NASA/TM–2015-217547

December 2015

E.G. Stassinopoulos, M.A. Xapsos, C.A. Stauffer

National Aeronautics and 

Space Administration 

Goddard Space Flight Center

Greenbelt, Maryland 20771

Forty-Year “Drift” and Change of the SAA
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NASA/TM–2015-217547

Decemember 2015

Epaminondas G. Stassinopoulos, Emeritus

NASA’s Goddard Space Flight Center, Greenbelt, MD

Michael A. Xapsos 

NASA’s Goddard Space Flight Center, Greenbelt, MD

Craig A. Stauffer

AS and D, Llc., AS and D- Inc Beltsville, MD

Forty-Year “Drift” and Change of the SAA
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1

Forty-Year “Drift” and Change of the SAA

E.G. Stassinopoulos

M.A. Xapsos

C.A. Stauffer

Abstract

1. A comprehensive study was conducted to evaluate the change in the location and size of the 

South Atlantic Anomaly (SAA) as a function of time, primarily in relation to trapped Van Allen Belt

proton populations. The study was limited to one altitude only (800 km), which is still within the

Earth's atmosphere but is removed from the atmospheric cut-off level located at about 100-200 km.

Introduction

2. Since its recognition and acknowledgement as an area of great importance to the space program,

particularly for Low Earth Orbits (LEO), the SAA has been the subject of much published research. 

Studies in the literature have dealt with the SAA’s drift, size, area, strength, and change. However,

most of these publications have been based on data generated by diverse instruments, designed for

research projects unrelated to the SAA and demonstrating varying degrees of sensitivity, accuracy,

reliability, and efficiency. Most of the observations have therefore been peripheral and incidental and

could not be expected to correctly and precisely define the size, form, area, strength, motion, or drift of

the SAA, or to have solved the problem of its existence, evolution, variability, and future.

3. Incidental information about the anomaly that can be gleaned from these sources, should be

taken into account, but cannot be considered a full or valid definition, description, or model of the

SAA, as is aptly demonstrated by the significant variations and discrepancies in the reported results and

conclusions. Additionally, many SAA related publications have shown inconsistencies in reference to

the geomagnetic field, the trapped particle models, and the relationship between them.

4. The SAA is really not an “anomaly” at all, but an apparent local depression of the Earth's

magnetic field. When first observed, the SAA was considered an “anomaly” of the field, an aberration

[1]. Later, the SAA was defined as stemming from the tilt, the eccentricity, and the displacement of the

dipole axis from the center of the Earth i.e. the SAA is determined by (a) the tilt of the magnetic dipole

axis to the axis of rotation (approximately ~11 degrees), (b) and is considered eccentric because it does

not pass through the center of the planet, and (c) is displayed (by about 500 km) away from the center

towards the North Pacific, thus producing a weaker magnetic field over Brazil and a stronger field over

the North Pacific. As a result of these conditions, the Earth-size magnetic unit-sphere lies outside the

northern physical Earth over the Pacific, which is closer to the magnetic center, resulting in a higher

local field strength at the surface of the globe, and inside the physical Earth over the south Atlantic,

which is farther away from the magnetic center, with a lower field strength at the local surface of the

globe. A uniquely different explanation for the existence of the SAA was given in one of the reviewed

papers.1

 1 Quote from APPENDIX A [4b]: “The anomaly arises from the eccentric displacement between the magnetic and

geographic poles of the Earth ....”
2

5. The Earth’s magnetic field is a combination of (a) a major dipole term, and (b) of minor

quadrupole and higher order components. The major dipole term of the magnetic field has a strong

radial dependence, dropping off inversely with the cube of the distance, as Br = B/r3, whereas the nondipole terms drop off inversely with the fourth power of the distance, and the dipole potential drops off

as 1/r2. The currently most widely used standard model for the magnetic field, globally, is the officially

established “International Geophysical Reference Field” (IGRF), that is issued every five years by the

“International Association of Geomagnetism and Aeronomy” (IAGA) [2]. The IGRF models contain 

first and second order time derivatives for temporal extrapolations, and are limited to a maximum of

5 years, because longer extrapolations introduce unacceptable errors and uncertainties in the results.

Each new 5-year model is based on current global measurements, correctly accounting for the changes

in the magnetic field for the 5-year period. A striking example of calculations, which vastly exceed the

limits of the field model projection-validity by several decades, is given in the frequently- cited

publications in the Footnote.2,3,4

6. The Earth’s magnetic field experiences two slow but important variations with time: (1) the drift

of the magnetic poles and (2) the secular variation of the field, a gradual, continuous, long range trend

of decreasing strength. The rate of change, of both variations, is not constant. Thus, the drift of the

north Magnetic Pole has increased over the last century and may change again in the near future. It is 

currently moving at about 40 km per year towards Siberia.

7. As mentioned in paragraph #6 above, the secular variation, expressed by the first and second

derivatives of the standard IGRF models, has been on a decreasing trend in terms of field strength for

several centuries [3] (Figures 1-9). This decreasing trend, with a varying rate, expressed as the

magnetic moment of the field, is plotted as a function of time in Figure 10 for the period of interest of

this study, i.e. 1970 to 2010.

 2 Quote from APPENDIX A [12a]: “The calculations presented here will be based on the IGRF for 1975 and fluxes of protons with

energies above 30 MeV as represented by AP8MIN.------- Figure 1 shows B and L contours in the SAA at 500 km altitude for epochs

1965(a), 1995(b), and 2025(c).” 3 Quote from APPENDIX A [14b]: “The most likely secular variation of the geomagnetic field is estimated and used to extrapolate

the geomagnetic field to the year 2100.” 4 Quote from APPENDIX A [14e]: “If the map of secular change from the IGRF coefficients (Figure 6) is used to extrapolate from

1995 to 1922 the charts compare favorably in the South Atlantic.”
3

Figure 1: Year 1600.

Figure 2: Year 1650.
4

Figure 3: Year 1700.

Figure 4: Year 1750.
5

Figure 5: Year 1800.

Figure 6: Year 1850.
6

Figure 7: Year 1900.

Figure 8: Year 1950.
7

Figure 9: Year 2000.

Figure 10: The change/year of the Earth’s dipole moment.
8

8. Based on calculations performed with the IGRF models, the SAA has “drifted” primarily westnorthwest and to a lesser extent north-northeast, over the four decades considered in this analysis. From

these calculations, though, the SAA appears to have only “expanded” in area into those directions, at

all altitude levels and for all proton energies, and not drifted in totality. The SAA has also extended a

narrow pointed protrusion toward the east, centered at about 20 degrees south geographic latitude, i.e. 

in the opposite direction from the “drift”. What has actually “drifted” is the peak proton flux position

within the frame of the SAA, which, of course, is of major importance to Low-Earth Orbiting missions 

(LEO).

9. Before addressing in more detail the issue of the “drift” of the SAA, some helpful background 

information and definitions are in order, which may provide explanations for some of the varied

questions, assumptions, and conclusions encountered frequently in the referenced literature.

10. Several papers have been published, dealing with the “Drift of the SAA”. Most of these reports

were based on measurements obtained from instruments not specifically designed or dedicated to: (a)

the study of trapped radiation levels (electrons and/or protons) within the SAA, (b) systematically

evaluate the effects of these particles on electronic systems, instruments, or components, or (c)

focusing on identifying the causes of the changes in the size, area, and drift of the South Atlantic

Anomaly. Nevertheless, single event effects and total doses experienced in the SAA have been

frequently mentioned in many of these papers

11. Such usually unrelated measurements of opportunity, however, may not accurately reflect the

actual and correct shape and location of the SAA. The most important parameter that defines the

domain of the SAA and the changes in its location with the passing of time, is the main dipole field,

which has frequently been neglected, overlooked, or ignored, while this role has been assigned, in some

publications, to the minor non-dipole field. In addition, the existence, shape, size, and drift of the SAA

are not controlled or influenced by the presence, absence, distribution, or intensity of the particle

radiation within its domain. Particles don't define the SAA, which as a local magnetic depression,

would still exist and change with time, even if there were no particles present at all.

12. Another comment frequently encountered in the reviewed literature is the reference to the effect

of the non-dipole field on the SAA, which by some is considered to be a major force driving the

changes. But in regards to the existence and evolution of the SAA and the corresponding trapped

radiation levels, the non-dipole field is, at best, of small significance, in comparison to the main dipole

field. All of the SAA’s trapped particles are part of the inner zone of the Van Allen belts, and are

hundreds to thousands of kilometers removed from the Earth’s surface, whereas most non-dipole field

measurements are made on or near the surface. Non-dipole field values decline rapidly as altitude

increases, dropping off with a rate inversely related to the fourth power of the distance, whereas the

dipole field changes as 1/r3. Hence the effects of the non-dipole field are of minimum importance to the

intensity level and location of the trapped, as well as the transiting, particle radiation, within the

magnetosphere and the inner zone of the Van Allen belts. 

13. Other causes, such as, Space Weather events, solar flares, coronal mass ejections, magnetic

storms, atmospheric conditions, and geomagnetic jerks, could briefly affect the SAA's particle

population and the magnetic field, and could produce small, short, and mostly localized variations in

the field strength. But such brief, minor, and locally-restricted excursions, although scientifically

valuable and important, do not significantly affect the prevailing basic shape and position of the SAA,

or its long-range evolution, and therefore have been ignored for the purpose of this study.
9

14. A review of several dozen published papers dealing with the anomaly, indicated that the

measurements (i.e. the data) which were used in the attempts to describe the SAA, were obtained from

a variety of instruments, detectors, or parts, each of which had a different sensitivity, resolution, and

accuracy in performance and calibration, These data were also obtained over different epochs and time

intervals (duration), for missions with varying trajectories (inclination, altitude, eccentricity, etc.). It is

to be expected that this variety of conditions, produces numerous results and conclusions that may not

reflect accurately the true shape and position of the SAA, nor to effectively predict and describe the

dynamics of its existence, which remains mainly a function of the dipole field. In addition, many

instances have been noticed, in which problems have been identified relating to (a) instruments and

their calibration, (b) detectors and their sensitivity or performance, (c) orbital trajectory generation, 

(d) data contamination, (e) data analysis and processing issues, (f) field model selection and coordinate

conversion, (g) flux calculations from models, (h) drift of the SAA, (i) origin of the SAA, etc. Some

examples of these problems are provided in Appendix A.

The Magnetic Field and its Effects

15. The solar wind encounters the Earth's magnetic field and creates the geomagnetic cavity that

defines the terrestrial magnetosphere (Figure 11). The interaction of the solar wind with the

magnetosphere sets up a multitude of currents. The sum of those currents represents the external

magnetic field of the Earth. The external field is vectorially superimposed onto the internal field of the

Earth. The combination of the two then becomes the total field which experiences:

• slow changes in the internal field (long-term effects), and

• fast changes in the external field (short-term effects). 

Figure 11: The Earth’s magnetosphere as sculpted by the solar wind.
10

Figure 12: The long-term change of the Earth’s dipole moment

16. The fact that the internal field is slowly changing, can be observed in two important ways:

• the drift of the magnetic poles and

• the shrinking of the field strength.

The rate of change of both of these variation is not constant in time,

The rate at which the internal field changes is called the “Secular Variation”. If the shrinking were to

continue at the present rate, the Earth could experience another magnetic reversal in about two

thousand years (see Appendix D for calculations). Short and rare disturbing events, such as

geomagnetic jerks, can alter prevailing normal conditions, and cause minor magnetic reversals, which

cannot be predicted in terms of frequency or intensity, and which, like CME’s, solar flares, and

earthquakes, cannot be modeled easily or reliably, so as to be of practical use to scientists, engineers,

and managers of the space program.

