the nature of the solar corona
the acceleration of the solar wind, and
the internal structure of the Sun
It is towards the first of these topics that my effort is directed and, in particular, the question of coronal heating.
It has being known for several decades that the solar corona has temperatures in excess of one million degress, however, the mechanism by which this is maintained is still an outstanding problem to-day. There have been many suggestions including, Alfven wave dissipation, turbulent cascades, currents sheets dissipation or nanoflaring and anomalous interruption of field-aligned currents. As of yet, none of these have proven to be entirely satisfactory.
The choice of wavelength region for observing various parts of the solar atmosphere is determined by the range of temperatures required to be studied. For example, in order to study the solar corona and the chromospheric-coronal transition region requires data shortward of 1600A region.
From previous missions we know of plasma structures varying in size from a solar radii to less than 1 arc sec (i.e. 725 km). For coronal heating the most critical are the small scale structures, mostly in the form of loops. Coordinated observations with the above suite of instruments (plus ground-based coverage) has the capacity to provide invaluable and unique data for solar physicists for the next decade which will enable major advancements in this area.
In addition to contributing to the picture of a finely structured transition region, HRTS has revealed the presence of a highly dynamic environment. For example, it was shown that transition region line profiles cannot be represented by a single Gaussian but instead showed a very complex velocity structure. In many instances only an average velocity was computed with the loss of relevant information on the dynamics. These multiple velocities were observed in many different locations ranging from sunspots, to active regions and sometimes in the so-called quiet regions. In fact, in `quiet' region, these multiple velocities were observed ~15% of the time and this may be a lower limit. The spectral lines concerned were those of lines formed at temperatures of 80,000 to 100,000K. These lines showed both red- and blue-shifted components and were present at both Sun center and at the limb. Flow velocities were both sub- and supersonic. However, the mechanism causing these flows and whether they extend to coronal temperatures is still a matter of speculation.
Thus the picture emerging from these observations is that the transition region should not be considered as static or stationary. Instead we may have a transition region comprising of an ensemble of small, nearly isothermal loops. The current array of instruments on SOHO has the potential of providing valuable observational data relating to temperatures structures ranging from twenty thousand degrees to over a million degrees. Such data can be used to provide important input not only for heating of the Sun's corona but to the heating of stellar coronae in general.
With high time resolution data over extended periods we can look at the possibility of detecting loop oscillations. In fact, based on SMM observations, oscillations in the UV continuum at 1370A have already being observed. The observed periods are all in the range 4 to 7 mins, consistent with the range of observed global photospheric periods. These observations were obtained using a 13x13 pixel raster (each raster of 10x10 arc sec) with a cycle time between rasters of 22 sec. It was normally found that only a small numbers of pixels show a periodicity while adjacent pixels were void of any periods. Such structures were therefore small, of the order of 10 to 20 arc sec and the question we wish to ask is why only some regions showed a periodicity. One can argue that these regions were simply resonating with the under-lying photospheric period, however, why only these regions and could this have implications for coronal heating (waves versus reconnection). We therefore plan an observational programme aimed at obtaining data as a function of temperature using both rasters and line profiles modes (the SMM data were taken only in the UV continuum at 1370A and therefore we do not have sufficient information on the temperature and/or height of these oscillations).
The above programme will be directed towards active and non-active regions, in particular loop structures. We plan to look at these structures both on-disk and those approaching the limb. Using high spectral resolution of selected lines with SUMER we can investigate profile changes, with CDS we can perform rasters plus line ratio diagnostics. With this data we can therefore investigate the microscopic transition region structure which has to be related to the local heating mechanism. This will then allow us to investigate how this microscopic structure can explain such characteristics of the transition region as the general redshift of EUV and UV spectral lines, the large-scale temperature stratification and the universal emission measure. On Hinode, we use EIS , SOT and XRT .
The above also involves the use of atomic physics in particular the data required for the various diagnostics line ratios. For this we use data from CHIANTI or ADAS .
The slaves involved in this work are: Rhona Maclean , Eoghan O'Shea , Eamon Scullion , Abhishek Srivastava , David Perez-Suarez and Srividya Subramanian Watch this space!