~10 nt/year

ASEN 5335 Aerospace Environment -- Radiation Belts 1

Equatorial field intensity in recent millenia, as deduced from measurements on archeological samples and recent observatory data.

~10 nT/year

Reversals have been documented as far back as 330 million years. During that time more than 400 reversals have taken place, one roughly every 700,000 years on average. However, the time between reversals is not constant, varying from less than 100,000 years, to tens of millions of years. In recent geological times reversals have been occurring on average once every 200,000 years, but the last reversal occurred 780,000

years ago. At that time the magnetic field underwent a transition from a "reversed" state to its present "normal state".

ASEN 5335 Aerospace Environment -- Radiation Belts 2A radiation belt is a population of energetic particles fairly-trapped by the magnetic field.

Map of magnetic striping of the seafloor near the Reykjanes ridge [Heirtzler, 1968]

Alfred Wegener

"The Origin of Continents and Oceans" [Wegener, 1929]


The Radiation Belts

A radiation belt is a population of energetic particles fairly-trapped by the magnetic field.


Review of Charged Particle Motions

• Gyromotion motion: =p2/2mB (1st), T_g~10-3 sec

• Bounce Motion: J= p||ds (2nd), T_b~100 sec • Drift motion: =BdA (3th) , T_d~103 sec


According to Faraday's Law of Magnetic Induction, a time rate of change of magnetic flux will induce an electric field ( and hence a force on the particle ):

Therefore, the requirement that no forces act in the direction of motion of the rotating particle demands that

B

t

E d

l

BB dA

dl

Bt

0


In other words, that the magnetic flux enclosed by the cyclotron path of the charged particle is constant:

Assuming B does not vary spatially within the gyropath,

where r = gyroradius = . We previously defined

the magnetic moment , implying

B B d

A const

B B dA Br2

mvqB

1

2

mv2

B


Or, = constant . This is called the first adiabatic invariant of particle motion in a magnetic field.

We should note that the above has assumed that is

constant within at least one orbital period of the particle. This is only approximately true, and the term "invariant" is also an approximation, but one that reflects the first-order constraints on the particle motion.

B m2v

2

q2B2

B

2m

q2 constNote: same assaying that K.E.does not changeif there are noforces parallelto v

First Adiabatic Invariant


Since , (i.e, the K.E. of the particle

remains constant since the only forces act to V), then must increase as B increases, and correspondingly the distribution of K.E.

between and changes:

If increases to 90° before the particle collides vigorously with the neutral atmosphere, the direction of will change sign (at the "mirror point") and the particle will follow the direction of decreasing B.

1

2mv

2 const

v

v||

v v sin

v|| v cos

v||


For a given particle the position of the mirror point is determined by the pitch angle as the particle crosses the equator (i.e., where the field is weakest) since

Therefore, the smaller eq the larger BM , and the lower down in altitude is the altitude of BM.

sin2 B

const

sin2 eqBeq

1

BM

BM Beq

sin2eq


Loss Cone and Pitch Angle Distribution

Obviously this will happen if eq is too small, because that then requires a relatively large BM (|B| at the mirror point).

The equatorial pitch angles that will be lost to the atmosphere at the next bounce define the loss cone, which will be seen as a depletion within the pitch angle distribution.

B

losscone

Particles will be lost if they encounter the atmosphere before the mirror point.


Magnetic Mirroring in a Dipolar magnetic Field

Trajectory of particleoutside the atmosphericbounce loss cone. Thisparticle will bouncebetween mirror points

Trajectory of particleinside the loss cone.this particle will encounterthe denser parts of theatmosphere (I.e., below100 km) and precipitatefrom the radiation belts.


where ds means integration along B and B = BM at M1, M2. Since

and m,v are constant, then

Second Adiabatic Invariant (Longitudinal Invariant)

The second adiabatic invariant says that the integral of parallel momentum over one complete bounce between mirror points is constant (this once again results from no external forces):

v|| v cos v (1 sin2 )

1/2 v 1B

BM

1/2

2mv||M 1

M 2 ds const

I2 1B

BM

M1

M 2

1/2

ds constsecondadiabaticinvariant


The second adiabatic invariant (I2 = const) assumes that B does not change appreciably during 1 bounce period (about 1 second).

