Showing papers by "M. J. Koomen published in 1982"

A magnetic cloud and a coronal mass ejection

Abstract: An interplanetary magnetic cloud observed by the Helios 1 spacecraft was found to be associated with a coronal mass ejection observed by the NRL Solwind coronagraph on the spacecraft P78-1. The magnetic cloud was observed on June 20, 1980, when Helios 1 was at 0.54 AU and nearly 90 deg west of the earth-sun line. This was associated with a large loop-like coronal mass ejection observed over the west limb on June 18, 1980, moving toward Helios 1. The speed of the front of the event at Helios 1 was (470 + or - 10) km/s, which is close to the mean transit speed (approximately 500 km/s). The magnetic cloud was similar to others described in the literature: The magnetic field strength was higher than average; the density was relatively low; the magnetic pressure greatly exceeded the ion thermal pressure; and the magnetic field direction changed through the cloud by rotating parallel to a plane which was highly inclined with respect to the ecliptic.

Observations of a comet on collision course with the sun.

Abstract: A brilliant new comet (1979 XI: Howard-Koomen-Michels) was discovered in data from the Naval Research Laboratory9s orbiting SOLWIND coronagraph. An extensive sequence of pictures, telemetered from the P78-1 satellite, shows the coma, accompanied by a bright and well-developed tail, passing through the coronagraph9s field of view at afew million kilometers from the sun. Preliminary orbital calculations based on the observed motion of the comet9s head and morphology of the tail indicate that this previously unreported object is a sungrazing comet and may be one of the group of Kreutz sungrazers. It appears from the data that the perihelion distance was less than 1 solar radius, so that the cometary nucleus encountered dense regions of the sun9s atmosphere, was completely vaporized, and did not reappear after the time of closest approach to the sun. After this time, however, cometary debris, scattered into the ambient solar wind, caused a brightening of the corona over one solar hemisphere and to heliocentric distances of 5 to 10 solar radii.

Observations of coronal structure during sunspot maximum

Abstract: This paper presents some of the results that have been obtained from the Kitt Peak observations of coronal holes and the NRL observations of coronal transients during the recent years near sunspot maximum (1979–1981). On the average, low-latitude coronal holes of comparable size contained 3 times more flux near sunspot maximum than near the previous minimum. In the outer corona, transients occurred at the observed rate of at least 2 per day, and quiet conditions persisted during less than 15% of the observed days. We describe a sample of the more than 800 events that we have observed so far, including the observation of a comet apparently colliding with the Sun.

Coronagraphic observations of two new sungrazing comets

Abstract: The discovery has recently been reported of a comet (Howard-Koomen-Michels: 1979 XI) that apparently collided with the sun on 30 August 1979. A report is presented of observations of two additional sungrazers that encountered the sun on 27 January 1981 and 20 July 1981, respectively. Like comet 1979 XI, these two new comets seem to have been members of the Kreutz group of sungrazers, and like 1979 XI the new comets did not reappear after their encounters with the sun. The discovery of three previously unreported comets during the initial 2.3 yr of satellite coronal observations suggests that sungrazers are much more common than one might suppose from the list of only nine known sungrazers observed during the years 1668-1970.

Simultaneous radio scattering and white light observations of a coronal transient

Abstract: Reports that a coronal transient observed by the Solwind coronagraph off the west limb of the sun was also seen in the spectral broadening observations of the Helois-2 2.3 GHz radio signal. These simultaneous data are compared and applied to studies of the shock front. UT difference images show a time difference of up to 2 h between shock and white light occurrences. Helios and Voyager shock speed measurements are consistent, implying that the event observed by Helios is a shock. These coronal transient data show that the shock front is ahead of the white light front, but additional observations of larger events will contribute information to the understanding of coronal transient evolution as they propagate away from the sun.