Leske, R A

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29th International Cosmic Ray Conference Pune (2005) 2, 113116Intensity Gradients of Anomalous Cosmic Rays Between 1 AU andVoyager During Solar Cycle 23R.A. Leske , A.C. Cummings , C.M.S. Cohen , R.A. Mewaldt , E.C. Stone ,M.E. Wiedenbeckand T.T. von Rosenvinge(a) California Institute of Technology, Mail Code 220-47, Pasadena, CA 91125 USA(b) Jet Propulsion Laboratory, MS 169-327, Pasadena, CA 91109 USA(c) NASA/Goddard Space Flight Center, Code 661, Greenbelt, MD 20771 USAPresenter: R.A. Mewaldt (ral@srl.caltech.edu), usa-leske-R-abs1-sh32-oralUsing ACE/SIS data we obtain measurements of the variation of the anomalous cosmic ray (ACR) O intensityat 1 AU throughout the solar cycle, including periods near solar maximum. After being suppressed by a factorof  100 or more from 2001 through 2003 during solar maximum, ACR intensities are once again recoveringat 1 AU, but their intensities are lower relative to higher rigidity galactic cosmic rays (GCRs) than they wereduring entry into solar maximum in 1998-2000. Comparison of the 1 AU ACR and GCR time variations withthose measured by both Voyager spacecraft in the outer heliosphere shows that both ACR and GCR large-scaleradial gradients are greater at solar maximum than at solar minimum, and that the ACR radial gradients aremuch larger than those of the GCRs, becoming very large inside of  5 AU.1. IntroductionMeasurements of anomalous cosmic ray (ACR) and galactic cosmic ray (GCR) intensities throughout theheliosphere during the solar cycle can be used to study the effects of solar modulation on cosmic ray transport.Many studies have shown that ACR and GCR intensities and their gradients vary with radial distance (e.g., [1]),heliographic latitude [2], phase [3] and polarity [4] of the solar cycle, and particle rigidity [5]. Most previousACR gradient studies that used a 1 AU baseline were not done under solar maximum conditions, since at 1AU near solar maximum ACR intensities are severely reduced and suffer contamination from frequent solarenergetic particle (SEP) events. To define quiet time periods for this study, we use a ratio of He intensities fromSIS on ACE [6] as illustrated in Figure 1. Large SEP events produce elevated intensities at both the higherand lower energies (horizontal and vertical axes), while smaller events affect only the lower-energy rate, mostoften when the higher energy rate is low (heavily modulated) near solar maximum. The line shown (low energyrate = high energy rate / 0.6) indicates the data cut adopted here to select days regarded as quiet. This cut isessentially equivalent to requiring that “quiet” days have a daily average He spectrum from 6-30 MeV/nucleonwith an index harder than about  		 , independent of phase of the solar cycle.Shown in the right panel of Figure 1 are oxygen spectra accumulated over the resulting quiet days duringthree time periods: solar minimum, near solar maximum, and the recent initial ACR recovery (see Figure3). Our solar maximum ACR oxygen intensity is almost a factor of 2 lower than that reported at the sametime and energy using Wind data [7]. Their quiet selection criteria (2.0-3.7 MeV/nuc He  2  10  (cm  sr sMeV/nuc)  ) may be less stringent and may have subjected their results to SEP contamination.2. DiscussionOur quiet time oxygen intensities are shown in Figure 2 along with earlier data going back 3 solar cycles. Whenplotted as shown in the figure, the Climax neutron monitor rate matches the ACR oxygen data during the A  0magnetic polarity cycle of the 1970’s and during the decline of the last A  0 cycle; differences in the ACR and114 R.A. Leske et al.Figure 1. Left panel: Daily-averaged 6.1-13.6 MeV/nucleon He intensity vs. 18.0-29.4 MeV/nucleon He. Right panel:Oxygen spectra accumulated over quiet days in the 3 time periods indicated.GCR recovery during 1992-1994 led to the conclusion that much of the GCR modulation occurs beyond thetermination shock [9]. The ACR oxygen fell short of the scaled neutron monitor during the last A  0 cycle,apparently due to the effects of particle drifts [10], and is doing so again during the initial ACR recovery inthe present cycle which has clearly begun. The lag in the ACR recovery relative to that of the neutron monitoris seen more clearly in the hysteresis plot in the right panel of Figure 2. The 108-day averages are connectedby a line which starts at the upper right during the 1997 solar minimum period, runs to the lower left alongthe upper trace, and starts back up along the lower trace. So far the recovery appears to be following the sameslope as the last decline, with the ACR O intensity scaling as the neutron monitor rate to the  25th power, butwith a significant offset between the decline and recovery tracks.During the period of the ACE measurements at 1 AU, the Voyager spacecraft were probing the outer heliosphereat distances of  50-95 AU. Figure 3 compares oxygen time profiles from the CRS instruments on Voyagers 1& 2 [11] and ACE/SIS. Although the oxygen intensities at 1 AU dropped by a factor of  100 between solarFigure 2. Left panel: Quiet time intensities of  8-27 MeV/nuc oxygen from ACE/SIS (circles), SAMPEX (squares), andearlier missions (OGO-5, IMP 6, 7, 8; diamonds) (see [8] for references) compared with the scaled Climax neutron monitorrate (histogram). Right panel: 7-29 MeV/nuc oxygen intensity vs. the Climax neutron monitor count rate.Intensity Gradients of Anomalous Cosmic Rays ... 