NASA/TM–2015-217547

Forty-Year “Drift” and Change of the SAA
E.G. Stassinopoulos, M.A. Xapsos, C.A. Stauffer

National Aeronautics andSpace Administration
Goddard Space Flight Center
Greenbelt, Maryland 20771

December 2015

NASA STI Program ... in Profile
Since its founding, NASA has been dedicated to theadvancement of aeronautics and space science. The
NASA scientific and technical information (STI) program plays a key part in helping NASA maintain thisimportant role.
The NASA STI program operates under the auspicesof the Agency Chief Information Officer. It collects,
organizes, provides for archiving, and disseminates
NASA’s STI. The NASA STI program provides accessto the NASA Aeronautics and Space Database and its
public interface, the NASA Technical Report Server,
thus providing one of the largest collections of aeronautical and space science STI in the world. Results
are published in both non-NASA channels and by
NASA in the NASA STI Report Series, which includes
the following report types:• TECHNICAL PUBLICATION. Reports ofcompleted research or a major significant phase of
research that present the results of NASA Programs
and include extensive data or theoretical analysis.
Includes compilations of significant scientific andtechnical data and information deemed to be ofcontinuing reference value. NASA counterpart ofpeer-reviewed formal professional papers but hasless stringent limitations on manuscript length andextent of graphic presentations.
• TECHNICAL MEMORANDUM. Scientificand technical findings that are preliminary or of
specialized interest, e.g., quick release reports,
working papers, and bibliographies that containminimal annotation. Does not contain extensive
analysis.
• CONTRACTOR REPORT. Scientific and technicalfindings by NASA-sponsored contractors andgrantees.

• CONFERENCE PUBLICATION. Collected
papers from scientific and technical conferences,
symposia, seminars, or other meetings sponsored orco-sponsored by NASA.
• SPECIAL PUBLICATION. Scientific, technical,
or historical information from NASA programs,
projects, and missions, often concerned withsubjects having substantial public interest.
• TECHNICAL TRANSLATION. English-languagetranslations of foreign scientific and technicalmaterial pertinent to NASA’s mission.
Specialized services also include organizing andpublishing research results, distributing specialized
research announcements and feeds, providing help
desk and personal search support, and enabling dataexchange services. For more information about theNASA STI program, see the following:• Access the NASA STI program home page at
http://www.sti.nasa.gov• E-mail your question via the Internet to
help@sti.nasa.gov• Fax your question to the NASA STI Help Desk at
443-757-5803• Phone the NASA STI Help Desk at 443-757-5802

NASA/TM–2015-217547

Forty-Year “Drift” and Change of the SAA
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

National Aeronautics andSpace Administration
Goddard Space Flight Center
Greenbelt, Maryland 20771

Decemember 2015

Notice for Copyrighted Information
This manuscript has been authored by employees of AS and D, Llc., with the National Aeronautics
and Space Administration. The United States Government has a non-exclusive, irrevocable,
worldwide license to prepare derivative works, publish, or reproduce this manuscript, and allow
others to do so, for United States Government purposes. Any publisher accepting this manuscriptfor publication acknowledges that the United States Government retains such a license in any
published form of this manuscript. All other rights are retained by the copyright owner.

Trade names and trademarks are used in this report for identification only. Their usage does notconstitute an official endorsement, either expressed or implied, by the National Aeronautics andSpace Administration.

Level of Review: This material has been technically reviewed by technical management

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 theSouth 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.

Introduction2.
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 forresearch 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 andcould 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 betaken 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 andconclusions. Additionally, many SAA related publications have shown inconsistencies in reference tothe 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 thedipole 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 thenorthern 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 andgeographic poles of the Earth ….”

1

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 officiallyestablished “International Geophysical Reference Field” (IGRF), that is issued every five years by the“International Association of Geomagnetism and Aeronomy” (IAGA) [2]. The IGRF models containfirst 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 thelimits 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 driftof the magnetic poles and (2) the secular variation of the field, a gradual, continuous, long range trendof decreasing strength. The rate of change, of both variations, is not constant. Thus, the drift of thenorth Magnetic Pole has increased over the last century and may change again in the near future. It iscurrently 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 forseveral centuries [3] (Figures 1-9). This decreasing trend, with a varying rate, expressed as themagnetic 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 withenergies 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 from1995 to 1922 the charts compare favorably in the South Atlantic.”

