Aug 1, 1997 - DepOt of Physics and Astronomy, University of New Mexico, Albuquerque ..... Embudo (27.3 (meters of water equivalent) and Socorro (New.
JOURNAL OF GEOPHYSICALRESEARCH,VOL. 102,NO. A8, PAGES17,433-17,443, AUGUST 1, 1997
Transverse cosmicray gradientsin the heliosphere and the solar diurnal anisotropy H. S. Ahluwalia and L. I. Dorman •' 2 DepOt ofPhysics andAstronomy, University of NewMexico,Albuquerque
Abstract.We describe a newscheme forcomputing theasymmetric partofthecosmic rayparticle
density gradient. Themethod makes useoftheecliptic components (A,, A ,) ofthecosmic raysolar diurnal anisotropy observed witha detector onspinning Earthandtheinterplanetary magnetic field sectorstructure in theheliosphere. We applythismethodology, tocompute themagnitudes and directions oftheasymmetric (G0•)particle density gradient inthehellosphere forselected intervals of timeduring the1965to 1993period.Datafromsixdetectors oftheglobalnetwork areusedforthis purpose. Theirmedian rigidityofresponse (P•) covers thefollowing range:10GV • R• _ 0 and(c) and(d) q A < 0, drivenby asymmetrictransverse gradientsandinterplanetary magneticfield (IMF) sectorstructureexistingin the heliosphere at a giventimeperiod. streaming observed by thedetectors onrotatingEarth,for the two magnetic polarity epochs in the solar northern hemisphere.The situation for q A > 0 epochis shownin Figures1a and1b, andthatfor q A < 0 epochis shownin Figureslc and1d,corresponding to northwardandsouthward
represent thecontributions fromthediffusionprocess to the diurnal anisotropy. One could use (7) to study Go contributions to the north-south anisotropy.S•larly, the differenceterms for the case of asymme•c gradients (G o=Go•)maybe writtenas
pointingasynnnetric gradients, respectively.The proton trajectoriesshownin (i), (ii), (iii), and(iv) are drawnin a planenormal totheHNCS. Thecorresponding contributions tOs_,olar di•umal anisotropy areshownbythedashed vectors
Ar*-Ar-=3 K.Goosinq• (9) 2
v
B x G inFigures 1b and1d. Forthiscase, K, changes sign acrossthe HNCS becausethe IMF polaritychanges. However,theparticledensitygradient doesnotchangesign
A0*3I•Grsino+K*G* 2Ao=-•[ cosO] (10)
across theHNcs. Hence theproduct K, Gochanges sign across theHNCS in (1) and(3). Thereforeonemayrearrange
(1),(2),and(3) as"sums" and"differences" of theamplitudes
oftheradial(A,), north-south ( A 0), andeaSt-west ( A ,) anisotropies. Thesumtermsare
A/
•
__3 [CV-I• G,+{K,,-K, )GOsinq• cos,](6)
2
v
rr G
cos
v
A0*+A0- 3 2
--
v
One notesthat (9), (10), and(11) representthe off-ecliptic
tothediurnalanisotropy. Our analysis is carded (7) contributions out by averagingdataover severalsolarrotations.In this
approximation, termscontaining theazimuthal gradient G,
A** +A*=--3v [K,,-Ki) Gr sinq• cosgt-K•G,] 2
(8)
may be droppedin (6), (8), and (10). Furthermore, our analysis isconfinedonly to the eclipticplanecomponents
(A,,A ,) ofthediurnal anisotropy (A) measured byaglobal The first term in (6) represems the contribution of the convection process, while the othertermsin (6) and(8)
networkof muondetectors andneutronmonitorsthereby coveringa rangeof rigiditiesin the galacticcosmicray
17,436
AHLUWALIA AND DORMAN:
COSMIC RAY TRANSVERSE
GRADIENT
spectrum.For the as3amnetric gradientcase,we therefore (minus sign) sectorsof IMF. Thusone shouldobtainan accurateestimateof the magnitude of the asyrmnetric part consider onlythesums (O03of thegradient.Furthermore, thevalueof O0.obtained from(14) shouldbe freeof theseasonal anddailyvariations
At* +At_3[CV - I•Gr] 2
v
(12)intheatmospheric temperature andthetemporal variation of
A; + A; _ 3
2
v (K•- K•)Gr sin•cos• (13)
One may use (12) and (13) for a detailedstudyof the contributions fromthediffusion andconvection processes and (9) and(11) to investigate the annualmeantimevariationsin theoff-ecliptic contributions to thediurnalanisotropy forthe epochswhen asyrmnetficgradients(G0•) persistin the heliosphere.The valuesof K • andspiralangle• maybe computed fromin situmeasurements of IMF recordedonthe Omnitapeavailableat the National SpaceScienceData Center (NSSDC), the Goddard Space Flight Center, Greenbelt, Maryland.Onecanthencomputethesteadystate valuesof Go,andtheratioa ( = K• /K, ) fromthemeasured values of the annual mean amplitudesof the ecliptic components of the diurnalanisotropy duringepochswhen asymme•cgradients aredominantin theheliosphere andby implicationsymmetricgradientsare absent. The reader shouldnotethatthedrffitheorydoesnotconcernitselfwith thepresence or absence of theasyrmnetfic gradient(G0•). Until recently,onecouldonlycomputethe annual mean values of the coupledparameterIqGr from the experimental data[RikerandAhluwalia,1987;Ahluwalia, 1988,1994,AhluwaliaandSabbah,1993b]. An application of (13) to thecosmicray datanowenablesoneto compute the valuesof theratio•, asa functionof time andprimary rigidity for a solarmagneticcycle,for the epochswhen asyrranetdc gradients persist intheheliosphere andsymmetric gradients areabsent.In thepast,it wasassumed that• = 0.1 isconaantand•dent of theprimaryrigidity[Ahluwalia andSabbah,1993b;Ahluwalia,1994]. Thisassumption may bequestionable. Similarly,onecancompute thevalueof the transverse gradientGo,in two differentwaysfrom (9) and (11) wifixmtmakinganyassumption aboutthevalueof •. The reader is remindedthat the radial anisotropy (A r)is significantly enhanced in the dataonlyfor q A > 0 epochs [Ahluwalia,1988, 1994]. In practice,to describethe caseof asymmetric
