There are two major enigmas in the outer solar atmosphere, the corona, that have persisted over many decades. Solar flares are explosions above the solar surface that sometimes exceed a hundred times the total energy consumption of all humanity since its existence.
Most surprising and not understood is why a significant fraction of the total energy first appears in the form of accelerated electrons and protons, far exceeding terrestrial accelerators in efficiency. And secondly, the corona at the surface is heated to millions of degrees also in apparently quiet times. The two enigmas may be closely related as both are generally agreed to be caused by the magnetic field. The heating may possibly occur in the form of a plethora of nanoflares, poorly observed tiny flare explosions.
Flare electrons accelerated to nearly the speed of light can readily be observed by their synchrotron emission in radio waves. They lose their energy by heating plasma in the solar atmosphere. In turn, plasma with temperatures of millions of degrees is observable in soft X-rays. Thus, radio and X-ray emissions seem to be related by cause and effect. This is confirmed by the fact that in most flares the radio emission peaks a few seconds before the X-ray emission. If the efficiencies and emission conditions were the same in all flares, one would expect the two emissions to correlate perfectly in peak luminosity.
Figure 2: Comparison of peak fluxes in soft X-rays and radio (3.6 cm wavelength). Upper limits derived from preflare observations are shown by red arrows. Two peaks in the early main are phase are indicated with a little red x, the main peak with a capital red X. (Reproduced with permission from Solar Physics)
The observed correlation is shown in Figure 2: flares with intense radio emission have a large luminosity in X-rays. The dashed line indicates the general relation. It is linear from microflares to huge stellar flares over a range of 5 orders of magnitude.
However, two types of solar flares deviate from the general relation. Nanoflares are small explosions in quiet regions of the Sun. Their radio emission is smaller in relation the X-ray emission than in regular flares, so their ratio is to the left of the dashed line in Figure 2. Thus they appear to be less efficient in accelerating electrons to high energies that emit radio emission. It was believed that this was a property of the quiet Sun, where the magnetic field is smaller than in active regions.
Figure 3: Light curves of preflares as observed in soft X-rays (6-12 keV) by RHESSI satellite (black) and the Nobeyama Radioheliograph (red) in Japan. (Reproduced with permission from Solar Physics)
Now we have observed three small flares before a major flare. They are known as preflares and are a hundred times smaller than the main-phase flare. They occurred in an active region at the same place as the main-phase flare a few minutes later. Do they follow in their X-ray/radio ratio the radio-poor trend of nanoflares or the ratio of active-regions regular flares?
In Figure 3 the three peaks of the preflare X-ray emission is shown. Also displayed is the observed radio luminosity. The latter does not show any peak. Thus the preflares were not detected at the limit of the radio telescope. They are radio-poor and thus follow the nanoflares.
The ratio of X-ray to radio emission of the three preflares is also shown Figure 2. As the radio emission was not detected, the result is an upper limit and indicated with arrows. In Figure 2, two early peaks and the main peak of the main phase of the flare are also indicated. They show ratios typical for flares.
In conclusion, the smallest flare explosions, such as nanoflares and preflares, are less efficient in accelerating radio emitting high-energy electrons. It is the unknown acceleration process that makes the difference. The more energy is released the higher energy electrons can achieve. This property excludes some acceleration models and favors others. For example, the avalanche model proposes that a flare is composed of a large number of elemental explosions that trigger more explosions of the same kind. It cannot explain the observed property. On the other hand, in the turbulence model, the abundance of high-energy electrons increases with power, thus predicting the observed behavior.
The coming years will see two space missions toward the Sun to further explore flares, coronal heating, and other phenomena. In 2018 the all-American small Parker Solar Probe will be launched. A year later a larger international mission led by the European Space Agency will follow. Both missions are equipped with a multitude of instruments to scrutinize the near-Sun medium and work together on complementary observation plans.
These findings are described in the article entitled Observations of Radio-Quiet Solar Preflare, published in the journal Solar Physics. This work was led by Arnold O. Benz from FHNW and ETH Zurich, Switzerland, in collaboration with Marina Battaglia (FHNW, Switzerland) and Manuel Guedel (University of Vienna).