I2 is a property of the field configuration and also of the mirror point (or equivalently, the equatorial pitch angle) since

I2 = const defines the surface, or shell, on which the

particle remains as it drifts around the earth.

This is called the longitudinal invariant surface or L-shell. Recall our previous discussion of the L-shell and its connection with invariant latitude.

BeqB

Msin2eq


solarwind

The Earth’s magnetic field is compressed on the daysideand drawn out on the night-side, so that the fieldconfiguration is zonally asymmetric

How does this affect particles as they drift around the Earth ?

Since the dominant adiabatic invariant governing particle motion is differentfor low and high pitch angle particles, we consider these separately.


1

2mv

2

sin2 B

= constantFirst adiabatic

Invariant:

[K.E. (& magnetic moment) of particle remains constant]

I2 1B

BM

M1

M 2

1/2

ds const

(integral of parallel momentum over one complete bounce between mirror points is constant)

Second adiabaticInvariant:

Review of Adiabatic Invariants 1 & 2


High pitch angle particles have mirror points not far from the equator. They are mostly affected by the magnitude of the B-field.

High pitch angle particles originating on the night-side, when drifting to the dayside, keep moving radially outward to stay at a constant B-value, since the dayside field is compressed with respect to the night-side field.

By the time these particles reach the noon meridian, they reach the boundary of the magnetopause and are lost.

High pitch angle particles originating on the dayside similarly descend on the night-side, but since they have such large pitch angles, they are not lost.

The above introduces a drift loss cone at high pitch angles at nighttime.

High Pitch Angle Particles


Low pitch angle particles travel long distances along a field line; the second adiabatic invariant is important for these particles.

They try to stay on field lines whose lengths are about the same for a given BM .

At a given equatorial distance fromEarth, day-side field lines are longerthan night-side field lines.

A low pitch angle particle on the(mainly outer) day-side field lines, when drifting over to the night-side, will seek higher and higher field lines.

These particles can find themselveson open field lines on the night-sideor be lost by other processes.

The above introduces a drift loss cone at small pitch angles at daytime.

Low Pitch Angle Particles


REGIONS OF STABLE ANDPSEUDOTRAPPING

• The stable trapping region is defined asthe volume within the Earth’s magneticfield where particles drift on closedpaths around the Earth.

• There are also two regions ofpseudotrapping in which particles candrift for some distance before being lostto the magnetosheath

• The nightside pseudotrapping region iscreated by large pitch angle particlesfollowing constant B contours

• The dayside trapping region is createdby small pitch angle particles that arelost onto open field lines of the tail lobe

• The size and locations of these regionschanges as the field changes in responseto the solar wind

Pseudo-Trapping Regions; Shell-Splitting

Thus, there are some regions from which trapped particles are unable to make a complete circuit of the earth, being lost en route in the magnetotail or beyond the magnetopause. These are called the pseudo-trapping regions.

This phenomenon is referred to as shell-splitting.

These particles, with highpitch angles beyond ~7 REare lost on the day-side

These particles,with smallpitch angles,and mirroringat low altitudes,are lost on thenight-side

These particles, with high pitch angles, descend on the night-side, but since they have such large pitch angles,they are not lost,and return.

These particles return to the night-side


Third Adiabatic Invariant

The third adiabatic invariant, or flux invariant, states that the magnetic flux enclosed by the charged particle longitudinal drift must be a constant:

(This is analogous to the application of Faraday’s law on p. 4, except in this case is due to longitudinal drift of particle)

In other words, as B varies (with longitude), the particle will stay on a surface such that the total number of field lines enclosed remains constant.

