On the drift of the South Atlantic Anomaly
O.R. Grigoryan, A.N. Petrov, V.V. Romashova
Scobeltsin Institute of Nuclear Physics, Moscow State University, Russia
Institute for Biomedical Problems, Moscow, Russia
Abstract. This work summarizes the previous study of South Atlantic Anomaly (SAA) drift. We used the large set of previously published results observed onboard SKYLAB, MIR, SALYUT-6, SHUTTLE spacecrafts and added our results observed onboard the MIR and ISS stations. The comparison of experimental results and modeling of magnetic field (IGRF/DGRF) and particle fluxes (NASA AP8/AE8) in SAA region is presented. The comparison of different approaches of center of SAA estimation and estimated drift speed are discussed.
The analysis of the experimental data obtained during charged particle fluxes investigation in the SAA region (inner Earth radiation belt) is presented. The long-term variations of charged particle fluxes are denoted to the SAA drift. One of the first reports devoted to the investigation of the Earth magnetic field variation is by Yukutake and Tachinaka . There was found westward latitudinal SAA drift during last 1400 years.
It has been known for more than 40 years that there is a weak geomagnetic field in the South Atlantic Ocean, known as the SAA. This coincides with a region of intense radiation in space near the Earth. This intense radiation causes damage to many spacecrafts in low Earth orbit and is a hazard to cosmonauts who are there.
Geomagnetic total field intensity in the SAA region has a minimum value of about 23,000 nT located about 700 km inland from the coast of Southern Brazil. The SAA is generally said to be due to the fact that the inclined dipole axis of the Earth is displaced in the direction of the northwest Pacific by 527 km.
The radiation environment causes several types of hazards, but here we will look at one of the most common. This is the sudden event upset (SEU), where, apparently, a single particle causes a piece of equipment to malfunction [see Heirtzler, 2002].
There is a geomagnetic field variation map shown in Figure 1. It was plotted by the World Data Center for Geomagnetism (Kyoto) according to the paleomagnetic data before the year 1900, according to the IGRF (International Geomagnetic Reference Field) model in the period from 1900 to 2000, and the extrapolation of the IGRF model up to the year 2100. Geomagnetic field intensity versus geographical coordinates plotted for different times shows clearly that Earth magnetic field drifts.
Several works [Badhwar G.D., 1997, Heynderickx, 1996, Sakaguchi et al., 1999] estimated the SAA drift (similar to magnetic field drift) to west and north. The westward drift rate ranges from 0.1 to 1.0 deg/year [Badhwar G.D., 1997].
The space satellites and stations are under impact of large fluxes of charged particles in the SAA region. The SAA existence is a consequence of shape of Earth’s magnetic field [see Heynderickx., 1996 and references therein]. The SAA region drifts due to long-term variations of geomagnetic field.
The NASA AP8/AE8 models of trapped radiation are often used to predict the particle fluxes in radiation belts. The main inconvenience of these models is the flux dependence on magnetic coordinates. If we use the magnetic maps generated for periods when model was made, we will make a mistake in particle flux distribution and in the SAA position estimation. This fact was proved during Shuttle space ships flight [Konradi et al., 1994].
The secular SAA drift can be estimated using three approaches [Heynderickx D., 1996]:
The position of maximum particle fluxes area depends on particle energy. While the particle energy increases, the maximum of particle flux moves closely to the minimum of geomagnetic field.
Position of the SAA before 1960 was estimated using the geomagnetic field models without the satellite data included. Therefore the Anomaly center could not be estimated precisely. If we use the SAA drift since 1960, we will found the motion to west mainly.
Westward drift of the minimum of geomagnetic field was estimated using the linear regression of Jensen and Cain  model of geomagnetic field and DGRF model for time period before 1960.
The results for time period after 1990 were extrapolated using the IGRF 90 model of geomagnetic field. Using the regression analysis the westward drift speed was estimated to be about 0.18 deg/year for the minimum of magnetic field. The same drift speed for maximum of particle flux was 0.29 deg/year. Using the dose registration during the Space Shuttle flights compared with AP8 MAX model calculation, the westward drift motion speed was estimated to be about 0.34 deg/year [Konradi A., 1994].
The dose registration results obtained onboard MIR and SKYLAB orbital stations were used by Badhwar . The estimated longitudinal drift was 6 0.5 deg per 21.2 years. This drift speed confirms the previous studies [Konradi A., 1994]. The results obtained at different altitudes (from 350 to 1400 km) shows that the drift speed is the same for all measured different altitudes.
In this work we used experimental results by Badhwar . A tissue equivalent ion chamber, called the Van Allen belt dosimeter (VABD), was onboard the SKYLAB in a 50° inclination orbit at altitudes varying from 420 to 450 km. The data from December 7, 1973, to January 8, 1974 were used, when the average altitude through the SAA was about 438 km. The ion chamber registered the dose rate. A tissue equivalent proportional counter (TEPC) was onboard the Base Block of the MIR orbital station (orbit inclination 51.6 degrees, altitude 400 km) since March 1995.
The data we analyzed were obtained by the RYABINA-2 device onboard the MIR Space Station (this device had measured neutron fluxes in the range from 0.25 eV to 1.9 MeV, using discharge detectors in 1991 and 1998); SPRUT-6 device onboard the same MIR SS (altitude 350 km, semiconductor detectors, 1999) measured radiation dose. The experiments had been also realized onboard the International Space Station (ISS, orbit inclination 51.6 degrees, altitude 350 km) by SCORPION-1 device (Geiger detector; neutron fluxes with energies from 0.1 eV to 1 MeV, 2003); System of Radiation Control (SRC) onboard ISS during 2005 included semiconductor detectors measured the radiation dose.
