Interplanetary scintillation observations for the solar wind disappearanceevent of May 1999

In this article we present ground-based interplanetary scintillation (IPS) measurements at 103 and 327 MHz for the period of the solar wind disappeara nce event of May 1999 as seen by various space probes. The solar wind veloc ity measurements at 327 MHz showed a variable solar wind velocity during th is period at a distance of similar to0.5 AU from the Sun. The average solar wind velocity from three radio sources varied in the range of 200-300 km s (-1). The scintillation index measurements at 103 MHz indicate that plasma density was very low in the interplanetary medium closer to the Earth and t hat the density was normal away from it during May 11-13. The scintillation index was enhanced significantly on May 14 after the disappearance event. The comparison with the in situ observations shows that the effect is drama tic in IPS observations. IPS and in situ measurements show that a large, te nuous, and slow plasma cloud engulfed our planet around this time, which co uld be because of a corotating low-density narrow stream. From the source ( Sun) point of view, this was mostly a normal plasma flow in most of the int erplanetary medium.