The high-energy proton fluxes in the SAA observed with REM aboard the MIR orbital station

Radiation Measurements 35 (2002) 489 – 497

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The high-energy proton %uxes in the SAA observed with REM aboard the MIR orbital station P. B.uhlera;∗ , A. Zehndera , M. Kruglanskib , E. Dalyc , L. Adamsc a Paul

Scherrer Institute, 5232 Villigen, PSI, Switzerland Institute for Space Aeronomy, 1180 Brussels, Belgium c ESA/ESTEC, 2200 AG Noordwijk, The Netherlands

b Belgian

Received 15 December 2001

Abstract During two years, from November 1994 to 1996, the particle detector REM measured the highly energetic electron and proton environment at the outside of the MIR orbital station. Using mission averaged data we investigate various aspects of the proton %uxes in the SAA. Comparison with the radiation belt model AP8 reveal important di;erences. c 2002 Elsevier Science Ltd. All rights reserved.  Keywords: Space radiation environment; MIR space station; Low Earth Orbit; South Atlantic Anomaly

1. Introduction For many years, the Russian orbital space station MIR has provided a platform in space for a diversity of scienti?c investigations among which were experiments to investigate the ionizing radiation environment the instruments and inhabitants of the station were exposed to. At the MIR orbit, at ≈ 400 km altitude and 52◦ inclination there are two main distinct regions of radiation contributing to the total radiation encountered—the South Atlantic Anomaly, SAA and the polar horns. Whereas the electrons of the polar horns are e;ectively absorbed by a few mm of aluminium (e.g. envelope of the station) the highly energetic protons in the SAA are much more penetrating(see e.g. Fig. 1). They can penetrate into the space station and form the main contribution of the radiation exposure to the crew members. For two years, from November 1994 to 1996 the particle detector REM (Radiation Environment Monitor) (B.uhler et al., 1996a) was mounted on the outside of MIR and measured the incident high energetic electrons and protons. ∗ Corresponding author. Westendstrasse 10 01187, Dresden, Germany. Tel.: +49-351-47-00-328. E-mail address: [email protected] (P. B.uhler).

In this paper, we focus on the SAA and summarize results obtained with data from REM.

2. Observations and data analysis REM consisted of two shielded solid state detectors, measuring the di;erential linear energy transfer of charged particles (B.uhler et al., 1996a). The apertures of the detectors had an opening angle of ±45◦ and were covered with spherical domes of aluminium and tantalum of 0.19 and 2:06 g=cm2 , respectively. REM was mounted on the outside of the MIR space station, ?xed on railings encircling the MIR core module, with the detector aperture directed perpendicular to the length axis of the station, toward open space. REM was shipped to the space station in September 1994 and was subsequently mounted outside the station by a cosmonaut. REM was programmed to accumulate data for a ?xed time of 32 s. The energy loss spectrum of each accumulation was binned into two 16-channel histograms, one for each detector, with channel energies ranging from 0:2 MeV cm2 =g (0:1 × minimum ionizing energy, MIP) to more than 2 GeV cm2 =g (1000 × MIP). The shielding de?ned the lower detection thresholds in the two detectors

c 2002 Elsevier Science Ltd. All rights reserved. 1350-4487/02/$ - see front matter
PII: S 1 3 5 0 - 4 4 8 7 ( 0 2 ) 0 0 0 7 9 - 3

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Fig. 1. Restricted dose rates averaged over the entire REM mission from November 1994 to 1996 in the heavy shielded (upper panel) and light shielded (lower panel) REM detectors aboard MIR.

for electrons of ≈ 0:7 and ≈ 2:6 MeV and for protons of ≈ 10 and ≈ 34 MeV, respectively. Extensive calibrations and numerical simulations including the approximate mass distribution of the space station have been carried out to determine the energy response of the instrument to electrons and protons (Ljungfelt, 1993; Hajdas et al., 1993). The response functions are used to compute the total energy deposited in the detectors (a quantity which in the following is referred to as ‘restricted dose’)

(B.uhler et al., 1996b), to compare measurements with predictions from radiation environment models (B.uhler et al., 2000), and also to extract approximate energy spectra of the incident particles (B.uhler et al., 1996a). In this paper, we mainly use mission averaged data which we bin into geographic bins and L–B bins. The good coverage obtained during the two years of operation guarantees good statistics in relatively small bins. During the operational time of the experiment no strong

P. B.uhler et al. / Radiation Measurements 35 (2002) 489 – 497

solar-terrestrial events occurred which could markedly change the inner belt proton population, however, the proton %uxes in the SAA have increased between November 1994 and November 1996 by roughly 25% (B.uhler et al., 1998a) according to the approach of solar minimum.