17. The trend of the secular variation of the internal field, mentioned earlier in paragraph #7, is

indicated in Figure 10 by the magnetic moment, plotted as a continuous function of time for the period

of interest to this study. The corresponding rate-change per year is shown in Figure 12, where the

magnetic moment obtained from the IGRF models, is plotted versus time.

18. Several external field models also exist. Two of the earliest and most popular models were the

Mead-Fairfield [4] and the Olson-Pfitzer [5] versions. These were later superseded by the more

sophisticated and improved Tsyganenko models [6, 7] that are now being widely used by the

international scientific community.5

 5 External field model effects were not considered in this review as they have no impact on the field strength in the domain of the SAA.

Their impact is only significant at distances greater than 4 Earth radii (at L-shells >4).
11

19. Fast changes of the total field are caused by strong and rapid modulations of the external field,

resulting from three dynamic and complex processes that cause rapid short-term changes in the

orientation and magnitude of the external field. 

a. The first process pertains to the interplanetary medium in which convection transfers solar

wind mass, energy, and electric field into the magnetosphere,

b. The second process deals with the magnitude and orientation of the interplanetary field which 

induces dynamic changes in the magnetosphere, resulting in magnetic storms, and

c. The third process involves diurnal rotation and yearly orbital motion (dipole wobbling).

20. The gradual and slow secular-variation changes of the internal field are accompanied by a long

term movement (drift) of the magnetic poles. This is shown for the “North Magnetic Pole” (NMP) in

Figure 13, in which the path of the NMP, since its discovery in 1831, has been plotted. During the last

century, the pole has moved a remarkable 1000 km. Since 1970, the NMP movement has accelerated

and is now drifting at more than 40 km per year. Furthermore, superimposed on the polar drift and on

the secular variation of the field, is the diurnal motion in the position of the magnetic pole. Figure 14

shows the 24-hour path of such a motion.

Figure 13: Term movement of the north magnetic pole.
12

Figure 14: Diurnal variation of the north magnetic pole.

21. A further concern of high altitude balloon projects, which have to deal with atmospheric or

space radiation, is the effect of a diurnal variation in the value of the magnetic shell parameter L,

particularly at high latitudes and/or high altitudes. An example of the range of this variation, at a

sample position, is shown in Figure 15 for a quiet environment (Kp = 0.0), at a high latitude and an

altitude of 36 km, for epoch 2001. The variation in L is significant, ranging from 7.8 to 10.5 re.

Figure 15: Diurnal variation of magnetic shell parameter L.
13

Dose Calculation Methods

22. In dealing with radiation measurements, another area of concern and source of possible errors

and discrepancies may be the method by which dose levels on sensitive targets within near-Earth

orbiting satellites, frequently mentioned in the referenced literature regarding the SAA, are calculated

from the spacecraft surface-incident space radiation. A quick and simple method, widely used in the

past, is the popular SHIELDOSE code [8], which does not involve elaborate solid angle sectoring or 3-

D ray tracing techniques, and considers only “slab” and “spherical” geometries (Figure 16). More

elaborate codes such as NOVICE [9] or GEANT-4 [10], have also been widely used to obtain results

with greater accuracy and detail.

Figure 16: Geometries used for dose calculations.

23. It should be noted that dose calculations for both slab and spherical shields (for a 4-pi exposure)

performed with the same given set of radiation data, will typically yield different results. The spherical

shield dose will be larger than the slab dose. For example, at a shield thickness of 200 mils, the

difference for electrons will be a factor of approximately 6, as shown in Figure 17 [8]. Since spacecraft

geometries are neither slabs nor spheres, the actual dose on a target inside the vehicle will be greater

than slab but less than sphere predictions. In any given case, the reality will usually lie between these

two extremes, as shown in Figure 18 for the polar orbit of the NPOESS (NPP) mission through the

SAA, at approximately 800 km altitude
14

Figure 17: Ratio of electron dose at center of aluminum sphere to twice the dose in a semi-infinite 

aluminum medium.

Figure 18: Total dose vs Al shield thickness.
15

Quality of Dose Data

24. With the great variety of instruments involved in measurements some data from the SAA show

noticeable variations for equivalent positions and times. This can happen for many reasons, such as: (a)

varying sensitivity and response of detectors, (b) their calibration, (c) their geometric factor, (d) their

threshold energy, (e) interference or contamination, (f) data analysis and processing method, etc. 

However, when evaluating sets of data for the same target and for the same conditions, particularly

when comparing dose data with similar results obtained from other sources, small differences of up to

50%, or even factors of two or more, are quite common, because they reflect mostly minor variations in

initial conditions, parameter definitions, accuracy of computations, and a multitude of other causes.

Examples include:

1. The quality of the orbit generator that was used to obtain the trajectory of the spacecraft [11].

6

2. The applied orbit integration step-size; that is, the density of trajectory positions used to

evaluate the satellite’s radiation exposure.

3. The magnetic field model used to convert positional geographic to magnetic coordinates.

4. The reference epoch of the field model (year) and the updating to the mission date (with the

secular variation terms of the model).

5. The influence of solar cycle activity.

6. The efficiency, sophistication, and complexity of the radiation transport code that propagates

the energetic particles through the shield(s) to the target.

7. The shield geometry, for example: spherical vs. slab (see the comments in paragraph #17)

8. Minor variations in shield composition, e.g. pure aluminum vs. aluminum alloy.

9. Minor variations in target composition, e.g. pure Si vs. SiO2.

10. A small difference in the slab shielding results if the target is assumed to be embedded within

the slab (e.g.: Case 1, in Figure 16, [8] vs. behind the slab (e.g.: Case 2, in Figure 16, [8].

11. The inclusion (or omission) of the smaller contribution to the total doses from the penetrating

Bremsstrahlung (braking radiation) that is generated by the slowing of the electrons in the 

material surrounding the target.

12. Solar proton contributions that have been included in the calculations, by the confidence level

considered: the probability that the predictions will not be exceeded by actual events.

13. The uncertainty factor of the standard radiation models which may or may not have been

applied to the results.

14. The fact that margins are occasionally applied to the results of calculations for projects or

missions with sensitive components.

However, in some cases, data concerning the SAA, collected from a variety of different instruments,

can be significantly different, even for similar basic conditions. This may be due to detector: (a)

sensitivity and response, (b) calibration, (c) geometry factor, and (d) contamination [noise,

interference], as well as the data processing and analysis efforts.

 6 Quote from APPENDIX A [1a]: “The position of RHESSI in latitude, longitude, and altitude is calculated via two different

algorithms. The first algorithm has three faults: (a) sometimes no numerical value is given at all, (b) on other occasions, the derived

altitude is suggested to be constant, equal to 6378.13 km for a while, being obviously wrong, and (c) the third defect is more delicate: 

the satellite seems to jump occasionally from its orbit at 540 km to 600 km to less than 500 km and back again within short time,

which is rather unrealistic behavior for a spacecraft”.
16

Space Radiation Models

25. In all the SAA studies reviewed, for comparison with on-orbit measurements, frequent

references are made to the standard NASA AP8 (proton) and/or AE8 (electron) Van Allen Belt

radiation models [11], that have been used for many years to calculate satellite exposure predictions.

Both types are published in two versions: for solar minimum and solar maximum conditions. These

series have been the best available models since the mid-1960s. However, the modeling of space

radiation data is not an exact science. The static models are commonly constructed by combining a

number of measurements obtained from different spacecraft, and acquired with different instruments, at

different times, and at different locations. These data are then combined to produce an approximation

of the environment at a specific period of time, as determined by the range of the measurements. It is

not uncommon to have data sets of electrons or protons that fortuitously share similar basic times and

locations of acquisition, but diverge significantly in the magnitude of their measurements (by factors of

2 to 10), due to instrument sensitivity, calibration, or efficiency factors.

26. These differences may also be due to many other factors, including variations in instrument

design, positional inaccuracy, and data processing conditions. Some of the challenges include:

1. correctly defining the location of each measurement in magnetic or geographic coordinates

2. the selection of the magnetic field model used in the specific process

3. the instrument calibration and efficiency factors, and

4. the data analysis efforts.

27. Comparing calculated space radiation data from several sources, whether in the form of fluxes, 

of accumulated doses, or effects on electronic systems, may often show large differences that cannot

easily be reconciled. These differences may sometime be due to the omission or the application of the

intrinsic “Uncertainty Factors”, attached to the radiation models (x2 for protons and x2-5 for electrons).

28. A new set of trapped radiation models, AP9 and AE9, is available for use while being further 

developed (Ginet [23]). The fundamental approach of these models is different in that it is Monte Carlo 

based and represents the radiation belt dynamics in a statistical way, thus allowing results to be 

calculated for different confidence levels. Another improvement is the quantification of uncertainties 

due to both space weather and instrument uncertainties. A feature not yet included that would have 

significant impact for LEO calculations is the solar cycle dependence of trapped protons (Houston 

[24]). Comparisons of AP8 to the mean environment of AP9 at 800 km altitude show the new model 

generally predicts higher fluxes, particularly at low proton energies and low orbital angles of 

inclination.

Ground Irradiation Tests and Calibration Shortcomings

29. The comments and conclusions expressed in the reviewed publications about the “drift” of the

SAA, are exclusively based on the unrelated mission-specific data collected by the large variety of

special instruments used, which were not designed or intended to measure the magnetic field or the

trapped/transient radiation in the SAA. To what degree these data are correct, accurate, and reliable is

another question. It was appropriately noted in some papers, that the instruments of old are not as

sophisticated as modern instruments. A constant area of concern for instruments is the process of test

and calibration, which may be negatively affected by a number of shortcomings. For example, particle

radiation facilities are restricted to providing only (a) a single source at a time, (b) a single energy at a
17

time, (c) a single (standard) beam direction, (d) a single species at a time, and (e) very high rates of

accelerated exposures. In contrast to these artificial conditions, the natural radiation in space consists of

a simultaneous, continuous, mostly isotropic incidence of all particle species populating a given region,

i.e. a breath of energies (complete spectral distributions), and of low rates arriving instantaneously

from all directions.

30. Consequently, because of these limitations, ground test simulations may not be equivalent to

space effects, and may lead to potentially inaccurate results and conclusions. Protons, electrons, cosmic

rays, and solar flare particles (intermittent only, if applicable), are simultaneously incident on a 

satellite, and continuous over the duration of an entire mission, but usually at a much lower rate than

the separate high-rate partial exposures at the irradiation test facilities. In an attempt to mimic space

conditions, the facilities may adjust particle species, energy, intensity, and micro-macro dose rates, at

any time, but can do so only for a single discreet value in each case.

The B and L Parameters and their Application

31. In order to obtain spacecraft encountered radiation levels for trapped Van Allen Belt protons or

electrons, orbital flux calculations were, until recently, being performed with the standard NASA AP8

and AE8, solar-min and solar-max models [11], which were constructed from space measurements on

the basis of the two magnetic parameters: B (the field strength in gauss or nano-tesla) and L, the

McIlwain drift shell parameter in Earth radii [12]. L is defined as the distance to the equatorial crossing

of the field line (line of force), passing through the position where the B measurement is made. The

concept of the B-L coordinate system is illustrated in Figure 19.

Figure 19: B-L Magnetic coordinate system. B=Magnetic field strength; L=Distance at equatorial 

crossing of field line.
18

32. However, in many applications, when using the AP8 to predict proton satellite radiation

exposures, calculations were mostly performed with a 1970’s magnetic field model, even for a

much later mission epoch (e.g. 2010). This was being done because the AP8 was constructed in

the 1970 epoch on the basis of the two magnetic parameters, B and L, that were valid for that

year, as well as because the orbital flux integrations, when performed for later missions with

more recent field models, would yield increasingly higher fluences, especially at lower orbit

altitudes (200-1000 km). These results were considered incorrect by some workers.

