Low energy protons on L 15 in 500 – 1500 km range icon

Low energy protons on L 15 in 500 – 1500 km range

НазваниеLow energy protons on L 15 in 500 – 1500 km range
Дата конвертации03.09.2012
Размер30.49 Kb.


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 (L1.15) 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 (-301.04. The characters of proton distributions with energies Ep>100 keV and Ep<100 keV are strongly different. There are features of altitude distribution connected with geomagnetic activity level: during quiet time at night hours all energy protons registered at all altitudes, at day hours the most intensive proton fluxes of were registered at 700-900 km; during disturbed time at night hours they are absent at 900-1100 km, whereas at day hours the essential high-energy proton fluxes appears only at 700 km


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:

  • the “band” exists permanently on L1.15 at the altitudes 200-1000 km;

  • the near-equatorial protons have energy from 10 keV to several MeV;

  • the distribution of proton fluxes have obvious longitudinal dependence (Hovestadt et al., 1972). Proton flux data from the SPRUT-V experiment confirm this fact. Figure 2 shows the longitudinal distribution of near-equatorial protons (the number of registered increases of proton fluxes with energy Ep=0.24-0.5 MeV as a function of geographical longitude) for the north and south hemispheres. There is an obvious asymmetry in the proton flux around the South Atlantic Anomaly (SAA) region (Biryukov et al., 1996);

  • protons with energies of tens and of hundreds keV respond differently to geomagnetic activity (Mizera et al., 1973);

  • the flux begins to increase after the Dst decrease. However, geomagnetic storms do not produce observable increases of equatorial protons (Greenspan et al., 1999);

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.

  • a north-south asymmetry in the occurrence of the proton events, as indicated in Figures 2 and 3, is apparent. Outside the SAA a slot is observed. It is obvious that the proton flux is absent directly at the equator (Biryukov et al., 1996);

  • a dependence of the proton flux on local time during the geomagnetic disturbances is observed: the number of events is considerably higher during 0-12h LT than during 12-24h LT (Greenspan et al, 1999). There is a sharp day-night asymmetry at altitudes ~200 km: the proton flux is absent on the dayside and it does not depend on the geomagnetic activity (Butenko et al., 1975);

  • there is a tendency of the proton flux increase with the altitude up to 400 km (Guzik et al., 1989).

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 L1.15 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 (L1.15) 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 h5%). 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 –30nTst<0 and Dst <-30nT respectively.

The main observational results:

  1. There is a difference between the low-energy (Ep<100 keV) and the high-energy (Ep=100-550 keV) proton behavior. This difference occurs in altitude, in MLT as well as in the geomagnetic activity level distributions.

  1. The altitude distribution (see Figure 4)

  • During the disturbed periods the low-energy protons are observed at all altitudes (500-1500 km), while high-energy protons occur at 500-700 km and at high altitudes (1200-1400 km) only in the night local time sector.

  • During the quiet geomagnetic conditions protons of all energies are observed at all altitudes in the night sector. On the dayside the low-energy protons appear at all altitudes with the exception of 1000 and 1100 km, whereas the high-energy ones are observed at 700-900 km only.

  1. Magnetic local time distribution (see Figure 5):

  • Directly at the geomagnetic equator (L<1.04) the high-energy proton flux is absent in all MLT sectors. However, it appears at L>1.04. The low-energy proton occurs sporadically, and mainly in night hours.

  • During geomagnetic disturbances the low-energy protons are localized, mainly in the sectors 2-4h MLT and 18-23h MLT. Those of higher energies are absent in the night hours.

  1. Proton flux strongly depends on MLT (see Figure 5)

  • The maximal proton flux is observed in the night and evening local time independently on the geomagnetic activity level.

  • The proton flux is essentially bigger at night hours than at day hours (except of 700-900 km altitudes).

  1. Geomagnetic activity dependence (see Figure 4)

  • During the geomagnetic disturbances the proton flux is essentially bigger for altitudes > 1100 km whereas the situation inverted for altitudes < 1000 km. The spectrum of protons is commonly harder for disturbed periods.

  • Fig. 5. Appearance of proton fluxes with energies Ep<100keV and Ep>100 keV as function of geomagnetic activity level, L and MLT (SPE-1 on Active). N is the number of passes.
    During the geomagnetic disturbances the low energy protons are observed at the dayside at any altitude (except of 700 km). For the same conditions in the night sector the protons of all energies occur at 500-700 km and > 1000 km, but they are absent at altitudes 800-1000 km.


There are several conclusions after studying of data obtained by ACTIVE experiment. The proton fluxes distribution on near-equatorial latitudes can be characterized such:

  1. The proton fluxes at L1.15 with tens and hundreds keV energies registered at 500-1500 km altitudes and have strong local time dependence: the most intensive fluxes of all energy protons were registered mainly at evening and night hours.

  2. The protons appears mainly at L>1.04, this fact confirm the result obtained on MIR station which inform us about the fluxes “slot” existence directly on geomagnetic equator.

  3. The characters of proton distributions with energies Ep>100 keV and Ep<100 keV are strongly different, it can be additional argue to prove hypothesis of their different generation.

  4. There are features of altitude distribution connected with geomagnetic activity level:

  • During quiet time at night hours all energy protons registered, in fact, at all altitudes, whereas they absent at 900-1100 km during disturbances.

  • During quiet time at day hours the most intensive proton fluxes of all energies were registered at 700-900 km, whereas during disturbances essential proton fluxes appears only at 700 km.


Biryukov A.S., O.R.Grigoryan, S.N.Kuznetsov, A.V.Ryaboshapka, and S.B.Ryabukha. Low-energy

charged particles at near equatorial latitudes according to MIR orbital station data. Adv.Space Res.,

10, 10189 (1996).

Butenko,V.S., O.R.Grigoryan, S.N.Kuznetsov, G.S.Malkiel, and V.G.Stolpovsky, >70 keV proton fluxes

at nearequatorial 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).

Gusev, A.A., et al., Dynamics of the low-altitude energetic proton fluxes beneath the main terrestrial

radiation belt, ^ J. Geophys. Res., 101, 19,659 (1996).

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).

Hovestadt,D., B.Hausler, and M.Scholer, Observations of energetic particles at very low altitudes near

geomagnetic equator. Phys.Rev.Lett.,. 28, 1340 (1972).

Kudela K., et al., Inner zone electron peaks observed by the “Active” satellite, J.Geophys.Res., 97, 8681


Miah,M.A. Observation of low energy particle precipitation at low altitude in the equatorial zone.

J. Atmos. Terr. Phys., 51, 541 (1989).

Mizera,P.F., and J.B.Blake, Observation of ring current protons at low latitudes. J.Geophys.Res., 78, 1058


Moritz,J. Energetic protons at low equatorial latitudes: A newly discovered radiation belt phenomenon

and its explanation. Z.Geophys., 38, 701 (1972).

Orsini,S., I.A.Daglis, M.Candidi, K.C.Hsieh, S.Livi, and B.Wilken. Model calculation of energetic

neutral atoms precipitation at low altitudes. J.Geophys.Res., 99, 13489 (1994).


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