As a sequel to our recent identification of the high-speed stream as the candidate structure in the solar wind at 1 AU, that is primarily responsible for the geomagnetic disturbances occasionally noticed after 'disparition brusques' (DBs) of solar filaments (Sastri et al., 1985), we report here that the streams, inferred to be recurrent in our earlier study, were consistently preceded by a stream interface, as expected of corotating streams. This observation substantiates the role of corotating streams of coronal hole origin in the apparent link between DBs and geomagnetic activity, and strengthens the view that DBs are not a unique source of geomagnetic activity.
In a very recent paper (Sastri et al., 1985), we have examined the possible origin of the association between the 'disparition brusques' (DBs) of quiescent solar filaments and geomagnetic activity that is recently reported from case studies (Joselyn and Bryson, 1980;Joselyn and Mclntosh, 1981) and statistical analyses (McNamara and Wright, 1982;Wright and McNamara, 1983). Our work dealth with the composite data base of interplanetary plasma and magnetic field parameters (King, 1977(King, , 1979(King, , 1983)), relevant to 30 clearcut instances when the DB of a filament (estimated size > 15 sq. deg.) was followed by a geomagnetic disturbance (Ap > 30) within 8 days (out of 104 DB events ascertained to have occurred over the period January 1967 through March 1978 with size > 15 sq. deg.). The study revealed that the geomagnetic disturbances noticed after DBs were almost invariably associated (in 28 out of the 30 events) with the passage of high-speed streams at the Earth, and a majority of the streams (19 out of the 28) were found to exhibit the ~ 27-day recurrence pattern, and so also the geomagnetic storms related to them. This finding is quite in accordance with the established fact that the transit of streams at the Earth leads, in general, to geomagnetic storms, and that the streams are a vital link between solar and geomagnetic activity (see Burlaga and Lepping, 1977;Legrand and Simon, 1981; Akasofu, 1981, and references therein). A similar role of streams was also evident in the case of the geomagnetic storms inferred as associated with only DBs by Joselyn and Mclntosh (1981) (see Table II of Sastri et aL, 1985). Besides, the delay in the onset of the geomagnetic disturbances w.r.t. DBs was found to depend not on the characteristics of the filament (e.g., size), but on the date of transit of the stream at the Earth, and there was an absence of a systematic spatial relationship between the potential sources of the streams (coronal holes and flares) and DB sites. These results based on near-Earth space craft measurements also received support from the interplanetary scintillation (IPS) observations. For example,