2-1-5 space radiation effect on...

1 Introduction

Solar activity and space environmentalchanges stemming from it are said to exercisevarious influences on human activities. Sincethe hazards posed by such phenomena canalso adversely affect the service life of artifi-cial satellites, caution must always be taken inproceeding with space development. Electron-ics parts used in artificial satellites shouldbasically be those having confirmed reliabilityand validation from various perspectives forspace purposes, and resistance to energeticparticles is a vital point regarding whether anyequipment in question can be used for spacepurposes. Therefore, when consideringwhether to use a part or component for a satel-lite, it is common practice to consider thingswhile paying attention to information aboutthe performance and functions of parts, alongwith their history of implementation in spaceequipment and information about their resis-tance to energetic particles. When any partlacking information about resistance to ener-getic particles is to be newly adopted, anaboveground testing device (accelerator orirradiator) should be used to irradiate anassumed amount of energetic particles expect-ed to be imposed during the part’s design ser-

vice life, with a task being conducted toensure that the equipment functions normally.This entails huge costs and labor, but is con-sidered a mandatory task in order to ensuresatellite reliability. When devising a finalsatellite system design, one should assume aquantity of space energetic particles applied tothe artificial satellite and proceed prudently,while checking based on model calculations orusing a similar means to determine whethereach part of the satellite can withstand thespace energetic particles. If sufficient reliabili-ty cannot be ensured, one must considerchanging parts and adding measures toincrease reliability (such as adopting standbyredundancy and the majority principle).

For the purpose of this paper, “energeticparticles” refer to charged particles and elec-tromagnetic waves having high energy stem-ming from solar flares and other space envi-ronments, and covers high-energy ions andelectrons, gamma rays, and X-rays. Thesehigh-energy particles and electromagneticwaves cause various interactions due to theirhigh energy. Figure 1, for example, showsimages taken of the sun’s external atmosphereby using a corona graph mounted on theSOHO spacecraft launched by the EuropeanSpace Agency (ESA). This telescope is spe-

49AKIOKA Maki

2-1-5 Space Radiation Effect on Satellites

AKIOKA Maki

Solar activity and space environment is considered as fundamental and important factors forspace system design and operation. Space and solar radiation is widely known as a cause ofmalfunction of electronics, surface charging and discharging, and deterioration of materials ofspace systems. Therefore, empirical model of solar activity and radiation is inevitable tools forreliability design of the systems. Observational data and information may be useful for operation.In this article, influence of space environment to space systems are briefly reviewed.

KeywordsSpace environments, Space system, Satellite

50 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009

cially designed to enable the observation oflean gases around the sun by artificially hidingthe sun proper. The series of images producedare likely to show white spots spread all overthe bottom two images. These white spotsindicate the noise generated when high-energyparticles accompanying a solar flare enter theCCD on the imaging plane of the coronagraph. The high-energy plasma particles accel-erated and released together with coronal massejection (CME; a phenomenon whereby solarcorona gases erupt) as shown in these fourimages generate a high level of noise in theCCD, thereby deteriorating the quality ofobservation considerably. The CCD functionsas a detector by reading electrons generated bythe photoelectric effect caused by light strik-ing a semiconductor. Energetic particles enter-ing and passing through the CCD surface willcharge the CCD — a condition that is read anddetected as a false signal. In strong events, theentire surface may become stark white, gener-ating noise that makes almost everything

invisible.Solar energetic particles do not generally

affect humans and aboveground systems,except the passengers aboard aircraft andother limited cases, and people probably neednot be overly concerned about such particles.Outer space, however, always entails issuesregarding the environment of space energeticparticles. As such, being careful about thespace environment is one process that cannotbe avoided to ensure reliability in outer spacesystems, and the advancement of electronicsequipment mounted in recent years raises con-cerns about such systems being easily affect-ed. The effects of the space environment onsatellites entail various mechanisms and aredifficult to describe in a uniform manner. Inthis paper, the author briefly describes themain effects of solar energetic particles andplasma.

