Figure 25

The first and only large CIR storm of cycle 24 as of this writing and its solar source region (coronal hole). The CIR was due to the high-speed stream from a large equatorial coronal hole (a) observed by SDO/AIA at 193 Å. At the photospheric level, the coronal hole had positive magnetic polarity (b) as observed by SDO/HMI. The outline of the coronal hole in (a) is overlaid on the magnetogram (b) to show the unipolar region. The CIR manifested as a region of enhanced magnetic field in in-situ observations (c). The three components of the IP magnetic field are shown in (d-f). The Bx component was negative indicating that the direction of the magnetic field was pointing away from the Sun consistent with the positive polarity in the photospheric magnetogram. The Bz component was negative for approximately 6 h and was responsible for the major storm. It should be noted that this type of magnetic structure in CIRs is atypical (see previous discussion). The solar wind speed increased from approximately 380 km/s first as a shock jump and then at the interface and finally in the high-speed stream (g). The temperature remained higher than the expected solar wind temperature (shown in red) throughout the interface and the fast wind (h). The density in the interface was high due to the shock compression reaching values in the range 10 to 30 cm⁻³ (i). Finally, the Dst index shows the evolution of the storm, reaching a peak value of −119 nT on June 1 at 9 UT (j).