# Presentation: 334740

## Diagnostics of Solar Wind Processes Using the Total Perpendicular PressureLan Jian, C. T. Russell, and J. T. Gosling

- How does the magnetic structure of the Sun lead to stream interaction regions (SIRs)?
- What are interplanetary coronal mass ejections (ICMEs)?
- Total perpendicular pressure as a solar wind diagnostic parameter
- Examples of SIRs
- Examples of ICMEs
- Conclusions

## Formation of Stream Interaction Regions

- Magnetic structure of corona controls solar wind velocity
- Solar wind speed is slowest near the heliospheric current sheet
- If current sheet is in rotational equator, there would be no fast-slow stream interaction
- When magnetic axis rotates away from rotation axis, the fast plasma can take over slow plasma
- Fast stream should be encountered after heliospheric current sheet crossing

## Evolution of Stream Interaction Regions

- SIRs arise from steady state streams on the solar surface
- As the streams move away from the Sun and collide, their signature evolves
- The density and magnetic field between the streams are compressed. The plasma temperature is increased
- If the velocity jump is sufficiently large relative to the compressional wave speed, then one or more shocks may arise: one forward, one reverse
- Far from the Sun, the SIRs coalesce

## Structure and Evolution of Interplanetary Coronal Mass Ejections

- Interplanetary Coronal Mass Ejections (ICMEs) arise from transient disturbances on the Sun
- ICMEs may play an important role in the evolution of the photospheric magnetic field during the solar cycle
- Magnetic structures (clouds, flux ropes) are seen in the center of many of these disturbances
- ICMEs often have leading shocks, declining velocity profiles, low proton temperatures

## Observational Characteristics of SIRs and ICMEs

- 1.	SIRs occur where Vsw is increasing, not decreasing
- 2.	ICMEs often occur where Vsw is decreasing because they are expanding
- 3.	SIRs often occur near the heliospheric current sheet
- 4.	SIRs should result in density and temperature maxima on the SIRs
- 5.	ICMEs often have magnetic flux rope signature, strong rotating field
- 6.	SIRs and ICMEs both strongly affect the direction of the solar wind flow

## What is total Perpendicular Pressure?

- Magnetic field exerts no pressure along its length but plasma does
- Magnetic field and plasma both exert a pressure force perpendicular to the field direction
- Total perpendicular pressure is B2/2o + ni kTi + nek Te
- Signatures of processes in the perpendicular pressure are usually much simpler than the signatures of the constituent components
- Irregularities in perpendicular pressure are smoothed by compressional waves
- Sudden discontinuities are caused by shocks

## Example 1: Perpendicular Pressure of a SIR

- Simple profile: acceleration phase followed by deceleration phase on increasing speed profile
- Component variations complex:
- magnetic pressure irregular
- thermal speed constant and density rise in acceleration phase
- sharp density drop at interface; density and thermal speed gradually decline in deceleration phase

## Example 2: Perpendicular Pressure of a SIR

- Perpendicular pressure enhancement bounded by shocks
- Acceleration phase has temperature decrease while pressure increases.
- Deceleration phase has declining thermal speed and density.
- Magnetic field qualitatively similar to pressure
- Lack of rotation in field and occurrence on increasing speed profile mark this as a SIR
- Pressure and field strength profile mimic those of ICME

## Example 3: Perpendicular Pressure of an ICME

- Leading shock followed by magnetosheath and magnetic obstacle
- Pressure profile much simpler than variations in components
- Declining velocity profile
- Rotating field in obstacle

## Example 4: Perpendicular Pressure of an ICME

- Leading shock with possible magnetosheath
- Pressure decreases monotonically after shock
- Obstacle beginning not clear
- Density increases initially while pressure decreases

## Conclusions

- It is force that drives the evolution of solar wind structures. The force is the gradient of the total perpendicular pressure.
- Total perpendicular pressure generally has a simple temporal variation, smooth except for shocks.
- The temporal variations of the components of total pressure are not simple.
- The perpendicular pressure can simplify the identification of different types of interactions.
- We recommend total perpendicular pressure as a useful diagnostic parameter for solar wind interactions.