# Presentation: 419811

## Particle Acceleration in the Heliosphere           --- Lessons from GEOTAIL

- Particle Acceleration in the Heliosphere           --- Lessons from GEOTAIL

- Oka, M.1,2,3 and  T. Terasawa2
- Kwasan Observatory, Kyoto Univ.
- Univ. of Tokyo
- ISAS / JAXA

- moka@kwasan.kyoto-u.ac.jp

## Particle Acceleration at Flares

- Many models have been proposed to account for non-thermal electrons of the loop-top sources.
- One of the candidates is the acceleration by shocks.
- However, it is still uncertain under which condition do shocks accelerate particles.

- Can a fast mode shock
- at the loop-top environment
- accelerate electrons?

- Parameter dependence of electron acceleration at shocks ?

## Particle Acceleration at Interplanetary Shocks

- Particle Acceleration at Interplanetary Shocks

- Solar Energetic Particles (SEPs)
- Detailed
-     studies
-     from ion compositions (e.g. Tylka et
-           al., 2005)

- Many studies on ions, but how about electron acceleration ?

## To obtain some hint for electron acceleration by shocks in the solar corona and the heliosphere....

## Geotail Spacecraft

- Launched in 1992 as a collaborative project by the ISAS and NASA.
- Designed to investigate distant magnetotail, but 40% of its orbital time is in the upstream side of the magnetosphere.
- Suitable for studying the solar wind, the bow shock, interplanetary shocks, and CMEs.

## Electron Acceleration at Interplanetary Shocks

- Though rare, interplanetary shocks associated with super flare events accelerate electrons.
- In the case of 21 Feb. 1994 event, the acceleration mechanism was clearly determined to be the Diffusive Shock Acceleration (Shimada et al., 1999).
- When and how do shocks accelerate electrons is the outstanding problem                      for space physics and astrophysics !!

- An example of
- shock observation
- by GEOTAIL

## Electron Accelerationat the Earth’s Bow Shock

- Large data set for the bow shock compared to interplanetary shocks.
-  Enables statistical study over wide range of parameters.
- Until today, *Gosling et al., 1989* is the only comprehensive work that studied electron acceleration at the bow shock.

*Gosling et al., 1989*

- Extend this work using the Geotail data.

- ISEE

## Our Statistical Study

- 78 ‘clean’ quasi-perp shock crossings
- From Jan 1995 – July 1997

- Earth

- magnetopause

- bow shock

- B

- n

- Bn

- `shock angle’

- >45 dgr
-   ‘(quasi-) perpendicular’

## Spectral Index 

- Hard spectrum
- for
- large Bn and
- large MA

## Whistler Critical Mach Number

- Kennel, 1985;
- Krasnoselskikh et al., 2002

- V1

- Vphase

- `Super Critical’
-     if  MA > Mw

## Spectral Index 

- Oka et al., in preparation

## Mechanism of Acceleration ?

- _Speculation_:
-    Different acceleration mechanism                  for super- and sub- critical shocks ?
- Sub-critical case:Diffusive Shock Acc. ?
- Super-critical case: Any other efficient way ?

- → Detailed Event Analysis

## A sub-critical shock

- Detailed analysis showed that this event can be well explained by the DSA.
-  (Oka et al, in preparation)

## 1 July 1996
MA~14, Bn~86 dgr
Candidates of acceleration
Drift acceleration
Ripple acceleration
1st order Fermi
2nd order Fermi
Surfing 

- MA~14, Bn~86 dgr
- Candidates of acceleration
- Drift acceleration
- Ripple acceleration
- 1st order Fermi
- 2nd order Fermi
- Surfing

- A super-critical shock

- Oka et al., in preparation

## Summary

- Recent Geotail results showed that electron acceleration at shocks is regulated by the whistler critical Mach number which is a function of both Alfven Mach number and shock angle.
- It is important to check this parameter dependence at other shocks.
- To test the shock acceleration model for the hard X-ray sources at solar flares, we need detailed and accurate information of the loop-top environment   Solar B
- For IPSs, we need larger set of examples of electron acceleration for interplanetary shocks  STEREO
- We are looking forward to collaborate with other S/C (ACE, WIND, Cluster,  and future missions such as MMS/SMART, MagCon, and SCOPE).