Reverberations of April 7, 1997

Science Nugget: August 24, 2001

Introduction

Sometimes the initial look at a given phenomenon doesn't reveal all of its subtleties. This may be hard medicine for a writer of science nuggets to swallow, since we're always having to meet our schedule. However it is also nice to see the information accumulating, and this nugget reviews several pieces of work on one of the first really nice flare/CME events of the current solar maximum: April 7, 1997.

Multifarious observations

  • Sigmoid -> arcade: Click on the link to the left to see Nariaki Nitta's comprehensive look at this phenomenon, as originally described by Sterling for the April 7 event. Surprisingly this initial discovery of a coronal sigmoid disappearing (see left below) was joined by the discovery (see right below) that the sigmoid sometimes appears instead! One speculation would be that the coronal structure at the heart of these flare/CME events is pretty stable and does not change as catastrophically as the middle and upper corona. Note that the illustrations just below don't refer to April 7, but to other examples.

    Bastille Day Not Bastille Day

  • An EIT wave: our April 7 event was one of the first for which the "EIT wave" phenomenon could be well observed. Click on the title here or the image below for a movie - reload it if you want to see it again, because it does not loop (sorry). These waves are either the direct manifestation of the CME liftoff, or else they may be signatures of global MHD shock fronts. Probably, in this case, the former.

    EIT wave

  • Delayed particles: flare/CME events release high-energy particles into the interplanetary medium, and this event was one used by Krucker et al. to discover late releases. Left below, particle counting rates; middle below, analysis of velocity dispersion; right below, the inference (white lines) regarding the location at which the EIT wave somehow triggered the proton injection. Well, this is pretty technical, but it is wonderful to see how we are beginning to use particles (as well as photons) to learn what's happening in the solar corona.

    Nice particle event Delayed protons Where the wave was

  • A "termination shock"? Finally, a recent report by Aurass, Vrsnak, and Mann (not yet published) points out a quite remarkable feature in the meter-wave spectrum of this event. They interpret it in terms of the standard reconnection cartoon (left below), which envisions a termination shock for the reconnection outflow. This makes the feature indicated in the middle plot below, according to the authors. The rightmost panel just puts it all in perspective, with the meter-wave images from the Nancay radioheliograph placing the new phenomenon precisely where the cartoon envisioned. Unfortunately it does not occur anywhere near when the cartoon suggests, but about an hour later. This discrepancy has our pundits stroking their greybeards in puzzlement. Alas, no theory is perfect!

    Large-scale usual cartoon Termination shock? Where the termination shock (?) was

    Last words

    One of the reasons this event continues to interest us is its lack of competition. Nowadays, as a survey of these nuggets will show you, there are many flare events. We are currently at the maximum phase of the solar cycle. This may cause problems of confusion. Such problems don't worry us so much for the April 7, 1997 event, which happened just at the end of the minimum of solar activity, when the corona was still quite empty. "More than four years ago" defines ancient history in this sort of research, well enough to make this a kind of Rosetta stone, it seems.

    August 24, 2001
    H. Hudson <hudson@isass1.solar.isas.ac.jp>.