Yohkoh returns from hibernation

Science Nugget: January 7, 2000


The close of each year finds Yohkoh beginning an interval of several days of low activity. This coincides with the Japanese view of the New Year as a proper time for a hiatus of human activity. For Yohkoh this means downlinks only via NASA ground stations, and no uplinks; also it means no quasi-real-time data. Readers of these pages will recall that last year (last millenium!) we were watching a huge coronal hole approaching the solar central meridian - would this concievably result in a "disappearing solar wind" again?

Yohkoh's alive and well

First, since this is a memorable nugget (Yohkoh survived Y2K, although many of the ground-based systems were a bit goofy), let us confirm that the images are still beautiful:

The new year's image (on the right) shows that the telescope and its complicated data processing all continue to work well. Actually this somewhat conceals the truth, because in fact the Yohkoh project did suffer from a great deal of Y2K software hassle, but fortunately not in anything critical.

In the "old" image on the left, one can see the spectacular coronal hole massively (or at least hugely) occupying disk center.

The solar wind has a healthily high pressure

In last week's nugget we speculated that this large equatorial coronal hole might turn the solar wind off again, as in the case described in that nugget. It only took a few days (at 700 km/s, actually only two and a half days) for the sun to dispel this idea. Below is a plot obtained from the on-line data provided by the ACE spacecraft:

where the red line shows the solar-wind speed surging almost to 800 km/s on the first day of the millenium. Our big coronal hole has emitted a high-speed stream, as is typical, and this is not a case like the remarkable case of May 6, 1999 at all.

So, a modest New Year's lesson from this might be... "Look before you leap" into scientific speculation. We still don't know much about the solar origins of the mysteriously disappearing solar wind. But if we do learn something, you will be the first to hear!

7-January-2000 H. Hudson (hudson@isass1.solar.isas.ac.jp)