One small step for a photon...

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Number: 28
1st Author: Eduard Kontar
2nd Author:
Published: 5 January 2006
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The famous "giant leap for mankind" was announced by Neil Armstrong to symbolize the stunning success of man's space adventure. The leap was mentioned in a figurative way. On the contrary the title above has a literal meaning. RHESSI has been designed to study particle acceleration and energy release in solar flares. However, RHESSI can only observe X-ray photons, not the particles. There is a large gap between electromagnetic emission and the actual particles that produce it. The purpose of this nugget is to help fill this gap.

From the dark ages of the Skylab observations, solar physicists have known that the typical X-ray spectrum of a solar flare broadly can be described as the sum of a thermal component and a nonthermal one. The former is attributed to a Maxwellian distribution of hot plasma trapped in a coronal magnetic loop source, while the later is a power-law nonthermal distribution of electrons stopping in the footpoints of the loop. The early missions had poor energy resolution and often insufficient sensitivity to make this clear. The astronomers were fitting a model to an observed photon spectrum, rather than inferring the true electron spectrum. This resulted in the orthodoxy - conventional wisdom - that all solar flare spectra can be well-fitted by a spectral model consisting of an isothermal plus double power-law mixture.

The high-quality RHESSI spectrum (animated example) gave us hope for a model-independent inference of the electron spectrum from the photon spectrum, and thus for better understanding of the life and death of energetic particles in solar flares. Electrons passing in the field of a proton can radiate photons via the process known as bremsstrahlung. This process creates the X-ray photons that RHESSI observes during a solar flare. Thus we know the basic physics well and only have to use it to learn what is happening.

One giant leap for electronkind!

The bremsstrahlung process can be understood via electrodynamic theory in physics, which gives us the probability of photon emission with energy e for a given electron with energy e'. To calculate the total X-ray spectrum one simply convolves the distribution of electrons with this known probability and gets a resulting photon spectrum. Unfortunately, every electron can emit photons of any lower energy; the lower the energy of the photon, the higher its probability for emission. This results in a broad continuum spectral distribution of photons. Therefore, the photon intensity at a given energy is produced by all electrons with energies higher than photons. Any photon at energy e can result from an electron at e' >e. We illustrate this with an animation (click on the frame below) that shows how the electron and photon spectra vary together.

Figure 1. Click on the above image to see how the electron and photon spectra vary together (takes a moment to load). The movie shows how subtle the X-ray spectral variations can be: A 100% change in the electron flux at 50 keV leads to almost imperceptible change in the X-ray photon spectrum. This animation shows why we need the high resolution and precise calibration of RHESSI, which allows us to unfold these effects for the first time. The mathematical methods involved in interpreting the spectra are generically called inverse theory and basically underlie all knowledge in astrophysics, since one learns most of the physics indirectly from the electromagnetic spectrum.

Discovering a dip

This knowledge has allowed us to find something quite new in the way solar flares accelerate electrons. The figure below takes a look at two RHESSI spectra. They look similar, but the electron distributions that generate them are shockingly different:

Figure 2. Two flare spectrum pairs: X-rays above (red) and electrons below (green). Our analysis has allowed us to take subtle differences in the X-ray spectra and show that the electrons that produce the X-rays have very different distributions. The electron distributions result from acceleration and propagation in the complex solar magnetic field. The features we are discovering thus may tell us new stories about the physics of these relatively unknown phenomena. Future science nuggets will discuss some of the physical explanations for these remarkable findings. Biographical note: Eduard Kontar is a Lecturer in Astronomy at the University of Glasgow and a PPARC Advanced Fellow.

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