solaractivityfor themuondetectors locatedonthe surfaceas wellasunderground [Ahluwalia,1993]. Onenotesthat(14) could also have been derivedjust from the geometryof streaming asdepictedin Figure1. Similarly,we couldsimplify(13) by substituting K = v •, / 3 and obtain
=
sin2
Themasterequation (15) maybe usedto calculatethe annual mean value of e (= •.-•/•.0 from the observations without
makinganyadditional assumptions (otherthanthatGo,* 0 and Go, -- 0); the annualmeanvaluesof the dimensionless product•., G, maybe computed in an independent manner fromtheexperimental data[Ahluwalia,1994, 1996]. Previous
Research
Earlyworkreportedin theliteratureonthistopicis mostlyqualitative[see$winsonet aL, 1991,andreferences therein]. It made use of the diurnal variationobserved worldwide
with a network of neutron monitors and muon
telescopes,suitablycorrectedfor the barometriceffect. However, no attentionwas paid to the variationsof the limitingprimaryrigidity(Re) abovewhichgalacticcosmic raysdonotcontribute to thediurnalanisotropy.Thecriterion used for selectingthe epochs,when asymmetricgradiem pointssouthwardandnorthward,arewell describedin the literature(e.g.,seethediscussion in AS 1). Figure1, which provides a schematic representation of GCR protonstreaming
patterns oneithersideof•e HNCSdrivenby•e asynunetfic
transverse gradients (G) andtheIMF (B) at Earth's orbitduringthepositivemagnetic polarity(q A > 0) (Figure la) in thesolarnorthernhemisphere (1969 to 1973) andthe negative polarity(q A < 0) (Figure1c) epochs(1965 to 1968) for the two sectorsof the IMF, can be used to understand
thesecriteria.Thecontributions to thediurnalanisotropy are
shown bythedashed vectors• (B x G), forthetwosolar magnetic polarities.If G pointssouthward persistently
duringa year,oneexpectstheannualmeandiurnalvariation amplitudefor the away(plussign)groupto be largerand earlierin phasethan1800LT (localdusk)(see(iv) in Figure l d) thanthatfor thetoward(minussign)group(see(iii) in (14) A, = (2 cos Figm'e1d). Thesituation isreversed for thecaseof northward gradient,thatis, the amplitudeof the Since the left-handside of (14) representsa pointingasymmetric meandiurnalvariationfortheaway(plussign)group difference, thecontribution fromthesymmetric gradien_[ts, if annual present, isminimized. Thereason isasfollows.If G is (see (i) in Figure lb) is smallerthanthat for the toward symmetric (it pointsin oppositedirections with respectto the (minussign)group(see(ii) in Figurelb). Hashimand HNCS), the particledensityis eithera minimum(q A > 0) Bercovitch[1972] usedthesecriteriaand an approximate tocompute themagnitude of Go..Theyanalyzed data or magnum (_qA < 0) at the HNCS. In this case,the method vector B x G is invariantacrossthe HNCS. Therefore from a varietyof cosmicray detectorsof the Canadian its contribution to thediurnalanisotropy cancelsoutwhena networkfor the 1967to 1968period. difference is taken between the east-westanisotropy Chenet al. [1991] approached thisproblemfroma amplitudesobservedin the away (plussign)andtoward different point of view than that presentedhere. They
gradients, we coulduse(5) to simplify(11) andwrite
AHLUWALIA AND DORMAN: COSMIC RAY TRANSVERSE GRADIENT
17,437
corresponding magnitudes of the Compton-Getting (CO) veclorforcorrecting thedatafor theorbitaleffect[Ahluwalia and Ericksen, 1970] are also listed in Table 1; being proportional to cos•,•, it hasthesmallest valuefor Yakutsk I C data. In termsof sensitivity, Deep River NM hasthe
computed thevalueof G0,for the 1953to 1988periodusing datafromthe five neutronmonitorsof the globalnetwork, assumingthat R• -- 100 GV for the whole time period. Actually,thevalueof R• variesovera widerange[Ahluwalia andSabbah,1993b].Moreover,theircomplicated (11) used tocompute thevalueof theasymmetric gradient[seeChenet at, 1991,p. 11,575]requiresa knowledge of theratioa. By contrash (14)is verysimpleandtherefore elegant.Also,it is independent of a. Chenet al. [ 1991] assumed that• = 0 for theircomputations; their(13) lookssimilarto our(14) butis in a differentcoordinate system.
