Interplanetary disturbances through the minimum to the maximum solar activities Original Research Article

A comparative study of three-dimensional propagation properties of transient interplanetary disturbances is made for two different phases of 11-year solar activity; early ascending phase (November 1997) and the maximum phase (June 1991). These events were apparently associated with solar flares. Principal data source is solar wind speed measurements by multi-station IPS observations at 327 MHz. Broad interplanetary disturbances with the maximum speed of 800 km/sec were observed in early November 1997, in association with a series of broad flare-associated CMEs observed by SOHO/LASCO coronagraphs. These disturbances propagated both in the northern and the southern hemispheres approximately in a symmetrical manner with respect to the heliospheric current sheet (or the streamer belt). The eruption of the streamer belt is suggested to be the principal sources of the CMEs and their interplanetary consequences. The propagation speed of the disturbance in the region along the heliospheric current sheet were considerably smaller than those in the region apart from the sheet. On the other hand, in the period of the solar maximum, high-speed interplanetary disturbances (about 1000 km/sec at 1 AU), associated with a series of six X10–12 class solar flares in June 1991, showed highly anisotropic characteristics; the enhanced solar wind speeds (>600 km/sec) were predominantly observed in the northern hemisphere of interplanetary space (or to the north of the heliospheric current sheet), in the same hemisphere where the solar flares took place. Close connection between these energetic solar flares and the associated interplanetary disturbances is suggested.