# Presentation: 880001

## B. Lavraud,1 M. F. Thomsen,1 B. Lefebvre,2 S. J. Schwartz,2 K. Seki,3 T. D. Phan,4 Y. L. Wang,1 A. Fazakerley,5 H. Rème,6 and A. Balogh2

**1: ***Space Science and Applications, Los Alamos National Laboratory, USA***2:***** ****Imperial College, London, UK*
**3:** *STELAB, U. of Nagoya, Japan*
**4: ***Space Sciences Laboratory, U. C. Berkeley, USA*
**5:***** ****Mullard Space Science Laboratory, Dorking, UK*
**6: ***CESR / CNRS, Toulouse, France*

_**Evidence for newly closed magnetosheath field lines **__**at the dayside magnetopause under northward IMF**_

## OUTLINE

- Motivation
- Event illustration Magnetic and electronic structure of the magnetopause
- Statistical results
- Conclusions

## Motivation: Formation of the Low Latitude Boundary Layer and Cold, Dense Plasma Sheet under north IMF

- Existence of dayside LLBL under north IMF

- [from *Seki et al*., 2005]

## Motivation: Formation of the Low Latitude Boundary Layer and Cold, Dense Plasma Sheet under north IMF

- Existence of dayside LLBL under north IMF
- Plasma sheet is denser and colder after a prolonged northward IMF: the Cold, Dense Plasma Sheet

- [from *Seki et al*., 2005]

## Motivation: Formation of the Low Latitude Boundary Layer and Cold, Dense Plasma Sheet under north IMF

- Existence of dayside LLBL under north IMF
- Plasma sheet is denser and colder after a prolonged northward IMF: the Cold, Dense Plasma Sheet
- Composition (O+) has confirmed its more direct solar wind origin

- [from *Seki et al*., 2005]

## Motivation: Formation of the Low Latitude Boundary Layer and Cold, Dense Plasma Sheet under north IMF

- Existence of dayside LLBL under north IMF
- Plasma sheet is denser and colder after a prolonged northward IMF: the Cold, Dense Plasma Sheet
- Composition (O+) has confirmed its more direct solar wind origin

- Two candidate mechanisms: 	- Double high-latitude reconnectionfor CDPS formation	 	- Kelvin-Helmholtz instability

- [from *Seki et al*., 2005]

## Motivation: Formation of the Low Latitude Boundary Layer and Cold, Dense Plasma Sheet under north IMF

- Existence of dayside LLBL under north IMF
- Plasma sheet is denser and colder after a prolonged northward IMF: the Cold, Dense Plasma Sheet
- Composition (O+) has confirmed its more direct solar wind origin

- K-H instability may not explain the formation of the dayside LLBL:

- Two candidate mechanisms: 	- Double high-latitude reconnectionfor CDPS formation	 	- Kelvin-Helmholtz instability

- Here, we focus on the occurrence of double high-latitude reconnection

- [from *Seki et al*., 2005]

## Illustration of MP crossing under northward IMF

- Heated electrons flowing both parallel and anti-//to magnetic field
- Low ion temperature: outside MP

- S/C trajectory

- Magnetopause

-  Potential signature of newly closed field lines

- Tions

- [*Onsager et al*., 2001][*Lavraud et al*., 2005]

## Structure of the magnetopause under northward IMF

_**Structure of the magnetopause under northward IMF**_

- Magnetopause

- Magnetosheath                flow

- Dayside Magnetosphere

- Lobes

- X

- Z

- Heated electrons

- Cold electrons

## Structure of the magnetopause under northward IMF

- High-latitude
- Reconnection

- Magnetopause

- X

- Z

- Cold magnetosheath electrons are heated at the magnetopause

- Magnetosheath                flow

- Heated electrons

- Cold electrons

_**Structure of the magnetopause under northward IMF**_

## Structure of the magnetopause under northward IMF

- High-latitude
- Reconnection

- Magnetopause

- X

- Z

- Cold magnetosheath electrons are heated at the magnetopause
- They may return to the magnetopause

- Magnetosheath                flow

- Heated electrons

- Cold electrons

_**Structure of the magnetopause under northward IMF**_

## Structure of the magnetopause under northward IMF

- High-latitude
- Reconnection

- Magnetopause

- X

- Z

- Cold magnetosheath electrons are heated at the magnetopause
- They may return to the magnetopause and form an observable layer on its outside

- Magnetosheath                flow

- Heated electrons

- Cold electrons

_**Structure of the magnetopause under northward IMF**_

## Structure of the magnetopause under northward IMF

- High-latitude
- Reconnection

- Magnetopause

- X

- Z

- Cold magnetosheath electrons are heated at the magnetopause
- They may return to the magnetopause and form an observable layer on its outside
- Uni-directional flows: reconnection in one hemisphere

