Why submillimeter?

RHESSI detects and images emissions from relativistic particles accelerated in solar flares, as described in many Nuggets in this series. The corresponding emissions at radio wavelengths, due to gyrosynchrotron emission (or possibly more exotic mechanisms) appear at the shortest wavelengths. These "submillimeter" wavelengths or THz (Terahertz) frequencies verge on the far infrared, and the detectors may be either bolometers (as in the IR) or heterodyne receivers (as in the radio bands). Figure 1 below shows the atmospheric transmission at 100-1000 GHz (3-0.3 mm) from an excellent site.

Figure 1: Atmospheric transmission at an excellent mountaintop site, where the "precipitable water vapor" is less than 0.5 mm equivalent thickness. The weak submillimeter transmission bands are at about 0.67 and 0.87 THz. At frequencies below 0.3 THz (wavelengths longer than one mm) the atmospheric transmission improves drastically.

As any astronomer can tell from the plot in Figure 1, these are very difficult wavelengths to observe at (just as RHESSI's observations of MeV photons are also extremely difficult technically). In astronomical parlance, the extinction is of order one magnitude per air mass! Nevertheless two observatories have now successfully made solar flare observations in the past solar maximum, and there have been simultaneous RHESSI observations as well. The two observatories are COSMA in Switzerland, and El Leoncito in Argentina.

Gyrosynchrotron radiation

Generally high-energy electrons in a solar flare can be detected either by thermal or nonthermal processes. The thermal process is free-free emission (the radio equivalent of X-ray bremsstrahlung; note that at these long wavelengths, free-bound emission is unimportant). The non-thermal process is synchrotron emission, and it is often thought of as "magneto-bremsstrahlung". This name captures the idea that a fast electron gyrates around the magnetic-field direction via the v x B force, and this angular acceleration necessarily involves the emission of electromagnetic waves, as originally predicted for astronomical purposes by Alfven and Herlofson as early as 1950.

The synchrotron emission spectrum is complicated for electrons that are not ultra-relativistic, which is not always the case for solar flares. But if we make this approximation for 10 MeV electrons, we find the characteristic emission frequency to be of order 100 THz for a magnetic field of only 100 G. So the THz frequency range (or even the true infrared) is the natural part of the radio spectrum to study high-energy electrons of the energies responsible for RHESSI's high-energy continuum photons.

First observations

Figure 2: Left: radio spectra observed at Gornergrat, displaying the "THz upturn" discovered in a different event observed at El Leoncito. This unexpected feature was thus detected at both of the solar submillimeter observatories. Right: RHESSI gamma-ray imaging of the same flare, as described in a. previous Nugget.

The highest frequencies show a striking spectrum: unlike the expectation of a decreasing intensity, normally ascribed in this context to a non-thermal process (synchrotron emission), we see an increasing intensity. The red contours in the RHESSI image show the locations of high-energy electrons possibly associated with this new radiation component.

Conclusion