Fig.1 Traces of CME observed and solar energetic particles that entered the CCD surface asobserved by the SOHO spacecraft

51AKIOKA Maki

but that it is important to comprehensivelycombine various factors as in a hybrid model,and discuss the issue including the propaga-tion process up through regions close to theearth.

So-called captured particles are thusnamed given their existence in a state wherethese particles are captured in the earth’s mag-netosphere. Two processes (the generation andacceleration mechanisms) have been con-ceived up to the time when the particles arecaptured. One process is considered to be theresult of particles entering the earth’s magnet-ic field from outer space, including those fromsolar flares. The other involves accelerationdue to a turbulence phenomenon such as anauroral storm or MHD waves in the earth’smagnetosphere. The generation and accelera-tion mechanisms require further study, eventhough the subject of much wider discussionthan solar flare particles.

Galactic cosmic rays are high-energy par-ticles entering from outside the solar system,and supernova explosions are considered onesource of those particles. Considering thatthese particles include some having extremelyhigh energy and entail a high percentage ofheavy particles, the action of each particle isstrong but the total quantity of particles issmall.

Normally, when considering the effects ofenergetic particles on a space system, we oftendiscuss and evaluate the issues as divided intothe three categories stated above. A standardengineering model is available for evaluatingenergetic particles. It can be used to performmechanical calculations, but appropriatedesign also requires an understanding of theorigin and characteristics of such particles. Forthat reason, a collection of knowledge andexperience concerning the space environmentnecessary for satellite design has been estab-lished as a standard for all of Europe (ECSSstandard).

2 Energetic particles around theSun and the Earth

Outer space is filled with high-energyenergetic particles stemming from the electro-magnetic interactions of plasma in the sun,interplanetary space, planetary magnetosphereand other regions, and from supernova explo-sions and other phenomena occurring outsidethe solar system. In addition to those generat-ed and accelerated as the result of a solar flare(also known as solar cosmic rays, solar high-energy particles, or otherwise), other particlesaccelerated and captured in particular regionsin the earth’s magnetosphere (otherwiseknown as the Van Allen Belt or radiation belt),and resulting from supernova explosions andother phenomena occurring in outer spacebeyond the solar system are called solar flareparticles, captured particles, and galactic cos-mic rays, respectively. These cannot be con-sidered physically exact classifications, butthese terms are often used on a practical levelfor satellite design. These classifications arethus used here. (Note that many researchersspecializing in the sun and space environ-ments feel particularly awkward about theterm “solar flare particles.”)

Solar flare particles are generated andaccelerated by a solar flare and CME thataccompanies it. Regarding the generation andacceleration mechanisms, previous observa-tions and theoretic studies have proposed vari-ous hypotheses. Still, this can be consideredan issue with much remaining to be elucidat-ed. Roughly divided, two major concepts havebeen discussed: one that holds that particlesare accelerated in the solar atmosphere wherea solar flare is generated, and one that holdsthat generation and acceleration are due toparticle acceleration in shock waves (inter-planetary shocks) generated by CME thataccompanies a solar flare. In recent years, ahybrid model intended to describe observationby combining both concepts has been the sub-ject of active discussions. The author person-ally believes that the issue of solar flare parti-cles is not a matter of selecting either concept,


3 Malfunction of semiconductordevices due to energetic parti-cles

Artificial satellites use computers, memo-ry, gate arrays, and other semiconductor inte-grated circuits for various uses. No artificialsatellite can therefore function without semi-conductor devices. However, highly integratedsemiconductor devices malfunction due to theentry of energetic particles, and this can beconsidered an issue that always accompaniessatellite development. For example, the posi-tion control system of a satellite uses datafrom position sensors as input, processes thatdata onboard by using its dedicated computer,and properly operates the actuator for positioncontrol (such as the reaction wheel andmomentum wheel), thereby maintaining posi-tion. As it were, a system subjected to closed-loop control by using a system consisting ofsemiconductor devices (including sensors) canbe considered the satellite’s position controlsystem. Various problems occur when thepositioning of a satellite cannot be correctlycontrolled, such as cases of insufficient com-munications or the difficulty in properly con-ducting an observation mission. Moreover, ifthe solar cell paddles can no longer be orient-ed toward the sun, the power generated willdecline, resulting in a fatal situation for thesatellite.

