The McClymont Jerk
From RHESSI Wiki
|1st Author:||Hugh Hudson|
|2nd Author:||Brian Welsch|
|Published:||14 January 2008|
|Next Nugget:||The microflare Height Distribution|
|Previous Nugget:||EIT Waves - Cadence issues|
Introduction: Sunquakes, Solar Tsunamis, and Helioseismic Waves
This Nugget deals with a most interesting aspect of the impulsive phase of a solar flare. The impulsive phase of a flare, observed by RHESSI in hard X-rays and gamma-rays, represents a sudden release of gradually stored energy. Obviously, this energy release makes waves. The waves described in last week's Nugget are in the corona, (see this APOD item for excellent movies and links of a coronal "tsunami.") Maybe more analogously, there are also large-scale waves propagating in the solar interior as described in another still earlier Nugget. These coronal and interior waves originate at about the same time and place but represent energy travelling in somewhat different wave modes.
The interior waves have been difficult to observe. After their discovery in 1996 by Kosovichev and Zharkovska, there were no observations for several years. Now we know of several examples.
The excitation of a coronal wave is simple enough in concept. There is a "magnetic explosion" (ie, a flare/CME) with substantial motions, and these must drive waves. For energy to couple into the interior is, at first glance, not so easy. Conservation of energy and momentum mean a negliglble solar recoil; if the Sun were solid, the energy of the atmospheric explosion would have to go into the expanding atmosphere. But the solar interior has finite wave speeds, and so in principle substantial energy transfer can result from the compression produced in the explosion. The details of this do not yet have a satisfactory theoretical treatment; part of the problem is that we do not understand the behavior of the photosphere during a flare (see this earlier Nugget). But in fact we don't have a Nugget yet that describes the most perplexing observation; this would be the 1.56-micron (infrared) "opacity minimum" flare phenomenon discovered by Y. Xu and collaborators in the Halloween flares of late 2003. We will try to capture this remarkable observation in a future Nugget.
What is the McClymont Jerk?
With this lack of certainty, there is room for another idea for the transfer of energy from the coronal perturbation into the deep solar interior. We call this the "McClymont Jerk," following McClymont's pioneering investigation of the cause of the flare-related sunspot displacement reported in 1993 (see Figure 1). The Jerk here is just a jerk, even though the word has a technical meaning. Neither McClymont nor we have worked this idea out in detail enough to know whether or not this derivative is important.
Figure 1: The original "jerk," taken from a 1993 paper by Bachtiar Anwar and colleagues. The plot at the left shows the stepwise displacement of a small sunspot during an X-class flare. The white-light flare emission affected the position measurement during the event itself, a remarkable phenomenon originally noted in the data by Loren Acton. The right panel shows simple descriptions of this stepwise motion as a result of a "jerk" on the solar interior caused by the coronal field restructuring. Figure 1, incidentally, shows the exquisite astrometry one can obtain even with a tiny telescope (the 5-cm aspect camera of the Yohkoh soft X-ray telescope, in this case). could easily measure a jump of one arcsecond in the position of one spot in a flaring region, using the other (non-moving) spots as reference. Note how little scatter there is in the time series of the centroid position of the moving spot. It is good to do astronomy in space if you can!
Figure 2 explains our view of the Jerk. The coronal magnetic field collapses to release energy. This launches a wave towards the surface of the Sun. Some fraction of the motion, on some time scale, penetrates to the interior. Effectively the dog is being wagged by its tail! Although no detailed theory exists, it is clear that this mechanism is quite different from the pressure pulse envisioned by the simple hydrodynamics of the explosion process. The jerk would probably have a more horizontal impulse, rather than a vertical one. It also could have a different geometry. Thus it will be interesting to compare the different predictions with the observations, when (a) we have better predictions, and (b) have better observations!
Figure 2: This shows the change in tilt of the solar magnetic field between the initial state Bi and the final state Bf. The black and white regions represent the flare ribbons in opposite polarities.
The sudden energy release of a flare in the solar atmosphere has many consequences. Global solar waves have now become much easier to observe, thanks to much-improved observations. They are among the most interesting phenomena because their excitation requires a large amount of energy, and so their structure may help us to figure out its sources. The interior waves are the most elusive; they could be excited directly by a hydrodynamic process, or (as we suggest here) by the McClymont Jerk which involves the magnetic field.
We expect data from the HMI instrument on the SDO mission to yield better understanding of flare-associated magnetic field changes, since it will provide vector magnetograms (instead of the line-of-sight magnetograms from SOHO/MDI) with relatively high cadence and a planned 100% duty cycle. These new data will allow us to expand upon the work of Sudol & Harvey (2005) and others in quantifying magnetic field changes during flares. As we discuss in our recent paper on this topic, measurements of flare-related changes in the magnetic vector should enable estimating changes in the Lorentz force on the photosphere.
Biographical note: Hugh Hudson and Brian Welsch are solar physicists at UC Berkeley. George Fisher also contributed to the paper this Nugget describes.