The restricted dose dREM is de?ned as the sum of deposited energy in a REM detector dREM =

4. Energy spectra The counting rate in channel i, ci (1 s−1 ) and the incident proton and electron %uxes, fp (E) and fe (E) (1 s−1 cm2 MeV) are related by the following equation:
 ∞ ci = fq (E)Gq (E; i) dE; (2) q=p;e

3. Dose rates

15 1:6 × 10−11
PEi ci (rad=s); V i=4

(1)

where the ?rst 3 channels (noise channels) and the last channel (cosmics, see e.g. (B.uhler et al., 1998a)) of each detector are excluded. In Fig. 1, dREM is plotted as function of geographic position. The map shows mission averaged data binned into longitude–latitude bins of 1◦ × 1◦ . The upper panel shows the heavy shielded detector and the lower panel, the light shielded. Note that the gray colored region is not covered by the MIR orbit. In both detectors the radiation in the SAA forms the major contribution to the absorbed dose. In the light shielded detector there is also a contribution from the electrons in the polar horns, which is e;ectively absorbed (factor ¿ 100) by the extra material covering the heavy shielded detector. Comparisons with predictions using the AE8/AP8 models (B.uhler et al., 2000) show that the model values in the SAA are by up to a factor ≈ 2 higher than the measured values.

491

0

where Gq (E; i) (cm2 ) is the geometric factor and represents the probability that a particle of species q with energy E is counted into detector channel i. In order to convert Eq. (2) and compute particle %uxes from the measured counting rates, the proton spectra are parametrized by a power law, fp (E) = fp (Ep; 0 ) × (E=E0; p )−p (E0; p = 30 MeV) and the electron spectra by an exponential function, fe (E)=fe (Ee; 0 )e−e (E−E0; e ) (E0; e = 1 MeV). The four free parameters fp (E0; p ), p , fe (E0; e ), and e are then determined by means of a 2 -minimization algorithm (B.uhler et al., 1996a). Since we only consider protons and electrons this method works where either particle species dominates the measurements. In Fig. 2, the computed proton spectral power-law index is shown as function of L and B. The color map shows the mission averaged data binned into L–B bins of 0:05RE and 0:005G. The spectral hardness is a function of B and L. The spectra are hardest at low L and large B and soften with increasing L and decreasing B. The same trend is also found for the AP8 model. The agreement between REM measurements and AP8 is good below L ≈ 1:6RE . At larger L-values however, the model spectra are signi?cantly softer than

Fig. 2. Spectral power-law index of the proton spectra above 10 MeV as function of L and B.

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the REM spectra (B.uhler et al., 2000). This is presumably the e;ect of strong solar–terrestrial events which occurred since 1970, like the one of March 1991 (Blake et al., 1992), which can change the inner radiation belt proton population signi?cantly for many years (Dyer and Rodgers, 1999). 5. Spatial distribution Normally the proton %uxes in the inner radiation belt are rather stable. Except for rare, very e;ective solar–terrestrial events which can cause signi?cant changes in the trapped particle population even in the inner radiation belt, the proton %uxes are subjected to slow variations linked with the solar cycle and the secular changes in the Earth’s magnetic ?eld. Whereas the solar cycle e;ect leads to a periodic variation of the trapped proton %ux intensities in the inner belt, with maxima around solar minimum and minima around solar maximum (Huston and P?tzer, 1998), the slow decrease of the low-order terms of the magnetic ?eld lead to a west-drift of the location of the magnetic ?eld minimum at a given altitude and with it, of the spatial distribution of the proton %uxes in the SAA. The spatial distribution is not only a function of epoch but also of energy. Since the proton spectra harden with

decreasing L and increasing B (Fig. 2) the spatial distribution of the SAA is shifted north–west with higher particle energies, according to the distribution of L and B at the MIR altitude. In order to determine ‘the location’ of the SAA, we binned the mission averaged proton %uxes at di;erent energies into longitude–latitude bins of 1◦ × 1◦ and ?tted the %uxes at ?xed longitudes and latitudes with a polynomial function of 5th order. The ?ts were then used to determine the locations of the maxima at given longitudes and latitudes and the ‘central location’ of the SAA. The results are shown in Fig. 3. The solid line show the maxima at 50 MeV and the dotted line, the maxima at 200 MeV, respectively. The diamonds indicate the location of the 50 and 200 MeV-proton %ux maxima as given by the NASA AP8 radiation belt model (Sawyer and Vette, 1976) for the year 1970, the epoch of the magnetic ?eld model originally used to construct AP8. For AP8 we used the same method as described above (?t with polynomial) although in many cases the AP8-curves at ?xed longitudes and latitudes have more than one local maximum, which is not observed with REM and seems to be an artifact of the model (Daly and Evans, 1993). A west drift of the ‘central location’ of the SAA since 1970 with an average drift rate of 0:32 ± 0:05◦ =y at 50 MeV and 0:31 ± 0:05◦ =y at 200 MeV, is obvious. Also a slight

