A gentle giant

Science Nugget: August 10, 2001

Introduction

First, glance at this plot of the X-ray brightness of the Sun and its variation during the period shown, in the middle of July 2001. What is your impression of the solar activity for these weeks? For the moment, ignore the arrows on the plot, please.

You may say, "Solar activity was not high, since the GOES background was much below C-level and there were only three M-class flares for these two weeks." You are quite right as far as flare activity is concerned. However, flares are not the only form of solar activity, as we have said several times in these nuggets. Actually, let's look at this LASCO movie, which covers nearly the same period as the above plot. How exciting these weeks were! Do you think the same?

Seeking the source of CMEs

In order to identify the source of remarkable events appeared in the LASCO movie, let us check with the SXT movie (but if you don't have a broadband internet connection, don't even think about the java version!!!) for the same period.

mpeg movie (1.74MB) and java script (22 MB!!)

By looking at this movie closely, you may see that many of major CME events are likely to have come from a large vague region, a little north of NOAA 9535 in the northern hemisphere.

Still image (17-July-2001)

What is interesting is that this region was not accompanied by distinct sunspots beneath it, but was located on the boundary of strong unipolar regions. NOAA gave this region a number, 9544, for only a few days (17-19, July), during which it produced no attractive events. Before and after these days, however, NOAA had no name for this region, in spite of its great activity described below. Closer views of NOAA 9544 below are the images taken from the BBSO Active Region Monitor .

Yohkoh/SXT Continuum Magnetogram

This region produced at least four major events observed with SXT. The timings of these events are shown by the arrows in the above plot of the GOES flux.

Description of the events

Event 1 : 10-Jul around 14:00UT. An exploding feature was seen on the north-east limb. As a remnant of this explosion, a large elongated loop like a flame remained visible for a few days after the event.

SXT movie java and mpeg (133kB) LASCO movie

Event 2 : 16-Jul around 08:30UT. Following flaring which occurred in newly emerged region NOAA 9545, the eastern edge of the giant region had flamed up. Then, at around 15:00UT, the whole region had brightened. In this event, the helmet streamer which pointed northward was completely blown away, as seen in the LASCO images.

java and mpeg (350kB) LASCO movie

Event 3 : 20-Jul around 04:00UT. This event is also interesting in the view that a large structural change in the neighboring corona observed with SXT. An eruptive front started from this region propagated toward north pole and activated the region next to the coronal hole. The material in the activated region was cooled down or blown away, then a new coronal hole appeared at the same location.

java and mpeg (192kB) LASCO movie

Event 4 : 23-Jul around 04:00UT. This must be the strongest event among the four introduced here. An apparent eruptive feature is seen on the west limb, and this is probably the source of the big CME observed with LASCO.

java and mpeg (127kB) LASCO movie

Closing comment

Here, we have reviewed the great activity of a region with no name (NOAA number) at the time of the event. All these events produced only a tiny enhancement, if any, in the GOES flux. Nor were they followed by any particle event which may affect the environment of the earth, even when they occurred on the disk. In this sense, this region is 'gentle'. However, this region is 'great', in the sense that it resulted in the significant CMEs and/or structural changes in the corona, since these phenomena are connected to the evolution of the sun as a variable star.

Now, we are looking at another disk passage of this region. Alas! As of 10-Aug., this region still keeps the elongated shape since the previous rotation. Even it looks like a huge sigmoid. Let us watch its behavior carefully.

August 10, 2001

A. TAKEDA <takeda@isass1.solar.isas.ac.jp>;