IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 43, NO. 2, APRIL 1996 369

Solar Flare Proton Evaluation at Geostationarv Orbits for Engineering Applications

E. G.‘ Stassinopoulos, G. J. Brucker, D. W. Nakamura, C. A. Stauffer, G. B. Gee, and J. L. Barth

Abstruct- This work presents the results of novel analyses of spacecraft solar flare proton measurements for solar cycles 20, 21, and 22. Solar events and cycles were classified and ranked by flnence and frequency of occurrence, and events were characterized by the mean energy of the proton spectral distributions. Spacecraft observations permitted a detailed study of event characteristics, such as special consideration of solar minimum flares and cycle variability. Tables and curves are presented to allow evaluations of potential threats to spacecraft survivability at GEO, particularly for types of flare environments that emulate solar cycle 22. Upsets for major events are calculated for several Bendel A parameter values and shield thicknesses, and effective energy thresholds of events are determined as a function of these variables. Critical fluence levels, required to cause errors, versus A are presented. SEU’s (single event upsets) of 93L422 devices on TDRS-1 are evaluated for various shielding conditions. Finally, upset dependencies on A and shield thickness are correlated with event fluences for threshold energies of >30, >50, and >60 MeV.

I. INTRODUCTION HE purpose of this work was to develop a method of T practical application for the assessment of effects of

solar flare proton events on electronic systems or devices. This work focused on the geostationary (GEO) orbits and their radiation environment. Because of the large number of expensive communication satellites in this region, it is of particular concern to the aerospace industry.

The data, obtained during solar cycles 20, 21, and 22, were collected from different sources [ I]-[9] and subsequently analyzed and compared. The proton measurements from the GOES series of satellites provided valuable data for this work because of their geostationary location and the time span they cover.

Those satellites, starting with GOES-5 launched in 1986 up to the most recent launch of GOES-8, were equipped with proton detectors covering a range of seven energy channels from a minimum energy of 0.6 to a maximum of 500 MeV. The launch of the GOES-5 satellite was close to the start of solar cycle 22, which now is about over. The major source of flare data prior to the GOES satellites has been measurements provided by the IMP and the OGO series of spacecraft. Some of these data have been used previously by other investigators

Manuscnpt received August 18, 1995; revised November 16, 1995. E. G. Stassinopoulos and J. L. Barth are with NASA-Goddard Space Flight

G. J. Brucker is with Radiation Effects Consultants, West Long Branch, NJ

D. W. Nakamura, C. A. Stauffer, and G. B. Gee are with SES, Inc.,

Publisher Item Identifier S 0018-9499(96)03326-6.

Center, Greenbelt, MD USA

USA.

Greenbelt, MD USA.

(Feynman et al. [I], [2], Stassinopoulos and King [3], and King [4]) to derjve predictive models for the interplanetary fluence of protons with energies > I O to >lo0 MeV. The period of that data base covers the years of 1963-1991.

II. ANALYSIS APPROACH AND METHODS

The intent of this study was initially to analyze only the major flare events that were observed in the vicinity of the Earth during the last three solar cycles (20, 21, and 22). As work proceeded, however, the study was expanded to include all recorded events in that three-cycle time period. The wealth of measurements provided by the proton detectors on the GOES satellites, particularly GOES-7, offered an opportunity to perform detailed analyses of flare events of solar cycle 22. These efforts consisted of identifying cycle 22 events from CD ROM’s containing corrected data provided by Wilkinson [8] and deriving differential energy spectra from the data of six of the detectors’ energy channels. These energy distributions were then used to obtain integral proton fluences for six threshold energies, ranging from 1-100 MeV. Integral proton fluences for cycles 20 and 21 were obtained from published data bases [l], [3], [5], [9] except for the solar minimum year of 1986, which is considered to be the last year of cycle 21 (Shea and Smart [5]). In this case, GOES-5 and -6 data were also considered.

To process the integral proton fluxes, two energy thresholds were selected, namely E > 10 MeV and E > 30 MeV, based on their impact on spacecraft design. These cutoff proton energies are important for the assessment of effects on electronic systems located on the surface or deeper in the spacecraft, respectively.

For the purpose of this paper, an arbitrary grouping and classification scheme of flare activity for solar cycles 20, 21, and 22 was devised based on the levels of integral proton fluences of composite events. These are grouped in bins of order-of-magnitude intensity levels, that is, by severity. Thus, five groups were established for the two selected cutoff energies: “EL” = extremely large, “VL” = very large, “L,” = large, “M’ = medium, and “S” = small. Solar cycles were classified according to the number of “EL” and “VL” events into seven categories ranging from “extremely mild” to “extremely severe,” with two sub-categories each: A and B.

The choice of fluence levels for the five intensity groups and the descriptions for the seven cycle classification categories are arbitrary. However, the distribution of all events for these choices of fluence values and energies was examined to determine the degree of randomness. This analysis indicated

0018-9499/96$05.00 0 1996 IEEE

370 EEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL 43, NO 2, APFUL 1996

I

A

c b

+---solar year

M a l e n d o r veal

g- o

+---solar year

M a l e n d o r year

E e

1107 1 /88 1/89 1/90 1/91 1192 1 /93 1194 1/95 1/96 1/97

-hvsib. 127, 1990

Cycle 20 Nov 1964 June 1976 Cycle 21 July 1976 Sept 1986 Cycle 22 Ocl 1966 7 7 7

E i & L E n r l

Fig. 1. Correlation of solar to calendar years for cycles 2G22.

that the distribution for these choices of level and description were indeed random, and consequently, any other selection would also lead to approximately the same outcome.