highestcounting rate(2.2x 10• eph).Thesedataformthe
Choice of CosmicRay Detectors
Data Analysis
Now thatwe havea methodology in place,we can carryoutsomecomputations for a widevarietyof detectors to updatethepreviousworkon the asymmetric gradients (see AS 1). We chosesix detectors fromthe globalnetworkto carryouttheanalysis presented in thispaper.Thischoicewas influencedby two considerations: thatthe detectors havea trackrecordof a reasonably stableoperation overtheperiod of our analysis (1965 to 1993) andthattheirresponse to the rigidityspectrum of the galacticcosmicrayscovera wider range.Theparticulars of thesedetectors arelistedin Table1. Theyconsist of neutron monitors(NM) locatedat DeepRiver (Can_ado), Climax(Colorado), andHuaneayo(Peru)aswell as the vertical-pointing underground muontelescopes (UT) at Embudo (27.3(meters ofwaterequivalent) andSocorro(New Mexico)(74 MWE) anda large(1 m diameter)ionchamber (IC) at Yakutsk (Russia). As discussedelsewhere [Ahluwalia, 1993], IC data have to be correctedfor the diurnal temperatureeffect due to the heating of the atmosphere by the Sun. The UT datafrom Embudoare
We use hourlycountingrate data from neutron monitorsand muon detectors(listed in Table 1) suitably corrected fortheatmospheric pressure variationsobservedat theirlocation.A 25 hourmovingaverageis subtracted from the hourlyrateto removeanylong-termtrendsin the data. The resultingdeviationsare expressed as a percentof the annual mean hourly rates. They are then subjectedto harmonicanalysisto obtainthe amplitudeandphaseof the diurnal variation. The daily harmoniccoefficientsare averaged overanappropriate periodfor a givenyearfor the "away"and"toward"IMF sectorgroupsto obtaintheannual mean valuesof the amplitudeand phaseof the diurnal variation.The ratiosof the diurnalvariationamplitudes for Deep River NM andfor EmbudoLIT areusedto determine thevalueof thelimitingrigidity(P•) of galacticcosmicrays that contributeto the observeddiurnalanisotropy duringa given year [Ahluwaliaand Riker, 1987a]. Geomagnetic bendingcorrectionis then appliedto the data. These
immune to this correction,while the correctionfor UT at
S(rx•o istinknown butisextx•tedto be small. Theeffective threshold rigidity(Ro)for thesedetectors liesin therange1.1 GV < Ro< 45 GV, andtheirmedianrigidity(1•) of galactic cosmic rayresponse spansa widerrange,10 GV g P• < 300 GV. Moreover,theirasymptotic latitudes of viewing(•,•) at 1• covera rangeon eithersideof the eclipticplane. The
mainstayof ourdetailedanalyses reported in thispaperand elsewhere.Note that UT at Socorrostartedoperationin March 1968 and that at Embudo started in December 1964
[Regener etal., 1970]. Otherswereoperational wellbefore 1965.
parameters arealsocorrected fortheCompton-Getting (CG) orbitaleffect[Ahluwalia andEricksen,1970].Theprocedure for converting thecorrected diurn_01 variationvectorintothe
free spaceanisotropy vectorA is described in detail elsewhere[Ahluwaliaand Sabbah, 1993b]. It is then a simplematterto determine the amplitudes of the east-west
(A ,) andradial(A r)anisotropies [Ahluwalia, 1988].
