 4 we build simple polynomial approximations (listed above) of proton flux dependence I(L) and I(B) and showed the product I(X(L,B), Z(L,B))=I(L)*I(B) of these approximation functions in Figure 9 in the XZ coordinates. The procedure is described below. In geocentric coordinates the point of origin is the Earth’s center. The ^ axis is to the North, X axis – one of directions in magnetic equator plane (we assume the field is axial-symmetric), we select Y axis so the XYZ is right-hand triple. The measurement unit is the Earth’s radius. There are relations between the magnetic latitude , radius vector module r and X, Z coordinates: X = r cos Z = r sin There is a simple relationship of XZ and L,B coordinates. If the Earth’s magnetic field is dipole there are such relations of r, coordinates with L, B coordinates as:  Figure 9. Approximation of the proton flux dependence on L and B obtained in the SAMPEX experiment in the geocentric XZ coordinates. r = L cos2.  The main features of the flux dependence on L,B are:
Conclusion Analyzing of set of experimental results concerning the fluxes of low-energy (tens keV – several MeV) protons at low altitudes we can say:
References Butenko, V.S., O.R. Grigoryan, S.N. Kuznetsov, G.S. Malkiel, and V.G. Stolpovsky, >70 keV proton fluxes at near equatorial region at low altitudes. Kosmich.Issled. (in Russian), 13, 508, 1975. Greenspan, M.E., G.M. Mason, and J.E. Mazur, Low-altitude equatorial ions: A new look with SAMPEX, J.Geophys.Res.,104, 19911, 1999. Grigoryan O., A. Petrov, K. Kudela: Near-equatorial protons: the local time dependence, WDS'02 Proceedings of Contribution Papers: Part II - Physics of Plasmas and Ionized Media, ed. by J. Safrankova, Praha, Matfyspress, p. 263-268, 2002. Grachev E., O. Grigoryan, J. Juchniewicz, S. Klimov, K. Kudela, A. Petrov and J. Stetiarova, Low energy protons on L1,15 in 500 – 1500 km range, Adv. Space Res, 30, 7, 1841-1845, 2002. Guzik T.G., M.A. Miah, J.W. Mitchell, and J.P. Wefel, Low-altitude trapped protons at the geomagnetic equator, J. Geophys. Res., 94, 145, 1989. Hedin, A. E., Extension of the MSIS Thermospheric Model into the Middle and Lower Atmosphere, J. Geophys. Res. 96, 1159, 1991. Hovestadt, D., B. Hausler, and M. Scholer, Observations of energetic particles at very low altitudes near geomagnetic equator. Phys.Rev.Lett.,. 28, 1340, 1972. Kovtyukh A.S., Panasyuk M.I., Modelling of space-energetic distribution of hot plasma at geostationary orbit, (in russian) preprint SINP MSU – 92-28/277, 1992. Kudela K., J.Matisin. Inner zone electron peaks observed by the “Active” satellite. Journal of Geoph. Res., .97, A6, pp 8681-8683, 1992. Mason, G.M., D.C. Hamilton, P.H. Walpole, K.F. Heuerman, T.L. James, M.H. Lennard, and J.E. Mazur, LEICA: A Low Energy Ion Composition Analyzer for the study of Solar and Magnetospheric Ions. IEEE Trans. Geosci. and Remote Sens., 31: p. 549-556, 1993. Mizera P.F., and J.B. Blake, Observations of ring current protons at low altitudes, J. Geophys. Res., 78, 1058, 1973. Moritz, J. Energetic protons at low equatorial latitudes: A newly discovered radiation belt phenomenon and its explanation. Z.Geophys., 38, 701, 1972. Pizzella, G. and L. A. Frank, Energy Spectrums for Proton (200 eV < E < 1 MeV) Intensities in the Outer Radiation Zone, J. Geophys. Res., 76, 88-91, 1971. |
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