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H2: SEP and GCR in heliosphere.


Solar Energetic Particles (SEPs) and Galactic Cosmic Rays (GCRs) are electrically charged particles with energies which significantly exceed those of typical solar-wind particles (typically 0.5 - 4 keV/nuc). As such they are bound to the interplanetary or heliospheric magnetic field (IMF) by the Lorentz force. Their gyro-radii depend on their energy and the strength of the magnetic field. Because of their large speed relative to that of the solar wind and their large gyro-radii, the local properties of the SEPs and GCRs are not determined by the local properties of the IMF, but at much larger scales. Thus, understanding and predicting the flux of SEPs and GCRs in the heliosphere requires knowledge of the IMF throughout the heliosphere and at the Sun. GCRs and SEPs reach the near-Earth space environment from two different “ends” of the heliosphere. GCRs which are predominantly accelerated in the galaxy enter the heliosphere from “outside” and their propagation is often modeled as diffusion through the heliospheric IMF and the heliosheath. SEPs on the other hand, are accelerated at the Sun and then distributed in the heliosphere. This “diffusion” or “distribution” process is called “particle transport” and is a topic of current research. Because of the long life of GCRs of hundreds of thousands of years, the GCR flux is affected only by transport processes. GCRs and SEPs are accelerated by supernova-driven shocks and by solar flares and coronal and CME-driven shocks, respectively. Because we consider the GCRs as a “background” which is modulated by solar activity at all time scales, their acceleration proper is not seen as a prime space weather issue, but understanding their transport is important for long-term predictions and possibly for also for terrestrial climate. On the other hand, SEP acceleration is an important topic for space weather, as the flux of SEPs can vary by many orders of magnitude. The respective roles of solar flares/reconnection and shocks in the deep corona are currently strongly debated, it is well possible that both play a key role in the energization of SEPs.

In order to be accelerated, the speeds of SEPs must exceed the ambient Alfvén speed; the more energetic a particle is, the more energy it gains in most acceleration models. Thus, it appears that SEPs are accelerated out of a “pool” of so-called suprathermal (because they are much faster than the thermal solar wind particles) particles, whose origin is unclear. Remnant solar flare particles, but also suprathermal solar wind particles have been considered for this “pool”. It is these particles which are injected into the acceleration process for SEPs.

Thus, to understand and predict the flux of SEPs, three crucial issues need to be understood: the injection of particles into the acceleration process, the acceleration process itself, and the transport of SEPs (and GCRs) to the location for which predictions are to be made. These issues are all current topics of scientific debate and significant progress in the scientific understanding of them needs to be made to allow reliable predictions of the SEP and GCR space weather component. The following paragraphs give more details.

Focused action tasks:


Acceleration: Transport: Inventory of available resources and on-going efforts.
Action plans up to COSPAR 2020

Assessment of Space Weather Modeling Capabilities
SEP focus evaluation team
CCMC logo National Aeronautics and Space Administration Air Force Materiel Command Air Force Office of Scientific Research Air Force Research Laboratory Air Force Weather Agency NOAA Space Environment Center National Science Foundation Office of Naval Research

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