Table1. Particulars ofCosmic RayDetectors StationLocation
Detector Latitude, Longitude, Altitude, R0, deg deg m GV
Rm, GV
;•m, deg
CG Corr, Counts/h %
Climax(CL)
NM
39.37N
106.18W
3400
3
11
22S
0.042
DeepRiver(DR) NM
46.10N
77.50W
145
1.1
16
14N 0.043
2.2x 106
Huancayo (HU)
NM
12.03S
75.33W
3400
13'
33
9S
0.044
1.8x 10•
Yakutsk(Yak)
IC
62.01 N
129.72E
105
1.7
67
57N
0.025
NA
Embudo(Emb) UT
35.20N
106.41W
2725
19
134
33N
0.041
3.3x 10•
Socorro (Soc)
34.04N
106.56W
1554
45
299
33N
0.038
6.7X 104
UT
4.3x 10•
NM, neutronmonitor;IC, ASK- 1 ionchamber[Dorman,1957];and UT, underground muontelescope.
*Ro forHuancayo is subject to a long-term systematic decrease duetothesecular variation ofthegeomagnetic field [Shea,1971].
17,438
AI-IL•••
AND DORMAN: COSMIC RAY TRANSVERSE GRADIENT
value.Theeorre•g valuesof Reareplottedin thelower halfofFigm'e2. Theepochsof thesolarpolarfieldreversals For a givenyear,thecorrected dataareseparated arealsoshown,andthepolaritiesin thenorthernhemisphere byN (outward)andS (inward)before into awayandtowardgroupsby usingIMF dataprocured ofthe Sunareindicated of thesunspot maxima from the National SpaceScienceData Center(NSSDC) andaftereachreversal.Theepochs locatedat the GoddardSpaceFlight Center,Greenbelt, (M) and minima (m) are shownby arrowson the top earlier,G0,pointssouthward Maryland. The criterionfor separating the daysintotwo horizontalscale.As discussed for the awaygroupexceedthatfor the groupsis describedelsewhere(see AS 1). Briefly,the whenthe amplitudes geocentric solarecliptic (GSE)coordinate system is employed toward groupand pointsnorthwardfor the reversecase. thisin mind,thefollowingcomments maybemade. forthisp• inwhichthex axispointstowardtheSunand Keeping 1. For the 1965to 1968period,the amplitudefor y axispointseastof theEarth-Sunline. IMF is assigned an thatfor thetowardgroup,indicating "away"polarityif its azimuthliesbetween90ø and180øand theawaygroupexceeds of a persistent southward pointinggradientin the assigned a "toward"polarityif theangleliesbetween270ø thepresence and 360 ø. This criterion is more restrictive than the heliosphere. Its magnitude is largestin 1968. 2. From1969to 1973,theamplitudefor thetoward hemispheric methodpopularized by Svalgaardand Wilcox with the [1974] and King [1976] which does not allow for the groupexceedsthatfor the awaygroup,consistent existence ofkinksin IMF linesastheyconvect pasttheEarth presenceof a persistentnorthwardpointing gradient, [McCrackenandNess,1966,andreferences therein].We followingtheonsetof solarpolarfieldreversalfromsouthto is largestin 1973. •,onsider onlydaysforwhichthereareat least12hourly northin 1969. Its annualmeanmagnitude 3. A noticeable 11 year as well as 22 yearperiodicity valuesfor the IMF components.The stringency of our is present in the amplitudes for the toward as well as away criterion reducesthe numberof days in each group groups. For example, the amplitudes for toward groupin particularlyafter1982whenISEE 3 left L• librationpoint after51 months (August1978to October1982)to embarkon 1965and1986 arelargerthanin 1976. 4. The gradient becomes southward againin 1974 its cometary mission, and the coveragefor in sire measurements of the interplanetary parmeters decreased andrevertsto beingnorthwardin 1975,with no discernible asymmetric gradiembeingobserved subsequemly until 1979 sharply. whenit becomesnorthwardfor 2 years(1979 to 1980) near themaximum in solaractivity.Atten•ard,it changes suddenly Results tobeingsouthward followingthesolarpolarfieldreversalin backto northward for another2 years Figure 2 depicts a plot of the annual mean 1980. Thenit switches (1982 to 1983), changing to southward againin 1984 and amplitudes ofthediurnalvariationobserved withDeepRiver acquiring a large magnitude. We note that thesudden1 year NM for away(solidcircles)andtoward(crosses) groupsfor conspicuous reversals in the direction of the gradient(from the 1965 to 1993 period. Experimentalerrorbarsare not the general trend) in 1974 and 1984, during the declining shownto avoidclutter;typicallythey are one tenthof the phasesof solar activity cycles 20 and 21, needs an explanation.We notethattherewasa burstof solaractivity 550 in thosetwo yearsasmeasured by sunspot numbers.Also, Gonzalezet al. [1996] notethatlow-latitudecoronalholes 235 • 450 distribution exhibitsa dualpeakfor solarcycle21, onepeak , .