The trailing edges of high‐speed streams at 1 AU

Abstract

The trailing‐edge rarefactions of 54 high‐speed streams at 1 AU are analyzed. The temporal durations of the trailing‐edge rarefactions agree with ballistic calculations based on the observed speeds of the fast and slow wind bounding the rarefactions. A methodology is developed to measure solar‐wind compression and rarefaction using the orientations of solar‐wind current sheets. One focus is to determine the signature that best describes the location of the trailing‐edge stream interface between coronal‐hole‐origin plasma and streamer‐belt‐origin plasma; based on the current‐sheet orientations, on the magnetic‐field strength, on the intensity of the electron strahl, and on the intensity of the negative vorticity, an inflection point in the temporal profile of the solar‐wind velocity is taken as the best indicator of the trailing‐edge stream interface. Computer simulations support this choice. Using superposed‐epoch analysis, the plasma properties and turbulence properties of trailing‐edge rarefactions are surveyed. Whereas the signatures of the coronal‐hole/streamer‐belt (slow‐wind/fast‐wind) boundary in the leading edge (corotating interaction region) stream interface are simultaneous, they are not simultaneous in the trailing edge, with ion‐charge‐state signatures occurring on average 13.7 h prior to the proton entropy signature. It is suggested that differences in the leading and trailing edges of coronal holes on the Sun might account for the differences in the leading and trailing edges of high‐speed streams at 1 AU: the formation timescales, heating timescales, and charge‐state‐equilibration timescales of closed flux loops in the corona might be involved.

Tests for coronal electron temperature signatures in suprathermal electron populations at 1 AU

“…Indeed as energy and momentum may be transferred across such boundaries, their structure should be better preserved in composition than, say, flow velocity. However, boundaries in ionisation state measurements are sometimes observed to be smoothed out at the trailing edge of solar wind streams, in a similar manner to velocity (Schwadron, 2005;Borovsky and Denton, 2016). Ko et al (2014) found O 7+ / O 6+ to be a better tracer of coronal origin than velocity observationally.…”

The response of the inner magnetosphere to the trailing edges of high‐speed solar‐wind streams

“…The authors in that study identified an inflection point in the solar wind velocity as the best indicator of the fast‐to‐slow‐wind stream interface. Fluid simulations supported this parameter as marking the fast‐to‐slow transition based on pressure balance; see Borovsky and Denton [ Figures 11 and 12] for a detailed description of the fluid simulations. Based on the findings from that study, it is clear that during (and after) the arrival of trailing edges of HSSs in the vicinity of Earth, the magnetosphere will be bathed in a different type of solar wind, with different plasma parameters, than those which are encountered on the leading edge of HSSs.…”

“…Figure contains a schematic diagram of the solar wind velocity during the passage of a single HSSs, including the location of the compression region (CIR) on its leading edge and rarefaction region on its trailing edge (see also Figures 1, 2, and 5 from Borovsky and Denton []). The precise location of the interface between fast plasma that is of coronal‐hole origin and slow plasma that originates in the streamer belt is generally straight‐forward to identify on the leading edge of a HSS (e.g., the east‐west flow deflection in the solar wind velocity), and challenging to identify on the trailing edge of a HSS.…”

“…The precise location of the interface between fast plasma that is of coronal‐hole origin and slow plasma that originates in the streamer belt is generally straight‐forward to identify on the leading edge of a HSS (e.g., the east‐west flow deflection in the solar wind velocity), and challenging to identify on the trailing edge of a HSS. As outlined in Borovsky and Denton [] a combination of parameters including the magnetic field strength, the intensity of the electron strahl, the orientation of current sheets, the intensity of the vorticity, the heavy‐ion charge states, the proton specific entropy, and the solar wind velocity profile all show distinct signatures in the trailing edge rarefaction region. The authors in that study identified an inflection point in the solar wind velocity as the best indicator of the fast‐to‐slow‐wind stream interface.…”

“…As fast solar wind catches up with the preceding slow wind it forms a compression region on the leading edge. The transition of the velocity back to slow solar wind results in a rarefaction region and a bend in the gradient of the velocity (also see Borovsky and Denton []). Changes in the state of the magnetosphere during the passage of the trailing edge have received little attention to date.…”