Fig. 3. Locations of the maxima at ?xed longitudes and latitudes of the measured proton %uxes at 50 MeV (solid line) and 200 MeV (dotted line). The diamonds indicate the location of the proton %ux maximum as given by the NASA AP8 radiation belt model (Sawyer and Vette, 1976) for the year 1970.

P. B.uhler et al. / Radiation Measurements 35 (2002) 489 – 497 Table 1 Location of proton %ux maximum in SAA Source REM, 50 MeV REM, 200 MeV AP8, 1970, 50 MeV AP8, 1970, 200 MeV

Longitude 0:5◦

−42:1 ± −44:3 ± 0:5◦ −34:2 ± 1:0◦ −36:6 ± 1:0◦

Latitude −33:0 ± 0:5◦ −32:0 ± 0:5◦ −34:0 ± 1:0◦ −34:0 ± 1:0◦

north-drift by 0:06 ± 0:05◦ =y can be noted. The results are summarized in Table 1. The drift rates correspond well with the value obtained by Badhwar (1997) using data from several Space Shuttle missions.

6. Loss cone At the MIR altitude the proton %uxes are a function of altitude. The deeper a particle dips into the atmosphere the higher the probability is that it interacts with an ambient particle and is lost. Since particles with small pitch angles dip deepest into the atmosphere there exists a limiting pitch angle LC , the loss cone angle, below which stable trapping is virtually impossible. This is clearly seen in Fig. 4, where the 50 MeV proton %uxes measured with REM are shown as functions of L and B. The dark lines indicate the location of the mirror points of particles with the labeled equatorial pitch angle. In the region covered by MIR the proton %uxes

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at a given L are very steep functions of the pitch angle. Over a few degrees, especially at low L-values, the %uxes decrease by several orders of magnitude. In order to determine the loss cone angle as function of L, the pitch angle dependence in each L-bin is ?tted with the integrated pitch angle distribution function proposed by Badhwar and Konradi (1990)  jp; 0  exp(−b);  ¡ 2 − LC ; jp () = (3) 0;  ¿ 2 − LC with  = (cos() − sin(LC ))=



(Beq )

  = − : 2 Since the REM measurements cover only a small fraction of the full pitch angle range, the normalization jp; 0 is only poorly determined by this method. However, the loss cone angle is well determined because the measurement cover the relevant range. The REM values of LC are listed in Table 2. In Fig. 5, 1=sin2 (LC ) is plotted versus L as a solid line with error bars. The dotted line is the function used by Daly and Evans (1993) to ?t the loss cone of the AP8 model. The loss cone angle can be translated to a limiting altitude hlim , below which particles are totally absorbed. The triangles and diamonds in Fig. 5 show the resulting loss cone angle if hlim was 120 km and 300 km, respectively. Although the di;erences in the resulting LC are rather small, the values for hlim = 120 km obviously ?t better—nearly

Fig. 4. 50 MeV proton %uxes in the SAA as function of L and B. The dark lines indicate the location of the mirror points of particles having an equatorial pitch angle of the labeled value.

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Table 2 Equatorial values of the loss cone angle LC

7. West–east asymmetry

L

LC (◦ )

1.125 1.175 1.225 1.275 1.325 1.375 1.425 1.475 1.525 1.575 1.625 1.675 1.725 1.775 1.825 1.875 1.925 1.975 2.025

79:34 ± 7:92 72:54 ± 5:50 67:63 ± 4:29 63:47 ± 3:17 60:27 ± 3:04 57:20 ± 3:19 54:57 ± 2:86 52:44 ± 2:47 50:40 ± 2:67 48:61 ± 2:60 46:97 ± 2:70 45:44 ± 2:71 44:13 ± 3:08 42:95 ± 3:03 41:79 ± 3:28 40:75 ± 5:18 39:74 ± 4:59 38:90 ± 5:37 37:95 ± 7:93

perfectly—the REM values. A value of hlim ≈ 300 km was found to best ?t the values found by Fischer et al. (1977), however using, data at lower magnetic latitudes at an earlier epoch.