•. occurringa year beforesolarmaximum(lVl) and a more one,occurring 2 to 3 yearsbeforesolarminimum 350 185• pronounced (m).Theyhypothesize thatlow-latitude coronalholesplayan 135 • importantrole in the solaroriginof transientinterplanetary 250 and geomagnetic enhancedactivity. Is this thenthe solar cause?Certainly,a detailedinvestigation is warrantedfor its 85 •' possiblecoronalmassejection(CME) connection. 150 5. The gradientdisappears againin the minimum 35 50 activityyearsof 1985 and 1986 (as measuredby sunspot 65 69 73 77 81 85 89 93 numbers), beco•g northward in 1987astheactivitypicked YEAR up, acquiringa largemagnitude.In 1988,it vanishedagain asa northwardgradient in 1989nearthesolar Figure 2. The amplitudes of theobserveddiurnalvariation andreappears areplotted inunitsof0.001%forthe 1965to 1993periodfor activitymaximumandthe onsetof anothersolarpolarfield the Deep River (NM) in the upper half of Figure 2. reversalepoch.Followingthereversalof the solarpolarity, Corresponding valuesof thelimitingprimaryrigidity(Re)are the gradientreversedsignalsoin 1992. In fact, the most featureof thedatais thatthegradientreversessign plottedin the lowerhalf of Figure2. The epochsof solar consistent polarfieldreversals (rectangular boxesnearlowerhorizontal in goingthroughtheepochsof thesolarpolarfieldreversals, of theepochdurationandthesolarpolarity.For scale)andthesunspot activitymaxima(M) andminima(m) irreslxx:tive are alsoshown(by arrowsat thetophorizontalscale). See example,it wassouthward in 1968 andnorthwardin 1972;
1MF Polarity
.
text for details.
northwardin 1979 and southwardin 1981; andnorthwardin
AHLUWALIA AND DORMAN: COSMIC RAY TRANSVERSE GRADIFNT 65
69
73
77
81
85
89
93
6O0
500
400
400
300• 100 •
-;oo 65
69
73
77
81
85
89
6. Or• notesthattherearequasi-periodicities in the observedtimevariations of R•, plottedin thebottomhalfof Figure2. It increases froma lowervaluenearsolarminima (1965, 1977, and 1986) to the largestvalue near solar maxima(1968to 1970,1978to 1980,and1989). Also,it has a highervaluein 1983,possiblycorrelated withtheenhanced valuesof the interplanetary parameters [Ahluwalia,1992]. F•, thevalueofR• showsa consistent decrease after eachobserved polarityreversalis completed.
Epochsof PersistentAsymmetricGradients
o
o
17,439
From our discussion in the sectionon results,it is
93
clear that there exist two conspicuous epochswhen YEAR asymmetric gradients persistin the heliosphere over an Figure 3. The amplitudes of the east-west anisotropyextendedtime periodduringthe entiretime frameof our
observed intheaway (A'e)andtoward (A',)sectors ofIMF analysis(1965 to 1993). Observationsindicatethat a
gradientis obtainablefrom 1965 to areplotted in unitsof 0.001%,forthe1965to 1993period. persistentsouthward 1968, and a northward gradient persists in theheliosphere for When A+,> A'e,theasymme•c gradient points southward; the 1969 to 1973 period. An interesting question arises as to it points northward ff A+e< A'•. The valuesofr• Go, the uniq• of this time frame. We defer this question for computed fromDeep RiverNM, usingmaster equation the present.Instead,we presentthe resultof our detailed (] 4), areplottedin thebottom half,withscaletotheright.
analysisfor thesetwo epochs, usingdatafromtheselected detectors listedin Table1,withthehelpof (14). 1989 and southward in 1992. So the presenceof the In Figure3, we haveplottedthe amplitude of the asymmetric gradient mayberelatedtothechange in thelevel east-west anisotropy (A,) obtained fromtheDeepRiverNM of thesolaractivity, itshemispheric asymmetry [Swinson et datafor the1965to 1993periodfortheaway(linejoining al., 1991], aswell asto the low-latitudecoronalholesandthe solidcircles) andtoward(linejoiningcrosses) groups; when large-scale rearrangement ofIMF following solarpolarfield A+,> A•, theasymmetric gradient pointssouthward, and reversals. A productiveresearcheffort can thereforebe mounted to investigate theseandotherpossibilities.
when A+,< A•, itpoints northward. Theobserved long-term variations inthetwocurves areverysimilartothosedepicted
Table2: Mean Values fortheEpoch ofAsynmaetric Gradient s(1965-1973) Period (epoch) Location/ A*,,days A',, days Detector
1965-1968
CL/NM
225
360
0.091 4- .029
(q A < 0)
DR/NM
286
466
0.096 4- .029
HU/NM
145
230
0.081 4- .040
Yak/IC
269
423
0.052 4- .016
Emb/UT
199
299
0.015 4- .031
Soc/UT
NA
NA
0.130 4- 0.079*
1969-1973
CL/NM
421
462
- 0.097 4- .018
(q A > 0)
DR/NM
523
589
- 0.049 4- .028
HU/NM
382
417
- 0.014 4- .023
Yak/IC
499
525
- 0.029 4- .011
Emb/UT
414
471
0.022 4- 0.016
Soc/UT
NA
NA
0.083 4- 0.048
,
NA',notavailable. *Dataonlyfor a partof 1968.