Note: Low altitude near-Earth space missions (below an altitude of about 1000 km) experience

only intermittent exposures to Van Allen belt radiations. That is, out of about 15-16 orbits per

day, 5-6 do not intersect the SAA, but bypass it completely, whereas 7-9 revolutions cross

through it, encountering the corresponding radiation. In addition, orbits with inclinations

smaller than 450 will not usually experience solar flare proton encounters, as these protons

generally do not penetrate to the low altitude orbits.

33. In an effort to prevent or minimize these occurrences, modifications were implemented in 1986

by Vette [13], which recommended that the radiation models not be accessed with the 1970 B and L

values, but with a new parameter B/B0 (obtained for the actual date of the mission) replacing B. B0 is

the field strength of the L-line at the magnetic equator. The use of B/B0 has been widely preferred since

the 1980’s, by domestic and international research communities. 7

34. Developing satellite exposure predictions based on the assumption that no changes in the

radiation levels at a specific position in space occurred between 1970 and 2010, and that no changes in

their magnetic field value should therefore be considered, did not solve the problem. 

Special Calculations

35. In order to evaluate the trend and the magnitude of the time-dependent variation of the B-L and

B/B0-L parameters within the SAA at the 800 km altitude level of this study, three pivotal positions

were selected along the 23 degree south geographic latitude line: one in the approximate center of the 

SAA and one each at the west and east periphery. Table 1 lists the geographic coordinates of these

points.

Table 1: Geographic coordinates of selected test positions.

Point Longitude Latitude

West - 90° - 23°

Center - 30° - 23°

East + 30° - 23°

36. In Figures 20 and 21, the changing magnetic coordinate values of these three points are plotted

for the five epochs specifically selected: 1970, 1980, 1990, 2000, and 2010, in terms of the B-L and

B/B0-L parameters, respectively. All calculations were performed with the IGRF models relating to the

above dates. It appears, that with both sets of parameters, the positions of the test points have moved

magnetically, during the 40 years, mostly towards higher proton intensity regimes.

 7 It should be noted that on a given field line (of any L value), B0 may not be the minimum field value, which may be located off the 

magnetic equator.
19

Figure 20: Changes in positional field values of B and L parameters.

Figure 21: Changes in positional field values of B/B0 and L parameters.
20

37. In Figures 22 (west), 23 (central), and 24 (east), the corresponding instantaneous AP8-Max

integral omnidirectional proton fluxes, are plotted for the three test position, for

energies > 30, >50, >100, and >200 MeV. They were obtained with the recommended B/B0 field

parameter. These figures show significant increases-with-time in the positional proton fluxes,

particularly at the lower energies (<200 MeV). Energies >200 MeV experience the smallest increases.

It is interesting to note that at the central position, at the epoch 2000, there occurs a reversal in the

rising trend, which results in lower flux predictions towards the epoch 2010. Figure 25 is a map, in B-L

space, of constant intensity contours for E>50 MeV protons from the AP8 model, in which the 40-year

change in the magnetic location of the three test points has been indicated. The composition and

distribution of the inner-zone Van Allen belt proton population, is likely not the same today, as it was in

1970, when the AP8 was constructed; the present calculations may not represent correctly the real

changes that have occurred, but serve only to indicate and approximate the possible trend in the

temporal evolution of the belt.

Figure 22: Variations of integral flux with time at indicated position.
21

Figure 23: Variations of integral flux with time at indicated position.

Figure 24: Variations of integral flux with time at indicated position.
22

Figure 25: B-L space map of AP8 proton model indicating the 40-year change in the magnetic location 

of the test positions.

38. Therefore, the B/B0 approach, devised to avoid obtaining larger fluxes for epochs later than

1970, did not solve the problem. For that reason, in popular and widely used computer codes (e.g.

SPENVIS), which calculate the orbit integrated vehicle encountered radiation belt fluxes, a simpler

and more practical, but limited method, was adopted to achieve that goal: assigning the original 1970

fluxes to all later epochs, regardless of the changes in the B or B/B0 values, and ignoring the fact that

the increase in the radiation levels is real, and is the result of the shrinking magnetic field, which brings 

down the larger fluxes from the higher altitudes. A comparison of sample calculations with the

SPENVIS method for a position near the center of the SAA (230 south, 2700 east) over 5 decades from

1960 to 2010, is given in Table 2. Several interim measurements, reported increased flux levels at

formerly tested positions, indicating better agreement with model predictions that use the B/B0

approach.

8,9,10,11

 8 Quote from APPENDIX A [5a]: “Heinderickx’s (1996) calculations of high-energy proton flux in the SAA region [with SPENVIS]

do not coincide with experimental results obtained on the MIR station.” 9 Quote from APPENDIX A [7c]: “Trapped proton results: It was noted that certain SAA passes (e.g. orbit 23 of STS-48 and orbit 40

of STS-44) were not predicted when using the recommended technique of employing the field model pertaining to the data from when

the models were created (i.e. 1970). However use of the 1991 geomagnetic field does predict peaks for these orbits as it accounts for

the steady drift of the SAA contours to the west due to evolution of the geomagnetic field. 10 Quote from APPENDIX A [7d]: “Trapped proton results: encouraging agreement is obtained between the three experiments

dispite the different design. For the region of the SAA, the LET spectra observed from KITSATA and POSAT have been compared

with each other and with predictions based on the AP8 model for solar minimum, allowing for proton stopping and slowing in the

detector. Figure 4 shows that agreement is good considering the uncertainties of a factor of two are inherent in allowing for spacecraft

shielding and particle atmospheres.” 11 Quote from APPENDIX A [8a]: “The AP8 model predictions were found to be in good agreement with the flight data”. [computed

with B/B0]
23

Table 2: Comparison of proton calculations for the indicated test position by two methods: 

SPENVIS (mission time independent) and time-dependent on Vette’s B/B0 approach.

Input SPENVIS AP8 Model Calculations SPENVIS AP8-MAX

B1 L1 B/B0 B2 L2 >30 >50 >100 >200 >30 >50 >100 >200

1960 0.23370 1.5743 2.9260 0.22939 1.5784 1.6562 1.4342 8.4231 1.5051 3.26832 2.78042 1.5882 2.90331

1970 0.22918 1.5833 2.9185 “ “ 2.9222 2.5312 1.3782 2.5151 “ “ “ “

1980 0.22412 1.5907 2.8944 “ “ 5.0152 4.0292 2.1822 4.0221 “ “ “ “

1990 0.21984 1.5904 2.8376 “ “ 6.5102 5.2242 2.6912 5.2951 “ “ “ “

2000 0.21683 1.5871 2.7816 “ “ 7.9882 6.1332 3.1612 6.3771 “ “ “ “

2010 0.21398 1.5817 2.7166 “ “ 9.0482 6.9342 3.5942 7.2421 “ “ “ “

Calculations for position: Latitude = -23°, Longitude = -90°, Altitude = 800 km

39. The consequence of miscalculating the radiation levels on spacecraft traveling through the inner

radiation belt, may not only jeopardize the success of a mission, but may also be hazardous to

biological systems on board (including humans). The most common effects of underestimating the

radiation exposure of a satellite, which may subsequently experience higher radiation levels than

expected, include: (a) damage to electronic components, circuits and systems, (b) parts failure, (c.) data

corruption, and (d) equipment malfunction.

40. What appears to occur in the lower magnetosphere, is that the shrinking field draws the larger

areas-of-occupation and the greater intensities from the higher altitudes to the lower altitude levels. 

This poses the question whether these greater trapped particle fluxes are sustained at the lower altitudes

or, if not, by what mechanism they are depleted, particularly in the 300-1000 km altitude domain. 

41. An example of that process is shown in Figure 26, where (a) the 900 km fluxes of the year 1970

[IGRF-1970] and (b) the 800 km fluxes of the year 2010 [IGRF-2010], are superimposed for the

1000-particle intensity contours of E>100 MeV protons, as obtained from the AP8-MAX model with

the B/B0 variable. These contours are almost identical in shape and size but the 2010 contour is shifted

towards the west-northwest direction, probably due to the drift of the magnetic poles.

Figure 26: AP8-MAX flux contours (E>100 MeV) in SAA for epoch 1970 (at 900 km altitude) and for 
24

epoch 2010 (at 800 km altitude).

42. It is likely, that the shrinking magnetic field lowers the mirror-points of the trapped particles

and that some mirror-points may drop into the atmospheric cut-off region, in which the particles will be

lost due to coulomb scattering by the atmospheric constituents, thus depleting the larger intensities

coming down from above. However, a quantitative evaluation of such a process, to determine the

magnitude and extent of the depletion, is not possible without detailed pitch-angle information, which

is not available for the AP8 data that have been used in the construction of the model. An answer to

this problem may be provided, after a new trapped particle model is constructed with current

measurements and field models.

43. An additional area of confusion and misunderstanding has been noted in some papers, regarding

the trapped particle mirror points, particularly in reference to the SAA. This issue applies to all trapped

particles, including those in the outer zone of the Van Allen belts12. Mirror point positions are

determined by equations that predict their location as a function of their pitch angle, which can have

any value <900. The predicted positions could be at any altitude, including on the surface of the Earth

or even at subterranean locations. The atmospheric cutoff would prevent particles from reaching their

mirror point below approximately 100 km in altitude. A relative display of the dip of the associated

magnetic field lines, guiding the particles to the lower altitudes, is presented in Figure 27 by McIlwain

[12], showing the drop observed below the Earth’s surface.

 12 Quote from APPENDIX A [12c]: “Since the gradient within the geomagnetic cutoff is very steep, within a short time period the

order of magnitude flux increases and even subterranean fluxes are predicted.” ---- “Not only does the flux increase with time, but

also increases disproportionally at low altitudes and even below the Earth’s surface. If real, such an increase would have implications

far beyond any one’s imagination.”
25

Figure 27: Intersections of shells of constant L with the surface of the Earth.

44. It is difficult to evaluate correctly, for protons and electrons, the conditions of the atmospheric

cutoff prevailing at any given time, in the inner zone of the Van Allen belt, without considering the

significant changes in the atmospheric density that occur with time and that are not fully identified.

These temporal variations are believed to reach about a factor of 200 between solar-max and solar-min

periods. In addition, diurnal variations are estimated to reach a factor of 10. These effects, however,

have no impact on the size, drift, and area of the SAA, and vice-versa, although in some papers the

opposite has been suggested.

44. Misconceptions regarding the “atmospheric cutoff” were noted in a few of the referenced

publications, which claimed that a dependent relationship exists between the cutoff and the magnetic

field, that is, that the cutoff and the field mutually affected each other. The atmospheric cutoff is in

essence a function of density, which in turn is a function of altitude and of solar activity conditions, and

not of the magnetic field. Therefore, a corresponding attempt to indirectly evaluate the “drift” of the

SAA based on these assumptions would be inaccurate.
26

45. Considering the dynamic nature of the Earth’s main dipole field, concerns about the validity of 

the three adiabatic invariants, the foundation of the B-L system being used in ordering the particle data, 

are justified. However, in the case of the IGRF, each new updated model issued, includes and accounts 

for all changes in the field, so that within its 5-year period, the expressed field is considered static, and 

hence, all three adiabatic invariants are valid, and the use of the IGRF is safe, correct, and appropriate. 

This observation may not be well understood in the community.13

46. Independent of any “drift” of the SAA, the SAA exhibits a very real and unavoidable increase

in size with time that implies longer transit times for spacecraft, and hence longer radiation

exposures. The accumulation of larger fluences, which is actually the major contribution to the higher

radiation doses predicted for orbits in the SAA, is in addition to the substantial increase of the flux in

the areas of highest energy particles. This is well illustrated in Figures 28, 29, 30, in which >10 MeV

proton intensities of 1000, 3000, and 5000 particles per centimeter square per second, are compared for

the years 1960 to 2010.

