Antipodal Flares

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Revision as of 16:48, 30 March 2013

Introduction

Antipodal flares

The RHESSI field of vew covers the whole Sun all the time, but its images can only extend to a few arc min (nominal FWHM about 3 arc min) (see a previous Nugget or the actual specifications for details). This means that very large-scale features, such as those noted recently in SDO/AIA observations (e.g., Ref. [1]), may at first glance have eluded discovery. We report here an analysis of large-scale variations that suggest the common occurrence of flares with an antipodal orientation, ie at opposite ends of a solar diameter. These events may have escaped earlier notice because of the uniqueness of our vantage point at Earth, prior to the adevnt of STEREO. RHESSI of course can only see such events at the exact limb, as illustrated Figure 1, since otherwise an antipodal flare counterpart would be occulted, and hence not be observable.

Figure 1: Hard X-ray images take from the RHESSI Browser's access to the Quicklook data archive. Top row shows EW antipodality; bottom row NS. Note that all four events occurred within the same day.

In Figure 1, the arc of the limb gives a rough idea of the flare location in azimuth around the limb. The existence of polar antipodal flares (lower row) was an unexpected result of the search described here.

Excitation of seismic disturbances

The near-simultaneous occurence of these antipodal pairs of events implies a causal connection, via Occam's Razor, in the body of the Sun - rather than in its tenuous corona. If so the best-known such link is the excitation of interior acoustic waves, first observed in 1997, but having been predicted by C. Wolff long before that time (Ref. [2]). Figure 2 (left) illustrates how the interior of the Sun refracts acoustic waves, allowing their propagation between opposite hemispheres. Because of the high temperature of the solar interior, the wave propagation time is only some tens of minutes.

Figure 2: Left, interior propagation of acoustic waves launched by a solar flare, at the point labeled "Far-side activity" (from D. Braun). Right, sketch showing how a small exchanges energy and momentum in a collision with a much larger mass.

2

Prior to these RHESSI results, it had not been realized that the antipodal focusing of waves induced by a flare at one point, could in fact trigger a comparably large energy release at the antipode. Note that in Figure 2 (left) the waves clearly disperse and do not focus intensely near a point, where one could imagine the triggering of energy release in a unstable region could occur. Something more is needed.

Heliogammonium

Another problem with the known properties of the flare-induced seismic waves is the fact of their existence in the first place. An apt analogy of a coronal disturbance and a photospheric response might be found in the thought-experiment of bouncing a table-tennis ball off of a bowling ball (see sketch in Figure 2, right panel). In such a collision between disparate masses, the requirement for conservation of momentum dictates that very little energy be coupled. The mass ratio of a flare and the Sun may be of order 10^-19), and yet the observed energy in flare seismic waves may be of order 10^-3 (e.g., ref. [3]) of the flare total!

We believe that RHESSI's observations of hard photons in antipodal flares, as illustrated above, points to a solution to this problem: a heretofore-unrecognized and weakly-interacting particle, created during the flare gamma-ray energy release, could serve to concentrate and transport the momentum associated with the flare eruption through the solar interior as a focused beam, enabling a resonance with the chance unstable region at the antipodal location. We tentatively call this new particle heliogammonium, noting the earlier discovery of helium itself in the Sun. The properties of the new heliogammonium particle remain to be elucidated, along with many other proposals for exotic new particles (see our earlier axion Nugget).

References

[1] "Long-range magnetic couplings between solar flares and coronal mass ejections observed by SDO and STEREO"

[2] "Free Oscillations of the Sun and Their Possible Stimulation by Solar Flares"

[3] "Seismic Emission from the Solar Flares of 2003 October 28 and 29"