The computer mounted onboard a satelliteis equipped with vast amounts of memory.Memory functions correctly when, in additionto what is called data, computer programs,parameters, and other facilities used to controlmemory execution are properly maintainedand updated. However, charged high-energyenergetic particles entering that area willadversely affect the charge status of semicon-ductor devices. In other words, as high-energyparticles enter, the materials and interactionsof such devices will undergo a certain percent-age of charge coming from outside (Fig. 2).This will entail a sudden memory error andcause various malfunctions in the satellite sys-tem, such as computer hang-up. Moreover, a

charge generated will cause continuity in partsthat should be insulated and may result in aphenomenon where current continues to flow(latch-up). If burning occurs due to this con-tinuous flow of current, permanent breakdownmay result. Therefore, devices that do notreadily cause latch-up should be selected.Moreover, in power MOSFET devices,burnout (i.e., burning due to energizationcaused by the generation of charged particles)and gate rupture phenomena as disordersstemming from energetic particles may alsooccur.

Changes in properties due to the ongoingeffects of energetic particles over long yearsare sometimes collectively referred to as the“total ionizing dose effects.” Typical exampleswould be the deteriorating characteristics ofCMOS devices and the deterioration of solarcell paddles. Figure 3 [1] shows some irradia-tion results of gamma-rays generated by acobalt-60 source on a high-performance CPU

Fig.2 Charges in a semiconductordevice due to the entry of high-energy particles

Fig.3 Example of a test of irradiatingenergetic particles on a CPU With a rise in the amount of energetic particlesirradiated on the CPU, current consumptionrises.

53AKIOKA Maki

used for civic purposes. The x-axis representsthe total amount of irradiation (equivalent toirradiation time), while the y-axis representsthe current flowing through the CPU. As theamount of irradiation rises, the amount of cur-rent also rises. Learning the amount of irradia-tion upon reaching the rated maximum currentof a device allows us to obtain a guide fordetermining the total amount of irradiationthat the device can withstand.

This test was conducted to consider thepossible use of the civic CPU as a data proces-sor mounted on a satellite. CPUs intended foruse in space are normally extremely expensiveand often fail to be approved for space purpos-es unless endorsed by flight data results. Thisentails structural problems where the latestdevices cannot be easily applied to outerspace. In other words, in case of a sufficientflight history, the CPUs concerned are likelyto be obsolete. Semiconductor devices thatundergo rapid advances encounter seriousproblems. Despite those concerns, if a high-performance civic device is to be used, thenone must judge whether to use it by conduct-ing tests to check its resistance to energeticparticles, such as a cobalt-60 irradiation testand heavy particle rays irradiation test asshown in Fig. 4.

To examine the resistance of a certainCPU device to energetic particles, the authoronce conducted a test where a comparativelylow-energy proton ray of about 10 MeV was

irradiated. At that time, under normal condi-tions, a cable is extended to another roomwhere the measuring system and data collec-tion system are arranged. However, a PC wasbrought to a location only a short distanceaway from the irradiation line for data collec-tion, and the author had a difficult time takingmeasurements because the PC hung up manytimes. Although the value 10 MeV is a regionwith relatively low energy, the author hasexperienced the occurrence of hang-up with acertain probability, and consequently felt theundesired effects of energetic particles.

4 Electric charging of satellitesand deterioration of membersby energetic particles

The outer space surrounding an artificialsatellite is filled with plasma. Due partly to theaction of electrons (Fig. 5) generated by thephotoelectric effect of sunlight, the surface ofthe satellite is in a state ready to be electricallycharged (surface electric charging). Moreover,comparatively high-energy particles in theradiation belt pass through the surface of thesatellite to reach its deepest areas. These parti-cles then apply an electric charge to the com-ponents and harnesses within the satellite, andmay be discharged for some kind of charge,thereby posing a hazard to the satellite. Toprevent such things from occurring, variousingenious ideas are adopted in the electrical

Fig.4 Status of a test where energetic par-ticles are irradiated on a CPUIrradiation of heavy particle rays (left) andirradiation of cobalt-60 (right)

Fig.5 Electric charging of satellite surfacesdue to the photoelectric effect ofsunlight


design of a satellite, but electrical chargingand the discharge thereof are probably respon-sible for more malfunctions occurring.

