Balloon Measurements of Charge Sign Dependent Solar Modulation

1. Balloon Measurements of Charge Sign Dependent Solar Modulation Paul Evenson John Clem University of Delaware

2. 1912: Discovery of Cosmic Rays • As his balloon ascended to 5300m, Viktor Hess observed the rate of discharge of his electroscope increase with altitude. • He concluded, "a radiation of very high penetrating power enters our atmosphere from above.“ • Initially Hess's theory about rays from space did not receive general acceptance, but increased research supported it. The newly discovered radiation was dubbed “Cosmic Rays" by Robert A. Millikan in 1925. Hess lands following a historic 5,300 meter flight. August 7, 1912 National Geographic photograph

3. Why Do We Still Fly Balloons?

4. Apart from the science…. • you can meet a better class of people.

5. Apart from the science…. • you can become famous

6. Apart from the science…. • you can develop self discipline

7. … but mostly you just get above the atmosphere • Technically, Hess was wrong, Nobel prize notwithstanding. Had he continued about 20,000 meters (and lived) he would have seen the radiation decrease again as he passed the Pfötzer maximum • Electrons and muons that dominate the radiation in the lower atmosphere result from the weak interaction, which was simply unknown at the time • Thus it took a long time to realize that the “cosmic rays” Hess saw are actually the products of the weak interactions of incoming positively charged hadrons • Only in the late 1940’s was it established that nuclei heaver than protons are present in these “primary cosmic rays”

8. Discovery of “Solar” Modulation • Using the network of the Compton ionization chambers Scott Forbush was the first to observed the 11-year cyclic variation of intensity and its anti-correlation with the solar activity cycle as measured by sunspot numbers. This discovery was reported in the late 40s. (World-wide cosmic-ray variations, 1937-1952. J. Geophys. Res. 59, 525, 1954) • Compton cosmic ray meters (also known as the Compton-Wollan-Bennett ionization chambers) are 12” diameter spherical ionization detectors filled with argon to 50 atmospheres. They measure the number of charged particles (primarily muons and electrons) that pass through the active volume Cosmic Ray Intensity Sunspot Number  Time (years) 

9. Modern View of Modulation • Different cycles are different • Hint of a 22 year periodicity

10. What is the Problem? • No known mechanism as static magnetic fields are impossible (1/r3) • A variable electric charge on the sun? • Curiously this works reasonably well in reproducing the changes in the spectral shape of the particles • It is still useful to characterize the modulation level with the single parameter Φ

11. Parker Found the Answer! • Support of the magnetic field is continuous and dynamic, not static or episodic • The hot solar corona expands, filling the solar system with plasma • Highly conductive plasma supports the magnetic field

12. The interplanetary magnetic field has large scale order, now called the “Parker Spiral”

13. Modulation can be described without considering the long range order • Parker (1965) showed that the spectra at Earth can be calculated by solving a Fokker-Plank equation in a spherically symmetric model of the interplanetary medium, including diffusion, convection and adiabatic acceleration • This works quite well reproducing primary spectra with a positive charge, and the steady state “force field” solution behaves as if there is a variable charge on the sun • Life was simple until GCR electrons came along

14. Cosmic Electrons • Electrons constitute only 1% of primary cosmic radiation, and were first identified in 1961 (Earl 1961, Meyer and Vogt 1961). • Initially it appeared that the modulation of electrons could be described by the same formalism as that for protons (Fisk 1971; Schmidt 1972; Fulks 1975) but this simple picture failed in the years 1973 and 1974 (Caldwell et al. 1975, 1977). • This was difficult to understand because it was thought that regions of outward and inward polarity were scattered randomly throughout the wind, following the pattern of magnetic flux seen on the solar surface • In such a field, positive and negative particles would have (statistically) exactly the same behavior. • The discovery that the dipole component of the magnetic solar field emerges in the solar wind led to a examination of the role played by gradient and curvature drifts, in the overall process of modulation (Jokipii and Levy 1977; Lee and Fisk 1981).

15. The Heliospheric Current Sheet • Magnetic fields in the solar wind are organized into large regions of similar polarity divided by a relatively thin “neutral” current sheet • Differences between the axis of solar rotation and the magnetic field dipolar axis cause ripples in the current sheet • At the equator, the tilted current sheet results in alternating regions of magnetic polarity (“sectors”)

16. . Reversals of the solar magnetic field occur every 11 years • Lack of axial symmetry in a magnetic field can produce charge sign dependence • Positron/electron and proton/antiproton comparisons provide a powerful differential way to study this

17. Electrons and Helium at 1.2 GV • Three separate effects are intermingled: • Charge sign dependence • Velocity dependence • Individual solar cycle variability

18. Charge Sign Dependent Modulation • Electrons and helium at a rigidity of 1.2 GV show differential modulation that is almost certainly due to charge sign • However the electrons move three times as fast as the helium and dynamical interactions might be the real cause

19. Electrons and Positrons • Electrons and positrons have the same mass, and nearly the same spectral shape. The only difference is charge sign • Solid line is the local interstellar space positron abundance calculated by Protheroe (1982). • Dashed lines are the predictions of Clem et al. (1996) for A+ (top line) and A- under the assumption that the electron/helium effect is only due to charge sign. • Solid symbols show data taken in the A+ state, while open symbols represent data taken in the A- state.

20. PAMELA Results • Highly precise PAMELA data agree with AESOP within errors • Their interpretation still does not seem to fully recognize charge sign dependence

21. Current Sheet Tilt Angle • Jokipii and Kota consider the tilt of the current sheet to be a very important parameter • Curvature drift is very rapid along the sheet, and the total distance to the “source” is a strong function of the tilt angle • Indeed, some charge sign effects clearly are a function of current sheet tilt angle

22. Helium and Electrons ~ 1.2 GV • Equally clearly, there are charge sign dependent processes operating that do not have a simple relationship to the tilt of the current sheet Solid points are for A_ and open points are for A+

23. Helium “Spike” in 1987 • The sudden transition at solar maximum remains remarkable • Spikes at solar minimum are also not explained • Note that the spike seems to precede any change in the magnetic field

24. Nucleon Modulation is Typically Better Described by Models than is Electron Modulation

25. Why is the Cosmic Ray Electron Spectrum so Strange? • Even with the new Voyager data, the upturn of the electron spectrum at low energy remains a mystery • Extreme flux in LIS • Anomalous electrons • Mean free path

26. Electron and Ion Mean Free Path • Wolfgang Droege has shown that dynamical effects can cause low energy electrons to have extremely long mean free paths compared to protons

27. Conclusions • Many aspects of solar modulation are understood • Mysteries remain, particularly when one considers charge sign • Essentially nothing is known about positron abundance in the region of the electron spectrum upturn