However, since most temporal fluctuations in B occur over time scales short compared to the longitudinal drift period (~ 30-60 minutes), the assumptions underlying this invariant law are usually not obeyed.

t

B B d

A const


Violations of the Invariant Constraints

The adiabatic invariants are said to be violated when electric or magnetic field variations take place near or above the adiabatic motion frequency in question, i.e.,

1/Tgyro, 1/Tbounce, 1/Tdrift

For instance, violation of the third invariant permits transport of the particles across field lines. If these violations occur frequently enough, in a statistical sense the net result can be thought of as radial diffusion.

Similarly, the paths of radiation belt particles are affected by collisions with neutral atoms and by E-M interactions of plasma waves. On time scales short compared to Tgyro and Tbounce, these interactions manifest themselves statistically in what is called pitch angle diffusion. (leading to diffusion into the loss cone.)


Radial diffusion transports radiation belt particles across the di-polar-like magnetic field lines in the radial direction.

Pitch angle diffusion alters the particle pitch angle (or equivalently, the mirror point location).

In both cases the earth's atmosphere is a sink; for radial diffusion by transport to very low L-shells, and for pitch angle diffusion by lowering the mirror points into the atmosphere.

A conceptual representation of pitch angle and radialdiffusion in Earth’s radiation belts. Diffusion occursin either direction, but in most cases there is a netdiffusion flux towards the atmosphere, because thatis where the net sink is.


Obviously trapping is not perfect, and there exist mechanisms for introducing particles into the radiation belts, as well as loss mechanisms. Before discussing these mechanisms, let us get a rough idea of the distributions of particles and their energies.

The Explorer I spacecraft carried a geiger counter to measure cosmic rays. However, there were times when the counter became saturated, and Van Allen and his group correctly concluded that this was the result of energetic particles. On the basis of these measurements, the 'radiation belts' were Defined, at that time consisting of an inner zone and an outer zone.


A Schematic View of the Locations of Radiation Belts

• Blue: inner belt, >100MeV protons, rather stable

• Purple: outer belt, 100s keV and MeV electrons and ions, not stable at all

• Slot region in between• Yellow: ACRs, stable• White line: Earth’s magnetic

field, approx. by a dipole field


Asymmetrical magnetic field and SAAdrift loss cone

The Earth’s magnetic field azimuthally asymmetrical with internal and external factors.

The internal magnetic field is sometimes approximated as an off-centered and titled dipole.

The magnetic field strength is much weaker at the south Atlantic area, called the South Atlantic Anomaly (SSA). It is a large sink of radiation belt particles. It results in the drift loss cone in the particle pitch angle distribution.


SAMPEX measured Anomalous Cosmic Ray Particles (Oxygen Nuclei, >200 keV/nucl)


Aerospace EnvironmentASEN-5335

• Instructor: Prof. Xinlin Li (pronounce: Shinlyn Lee)• Contact info: e-mail: [email protected] (preferred) phone: 2-3514, or 5-0523, fax: 2-6444, website: http://lasp.colorado.edu/~lix• Instructor’s office hours: 9:00-11:00 am Wed at ECOT 534;

before and after class Tue and Thu.• TA’s office hours: 3:15-5:15 pm Wed at ECAE 166

• Read Chapter 4&5 and class notes• HW4 due 3/13• Quiz-4 today, close book.

mailto:[email protected]

http://lasp.colorado.edu/~lix


Sources and Sinks of Radiation Belt Particles

The following processes are involved:

Injection of charged particles into the trapping region

Radial diffusion or radial transport within the region

Local acceleration of particles to high energy

Loss processes removing particles from the trapping region (loss through magnetopause and loss by precipitating into atmosphere).

Inner Zone < 2.5 RE

Production: Galactic cosmic-ray proton impinge on neutral atoms neutron decay (half life time ~ 10 min) proton and electron.

Loss Mechanisms: Coulomb collisions loss cone scattering

Charge exchange Energetic neutron escapes

(H+ energetic + H Henergetic + H+)

These processes explains well the inner proton belt.


