LOW ENERGY PROTONS ON L1.15 IN 500 – 1500 km RANGE
E. Grachev1, O. Grigoryan1, J. Juchniewicz4, S. Klimov2, K. Kudela3, A. Petrov1, J. Stetiarova3
1Skobeltsyn Institute of Nuclear Physics Lomonosov Moscow State University, 119992 Moscow, Russia
2Space Research Institute Russian Academy of Sciences, 117810 Moscow, Russia
3Institute of Experimental Physics, Slovak Academy of Sciences, 04353 Kosice, Slovakia
4Space Research Centre, Polish Academy of Sciences, Warsaw, Poland
The results of low-energy (<1MeV) protons investigation near geomagnetic equator (L1.15) at different altitudes are presented. Used data from Active satellite (1989-1991, altitude 500-2500, inclination 82о, protons with energy Eр=50 – 500 keV). The proton spectra were obtained during absolute quiet period (-30
Fig. 1. Example of near-equatorial proton registration by SPRUT on MIR. Ep1=0.1-0.24 MeV, EP2=0.24-0.5 MeV, EP3=0.5-1.0 MeV, EP4 > 1.0 MeV
The existence of some analogue of the proton belt composed of low-energy particles in the region of geomagnetic equator at low altitudes (<1000 km) was discovered by “Azur” satellite (Moritz, 1972; Hovestadt et al., 1972). This fact was confirmed by several satellite experiments later indicating the constant band of low-energy protons near the equator at low altitudes. Figure 1 shows an example of near-equatorial proton increases observed by the SPRUT-V experiment on MIR station (altitude ~400 km). This device measured protons in the energy range 0.1–5.0 MeV (Biryukov et al., 1996). It was installed on MIR in 1991. The maximum flux of protons (E=0.24-0.5 MeV) is shown by arrows.
The main features of the near-equatorial formation, obtained in several experiments are listed below:
Fig. 2. The longitude occurrence patterns (separately for north and south hemisphere) of the proton events near equator as observed by SPRUT-V on MIR.
Fig. 3. The L occurrence pattern of proton events by SPRUT-V on MIR. +L and –L correspond to north and south hemispheres.
It can be suggested that the ring current (and/or radiation belt) protons are the sources of the observed near-equatorial proton features. These protons reach low altitudes after the double charge-exchange reaction with the neutral atoms (mainly hydrogen), namely in “geocorona” on the altitudes 1.5-10.0 Re (Re – Earth radius), and they interact with the oxygen atoms at low altitudes (Moritz, 1972).
The experimental data on near-equatorial proton observations are obtained mainly in the narrow altitude intervals: 400 and 1000 km by “Azur” (Hovestadt et al., 1972; Moritz, 1972), 400-470 km by “OVI-17” (Mizera, 1973), 200 km onboard the “Kosmos-24” satellite (Butenko et al., 1975), 170-290 km by S81-1 (Guzik et al., 1989), 320-850 km by OHZORA (Gusev et al., 1996), 520-670 km by SAMPEX (Greenspan et al., 1999). The orbit of “Intercosmos-24” satellite (Active, apogee 2500 km, perigee 500 km, inclination 82.6o) allowed to check the spatial and temporal distribution of near-equatorial protons in relatively large range of altitudes. We present some experimental results of the low-energy (55-550 keV) protons registered by the SPE-1 instrument (Kudela et al., 1991) on the Active satellite near the geomagnetic equator at the altitudes 500-1500 km, namely the differential spectra at various altitudes during the quiet and disturbed geomagnetic conditions separately, and the proton flux dependence on magnetic local time (MLT). Presented data analysis is based on ~5000 passes through the geomagnetic equatorial region during the year 1990 (almost 3000 orbits). The cycle of the apogee’s latitude was ~ 5.5 months and of the local time it was ~3 months. All local time sectors near equator were covered within 115 days. The variation of the attitude with respect to the magnetic field was smooth, and periodicities of the satellite axial variations (with respect to the nominal orientation) were 15-20 min. Since these periodicities are long, rapid particle flux variations, which were observed, were not caused by variations in the orientation. Measurements of proton fluxes were obtained with the use of single Si surface barrier detectors. Three pairs of detectors measured at different angles. The axes of the detectors were 99, 69 and 39 (detectors 1, 2 and 3) with respect to the zenith axis of the satellite. Thus proton flux is measured at 3 different pitch angles. Here we use detector 2 (energy channels 55.2-63.9 keV, 63.9-78.6 keV, 78.6-103.0 keV, 103.0-144.0 keV, 144.0-213.0 keV, 213.0-330.0 keV, and 330.0-564.0 keV respectively) for the analysis.
Fig 4 Differential energy spectra of protons (SPE-1 on Active) in the range 55-550 keV on L1.15 for different levels of geomagnetic activity, in different sectors of MLT and at selected altitudes (labeled on the top). The errors are marked by the vertical lines.
Figure 4 shows differential spectra of the near-equatorial protons (L1.15) in the energy range 55-550 keV. The proton flux is analyzed at different altitudes (h=500-1500 km) as a function of energy, MLT and the level of geomagnetic activity. The figure shows average proton flux observed within the narrow altitude range (selected altitude value h5%). The flux is shown for different geomagnetic activity levels on the nightside (N, 20-24-6h MLT) and on the dayside (D, 6-20h MLT). Quiet (Nq, Dq) and disturbed (Nd, Dd) conditions correspond to –30nT
The main observational results:
There are several conclusions after studying of data obtained by ACTIVE experiment. The proton fluxes distribution on near-equatorial latitudes can be characterized such:
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Greenspan,M.E., G.M.Mason, and J.E.Mazur, Low-altitude equatorial ions: A new look with SAMPEX,
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radiation belt, ^ , 101, 19,659 (1996).
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