Until now, we treated the data as omnidirectional which was justi?ed by the fact that mission averaged data were used and it can be assumed that throughout the two years of operation, all directions have been equally sampled. However, the detector is most sensitive to particles coming through the aperture. By sorting the data according to the orientation of the detector it is possible to get a measure of the %ux anisotropy (B.uhler et al., 1998b). In Fig. 6, REM counting rates at L ≈ 1:325 are plotted as function of B for the detector looking west (asterisks) and east (diamond) in the local ˜ =˜r × ˜B, with ˜r the vector connecting the mirror plane, (west centre of the Earth with the position of the space station). The dotted lines show background corrected rates, where the background was deduced at large B-values and where the measurements are dominated by cosmics. The plotted detector channel is sensitive to ¿ 200 MeV-protons. Over a large B-range, covering roughly three decades of counting rates, the west–east ratio is fairly constant. The west–east anisotropy is a consequence of the ?nite gyro radius of the trapped particles. At a given point in space, the guiding centres of particles arriving from di;erent directions have di;erent locations. If the di;erence in guiding centre position is large compared with the scale length of the spatial %ux gradient, the observed %ux distribution can be expected to be anisotropic (Lencheck and Singer, 1962).

Fig. 5. Loss cone angle LC as function of L. The solid line with error bars are the REM values, the dotted line is the (Daly and Evans, 1993) function, and the triangles and diamonds are the values computed for minimum mirror-point altitudes of 120 and 300 km, respectively.

P. B.uhler et al. / Radiation Measurements 35 (2002) 489 – 497 1.30 < L [RE] < 1.35

1000.00

West East

count rate [1/sec]

100.00

10.00

1.00

0.10

0.01 0.190

0.200

0.210

0.220 B [Gauss]

0.230

0.240

0.250

Fig. 6. REM counting rates for east- (diamonds) west- (asterisks) looking detector at 1:3 6 L ¡ 1:35 as function of B. The values at high magnetic latitude, where the detections are dominated by cosmics, is used to determine the background counts. The dotted lines show the background corrected counting rates. The detector channel plotted here is sensitive to protons with energies above 200 MeV.

Since the e;ect scales with the gyro radius, the anisotropy is best observed with high momentum particles at low magnetic ?elds, thus aboard MIR with protons in the SAA. The gyro radius of 100 MeV protons at B = 0:2 G is of the order of 100 km. A simple relation was found by Lencheck and Singer (1962) to express the west–east %ux ratio as function of the altitude di;erence dh of the guiding centres of particles arriving from opposite directions, caused by the ?nite gyro radius. Under the assumption that the %ux is approximately inversely proportional to the local atmospheric density and the atmospheric density varies as a power law function of altitude (atm ˙ h−n ), the west–east %ux ratio can be expressed as  n h0 + dh=2 jw =je = (4) h0 − dh=2 with h0 the altitude of the point of detection. dh is given by dh = rgyro (E) cos(I ) sin(&);

(5)

where I is the dip angle of the local magnetic ?eld line, and & is the azimuthal angle, where & = −90◦ is east and & = +90◦ is west. In order to verify relation (4) we computed the relevant quantities for the B–L bins with the lowest B-value at a given L covered by MIR. The results are shown in Fig. 7. The two uppermost panels show the di;erential proton %ux at 30 MeV and the spectral power law index as function of L for east (diamonds) and west (asterisks) looking detector. The following three panels show (t.t.b.) the %ux ratio at 50 and 200 MeV, the cosine of the dip angle of the magnetic ?eld line, cos(I ), and the altitude power law index n,

495

computed with Eq. (4) using the observed west–east %ux ratio. The lines without symbols are values computed with the smoothed curves shown in the two uppermost panels as dashed lines. Note, that the spectra of the protons coming from east (low %uxes) are softer than the spectra of the particles coming from west (high %uxes). This is expected since the highly energetic particles coming from east dip to the lowest altitudes and are therefore most a;ected by the interaction with atmospheric particles. The west–east ratio, obviously a function of L, is largest at low L-values, decreases with increasing L, and becomes 1 above L ≈ 1:7 RE . The only quantity in Eq. (4) which should vary with L is the dip angle I . However, its L-dependence cannot fully account for the observed L-dependence of the %ux-ratio. In particular, it cannot account for the fact that the %ux ratio becomes 1 at larger L-values. This de?ciency must be compensated by an L-dependence of n, which decreases with L and becomes 0 above L ≈ 1:7 RE . In the simplest form of the Lencheck and Singer theory (Lencheck and Singer, 1962), the power law index n is assumed to be a characteristic quantity of the altitude dependence of the atmospheric density and as such would not depend on geomagnetic parameters such as B and L. However, since the interaction of the protons with the atmospheric particles takes place everywhere along the particles trajectory, the ‘e;ective’ parameter could be expected to be a function of the drift shell (Heckman and Brady, 1966). The simple model gives satisfactory results at speci?c locations (Heckman and Nakano, 1969; B.uhler et al., 1998b) or for mission averaged measurements (Sakaguchi et al., 1999). However, to our knowledge these are the ?rst measurements of the spatial distribution of the west–east proton %ux ratio, indicating that a re?ned model is needed to account for the observed spatial dependence. 8. Conclusions Using mission averaged data from REM aboard MIR (November 1994 to 1996) we gained a picture of many aspects of the highly energetic proton %uxes in the SAA at close to solar minimum conditions. We determined the geographic location of the SAA—which is a function of particle energy, its westward drift rate, the spectral hardness as function of L and B, the loss cone angle as function of L, and also investigated the west–east ratio of the proton %uxes as function of L and B. Comparison with the AP8 model reveal that several adjustments of the model could be useful and also sets some requirements for new, to-be-developed models. In this particular case (close to solar minimum, E ¿ 10 MeV) the solar-minimum models overestimate the proton %uxes in the SAA by up to a factor 2. Since the proton %uxes in the SAA vary with the solar cycle, models adjustable to the solar-cycle phase are to be striven for (Huston and P?tzer,