115Figure 3. Oxygen intensity (left) (mostly ACRs during solar minimum, but with no GCR background subtraction) andGCR carbon (right) vs. time from ACE (circles), Voyager 1 (diamonds), and Voyager 2 (squares). The three time periodsused for the gradient study are indicated between vertical dashed lines.minimum and maximum, there was hardly any change in the outer heliosphere; at Voyager 1 the intensitiesactually increased near solar maximum (largely due to Voyager 1’s increased radial distance compared withsolar minimum). At solar maximum, the ACR intensities at Voyager 1 were  1000 times greater than those at1 AU, while at solar minimum the rates in the two locations were within a factor of  10 of each other. Thestart of the ACR recovery at 1 AU is followed  6 months later at Voyager 2, while almost no residual ACRmodulation exists at Voyager 1 so there is no recovery. Unlike ACR O, at solar minimum the GCR carbonintensities are practically the same at 1 AU as in the outer heliosphere, and even during solar maximum therates are only a factor of  5 higher at the Voyagers than at ACE. As noted before for GCR He [12], the intensitydecline of GCR C from 1999-2001 moves out through the heliosphere as it first appears at ACE and then  1year later at Voyager 2 and finally Voyager 1. The recent recovery in GCR intensities at 1 AU does not seem tohave reached either Voyager 1 or Voyager 2, indicating that the recovery at 1 AU was effective only inside 75AU and that most of the GCR modulation at Voyager 1 occurs in the heliosheath.Figure 4. Intensity versus radial distance for ACR oxygen (left) and GCR carbon (right) from ACE (circles), Voyager 1(diamonds), Voyager 2 (squares), and Ulysses (stars; see text) during the three time periods of interest. For solar minimum,values from an earlier study [4] (triangles) and without latitudinal gradient corrections (crosses) are also shown.The “solar minimum” and “solar maximum equilibrium” periods we select are indicated in Figure 3. BothACR and GCR intensities at 1 AU were somewhat higher during our “solar maximum equilibrium” periodthan during intervals on either side of it, but during the period we chose the intensities seem to have stabilizedthroughout the heliosphere. Also, the inferred mean free paths for ACR He and O in the outer heliospheremore nearly resemble solar maximum values during this period than they did during late 2000/early 2001 [13].116 R.A. Leske et al.Summing over the selected periods and, for the ACRs, subtracting the GCR O background indicated in Figure1, results in the intensities shown versus radial distance from the Sun in Figure 4. To help cover the wide gapbetween 1 and  50 AU, online rate data from COSPIN/LET on Ulysses were obtained. Using COSPIN/LET Hand He daily-averaged intensities, quiet days were selected in a manner analogous to that illustrated in Figure1 and the accumulated 7.1-39 MeV/nucleon CNO intensities in our 3 time intervals were calculated. Estimatedbackground due to GCR O, N, and C as well as ACR N was subtracted and a correction applied to accountfor the different energy intervals in the Ulysses and ACE data. For the A  0 solar minimum we corrected allthe data shown for a latitudinal gradient (  1.3%/  for ACRs,  0.3%/  for GCRs). Although a larger negativegradient is expected during A  0, the recovery is far from complete and it is uncertain whether this gradient isestablished yet or not; so far we have not corrected the recent recovery data.3. ConclusionsThroughout the heliosphere, the ACR radial gradients are found to be greater at solar maximum than solar min-imum, with the initial stages of the recent recovery falling somewhere in between. The ACR radial gradientsbetween  5 and  95 AU are nearly independent of radial distance in all 3 periods, and become much greaterinside of 5 AU [3]. While the intensity of ACR oxygen varies by nearly 4 orders of magnitude between 1 AUat solar maximum and the outer heliosphere at solar minimum, GCR carbon varies by less than a factor of 10,and the steep increase in ACR radial gradients in the inner heliosphere is not seen in the GCR gradients. Likethe ACR gradient, the GCR radial gradient is greater during solar maximum than during solar minimum [14].The lack of recovery so far in the outer heliosphere GCR C intensities suggests that the heliosheath has not yetrecovered towards solar minimum conditions.4. AcknowledgementsThis work was supported by NASA at Caltech (under grants NAG5-6912 and NAS7-03001), JPL, and GSFC.We acknowledge the Ulysses Data System for COSPIN/LET data and the National Science Foundation (grantATM-9912341) for Climax neutron monitor data.References[1] W.R. Webber et al., 16th ICRC, Kyoto (1979) 5, 353.[2] K.J. Trattner et al., Astron. Astrophys. 316, 519 (1996).[3] A.C. Cummings and E.C. Stone, Adv. Space Res. 23, 509 (1999).[4] A.C. Cummings et al., Geophys. Res. Lett. 22, 341 (1995).[5] F.B. McDonald, Space Sci. Rev. 83, 33 (1998).[6] E.C. Stone et al., Space Sci. Rev. 86, 357 (1998).[7] D.V. Reames and F.B. McDonald, Ap. J. Lett. 586, L99 (2003).[8] R.A. Leske et al., in ACE-2000 Symposium, AIP Conf. Proc. 528, 293 (2000).[9] F.B. McDonald et al., J. Geophys. Res. 105, 1 (2000).[10] M.E. Pesses, J.R. Jokipii, and D. Eichler, Ap. J. Lett. 246, L85 (1981).[11] E.C. Stone et al., Space Sci. Rev. 21, 355 (1977).[12] F.B. McDonald et al., 27th ICRC, Hamburg (2001) 10, 3906.[13] A.C. Cummings and E.C. Stone, 28th ICRC, Tsukuba (2003) 7, 3897.[14] F.B. McDonald et al., 28th ICRC, Tsukuba (2003) 7, 3965.

Intensity Gradients of Anomalous Cosmic Rays Between 1 AU and Voyager During Solar Cycle 23

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