H3: Radiation Environment in Heliosphere: SEPs and GCRs

Moderator:

Jingnan Guo (University of Science and Technology, China), jnguo@ustc.edu.cn

Co-Moderator:

Lulu Zhao (University of Michigan, USA), zhlulu@umich.edu

Advisor:

Robert Wimmer-Schweingruber

Impact:

Human Exploration
(Aero)Space Assets Functions

Liaisons:

S3 Cluster: Sophie Murray

Human Exploration: Eddie Semones, NASA/JSC Space Radiation Analysis Group, edward.j.semones@nasa.gov

Introduction:

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.

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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.

Objectives:

To understand, characterize and predict the flux of SEPs. To achieve these objectives three crucial issues need to be understood:

the injection of particles into the acceleration process,
the acceleration process itself,
the transport of SEPs (and GCRs) to the location for which predictions are to be made.

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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.

Injection:

Composition of suprathermal particle population (Solar Orbiter and Parker Solar Probe)
Spatial and temporal distribution of suprathermal particles (Solar Orbiter and Parker Solar Probe)
Processes affecting the suprathermal particle population (Solar Orbiter and Parker Solar Probe)
Role of turbulence in maintaining the suprathermal particle population (Solar Orbiter and Parker Solar Probe)
Kinetic models of particle injection

Acceleration:

Where are particle accelerated to the energies relevant for space weather
Relative role of flares, reconnection, and coronal shocks
Role of turbulence, compression, shocks
Realistic 3D hybrid models for ongoing acceleration in corona and inner heliosphere

Transport:

Understanding of turbulence evolution throughout the heliosphere
3D models of the heliospheric magnetic structure
Multi-spacecraft measurements of SEPs and GCRs distributed in the heliosphere (Solar Orbiter, Parker Solar Probe, L5, Mars, Jupiter, etc.)
Models of 3D particle transport in corona, inner and outer heliosphere

Essential Space Environment Quantities / Forecasting Problems:

Action topics:

For list of action topics please click "Read more"

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Observational signatures of pre-existing suprathermal particle population (including composition, and spatial and temporal distribution).
Understanding and modeling processes responsible for maintaining suprathermal seed population.
Understanding role of reconnection, turbulence, compression & shocks in particle acceleration.
Understanding and quantifying relative role of flares and coronal shocks in accelerating particles to the energies relevant for space weather impacts.
Modeling of ongoing particle acceleration and transport in corona, inner and outer heliosphere.
Multi-spacecraft measurements of SEPs and GCRs distributed in the heliosphere (Solar Orbiter, Parker Solar Probe, L5, Mars, Jupiter, etc.).
Assessment and improvement of capabilities to forecast temporal evolution of SEP fluxes at different locations in heliosphere.
Automatization and improving operational forecasts.
Generating inputs to global space weather roadmap.

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