F-HUNTERS OBSERVING CAMPAIGN

Are you an amateur of solar observations? Do you have equipment suitable for safe solar observations? Are you interested in solar astrophotography and image processing techniques? Are you amazed by majestic power of solar flares and eruptions? Do you want to contribute to the database of amateur observations dedicated solely to these explosive phenomena in solar atmosphere? If you answered “yes” to at least some of these questions, participate in F-HUNTERS observation campaign and start your hunt for flares today!

How solar flares look like?

Dominik Gronkiewicz

What we collectively describe as a “solar flare” is a set of multiple phenomena in all layers of solar atmosphere: corona, chromosphere and even photosphere. Solar flare is caused by a sudden release of enormous amounts of energy, previously accumulated in twisted magnetic fields of active regions, into the solar atmosphere. Because of their magnetic nature, they are more likely to happen in proximity of sunspots, where magnetic field is stronger. The duration of solar flares varies from several minutes up to a few hours, during which substantial amounts of electromagnetic radiation (starting from radio, visual radiation, up to X-ray and even gamma ray) and highly energetic protons, electrons and ions (with velocities up to 70% of speed of light!) are released.

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In the typical scenario of a solar flare, the event starts with the impulsive phase associated with rapid injection of huge amounts of energy into the lower atmosphere. The plasma in footpoints of magnetic structures is warmed up and the brightness of flaring regions in H-alpha increases. This is also followed by filling the magnetic loops with hot plasma which is a strong X-ray emitter, as we can observe on GOES X-ray flux lightcurve (see below). Moreover, the solar flare classification is based on their maximum flux registered by GOES in its softer (red) channel. (Notice the letters next to the right axis of the plot.)

The impulsive phase is followed by the decay or gradual phase, when emission gradually subsides to the initial state. When forementioned hot loops cool down enough, they become visible in H-alpha line as post-flare loops.

Fig. Loops filled with hot plasma at temperature of several milion kelvins observed in X-ray by HINODE. (source)

We can observe the chromospheric plasma using narrowband filters with transmission centered on strong spectral lines, such as H-alpha or calcium K line. The flare should appear as two slowly evolving bright spots or ribbons. Typically, the separation of flaring ribbons should increase with time, as energetic particles are injected into more outward loops of magnetic arcade.

Fig. 28 October 2003 X17 flare observed in H-alpha in Bialkow Observatory, Poland.

Fig. Flare ribbons observed in ultraviolet by IRIS.

Fig. Solar flare visible in Ca-II K line (source)

Even weak solar flares should be observable using even cheapest amateur H-alpha or Ca-K telescope. However, there is a class of exceptionally strong events, in which the process of energy release during the impulsive phase is so powerful that it drives much denser photosphere to shine. We call these extremely rare and short brightenings as “white light flares”, even though their colour is bluish-white due to contribution of hydrogen. Historically, the first solar flare ever observed was a white light flare spotted by lord Carrington in 1859 (click here to read original paper by Carrington). Even though Carrington's flare was of exceptional energy, keep in mind that even C-class flares have been shown to cause enhancements in white light emission.

Fig. White light flare observed by Thierry Legault on 28th October 2003 (source).

The release of magnetic energy is usually associated with significant reconfiguration of magnetic field structure, which can manifest as motions of enormous amounts of plasma, such as prominence eruptions and coronal mass ejections. Coronal mass ejections (CMEs) are particularly important for our civilization, since the impact of a large CME may cause failure of electric circuits aboard satellites and even on Earth.

Fig. Eruption of a big prominence observed by SDO in extreme ultraviolet. (source: Solar Dynamics Observatory)

Several other flares and prominences photographed by coronograph in Bialkow Observatory, Poland.