We used identical Geiger detectors in the RYABINA-2 and SСORPION-1 experiments for registration of proton fluxes with energy Ep>50 MeV. We used identical equipment (semiconductor detectors with the same characteristics) in the SRC and SPRUT-6 experiments.
Also we used experimental results obtained onboard SS “Salyut-6”. Techniques were used to detect neutrons with energies up to several MeV. Neutrons are moderated in polyethylene, and detected by a LiI(Eu) crystal. For charged particle rejection, the LiI(Eu) crystal is surrounded on all sides by a plastic scintillator. A LiI(Eu) crystal with dimensions 29.4×4 mm2, enriched by 6Li up to 89%, was used. An anticoincidence plastic scintillator had the dimentions of 60×34 mm2. The mean thickness of the polyethylene moderator was 7 cm [for more details see Shavrin, et al. 2002].
The study of the SAA drift was done using several experimental data sets obtained by different satellite experiments. We compare the original results with recent experimental results using observations of charged and neutral particles fluxes in the SAA region.
We analyzed the experimental results observed by SPRUT-6 experiment onboard the MIR Space Station 1999 and SRC device onboard ISS 2005. Figure 2c shows the lines of equal dose intensity. We used the results published by Badhwar  and added the SPRUT-6 results to this map.
a) Results of three approaches (points 1, 2, 3 as described in text) of SAA center estimation as function of time (increasing to the left) from 1945 to 2000 [Heynderickx, 1996]. Also the map of model IGRF 1945 magnetic field and the experimental magnetic field map obtained by SPRUT-6 in 1999 are shown.
b) Proton (Ep>50 MeV) flux map in SAA region obtained by RYABINA-2 experiment in 1991. The contour level values are 1-20, 2-100, 3-200 s-1.
c) Estimation of SAA center obtained in dose registration experiments, 1 – SKYLAB, 2 – experiment onboard MIR station [Badhwar, 1997], 3 – SPRUT-6 – the SINP MSU experiment onboard MIR.
Figure 2b shows that the location of the maximum of the proton fluxes (Ep>50 MeV) in the region of the Anomaly obtained in the RYABINA-2 experiment unmatches the location of the energetic proton fluxes shown in Figure 2a [Heynderickx D., 1996]. Comparison of these figures shows that the location of the maximum proton fluxes in the SAA region obtained by the RYABINA-2 (1991) experiment agrees with the location of the minimum geomagnetic field obtained by the SPRUT-6 (1991) experiment.
We can see that the location of maximum proton flux calculated using the models of ^ and Vette,  and Vette  are not coincide with the experimental (RYABINA-2, SPRUT-6), localization of the SAA region (experiment onboard MIR station, see Figure 2a and 2b).
It is known that the energetic proton fluxes make the major contribution to the radiation dose in the inner radiation belt. From Fig. 2a and 2c, it appears that the location of the maximum proton fluxes in 1991 disagrees with the location of the maximum of the radiation dose modeling in 1995.
Figure 3 Neutron flux in the SAA region observed by RYABINA-2 experiment in 1991, the SCORPION-1 data obtained in 2003 and “Salyut-6” station data [Shavrin, et al. 2002] obtained in 1979.
Figure 3 shows the maps of neutron flux in the SAA region obtained by RYABINA-2 experiment in 1991, the SCORPION-1 data obtained in 2003 and “Salyut-6” station data [Shavrin, et al. 2002] obtained in 1979. We can see that maximum of particle flux drifts to westwards.
Figure 3 shows that westward drift of the SAA region does exist. This fact was proved by several satellite experiments. We can assume that the SAA drift is connected with the drift of geomagnetic field.
Analysis of several satellite and space station experiments (40 years of observations) registering charged and neutral particles in the SAA zone we can formulate results:
The authors thank referees for their assistance in evaluating this paper.
Badhwar G.D., W. Atwell, B. Cash et al., Intercomparison of Radiation Measurement on STS-63, Rad. Meas., 26(6), 901-916, 1997.
Badhwar G.D, Drift rate of the South Atlantic Anomaly, J. Geophys. Res., 102, 2343-2350, 1997.
Jensen D.C. and J.C. Cain, An interim geomagnetic field, J. Geophys. Res., 67, 3568-3569, 1962.
Heirtzler J.R., The future of the South Atlantic Anomaly and implications for radiation damage in space, J. Atmosph. and Solar-Terrestrial Phys., 64, 1701-1708, 2002.
Heynderickx D., Comparison between methods to compensate for the secular motion of the South Atlantic Anomaly, Rad. Meas., 26, 369-373, 1996.
Konradi A., G.D. Badhwar, and L.A. Brady, Recent Space Shuttle observation of the South Atlantic Anomaly and the radiation belt models, Adv. Space Res., 14, 911-921, 1994.
Sakaguchi T., T. Doke, N. Hasebe, J. Kikuchi, S. Kono, T. Takagi, and K. Takahashi, S. Nagaoka, T. Nakano, and S. Takahashi, G. D. Badhwar, Measurement of the directional distribution of incident particles in the Shuttle-Mir mission orbit, J. Geophys. Res., 104, 22,793-22,799, 1999.
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