Since the differential energy spectra for cycle 22 were available, another potential characteristic of an event's severity and impact on satellite designs was generated, that is, the mean energy of the spectra. To obtain these, calculations utilized the usual general definition of a mean value, namely, a = sg $(E)ESE/ sg $(E)SE where a is the mean energy in MeV, E is the particle energy in MeV, $(E) is the proton distribution in particles/cm2-MeV, and El is the threshold energy.

To obtain fluence-per-event averages for a group of events at a given intensity level, the total fluences for that defined intensity level of every cycle were normalized by the corre- sponding number of events, for the selected threshold energies of E > 10 and E > 30 MeV.

Several tables were generated containing the classification and grouping of events by fluences treated in various ways. The ultimate objective of these efforts was to determine if a practical engineering approach to using solar flare data in spacecraft designs may be accomplished.

For that purpose, several software programs were used to obtain proton upsets from solar flare event fluences. These codes were NOVICE [lo] and COSMIC [11], in addition to both the Bendel one- [12] and two- [13] parameter models, which were utilized in calculating upsets versus shield thick-

Sunsoot Numba Solar Maximum: Feynman et ai., m, Vol. 98, No. AB, 1993

- - Cycle 20: 1968.9 - - Cycle21: 1979.9 - - Cycle 22: 1989.9

ness. In this process, the differential energy spectra from the major solar cycle 22 events were employed, as developed from the analyses of GOES spacecraft data. These environments were the primary inputs to the codes.

HI. RESULTS AND DISCUSSION

Flare events and their integrated proton fluences for four solar cycles (19, 20, 21, 22) have been identified by various investigators. Based on their results, classification of cycles in terms of solar flare proton (SFP) events at 1 AU' in the vicinity of the Earth can be made with some confidence, at least for the last three cycles (20, 21, 22). The sources of data in this investigation were Stassinopoulos and King [3] for cycle 20, Shea and Smart [SI and Feynman et aZ. [l], and Goswami et al. [9] for cycles 19, 20, and 21, and NOAA [SI, which provided the GOES 5 , 6, and 7 measurements for cycles 21 and 22.

Fig. 1 allows a quick correlation between solar years and calendar years for the three investigated cycles. The active years and the quiet years of each cycle are clearly identified. The time of maximum sunspot number (Feynman et al. [l]) and the start and end of each cycle (Shea and Smart [5]) are also indicated. Fig. 1 shows that the time profiles of solar cycles described by calendar years are not the same as solar years. Both types of descriptions have been used by various investigators to sort and present event data. Generally, the electronics community associates solar events with calendar

' AU = astronomical unit.

STASSINOPOULOS et al.: SOLAR FLARE PROTON EVALUATION AT GEOSTATIONARY ORBITS

Group

EL

L M S

371

E > 10 MeV E > 30 MeV

cutoff cutoff Fluence Cvcle 20 Cycle 21 Cvcle 22* Fluence Cycle 20 Cvcle 21 Cycle 22”

(10’”) 1 0 5 (io9) 1 0 8

(107 5 7 8 (lo8) 5 9 5

(108) 10 18 15 (io7) 23 I8 21

(107 __ 1 - 3 19 (104 - 1 22- 20 (103 27 24 24 (IO6) 14 24 24

Total 44 52 71‘* Total 44 51*** 78

TABLE I DEFINITION OF EVENT INTENSITY LEVEL GROUPS, NUMBER OF OBSERVED EVENTS, AND CLASSIFICATION OF SOLAR CYCLES

Defmition of Event GrouDs for ComDosite Events (event-integrated fluences)

$(E> 1 OMeV) $(E>30MeV) EL = extremely large: > 10’’ p/cm* > IOp p/cm2

VL = very large: > io9 p/cm2 > 10’ p/cm2

L = large: > lo8 p/cm’ > io’ p/cm’

M = medium: > IO7 p/cm’ > IO6 pkm’

S = small: > lo6 p/cm* > lo5 p/cm’

Classification of Solar Cvcles in terms of SFP even

1A Extremely Mild A 1B Extremely Mild B 2A VeryMildA 2B VeryMildB 3A MildA 3B MildB 4A AverageA 4B AverageB 5A SevereA 5B SevereB 6A VerySevereA 6B Very SevereB 7A Extremely Severe A 7I3 Extremelv Severe B

E L E L M S 0 4-6 0 >6 1 4-6 1 >6 2 4-6 2 >6

3 4-6 3 >6 4 4-6 4 >6 5 4-6 5 >6

>5 4-6 >5 >6

at the E

> I O MeV

=Cycle 21 = Cycle 20

= Cycle 22

rth*: >30 MeV

= Cycle 21 = Cycle 20

= Cycle 22

* The Occurrence of EL & VL events onlv is used in the classification of solar cvcles 20.21. and 22.

