# Presentation: 653706

## Modeling and Simulations in Space Plasma Physics

- S. Peter Gary
- Group ISR-1
- Los Alamos National Laboratory
- 21 July 2008

## What is a plasma?

- Space is permeated by plasma: a gas of ionized particles.
- If you heat a solid, it will melt to form a liquid.
- If you heat a liquid, it will vaporize to form a gas of molecules or atoms.
- If you heat a gas hot enough, the atoms will break down into electrons and ions, and a plasma will form.

## Space Plasmas are Different

- Local properties of neutral fluids, like air and water, are determined by collisions.
- Local properties of plasmas are strongly affected by electric and magnetic forces, so they behave very differently than neutral fluids.
- Space plasmas are called “collisionless” if they are:
- Sufficiently hot and
- Sufficiently tenuous.

## Slide 4

## Waves in Space Plasmas

- Light waves
- c = 3 x 105 km/s
- Fluctuating electric and magnetic fields.
- Do not interact with plasma at high frequencies
-  Alfven waves
- vA ~ 40 km/s
- Fluctuating fields and plasma velocities
- Strong plasma interactions

## Space plasma regimes include:

- Solar corona: The hot solar atmosphere.
- Solar wind: The corona is the source of this hot plasma blowing out into space.
- Terrestrial ionosphere: Cold plasma just above the Earth’s neutral atmosphere.
- Terrestrial magnetosphere: Plasma from the solar wind and ionsophere trapped in the Earth’s magnetic field.
- Terrestrial magnetotail: The force of the solar wind pulls the Earth’s magnetic field out to a long “tail.”

## Solar corona

- SOHO/EIT image in Fe IX/X at 171 A: The corona at T ~ 1,000,000 K.

- *  High Latitudes: Open coronal holes.
- *  Low Latitudes: Active regions with closed coronal loops.

## Solar wind

## Magnetosphere and Magnetotail

## How does science work?

- Many scientific studies examine small, isolated phenomena.
- Observations
- Build an experiment
- Make measurements
- Analyze data
- Draw conclusions
- Theory
- Develop a formalism
- Solve the problem
- Draw conclusions
- Comparison of observations and theory leads to...
- More observations
- More theory

## How does space science work?

- Observations
- Build an experiment
- Make measurements
- Analyze data
- Draw conclusions
- But with the connectedness of space plasmas, theories of isolated phenomena may not apply.  So if comparing theory and observation doesn’t work...
- More observations
- Build another experiment
- Make more measurements
- Analyze more data
- Draw more conclusions

## What’s missing from this picture?

- In a laboratory, one can build a controlled experiment to test a theory.
- In space science, there are almost no controlled experiments.  We build probes, and send them to measure what is out there.  It is very difficult to compare space observations and theory.
- Computer models and simulations have the capability to represent the large-scale properties of space plasmas.
- Thus, computations are an essential link between space observations and theory.

## Theory, Simulations, and Modeling

- Theory: pencil, paper, equations.  Basic physical questions.
- Simulations: computers, equations.  Representations of small, fundamental systems.
- Modeling: computers, equations.  Representations of large, complex systems.

## A Modeling Example

- In the Earth’s magnetosphere, the fundamental process of “magnetic reconnection” allows transfer of magnetic energy to plasma energy.
- Reconnection leads to magnetospheric substorms, which can affect satellites and sometimes Earth-bound electrical transmission systems.
- The following illustration shows a magnetotail computer model in which reconnection leads to strong plasma acceleration.

## Slide 15

## Magnetotail Model: Interpretation

- Magnetic field lines in the northern magnetotail point in the opposite direction from field lines in the southern magnetotail.
- If we allow reconnection in the model, the computations show magnetic energy is converted to plasma kinetic energy, in agreement with observations.
- But the model does not explain why reconnection takes place; it only demonstrates the consequences of that process.

## Space Plasma Simulations

- Simulations represent small scale physics, e.g. the interactions between charged particles and electromagnetic fields.
- Plasma simulations consider the motion and interaction of a large number of charged particles.  By averaging over the velocities of all particles, the results of simulations can be compared against the results of large-scale models.
- Computers are essential: a small simulation uses a million particles, and a large simulation uses a billion particles.

## Simulation Example: Reconnection

## Reconnection Simulation: Interpretation

- Plasma simulations like this one show that reconnection can originate in small regions of weak magnetic field where electrons are not constrained by the magnetic field.
- The simulations show that reconnection can begin in a small region, and that this can trigger the spreading of reconnection to the much larger volumes represented by the computer models of the magnetotail.

## Another Simulation Example

- The solar corona is much hotter (T~106 K) than the visible surface of the Sun (T~104 K).
- How can this be?
- Large-scale models cannot explain this mystery.
- Computer simulations of the small-scale physics of the corona may help answer this question.

## Heating of the Corona: A Hypothesis

- Magnetic field lines extend from the solar surface into the corona.
- Turbulence on the solar surface drives Alfven waves which propagate into the corona.
- Hypothesis: These waves heat plasma in the corona.

## Simulation: Alfven Wave Heating

- We do a “numerical experiment.”
- We drive Alfven waves in the system.
- We study the response of the particles.
- We pay special attention to the positive ions, which are observed to be more strongly heated than electrons.
- We find strong ion heating.

## Slide 23

## Simulations: Alfven Wave Heating of Ions

- Simulations show:
- Ions are heated by Alfven waves
- Ions are heated in directions perpendicular to magnetic field.
- Both results are consistent with observations of the corona.
- Can we do better than “are consistent with?”
- We carry out an ensemble of simulations, and compare against solar wind observations, which are much more detailed than coronal measurements.

## Observations vs. Simulations

## Observations vs. Simulations

- Perpendicular heating of solar wind alpha particles increases as the alpha/proton relative speed becomes more negative in both observations and simulations.
- This strongly suggests that Alfven waves are heating heavy ions in the solar wind, and possibly also in the corona.
- We need more observations and more simulations before we can claim real agreement between the two.
- Using solar wind observations, we can test our simulation results in detail, and then extrapolate back to the corona with more confidence that we have the right physics.

## Simulation: Electron Beam Scattering

- Hot electrons from the solar corona stream into space and are measured in the solar wind near Earth.
- Gary and Saito have recently simulated a burst of such electrons.  Prediction: There should be a characteristic broadening of the electron beam:

## Observations vs. Simulations

## Summary

- Models and simulations are critical tools in linking space physics observations with fundamental plasma physics theories.
- The interpretation of output from computer models and simulations relies heavily on computer graphics.  Assisting a staff member in that interpretation could be an interesting project for a computer-capable high school student.