2

Figure 1: Year 1600.

Figure 2: Year 1650.
3

Figure 3: Year 1700.

Figure 4: Year 1750.
4Figure 5: Year 1800.

Figure 6: Year 1850.
5Figure 7: Year 1900.

Figure 8: Year 1950.
6

Figure 9: Year 2000.

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

7

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. Fromthese 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 anarrow 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 backgroundinformation 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) systematicallyevaluate 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 beenfrequently mentioned in many of these papers
11.
Such usually unrelated measurements of opportunity, however, may not accurately reflect theactual and correct shape and location of the SAA. The most important parameter that defines thedomain 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 somepublications, 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 effectof the non-dipole field on the SAA, which by some is considered to be a major force driving thechanges. But in regards to the existence and evolution of the SAA and the corresponding trappedradiation 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 fieldmeasurements 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 theintensity level and location of the trapped, as well as the transiting, particle radiation, within themagnetosphere 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 scientificallyvaluable 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.
8

14.
A review of several dozen published papers dealing with the anomaly, indicated that themeasurements (i.e. the data) which were used in the attempts to describe the SAA, were obtained froma variety of instruments, detectors, or parts, each of which had a different sensitivity, resolution, andaccuracy in performance and calibration, These data were also obtained over different epochs and timeintervals (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 notreflect accurately the true shape and position of the SAA, nor to effectively predict and describe thedynamics 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 andtheir 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 thatdefines the terrestrial magnetosphere (Figure 11). The interaction of the solar wind with themagnetosphere 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.
9

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 tocontinue 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, whichcannot be predicted in terms of frequency or intensity, and which, like CME’s, solar flares, andearthquakes, 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, isindicated 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 themagnetic 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 theMead-Fairfield [4] and the Olson-Pfitzer [5] versions. These were later superseded by the moresophisticated and improved Tsyganenko models [6, 7] that are now being widely used by theinternational scientific community. 5
5External 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).

10

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 theorientation 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, andc. 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 longterm 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 lastcentury, 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.

11

Figure 14: Diurnal variation of the north magnetic pole.
21.
A further concern of high altitude balloon projects, which have to deal with atmospheric orspace 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 asample position, is shown in Figure 15 for a quiet environment (Kp = 0.0), at a high latitude and analtitude 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.
12

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-Earthorbiting 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 thepast, is the popular SHIELDOSE code [8], which does not involve elaborate solid angle sectoring or 3D ray tracing techniques, and considers only “slab” and “spherical” geometries (Figure 16). Moreelaborate codes such as NOVICE [9] or GEANT-4 [10], have also been widely used to obtain resultswith 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 sphericalshield dose will be larger than the slab dose. For example, at a shield thickness of 200 mils, thedifference 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 greaterthan 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

13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.

14

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) theirthreshold 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 ininitial 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 toevaluate 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 thesecular 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 thematerial surrounding the target.
12. Solar proton contributions that have been included in the calculations, by the confidence levelconsidered: 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”.

15

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 spaceradiation 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, atdifferent times, and at different locations. These data are then combined to produce an approximationof 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 andlocations 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 coordinates2.
the selection of the magnetic field model used in the specific process3.
the instrument calibration and efficiency factors, and4.
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 Carlobased 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 havesignificant 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 ofinclination.

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 thetrapped/transient radiation in the SAA. To what degree these data are correct, accurate, and reliable isanother 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 a16

time, (c) a single (standard) beam direction, (d) a single species at a time, and (e) very high rates ofaccelerated exposures. In contrast to these artificial conditions, the natural radiation in space consists ofa 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 tospace 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 asatellite, 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 spaceconditions, 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 orelectrons, 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, theMcIlwain drift shell parameter in Earth radii [12]. L is defined as the distance to the equatorial crossingof 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.

17

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 amuch 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 thatyear, as well as because the orbital flux integrations, when performed for later missions withmore 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 perday, 5-6 do not intersect the SAA, but bypass it completely, whereas 7-9 revolutions crossthrough 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 sincethe 1980’s, by domestic and international research communities. 7
34.
Developing satellite exposure predictions based on the assumption that no changes in theradiation levels at a specific position in space occurred between 1970 and 2010, and that no changes intheir 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 thesepoints.
Table 1: Geographic coordinates of selected test positions.
Point
West
CenterEastLongitude
- 90°
- 30°
+ 30°

Latitude
- 23°
- 23°
- 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 theabove 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 themagnetic equator.

