Members of the solar research group of the Radio Warning Services Section of the National Bureau of Standards' Boulder (Colo.) Laboratories have been appraising an unusual solar event which occurred on June 9, 1959. A severe radio blackout of very long duration, and large radio noise outbursts on a number of wavelengths, began at 1630 hours, UT. No solar flare could be seen on the sun's disk, although a prominent flare would normally be apparent at the time of the radio disturbance. It now appears that the reason for this anomaly can only be conjectured until solar behavior is known in much greater detail than at present.
The understanding of the association between solar events, ionospheric disturbances, and geomagnetic storms has played an important part in the Bureau's radio propagation prediction services, particularly in the radio disturbance warning network. Part of this prediction work, based on the analysis and evaluation of solar radio data received from many sources, is the deternfination of the relationships between various solar events, radio fadeout, and temporary changes in the earth's lnagnetic field. The unusual solar event of June 9, colnpletely at variance with the experience of many years, has caused the Bureau to question the validity of some of what were considered to be established relationships.
As soon as the severity of the blackout of June 9 became apparent, radio and optical observations were intensified. The initial position of the radioburst source on the sun's disk was established, by high resolution scan at 10.7 cm, to be N 24, E 90: In the light of the H-alpha (a) spectral line, only jets and bright loops were observed at this position. It was not until almost 1ยฝ hours after the first event that the expected Ha flare became observable at N 19, E 90. Ordinarily the flare is seen at the same time as the radio disturbance is observed.
Complete blackout of the Bureau's radio station, WWV, occurred at a lminber of receiving locations. Cosnfic noise absorption of an outstanding nature (3 +) was evidenced by the great drop-off in received signal strength, and also by the unusually slow onset of the absorption. Solar radio bursts at 18 Mc were classified as 3 + in importance, and bursts at high frequencies (4.3 ram, 10.7 cm.) were very strong. These observations classify the ionospheric events as of relatively great importance (3 +).
Past observations of such outstanding events have, in almost all cases, revealed the presence of an Ha flare of major importance. The absence of such a ttare in itself establishes the June 9 event as a most unusual one. The anomaly raises a number of serious problems concerning the understanding of solar-terrestrial relationships.