312 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL 43, NO 2, APRIL 1996

TABLE II TOTAL NUMBER OF EVENTS, TOTAL FLUENCES BY EVENT CATEGORY, AND AVERAGE fiUENCES PER EVENT AND CYCLES FOR E >

IO AND E > 30 MEV THRESHOLD ENERGIES BASED ON SOLAR YEARS (**TJE NOTATION a.aaaab IMPLIES a.aaaa x l o b )

Total No. of Events: bv Cateporv U V L L M s - Total

Cvcle20: 1 5 10 27 1 = 44

,Qcle21: 0 7 18 24 3 = 52

Cvcle22: 5 8 15 24 19 = 71

3 -Cycle: 6 20 43 75 23 I = 167 *

Total No. of Events: by Cateaorv E L Y L L M S Total

Cycle20: 1 5 23 14 1 = 44

Cvcle21: 0 9 18 24 0 = 51 *

Cvcle22: 8 5 21 24 20 = 78

3-Cvcle: 9 19 62 62 21 = 173 *

‘Seven (7) events for E > 10 MeV are below cutoff fluence of 1 E6 p/cmsq

*one event has only > 10 MeV data: > 30 MeV data are missine

3 - Crcle 8 3928’O 4 3931” 1 3433” 2 77229 I l505* = 14418” 3 - Cycle 2 9344“ 4 79169 2 0649 2 7743’ 9 85836 = 3 6488”

(b) (e )

E L V L L M - S

0 17571’ 29983’ 34083- 676676

3 - Cvcle 1 3988” 2 19669 3 1240’ 3 6963’ 5 00226

(C)

years, whereas the more correct method to apply would be solar cycle years. For example, it is evident from Fig. 1 for cycle 22 that the calendar year 1987 covers part of a solar minimum year and part of a solar maximum year. Thus, sizable events occurring in a calendar year may actually be within a minimum or maximum solar cycle year.

Table I(a) lists the five intensity level groups of events established for this study, ranging from “S” (small) to “EL” (extremely large), with the corresponding fluence levels for the two threshold energies, each higher level increasing by one order of magnitude in intensity. The definitions of the five levels of event-integrated fluences shown in Table I(a) were selected to cover the ranges from > lo6 to > 1O1O p/cm2 for the total number of events in each cycle with threshold energy >10 MeV. It turned out that for the higher threshold of >30 MeV, the fluence range decreased by about an order of magnitude. Thus, the range of minimum to maximum for the higher energy was >lo5->lo9 p/cm2. Table I(b) contains the number of events in each group defined in Table I(a) for all three solar cycles, including the cycle totals. As stated above, Fig. 1 clarifies the differences that may occur in a determination of the number of events per cycle and fluence

430’ 28632’ 36993’ 43350‘ 44942’

3 - Cycle: 3 26W9 2 5219’ 3 3298’ 4 4747‘ 4 6944‘

(0

level. If calendar years were used, then the values may disagree with other investigations. In this paper, event classification is based on solar years, as listed in Table I(b). Cycle totals sorted by threshold energies of >10 and >30 MeV are not significantly different for cycles 20 and 21, but they are slightly different for the higher energy in the case of cycle 22.

Table I(c) presents the classifications of the three inves- tigated solar cycles. They show that in this scheme, cycle 21 is described as “extremely mild” (class lB), although the number of VL and L events are greater than those for cycle 20 for E > 10 MeV. Of course, cycle 20 experienced one EL event, which is the very well known 1972 flare initially referred to as “Anomalously Large.” This places cycle 20 into the “very mild” (2A) category. In contrast, cycle 22 is in the “very severe” (6B) class for >10 MeV and “extremely severe” (7A) class for >30 MeV as a consequence of its multiple EL events.

Another way to rank the cycles would be by dividing the fluence-totals per cycle by the total-number-of-events per cycle, ~T,”ET, and normalizing by a factor of los for E > 10 MeV and lo7 for E > 30 MeV. Thus, using the data from Table I1 (to be discussed below), one obtains numerical

STASSINOPOULOS et al.: SOLAR FLARE PROTON EVALUATION AT GEOSTATIONARY ORBITS 373

85 80 75

; 55 5 50

8 40 - 35 30

$ 25

15 10

5 0

g 45

5 20

n I 20 21 22

Cycle

I 20 21 22

Cycle

Fig. 2. Total number of events and fluences (for E > 10 and E > 30 MeV) per cycle by intensity leve1.

values to associate with the three cycles:

E > 10 MeV E > 30 MeV Cvcle 21: 3.56 5.75

comparison of flare activity and severity for the three cycles. As pointed out above, the total number of events is greater for cycle 21 than 20, whereas the > 10 MeV fluences are about the same for both cycles. Cycle 22 exceeds the severity of

Cycle 20: 5.07 15.66 both these cycles by a significant factor. Cycle 22: 14.55 34.19 Fig. 3(a) and (b) allows a quick correlation between active

and quiet solar years for the three cycles. As in Fig. 1, the

of each cycle [5] are also indicated. Here, bar charts are generated for every year of each

cycle with specific symbols (patterns) designating the different intensity levels. The increased resolution of these plots takes into account the beginning and termination of each cycle. One further detail in these plots is to identify the nonactive years on either side of the active Period (defined according to [I] as minimum years). For example, cycle 20 shows two minimum years at the beginning of the cycle and three minimum years at the end of the cycle. Thus, cycle 20 lasted for approximately 12 years, cycle 21 covered about 10 years, and for cycle 22,

The ranking Of cyc1es by the ratio Of time of maximum sunspot number [I] and the and end fluence per total-number-of-events is more representative of the true severity of the cycles, and provides a more realistic comparison. But whatever method of ranking is employed, solar flare have great similarity to earthquakes: they can be very mild or very Severe but one never knows beforehand when hey *ill happen or how strong they will be. In both cases, accurate predictions are impossible. ~ 1 1 one can do is study them, evaluate them after they have occurred, and probabilistically make predictions.

