Kazuyuki Hakamada

Low‐speed solar wind from the vicinity of solar active regions

We have investigated the origin of low-speed winds observed in association with active regions near the equator at times of solar activity minimum. The solar wind velocity distribution on a source surface at 2.5 Rs is derived by interplanetary scintillation tomographic analysis, and compact low-speed regions in it are investigated in relation to active regions and large-flux-expansion regions. We show that although the low-speed regions tend to be located near active regions, they are more closely associated with large flux expansion from the vicinity of active regions. We find that slow solar wind does not arise from closed magnetic loops above an active region, but instead the low-speed stream originates from the vicinity of one polarity side of the active region. Therefore the low-speed stream, unlike the helmet streamer, has a single magnetic polarity. This can explain why compact low-speed streams are often not associated with a heliospheric current sheet.

Latitudinal velocity structures up to the solar poles estimated from interplanetary scintillation tomography analysis

The Ulysses spacecraft observed high-speed wind at high latitudes up to 80° and found that the high-speed solar wind increased in velocity gradually with latitude and that the velocity had asymmetry between Northern and Southern Hemispheres. We have investigated the velocity increase up to the polar regions for the Carrington rotations of 1908-1915 in the year 1996. For this purpose we have made tomographic analyses of the latitudinal structure of the solar wind speed using interplanetary scintillation data obtained at heliocentric distances of 0.1-0.9 AU and latitudes up to 90°. The tomographic analysis method was modified from its previous version [Kojima et al., 1998] so that it could obtain more reliable solutions with better sensitivity in the polar region than the previous method. The results from the observations in 1996 showed that the velocity increased with latitude and had the N-S asymmetry as observed by Ulysses. These features persisted during the period analyzed. Since the asymmetry was found in rather short period observations of several Carrington rotations and at distances within 0.9 AU, it is caused neither by temporal evolution of the solar wind structures nor by interactions in the solar wind in interplanetary space. These global latitudinal velocity structures agree qualitatively with the magnetic flux expansion factor.

Resolving the enigmatic solar wind disappearance event of 11 May 1999

[i] On 11 and 12 May 1999, the Earth was engulfed by an unusually low-density (<1 cm -3 ) and low-velocity (<350 km s -1 ) solar wind for a period of over 1 day. Extensive studies of this unusual event that occurred during Carrington rotation 1949 (CR1949), using both ground-based and space-based in situ observations, have not as yet been able to identify the cause or the solar source of this event. Using solar wind velocity measurements from the four-station IPS observatory of the Solar-Terrestrial Environment Laboratory (STEL), Toyokawa, Japan, we investigate the structure of the solar wind in May 1999 during CR1949. IPS observations from STEL were used to make tomographic velocity maps to identify and delineate the extent and morphology of the stable solar wind flows during CR1949 in the vicinity of the Earth. Combined with in situ measurements of the interplanetary magnetic field (IMF), potential field computations of the solar magnetic fields in the period, and HeI 10830A observations of coronal hole boundaries during CR1949, we have identified the source region of the unusual flows and have shown that the flow responsible for the disappearance event was a stable unipolar flow originating in the vicinity of a large midlatitude active region AR8525, located at ∼18°N and between heliographic longitudes 280° and 300°. Earlier workers have speculated that such events may be caused by the large-scale restructuring of the solar magnetic field at the maximum of each solar cycle. However, by identifying the solar source and nature of this event, we believe that at least in this particular case, the association with global, large-scale solar phenomena like the periodic 11-year solar polar field reversal is most likely to be coincidental.

Solar Wind Properties from IPS Observations

Since Hewish et al. (1964) discovered the interplanetary scintillation (IPS) phenomena, the IPS method has been used as one of the few devices which can be used to observe solar wind in three-dimensional space. However because of the line-of-sight integration effect of IPS, solar wind had to be studied with blurred images. In the late 1990s new methods of IPS observation and analysis which can deconvolve the line-of-sight integration effect were developed independently by a group at University California at San Diego (Grall et al., 1996) and a group at the Solar-Terrestrial Environment Laboratory, Nagoya University (Asai et al., 1998; Kojima et al., 1998; Jackson et al., 1998) . Today we can obtain unbiased solar wind images with high spatial resolution from IPS observations. The Ulysses spacecraft has been observing detailed structures of solar wind in three dimensions since its launch in 1990. However, Ulysses takes ten months even to make a rapid latitudinal scan from the south to north poles. IPS measurements have several advantages in comparison with spacecraft measurements. It can observe three-dimensional solar wind in a short time, and the observations can be carried out consistently over a solar cycle. Making use of these advantages of IPS, we have been studying several interesting solar wind features observed by Ulysses; namely, whether they are stable structures and how they depend on the solar cycle. We introduce these studies and propose a model to determine the solar wind velocity structure.

A simple method to compute spherical harmonic coefficients for the potential model of the coronal magnetic field

It is impossible to make a direct measurement of the coronal magnetic field from the ground. The coronal magnetic field is, then, usually inferred by extrapolation of the observed photospheric magnetic field. The so-called ‘potential model’ has been used for this extrapolation. We have to solve the Laplacian equation of the magnetic scalar potential. This magnetic scalar potential can be expanded into a spherical harmonic series. In this paper, new simple recursion formulae are proposed to solve the Laplacian equation; that is, to determine the spherical harmonic coefficients.

Polar low-speed solar wind at the solar activity maximum

The tomographic analysis of interplanetary scintillation (IPS) showed that low-speed winds (< 370 km s -1 ) emanated out from the polar region at the last solar activity maximum. In order to investigate the origin of those low-speed winds, we compared the velocity distribution derived from the IPS tomographic analysis to the magnetic field structure derived from the potential field analysis. We found that the polar low-speed winds appeared for a short period just before and after the disappearance of polar open fields. When the polar coronal hole shrank very small before its disappearance, the coronal polar open field was encircled by large-scale closed loops and became super radially diverging field into the interplanetary space. A low-speed region appeared in this diverging polar magnetic field region. This situation is a condition very similar to the compact low-speed streams associated with equatorial active regions, which were found by Kojima et al. [1999]. After the open field regions had disappeared from the pole, the polar regions were occupied with closed loops. These closed loops were overlapped by the magnetic field which fanned out from the midlatitudes. A low-speed streamer located above these closed loops even after the polar open field had disappeared. The velocities of polar low-speed streams before polar hole disappearance were much lower than those after disappearance. This result suggests that the physical conditions to generate much lower speed streams are closely associated with large expansion from small open field regions encireled by large-scale closed loops. Finally, a reliability of the IPS measurement of polar low-speed wind was examined by simulating synthetic IPS observations in hypothetical model polar streams.

Relationship Between Solar-Wind Speed and Coronal Magnetic-Field Properties

We have studied the relationship between the solar-wind speed [V]

Origin of the slow solar wind

[V]

Solar wind speed and He I (1083 nm) absorption line intensity

and the coronal magnetic-field properties (a flux-expansion factor [f

LONG-TERM TREND OF SOLAR CORONAL HOLE DISTRIBUTION FROM 1975 TO 2014

f