Coronal Magnetic-Field Configuration Associated with Pseudostreamer and Slow Solar Wind

Abstract

The global distribution of the solar wind speed \(V\) is closely related to the configuration of the coronal magnetic-field, and the expansion factor \(f\) of the flux tube is known as a parameter for determining \(V\). However, the inverse relation between \(f\) and \(V\) does not hold for pseudostreamers, which separate open-field regions with the same polarity. In the present study, we examined the magnetic-field configuration of pseudostreamers using the potential field (PF) model analysis of magnetograph observations for six Carrington rotations (CRs) in Cycle 23 and compared it with \(V\) data derived from interplanetary scintillation observations. We calculated the parameter \(S\), which represents the relative angular distance of foot points on the photosphere magnetically connected to adjacent pixels on the source surface and \(f\) from PF model analysis and discriminated areas of helmet and pseudostreamers on the source surface by selecting large values of \(S\). Although the overall correlation between \(S\) and \(V\) was very poor, helmet and pseudostreamers with large \(S\) values were exclusively associated with slow \(V\). Furthermore, helmet and pseudostreamers were associated with large and small values of \(f\), respectively. This suggests that \(S\) enables a better discrimination of slow-wind sources associated with pseudostreamers than \(f\). We calculated the distance from the streamer boundary (DSTB) on the source surface using data of helmet and pseudostreamers to compare with \(V\) data. Calculated DSTB data exhibited significant correlations with \(V\) data except for the solar maximum period. The average of correlation coefficients between DSTB and \(V\) over five CRs excluding one at the solar maximum were 0.69, higher than that between the distance from the coronal hole boundary (DCHB) and \(V\). This suggests that DSTB acts as a better parameter for determining \(V\) than DCHB. We demonstrated that \(f\) for pseudostreamers tended to reach a maximum at a height lower than the source surface (2.5 \(R_{\odot }\)). This provides important insight into the formation process of the slow solar wind in pseudostreamers.