Figure 28: High intensity area of 1000 particle E>10 MeV proton flux for epochs 1960 and 2010.

 13 Quote from APPENDIX A [12b]: “The above described use of the trapped radiation environment models is also based on the

assumption that B and L, being derived from the first and second adiabatic invariants, are themselves invariant. In a static magnetic

field, this is certainly true and this coordinate system is very successful in ordering particle data and is, therefore, incorporated into

the models of the Earth’s trapped radiation environment. However, since the Earth’s dipole is decreasing with a characteristic time of

about 1000 years, when calculating particle flux transformations, all three adiabatic invariants have to be taken into account properly.
27

Figure 29: High intensity area of 3000 particle E>10 MeV proton flux for epochs 1960 and 2010.

Figure 30: High intensity area of 5000 particle E>10 MeV proton flux for epochs 1960 and 2010.

47. A more detailed comparison of the SAA's location and area at 800 km altitude, as outlined by

the proton population between the years 1970 and 2010, is shown in Figures 31-38 for energies E>50

MeV, 39-45 for E>100 MeV, 46-50 for E>200 MeV, and 51-53 for E>300 MeV, for integral proton flux

contours of 10, 100, 300, 1000, 3000, and 5000 particles per centimeter square per second. This

arrangement is illustrated in Table 3.
28

Figure 31: Location of E>50 MeV flux of 10 protons at 800 km altitude for epochs 1960 and 2010.

Figure 32: Location of E>50 MeV flux of 100 protons at 800 km altitude for epochs 1960 and 2010.
29

Figure 33: Location of E>50 MeV flux of 200 protons at 800 km altitude for epochs 1960 and 2010.

Figure 34: Location of E>50 MeV flux of 500 protons at 800 km altitude for epochs 1960 and 2010.
30

Figure 35: Location of E>50 MeV flux of 1000 protons at 800 km altitude for epochs 1960 and 2010.

Figure 36: Location of E>50 MeV flux of 2000 protons at 800 km altitude for epochs 1960 and 2010.
31

Figure 37: Location of E>50 MeV flux of 3000 protons at 800 km altitude for epochs 1960 and 2010.

Figure 38: Location of E>50 MeV flux of 5000 protons at 800 km altitude for epochs 1960 and 2010.
32

Figure 39: Location of E>100 MeV flux of 10 protons at 800 km altitude for epochs 1960 and 2010.

Figure 40: Location of E>100 MeV flux of 100 protons at 800 km altitude for epochs 1960 and 2010.
33

Figure 41: Location of E>100 MeV flux of 300 protons at 800 km altitude for epochs 1960 and 2010.

Figure 42: Location of E>100 MeV flux of 500 protons at 800 km altitude for epochs 1960 and 2010.
34

Figure 43: Location of E>100 MeV flux of 1000 protons at 800 km altitude for epochs 1960 and 2010.

Figure 44: Location of E>100 MeV flux of 2000 protons at 800 km altitude for epochs 1960 and 2010.
35

Figure 45: Location of E>100 MeV flux of 3000 protons at 800 km altitude for epochs 1960 and 2010.

Figure 46: Location of E>200 MeV flux of 10 protons at 800 km altitude for epochs 1960 and 2010.
36

Figure 47: Location of E>200 MeV flux of 100 protons at 800 km altitude for epochs 1960 and 2010.

Figure 48: Location of E>200 MeV flux of 300 protons at 800 km altitude for epochs 1960 and 2010.
37

Figure 49: Location of E>200 MeV flux of 500 protons at 800 km altitude for epochs 1960 and 2010.

Figure 50: Location of E>200 MeV flux of 1000 protons at 800 km altitude for epochs 1960 and 2010.
38

Figure 51: Location of E>300 MeV flux of 10 protons at 800 km altitude for epochs 1960 and 2010.

Figure 52: Location of E>300 MeV flux of 100 protons at 800 km altitude for epochs 1960 and 2010.
39

Figure 53: Location of E>300 MeV flux of 500 protons at 800 km altitude for epochs 1960 and 2010.

Table 3: Arrangement of Figures 31-53 by intensity and energy at 800 km altitude for epochs 

1960 and 2010.

E(MeV): > 50 MeV > 100 MeV > 200 MeV > 300 MeV

Intensity

(p/cm2-sec) Figure Number

10 1a 2a 3a 4a

100 1b 2b 3b 4b

300 1c 2c 3c 4c

500 1d 2d 3d

1000 1e 2e 3e

2000 1f 2f

3000 1g 2g

5000 1h

48. All plots indicate only minimal changes in the location of the southern boundary of the SAA,

but display a substantial displacement in the west, north, and east directions. At all energies selected for

plotting (>50 MeV, >100 MeV, >200 MeV, >300 MeV), significant expansion for all intensity levels

occurs to the “north-northwest” and the “west-southwest” side of the SAA. Particularly noteworthy is

the sharp protrusion mentioned before, that is most pronounced at the intensities displayed in

Figures 32-35, 40-41, 47, and 51. As is to be expected, the areas occupied by the higher energy protons,

are smaller in size than those of the lower energies. For increasing energies, the maximum intensity

areas become progressively smaller, reflecting the drop in the proton spectrum.
40

49. An important feature of the maps for epoch 2010 is the appearance of higher intensity contours

in the center of the SAA, contours that either are not present in the maps for epoch 1970, or appear only

as small “islands”. In this case, the 2010 maps indicate a significant increase in the area occupied by

these high intensities, as compared to the 1970 maps (see Figures 37-38, 42-43, 50, 53), correctly

contributing to the increased flux and dose predictions, questioned by many of the models users. A

more direct display of that observation is provided by a set of plots for particle energies of 10, 30, 50,

100, 200, and 300 MeV, respectively, in Figures 54 to 59, for the two epochs being compared (i.e. 1970

to 2010).

Figure 54: Comparison of proton fluxes of E>10 MeV for epochs 1960 and 2010 at 800 km altitude.
41

Figure 55: Comparison of proton fluxes of E>30 MeV for epochs 1960 and 2010 at 800 km altitude.

Figure 56: Comparison of proton fluxes of E>50 MeV for epochs 1960 and 2010 at 800 km altitude.
42

Figure 57: Comparison of proton fluxes of E>100 MeV for epochs 1960 and 2010 at 800 km altitude.

Figure 58: Comparison of proton fluxes of E>200 MeV for epochs 1960 and 2010 at 800 km altitude.
43

Figure 59: Comparison of proton fluxes of E>300 MeV for epochs 1960 and 2010 at 800 km altitude.

50. Another way to show the effects of the field changes on the radiation evaluations for spacecraft

is via the relative distribution of B-L points visited by the orbit. These results were obtained from the

identical polar trajectory at 800 km altitude. Figures 60 to 63 list the B-L combinations experienced by

the orbit during the years 1960 and 2010, for particles with energy of 10, 50, 100, and 300 MeV,

respectively. The blue numbers reflect 1960, while the red numbers reflect 2010. The numbers indicate

how many times each combination was encountered. As expected, it is apparent, that the same 

trajectory, for the same geographical positions, experienced, different combinations of B-L values at all

orbit points in the 2010 calculations, but also encountered new combinations and dropped others. The

quantity of points listed correlate with the size of the SAA area occupied by the particles of the

indicated energy. Many of the dropped positions lie in the lower intensity regions of the model and

many of the gained positions lie in the higher intensity regions.
44

Figure 60: Comparison of orbital SAA B-L-bins for epochs 1960 and 2010 of E>10 MeV protons.
45

Figure 61: Comparison of orbital SAA B-L-bins for epochs 1960 and 2010 of E>50 MeV protons.
46

Figure 62: Comparison of orbital SAA B-L-bins for epochs 1960 and 2010 of E>100 MeV protons.
47

Figure 63: Comparison of orbital SAA B-L-bins for epochs 1960 and 2010 of E>300 MeV protons.

Solar Cycle and Events

51. Frequently, the minimum and maximum activity phase of the solar cycle, and of the

corresponding versions of the AP8 and AE8 models, are mentioned and invoked in the SAA literature

to explain the obtained data or justify the author’s conclusions, particularly in regards to

environmental conditions and their effects on scientific instruments or measurements.

52. Several topics are considered in this section: (a) solar cycles, (b) solar activity, and (c) effects

on the SAA (radiation, size, location, drift). In modern times, when referring to solar cycles, the

prevailing concept is to think of them in terms of a fixed 11-year duration, which is the typical average.

The actual length of a solar cycle is unpredictable, however, and may vary from a short <9-year to a

long >13-year period. There does not yet exist a sure and reliable way to accurately predict the duration

of a future cycle.
48

53. The active phase of a solar cycle is defined as a seven-year period, starting 2.5 years before the

absolute maximum sunspot peak and ending 4.5 years after. The remaining time is classified as “solar

minimum”. Consequently, since the active phase is constant, and solar cycles vary in length, the solar

minimum period is (by necessity) a cycle variable. A presentation of that solar cycle reality is

provided in Figure 64 for the 5 most recent cycles, in which the cycles are numbered and the periods

are indicated.

Figure 64: Activity and duration of solar cycles 19-23.

54. Another aspect of solar cycles is their activity level. It is obvious from the recorded sunspot

totals shown in Figure 64, that the activity level in each cycle is quite different. Historical records show

a time at which no sun-spots appeared on the sun: the Maunder Minimum. This indicator of activity,

however, did not exactly correspond to the occurrence of solar flare proton events (SPE) or coronal

mass ejections (CME), which may have extended from small to extremely large in duration and

intensity (CME’s happen predominantly during the active phase, although some SPEs do occur also in

the quiet [“inactive”] phase). As mentioned before in paragraph 16, these events cannot be predicted

with accuracy and confidence. A probabilistic statistical prediction approach was developed by King

[14] as a function of (a) mission duration during the active period and (b) the confidence level that

assumes a predicted number of extremely large events will not be exceeded.

55. Subsequently, a new approach was developed based on the maximum entropy principle that

accurately described the complete distribution of solar proton event magnitudes as a truncated power

law (Xapsos [19]). This led, in conjunction with other statistical methods, to models of worst case

events (Xapsos [20]), and cumulative fluences during missions for both protons (Xapsos [21]) and

heavy ions (Xapsos [22]), In each case model inputs are the time period of the space mission and

desired confidence level, as was the case for the King Model.

19 20 21 22 23

YY/MM

10/03

YY/MM

11/07

YY/MM

09/07

Sunspot Number
49

56. A final concern regarding solar particle events, and their impact on the Earth’s radiation

environment, pertains to the penetration of solar protons into the SAA, especially from the occurrence

of major flares or CMEs. Typical spectra of extremely large proton events are shown in Figure 65,

which indicates their steep decline: the spectra do not exceed energies greater than a few hundreds of

MeVs protons, because of their charge, are being deflected by the magnetic field in a process that

involves the ratio of their momentum-over-charge, i.e. the “magnetic rigidity”. In order to overcome

the magnetic rigidity and penetrate into the low altitude regime of the SAA, protons need to have at

least an energy of about E = 2900 MeV (i.e. 2.9 GeV). This rigidity relationship is illustrated in

Figure 66.

14,15 It appears these GeV protons cannot be trapped in the Van Allen Belt.

Figure 65: Typical spectra of extremely large solar proton events.

 14 Quote from APPENDIX A [4a]: “In this region [SAA] the shielding effects of the geomagnetic field have been severely compromised, 

thus allowing high energy particles trapped in the Van Allen radiation belts to penetrate deep into the upper atmosphere to altitudes 

below 100 km.” 15 Quote from APPENDIX A [13a]: “However, other satellite missions, although designed for Earth observation, have also provided 

valuable information on solar flux. In fact, energetic particles (such as protons) reaching altitudes between 500 and 800 km produced 

signals by causing spikes (or outliers) in the measured radiances, as the case of NASA MOPITT ....”.
50

Figure 66: L-shell contours with rigidity imposed energy penetration limits.