There are 3 types of cosmic rays of interest here:

Galactic Cosmic Rays

Anomalous Cosmic Rays

Solar Energetic Particles


Cosmic Rays

Cosmic rays are high energy charged particles, originating in outer space, that travel at nearly the speed of light and strike the Earth from all directions. Most cosmic rays are the nuclei of atoms, ranging from the lightest to the heaviest elements in the periodic tables, but dominated by protons (89% hydrogen, 10% helium, and about 1% heavier elements). Cosmic rays also include high energy electrons, positrons, and other subatomic particles.

The term “cosmic rays” usually refers to galactic cosmic rays, which originate in source outside the solar system, distributed throughout and possibly beyond our Milky Way galaxy.

Discovery and Early Research: Cosmic rays were discovered in 1912 by Victor Hess, when he found that an electroscope discharged more rapidly as he ascended in a balloon. He attributed this to a source of radiation entering the atmosphere from above, and in 1936 was awarded the Nobel prize for his discovery. For some time it was believed that the radiation was electromagnetic in nature (hence the name cosmic “ray”). However, during the 1930’s it was found that cosmic rays must be electrically charged because they are affected by the Earth’s magnetic field (How was this known?).

Studying the energetic particle population is very important for two reasons:

These particles represent considerable hazard for both humans and radiation-sensitive systems in space, because they can penetrate through large amount of shielding materials.

They carry information about the large-scale properties of the heliosphere and the galaxy.


Cosmic Ray Energies and Acceleration

The energy of cosmic rays is usually measured in units of MeV and GeV. Most galactic cosmic rays have energies between 100 MeV (corresponding to a velocity of protons of 43% of the speed of light) and 10 GeV ( 99.6% of the speed of light). The highest energy cosmic rays measured to date have had more than 1020 eV, equivalent to the kinetic energy of a baseball traveling at about 100 mph!

It is believed that most galactic cosmic rays derive their energy from supernova explosions, which occur approximately once every 50 years in our Galaxy. To maintain the observed intensity of cosmic rays over millions of years requires that a few percent (even >10%) of the more than 1051

ergs released in a typical supernova explosion be converted to cosmic rays.

The energy contributed to the Galaxy by cosmic rays (~1eV/cm3 ) is about that contained in galactic magnetic fields, and in the thermal energy of the gas that pervades the space between the stars,


Cosmic Rays in the Galaxy

Because cosmic rays are electrically charged they are deflected by magnetic fields, and their directions have been randomized, making it impossible to tell where they originated. However, cosmic rays in other regions of the Galaxy can be traced by the electromagnetic radiation they produce. Supernova remnants such as the Crab Nebula are known to be a source of cosmic rays from the radio synchrotron radiation emitted by cosmic ray electrons spiraling in the magnetic fields of the remnant. Show above is an image of the Crab Nebula

in the X-ray band. In the center lies the powerful Crab pulsar, a spinning neutron star with mass comparable to our Sun but with the diameter of only a small town. The pulsar expels particles and radiation in a beam that sweeps pass the Earth 30 times/sec. The supernova that created the Crab Nebula was seen by ancient Chinese astronomers and possibly even the Anasazi Indians in 1054, perhaps glowing for a week as bright as the full moon.

Observations of high energy (10 MeV – 1000 MeV ) gamma rays resulting from cosmic ray collisions with interstellar gas show that most cosmic rays are confined to the disk of the Galaxy, presumably by its magnetic field. Observations show that, on average, cosmic rays spend about 10 million years in the Galaxy before escaping into inter-galactic space.

ASEN 5335 Aerospace Environment -- Radiation Belts 38 kinetic energy (MeV/nucleon)

100

10-1

10-2

10-3

10-4

10-5

10-6

10-7

101

SSMIN

SSMAX

101 102 103 104 105 106

Differential Flux

Particles

m2 ssr(MeV / nucleon)

The figure to the right Illustrates differential energy spectra for GCR outside the magnetosphere at maximumand minimum solar activity.

GCR typically consists of 108-109 eV particles, but some GCR particles can have energies as high as 1020 eV.


