Nano-Flaring & Doppler Line-shifts
Several years ago, John Butler and myself derived the radiative energy output from the `quiet' corona of some twenty odd stars, similar in many ways to our own Sun, and found that there was a one-to-one correspondence with the total amount of energy radiated during flares on these stars over a period of say one day. This suggested that a flare-like process was an important component in heating the corona. This has also being applied to the Sun.

We have found that nano-flaring and one of the most interesting problems in solar physics, i.e. the observed redshifted emission of lines formed at transition region temperatures, are related.

Doppler-shift v. Te

The velocities derived increase from 0 km/s at 20,000K to a red-shift of 10 km/s at 190,000 K for the quiet Sun, and to 15 km/s 100,000 K for the active region. At higher temperature an opposite behaviour is observed. In the quiet Sun a blue-shift of -2 km/s is observed at the Ne VIII formation temperature (600,000 K), while in the active region, a blue-shifted value around -8 km/s is observed for the same spectral line.

Observations are compared to numerical simulations of the response of the solar atmosphere to an energy perturbation of 4 10E24 ergs where we represent the energy release during magnetic reconnection in a 1-D semi-circular flux tube. The temporal evolution of the thermodynamic state of the loop is converted into C IV 1548, O VI 1032 and Ne VIII 770 line profiles in non-equilibrium ionization.

Model C IV & Ne VII profiles

C IV, O V & Ne VII Doppler-shift v. Time

Performing an integration over the entire period of simulations, a redshift of 6 km/s is found in C IV, while a blue-shift of 2 km/s and 10 km/s were derived for O VI and Ne VIII, respectively, in reasonable agreement with observations.

We have now followed up our early observations with some long time-series studies involving a line formed at 250,000K. This is extremely interesting because we can clearly see a series of short impulses, each lasting only about 30 seconds, occurring every 200 seconds and sometimes more often.

We believe this is strong evidence in favour of nano-flaring and are currently up-dating our modelling codes to make it easier to change and play around with various input parameters, such as, input energy, duration of the nano-flare, starting electron density, starting electron temperature, etc. Also, we are now using a 2-D code involving magnetic reconnection.

Although we believe that nano-flaring is a prime source for coronal heating, we do not believe that it is the main one. In-fact, it's highly likely that several different mechanics are working simultaneously, depending perhaps on the local conditions of the plasma. For example, as shown above for observations taken close to the solar limb we see a periodicity in the intensity of 20 to 30 minutes which takes us back to some of the early theories on wave heating. So although we have at present learned a lot from the SoHO & TRACE, there is a lot more work to be done, not only observationally, but from the modelling side.