57. The question of energetic proton penetration into the inner zone of the Van Allen belts, and

hence the SAA, is of particular importance because it is frequently mentioned in several publications,

which claim that these particles affected their measurements in the SAA and had an impact on the

SAA’s evolution. However, the “rigidity principle” applies to the inner zone of the radiation belts, and

consequently the SAA; protons must have energies greater than 2.9 GeV (as mentioned above) at least,

to be able to reach the low altitude range of the anomaly.

Cosmic Rays

58. A few papers mentioned concern about the penetration of cosmic rays into the SAA region.

Solar and galactic cosmic rays are high-energy heavy ions: 90% hydrogen, 9% alpha particles, and 1%

nuclei of heavier elements. In order for these particles to reach the inner zone of the Van Allen belts,

and hence the SAA, they must have energies in excess of 1.15 GeV per nucleon. All cosmic rays with

lower energies are deflected by the Earth’s magnetic field (rigidity cutoff). Most workers consider the

galactic cosmic rays reaching the vicinity of the Earth (about 1 AU) as fully ionized, which means

maximum deflection. As a consequence, very few of these particles reach the SAA. A solar cycle

variation has been observed in the cosmic ray flux levels between solar minimum and solar maximum.

During the active phase of the solar cycle, the cosmic ray intensities are about a factor of two or so 

lower than during solar minimum. It is obvious, considering Figure 66, that more cosmic rays have

access to the higher latitude regions than near the equator, with a free unimpeded penetration over the

poles, where open field lines connect directly to the interplanetary medium.

16

 16 As a consequence of this shielding effect, neutrons, the cosmic ray daughter products in the atmosphere from collisions with the

atmospheric constituents nitrogen and oxygen, are more numerous at high latitudes than near the equator [15].
51

Figure 67: Schematic of the distribution of fission electrons from the STARFISH and Soviet tests in 

magnetic coordinates.

Artificial Radiation Belt

59. A rare but serious threat to space missions travelling through the inner zone of the radiation

belt, and especially the SAA, can be another exo-atmospheric explosion of a nuclear device, as for

example, the famous US “STARFISH PRIME High Altitude Weapons Test” on July 9, 1962, over

Johnston Island in the Pacific (at ~400 km altitude, 1.4 megaton yield) [16]. A similar Soviet highaltitude nuclear test was conducted over Semipalatinsk in Kazakstan on October 28, 1962. A schematic 

of the distribution of the fission electrons from those two tests is shown in Figure 67, in terms of the L

parameter and the magnetic latitude. The STARFISH event injected about 1029 fission electrons into

the magnetosphere, raising the intensity of the inner zone electron population by several orders of

magnitude. Within the following six months, 10 satellites were lost from the effects of the increased

radiation levels.

60. A map of the STARFISH threshold-energy of E>0.28 MeV, as a function of magnetic shell

parameter L, for the artificial electron cutoff times, is shown in Figure 68 [17]. Figure 69 shows the

integral STARFISH enhanced Van Allen belt electron fluxes before and after the Soviet event, in

regions from L of ~1.8 – 4.0, for particles with energies of E>0.5 and E>1.9 MeV [18]. The

STARFISH electrons were mostly distributed over an L-range from 1.1 – 2.2 er, whereas the Soviet

particles ranged from L=1.8-4.0, because of the higher test latitude. Significantly, the STARFISH 2-

MeV electrons lasted about 8 years, before decaying back down to the natural background levels.

Figure 70 shows the average lifetime of the 2 MeV fission electrons from the STARFISH and the

Soviet experiments. The data suggest that longevity is maximum at low L values, decreases rapidly

toward the slot region, and is in the range of weeks, and perhaps only days, at larger L values

(approximately 10-12 days at Geostationary Earth Orbits [GEO]).
52

Figure 68: Decay cutoff times in months for the STARFISH model as a function of threshold energy 

and L paramets.

Figure 69: Integral Van Allen belt electrons before and after the Soviet event.
53

Figure 70: Comparison of the average lifetimes of E>2 MeV electrons for the STARFISH and 

Soviet tests.

Conclusion

61. In conclusion, the effects of the differing radiation exposures experienced by satellites over long

mission durations in near-Earth space, and particularly within the SAA, have to be expected, because

the Earth's magnetic field will continue to change. A better approach to calculating intensity, exposure,

and risk, would be to use a dynamic radiation model that takes into account and accommodates these

natural field variations. Until a dynamic model becomes available, calculations performed with current

models for later epochs (projections), will always yield higher values because of (a) the increasing size

of the SAA, (b) the higher intensity patches that will appear and increase in size, and (c) longer

exposure times during crossings of the larger area of the expanding anomaly. 

62. We only have two sets of models available that describe and define the near-Earth space

environment: (a) geomagnetic field models, and (b) particle radiation models. In a process of

evolution, these have been periodically updated and re-issued in an improved and/or expanded form, as

new information is established and has been officially confirmed. In the meantime, whether perfect or

not, we continue to use the current models for two reasons: (a) we have no other, better tools, and (b)

they provide a common, acceptable base of reference for the entire international science community.

63. It would be preferable and prudent to follow a more realistic and conservative approach in

evaluating spacecraft radiation exposures, by using the latest existing models until better ones become

available, even if that would involve possibly over-predicting the vehicle encountered particle fluences.

It is better to be on the safe side, than to risk failure, by introducing makeshift remedies that don’t

correspond to the actual reality, and underestimate the vehicle-encountered radiation (Example: Table 

2)
54

In Summary

64. The drift of the SAA: In most papers from the SAA literature, data from experiments unrelated

to the SAA were used. The primary interest and concern of the authors of these papers appeared to be

the drift of the SAA and the effects of the SAA’s radiation on LEO missions. As a result, an

unexpectedly large spread in the derived SAA drift rates was observed. These unusual findings could

be due to the great variety in acquisition methods, to the criteria used for the selection of data, and to

the data processing techniques. The result is that the SAA drift values appear to be spread over an

abnormally large range. Specifically, the minimum drift rate reported is 0.170/yr W and 0.080/yr N, and

the maximum drift rate reported is 0.660/yr W and 0.220/yr N. The difference is almost a factor of 4 for

the W direction, and approximately a factor of 3 for the N direction, which, in both directions, is much

larger than expected, and which probably reflects an inconsistent approach, using non-specific data, to

defining and describing the SAA. The corresponding data sources are listed in Table 4.

65. The Size of the SAA: In several publications authors’ claim to have established the size of the

SAA based on analyses of data from their unrelated experiments. In rare instances, the authors also

imply that there is no altitude dependence. However, the accurate size of the “anomaly” cannot be

defined in a single frame of reference from non-specific data, because the SAA has a different shape

and occupies a different area at any given time, when plotted in reference to the variables (a) altitude,

(b) particle species (electrons, protons), (c) particle energy (high, low), and (d) effects on electronics.

Thus, many maps could be produced for temporally and spatially identical conditions, varying in the

detail of the size and the shape of the SAA, depending on the type and quality of the data used.
55

Table 4: SAA North and West drift values reported in the literature.
56

Some Afterthoughts 

Judged by the plethora of published papers in the topic, there is a very significant interest in the

scientific community regarding the SAA. Concerns about the “anomaly” arise from a breadth of

specialties.

Because all information to date has been obtained in a peripheral way, i.e. using opportunistic data

from unrelated experiments, a large variety of opinions, results, and conclusions have been presented.

These findings have sometimes been mutually contradictive, misleading, or confusing. What is needed

is an independent research project specifically exploring the unknowns of the SAA, i.e. a dedicated indepth study of the “anomaly”.

A special exclusive examination of the SAA, with instruments specifically designed for that purpose,

would focus primarily on its: (a) nature, (b) source, (c) dynamics, (d) evolution, (e) variability, and (f)

its future, and would lead to a more reliable definition and model of the “anomaly”.

Such research would also address the causes effecting slow (long range) and fast (short range) changes

of the SAA, such as: (1) space weather events, (2) solar flares and CMEs, (3) magnetic storms, (4)

atmospheric conditions, (5) geomagnetic jerks and reversals, (6) solar cycle effects, (7) non-dipole

contributions/effects, etc.

The currently available information on these topics is largely incomplete, inadequate, and inconsistent,

having been collected by a variety of diverse instruments, on missions dedicated to other important

scientific research, unrelated to the SAA. The data from these missions/instruments, and the

concomitant conclusions drawn, are seriously undermined as a result of: (a) the type of instrument used

and its sensitivity and calibration, (b) the quality of the measurements (contamination, noise), (c) the

data analyses and processing methods (Weibull. Gaussian, Gumbel, etc.), (d) the environmental particle

species (electrons, protons, cosmic rays), (e) the altitude in the SAA (200-1500 km), (f) the time- and

epoch-effects on the measurements (day-night, solar cycle, diurnal, seasonal, etc.), and (g) the magnetic

variations.

Finally, from the review of about two dozen publications about the SAA, it appears that frequently

references are made to data, arguments, or statements in other similar documents, and opinions are 

being repeated, that are not based on sound scientific procedures or principles, thus promulgating

unconfirmed assumptions and theories.
57

References

1. Kurnosova, Kolobianina, Logachev, Razorenov, Sirotkin, Fratkin, “Discovery of radiation

anomalies about South Atlantic at heights of 310-340 km”, Planet. Space Sci., 9, 513-516,

August 1962.

2. International Geomagnetic Reference Field (IGRF), International Association of Geomagnetism

and Aeronomy (IAGA), 1968.

3. Retrieved from: wdc.kugi.kyoto-u.ac.jp/igrf/anime/index.html 

4. Fairfield, D.H. and G.D. Mead, “Magnetospheric Mapping with a Quantitative Geomagnetic Field

Model”, J. Geophys. Res., 80, 535, 1975.

5. Olson, W.P., and K.A. Pfitzer, “A Quantitative Model of the Magnetospheric Magnetic Field”,

J. Geophys. Res. 79, 3739, 1974.

6. Tsyganenko, N.A., “A magnetospheric magnetic field model with a warped magnetic tail current

sheet”, Planet. Sci., v. 37 (1), pp. 5-20, 1989.

7. Tsyganenko, N.A., “Modeling the Earth’s Magnetospheric Magnetic Field Confined Within a

Realistic Magnetopause”, J. Geophys. Res., 100, 5599-5612, 1995.

8. Seltzer, S.M., “SHIELDOSE: A Computer Code for Space-Shielding Dose Calculations”, National

Bureau of Standards, NBS Technical Note 1116, US Government Printing Office, Washington,

D.C., 1980.

9. Jordan, T., “A Radiation Transport/Shielding Code: User’s Guide”, Experimental and

Mathematical Physics Consultants, Gaithersburg, Maryland, 1987.

10. Agnostelli et al., “GEANT4-A Simulation Tool Kit”, Nuclear Instruments and Methods in Physics

Research, Vol. A, No. 506, pp. 250-303, 2003.

11. Sawyer, D. and Vette, J., “AP-8 trapped proton environment for solar maximum and solar

minimum”, National Space Science Data Center, Report 76-06, Greenbelt, Maryland, 1976.

12. McIlwain, C., “Coordinates for mapping the distribution of magnetically trapped particles”,

J. Geophys. Res., 66, 1961.

13. Vette, J.I., and Sawyer, D. M., “Short Report on Radiation Belt Calculations”, NSSDC, NASAGSFC, Greenbelt, MD, 1986.

14. King, J.H., “Solar Proton Fluences for 1977-1983 Space Missions”, J. Spacecraft Rockets, 11, 401,

1974.

15. Normand, E., and Baker, T. J., “Altitude and latitude variations in avionics SEU and atmospheric

neutrons.”, IEEE Trans. Nucl .Sci., 40, 6, 1993.

