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• Quiet Sun Workshop Summary
• Coronal Heating Studies
• GIS Slit Anomaly - Alternative GIS Studies

Recently a workshop entitled 'The Quiet Sun: Transient Events and Coronal Heating' was held at MEDOC in Paris. Approximately, 25 known participants attended although on the first day we started off with a few extras! The aim of the meeting was to understand the relationships between the many reported quiet Sun transient events, e.g. explosive events, network flares, blinkers, etc. In particular, to determine whether all these types of phenomena are just manifestations of the same event detected by different techniques in different wavelength bands, or whether they are fundamentally different? To do this participants were encouraged to analyse particular named data sets. Each of the named data sets contained CDS data. Furthermore, since CDS can observe in chromospheric, transition region and coronal wavelengths it was particularly useful for comparing observations in these regions.

The results from the workshop strongly suggest that the SOHO/CDS EUV transient brightenings, blinkers, network and cell brightenings are all the same phenomena just observed using wide slit, rastered and single slit data, respectively. However, various members of the workshop have agreed upon a strategy to confirm (or deny) this.

There was also an animated discussion as to whether CDS blinkers were simply made up of many SUMER brightenings and SUMER explosive events. There was certainly some evidence that this may well be the case and work has started to find specific cases where a CDS blinker occurs directly above an explosive event observed in SUMER. Although there was a good data set where both SUMER and CDS were observing the same area, the SUMER slit, which was longer than the CDS slit, showed its greatest activity outside the CDS field of view!

A number of other discussions took place covering jetting phenomena and network flares/heating events/nanoflares. Further details of the workshop and the conclusions can be found on the following website.


Solar Cycle Observations with SOHO CDS - Implications for Coronal Heating

Carl Foley

A widely used diagnostic tool for coronal heating enthusiasts has been the measurement of the coronal temperature, and its variation with height in the corona and along coronal loops. Previous work has generally found that the coronal temperature increases as a function of height above the limb and along the loops. This has demonstrated a requirement for heating up to and including the maximum observed height range of almost 2 solar radii. More recently Aschwanden & Nitta., (2000) have demonstrated that heating at heights above the first gravitational scale height (a few x 10Mm) is not required, and the observed positive temperature gradient may simply be a result of the superposition of many flux tubes of different temperature which are heated at their base.

CDS now allows us to view the relative distribution of individual ions as function of height in the corona. This enables us to examine the distribution of plasma in many regions and structures. The Grazing Incidence Spectrometer (GIS) of CDS is operated in a photon counting mode. This allows events to be discriminated by pulse height, thereby allowing cosmic ray hits to be excluded from the accumulated spectra. The GIS is therefore able to obtain deep exposures relatively free of cosmic ray background far off limb.

In a recent paper (A&A in press) Foley, Patsourakos, Culhane, & Mackay 2001 we evaluate the relative abundance of Fe IX through Fe XV ions for coronal streamers observed close to the solar minimum and maximum of the current solar cycle (23), on 1996 July 8 and 1999 August 5 respectively. The most abundant (adjacent in Te) ions were then used to establish the best representation of the apparent temperature profile with height. The derived temperature profile and its variation from solar minimum to maximum are used to infer how the corona evolves and its associated energy requirements. For comparison we have also included solar maximum data for cycle 22 (1992, October 3) obtained from Yohkoh observations which were the basis of Priest et al., 1998, 2000. A figure from this paper is displayed below and highlights the difference in the coronal temperature between solar minimum and maximum.

Using SUMER data, Schuhle et al. (2000), found that the emission from the transition region (TR) increased by a factor of 2 from solar minimum to solar maximum. Classical models of the Transition region for the quiet Sun (e.g., Gabriel 1976) indicate that TR radiative losses are powered from the coronal conductive flux from the overlying corona. The enhanced transition region emission reported by Schuhle et al. (2000) is consistent with the increase of the coronal temperature over the solar cycle which we observe and present here.

The variation of the Sun's irradiance was covered in issue 1 of the CDS newsletter. To prodce the irradiance full sun spectral scans were obtained (FSUN) on a regular (monthly) basis. The evolution of the Sun as recorded in these observations are displayed above for Mg IX and Fe XVI. From these observations we can see that the major evolution of the sun in the EUV is associated with the active region emission. Over time the active regions emission appears to become more extended as more sunspots emerge and migrate to lower latitudes.

During this time period the magnetic field associated with these active regions is transported to the surrounding quiet sun (and ultimately the polar regions) by the Sun's meridional flow. This is illustrated below in the synoptic map produced from Kitt Peak magnetograms over the rise of the current solar cycle (courtesy of Duncan Mackay and Karen Harvey).

This process supplies and replenishes the quiet sun magnetic field and dictates the characteristic emergence of features over the solar cycle such as high latitude streamers and low latitude coronal holes.

Synoptic observations of the quiet sun have been obtained on a regular basis with the Grazing Incidence Spectrometer. The dramatic effect of the transport of magnetic flux is characterised by more than an order of magnitude enhancement observed in the flux of Fe XV and XVI. This is consistent with the offlimb temperature measurements which we discuss above.

These observations together support the hypothesis that the positive temperature gradients which we observe are real and not due to the effect of supersition of different isothermal flux tubes with different scale heights. The solar cycle evolution of the transition region emission is a result of the increased energy deposition into the overlying corona over the cycle.

GIS Slit Anomaly - Alternative GIS Studies

Carl Foley

Whilst the problem with the slit mechanism is resolved it is still possible to perform observations with the GIS which do not move in the N-S direction, which are either sit and stare observations or simple E-W scans. A strength of using the GIS is that it is operated in a photon counting mode. This allows photon events to be discriminated by pulse height, thereby allowing cosmic ray hits to be excluded from the accumulated spectra. The GIS is therefore able to obtain deep exposures relatively free of cosmic ray background. An example of this is illustrated below for an offlimb observation made with both the NIS (top row), and GIS (bottom row) on the 5th August 1999. This observation was of the base of a coronal Streamer as delineated in the LASCO C2, MLSO, EIT 284 A, composite image.

I have summarized studies which are ok to run whilst the slit anomaly problem is resolved here. The GEWS3 studies, are ideal for off limb observations and can be used in sequence with careful pointing to map large off field regions as is illustrated in the figure above.