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Fig. 7. Parametrization of the west–east e;ect as function of L in the B–L bins with the lowest B-value encountered by MIR at a given L. The two uppermost panels show the proton spectral normalization at 30 MeV and the power law spectral indices for west (asterisks) and east (diamonds) looking detector. The following panels show the west–east ratio, cosine of local magnetic ?eld line dip angle, and altitude power law index n at di;erent energies. The lines without symbols are values computed with the smoothed curves shown in the two uppermost panels as dashed lines.

1998). The westward drift of the SAA is a consequence of the secular variations of the Earth’s magnetic ?eld. In order to correct the existing models for this e;ect, the model distribution can be shifted according to the required epoch (Heynderickx, 1996). However, this only compensates for the shift, but does not correct for the changed ?eld con-

?guration which e.g. a;ects the loss cone angle. The fact that strong solar–terrestrial events can cause long lasting changes in the inner belt, as notable e.g. in the hard REM proton spectra at the outer %ank of the inner radiation belt, calls for a regular update of the models. Finally, the proton %uxes in the SAA are anisotropic. Models to compute

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directional %uxes from the omnidirectional model %uxes are available (e.g. Kruglanski and Heynderickx, 1999), but cannot account for the observed spatial distribution (B.uhler et al., 1999) in a consistent way. Acknowledgements We thank L. Maslennikov, N. Shvez, and M. Beliaev from S.P. Korolev Rocket and Space Corporation Energia for their assistance in obtaining and preparing the data. This study was supported by ESA/ESTEC/WMA Technology Research Contract 11108/94/NL/JG(SC). References Badhwar, G.D., 1997. Drift rate of the South Atlantic Anomaly. J. Geophys. Res. 102, 2343. Badhwar, G.D., Konradi, A., 1990. Conversion of omnidirectional proton %uxes into a pitch angle distribution. J. Spacecraft Rockets 27 (3), 350. Blake, J.B., Kolasinski, W.A., Filius, R.W., Mullen, E.G., 1992. Injection of electrons and protons with energies of tens of MeV into L ¡ 3 on March 24, 1991. Geophys. Res. Lett. 19, 821. B.uhler, P., Ljungfelt, S., Mchedlishvili, A., Schlumpf, N., Zehnder, A., Adams, L., Daly, E., Nickson, R., 1996a. Radiation Environment Monitor. Nucl. Instr. and Meth. Phys. Res. A 386, 825. B.uhler, P., Desorgher, L., Zehnder, A., Daly, E., Adams, L., 1996b. Observations of the low Earth orbit radiation environment from MIR. Rad. Meas. 6, 917. B.uhler, P., Daly, E., Desorgher, L., Hajdas, W., Zehnder, A., Adams, L., 1998a. Measurements of the radiation belts from MIR and Strv 1994 –1997. IEEE Trans. Nucl. Sci. 137, 108. B.uhler, P., Desorgher, L., Zehnder, A., Daly, E., Adams, L., 1998b. REM measurements aboard MIR during 1995. Adv. Space Res. 21, 1645. B.uhler, P., Zehnder, A., Kruglanski, M., Daly, E., 1999. Energy and spatial dependence of the east–west e;ect observed by MIR-REM. Proceedings of the Space Radiation Environment Workshop, November, DERA, UK. B.uhler, P., Desorgher, L., Zehnder, A., Glover, A., Daly, E., 2000. Measurements of the space radiation environment with REM.

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