16. Hess, W.N., “The Effects of High Altitude Nuclear Explosions”, NASA TN D-2402, September 

1964.

17. Stassinopoulos, E.G., and P. Verzariu, “General formula for decay of STARFISH electrons”, 

J. Geophys. Res, vol.76, pp. 1841-1844, 1971.

18. Barth, J.L., C.S. Dyer, and E.G. Stassinopoulos, “Space, Atmospheric, and Terrestrial Radiation

Environments”, IEEE Trans. Nucl. Sci., vol. 50, no. 3, June 2003.

19. Xapsos, M.A., Summers, G.P., and Burke, E.A., “Probability Model for Peak Fluxes of Solar

Proton Events”, IEEE Trans. Nucl. Sci., 45, pg. 2948-2953, December 1998.

20. Xapsos, M.A., Summers, G.P., Barth, J.L., Stassinopoulos, E.G., and Burke, E.A., “Probability

Model for Worst Case Solar Proton Event Fluences”, IEEE Trans. Nucl. Sci., 46, pg. 1481-1485,

December 1999.

21. Xapsos, M.A., Summers, G.P., Barth, J.L., Stassinopoulos, E.G., and Burke, E.A., “Probability

model for Cumulative Solar Proton Event Fluences”, IEEE Trans. Nucl. Sci., 47, pg. 486-490,

June 2000.
58

22. Xapsos, M.A., Stauffer, C., Jordan, T., Barth, J.L., and Mewaldt, A., “Model for Cumulative Solar

Heavy Ion and Linear Energy Transfer Spectra”, IEEE Trans. Nucl. Sci., 54, pg. 1985-1989,

December 2007.

23. Ginet, G.P., et al., “AE9/AP9 and SPM: New Models for Specifying the Trapped Energetic Particle

and Plasma Environment”, Space Sci. Rev., 179, pg. 579-615, 2013.

24. Huston, S.L., “Space Environments and Effects: Trapped Proton Model”, Report NAS8-98218, The

Boeing Co., Huntington Beach, CA, January 2002.
59

List of Projects, Instruments, Missions

(mentioned in SAA papers (re: source of data used by authors)

a. AMPTE = University of Surrey Satellite (UK)

b. ATSR = Along Track Scanning Radiometer (ESA)

c. CEASE = Compact Environment Anomaly Sensor (USAF)

d. CREDO = Cosmic Radiation Environment and Dosimetry (UK)

e. DEMETER = Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions

(CNES)

f. DMI = Doppler Michelson Interferometer (SWIFT, Canada)

g. DORIS = Doppler Orbitography and Radiopositioning Integrator by Satellite (CNES)

h. DSLP = Dual Segmented Langmuir Probe (ESA)

i. ENVISAT = Environmental Satellite (ESA)

j. ISIRO = ???

k. JASON == Ocean Surface Topology Mission (NASA-CNES) 

l. MEPED = Medium Energy Proton and Electron Transport Studies (CANADA)

m. MODIS = Moderate-Resolution Imaging Spectroradiometer (NASA)

n. MOPITT = Measurement of Pollution in the Troposphere (NASA)

o. NYULIN = Dosimeter on Board MIR Space Station (Bulgarian)

p. OERSTED = Geomagnetic Research Satellite

q. PAMELA = Payload for Antimatter Exploration and Light-nuclei Astrophysics (Russia, Germany,

Italy, Sweden)

r. POES = Polar Operational Environmental Satellite (NOAA)

s. PROBA-2 = Project for Onboard Autonomy (ESA)

t. RHESSI = Ramaty High Energy Solar Particle Spectroscopic Imager (NASA)

u. SAMPEX = Solar Anomalous and Magnetospheric Particle Explorer (NASA)

v. SEM-2 = Space Environment Monitor (NOAA)

w. SREM = Space Radiation Environment Monitor (India)

x. SWIR = Short Wave Infrared Sensor (ESA)

y. TED = Total Energy Detector (POES)

z. TERRA = Earth Observing Satellite (NASA)

aa. TPMU = Thermal Plasma Measurement Unit (ESA)

bb. TSX-5 = Tri-Service Experiment (USAF)

cc. UARS = Upper Atmosphere Research (UK)

dd. UoSAT = University of Surrey Satellite (UK)
60

Appendix A.

Important Statements

Good or Bad --- Right or Wrong

1. Hell, Natalie, “The Evolution of the South Atlantic Anomaly Measured by RHESSI”, Erlanger Centre for Astroparticle

Physics, Dr. Remeis-Sternwarte Bamberg, Friedrich-Alexander-Universitaet, Erlangen-Nuernberg, Germany, 2010

a) Quote: “The position of RHESSI in latitude, longitude, and altitude is calculated via two different

algorithms. The first algorithm has three faults: (a) sometimes no numerical value is given at all, (b)

on other occasions, the derived altitude is suggested to be constant, equal to 6378.13 km for a while,

being obviously wrong, (c) the third defect is much more delicate: the satellite seems to jump

occasionally from its orbit at 540 km to 600 km to less than 500 km and back again within short

time, which is a rather unrealistic behavior for a spacecraft. In contrast, the second algorithm produces

no such outliers for three months in 2006: from March to May the position calculation completely

failed, claiming an altitude of minus earth radius.”

2. Smart, D.F. and M.A. Shea, “Comment on the use of GOES solar proton data and spectra in solar proton dose

calculations”, Radiation Measurements, 30, 327-335, 1999

a) Quote: “Appendix: Conversion from CD-ROM high energy proton flux data to corrected proton

flux developed by Sauer (1995): Procedure: Convert data back to counts, correct for side penetration

and apply new geometric factors to obtain corrected flux. Calibration: The greatest unresolved

problem is the response of the HEPD instrument. The threshold energy adjustment and the geometric

factor adjustment proposed by Sauer (1995) are a good step. These are helpful resolving some of the

discrepancies in P8, P9, and P10 data”.

3. Fuerst et al., “Temporal Variations of Strength and Location of the South Atlantic Anomaly as Measured by RXTE”,

Earth and Planetary Science Letters, 10 February 2009

a) Quote: “The AP8/AE8 models are based on data taken with older instruments which were not

necessarily as sophisticated as modern devices”.

b) Quote: “On shorter time scales, solar coronal mass ejections (CME) can cause geomagnetic storms and

inject electrons and protons into the atmosphere. Asikainen and Mursula (2005) have shown that

during the great storm of March 21, 2001 electrons became trapped in the SAA and drifted around

the Earth with the SAA.”

c) Quote: “The drift rate [of the SAA] depends strongly on the accuracy of the models [AP8/AE8]’.

d) Quote: “We note that while some properties of the SAA are altitude dependent, in the following we will

concentrate on the shape and position of the SAA, which have been shown to be independent of

altitude.”

4. Mozzoni, David, “The Changing Geomagnetic Field from the Ionosphere to the Core-Mantle Boundary”,

GeoForschungs Zentrum Potsdam (GFC), Scientific Technical Report STR08/02.

a) Quote: “In this region [= the SAA] the shielding effects of the geomagnetic field have been severely

compromised, thus allowing high energy particles trapped in the Van Allen radiation belt to penetrate

deep into the upper atmosphere to altitudes below 100 km. Compromise = a change that makes something worse and that is not done for a good reason

b) Quote: “The anomaly arises from the eccentric displacement between the magnetic and geographic

poles of the Earth ...”
61

c) Quote: “The lack of the natural shelter afforded by the geomagnetic field is more than simply a curiosity

of the region. It represents a real hazard, which can be and does have direct impact on spacecraft,

orbiting the Earth with inclinations between 350 to 600, exposing them to long periods (several

minutes) of stronger than usual radiation during each orbit.

d) Quote: “In fact orbiting spacecraft, while traversing the region, may be bombarded by particles with

energies up to 10 MeV, at rates as high as 3000 cm-2 sec-1, but it only takes particle energies of 1 MeV

to have the potential to cause spacecraft damage.

e) Quote: “Humans orbiting in the ISS or in spacecraft (like the shuttle), as a result, are potentially subject

to increased ionizing radiation dose rates as well, which can be biologically harmful.

f) Quote: “The effect can be shown to a lesser degree, even on airplanes that fly through the region.” 

[= the SAA]

5. Grigorian, O.R., A.N. Petrov, V.V. Romashova, “On the Drift of the South Atlantic Anomaly”, WDS’05 Proceedings

of Contributed Papers, Part II, 251-256, 2005

a) Quote: “Conclusions: 1. Heinderickx (1996) calculation of high-energy proton flux in the SAA region

do not coincide with experimental results obtained on board the MIR station”. 2. Experimental distribution

of high-energy proton fluxes obtained onboard the MIR station agrees with a magnetic field

distribution obtained at the same station experimentally.” 3. Comparison of the position of SAA using

dose rate and proton flux distribution in the SAA region shows that positions of these distributions do

not correspond.”

6. Grigorian, O.R., V.V. Romashova, A.N. Petrov, “SAA drift: Experimental Results”, Advances in Space Research 41

(2008) 76-78

a) Quote: “Conclusions: and summarising the observations onboard various satellites [COSMOS-484,

1972; Intercosmos-24, 1990] and orbital stations [SALUT-6, 1979; MIR 1991 & 1998; ISS, 2002], we

can formulate the following conclusions: 1., 2. The comparison of electron fluxes reveals that there is a

northward drift of the SAA at the same time with westward. The northward drift of the SAA protons

is not observed according our data.”

7. Dyer, C., A. Sims, C. Underwood, “Radiation Belt Observations From CREAM and CREDO”, American Geophysical

Union, Geophysical Monograph 97, 1996

a) Quote: “Abstract: The LEO observations have shown the drift of the SAA and an altitude dependence

of trapped protons which is at variance with the AP8”.

b) Quote: “Discussion: 1., 2. The increase in the SAA protons with altitude is less than predicted by the

AP-8 model”.

c) Quote: “Trapped Proton Results: It was noted that certain SAA passes (e.g. orbit 23 of STS-48 and

orbit 40 of STS-44) were not predicted when using the recommended technique of employing the field

model pertaining to the data from which the models were created (i.e. 1970). However, use of the

1991 geomagnetic field does predict peaks for these orbits as it accounts for the steady drift of the

SAA contours to the west due to evolution of the geomagnetic field (Fig. 1 = 1970 field [orbit misses SAA],

Fig. 2 = 1991 field [orbit intersects SAA]).”

d) Quote: “Trapped Proton Results: Encouraging agreement is obtained between the three experiments

despite the different design. For the region of the SAA, the LET-spectra observed from KITSAT and

POSAT have been compared with each other and with predictions based on the AP8 model for solar

minimum, allowing for protons stopping and slowing in the detector. Figure 4 shows that agreement is

good considering that uncertainties of a factor of two are inherent in allowing for spacecraft shielding and

particle atmospheres.”
62

8. Underwood, C., N.K. Oldfield, C.S. Dyer, A.J. Sims, “Long-term Trends in the LEO Radiation Environment as

Measured by Radiation Monitors On-board Three UoSAT-Class Micro- Satellires”, ESA Symposium Proceedings on

“Environment Modeling for Space-based Applications”, ESTEC, Noordwijk, NL, 18-20 September 1996.

a) Quote: “The AP8 model predictions were found to be in good agreement with the flight data. Figure

11 shows that the fit between the AP8MIN model predictions and the KITSAT-1 CRE [800 km] flight

data is very good. Similarly, the fit between the AP8MIN predictions and flight results for PoSAT-1

[1330 km] is good, although here the predicted flux is slightly low.’

b) Quote: “The CRE measurements of the trapped proton environment in the SAA, generally, show good

agreement with the predictions of the AP8MIN model, provided the proper account is taken of the very

high-LET, low-energy protons, slowing and stopping in the detector.”

c) Quote: “It would therefore appear that the AP8 model gives a reasonable basis for modeling the

trapped proton environment of these low-Earth orbits in order to make predictions of single-event

effect rates -- at least to within a factor of two”. 