- Magnetosheath                flow

- Heated electrons

- Cold electrons

_**Structure of the magnetopause under northward IMF**_

- Open in solar wind

## Structure of the magnetopause under northward IMF

- High-latitude
- Reconnection

- Magnetopause

- X

- Z

- Cold magnetosheath electrons are heated at the magnetopause
- They may return to the magnetopause and form an observable layer on its outside
- Uni-directional flows: reconnection in one hemisphere
- Bi-directional flows:newly closed field lines

- Magnetosheath                flow

- Heated electrons

- Cold electrons

_**Structure of the magnetopause under northward IMF**_

- Open in solar wind

- Closed in other hemisphere

## Structure of the magnetopause under northward IMF

- Magnetopause

- X

- Z

**Ion**
**edge**

- _and_

**Electron****edge**

- High-latitude
- Reconnection

- Velocity dispersion effect:Electrons are faster than protons                     andPlasma convection is sunward
-  Creation of the magnetosheath boundary layer with spatially separated electron and ion edges

_**Structure of the magnetopause under northward IMF**_

- _Velocity __dispersion_

## Structure of the magnetopause under northward IMF

- Spacecraft time series:

- <T>

- _Velocity __dispersion_

- Ion T

- e- temp.

- Sheath electron boundary layer

_**Structure of the magnetopause under northward IMF**_

- Magnetopause

- X

- Z

**Ion**
**edge**

- _and_

**Electron****edge**

- High-latitude
- Reconnection

## Structure of the magnetopause under northward IMF

- ?

- +0
- -

- e- // heat flux

- <T>

- Ion T

- e- temp.

- Sheath electron boundary layer

- Newly closed field lines:- Low ion temperature- High electron temp.- Low absolute e- heat flux

- Ion temperature may be used for “magnetopause” identification

_**Structure of the magnetopause under northward IMF**_

- Magnetopause

- X

- Z

- High-latitude
- Reconnection

-  Such observation ought to occur only under northward IMF

## Objectives

- Survey all 2001 – 2003 Cluster dayside magnetopause crossings under steady IMF
- Analysis of low absolute heat flux (bi-directional), heated electrons outside the magnetopause (ion T)
- Investigation of its statistical dependence on the local (FGM) and IMF clock angle

## Statistical criteria

- Investigate dayside MP crossings 2001- 2003 (S/C 3)
- Selection of events with steady IMF direction: clock angle variations < 20° over 30 m. interval
- 56 events were selected (with good data)
- Each event was analyzed for times when low heat flux, heated electrons are observed outside the magnetopause:
-                    CRITERIA                                      GOAL- Tions < Tsheath + (TLLBL – Tsheath)/4             outside MP- |Q//elec| < 15 μW/m2                                                     bidirectional- T//elec/T//sheath > 1.1                                    heated

## Statistical criteria

- Investigate dayside MP crossings 2001- 2003 (S/C 3)
- Selection of events with steady IMF direction: clock angle variations < 20° over 30 m. interval
- 56 events were selected (with good data)
- Each event was analyzed for times when low heat flux, heated electrons are observed outside the magnetopause:

## Statistical results: normalized distribution of clock angles

- The normalized clock angle distribution shows that such bi-directional electrons are primarily observed under northward IMF
- The local clock angle range is limited to ~[-60o ; 60o]

- 60o

- Magnetosphericfield lines

- Magnetospheric lobes

- Field line at 40o from North direction

- Cusp

- North

- Dusk

## Conclusions (1/2)

- A layer of heated magnetosheath electrons is often observed outside the magnetopause
- A statistical survey evidenced that this layer often contains low heat flux, bi-directional heated electrons
- Such electron distributions are primarily observed when the magnetosheath field has a northward component (clock angle range: ~ -60o to 60o)
- This can be attributed to reconnection having occurred in both hemispheres

## Conclusions (2/2)

- Visual examination of the events shows that 11 out of 19 events (2002-2003) with clear northward IMF show a distinguishable newly closed layer
- Double high-latitude reconnection do commonly occur
- It may lead to the formation of the dayside low latitude boundary layer (LLBL)
- Its efficiency, in terms of plasma transfer, is still to be investigated in comparison with other candidate processes such as the Kelvin-Helmholtz instability

- These findings show that double high-latitude reconnection is a tenable mechanism for LLBL and Cold Dense Plasma Sheet formation under northward IMF

## Benoit Lavraud, Edward E. Pogue, Joseph E. Borovsky et al.

*Space Science and Applications, Los Alamos National Laboratory, USA*

_**Observation and simulation of asymmetric flow acceleration in the low-Beta magnetosheath:**_
_**A slingshot effect?**_

## Illustration of flow asymmetry in low-Beta sheath

-  The flow asymmetry follows the IMF orientation

- BATS-R-US MHD simulation:

## Large acceleration event under low-Beta conditions

- Close to the magnetopause, magnetosheath plasma is accelerated up to 1000 km/s while the solar wind at the time is slower than 700 km/s

- PEACE

- CIS

## Interpretation: A slingshot effect?

- Shock

- Sheath

- Acceleration

- Deceleration?

## Conclusions (2/2)