Cosmic Rays in the Solar System

Just as cosmic rays are deflected by the magnetic field in interstellar space, they are also affected by the IMF embedded in the solar wind, and therefore have difficulty reaching the inner solar system. Spacecraft venturing out towards the boundary of the solar system have found that the intensity of galactic cosmic rays increases with distance from the Sun. As solar activity varies over the 11 year solar cycle the intensity of cosmic rays at Earth also varies, so does the inner radiation belt particles, in anti-correlation with the sunspot number. Ionosphere expansion also plays a key role …..

SAMPEX measured protons (19-27.4 MeV) and the sunspot numbers

The Sun is also a sporadic source of cosmic ray nuclei and electrons that are accelerated by shock waves traveling through the corona, and by magnetic energy released in solar flares. The solar particle events are more frequent during the active phase of the solar cycle. The maximum energy reached in solar particle events is typically 10 to 100 MeV, occasionally reaching 1 GeV (roughly once a year) to 10 GeV (roughly once a decade). Solar energetic particles can be used to measure the elemental and isotopic composition of the Sun, thereby complementing spectroscopic studies of solar materials.


Effects of Starfish lasted until the early 1970’s

Telstar wasLaunched 1 day afterStarfish, andwas the firstsatellitefailure due to radiationexposure.Telstar receiveda totalradiationdose 100times thatexpected.


Anomalous Cosmic Rays (ACRs) Since the beginning of the space age, it

was known that two main sources of energetic particles that pervade the interplanetary space:

(a) Galactic Cosmic Rays (GCR), originated from supernova explosions, which occur approximately once every 50 years in our galaxy.

(b) Solar Energetic Particles (SEP) , from solar flares or CME.

So the Anomalous Cosmic Rays (ACRs) belong to neither of them by definition.

In 1973, the anomalous excesses of several elements in low-energy cosmic rays led to the discovery of this so called ACRs.

For Examples: O/C > 30, He/H>1

Fisk et al. [1974] proposed the origin of the ACRs.


Origins of ACRs


Origin of Anomalous Cosmic Rays


Trapping ACRs

ACRs singly charged, picked up by solar wind, heading to the termination shock, where some of them can be further energized and some of them come back.

Gyroradius inversely proportional to the number of the charge. A singly charged ion can be further stripped of its electrons when it happens to skim the Earth’s atmosphere, the gyroradius is reduced many times and the ion can become trapped.

This scenario was predicted far in advance [Blake et al., 1978].

First evidence from Russian COSMOS satellites.

SAMPEX pin pointed the location of the narrow belt of ARCs.


Solar Cycle Variations of ACRs


Sinks of ACRs

Loss Mechanisms: Coulomb collisions loss cone scattering

Charge exchange Energetic neutron escapes

SAA is the largest sink.


The SAMPEX (Solar Anomalous and Magnetospheric

Particle Explorer) Satellite Mission

Launched July 3, 1992; Polar elliptical non-Sun-synchronous Orbit; inclination 82°, 520 km x 670 km (now at about 450x550)

SAMPEX was designed to study energetic particles in the geospace environment, including GCR and ACR.

SAMPEX discovered a new "radiation belt" consisting of trapped Anomalous Cosmic Ray particles.

The ACR of order > 15 MeV are capable of penetrating farther into the magnetosphere than the multiply-charged GCR of similar energy. They then become trapped. (The GCR stop at higher L-shells and enter the atmosphere)

The ACR belt is located around L = 2, or within the inner van Allen proton radiation belt.


THE FOLLOWING FIGURE ILLUSTRATES THE GEOGRAPHICAL DISTRIBUTION OF OXYGEN IONS DETECTED BY SAMPEX,

WHICH CONSIST OF THREE DISTINCT POPULATIONS:

1. For latitudes > 60° there is a mixture of GCR and ACR which have directly penetrated the magnetic field.

2. Between 50° and 60° there is a mid-latitude component composed of multiply-ionized ACR, and also possiblely highly-charged GCR. The cutoff latitude is strongly dependent on the charge states.