Editor: which is the value of the uncertainty factor of the model.

9. Ginet, G.P., Dan Madden, B.K. Dichter, D.H. Brautigam, “Energetic Proton Maps for the South Atlantic Anomaly”,

AFRL-RV-HA-TR-2008-1060.

a) Quote: “Proton beam calibration for the telescope, validating the computer model of the telescope,

have not yet been performed.”

b) Quote: “Our simple application of the Monte Carlo results assumes an incident isotropic flux which is

not necessarily consistent with observation.”

c) Quote: “Uncertainties in the geometric factor and threshold energies due to the sensor modeling

process, were estimated by using different characterizations of the proton spectrum when integrating the

channel response functions”.

d) Quote: “Given the disparity in geometric factors it is reasonable to assume that the response in the

anomaly is predominantly from protons”.

10. Casadio, S., and O. Arino, “Monitoring the South Atlantic Anomaly using ATSR Instrument Series”, Advances in

Space Research, Vol. 48, Issue 6, pp.1056-1066, 2011.

a) Quote: “The comparison results, reported in Table 2, show that the SAA area and strength are

essentially insensitive to the presence of fires while the SAA peak location is shifted westward by 10

and northward by 1.40. Thus the SAA peak coordinates evaluated from SWIR data are expected to be

biased by similar amounts with repsect the VIS retrievals”.

11. Konradi, A., G.D. Badhwar, L.A. Braby, “Recent Space Shuttle Observations of the South Atlantic Anomaly and

the Belt Models”, Advances in Space Res., Vol. 14, No. 10, pp. 911-921, 1994.

a) Quote: “ We know from both part exposure and the current data set that the radiation is very anisotropic

since it not only has highly peaked pitch angle distribution, but also exhibits a strong east-west effect.

This, combined with the uneven shielding distribution around the spacecraft from pass to pass, may

produce different measured dose rates even for identical space points sampled at different times.”

12. Konradi, A., A.C. Hardy, W. Atwell, “Radiation Environment Models and the Atmospheric Cutoff”, JGR, Vol. 24,

No. 3, May-June 1987.

a) Quote: “The calculations presented here will be based on the IGRF for 1975 and fluxes of protons with

energies above 30 MeV as represented by AP8MIN. ----- Figure 1 shows B and L contours in the SAA

at 500 km altitude for epochs 1965(a), 1995(b), and 2025(c).
63

b) Quote: “The above described use of the trapped radiation environment models is also based on the

assumption that B and L, being derived from the first and second adiabatic nvariants, are themselves

invariant. In a static magnetic field, this is certainly true and this coordinate system is very successful in

ordering particle data and is, therefore, incorporated into the models of the Earth’s trapped radiation

environment. However, since the Earth’s dipole is decreasing with a characteristic time of about 1000

years, when calculating particle flux transformations, all three adiabatic invariants have to be taken

into account properly.

c) Quote: “Since the gradient within the geomagnetic cutoff is very steep, within a short time period the

order of magnitude flux increases and even subterranean fluxes are predicted.” ---- “Not only does the

flux increase with time, but also increases disproportionally at low altitudes and even below the Earth’s

surface, If real, such an increase would have implications far beyond any one’s imagination.”

13. Casadio, S.O., O. Arino, O. Serpe, “Monitoring the South Atlantic Anomaly Using ATSR”, Living Planet

Symposium, 27 June – 2 July, 2010, Bergen, Norway

a) Quote: “However, other satellite missions, although designed for Earth observation, have also

provided valuable information on solar flux. In fact, energetic particles (such as protons) reaching

altitudes between 500 and 800 km produce detectable signals by causing spikes (or outliers) in the

measured radiances, as the cases of the NASA MOPITT instrument on TERRA and DORIS on

JASON-1.”

b) Quote: “Monitoring the solar particle flux reaching the inner belt of the Earth’s magnetic field and in

particular in correspondence of the SAA.

c) Quote: “Regarding the accessibility of solar energetic particles to the SAA.”

d) Quote: “RHESSI, SAMPEX, NYULIN, DMI, ISIRO, PROBA, SREM, SEM-2, POES, MEPED,

PROBA-2, TPMU, DSLP: all these satellite missions are dedicated to the detection and

characterization of extraterrestrial particles reaching the inner belt of the Earth’s magnetic field.

14. Heirtzler, J.R., “Future Radiation Damage in Space Due to the South Atlantiv Anomaly”. Laboratory for

Terrestrial Physics, NASA GSFC, Greenbelt, MD 20771, 2000

a) Quote: “The geographic limits of the South Atlantic Anomaly, as defined by radiation damage, are

compared to contours if geomagnetic total field intensity, as defined by the 1995 IGRF, for the present

and recent past.

b) Quote: “The most likely secular variation of the geomagnetic field is estimated and used to

extrapolate the geomagnetic field to the year 2100.

c) Quote: “The SAA greatly influences the shape of the geomagnetic field lines.”

d) Quote: “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.

e) Quote: “If the map of secular change from the IGRF coefficients (Figure 6) is used to extrapolate

back from 1995 to 1922 the charts compare favorably in the South Atlantic.”

15. Csaadio, S.O., and O. Arino, “Monitoring the South Atlantic Anomaly using ATSR historical data sets”, 2013

a) Quote: “The ATSR’s provided consistent SWIR measurements. The SWIR data could be used for fire

detection. But how to discriminate fires from the SAA noise? Not easily, because the SAA “spikes”

cannot be unambiguously distinguished from that of the fires.”

Etc. etc. etc.
64

Appendix B

Comments on Appendix A

1. Hell 2010: problems with orbit generator (trajectory calculations)

2. Smart and Shea 1999: problems with data processing and instrument calibration (geometric 

factor, threshold energy)

3. Fuerst 2009: (a) old vs. new instruments (sophistication of devices)

(b) electrons from the CME, trapped in the SAA, drift around the Earth with the SAA

(c) drift rate [= of SAA] depends strongly on the accuracy of the models

4. Mozzoni ????: (a) proton penetration to altitudes below 100 km

(b) the SAA arises from the displacement of the magnetic and geographic poles

5. Grigorian 2005: (a) SPENVIS High energy proton calculations don’t coincide with experiment

results on MIR

(b) position of SAA does not correspond to dose rate and flux distribution

6. Grigorian 2008: 

(a) the North drift of SAA’s protons is not observed in their data from many satellites

(b) the altitude dependence of protons is at variance with the AP8 model 

7. Dyer 1996: 

(a) increase of SAA’s protons with altitude is less than produced by the AP8 model

(b) certain shuttle SAA passes were not predicted when using recommended techniques for

using the field model of 1970 [= SPENVIS], however, use of the 1991 model does [=correctly] predict

peaks, as it accounts for the drift, due to the evolution of the field from 1970 to 1991

(c) trapped proton results from KITSAT and POSAT compared with predictions from AP8-

MIN show good agreement

8. Underwood 1996: 

(a) AP8 predictions are in good agreement with flight data

(b) KITSAT-1 CRE proton measurements in the SAA show very good agreement with AP8-

MIN predictions

(c) it appears the AP8 gives a reasonable basis for trapped proton modeling on LEO

9. Ginet 2008: (a) instrument calibration was not performed for telescope

(b) assumed indirect isotropic flux not consistent with observations

10. Cassadio 2011: SAA peak coordinates from SWIR data expected to be biased

11. Konradi 1994: radiation in SAA is anisotropic, and uneven shielding around spacecraft from pass 

to pass may yield different doses, even foridentical points, sampled different times

12. Konradi 1987: 

(a) field calculations for 1965, 1995, and 2025 were performed with the IGRF 1975 model

(b) expressed concern about adiabatic invariants: seams not to be aware that the IGRF models 

are static over their 5-year validity period and that all 3 adiabatic invariants are valid
65

13. Casadio 2010: 

(a) solar flux reaching altitudes between 500 and 800 km in the SAA

(b) monitoring solar particle flux reaching the inner belt, in particular the SAA

(c) regarding the accessibility of solar energetic particles to the SAA

(d) RHESSI, SAMPEX, NYULIN, DMI, ISIRO, PROBA, SREM, SEM-2, MEPED,

PROBA-2, TPMU, DSLP: all dedicated to the detection and characterization of 

extraterrestrial particles reaching the inner belt of the magnetic field

14. Heirtzler, J.R.

(a) the geographic limits of the SAA, as defined by radiation damage

(b) the most likely secular variation of the geomagnetic field is estimated and used to

extrapolate the field to 2100 [!!!!]

(c) the SAA greatly influences the shape of the geomagnetic field lines

(d) 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) [SUDDEN ????]

(e) if the map of secular change from the IGRF coefficients (Figure 6) is used to extrapolate

back from 1995 to 1922 the charts compare favorably in the South Atlantic. [!!!!]
66

Appendix C

Some Special Comments on the Reviewed Publications

1. Data concerning the SAA, collected by different instruments at different times, have yielded different

results, for similar conditions, even when measured at identical points in space. The causes include: (a) the

large variety of instruments, (b) their specific sensitivity and response, (c) their calibration and geometric

factors, (d) their threshold energy, (e) their contamination by noise/interference, and (f) their accuracy.

Contributing could also be the differences in processing and analyzing the data, the relevance and

interpretation of the data, and the assumptions made may also contribute to this great variety of results. The

examples below highlight some of the challenges faced when using varied instruments and peripheral data.

The listed examples refer to the items contained in section Appendix A.

Example: #2 Smart and Shea

Quote: “Appendix: Conversion from CD-ROM high energy proton flux data to corrected proton flux

developed by Sauer (1995): Procedure: Convert data back to counts, correct for side penetration and

apply new geometric factors to obtain corrected flux. Calibration: The greatest unresolved problem is the

response of the HEPD instrument. The threshold energy adjustment and the geometric factor adjustment

proposed by Sauer (1995) are a good step. These are helpful resolving some of the discrepancies in P8, P9,

and P10 data”.

Example: #3 Fuerst et al.

Quote: “The AP8/AE8 models are based on data taken with older instruments which were not necessarily

as sophisticated as modern devices.”

Example: #9 Ginet 2008

Quote: “Proton beam calibration for the telescope, validating the computer model of the telescope, have

not yet been performed”.

Quote: “Uncertainties in the geometric factor and threshold energies due to the sensor modeling process,

were estimated by using different characterizations of the proton spectrum when integrating the channel

response functions.” 

Quote: Given the disparity in geometric factors it is reasonable to assume that the response in the

anomaly is predominantly from protons.”

2. Based on the assumptions presented in some of the of SAA related publications, not all authors may be

familiar with the special physics, the processes, and the conditions involved with the SAA and its origin,

area, size, drift, strength, evolution, and accessibility to solar energetic particles. Publications that mention

“extraterrestrial energetic particles”, solar or galactic in origin, that are supposedly capable of reaching (i.e.

penetrating) the SAA, to altitudes below 100 km, within the inner zone of the Van Allen belts, may not be

considering the current understanding of atmospheric cutoff and magnetic rigidity.

Example: #4 Mozzoni, David

Quote: “In this region [= the SAA] the shielding effects of the geomagnetic field have been severely

compromised, thus allowing high energy particles trapped in the Van Allen radiation belt to penetrate

deep into the upper atmosphere to altitudes below 100 km.”
67

3. The SAA, when first detected, was considered an “anomaly” of the field, an aberration. But the SAA is

really not an anomaly at all, just an apparent locally weaker area of the Earth’s magnetic field over Brazil.

