T. Ohmi

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.

Time‐dependent tomography of hemispheric features using interplanetary scintillation (IPS) remote‐sensing observations

We have developed a Computer Assisted Tomography (CAT) program that modifies a time‐dependent three‐dimensional kinematic heliospheric model to fit interplanetary scintillation (IPS) observations. The tomography program iteratively changes this global model to least‐squares fit IPS data. The short time intervals of the kinematic modeling (∼1 day) force the heliospheric reconstructions to depend on outward solar wind motion to give perspective views of each point in space accessible to the observations, allowing reconstruction of interplanetary Coronal Mass Ejections (CMEs) as well as corotating structures. We show these models as velocity or density Carrington maps and remote views. We have studied several events, including the July 14, 2000 Bastille‐day halo CME. We check our results by comparison with additional remote‐sensing observations, and observations from near‐Earth spacecraft.

Large-scale structure of solar wind turbulence near solar activity minimum

Abstract Interplanetary scintillation measurements at 327MHz taken in 1997 are used to study the large-scale heliospheric structure near the solar activity minimum. Our data show that density fluctuations in the fast polar wind is much less than those of the slow wind around the equator. The reduction ratio of the fast wind turbulence is estimated to be about 0.53 from the model calculation. While an inverse relation between wind speed and density fluctuations appears to exist, the correlation is weak, and there are significant discrepancies in the locations of high turbulence and slow wind speed. This discrepancy might come from the integration effect along the line-of-sight and/or the effect of the stream-stream interaction.

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.

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.

Origin of the slow solar wind

Abstract The origin and location of the slow wind are interesting subjects on their own and also they are important in the matter of the origin of the fast wind especially if the slow wind originates directly from a coronal hole. We have been studying the solar wind whose speed is comparative to or a little slower than that measured in the heliospheric plasma sheet. We derived solar wind velocity distribution maps on the source surface using an interplanetary scintillation tomography analysis, and low speed regions in them were investigated in relation to a coronal hole using potential field magnetic field lines. Slow solar wind was found to originate from an equatorial coronal hole located in the vicinity of active regions and also from a polar coronal hole which was about to disappear at solar activity maximum. The properties of the slow solar wind from an equatorial coronal hole were investigated using spacecraft in situ measurements. Magnetic polarity in this wind was uniform, as expected from a potential field analysis. The helium relative abundance was as large as the fast wind, and variances of density, velocity, and helium abundance were as small as the fast wind from a large coronal hole. In spite of the small equatorial coronal hole origin, the density and ion freeze-in temperature were as large as observed for the slow wind in the heliospheric plasma sheet.

Low-speed solar wind associations with active regions near solar minimum

Abstract Solar wind velocity distributions were derived on the source surface from interplanetary scintillation observations using a tomographic analysis method to investigate the positional relation of compact low-speed regions with active regions and large-flux-expansion regions. It is shown that the compact low-speed regions are better associated with large-flux-expansion regions, and they are found near active regions.

Solar wind imaging facility (SWIFT) for space weather research

The interplanetary scintillation (IPS) method can observe the dynamics and structure of the solar wind in three dimensions with a relatively short time cadence (<1 day) using IPS radio sources distributed over the sky. Because of this advantage over in situ measurements, we have been conducting multi-station 327 MHz IPS observations at the Solar-Terrestrial Environment Laboratory. The IPS measurement is a line-of-sight integration which is a convolution of the solar wind structures, the distance of these from the Earth and other diffraction effects present along the line of sight. We have recently succeeded in developing a method to deconvolve the line-of-sight integration effects using a computer-assisted-tomography (CAT) technique to obtain solar wind speed and electron density fluctuations. The CAT analysis not only retrieves three-dimensional solar wind parameters, but also provides better spatial resolutions than previous analysis techniques. The present IPS system at STELab observes several tens of IPS sources a day. To make solar wind observations with higher spatial and temporal resolution using the CAT method, we need more perspective views of the solar wind. Therefore, we are planning a new UHF antenna with an effective collecting area of 5500m^2 that will observe more than 100 IPS sources per day. The antenna is designed with a tolerance for radio noise interference and high aperture efficiency. Based on the successful development of the IPS CAT analysis, we are presently continuing a US-Japan cooperative project for space weather research between UCSD/CASS and STELab. This project with the new antenna will enhance IPS/US cooperation including future comparative analyses of data from the Solar Mass Ejection Imager (SMEI) and from STEREO.

Solar wind velocity structure around the solar maximum observed by interplanetary scintillation

Ulysses observed a latitude structure of solar wind in its second fast latitude scan and found that the global structure of solar wind near the solar maximum is significantly different from that in the solar minimum. Also soon after the solar maximum, Ulysses measured that the fast solar wind which has magnetic polarity of the new solar cycle appeared at high latitude in northern hemisphere. This fast wind appeared and disappeared a few times. We introduced a new tomographic algorithm, time‐series tomography, to reconstruct IPS velocity map using all data observed in the year from 1998 to 2001 and analyzed the variation of the solar wind structure through these four year. Especially in 2001, we compared the Ulysses’ fast latitude scan data. As results, it is found that disappearance and recovery of the fast solar wind around the north pole precedes that around the south pole for several months. And also found that the IPS observation shows high level agreement to the Ulysses observation especially for hig...