JOURNALOF GEOPI-IY$ICALRESEAR½I-I VOL. 70, NO. 13 JULY 1, 1965 The Responseof High Altitude Ionization Chambers during the 1954-1965 Solar Cycle R. ]H].CALLENDER, J. R. MANZANO,1 AND J. R. WINCKLER Schooloi Physicsand Astronomy,University of Minnesota,Minneapolis Abstract. The responseof an integratingionization chamberat 10 g/cm•' depth in the atmosphereto particlesof various rigidities is evaluated by using the changeof ionization with latitude.This procedureyields the differentialresponse curves at solarminimum and solar maximumand also the mean rigidity of responseat any given latitude. For high latitude and Minneapolis,the mean responsesare 2.5 bv and 3.2 by, respectively,at solarminimum and 3.6 and 3.8 at solar maximum. The solar cycle effect at 10 g/cm= is evaluatedusing 250 balloon flightsat Minneapolisand at high latitude. Total ionizationminimum lags about 18 months behind sunspotmaximum. The high altitude ionization when comparedwith neutron moni- tors formsa singlecorrelation curve over the solar cyclewith significantdeviations occurring only for a few monthsin 1957.We concludethat the relativerigidity effectsbetween 3.6 and 15 by are very similar during the decreasingand increasingphases of the solar cycle. Com- parisonof data from ionizationchambers of Pioneer 5 and Mariner 2 with data from de- tectors on earth shows no definite gradient effects. The larger intensity changesshow a correlationbetween earth and deep space,but real fluctuationsof smaller amplitude are frequently not correlated. Introduction. Integrating-type ionization such as the solar cycle and Forbush-type chambershave long beenused for studyingvari- modulations.Also, attempts have been made to ous aspects of energetic particles and other study the spacegradient of primary intensity in radiation both in the high atmosphereon bal- the solar system near the earth's orbit using loonsand, more recently, in free spaceon satel- ionization chambers on space probes [Neher lites and spaceprobes [Winckler, 1960; Neher and Anderson., 1964; Arnoldy et al., 1964]. and Anderson,1962; Arnoldy et al., 1964]. If Since many months of flight data are required, the general nature of the radiation is known, reliability of ion chambersis a great.asset here. then this instrument can give the mean energy If the ionization chamber is exposed to a by measuringthe loss rate by ionization inte- complex radiation environment, a detailed in- grated over the incident spectrum. This in- terpretation usually becomes impossible.For formation has been of use in the study of aurora.1 example, attempts to determine the electron and solar X rays and solar cosmicray protons flux in some parts of the Van Allen radiation at high altitudesand in space [Winckler, 1962; belts have failed because of the mixture of elec- Mosley et al., 1962; Winckler, 1963]. If the trons, X rays, and protons [Arnoldy et al., ionization chamberis supplementedby a Geiger 1962]. Another case is that of an ionization counter having similar stopping power for the chamber on a balloon at high altitude respond- radiation under investigation,then the average ing to galactic cosmicrays. The responseis due ionizationper countis a significant,quantity for partly to direct primaries but also to secondary estimating the mean energy of the radiation particles produced in the atmospherictransi- and, in some cases,its nature [Ho[mann and tion of the primaries, and possiblyto re-entrant Winckler, 1963]. Becausesuch ionization cham- albedo particles. Attempts to unravel these.ef- bers are capableof long-term accuratecalibra- fects and obtain any detailed information about tion and standardization,they are useful for the primary particles from a single total ioniza- studiesof primary cosmicray time variations tion rate measured at high altitudes from a balloon seem useless. • NASA Argentine Space Research Committee However, if high altitude ionization measure- Fellow on leave from the University of Tucuman. ments are made over a range of geomagnetic 3189 3190 CALLENDER, MANZANO, AND WINCKLER latitudes, then with knowledgeof the local mag- where Z is the atomic number of the incident netic rigidity cutoff a responsecurve for the pa.rticle, Px is the cutoff rigidity at the instru- ionization chamber can be constructed. The ment, P is the rigidity, x is the atmospheric ionization ra.te I is related to the primary depth of the ion chamber,and $, (P, x) is its rigidity spectrumD incidenton the earth by the specificyield function. relation: Differentiating with respect,to cutoff rigidity, we obtain the differential responsecurve which is a function of depth, time, and rigidity of the primary particles, but no longer has reference I = •z f• Dz(P,t)Sz(P, x)dP (1) 300 250 ION CHAMBER I0 Mb 1958 • 200 E • 150-- z _o I00-- 50- O0 I 2 5 4 5 6 7 8 9 I0 II 12 13 14 15 16 17 PX GV Fig. 1. Latitude variation of total cosmic ray ionization at 10 g/cm-• atmosphericdepth. 