Weekly Notes from the Yohkoh Soft X-Ray Telescope

(Week 38, 2001)

Instrument Status

See the weekly operations report for details.

In the display above (click to enlarge), the colored lines show the times of SXT images that currently are on-line at ISAS. The purple lines are flare mode. The gaps will be considerably reduced when the NASA telemetry arrives and gets reformatted. For a summary index listing of the weekly science reports in this raw form, click here. For a cleaned-up version of earlier nuggets, back to our beginning in 1997, please see any of the following Internet mirror sites: Japan; U.K.; U.S.A. (California); U.S.A. (Montana). The figures and text in these nuggets are intended for public use. There is a search facility and also a topical index.

Science Nugget: September 21, 2001

A Week in the Life of a Sigmoid

Introduction

This week we go back to a set of observations which were made in August of 1999, as part of 'Whole Sun Month 3', where, jointly with many other instruments, we followed an active region as it crossed the solar disk. This active region happened to be sigmoidal (see the topical index ) for other sigmoid nuggets). Sigmoidal active regions are those in which there is an overall `S' shape, often most clearly seen by SXT, and they are of particular interest to us because of the association of sigmoids and solar eruptions.

We have many `snapshot' observations of the eruption stage in a sigmoid's life, such as this classic, but these observations allowed us to see how the region behaved over time. There was a whole pile of observations, but we will concentrate on two types; the SXT observations, and H alpha observations which show up the filament

A Reminder

Here is the standard 'magnificent cusp' image. What do these pointy loops mean to us? We believe that in the solar corona, the shapes of the emitting plasma that we see are almost totally controlled by the magnetic field, so the pointed shape of a cusp indicates that magnetic field lines are converging, while the fact that a cusp is so bright implies heating is taking place, or has taken place somewhere nearby. Both of these are consistent with magnetic reconnection taking place or having taken place. As regular readers will know, reconnection which is the mechanism of choice for solar flares. For examples of 'Standard' flare cartoons involving cusps see here or here or here , to name but a few.

Nested Cusps

Our attention was caught by this small but very distinct cusp which appeared in data on September 13th (RHS), apparently arising from the interaction of two sets of loops side-by-side (LHS).

As well as its lovely shape, it was noticeable because within the main outer cusp there seems to be a smaller inner cusp. This is a feature which was also seen in one of the greatest cusps of all time

So How Does it Happen?

Remembering that the cusp implies magnetic reconnection, how can two cusps appear together? In the `standard model', field lines below the cusp are rounded, already reconnected field lines, so one cusp cannot be directly below the other as the geometry of the field would not permit this. So perhaps we see two cusps in different different vertical planes, but formed at two very different heights by two separate but related reconnection events. An upper and a lower reconnection region is a feature of magnetic breakout models, e.g. here, though the models as they stand also have problems explaining the configuration we see.

On the other hand, a twisted loop viewed at an angle can also look like a cusp, as an experiment with a piece of wire, or the four figures below will easily show. So maybe all we are seeing is a small twisted loop orientated at an angle to the plane of the main cusp. But this forces us to think in three dimensions about the flare, which many models can not yet do. And we also have to explain the hot loop low down and at an angle... could it be related to the presence of a flux rope which many people think is what makes the active region go unstable in the first place?

A loop which is semicircular in the back plane but with a small twist in the bottom plane will look like a cusp at certain viewing angles.

As we write this nugget on Friday September 14th, thousands mourn family and friends who were lost in the terrorist attacks on the World Trade Center, the Pentagon, and UA Flight 93, and millions honour the victims with a day of remembrance. From the fourth floor of D-Toh, we also express our deepest sympathy.

September 14, 2001
L. Fletcher ( lyndsay@astro.gla.ac.uk ); H. Hudson ( hudson@isass1.solar.isas.ac.jp )