3. A 8000 km-long band southeast of the South Atlantic Anomaly (L is about 2 here).

Note: The following figure is given in terms of cutoff energy. For cosmic rays to reach a spacecraft in Earth orbit, they must penetrate the Earth’s magnetic field, which tends to deflect the (charged) particles. However, this tendency is opposed by the energy of the particles as they move at high velocity towards the Earth. A particle’s penetrating ability is determined by its momentum divided by its charge, and this quotient is referred to as its ‘magnetic rigidity’. A cosmic ray will require a minimum magnetic rigidity to reach each point within the Earth’s magnetic field. Particles below the minimum will be deflected and this minimum is called the geomagnetic cutoff value.


SAMPEX Observations of Untrapped Oxygen Ions

This figure illustrates one of the important causes of spatial variability of cosmic radiation -- the geometry of the terrestrial magnetic field.

There isalso aheight-dependent shielding provided by the atmosphere.


Outer Zone L ≥ 3 RE

Source : Most probably solar wind/magnetospheric particles (in the case of H+) which have undergone acceleration, for instance in the tail region.

Indirect evidence for this lies in the strong correlation with solar activity (see following figure).

-- In the case of O+, source is probably the ionosphere

Loss Mechanism : Pitch angle diffusion, i.e., plasma waves cause violation of the first adiabatic invariant, implying diffusion into the loss cone and entry into the atmosphere.

Note: High-energy protons are not found in the outer belts because their gyroradii mv/qB are very large (100’s to 1000’s of km), and at the mirror points the gyroradii are large enough to bring them into the atmosphere. Only low-energy protons can remain at high altitudes. The high-energy protons only remain trapped where B is large.


Outer Radiation Belt Observations by SAMPEX (Solar Anomalous and Magnetospheric Particle Explorer)


(Courtesy of M. Looper)


ENERGETIC PARTICLE/RADIATION NOMENCLATURE

There exist several ways to express particle flux (J):

J(E) = unidirectional differential intensity

(particles/cm2/s/str/MeV)

= flux of particles (# / time) of a given energyper unit energy level in a unit solid angleabout the direction of observation, incidenton a unit area perpendicular to the direction of observation.

J(>E) = unidirectional integral intensity

steradian (sr) = angle subtended at the center of a sphere of unit radius by unit area of the surface of a sphere = unit of solid angle.

The solid angle encompassing all directions at a point is given by the total area of a circumscribed sphere 4r2 divided by the radius squared, or 4 sr.


Very often, the omnidirectional fluxes are expressed as

or

or

where Eo = spectral e-folding energy and spectral index. These representations will be used later when we discuss shielding.

J(E) JoE

J Jo exp EEo

J( E) JoE 1

1

J JoEo exp EEo


J(>E) = unidirectional integral intensity

= intensity of particles with energy greater than a threshold energy E

= = particles/cm2/s/str

Omnidirectional intensities are J(E) or (J>E) integrated over 4 steradians solid angle.

J (E)dEE

Omnidirectional Units

J(E)particles /cm2/s/MeV

(J > E)particles/cm2/s

E E

Differentialenergy spectrum

Integralenergy spectrum


The trapped flux environment specification modelscurrently in use at NASA are

-- AP8MAX, AP8MIN: proton models, solar max/min

-- AE8MAX, AE8MIN: electron models, solar max/min

These models are based largely on satellite data taken between 1960 and 1970; consequently, given the secular variation in earth's magnetic field, one must use the proper epoch magnetic field, i.e.,

-- IGRF 1960 80-term model for SSMIN

-- Hurwitz USCGS 1970 field for SSMAX

Due to the tendency to obey the adiabatic invariants, the two parameters B and L (or equivalently, the invariant latitude) form a convenient 2-dimensional space upon which to map 3-dimensional particle flux distributions. Some examples from the AP8 and AE8 models follow.


Invariantlatitude


The AP8 and AE8 Models also contain some local time dependencies; although these were derived by "binning" data from several satellite missions, there are many regions of spotty data coverage.

The "uncertainty" generally attributed to these models is about a factor of two for 2 to 5 year averages.

It is important to note that these models represent average statistical distributions, and there exists much variability about such mean values.

