As promptly reported in previous report, the Circulating Current was observed, that is, rapidly retrograding dark spots of SEB Disturbance southern branch made U-turns at South Tropical Disturbance (STrD).

The Circulating Current is a structure where two jetstreams on SEBs and STBn was reconnected by STrD. Though, retrograding jetstream flow of SEBs was reflected at the p. end of STrD and was changed to prograding motion on STBn, it is invisible unless any marking exists as a tracer. The Circulating Current is hard to observe, because, in addition to STrD itself being a rare phenomenon, dark spot activity on SEBs is required simultaneously and spots sometimes disappear or were distorted in making U-Turn. This phenomenon was discovered in 1919, and actual U-turn of spots was observed from 1932 to 1934. Then, Voyager 2 caught the moment that STrD was formed in 1979, but spots disappeared within STrD and never prograded on STBn. Also, the Circulating Current was strongly suggested in 1993, when dark portion of STrD moved anticlockwise, but no spot was observed. Therefore, it is the first time in 73 years that actual U-turn of dark spot is observed.

SEB Disturbance provided plenty of dark spots retrograding at +4 deg./day along SEBs. STrD-1 located 140 deg. following source region of SEB Disturbance, and it was expected that spots would make U-turn at STrD-1 by the Circulating Current.

Leading spot (a) of southern branch reached STrD-1 on Jul. 2. It moved southward along dark column of STrD-1 in a few days, and was confirmed on Jul. 6 that it begun to prograde along north edge of STBn. Following spot b, which reached STrD-1 around Jul. 5, also made a U-turn in the same way, and began to prograde along STBn by Jul. 8. This motion of the spots was observed scrupulously and in detail by Miyazaki. He is the only observer who succeeded in observation of the spots in such not only spatial resolution but also time resolution.

After that, following spots reached STrD-1 one after another. Each spot tended to undergo a great change during U-turn, because spot c disappeared, and spot f and g merged into single spot. Also, spot a and b merged around Jul. 17 after U-turn, since latitude and prograding speed of the spots on STBn were uneven (There was a tendency for a spot with higher latitude to move faster). By the end of July, 10 spots of a-k reached STrD-1 (spot d and e merged before U-turn), and only 5 spots of a+b, d+e, f+g, h, and i were prograding on STBn. Interestingly, all of original spots were grey, while STBn spots after U-turn were slightly reddish. Moreover, latitude of spots after U-turn was -25 or -26 deg. in latitude, evidently lower than normal STBn jetstream spots which located at -28 deg. in latitude.

Our next expectation is whether prograding STBn spots will return to SEBs by making U-turn again when they reach STrD-2. The historical example shows that spots disappeared in a short term and never made U-turn again. Many spots were obsereved on STBn jetstream this apparition too, two of them reached STrD-2 in mid-July. Though they passed through STrD-2 disappointingly without making U-turn, the spots of Circulating Current may show any interaction, since latitude of them are lower than normal STBn jetstream spots, as described above.

(2) Saturn

Saturn will be at solar conjunction on Aug. 21, and was visible short time just after sunset in the low western sky. An enthusiastic observer sent us the last observation in this apparition in spite of poor condition.

As tilt of Saturnian ring were decreasing, a light zone, probably NTZ, appeared from north of ring A. When Saturn appears in the east sky of early morning late in September, the ring will become much narrower. Details of northern hemisphere will gradually become clear.

(August 9, K. Horikawa)

(Translated on January 23)

[Fig.1] U-turn of SEBs spots by Circulating Current

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