What coronal parameters determine solar wind speed? M. Kojima, M. Tokumaru, K. Fujiki, H. Itoh and T. Murakami Solar-Terrestrial Environment Laboratory, Nagoya University K. Hakamada Department 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 m 2 Freq. 327 MHz

Four-station system for IPS 102km 126km 131km 98km 109km

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 scale size. Wang and Sheeley (1990) the flux expansion rate is inversely proportional to the solar wind speed. Fisk et al. (1999) reconnection of emerging magnetic fields in supergranules supplies Poynting flux to accelerate the solar wind.

CH size vs. V

? ? potential field neutral line after Kojima et al., 1999

Oct dependence on polar coronal hole scale size 1991

CH size vs. V pola rCH Polar CH Equatorial CH

B 10G small expansion B weak small expansion B weak medium expansion B 20G large expansion CH size

We investigate the relation between the solar wind velocity and 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 photosphere HeI, open B region Data from the National Solar Observatory (NSO) at Kitt Peak Potential magnetic field lines

Analysis 1 Data period:

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.

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 , CR1844 : Jun , CR1855 : Apr , CR1870 : Jun , CR1887 : Sep , CR1898 : Jul , CR1901 : Sep , CR1909 : May , CR1925 : Jul , CR1939 : Aug , CR1950 : May , CR1964 : Jun , CR1976 : May , Analysis 3 Data period:

Mesh size:

Solar minimum phase

V=236.7×B/f

Solar maximum phase

V=163.6×B/f V=143.1×B/f B p ≦１５ G (High Latitude) B p ＞１５ G (Low Latitude)

Physical meaning of the parameter B/f Energy supply ー B Efficiency ー Flux expansion f slow fastslower CH size What is the role of the coronal hole size in determining the solar wind speed ?

Flux expansion + B energy after Hirano (2004)