18

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.

19

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, forenergies > 30, >50, >100, and >200 MeV. They were obtained with the recommended B/B0 fieldparameter. 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 therising 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-yearchange in the magnetic location of the three test points has been indicated. The composition anddistribution of the inner-zone Van Allen belt proton population, is likely not the same today, as it was in1970, when the AP8 was constructed; the present calculations may not represent correctly the realchanges that have occurred, but serve only to indicate and approximate the possible trend in thetemporal evolution of the belt.

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

20

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

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

21

Figure 25: B-L space map of AP8 proton model indicating the 40-year change in the magnetic locationof the test positions.
38.
Therefore, the B/B0 approach, devised to avoid obtaining larger fluxes for epochs later than1970, 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 thatthe 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 from1960 to 2010, is given in Table 2. Several interim measurements, reported increased flux levels atformerly 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 whenthe models were created (i.e. 1970). However use of the 1991 geomagnetic field does predict peaks for these orbits as it accounts forthe 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 comparedwith each other and with predictions based on the AP8 model for solar minimum, allowing for proton stopping and slowing in thedetector. Figure 4 shows that agreement is good considering the uncertainties of a factor of two are inherent in allowing for spacecraftshielding and particle atmospheres.”11
Quote from APPENDIX A [8a]: “The AP8 model predictions were found to be in good agreement with the flight data”. [computedwith B/B0]22

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.
B1

Input
L1

B/B0

SPENVIS

B2

L2

AP8 Model Calculations

>30

>50

>100

>200
1

2

2

1

SPENVIS AP8-MAX

>30

>50

2

2

>100

>200

2

1960

0.23370

1.57432.92600.22939

1.5784

1.656

1.434

8.423

1.505

3.26832.78041.588

2.903311970

0.22918

1.58332.9185““2.92222.5312

1.37822.5151

“

“

“

“

1980

0.224121.59072.8944““5.01524.02922.18224.0221

“

“

“

“

2

2

2

1

1990

0.219841.59042.8376

“

“

6.510

5.224

2.691

5.295

“

“

“

“

2000

0.21683

1.58712.7816““

7.98826.13323.1612

6.3771

“

“

“

“

2010

0.21398

1.58172.7166““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 innerradiation belt, may not only jeopardize the success of a mission, but may also be hazardous tobiological systems on board (including humans). The most common effects of underestimating theradiation 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 for23

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 belost 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 themagnitude 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 tothis 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 belts 12. Mirror point positions are
determined by equations that predict their location as a function of their pitch angle, which can haveany value <900. The predicted positions could be at any altitude, including on the surface of the Earthor even at subterranean locations. The atmospheric cutoff would prevent particles from reaching theirmirror point below approximately 100 km in altitude. A relative display of the dip of the associatedmagnetic 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 theorder of magnitude flux increases and even subterranean fluxes are predicted.” ---- “Not only does the flux increase with time, butalso 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.”24

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 atmosphericcutoff prevailing at any given time, in the inner zone of the Van Allen belt, without considering thesignificant 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 inessence a function of density, which in turn is a function of altitude and of solar activity conditions, andnot of the magnetic field. Therefore, a corresponding attempt to indirectly evaluate the “drift” of the
SAA based on these assumptions would be inaccurate.

25

45.
Considering the dynamic nature of the Earth’s main dipole field, concerns about the validity ofthe 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, andhence, 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 increasein 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 inthe 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 forthe 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 theassumption 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 ofabout 1000 years, when calculating particle flux transformations, all three adiabatic invariants have to be taken into account properly.

26

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.

27

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.

28

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.

29

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.

30

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.

31

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.

32

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.

33

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.

34

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.

35

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.

36

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.

37

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.

38

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
Figure Number(p/cm2-sec)
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 forplotting (>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.

39

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 bythese 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.

40

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.

41

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.

42

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 spacecraftis via the relative distribution of B-L points visited by the orbit. These results were obtained from theidentical 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 allorbit 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 theindicated energy. Many of the dropped positions lie in the lower intensity regions of the model andmany of the gained positions lie in the higher intensity regions.

43

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

44

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

45

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

46

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 thecorresponding 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 toenvironmental 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, theprevailing 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 along >13-year period. There does not yet exist a sure and reliable way to accurately predict the durationof a future cycle.

