SH33A-1191

GEOTAIL Observation of Interplanetary Shock-Magnetic Hole Interaction on 25 September 2001: An Overview

T. Terasawa, M. Oka, K. Nakata, Y. Tanaka, Y. Saito, T. Mukai, Y. Kasaba, H.Hayakawa, A. Matsuoka, K. Tsuruda, K. Ishisaka, H. Kojima, H. Matsumoto, K. Keika, M. Nose, and R. W. McEntire

Magnetic holes (MHs) in the solar wind are isolated depressions in the magnitude of the interplanetary magnetic field, which have attracted recent observational and theoretical attentions. On 25 September 2001, we observed an interaction event between an interplanetary shock (IPS) and a MH using the GEOTAIL data (plasma and energetic particles, magnetic and magnetic fields, and plasma waves) as well as those from the other spacecraft (SOHO, ACE, Genesis, and WIND). What we observed were, (1) within the MH the solar wind plasma density and temperature respectively showed a decrease and increase keeping the pressure balance condition with the dropping magnetic pressure, (2) the solar wind velocity showed a significant decrease in the region surrounding the MH, (3) the nonthermal ions were trapped within the MH structure, and (4) enhancement of magnetic turbulence in the MH.

(Preliminary)

* The apparent shock velocity along the X axis, Vapp =Vshock / cos n cos n ~ 957 km/s

宇宙空間・磁気圏物理学コロキウム 2005/7/7 担当寺沢

LEP overview

25-26 September 2001

Bow shock

IP shock

25-26 September 2001

02 03 04 05 06 JST

IP shock

(a) (b)

GEOTAIL

magnetic holeat ACE

magnetic holeat GEOTAIL

ACE-GEOTAIL (time adjusted by the IPS)IPS

Apparent velocity of the MH:

Minutes after 19:00UT on 25 Sept 2001

Apparent velocity of the MH = r [Ace-Geotail] / t = 212.6Re / 28min ~ 807km/s … this value is much faster than Vsw at Geotail. Possibilities:(1) The estimation of t~28min is not correct (mis-identification of the MHs at Ace/

Geotail)?

(2) Geometric effect: When considering the above two possibilities (1) and (2), we have implicitly assumed that the normal direction of the MH was close to the X axis. However, if this direction was oblique to the X axis, we should take into account of the geometric effect (Figure 7 in the next page).

Figure 7(a)

=(-520, -130, 20) km/s… here we use the measured solar wind velocity at Geotail.

~800 km/s along the Ace-Geotail line

VMH,n = Vsw cos 1

Vapp =VMH,n / cos 2

The normal vector of the magnetic hole surface is taken in the direction of (MH,n, MH,n).

Latit

udin

al a

ngle

Longitudinal angle

Vapp =Vsw cos 1 / cos 2

Figure 7(b)

WIND-GEOTAIL (time adjusted by the IPS)

magnetic holeat WIND?

IPS

magnetic holeat GEOTAIL

magnetic holeat ACE

magnetic holeat GEOTAIL

ACE-GEOTAIL (time adjusted by the IPS)IPS

?

WIND-GEOTAIL (time adjusted by the IPS)

magnetic holeat WIND

IPS

magnetic holeat GEOTAIL

The magnetic hole was seen to be overtaken by the IPS. … A consistent picture between ACE and Geotail.

However, the consistent picture seen between ACE and Geotail (left panel) is lost when we compare data from Wind and Geotail. Why?

Magnetic hole at GeotailIPS