Charge Sign Dependence in Cosmic Ray Solar Modulation John Clem and Paul Evenson Bartol Research Institute, Department of Physics and Astronomy, University of Delaware Abstract: The steady flux of cosmic rays entering the heliosphere provides a unique probe for studying the magnetic fields carried by the solar wind. Fluctuations in these magnetic fields produce an anti- correlation between cosmic ray intensities observed at Earth and the level of solar activity (solar modulation). Cosmic ray electrons and nuclei respond differently to solar modulation, with the differences being clearly related to reversals of the solar magnetic field, which occur every eleven years. If the large scale heliospheric magnetic field has certain types of structures, the charge sign of cosmic ray particles can affect their propagation. Careful study of the behavior of cosmic ray positrons, relative to negative electrons (which have identical masses) allows for a definitive separation of the effects due to charge sign from the other possible effects. As part of a on-going investigation of charge-sign dependence in solar modulation, cosmic ray positron fraction in the energy range of 0.6 to 4.5 GeV was measured on a NASA balloon flight from Kiruna, Sweden to Victoria Island, Canada during June 2-6, Measurements from this flight are compared to previous results and models. NASA grants: NNG05WC08G,NNX08BA62G Solar modulation is the result of magnetic field fluctuations in the solar wind, scattering cosmic rays out of the solar system and/or decelerate them. Observations have shown cosmic ray electrons and nuclei respond differently to solar modulation, with the differences clearly correlated to reversals of the solar magnetic field, occurring every eleven years. Measurements of positrons, relative to negative electrons allows for a definitive separation of the effects due to charge sign from the other possible effects. The primary goal is quantify these effects through a serials of observations using the same instrument and applying the resulting constraints to heliospheric propagation models. Introduction Outward flowing solar wind and solar rotation produce a spiral geometry of the interplanetary magnetic field lines. A+ is shorthand for the case where the dipole has a positive projection on the solar rotation axis is positive whereas the opposite projection is termed A-. Reversals of the solar magnetic field occur every 11 years. q>0q<0 q>0 Drift Directions Particles moving on a curved magnetic field line experience a centrifugal force due to the field curvature that causes the guiding center drift perpendicular to both the centrifugal force and B -- either toward or away from neutral current sheet depending on the particle charge sign and polarity epoch. Time profile of helium and electron fluxes at a rigidity of ~1.2 GV Filled Symbols Open Symbols Magnetic Polarity ● As shown in the above figure the response of electrons and nuclei to changing conditions in interplanetary space is qualitatively similar but quantitatively different. ● Fluxes are low when the sun is active and high when the sun is inactive, however particles with opposite sign to the polarity state reveal a narrower time profile than those with like charge- sign. ● This effect could be explained as the result of the differential replenishing rate of cosmic rays in a depleted heliospheric cavity that depends on the drift direction. ● The electron profile in the 1990s seems to be broader than the helium profile observed in the 1980s possibly due to positron component in the electron observations. A series of balloon observations of electrons with the LEE begun in 1968 at the University of Chicago and has continued at the Bartol Research Institute since The data from these balloon flights have been used to study solar modulation of electrons with energies up to ~ 20GeV. This data were also used to help calibrate spacecraft instruments such as OGO- 5, Ulysses/KET and ICEE-3/ICE Flight Log for LEE For Decades the Cosmic Ray Electron “Standard Candle” Has Been LEE Radial Gradient (1 To 5 AU ) of Cosmic Ray Electrons at 1.2 and 2.5 GV ● Prior studies: McDonald et al 1997, Fujii and McDonald 1997, Heber et. al ● Our analysis uses LEE, ICE, and Ulysses data. – Squares: Gradients of positive charged particles. – Circles: Gradients of negative charged particles. – Red: 1.2 GV – Green: 1.7 GV – Black 2.5 GV Electron Latitude Gradient ● Fractional deviation between the observed LEE fluxes and KET fluxes of 1.2GV electrons as a function of helio-latitude of the Ulysses spacecraft. ● Latitude gradient appears after the solar polarity reversal In August of 2002 LEE flew on a new 60mcf balloon reaching float at 161kft (0.9 mbar). The high altitude provided a low background environment allowing measurements of primary electrons with energies as low as 20MeV. ● Observations of cosmic ray positrons, relative to negative electrons (which have an identical relationship between velocity and rigidity) will allow a definitive separation of effects due to charge sign from effects arising from velocity differences. ● The AESOP instrument was designed and built specifically for this goa. To measure the positron abundance in electrons from 200MeV to 5GeV over a full 22 year cycle. ●Since electrons and nuclei have greatly different charge/mass ratios, the relation of velocity and magnetic rigidity is very different for these two particle species. ● Consequently, a fully quantitative determination of the charge- sign-dependent component of solar modulation has yet to be made, however significant progress has been made. AESOP detects electrons with plastic scintillators (T1, T3), anticoincidence)(G) and a gas Cherenkov detector (T2). It measures the electron energy in a lead glass (T5) calorimeter. A final scintillator (T6) assists in particle identification and energy determination by counting the number of particles that escape the calorimeter. A permanent magnet and a digital optical spark chamber hodoscope (SC 1,2,3) determine the charge sign and momentum of the electrons. AESOP Anti-Electron Sub-Orbital Payload Vertical axis: Energy measured in the Pb-Glass (T5) calorimeter Horizontal: Deflection in the magnet in units of inverse rigidity. Curve represents the ideal instrument response for positrons (positive side) and electrons (negative side). Red symbols are events tagged as high energy protons based on signals in T4 Particle ID and energy of each event are assigned using a likelihood analysis AESOP 2006 Flight The world summary of positron observations including AESOP data Pamela group has submitted a paper presenting new positron results so we look forward incorporating these into our analysis. Consider data at ~1.2GV and correlate in Time 13 Abstract #2368 Time dependence of positron abundance (black) and anti- proton ratio (red) at a rigidity of roughly 1.25GV. Black line is a Positron abundance prediction based on the analysis of Clem et al. (1996). Red line is an antiproton/proton ratio model Bieber et al. (1999). Dashed lines are the predicted results for future observations. Anti-protons were measured by the series of BESS balloon flights Expected 2009 Flight SUMMARY The cosmic ray positron abundance spectrum was measured by AESOP on a five-day balloon flight, launch from Kiruna, Sweden on June 2, These observations are qualitatively consistent with the AESOP results from the 2000 and 2002 flights, which revealed a significant decrease in the positron abundance at the time of the solar polarity reversal. However the 2006 flight results at 1.25GV suggest the predicted magnitude of the decrease in the Clem et al. (1996) model may be somewhat too large. Likewise the 2005 Pbar observation by the BESS balloon flight suggest the predicted levels Bieber et al. (1999) are too high. An ongoing effort to analyze this difference will provide an improved description of charge sign dependence in modulation. LEE has maintain the standard candle of low every electron observations over the past few decades. We hope to keep it burning and provide an intercalibration with active PAMELA experiment. To put these observations in context with previous and succeeding measurements related to this work, the LEE and AESOP instruments will provide a means to link PAMELA measurements with the past and future. LEE and AESOP payloads are schedule to re-fly from Sweden May 2009 For references please see