The bar charts in Fig. 2(a) and (b) summarize the total number of events and fluences by intensity level for the three cycles and the two energies. These bar charts allow a quick

314

2 4 6 B I O I 2 2 Year In Cycfe 20

IEEE TRANSACI'IONS ON NUCLEAR SCIENCE, VOL. 43, NO. 2, APm 1996

4 6 8 3 0 2 4 6 8 1 0 Year In Cycle 21 Year In Cycle 22

25

25

k

2 5

.a s

0

. . . . . . . . . . . . . . . . . * r i O s 1 ~ . .

. . . . . . . . . . . . . . . . . . . . . . I . . . . . . I . . . . . . . . . . . . . .

44 E v a & + + 78 Events

EL ffxtrernttiy Larwj ............* r 3 OB

VL (Very Large) .... I ...*....... t (tarpel l.._..l_..-..l. ..._...

t I os I a7

M &fMedium) -.._ ...._..........._.. @ > f as

1 2 4 6 8 ' I O 1 2 1 2 4 8 8 1 0 1 . 2 4 6 8 101 Year In Gvcle 20 Year In Cvcfe 21 Year In Cvcie 22

(b)

Fig. 3. Number of events (for (a) E > 10 MeV and (b) E > 30 MeV) by intensity level for cycles 20-22 by solar years. *Sunspot maximum: Cycle 20 = 1968.9, Cycle 21 = 1979.9, Cycle 22 = 1989.9 (Ref. Feynman et al., JGR, Vol. 98, No. A8, 1993). **Seven (7) E = 10 MeV events are below cutoff value: total number of composite events are 174. ***For one (1) event, E = 30 MeV data are not available: total number of composite events are 174.

the end remains to be seen. At this time, it appears that the duration of 22 may be either 10 or 11 years. Although, in the case of the three cycles analyzed in this study, the duration varied from about 10-12 years, cycle lengths could possibly range from 9-13 years. Sunspot maxima dates are indicated in the charts using data from [ 11. There is no way one can

predict with confidence the length of the current or the next solar cycle.

In contrast to this presentation, Fig. 4 is a bar chart showing the proton fluences for each solar year for both >10 and >30 MeV energies. There is an important fact to note in the minimum years. That is, the number of events for the years 11

STASSINOPOULOS et al.: SOLAR FLm PROTON EVAL.UNI0N AT GEOSTATIONARY ORBlTS

I cycle 20 t cycle21 I 1 I O ”

10‘0

104

i o 7

1 06

105

J ” I I 1 I i 5 ! ! I , i , I I I I I I I l r . I I I I I I I

375

2 4 6 8 ‘ 0 1 ~ 2 4 6 8 I 0 1 2 4 6 8 1 0 1 Year In Cycle 20 Year In Cycle 21 Year In Cycle 22 I - - - I_ __ ___I_- __ - -- l__l

Fig. 4. Cycle 22 = 1989.9 (Ref. Feynman et al., JGR, Vol. 98, No. A8, 1993).

Annual fluences (for E > 10 and > 30 MeV) per solar year for cycles 20-22. *Sunspot maximum: Cycle 20 = 1968.9, Cycle 21 = 1979.9,

and 12 (minimum years) in cycle 20 is equal to or greater than the active years four and eight of that cycle, and the fluence for E > 10 MeV protons of minimum year 11 is higher than that of both these active years, while the fluence of year 12 (minimum) is higher than the fluence of year four (active). It is also worth noting that many other minimum years show similar significant activity.

Table II(a) repeats the total number of events by category, for convenience, while Table II(b) contains the total fluences of all events within each event group, and Table II(c) contains these fluences normalized by the total number of events to obtain average-per-event fluences. As pointed out above, cycle 21 is very close in severity to 20 even though it did not experience an EL event. From Table II(b) it can be seen that the total fluence for E > 10 MeV summed over all groups is 2.23 x lo1’ particles/cm2 for cycle 20 and 1.85 x lo1’ particles/cm2 for cycle 21, whereas, for E > 30 MeV in Table II(e), it is 6.89 x lo9 and 2.93 x lo9 particleskm’, respectively.

Table II(c) and (f) indicate that in most cases of event- groups and cycles, the average fluence numbers are equal to or lower than the means of their range: 5 x loN, where the exponent N varies from 5-10, as defined in Table I(a). The only exceptions are the small-event averages of cycles 20 and 21 for E > 10 MeV, and the medium and small averages of cycle 20 for E > 30 MeV.

Another interesting observation is that the three-cycle averages-per-event by intensity level (last row in Table II(c)) are a very reasonable approximation of these events that can

be used for predictive purposes in a suitable model. The maximum difference in any group, for any cycle, is less than 53%, with one exception: the small events for E > 30 MeV (Table II(f)), with a difference of about 86%. This is an indication, at least for these three cycles, that the intensity range of the events is relatively well defined and no extreme variations have occurred so far in the important groups of intensity levels.