17,440
AI-R,UWALIAAND DORMAN: COSMIC RAY TRANSVERSE GRADIENT
in Figure 2, althoughthe separation betweenthemis more
pronounced. Forexample, the valueof A+, in 1968is (0.69ñ 0.05)%,andthatofA', in 1973is,(0.53ñ 0.03)%. Error bars are not shown to avoid clutter. The data for the two
150
1965-1968
100
,
ß
I•
ß
I
-----N---
kl
epochs(1965 to 1968 and 1969 to 1973) are averaged separately foreachdetector, andcomputations arecarriedout withthehelpof (14). Thevaluesof thedimensionless product rc Go.,computed fromDeepRiverNM datafor the 1965to 1993 period,are plottedin the lowerhalf of Figure3. Its magnitudeis largein 1965, 1968, 1973, 1982, 1984, and
1987whenthevalues ofA+,andA', arewellseparated. Theresults of ourcalculations arepresented in Table 2. Thenumberof daysavailable for analysis for eachdetector for eachgroupduringeachepocharelistedin thethirdand fourth
columns of Table
2. The mean values of the
dimensionless parameter r• G0.computed from(14) for each detector foreachelxx:harelistedin thefifthcolumn.Socorro LIT dataareonlyavailablefor a partof 1968. For the 1969 to 1973 period,themeanvalueof rcG0afor SocorroLIT is computedfrom the values of the annualmean diurnal variationamplitudes for thetwoIMF polarities published by Swimonetal. [ 1986]. Theydidnotstatethenumberof days overwhichthemeanwastakenin a givenyear.As expected, Go.hasapositive sign(pointingsouthward) for eachdetector for the q A < 0 epoch and a negativesign (pointing northward)for the q A > 0 epochfor thefirstfourdetectors (10 GV a R• a 67 GV). The associated experimental errors arelargehere. Evenso,a significant rigiditydependence is quiteapparentin thedata. Notethatthemagnitude of G0.
1973
-lOO,,'•", 11
I 16
I 33
I 67
I 134
299
Median GCRProton Rigidity, Rm(GV) Figure4. Thecomputed valuesof r• G0•areplotted(in units of0.001%)fortheetxx•hs ofthenegative(N) andpositive(P) solarmagnetic polaritiesduringthe 1965to 1973period,as a functionof themediangalacticcosmicray (GCR) proton rigidity(R•. A straightlineis drawnthrough thedatapoints for q A ) 0 epochto emphasize thesteeper dependence on rigidity. Note that our resultsmay be consistent with a southward pointingasymmetric gradientat largeoff-ecliptic distance (&1 Aid). Theymayalsosupportthe view that • 0 at largeGCR protonrigidities(R• & 300 GV) since e•ental errors(notshown)arelarge(seeTable2). See text for details.
appears todecrease morerapidlywithincreasing 1• forq A > 0 epoch.Ourdetailed investigations arestillin progress. Rigidity Dependenceand Time Variations ForSocx•oUT, G0a• 0 withintherangeof largeassociated To investigate how the asymmetric gradient(Go.) experimentalerrors; indicatingthat G0. dependsmore stronglyon thecosmicrayrigidity(R) whenRm& 1O0GV, behavesover a solarmagneticcycle (1965 to 1987), we thedimensionless productr• Go.for thesixyearly perhaps vanishingat P• & 300 GV. Thismighthelpexplain evaluated whenA+,and•, arewellseparated in Figure3, the difficultiesencounteredby theSwimon et al. [1986] intervals analysis.Theydid notmakequantitative estimates butonly namely,1965,1968,1973,1982, 1984,and1987. Its annual presenteda qualitativediscussion.The valuesat higher meanvaluefor theseyearsis listedin Table3 for the six listedin Table 1. Theycovera decentrangein the rigidities(R• & 100 GV) mayevensupporttheview thatthe detectors gradient reversessignat largeoff-eclipticdistances (&1AU). galacticcosmicray spectnma.One notesthe following This is depictedin Figure4 wherea straightline is drawn features. 1. At lowerrigidities(R• a 67 GV), themagnitude through thedatapoints(P)forq A > 0 epoch(1969-1973)for r• Go.remainsnearlyconstant foreachof the6 the purposeof guidingthe eye to emphasize the steeper oftheproduct gradient(Go.) is inversely depen• onGCRprotonrigidity(R) of r• G0..Swinsonet years. Thereforethe asymmetric to GCRprotonrigidity(R). For 1968and1987, al. [1986]notedthatUT at the southernhemisphere site of prolx)rtional theresults Hobart (R• = 30 GV, Rm = 184 GV, and •'m=43S) thisrelationis validup to R• a 300 GV, although with Go.