The Solar X-ray Limb III
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== Conclusions == | == Conclusions == | ||
| + | After much hardship we managed to show that the method works in principle and we can report the first measurement of the X-ray limb height. You can read all the details in the forthcoming ApJ publication ([http://adsabs.harvard.edu/abs/2017arXiv170511044B Battaglia et al. 2017]). However, many questions remain and new ones have been raised: | ||
| + | |||
| + | * Why is the observed height above the expected height from the modelling? | ||
| + | * Is the VAL-C model simply not applicable for flaring atmospheres and a more realistic model might fit better? | ||
| + | * How much is the inferred height affected by time-variability and substructures of the chromosphere? | ||
| + | * Why is it so exceedingly difficult to find suitable flares for this kind of analysis? | ||
| + | |||
| + | The next steps thus have to be better modelling and a systematic search for a larger number of events over the whole duration of the RHESSI mission to confirm this first detection and answer the above questions. | ||
Revision as of 14:22, 8 June 2017
Contents |
Introduction
How big is the Sun? This question has to be answered with "it depends", since the observed diameter is wavelength dependent. The IAU nominal value for the solar radius is given as 695.7 km (Prsa et al. 2016). This value is based on optical measurements and depends on details of the atmosphere and radiative transfer models. In two previous nuggets we described a new technique that uses RHESSI visibilities of occulted solar flare sources to measure the height of the X-ray limb (The X-ray Limb) and reported on preliminary analysis of a suitable solar flare (The Solar X-ray Limb II). As reported in the second nugget there were some problems with the chosen flare, but having found a better suited candidate we can finally claim success!
The event
The event that lead to our success, SOL2011-10-20T03:25, is the perfect candidate for several reasons. AIA context images provide independent confirmation that the event was indeed occulted. In addition, RHESSI's detector 2 was working well enough to provide usable data for this kind of work. This allowed us to perform a set of rigorous tests to confirm that the observed peak in visibility amplitude is indeed the signature of a limb-occulted source.
The height of the X-ray limb
We find (averaged over the measurements from G1 and G2) the X-ray limb radius a R_{X−ray} = 964.05\pm 0.15\pm 0.29 arcsec. The error includes both a statistical error from the visibility fits and a systematic error that includes the phase scatter of the visiblities that are associated with the limb, the length of the metering tube that holds the grids, and the accuracy of the Sun-center determination from the solar aspect system. How does this height compare with the optical limb height and with predictions for the X-ray limb height from models? This is shown in Figure 2. For the comparison, the standard VAL-C model (Vernazza et al. 1981) was used.
Conclusions
After much hardship we managed to show that the method works in principle and we can report the first measurement of the X-ray limb height. You can read all the details in the forthcoming ApJ publication (Battaglia et al. 2017). However, many questions remain and new ones have been raised:
- Why is the observed height above the expected height from the modelling?
- Is the VAL-C model simply not applicable for flaring atmospheres and a more realistic model might fit better?
- How much is the inferred height affected by time-variability and substructures of the chromosphere?
- Why is it so exceedingly difficult to find suitable flares for this kind of analysis?
The next steps thus have to be better modelling and a systematic search for a larger number of events over the whole duration of the RHESSI mission to confirm this first detection and answer the above questions.
