Acceleration in the Heliosphere

Since the discovery by Victor Hess on a balloon flight in 1912 that the intensity of energetic radiation increases with altitude, it has been known that cosmic rays are an important ingredient of the space environment. The space era has opened this important phenomenon to detailed study. The extension of optical astronomy into the radio regime in the 1940’s, and later with space-based telescopes into the more energetic X-ray and γ-ray regimes, has provided ample evidence for humongous and violent objects in the Universe that generate floods of extremely energetic particles, which ultimately form the cosmic-ray population in our galactic neighbourhood. Our living environment on Earth can be affected by this potentially dangerous radiation and its mutation-driving impacts, but it is very effectively protected by a three-layer shield system: (1) the heliosphere with the solar wind and its embedded magnetic field; (2) Earth’s magnetic field; (3) Earth’s atmosphere. Satellites and space probes have gathered information about the variation in the radiation intensity and spectra in response to changes in the interplanetary medium and solar activity. They have also provided us with in-situ observations of sources, acceleration, and transport of energetic particles generated in our immediate neighbourhood, in and around Earth’s magnetosphere, at the Sun, and throughout the heliosphere. Our ability to model and scale acceleration processes enables us to understand not only our own environment, but also that of distant particle accelerators, which cannot be studied in-situ. As shown in Figure 1, the average fluence spectra in interplanetary space extend from the solar wind up to several 100 MeV/atomic mass unit [amu] with a power law for most of the range similar to cosmic rays. Shock waves, or structures where the solar wind flow is abruptly decelerated from superto sub-sonic speeds, have been identified as powerful particle accelerators in the heliosphere. They serve as a model for supernova blast waves, which accelerate galactic cosmic rays. With ever-increasing sophistication in spacecraft instrumentation, we make good use of the laboratory on our front doorstep by simultaneously observing source and energetic-particle populations as well as magnetic and electric fields near shocks. Yet the first step from the bulk plasma into the accelerated distribution (marked with ? in Fig. 1) is still very much under debate. 165