47

Sunspot Number

53.
The active phase of a solar cycle is defined as a seven-year period, starting 2.5 years before theabsolute 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 isprovided in Figure 64 for the 5 most recent cycles, in which the cycles are numbered and the periods
are indicated.

YY/MM
10/03

YY/MM
09/07

21

22

YY/MM
11/07

19

20

23

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 coronalmass ejections (CME), which may have extended from small to extremely large in duration andintensity (CME’s happen predominantly during the active phase, although some SPEs do occur also inthe quiet [“inactive”] phase). As mentioned before in paragraph 16, these events cannot be predictedwith 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 thatassumes 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 powerlaw (Xapsos [19]). This led, in conjunction with other statistical methods, to models of worst caseevents (Xapsos [20]), and cumulative fluences during missions for both protons (Xapsos [21]) andheavy ions (Xapsos [22]), In each case model inputs are the time period of the space mission anddesired confidence level, as was the case for the King Model.

48

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 occurrenceof 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 thatinvolves 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 atleast 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 ….”.

49

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, andhence 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, andconsequently 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 withlower energies are deflected by the Earth’s magnetic field (rigidity cutoff). Most workers consider thegalactic 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 solower than during solar minimum. It is obvious, considering Figure 66, that more cosmic rays haveaccess to the higher latitude regions than near the equator, with a free unimpeded penetration over thepoles, 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 theatmospheric constituents nitrogen and oxygen, are more numerous at high latitudes than near the equator [15].

50Figure 67: Schematic of the distribution of fission electrons from the STARFISH and Soviet tests inmagnetic 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 forexample, the famous US “STARFISH PRIME High Altitude Weapons Test” on July 9, 1962, overJohnston 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 schematicof the distribution of the fission electrons from those two tests is shown in Figure 67, in terms of the Lparameter 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 ofmagnitude. 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 shellparameter L, for the artificial electron cutoff times, is shown in Figure 68 [17]. Figure 69 shows theintegral STARFISH enhanced Van Allen belt electron fluxes before and after the Soviet event, inregions 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 2MeV 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 theSoviet 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]).
51

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.

52

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 thesenatural 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 sizeof 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 spaceenvironment: (a) geomagnetic field models, and (b) particle radiation models. In a process ofevolution, 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 ornot, 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 inevaluating 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)
53

In Summary64.
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 anabnormally large range. Specifically, the minimum drift rate reported is 0.170/yr W and 0.080/yr N, andthe 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 muchlarger than expected, and which probably reflects an inconsistent approach, using non-specific data, todefining 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 shapeand 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 thedetail of the size and the shape of the SAA, depending on the type and quality of the data used.

54

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

55

Some Afterthoughts
Judged by the plethora of published papers in the topic, there is a very significant interest in thescientific 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 datafrom 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 neededis 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) changesof 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 importantscientific research, unrelated to the SAA. The data from these missions/instruments, and theconcomitant 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) thedata 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- andepoch-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 arebeing repeated, that are not based on sound scientific procedures or principles, thus promulgating
unconfirmed assumptions and theories.

56

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 aRealistic 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 andMathematical 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.

57

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.

58

List of Projects, Instruments, Missions(mentioned in SAA papers (re: source of data used by authors)
a.
b.
c.
d.
e.

AMPTE = University of Surrey Satellite (UK)
ATSR = Along Track Scanning Radiometer (ESA)CEASE = Compact Environment Anomaly Sensor (USAF)
CREDO = Cosmic Radiation Environment and Dosimetry (UK)
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)59

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, 2010a) 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 jumpoccasionally from its orbit at 540 km to 600 km to less than 500 km and back again within shorttime, 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, 1999a) 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 geometricfactor adjustment proposed by Sauer (1995) are a good step. These are helpful resolving some of thediscrepancies 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 2009a) Quote: “The AP8/AE8 models are based on data taken with older instruments which were notnecessarily as sophisticated as modern devices”.
b) Quote: “On shorter time scales, solar coronal mass ejections (CME) can cause geomagnetic storms andinject electrons and protons into the atmosphere. Asikainen and Mursula (2005) have shown thatduring the great storm of March 21, 2001 electrons became trapped in the SAA and drifted aroundthe 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 willconcentrate on the shape and position of the SAA, which have been shown to be independent ofaltitude.”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 severelycompromised, 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 reasonb) Quote: “The anomaly arises from the eccentric displacement between the magnetic and geographic
poles of the Earth …”

