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J.G. Doyle, Research Astronomer

During 1998 Dr Gerry Doyle was involved with three PPARC PDRA research grants, and was also a co-organiser of two international conferences (the COSPAR Symposium `Activity Observed in the Sun and in Similar Stars', and the Armagh Cool Star Workshop `Solar and Stellar Activity: Similarities and Differences'). He was Associate Scientist with the CDS and SUMER instruments on SOHO. Many refereed and other papers or research notes were published during the year, while research visits were undertaken to NASA (Goddard), and the DIAS (Dublin). He acted as external examiner to a Ph.D. student at the University of Glasgow.

Gerry Doyle leads the solar physics group at Armagh, with a particular focus on using data from satellites such as SOHO to infer the structure of the Sun's surface layers and its outer atmosphere or corona. One of the main questions in this field is how the corona is heated to temperatures in excess of one million degrees compared to the surface temperature of 6,000K. Over the years many different theories and models have been proposed, most of these based around wave-type heating.

Nano-flares In the mid-eighties, John Butler and Gerry Doyle, while comparing the radiative energy output from the quiet corona of some twenty stars similar in many ways to our own Sun, found that there was close to a one-to-one correspondence with the total amount of energy radiated by flares on these stars during a period of say one day. We suggested the term micro-flaring to describe this result, and proposed that the outer atmospheres of these stars were heated via flare-like activity.

At around the same time, an astronomer (E. Parker) in the USA was working on a similar idea but applied to the Sun. His suggestion was similar to ours, but the scale was slightly different. He suggested that the outer atmosphere was heated by a large number of very small events, each having the energy of only a billionth that of a large solar flare, hence the term nano-flare.

What has SOHO seen? SOHO is a joint ESA/NASA mission, which was launched in 1996 with an array of different instruments, designed to study various aspects of solar activity, including heating of the solar corona. Over the last few years, we and others have obtained many different sequences of observations which have allowed us to search for fast time-scale enhancements in radiative intensity, line-shifts (i.e. the line centre being shifted from its normal rest wavelength), and whether there is evidence that these events recur with certain periods.

Previous observations have shown that there is a slight shift from the rest wavelength of lines formed between the chromosphere, which has a temperature of 20,000K, and the corona. However, all of the previous work suggested that this line shift should disappear at about 250,000K. With SOHO we have shown that this is not the case, in fact the line shift changes from being red-shifted below 400,000K (i.e. it is moving away from us, in other words down towards the surface of the Sun) while hotter gas (at temperatures in excess of 400,000K) moves towards us, i.e. upwards, away from the Sun.

We have now started to develop models to explain this. Our present models suggest that some form of magnetic reconnection takes place at a temperature of about 250,000K, thereby producing mostly down-flowing material below this value (For example, the strong CIV line formed at 100,000K shows precisely this behaviour.) On the other hand, the strong NeVIII line formed at 500,000K shows the opposite behaviour. Output from detailed spectral modelling clearly shows that both lines are initially at their rest position, but following a nano-flare the spectral lines start to move in opposite directions at a few km s^-1 in very good 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 one can clearly see a series of short impulses, each lasting only about 30 seconds, occurring every 200 seconds and sometimes more often. We believe that this is strong evidence in favour of micro-flaring and are currently up-dating our modelling codes to make it easier to vary the key input parameters such as energy, duration of the nano-flare, initial electron density and temperature etc.

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 is likely that several different mechanisms are working simultaneously, depending on the local conditions of the plasma. For example, observations taken from regions close to the solar limb (again taken in a spectral line formed at 200,000K) show a clear periodicity of perhaps 20 to 30 minutes in the intensity, which takes us back to some of the early theories on wave heating. Although a great deal has been learned about the Sun from SOHO, there is still a lot more to be done, not only observationally, but also from theoretical modelling.