• 0. Sothegradient continuesto recordthe presence of the southward pointing atR• • 300 GV areconsistent muchmorerapidlyat P• & 100 GV. asymmetricgradientfor the entire 1965 to 1983 period, mustdecrease 2. For 1965 and 1982,themeanvalueof rcG0.is irrespectiveof the observedepochsof the solarpolarfield reversals (two)andthesolarmagneticpolarity.We notethat abouthalf as large as for the other4 years. However,no dependence is discernible on the levelof the HobartLIThasa largevalueof themedianasymptotic latitude systemmatic solar activity. No explanation is available atpresent forthis of viewingin the southern hemisphere.Togetherwith our results for LIT at Embudo (•,m= 33N) and Socorro pttzzlingresult. 3. At R• a 67 GV, thesignof thegradient agrees (•'m= 33N), thissuggests thatat largeoff-eclipticdistances, theasymme•cgradient (forlargevaluesofl•) alwayspoints for 6 years, except for HuancayoNM in 1973; its southward, at leastduring1965to 1973. If true,thisis very experimental errorsarethelargestamongNM. Thereis an intriguing. What is the underlying physicalcauseof this? evidenceof signreversalat higherrigiditiesfor theyears Thesenew resultsareveryexciting.Theyencourage usto 1973, 1982,and1984duringwhichhigh-speed solarwind areobservedat Earth'sorbit [AhluwaliaandFikani, undertake amorecomprehensive analysis of thedataobtained streams betweenthetwophenomena? withtheunderground muontelescopes of theglobalnetwork. 1991]. Is therea connection
AHLUWALIA AND DORMAN: COSMIC RAY TRANSVERSE GRADIENT,..
17,441
Table3: MeanValuesoftheAsymmetric Gradients forSelected Intervals Location/
Average Values ofrc G0a' %
Detector
1965
1968
1973
1982
CL/NM
0.08 ñ 0.04
0.15 ñ 0.05
- 0.14 ñ 0.03
- 0.11 ñ 0.05
0.09 ñ 0.05
- 0.12 ñ 0.04
DR/NM
0.09 ñ 0.05
0.17 ñ 0.04
-0.13
-0.10
ñ 0.06
0.13 ñ 0.06
-0.14
ñ 0.05
HU/NM
0.05 ñ 0.08
0.16 ñ 0.08
-0.07
ñ 0.08
0.10
-0.09
ñ 0.07
Yak/IC
0.08 ñ 0.03
0.10 ñ 0.02
-0.12
-0.07
ñ 0.03
0.11 ñ 0.04
-0.10
ñ 0.03
Emb/UT'
0.03 ñ 0.04
0.10 ñ 0.05
-0.02 ñ 0.03
-0.05 ñ 0.06
-0.07 ñ 0.05
-0.24 ñ 0.12
SOC/UT'
NA
0.13 ñ 0.08
0.04 ñ 0.10
0.04 ñ 0.08
- 0.09 ñ 0.06
NA
ñ 0.03
0.05 ñ 0.05
ñ 0.02
1984
ñ0.12
1987
NA, not available.
'Computed fromdatapublished bySwinson etal. [1986]. Oneneeds toexplorehowa finestructure is inducedin Go,on such occasionsat large off-eclipticdistances(e I AU). However, becatzse oflargeexperimental errors,onecouldalso conclude thatGo,= 0 at largeoff-eclipticdistances. We havecomputedthe valuesof the asymmetric
be discernedbetweenGo. valuesand the epochsof solar activitymaxima(M) andminima(m), although thedirection of Go,changes consistently aftereachobserved epochof solar polarfieldreversal(shadedboxesnearhorizontalscaleat the bottom)in thesolarnorthernhemisphere. It is possiblethat gradient (G• fromDeepRiverNM dataforthe1965to 1993 solarrelationships aremorecomplexandmaybe discovered period.Theyareplottedin Figure5. Thevaluesof re in this if the datastatistics areimproved.Thiswill be donein a timeintervalwerecalculated usingannualmeanvaluesof the futureupdate. iM (B) fromtheOmnitape.Theycovertherange5.1 nT _< B < 9.1 nT; the lowest value of B occurs in 1965 and the
Conclusions
highestoccursin 1991. Thecomputed valuesof Go,exceed 2o level for 4 yearsonly,namely,1968, 1973, 1984, and 1987. The corresponding + 1o error bars are plottedin Figure 5. There are manymore valuesabove 1o level. Extreme valuesof Go,rangefrom(3.0 ñ 0.8) % / AU in 1968 to(- 2.4+ 0.8) % / AU in 1987. No obviousrelationship can