60c) Quote: “The lack of the natural shelter afforded by the geomagnetic field is more than simply a curiosityof 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 withenergies 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 Proceedingsof Contributed Papers, Part II, 251-256, 2005a) 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 distributionof 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) 76-78a) 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 anorthward 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 GeophysicalUnion, Geophysical Monograph 97, 1996a) Quote: “Abstract: The LEO observations have shown the drift of the SAA and an altitude dependenceof 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 theAP-8 model”.
c) Quote: “Trapped Proton Results: It was noted that certain SAA passes (e.g. orbit 23 of STS-48 andorbit 40 of STS-44) were not predicted when using the recommended technique of employing the fieldmodel pertaining to the data from which the models were created (i.e. 1970). However, use of the1991 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 isgood considering that uncertainties of a factor of two are inherent in allowing for spacecraft shielding andparticle atmospheres.”

61

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] flightdata 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 thetrapped 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 modelingprocess, were estimated by using different characterizations of the proton spectrum when integrating thechannel response functions”.
d) Quote: “Given the disparity in geometric factors it is reasonable to assume that the response in theanomaly is predominantly from protons”.
10. Casadio, S., and O. Arino, “Monitoring the South Atlantic Anomaly using ATSR Instrument Series”, Advances inSpace 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 areessentially 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 bebiased 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 andthe 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 withenergies 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).

62b) Quote: “The above described use of the trapped radiation environment models is also based on theassumption 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 inordering 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 theorder of magnitude flux increases and even subterranean fluxes are predicted.” ---- “Not only does theflux 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, Norwaya) 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 themeasured 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 inparticular 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 andcharacterization 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, 2000a) Quote: “The geographic limits of the South Atlantic Anomaly, as defined by radiation damage, arecompared 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 themost common. This is the Sudden Event Upset (SEU), where, apparently, a single particle causes apiece of equipment to malfunction.
e) Quote: “If the map of secular change from the IGRF coefficients (Figure 6) is used to extrapolateback 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”, 2013a) 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.

63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 (geometricfactor, 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 experimentresults 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 forusing the field model of 1970 [= SPENVIS], however, use of the 1991 model does [=correctly] predictpeaks, 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 AP8MIN 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 AP8MIN 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

64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 extrapolateback from 1995 to 1922 the charts compare favorably in the South Atlantic. [!!!!]

65

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 geometricfactors, (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 andinterpretation 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 andapply 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 theanomaly is predominantly from protons.”
2. Based on the assumptions presented in some of the of SAA related publications, not all authors may befamiliar 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 severelycompromised, thus allowing high energy particles trapped in the Van Allen radiation belt to penetrate
deep into the upper atmosphere to altitudes below 100 km.”66

3. The SAA, when first detected, was considered an “anomaly” of the field, an aberration. But the SAA isreally 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 ofthe 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 ofthe 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 polesof the Earth …..”.
4. Regarding the geomagnetic field models, the internationally accepted and widely used IGRF model isbeing 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-yearversion, 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 withenergies 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 thegeomagnetic field to the year 2100.”
Quote: “If the map of secular change from the IGRF coefficients (Figure 6) is used to extrapolate backfrom 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 5year 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 theassumption 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 inordering 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.”67

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 samemagnetic 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 majorpart 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 inparticular 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 injectelectrons and protons into the atmosphere. Asikainen and Mursula (2005) have shown that during the great storm ofMarch 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 theirinstruments , 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 atmosphereto 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.”

68

Quote: “Monitoring the solar particle flux reaching the inner belt of the Earth’s magnetic field and in particular incorrespondence 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 inmost 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 muchaltitude dependent. This fact is easily demonstrated with any field model used, any proton model considered, anyenergy 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 concentrateon 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 andshape 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, thealtitude, 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 influenceand power that can impact physical processes, as the quotation below implies. In reality, the opposite is true, i.e. thegeomagnetic 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”.

69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 themagnetic 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 withthe 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 theequator. (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, thesecalculations imply a disappearance of the field, i.e. a possibility magnetic reversal, as experienced
many times before on geologic scales.

70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.

71

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 thanon 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 becompletely 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

72