This area is the consequence of simple geometry, namely the tilt, the eccentricity, and the displacement of

the dipole axis by about 500 km toward the North Pacific. As a result, the magnetic unit sphere lies outside

the northern physical Earth over the Pacific, and therefore has a higher local field strength at the surface of

the globe, because it is closer to the dipole center, it lies inside the physical Earth over the South Atlantic

and has a lower local field strength at the surface of the globe, because it is farther away from the center of

the dipole.

Apparently, though, there is still prevailing some confusion in the community about this topic, judging by

the following statement:

Example: #4 Mozzoni, David

Quote: “The anomaly arises from the eccentric displacement between the magnetic and geographic poles

of the Earth .....”.

4. Regarding the geomagnetic field models, the internationally accepted and widely used IGRF model is

being updated and re-issued every 5 years. It has accounted for long scale changes of the field, which

occurred during each 5-year time period. The secular variation terms, accompanying every new 5-year

version, however, should not be used to extrapolate the field into the future for more than the 5 years,

maximum.

Example: #12 Konradi, A., A.C. Hardy, W. Atwell

Quote: “The calculations presented here will be based on the IGRF for 1975 and fluxes of protons with

energies above 30 MeV as represented by AP8MIN. ---- Figure 1 shows B and L contours in the SAA at

500 km altitude for epochs 1965(a), 1995(b), and 2025(c).

Example: #14 Heirtzler, J.R.

Quote: “The most likely secular variation of the geomagnetic field is estimated and used to extrapolate the

geomagnetic field to the year 2100.”

Quote: “If the map of secular change from the IGRF coefficients (Figure 6) is used to extrapolate back

from 1995 to 1922 the charts compare favorably in the South Atlantic.”

5. The adiabatic invariants, that are the basis for McIlwain’s magnetic shell parameter “L”, require a static

magnetic field for all three to be valid. In the case of the IGRF models, all changes in the field over the 5-

year period of each new model issued, have been accounted for; the field is indeed static over that time,

and its use is perfectly safe and correct, in terms of the three adiabatic invariants.

Example: #11 Konradi, A., A.C. Hardy, W. Atwell

Quote: “The above-described use of the trapped radiation environment models is also based on the

assumption that B and L, being derived from the first and second adiabatic invariants, are themselves

invariant. In a static magnetic field, this is certainly true and this coordinate system is very successful in

ordering particle data and is, therefore, incorporated into the models of the Earth’s trapped radiation

environment. However, since the Earth’s dipole is decreasing with a characteristic time of about 1000

years, when calculating particle flux transformations, all three adiabatic invariants have to be taken into

account properly.”
68

6. The SAA is nothing more than an integral part of the inner zone of the Van Allen belts. It is not an

independent, separate, or special entity. All the rules that apply to the inner zone, apply also to the SAA. It

has the same atmospheric cutoff, the same intensity and distribution of trapped particles, the same

magnetic field effects, and experiences the same penetration of very high energy cosmic rays. Specifically,

most solar-flare and CME protons, being deflected by the magnetic field (rigidity), cannot reach a major

part of the inner zone, and hence the SAA, because in order to do so they need to have energies in excess

of 2.9 GeV. These energies have not been detected in such events.

Example: #13 Casadio et al.

Quote: “Monitoring the solar particle flux reaching the inner belt of the Earth’s magnetic field and in

particular in correspondence of the SAA.

7. Although trapped electrons and protons drift around the Earth in opposite directions, the drift of the

SAA is strictly a local phenomenon, centered at the South Atlantic over Brazil.

Example: #3 Fuerst et al.

Quote: “On shorter time scales, solar coronal mass ejections (CME) can cause geomagnetic storms and inject

electrons and protons into the atmosphere. Asikainen and Mursula (2005) have shown that during the great storm of

March 21, 2001, electrons became trapped in the SAA and drifted around the Earth with the SAA.”

8. Incorrect assertions about the SAA may stem from using data from unrelated spacecraft missions, and their

instruments , and measurements. In the examples below none of these missions/satellites were either dedicated,

designed, qualified, or intended for a study of the SAA. Page 58 lists many unrelated projects, frequently

referenced in the reviewed literature, and describes their correct mission definitions.

Example: #13 Casadio et al. 2010

Quote: “ RGESSI, SAMPEX, NIULIN, DEMETER, DMI, ISIRO, PROSA, SREM, SEM-2, POES, MEPED,

TPMU, DSLP = all these satellites are dedicated to the detection and characterization of extraterrestrial energetic

particles reaching the inner belts of the Earth’s magnetic field.”

Quote: “However, other satellite missions, although designed for Earth observation, have also provided valuable

information on solar flux. In fact, energetic particles (such as protons) reaching altitudes between 500 and 800 km

prodiuce detectable signals by causing spikes (or outliers) in the measured radiances, as the cases of the NASA

MOPITT instrument on TERRA and DORIS on JASON-1.”

9. A far more serious and potentially dangerous, misrepresentation is the issue of solar proton event particles

penetrating the inner zone of the Van Allen belts (aka: the SAA), and reaching altitudes below 100 km. One paper

claims effects on airplanes that fly through the SAA, an impossibility.

Example: #4 Mozzoni, David

Quote: “In this region [= the SAA] the shielding effects of the geomagnetic field have been severely compromised, thus

allowing high energy particles trapped in the Van Allen radiation belt to penetrate deep into the upper atmosphere

to altitudes below 100 km.”

Quote: “The effect can be shown to a lesser degree, even on airplanes that fly through the region.” 

Example: #13 Casadio et al.

Quote: “Regarding the accessibility of solar energetic particles to the SAA.”

Quote: “However, other satellite missions, although designed for Earth observation, have also provided valuable

information on solar flux. In fact, energetic particles (such as protons) reaching altitudes between 500 and 800 km

prodiuce detectable signals by causing spikes (or outliers) in the measured radiances, as the cases of the NASA

MOPITT instrument on TERRA and DORIS on JASON-1.”
69

Quote: “Monitoring the solar particle flux reaching the inner belt of the Earth’s magnetic field and in particular in

correspondence of the SAA.”

10. Some authors are confused as to the nature of the SAA and of the inner zone of the Van Allen belts, which in

most cases are considered two different entities that interact with and influence each other. It is not understood that

the SAA and the inner zone of the Van Allen belts are one and the same, with no physical differences, having only

different names. The fact that the SAA is, in essence, not even a true anomaly, is often overlooked.

Example: #14 Heirtzler, J.R.

Quote: “The SAA greatly influences the shape of the geomagnetic field lines.”

Example: #4 Mozzoni, David

Quote: “The anomaly arises from the eccentric displacement between the magnetic and geographic poles of the

Earth ... “.

Quote: “The main feature visible in Figure 3.1 is represented by the large red region, indicating an elevated proton

flux count corresponding to the SAA, which arises from the relative proximity of the inner radiation belt.”

11. Another challenging feature of the SAA is that both its shape and size, as well as its position, are very much

altitude dependent. This fact is easily demonstrated with any field model used, any proton model considered, any

energy selected, and for any time period involved: in each case the shape of the SAA is different at every altitude.

Example: #3 Fuerst et al.

Quote: “We note that while some properties of the SAA are altitude dependent, in the following we will concentrate

on the shape and position of the SAA, which have been shown to be independent of altitude.”

12. The use of peripheral data from unrelated experiments cannot accurately define uniquely the real size and

shape of the SAA. As stated before, the anomaly can have (at the same time), multiple, varying shapes and sizes,

depending on the parameters selected for plotting (e.g. the field model, the particle model, the particle energy, the

altitude, or special data sets from an unrelated experiment, etc.).

Example: #14 Heirtzler, J.R.

Quote: “The geographic limits of the South Atlantic Anomaly, as defined by radiation damage, are ......”.

13. A further misconception is the assumption that the SAA , which is just a name defining a region, has influence

and power that can impact physical processes, as the quotation below implies. In reality, the opposite is true, i.e. the

geomagnetic field lines influence the shape of the SAA (in essence: the dipole field)

Example: #14 Heirtzler, J.R.

Quote: “The SAA greatly influences the shape of the geomagnetic field lines”. 
70

Appendix D

Future Evolution of the Earth’s Magnetic Field

A special study was performed with data obtained from the officially established “International

Geomagnetic Reference Field” (IGRF), issued in updated form every 5 years by the “International

Association of Geomagnetism and Aeronomy” (IAGA).

The model is equipped with first and second order time derivatives, describing the secular variation of

the field. Each new 5-year issue is based on current global measurements of the field.

The geomagnetic field is composed of a major dipole term and of additional lesser non-dipole terms

(quadrupole and higher). 

The field experiences two significant variations that are not constant in time: (a) the drift of the

magnetic poles and (b) the gradual decline in the field strength.

The study was based on eleven versions of the IGRF, from 1960 to 2010, using data calculated with

the secular variation terms in each model period.

All these field models are widely being used in research and application, involving the radiation of the

Van Allen belt particle models, where a primary concern is the use of the parameters B and B0, as

obtained from the updated IGRF field, at epochs later then the construction dates of the particle

models. 

B0 is the field strength at the point where the magnetic field-line intersects the magnetic equator, but it

is not necessarily the location of the absolute minimum B value on that field-line, which may be off the

equator. (Figure 1)

Specifically, the exact values are respectively:

B(1960) = 21300.4 nT, B(2010) = 19358.7 nT, Differ. = 1941.7 nT

B0(1960) = 18267.9 nT

B0(2010) = 16819.0 nT

Differ. = 1448.9 nT

Thus: 1941.7 – 1448.9 = 492.8 nT: is the relative decrease of B vs B0 in 50 years

Then: 19358.7 ÷ 492.8 = 39.283: is the number of 50-year periods to reach equal values 

And: 39.283 × 50 = 1964.15: is the number of years, when B will be equal to B0.

Assuming that the change in the field remains constant during this time, which is questionable, these

calculations imply a disappearance of the field, i.e. a possibility magnetic reversal, as experienced

many times before on geologic scales.
71

Figure 1: Magnetic coordinate system.

A plot of the B and B0 intensities from 1960 to 2010 (Figure 2), obtained from the corresponding

eleven IGRF 5-year models, clearly indicates a gradual convergence of the positional field strength to

the relative B0 value. It is obvious that B decreases faster than B0 by about 500 nT in 50 years.

Figure 2: B and B0 magnetic field intensities from 1960 to 2010.
72

Appendix E

General Information on SAA Radiation

Inclination Dependence

• Greatest flux is experienced in the inclination range 00<i<350.

• For i>350

, fluxes rise gradually until about i=600.

• Over i=600, inclination has little effect on flux levels.

Altitude Dependence

• Largest fluxes occur between 200-600 km, increasing as altitude rises.

• For altitudes h>600 k m, fluxes increase more gradually.

• Over about 1500 km, orbits are essentially outside the SAA.

Peak Flux Positions

• Location of peak fluxes depends on particle energy, for both electrons & protons.

Magnetic Rigidity Protection

• Shielding by geomagnetic field from cosmic rays and solar event particles is dependent more on inclination than

on altitude. The degree of inclination is most important for shielding from these particles.

• Inclinations up to about 450 are almost totally shielded.

• Inclinations greater than 450 experience progressively larger exposures.

• As inclinations reach 900

, the satellite is outside the closed magnetic field lines and is fully exposed to cosmic

rays and solar event particles in the polar regions.

• During major solar particle events or magnetic storms, satellites with inclinations below ~450 may not be 

completely shielded, due to the compression of the field lines, resulting in solar flare and cosmic ray particles

reaching previously unattainable altitudes and inclinations.

• As altitude increases the regions of space accessible to cosmic rays and solar event particles gradually

increase also.

Systemic Changes

• Solar cycle variations

• Seasonal variations

• Diurnal variations

• Magnetic field secular variations

Occasional Variations

• Solar events (flares, CMEs)

• Magnetic storms

• Geomagnetic jerks

• Space weather effects