A Statistical Analysis of the Effects A Statistical Analysis of the Effects of Fast Plasma Flows on the Pi 2 of Fast Plasma Flows on the Pi 2

Pulsations and Its Associated Pulsations and Its Associated Magnetospheric ReconfigurationMagnetospheric Reconfiguration

Tung-Shin Hsu1 and R L McPherron1, Yasong Ge1, Joseph Borovsky2, Hui Zhang1, Vassilis Angelopoulos1, Christopher Russell1

1.IGPP/UCLA2.Las Almos National LabGEM 2008, Midway, Utah

June 22 – June 27, 2008

Selection of THEMIS Flow Burst Events• All of the events are beyond -5 Re

• CCNV nightside Pi 2 is used.

• Vxgsm is used for selection of events.

• A sliding 5-min time windows is used for selection of possible flow bursts events.

• Threshold value is set at 150 km/sec.

• Inside this 5-min time window, there must be at least 30 second beyond 150 km/s.

• Visual inspection is used to selected the onset time instead of the maximum Vx peak time.

• If multiple S/C detect flow, the earliest flow onset time is used.

Superposed THEMIS Vx at Different Location

Superposed Bz at Different Locations

Superposed Pi 2 Power at Flow Burst

Superposed Pi2 power and Vx

• Pi2-Flow: 45%, Pi2-No Flow: 25%, Pi2-weak Flow: 30%

• The superposed Vx and Pi 2 power by Pi 2 onsets is very similar to the behavior by plasma flow onsets.

• This strongly suggests that Pi 2 and plasma flow have a very close relationship.

Near Earth Neutral Line (?)

[Shiokawa et al., 1997; Zhang et al., 2007; Birn et al., 1999; Roux et al., 1991; Pu et al., 1997; Cao et al., 2008]

• It seems like that the flow braking as well as the Bz dipolarization occurs in the 8~15 Re in the tail.

• This is consistent with the suggestion of the flux pile up neutral line model.

Introduction• Pi 2 is important for substorm

studies.

• Several case studies has suggested Pi 2s are associated with plasma flows.

• Hsu and McPherron [2007] suggested that substorms Pi 2s have high probability of flow association.

• However, a recent statistical study [Kim et al. 2007] suggests that only 6% ~26% of probability of flow associated Pi 2s.

• This probability seems too low if the Pi 2s are generated by the tail plasma flows.

• With the launch of the THEMIS, we can re-examine this question with multi-points observations

Position and Time Separation of Plasma Flow Events

It seems that the time separation of the Plasma flow events is bout 6~8 minutes

Pi 2- Flow Relationship Is Not Simple

• It seems that the plasma flows are decelerated toward the earth.

• Inside the synchronous orbit, there is no flow detection.

• The flow speed are approximately the same at 21 to 25 Re.

• The braking of the plasma flows starts at 19 Re and is most significant from 15 to 9 Re.

Superposed THEMIS Vx at Different Location

Summary (1)

• Some of the plasma flow bursts may not penetrate deep enough to the near-Earth region to generate Pi 2s.

• When there is a plasma flow burst, there is a high probability to see ground Pi 2s (~75%).

• When there is a Pi 2, there is reasonable chance (45%) to see plasma flows in the tail

• The Pi 2s duration is corresponding well with the plasma flow duration.

• Pi 2s and plasma flows can cause intensification of auroral electrojets.

Summary (2)

• This suggests that the plasma flows is very likely the source of the Pi 2s.

• The plasma flows are very likely carrying magnetic flux and are associated with magnetospheric reconfiguration.

• The reconnection site seems at ~20-25 Re.

• The flux pile up region (flow braking region) seems at ~8-15 Re in the tail.

• The cluster between flow bursts and AL onsets suggests flow bursts is important to substorm dynamics.

Superposed Bz at Different Locations

• The dipolarization is not obvious as the S/C goes further from the Earth.

• From 7 Re to 15 Re, there is a significant dipolarization while the flows are clearly decelerated.

• Inside the 6.6 Re orbit, the flow is barely detected as well as the dipolarization (SCW effect?)

Probability of Vx with Pi2 • The probability of

association between flow bursts and Pi 2 is higher when more than one S/C detected flows.

• This may suggest that some of the flow may not penetrate to the near earth region to initiate Pi 2 waves

• It is not unusual that only 3~4 S/C has plasma observation.

Time Delay between AL onsets and Flow Bursts

There is a cluster between AL Onsets and flow bursts whichmay suggest flow bursts is Important to substorm onsets.

How about Pi2 Events-Flows Association?

Selection of Pi 2 Events• Only substorm associated Pi 2s

are examined.

• A +/- 2 hours time windows centered by AL onset is created for examination of the association between Pi 2s and plasma flows.

• Every Possible Pi 2s are picked and marked the possible association.

• If there is a auto detected plasma flow event within +/- 5 min, it is marked as an associated event.

• Total: 774 Pi 2 events.• 353 associated with flow (45%)

Pi 2s without Plasma Flows

Pi 2s with Plasma Flows

• The duration of the plasma flow is about 5~10 minutes.

• The Pi 2s duration is also about 5~10 minutes

• The near earth region shows plasma perturbation when no plasma flows associated with Pi 2s

• The deceleration of the plasma flow is also observed in the superposed curves (by Pi 2 onsets).

Intensification of Auroral Electrojet with Plasma Flows