The following figure illustrates about a year's worth of hourly 1.9-MeV omnidirectional electron fluxes at L=6.6 at the equator during midnight.

-- Note the variation about the AE Model value (represented by the horizontal line)

-- Note the bias towards the large magnetic storms,as a result of averaging fluxes for the model


The following figure illustrates 10-day average electron fluxes for energies > .28 MeV, for several L-values.

Inside L=1.8 (generally referred to as the inner electron zone) the time variations are quite small, demonstrating the usefulness of an average model in this region.

In contrast, outside L = 1.8, the fluxes at L = 2.2 vary greatly with time due to geomagnetic activity.

The steady decay of flux levels in this figure is due to the decay of residue from the artificial Starfish injection event (nuclear explosion) of July, 1962.

-- 1.4 megaton nucelar explosion 248 miles over JohnstonIsland on July 9, 1962

-- widespread aurora occurred in the central Pacific

-- within a few days, the trapped energetic particles damagedsolar panels on several weather and communications satellites

-- within 7 months, Starfish destroyed 7 satellites due to solarcell damage


Effects of Starfish lasted until the early 1970’s

Telstar wasLaunched 1 day afterStarfish, andwas the firstsatellitefailure due to radiationexposure.Telstar receiveda totalradiationdose 100times thatexpected.


The CRRESRAD Model

Based upon the CRRES (Combined Release and Radiation Effects Satellite) Mission.

-- complete complement of radiation environment sensors (dosimeter, e- and H+ monitors, etc.)

-- July 25, 1990 - 12 October 1991; 18.1°, 350 km x 33,000 km orbit (a.k.a Geotransfer orbit)

PC- based software program that provides estimates of the dose behind 4 shielding thicknesses of hemispherical aluminum shielding (0.57, 1.59, 3.14, and 6.08 gm/cm2) --- corresponding to electron energies > 1, 2.5, 5, and 10 MeV and proton energies > 20, 35, 52, and 75 MeV. Different levels of magnetic activity are included.

Comparisons indicate that in some cases the differences between CRRESRAD and equivalent results for AP8 and AE8 can be substantial (see following figures).


Comparison of dose rate along magnetic equator as a function of L for quiet and active CRRES dose models and for AP8MAX.

Due to variableProton belt L = 2- 4In CRRES, not in AP

L-Shell (in RE)


Comparison of dose rate along magnetic equator as a function of L for quiet and active CRRES dose models and for AP8MAX/AE8MAX.

Absence of > 5 MeVElectrons in AE model

L-Shell (in RE)

ACTIVE CRRES MAP


Near the coast of Brazil, a decrease in the intensity of earth's magnetic field exists called the South Atlantic Magnetic Anomaly. This causes an increase in the energetic particle fluxes encountered, for instance, at LEO. Proton fluxes near 300 km associated with the anomaly are shown in the following figure, with the ground track of a 30° inclination satellite superimposed.

Since the magnetic field is weaker, and particles mirror at a constant magnetic field strength, these particles find themselves mirroring at much lower altitudes in this geographical region.


particles from the low-altitude extension of the radiation belts (or "horns") are apparent at high latitudes.

In addition to the SAA,in LEO, radiation beltparticles are also encountered athigh latitudes.


As discussed previously in connection with solar flares, a few times each solar cycle, SCR's consisting of electrons, protons and heavy nuclei can achieve energies of 107 - 109 eV (10-100 MeV) during large flares.

SOLAR ENERGETIC PARTICLES

Flare-associated energetic particle events are associated with a number of descriptive phrases:

solar cosmic rays (SCRs) solar proton events solar electron events polar cap absorption (PCA) eventsground-level events (GLEs)


Exposure to GCRs/SCRs

-------------------------------------------------------------------------------high-altitude/polar |

geostationary orbits | unattenuated exposure----------------------- |----------------------------------------------low-altitude |polar orbits | intermittent exposure----------------------- |----------------------------------------------low-altitude/low- | well shieldedinclination orbits | up to 10 MeV----------------------- |----------------------------------------------

low-altitude/ | well shielded equatorial orbits | up to 10 GeV

--------------------------------------------------------------------------------

Low-altitude low-inclination orbits experience almost no dose variations due to the strong shielding imposed by the combined effects of the atmosphere and geomagnetic field.NOTE: For very thin shields (< .3g/cm2) trapped electrons are more important than trapped protons (critical during EVAs).NOTE: the very high energy GCR can pass through human tissue with almost no effect. The particles that do pose danger are those that are stopped by tissue. As these particles decelerate, their energy is converted to EM radiation. This radiation can ionize atoms, in a crew member's body for instance.