Note that in Tables II(b), (c), (e) and (0, four decimal places were deliberately retained to allow summations, interpolations, and calculations of differences. However, the final results are only significant to two decimal places.

Table 111 compares the total number of events and fluences considered in this work to that of [5]. Cycle 19 was included in their work, but not in this paper. The differences in the number of events listed for the two studies are mainly due to selection of some consecutive events as discrete by Shea and Smart, whereas this study counted events by the availability of fluences, which are identified as composite events. However, it appears that the total fluences for the two common cycles and the two limiting energies are in very good agreement.

Fig. 5 contains plots of mean energies 0: (alpha) in MeV versus proton threshold energy for spectra of select events truncated at the plotted energies. These mean energies were calculated from the differential energy spectra derived from the data obtained from the six energy channels of the GOES- 7 spacecraft detectors. Alpha represents the hardness of a spectrum. The larger the value of alpha, the greater the energy

316 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 43, NO. 2; APRIL 1996

160

1 4 0

I20 - - 2 E g 100 W p. (I)

’0 P 5 80

x

C m

f 8

60

40

20 I 1 5 20 25 30 35 40 45 50 55 60

Proton Threshold Energy Of Spectrum (> MeV)

Fig. 5. in cycle 22.

Mean energies versus threshold energies for spectra of selected events

content of the particle distribution. Thus, event spectra of about equal fluences but with high alphas are more of a threat to spacecraft electronics relative to TID (total ionizing dose) and SEE’S (single event effects). It can be seen from Fig. 5 that the May 6, 1990 flare that occurred about two months prior to the launch of the CRRES spacecraft is an event with an alpha of about 100 MeV for a >30 MeV threshold. This event contained a relatively large number of high-energy protons capable of upsetting radiation sensitive memory devices. Al- though the fluence levels were not very high for this May Bare and significantly less than the September 1989 EL flare shown in Fig. 5, its alpha was much higher than that of the EL event (about 60 MeV). The May flare was also identified in another publication [14] as a most probable cause of the apparent residue of the recently rediscovered second proton belt peak, as measured by upsets in test memories on the CRRES spacecraft, launched in July 1990.

As the major objective of this study is to provide a practical engineering approach to predicting upsets in space during solar maximum periods, all events derived from the analyses and described as EL and VL events were included in the calculations of upsets. Fig. 6(a)-(c) are plots of upsets versus the Bendel “A” parameter for a 4-7r spherical shield thickness of 100 mils A1 and for the 11 largest events that occurred in solar cycle 22. The three frames include all events that were classified by their proton fluences of E > 10 and > 30

io3

1 02

1

1 00

1 R-1

10-2

I 0-3

Y l e

Bendel “A’ Parameter

(4

102

1 01

1 00

1 0-1

10‘2

I 0-3

i 0-4

I 0-5

1 0-8 10 15 20 25 30

Bendel “A” Parameter

(b)

102

1 0‘

1 00

IO-’

1 0-2

1 cr3

i 04

1 5-5

1 0-6

1 0 7

10-8 10 1 5 20 25 30

Bendel “A” Parameter

(c)

Fig. 6. 100 mils shield thickness.

Number of upsets versus Bendel A parameters for cycle 22 events:

upsets in Fig. 6(a) cover a wide range of values, from to lo3 SEUshit. event. In view of this range, a memory device sensitive to proton upsets may be defined as “soft” if it has an “A” value 520, whereas a “hard” part is one with an “A” 230 MeV. However, it should be noted that a memory system with a large number of bits may experience many upsets during a solar event, even if it is “hard.” For example, a 16-Mbit device

MeV as EL and VL, as described previously. Note that the with a Bendel “A” parameter of 30 may experience one or

STASSINOPOULOS er al.: SOLAR FLARE PROTON EVALUATION AT GEOSTATIONARY ORBITS 311

I oa

1 02

1 0’

100

1 0-1

1 0-2

I 0-3

10-4

10-5

1 0-6

1 0-7 10 15 20 25 30

Bendel ‘A‘ Parameter

(a)

102

I 0‘

1 00

10-1

1 0-2

103

I 0-4

10-8

10-8 10 15 20 25 so

Bendel ’A’ Parameter

(b)

Bendel ‘A’ Parameter

(C)

Fig. 7. Number of upsets versus Bendel A parameters for cycle 22 events: 300 mils shield thickness.

two upsets from the smaller March 6, 1989 event, but could suffer as many as 640 upsets from the October 30, 1992 event. Clearly, an event similar to the October 30, 1992 flare, shown in Fig. 6(a), would cause a maximum number of upsets for any system and most “A” values.

Fig. 7(a)-(c) illustrate the effects on the number of upsets of an increase in shielding thickness to 300 mils for the same set of events. Obviously, the impact of such a change in

shielding on the number of errors is not very much. Thus, for the worst-case event of cycle 22, the October 30, 1992 flare, and for an “A” value of 15, the decrease in SEU’s is only 27% for this increase in shield thickness of 200 mils. At an A = 30, the corresponding change in the number of upsets is only 67%. Note that the October 1992 event produced a higher number of upsets than the October 1989 event, which heretofore was considered to be the largest flare of cycle 22. These data confirm again that shielding does not provide an efficient method for protecting devices against upset by high- energy protons. If a designer has an “A” parameter and a shielding estimate for his part, close to the values used in this analysis, then he can use Figs. 6 or 7 to predict the number of upsets for any given event. For example, assume A = 15 and a shield thickness of 300 mils Al, and that the integral fluence specification is equal to the value of the October 30, 1992 event. Fig. 7(a) indicates that the number of upsets is one. If the error specification for the memory system calls for no errors and an overdesign or safety factor of 10, then Fig. 7(a) shows that a memory with an A = 17 will satisfy both of these requirements.