In thisp•, wehaveupdated theearlieranalysis of the asymmetricgradients(AS 1) by takingaccountof the secularvariationsof Re and usingdata from cosmicray detectors whichcovera widerrangeof galacticcosmicray spectrum.This comprehensive studyhasyieldedseveral additional insights, helpedby computations carriedoutwith thehelpofmaster equations developed by a newmethodology which is basedon empiricalconsiderations. Thisefforthas also encouraged us to expandthe scopeof our future investigations in searchof morecompleteanswers to someof thecrucialquestions raisedby thepresentinvestigation. Our carefulfindingsaresummarized below. 1. There are two conspicuousepochs when asymmetricgradientspersistin the heliosphereover an extendedtime period;southward from 1965 to 1968 and northward from1969to 1973. We havecomputed thevalues ofr•Go,for• aswellasotherselected intervals duringthe
64
68
72
76
80
84
88
92
500
:_ •rM 40Oß
m•' •M
m•r •'M -•400
300
3OO
•
200
2OO
o
lOO
o
.--
ß
e
--
0
IO0 0
-100
-100
._o -200
-2OO
E -300
-30O
'400
-• I I I I • I I • • I I • • I I I I I I I I I I I I I I I' 64 68 72 76 80 84 88 92 YEAR
'400
Figure5. Valuesof Go,computed fromDeepRiverNM data for the 1965 to 1993 period are plotted along with experimentalerror bars (ñ 1o) for selecteddatumpoints. Epochsof solaractivitymaxima(M) andminima(m) are shownby arrowsonthetophorizontalscaleaswell asthose
of solarpolarfieldreversals (rectangular shaded boxesnear thebottomhorizontalscale). Seetextfor details.
1965to 1993period.We findthattheannualmeanvaluesof r• Go,arewell definedoverthelowerrigidityrangeof the galacticcosmicray spectrum,where Go, is inversely proportional tothegalactic cosmicrayprotonrigidity(R). At higher rigidities(R e 100GV), Go,mayfall offmoresteeply and may evenreversesignat large(• I AU) off-ecliptic distances.No systemmatic dependence of Go, on solar activitycycleis descernible in our results. This puzzling resultcannotbe explainedat present. 2. Sudden,conspicuous,short-lived(1 year) reversals oftheasymmetric gradient areobserved in 1974and 1984. Theymayarisefrom suddenburstsof solaractivity
17,442
AI-IL•••
AND DORMAN: COSMIC RAY TRANSVERSE GP•DIENT
during the decliningphaseof a solar activitycycle;the Ahluwalia,H.S., andJ.F.Riker, The diurnalanisotropy of possible connection withCMEsneedsto be investigated. The cosmicraysandthe heliospheric transportparmeters, most consistentfeatureof the data is that the asymmetric Planet.SpaceSci.,35, 45-50, 1987b gradient reverses sign, even if briefly, ingoing through the Ahluwalia, H.S., and I.S. Sabbah, On cosmic ray epochs ofthesolarpolarfieldreversals atlowerGCRproton asymmetrical latitudinalgradient,Ann. Geophys.,11, 763-773, 1993a. rigidities (R=< 67 GV) irrespective of theepochdurationand thesolar magnetic polarity. Ahluwalia,H.S., and I.S. Sabbah,Cosmic ray diurnal 3. We havecomputed thevaluesof Go-fromDeep anisotropy for a solarmagneticcycle,Planet.SpaceSci., RiverNM data(P• = 16 GV) for the 1965to 1993period. 41, 113-125, 1993b. Extreme values range from(3.0+ 0.8)%/ AUin1968to(- Ahluwalia,H.S., andS.S.Xue, Interplanetary parameters for 214+ 0.8) % / AU in 1987. Therearemanymorevaluesof a solarmagl•ticcycle,Conf Pap. Int. CosmicRay Conf Go,whichexceed + 1o. We notethattheprospects of a better 23rd, 3, 563-566, 1993a. coverageof interplanetary parameters at Earth'sorbithave ^hluwalia,H.S., andS.S.Xue, Is therea persistent northimprovedrecentlywith the launchof new spacecraft.This southasymmetry of IMF spiral?,Conf Pap.Int. Cosmic mightenableusto showthatasynunetric transverse gradients Ray Conf 23rd, 3, 567-570, 1993b. arecommonplace in theheliosphere. Axford,W.I., Themodulation of galacticcosmicraysin the interplanetary medium,Planet.Space.Sci.,13, 115-130, Acknowledgments. L.I.D. is gratefulfor the support 1966. receivedfrom the Universityof New Mexico. We thank Bieber,J.W., andJ. Chen,Cosmicray diurnalanisotropy, RogerC. Ygbuhay for technicalassistance. Comments made 1936-88: Implications for drill andmodulation theories, by therefereeswereuseful. Astrophys. ,I., 3 72, 301-313, 1991. The Editor thanks J. E. Humble and another referee for their
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