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The Forefront of Space Science

Source of Solar Wind Revealed by HINODE
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As stated above, the gas definitely ascends along the magnetic lines. If the magnetic lines were bent and returned to the solar surface, however, the gas could not escape from the Sun to enter interplanetary space. To clarify this issue, we investigated the ends of the magnetic lines as shown in Fig. 3. We used the magnetic-field map of the photospheric plane of the Sun, which was retrieved by MDI instrument onboard SOHO satellite, and drew magnetic lines assuming a potential magnetic field. The yellow-colored magnetic lines in the figure are ones that reach up to the “source surfaceEsphere, which is set up at a distance of 2.5 times the radius of the Sun. These magnetic lines are thought to spread out into interplanetary space, and not return to the Sun. Meanwhile the blue-colored lines are “closed-magnetic linesEthat return to the solar surface. (N.B.: the closed magnetic lines in the polar regions are not correct because we were unable to measure the magnetic field in those regions, so drew the lines by supposition.) From this figure, it can be seen that the magnetic lines in the outflow area indicated by the white arrow spread out into interplanetary space. In other words, it suggests that the gas flows out along the magnetic lines and could be the solar wind. Assuming that all the gas observed in this area would come out into interplanetary space, about a quarter of the mass released by the solar wind per unit time is supposed to be covered by this area.

Figure 3

Figure 3. Profile of the magnetic lines of the entire Sun during the observation period (assuming potential magnetic field).
Yellow shows the magnetic lines that extend out into interplanetary space. Blue shows the closed magnetic lines, of which both ends are rooted on the solar surface. The site of coronal gas outflow is indicated by the white arrow.


The XRT onboard HINODE discovered that the solar wind flowed out from the active region next to the coronal hole. In fact, the possibility that such area might be one cause of low-speed solar wind was suggested by the interplanetary-scintillation observation utilizing radio waves from quasar, etc., which was conducted by the solar wind group of the Nagoya University. They reported that its possible origin could be the edge of the active region adjacent to a coronal hole, where a bundle of magnetic lines ascend and rapidly spread out. Our observation with HINODE succeeded in actually witnessing the site where the wind (specifically, low-speed solar wind) flows out. Based on this observation, we expect significant progress in research on the still-mysterious site of solar-wind occurrence and its acceleration mechanism.


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