Contours for electron fluxes above1 MeV at Jupiter

Contours for proton fluxes above1 MeV at Jupiter

Mars and Venus have only some spotty magnetic fields; Mercury has weak magnetic fields, and no radiation belts. All the giant outer gas planets possesses radiation belts.

The Pioneer and Voyager 1 and 2 spacecraft encounters with Jupiter have led to the first model Jovian radiation belt models, below.

>> Earth (~104) for 1 MeV Comparable to Earth for 1 MeV at lower L-shells (RE)

ASEN 5335 Aerospace Environment -- Radiation Belts 94 kinetic energy (MeV/nucleon)

100

10-1

10-2

10-3

10-4

10-5

10-6

10-7

101

SSMIN

SSMAX

101 102 103 104 105 106

Differential Flux

Particles

m2 ssr(MeV / nucleon)

The figure to the right Illustrates differential energy spectra for GCR outside the magnetosphere at maximumand minimum solar activity.

GCR typically consists of 108-109 eV particles, but some GCR particles can have energies as high as 1020 eV.


Cosmic Rays in the Solar System

Just as cosmic rays are deflected by the magnetic field in interstellar space, they are also affected by the IMF embedded in the solar wind, and therefore have difficulty reaching the inner solar system. Spacecraft venturing out towards the boundary of the solar system have found that the intensity of galactic cosmic rays increases with distance from the Sun. As solar activity varies over the 11 year solar cycle the intensity of cosmic rays at Earth also varies, so does the inner radiation belt particles, in anti-correlation with the sunspot number. Ionosphere expansion also plays a key role …..

SAMPEX measured protons (19-27.4 MeV) and the sunspot numbers

The Sun is also a sporadic source of cosmic ray nuclei and electrons that are accelerated by shock waves traveling through the corona, and by magnetic energy released in solar flares. The solar particle events are more frequent during the active phase of the solar cycle. The maximum energy reached in solar particle events is typically 10 to 100 MeV, occasionally reaching 1 GeV (roughly once a year) to 10 GeV (roughly once a decade). Solar energetic particles can be used to measure the elemental and isotopic composition of the Sun, thereby complementing spectroscopic studies of solar materials.


Trapping ACRs

ACRs singly charged, picked up by solar wind, heading to the termination shock, where some of them can be further energized and some of them come back.

Gyroradius inversely proportional to the number of the charge. A singly charged ion can be further stripped of its electrons when it happens to skim the Earth’s atmosphere, the gyroradius is reduced many times and the ion can become trapped.

This scenario was predicted far in advance [Blake et al., 1978].

First evidence from Russian COSMOS satellites.

SAMPEX pin pointed the location of the narrow belt of ARCs.


Charged Particle Motions in Earth’s Magnetic Field

• Gyromotion motion: =p2/2mB (1st), T_g~10-3 sec

• Bounce Motion: J= p||ds (2nd), T_b~100 sec • Drift motion: =BdA (3rd) , T_d~103 sec


Equatorial field intensity in recent millenia, as deduced from measurements on archeological samples and recent observatory data.

~10 nT/year

Reversals have been documented as far back as 330 million years. During that time more than 400 reversals have taken place, one roughly every 700,000 years on average. However, the time between reversals is not constant, varying from less than 100,000 years, to tens of millions of years. In recent geological times reversals have been occurring on average once every 200,000 years, but the last reversal occurred 780,000

years ago. At that time the magnetic field underwent a transition from a "reversed" state to its present "normal state".

~10 nt/year

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