What coronal parameters determine solar wind speed?

What coronal parameters determine solar wind speed?

M. Kojima, M. Tokumaru, K. Fujiki, H. Itoh and T. MurakamiSolar-Terrestrial Environment Laboratory, Nagoya University

K. HakamadaDepartment of Natural Science and Mathematics, Chubu University

In order to model the solar wind acceleration it is important to find a universal relation between the global properties of the solar wind and corona holes. However most coronal holes are at high latitudes where spacecraft cannot access, with the exception of the Ulysses.

In this study we identify the relation between the wind velocity and

the coronal magnetic condition using the IPS tomographic measurements, which can derive an unbiased solar wind velocity map over all latitudinal ranges,.

Aperture 　 2000 m2

Freq. 　 327 MHz

Four-station system for IPS

102k

m

126k

m

131km

98km

109k

m

Interplanetary scintillation measures an integration of the solar wind speed V and density turbulent level △Ne distributed along a line of sight.

Earth

☆Radio source

Computer Assisted Tomography analysis

This technique can retrieve not only unbiased solar wind parameters but also provides high spatial resolution.

The IPS los integrations provide us with perspective views of 3D SW structures at various different view angles using both the solar rotation and solar wind outward motion.

CT Analysis

1996

• Nolte et al. (1976) 　 the velocity depends on the coronal hole sc

ale size.

• Wang and Sheeley (1990)　 the flux expansion rate is inversely proport

ional to the solar wind speed.

• Fisk et al. (1999)　 reconnection of emerging magnetic fields i

n supergranules supplies Poynting flux to 　 accelerate the solar wind.

CH size vs. V

?

?

potential field neutral line

after Kojima et al., 1999

after Kojima et al., 1999

Oct. 1999

2000

dependence on polar coronal hole scale size

1991

CH size vs. V

polarCH

Polar CH

Equatorial CH

789 68

B 10Gsmall expansion

B weaksmall expansion

B weakmedium expansion

B 20Glarge expansion

CH size

We investigate the relation between the solar wind velocityand the coronal magnetic condition for various kinds of coronal holes, which have different properties of magnetic field intensity B and flux expansion rate f.

We make correlation analysis between the velocity V and a) the inverse of an expansion rate 1/f, b) photospheric magnetic field intensity B, and c) the ratio between these two parameters B/f.

V-map on the source surface from IPS tomography

Coronal holes on the photosphereHeI, open B region

Data from 　 the National Solar Observatory (NSO) at Kitt Peak

Potential magnetic field lines

Analysis 1Data period: 1995-1996

Although the polar coronal hole extended to lower latitudes than 60 degrees, we examine the coronal hole area at latitudes higher than 60 degrees because the boundary region has a complex structure. If there is a extension from a polar coronal hole toward the equator beyond a latitude of 40 degrees, it is treated as an independent mid-latitude coronal hole.

2

Flux expansion rate source surface source surface

photosphere photosphere

A R

A R

Flux expansion rate

after Hirano et al. (2004)

strong B

weak B

Magnetic field energy

after Hirano et al. (2004)

large expansion

small expansion

Flux expansion + B energy

after Hirano et al. (2004)

1. CR1830 : Jun. 11.4556, 1990. 2. CR1844 : Jun. 28.2611, 1991. 3. CR1855 : Apr. 23.4361, 1992 4. CR1870 : Jun. 6.4778, 1993. 5. CR1887 : Sep. 12.9590, 1994. 6. CR1898 : Jul. 10.0910, 1995. 7. CR1901 : Sep. 29.8104, 1995. 8. CR1909 : May 5.2840, 1996. 9. CR1925 : Jul. 15.5059, 1997. 10. CR1939 : Aug. 1.3214, 1998.11. CR1950 : May 28.5213, 1999. 12. CR1964 : Jun. 13.3296, 2000.13. CR1976 : May 6.7222, 2001.

Analysis 3Data period: 1990-2001

Mesh size:

11

2

,

,

Flux expansion rate r photosphere source surface

r source surface photosphere

B R

B R

2

,,

r photospherephotospherer source surface

source surface

BRB

R f

Solar minimum phase

V=236.7×B/f +473.4

Solar maximum phase

V=163.6×B/f +419.9 V=143.1×B/f +376.5

Bp≦ １ ５ G (High Latitude)

Bp＞１５G (Low Latitude)

Physical meaning of the parameter B/f

• Energy supply ー B

• Efficiency ー Flux expansion f

slow

fast slower slower

CH size

What is the role of the coronal hole size in determining the solar wind speed ?

Flux expansion + B energy

after Hirano (2004)