Energetic particles may change the materi-al characteristics of the members of a satellite.Examples include changes in the refractionindex and browning (coloring) of the vitreousmaterials of optical equipment. As highlyenergized energetic particles enter, their highmomentum may cause certain changes in theatomic structure of the surrounding materials.This is known as displacement damage and, inthe case of optical vitreous materials, theseparticles can cause such physical properties asthe refraction index to deteriorate (change).The temperature dependence of the refractionindex is one characteristic that cannot beignored in ensuring the performance of high-precision optical systems. Therefore, the pos-sibility of changes in those systems in outerspace over the years must be carefully andproperly considered.

Since vitreous materials include leadoxide, such materials are also known to deteri-orate in terms of transmittivity due to the irra-diation of energetic particles. Figure 6 showshow various materials change as exposed toenergetic particles with human intervention.The five samples shown in the figure wereexposed to the same level of energetic parti-cles and significant changes in the status ofcoloring due to the different types of vitreousmaterials can be seen.

Deterioration and changes in vitreousmaterials used in optical equipment due toenergetic particles may affect ultimate perfor-mance. For that reason, they may become agreat issue in design particularly for tele-scopes mounted in satellites whose mission isto observe astral bodies and the earth, alongwith star trackers (i.e., the bus equipment ofsatellites), earth sensors, and other opticalequipment. As the result of optical designwhere designs are adopted based on vitreousmaterials lacking sufficient information abouttheir resistance to energetic particles, a testusing energetic particles must be conducted bya specific organization on its own. Quartz is

apparently resistant to energetic particles,while flint and other vitreous materials withhigh refraction indexes tend not to be resistantto energetic particles.

Solar energetic particles and space ener-getic particles also significantly deterioratesolar cell paddles. Great care is therefore exer-cised in the design and operation of powersupply systems as well. However, an appropri-ate model must be used to prevent an exces-sive evaluation of the effects of such particles.Regarding the method of evaluating the deteri-oration of solar cell paddles due to energeticparticles, Japan previously proposed a methodbased on data obtained from space environ-mental monitoring equipment mounted on ameteorological satellite, and ISO is now in theprocess of deliberation to make that method aninternational design standard [“Methods forCalculation of Solar Cell Degradation due toEnergetic Particles (ISO Draft)].

5 Conclusion

As electronics parts become moreadvanced along with a rising degree of inte-gration, such parts generally decline in resis-tance to energetic particles. In addition, moredevices now operate at low voltages to reducepower consumption, and this raises anotherconcern from the standpoint of resistance toenergetic particles. Many FPGAs and other

Fig.6 Changes in transmittivity after anirradiation test with energetic parti-cles due to differences in vitreousmaterials

55AKIOKA Maki

parts having many gates are extremely fragileto energetic particles. In order to realizeimplementations as advanced as abovegroundtechnologies in outer space, understanding theeffects of energetic particles becomes increas-ingly important. At present, a standard effortis under way to make designs with sufficientlikelihood while referring to engineering mod-

els. However, in order to enhance the degreeof freedom in design and reduce the cost ofdeveloping satellites and related equipment,technologies must be developed for flexiblychanging the operation status while flexiblyreferring to observation data about the spaceenvironment in real time.

References001 M. Akioka, “Light and Wind from Our Sun”, Gijyutsu-Hyoron-Sha, Co. Ltd, pp. 145–146, 2008. (In

Japanese)

AKIOKA Maki, Dr. Sci.

Senior Researcher, Project PromotionOffice, New Generation NetworkResearch Center

Solar Physics, Optical System, SpaceWeather

2-1-5 space radiation effect on...

Documents