• Document: Charge Sign Dependence of Cosmic Ray Modulation Near a. Rigidity of1gv. J.M. Clem, P. Evenson 1, D. Huber 2 and R. Pyle
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1 Charge Sign Dependence of Cosmic Ray Modulation Near a Rigidity of 1 GV J.M. Clem, P. Evenson1, D. Huber2 and R. Pyle Bartol Research Institute, University of Delaware, Newark, DE 19716, USA C. Lopate, and J.A. Simpson LASR, University of Chicago, Chicago, Illinois, 60637, USA Short title: CHARGE SIGN DEPENDENCE OF COSMIC RAY MODULATION 2 Abstract. New observations of electron uxes made in 1997 and 1998 extend our ongoing investigation of the relative modulation of positively and negatively charged particles. We compare electron uxes measured on high altitude balloon ights with continuing observations of helium uxes from the IMP-8 spacecraft, and present new measurements of the primary cosmic ray positron abundance in 1997 and 1998. Electron uxes during the 1984-1990 period show a at topped distribution, whereas the positively charged He uxes show a peaked distribution, with the peak in 1987. This is expected from modulation theory, including the role of drifts when the northern heliospheric magnetic eld is inward, and the southern heliospheric eld is outward. From 1990 to 1999, data are consistent with an inverse relationship, but electron data are too sparse to allow a de nitive statement. Near a rigidity of 1 GV, the relative abundance of electrons and helium nuclei is a weak function of the tilt angle of the heliospheric current sheet. 3 1. Introduction Although the sun has a complex magnetic eld, the dipole term nearly always dominates the magnetic eld of the solar wind. The projection of this dipole on the solar rotation axis (A) can be either positive, which we refer to as the A+ state, or negative, which we refer to as the A; state. Near each sunspot maximum, the dipole reverses direction, leading to alternating magnetic polarity in successive solar cycles. Babcock (1959) was the rst to observe a change in the polarity state when he observed the northern (southern) polar region change to positive (negative) polarity, that is a transition to the A+ state. Many modulation phenomena have di erent patterns in solar cycles of opposite polarity. Possibly the most striking of these is the change in the ux of electrons relative to that of protons and helium when the solar polarity reverses (Evenson and Meyer 1984; Garcia-Munoz et al. 1986; Ferrando et al. 1996). Electromagnetic theory has an absolute symmetry under simultaneous interchange of charge sign and magnetic eld direction, but positive and negative particles can exhibit systematic di erences in behavior when propagating through a magnetic eld that is not symmetric under re ection. Two systematic deviations from re ection symmetry of the interplanetary magnetic eld have been identi ed { one in the large-scale eld, the other in the turbulent, or wave component. The Parker eld has opposite magnetic polarity above and below the helio-equator, but the spiral eld lines themselves are mirror images of each other. This antisymmetry produces drift velocity elds that (for positive particles) converge on the heliospheric equator in the A+ state 4 or diverge from it in the A; state. (Jokipii and Levy 1977, Jokipii 1997). Negatively charged particles behave in the opposite manner, and the drift patterns interchange when the solar polarity reverses. Alternatively, systematic ordering of turbulent helicity discovered by Bieber, Evenson, and Matthaeus (1987) can cause di usion coecients to depend directly on charge sign and polarity state. Accurate measurements of the relative modulation of negative and positive electrons (negatrons and positrons) are beginning to enable a more precise investigation of the \pure" charge sign dependence of modulation. One major nding of these studies is that the positron abundance is at most 20% in data taken during the A+ polarity decade of the 1990's. Since this polarity state is the one expected to enhance the positron uxes, this reduces the impact of concerns such as that voiced by Moraal et al. (1991) that the positron content of the electrons might be so high as to challenge their use as negative particles in charge sign studies. Historical data on the di erential modulation of electrons and nuclei can now be approached with new con dence as a way to study the lack of re ection symmetry in solar wind magnetic elds. 2. Observations In this paper we report new measurements of the 1.2 GV electron ux (both negatron and positron) taken in 1997 and 1998, by the balloon borne payload LEE/AESOP (Hovestadt et al. 1970; Clem et al. 1996). In the Appendix, we discuss improvements in the AESOP instrument and data analysis process over that presented by Clem et al. (1996). We also present an extension of measurements made by the

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