80 ION CHAMBER I0 Mb 60 1958 0 I I I I I I I I I I I I I I 0 I 2 $ 4 5 6 7 8 9 I0 II 12 I$ 14 15 Px GV Fig. 2. Differential responsecurve of an integrating ionization chamber at 10 g/cm2 atmos- pheric depth. This responsecurve is characteristicof sunspotmaximum. IONIZATION CHAMBERS AT HIGH ALTITUDES, 1954-1965 3191 to the geomagneticfield if the Px are correct. is approximatelytrue over a considerablerange of rigiditiesfor protons and e• particles [see OI/OP= - •Z Dz(P,t)S,(P, x) (2) McDonaldandWebber, 1959])• this becomes If the spectrumsof the different Z compo- nentsof the primariesare linearly related (this OI/OP = --D(P, t)$(P, x) (3) 6OO 5OO e\e ION CHAMBER I0 Mb 4OO 300 20O I00 0 I I I I 0 1.0 2.0 3.0 4.0 5.0 Px GV Fig. 3. Latitude variation of 10 g/cm" total ionizationat sunspotminimum and sunspot maximum. 300 •. IONCHAMBER 200 ß • I0Mb •oo • • • 1958 •. o i i i i I 0 1.0 2_.0 $.0 • 4.0 5.0 6.0 , Px •v . Fig. 4. o•mbe• •t •0 •/om • •osp•e•Jo dept•. 3192 CALLENDER, MANZANO, AND WINCKLER / 1955 MINNEAPOLIS --[McDONALD8•WEBBER] 1958 MINNEAPOLIS D58 [McDONALD8•WEBBER] o.ol I I I I I I IIJ I I I I I IIIJ I I I • • •11 0.1 1.0 t0 I00 P GV Fig. 5. The total primary rigidity spectrumat sunspotminimum and sunspotmaximum. Solid curve, direct particle measurements;points on line, solar cycle effect derived from ionization chamber. where all linear factors are now included in the TABLE 1. Mean Rigidity of Response S. (0I/OP will be plotted positive; e.g.,see Fig- ure 2.) BecauseS is not expectedto be time de- Mean Rigidity, bv pendent but to dependonly on the atmospheric Sunspot Sunspot transition phenomena,we can obtain from (3) Min. Max. the relative time variations in the spectrum D Instrument (1954) (1958) once the response curves OI/OP have been measured as a function of time. Ogo-A ion chamber If an ion chamber measurement is made at a (free space) 2.1 2.9 single latitude, the mean or effectiverigidity of High latitude ion primary particles representedby the total ion- chamber (10 mb) 2.5 3.6 ization rate can be computed from the response curve by the relation' Minneapolis ion chamber (10 mb) 3.2 3.8 High latitude neu- tron monitor - (4) (sea level) Approx. 15 Approx. 15 SinceP dependson the spectralshape, it will Evaluation of ion chamber responsecurves. vary, for example, with solar cycle modulation In the ideal case,the responsecurve of a high effects. The entire procedure above is exactly altitude ion chamber should be obtained from analogousto. the treatment of ground level a set of simultaneousreadings distributed from neutron monitor data, where the details of the the equator to the pole at the desired atmos- atmospherictransition processalso are obscure pheric depth (say, 10 mb pressure).An ap- and are never evaluated in detail. The balloon proximation to this is provided by two sets of ionization chambers, however, when flown at observations,one made in 1954 a•d one in 1958 high latitude and high altitude, extend the re- by H. V. Neher and co-workers[Neher, 1956; sponseto considerablylower primary rigidities Neher and Anderson,1962; data for 1958 were than are obtainable even with high latitude obtainedin a preprint from W. R. Webber]. neutron monitors. Cutoff rigiditiesfor the locationof the flights IONIZATION CHAMBERS AT HIGH ALTITUDES, 1954-1965 3193 were obtained using a Cerenkov scintillator a•d Geiger counters carried in balloons to an average altitude of 5 rob. Both the July 1955 and the July 1958 rigidity spectrum curves were made at Minneapolis. Using the previ- ously derived differential responsecurves and 15 the known spectrum of one period, the spec- trum of another period can be found, since I0 Note that the primary spectrum of 1955 has been given, but, as the previous intensity 5 plots have shown, a significant change in the - F IIFEB, 1958 _ primary spectrum from 1954 to 1955 is not, expected to. have occurred. The light curve in - q- - Figure 5 is the spectrum derived using the I000 400 2:00 I00 40 2:0 I0 above formula. The agreementis satisfactory. ATMOSPHERICDEPTH (GM/CM •' ) Mean responserigidity. The meanrigidity P Fig. 6. Normalized rate of ionization chamber of responseof an ion chamber flown at any ascendingin the atmosphere.The histogram repre- latitude can now be computedfrom the differ- sents every pulse of the ionization chamber and ential responsecurves of Figures2 and 4 using therefore showsall the original data complete with equation 4. The results for a number of eases the fluctuations normally seen on such a measure- are shown in Table 1. These include balloon ion ment. The value at 10 mb is estimated by smooth- ing a line through the histogram.