Regarding attenuation properties of protons in aluminum, Fig. 8 displays a plot of proton range versus energy. The plot shows that increasing the shield from 100-300 mils truncates a proton spectrum by raising the cutoff energy of the incident spectrum from about 22-40 MeV. Thus, all incident protons with initial energies less than 40 MeV are eliminated by the shielding. Of course, the degradation of high energy protons to lower values can supply particles <40 MeV. The cutoff can be more easily seen from Fig. 9(a)-(c), which contain plots of upsets for truncated spectra of the October 19, 1989 event; that is, the lower energy sections of the input spectra were eliminated from the calculations. The three frames correspond to three “A” values of 5, 15, and 30 MeV for the two basic shields of 100 and 300 mils. The plots illustrate that the number of errors remains constant up to some higher-energy cutoff where the number commences to decrease. This cutoff energy is a function of shield thickness and “A” value. For example, consider Fig. 9(a), A = 5 , shield = 100 mils, where the elimination of all protons of energy equal to or less than 30 MeV does not decrease the errors. However, if the spectrum is truncated at 40 MeV, the upsets start decreasing. For a thickness of 300 mils, the cutoff of the lower energy part of the spectrum has to reach 60 MeV before the SEUs decrease. It is evident that even soft devices ( A = 15) are affected only by protons with energies E > 40 MeV whether behind 100 or 300 mils of shielding.

Fig. 10 shows the proton fluence required to produce one upsethit versus mean energy for a range of “A” values from 7-16. The graph allows the designer of a space system to determine whether a device with a known “A” value will be upset by a specific solar flare characterized by its fluence and mean energy a. The integral fluences of all cycle 22 events are only plotted in Fig. 10 for >10 and >30 MeV threshold energies to provide the reader with information on the severity and frequency of solar flare proton occurrences in that cycle. For example, assume that a 1-kbit part has an “A” of 16 MeV and the designer is required to design a system that should not

318

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$&s

TABLE III COMPARISON OF NUMBER OF EVENTS AND TOrAL mCLE FLUENCES FROM SHEA AND SMAFS AND THtS STUDY

E>lOMeV

- 44

52

TOTAL EARTH-ENCOUNTE RED SOLAR-CYCLE EVENTS -AND PROTON FLjTENCE

@ m d on Solar Years)

E>30MeV

- 9.2 x IO" 1.8 x IO'"

44 51- 1.85 x 10" 2.93 x IO9 ' 1.8 x IO'" 2.8 x lo9

19

20

21

22 71- 78 - 1.03 x 10" 2.67 x 10" - --

* For one (1) event, E >30 MeV data are not available .. Seven (7) E >10 MeV events are below cutoffvalue of 10' $cm2

+ Number of composite events * Number of discrete events

Note: allShea&Smartdataarefromref.[4]

Proton Energy (MeV)

Fig. 8. Proton range versus proton energy.

upset for an exposure to lo9 particles/cm2 and for a threshold energy >30 MeV with a mean energy (Y of 80 MeV. From Fig. 10, it is evident that the critical fluence for a single upset and for a single bit system is lo1' protons/cm2. However, the system in this example is assumed to have lo3 bits. Thus, the critical fluence must be divided by the number of bits to yield

a critical fluence of lo7 protons/cm2 to produce one upset. Since the fluence specification is lo9 protons/cm2, #is p& cannot satisfy the requirements. Event spectra with low values of alpha require higher fluences to produce an upset. Note that for a large memory part, such as lo6 bits, the fluence to cause one upset is reduced by a factor of a million.

STASSJNOPOULOS ef al.: SOLAR FLARE PROTON EVALUAmON AT GEOSTATIONARY ORBlTS 319

Since Stapor et al. [13] have shown that in most cases, particularly for the more modern, small feature size parts, the Bendel two-parameter approach is more correct (better fit to measurements), both the one- and two-parameter methods of calculating upset rates were used to determine and compare the error dependence on shield thickness. Actual space measure- ments of SEU’s obtained from controlled memory bits (i.e.. 93L422 devices) on board the TDRS-1 spacecraft were used to make the determination. Fig. 11 shows the results of this exercise. The 42 and 48 upsets that occurred on Page two of this memory system (Normand and Stapor [15]) are indicated on the plots for the two methods of calculation. The work of Croley et al. [16] claims that 243 SEU’s were observed on TDRS-1, of which 176 supposedly were caused by heavy ions and that only 72 were due to protons. In contrast, the work of Normand and Stapor [15] quotes a corrected total of 239 upsets for the four pages of the memory system on TDRS-1. Their analysis indicates that the most reliable upset data are the 90 upsets from Page two of the memory system. These latter SEU’s were used in the present analysis to create Fig. 11. Normand and Stapor also considered that only four of the total number of SEWS were produced by two single events attributed to heavy ions.

Those investigators found that the best fit of measurements to predictions was obtained for a shield thickness of 500 mils, which is in agreement with the range of 330 to 500 mils shown in Fig. 11 for the two-parameter Bendel model. The shield thicknesses are higher for the two parameter approach than for the one. We believe that the higher shield thicknesses probably represent the actual shielding condition. This investigation is continuing, and the plan is to apply the more complex two- parameter Bendel model to additional, controlled space data as they become available. In addition, the events categorized as S, M, and L will be treated in a similar manner to the EL and VL events in the next study.

During the course of this investigation, the authors were searching for some method of presenting event data combined with upset and shielding data that would simplify and order the information. Figs. 12 and 13 show an attempt to accomplish this objective. Upsets (using the Bendel one-parameter model) versus integral event fluences for threshold energies >30, >50, and >60 MeV are shown in these figures. The value of the “A” parameter is 17 MeV (about the value for the 93L422 device) and the shield thicknesses are 100 and 300 m i l s Al in Fig. 12(a) and (b). Fig. 13(a) and (b) contains similar plots for the same two thicknesses, but now the value of “A” is increased to 30 MeV. The “A” values are selected to represent the “soft” (more sensitive) and “hard” (less sensitive) devices. The two shield thicknesses are chosen since they are the more practical values found in most spacecraft. In terms of shield thickness, whether the value of “A” is 17 or 30 MeV, the thicker shield lowers the number of upsets, as would be expected. Relative to the dependence on “A” value, whether the shield thickness is 100 or 300 m i l s AI, the larger “A” yields a lower number of errors, again as would be expected. The points in Figures 12 and 13 represent the integral fluences of cycle 22 events, given for the above indicated threshold energies.

Bendel Parameter ‘A’ = 5

11 I I I I I I I I ~ ~

“ I I I I I I I I I ! ! I

I I I I I I I I I I I I I I I I

1m I I I I I I I I I I I

Enerav Renee (MeV)

( 4

Fig. 9. Upsets for truncated spectra of the October 19, 1989 event for A values of 5, 15, and 30 MeV and for two basic shields of 100 and 300 mils Al.

Considering the ordering and scatter of the data points in these plots, the overall best fits occur at the higher thresholds of E > 50 and > 60 MeV, and for these energies, the scatter is greater for A = 30 and lower for A = 17. Thus, for the “SOW devices with A values of about 17 MeV, the upsets correlate reasonably well with integral event fluences for the higher-energy thresholds. For the “hard” parts, A = 30 MeV

380

Shield Thickness

( m w 100 300

LEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 43, NO. 2, APlUL 1996

Energy (MeV) 50 > 60

k m k m 4.056”’ 0.9006 2.775.’ 0 8245 7.411‘’2 1.0690 5.399.’’ 0.9949

T-LE N

ON BENDEL A, ENERGY, AND SH~ELD TK[CKNEsS EQUATIONS TO CALCULATE SOLAR FLARE PROTON h U c U , UPSI%S/BIT.EVENI. AND T H E R DEPENDENCE

1 U k$,” I U = numberofpredictedupsetdbit-event

9, k = regression weEcient m = powerexponent

= event integrated integral solar flare proton fluence

Fipure 12:*

Fimre 13:* I A = 30 MeV mendel Parameter) ~ 7

Shield Energy (h4eV) Thickness

(mils] 100 3 00

* Data relate to Figures 12 and 13

and these same energies, the correlation is sufficient to make the methods practical.

In order to provide a practical means of using these data to evaluate future events, straight lines were fitted to the points for each threshold energy. These lines represent the dependence of upsets on integral fluence, as expressed by the eight equations shown in Table IV. An example to illustrate the meaning of these curves is as follows. Assume an integral fluence from an event in the next solar cycle 23 with threshold energy >50 or >60 MeV, and select an appropriate shielding thickness and Bendel parameter close to the values of the plots. Then, insert the integral fluence into the equation corresponding to the chosen parameters and obtain the number of upsets to be expected.

Iv. SUMMARY AND CONCLUSION

The results show that the ranking of cycles and the grouping of events by fluences, and the use of mean energies can be used to evaluate the threat to electronic systems of spacecraft operating in orbits at GEO. Derivation of differential energy spectra for approximately 70 events based on the GOES-7 measurements was accomplished. These spectra were used to generate integral fluences for several limiting energies.

Length of cycles, identification of solar quiet and active years per cycle (as related to calendar years), and severity of quiet year events (solar min years)-reflecting the rela- tive importance of intensity level groups-were addressed. Relevance of event fluences to electronic device errors was established in terms of threshold proton energy and Bendel “A” parameter.

The worst-case flare of cycle 22 relative to generating SEU’s is the October 30,1992 event rather than any of the 1989 flares, particularly the October 19, 1992 flare, which heretofore was considered to be the worst event of cycle 22. Relatively intense and significant flares can occur during “minimum” solar years. The threshold energies that generated the greater number of upsets are the >40, >50, and >60 MeV. Upset data obtained with the 93LA22 memory devices on the TDRS spacecraft during the October 19, 1989 flare suggest that effective shield thicknesses of 330 to 500 mils A1 shielded these memories. It appears that errors in devices with “A” values of 17 and 30 for shields of 100 and 300 mils A1 can be correlated with and ordered by integral proton fluences of solar flare events with threshold energies of >30, >50, and >60 MeV. Descriptions with examples were given to demonstrate the application of the data and the use of the figures.

STASSINOPOULOS et al.: SOLAR FLARE PROTON EVALUATION AT GEOSTATIONARY ORBITS 381

Mean Energy a (MeV)

Fig. 10. Critical solar flare proton fluence versus mean energy for cycle 22, with a grid of Bendel A value curves for E > 10 and > 30 MeV.

Shield Thickness (mils) AI

Fig. 11. Upsets versus shield thickness for Bendel A and A + B of the October 1989 event for TDRS-1 results (determination of effective shield thickness).

382

Bendel “A” = 17: Aluminum Shielding = 100 Mns

IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 43, NO. 2, AF” 1996

Bendel “A‘ = 17; Aluminum Shielding = 300 Mils 100

I 0-4 1 06 i 07 108 1 09 10’0 106 I 07 1 os 1 09 10‘0

integral Proton Fluence (#/cm2) Integral Proton Fluence (#/me)

(a) @)

Fig. 12. Upsets versus integral proton fluences for cycle 22 events: (a) 100 and @) 300 mils, E > 30, 50, and 60 MeV, Bendel A = 17.

Bendel “A” = 30; Aluminum Shlelding = 100 Mils

10-8 106 I 07 108 I 09 I O I O

Bendel ‘A” = 30; Aluminum Shielding = 300 Mils

?io5 m

$ L

2 IO* 5

m m m 3 1 0-7

- -

1 0-a I06 I 07 1 08 1 OS

Integral Proton Fluence (#/cm2) Integral Proton Fluence (#/cm2)

(a) @)

Fig. 13. Upsets versus integral proton fluences for cycle 22 events: (a) 100 and @) 300 mils, E > 30, 50, and 60 MeV, Bendel A = 30.

Finally, calculations using the materially attenuated solar flare spectra have confirmed again that shielding does not provide an efficient method for protecting sensitive devices

spheric Administration, Boulder, CO, 1994. PI J. N. Goswami, R. E. M c G u h R. C. Reedy, D. L d and R. Tna, ‘‘Solar

flare protons and alpha particles during the last three solar cycles,” J. Geophysical Research, vol. 93, no. A7, pp. 7195-7205, July 1, 1988.

[lo] T. J. Jordan, “NOVICE A Radiation Transport/Shielding Code, User’s against upsets by high-energy protons.

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J. Feynman, T. P. Armstrong, L. Dao-Gibner, and S. Silverman, “New interplanetary proton fluence model,” J. Spacecraft and Rockets, vol. 27, no. 4, pp. 403410, July-Aug. 1990. E. G. Stassinopoulos and J. H. King, “Empirical solar proton model for orbiting spacecraft applications,” IEEE Trans. Aerospace and Electronic Systems, vol. AES-IO, no. 4, pp. 442450, July 1974. J. H. King, “Solar proton fluences for 1977-1983 space missions,” J. Spacecraft and Rockets, vol. 11, no. 6, pp. 401408, June 1974. M. A. Shea and D. F. Smart, “A summary of major solar proton events,” Solar Physics, vol. 127, pp.’297-320, 1990. -, “Significant proton events of solar cycle 22 and a comparison with events of previous solar cycles,” Advance Space Research, vol. 14, no. 10, pp. 631-638, 1994. __, “Recent and Historical Solar Proton Events,” Radiocarbon, Vol. 34, No. 2, pp. 255-262, 1992. D. Wilkinson, Private Communication, Solar-Terrestrial Physics Divi- sion, National Geophysical Data Center, National Oceanic and Atmo-

pp. 13281-13294, Aug. 1, 1993.

Guide,” E.M.P. Consultants Report, Jan. 1990. [ll] R. A. Reed, P. T. McNulty, W. J. Beauvais, W. G. Abdul-mer, E.

G. Stassinopulos, and J. L. Barth, “A simple algorithm for predicting proton SEU rates in space compared to the rates measured on the CRRES satellite,” IEEE Trans. Nucl. Sei., vol. 41, no. 6, pp. 2389-2395, Dec. 1994.

[12] W. L. Bendel and E. L. Petersen, “Proton upsets in orbit,” IEEE Trans. Nucl. Sci., vol. NS-30, no. 6, pp. 4481-4485, Dec. 1983.

[13] W. J. Stapor, J. P. Meyers, J. B. Langworthy, and E. L. Petersen, ‘Two parameters bendel model calculations for predicting proton induced upset,” IEEE Trans. Nucl. Sci., vol. 37, no. 6, pp. 19661973, Dec. 1990.

[14] E. G.Stassinopoulos, G. J. Brucker, C. A. Stauffer, and A. Meulenberg, “Chronology of generation, variation and disappearance of a second proton belt as determined by effects in electronic devices,” IEEE Trans. Nucl. Sci., vol. 42, no. 6, Dec. 1995.

[15] E. Normand and W. J. Stapor, “Variation in proton-induced upset rates from large solar flares using an improved SEU model,” IEEE Trans. Nucl. Sei., vol. 37, no. 6, pp. 1947-1952, Dec. 1990.

[16] D. R. Croley, H. B. Garrett, G. B. Murphy, and T. L. Garard, “Solar particle induced upsets in the TDRS-1 attitude control system RAM during the October 1989 solar particle events,” ZEEE Trans. Nucl. Sei., vol. 42, no. 5, Oct. 1995.