The aim of this STEREO/SECCHI-led JOP is to use the singular vantage points of the STEREO spacecraft coupled to observations from SOHO, TRACE and Hinode in the Sun-Earth line to produce a unique dataset examining the time-dependent nature of the solar atmosphere.
We know already that this magnetised, plasma environment is highly structured and dynamic. Here we concentrate upon two important aspects:
1. the detection and characterisation of guided wave-modes within active region loops and;
2. the determination of the nature (birth, life, death) of small scale EUV brightenings.
Outlining the significance of these dynamic features is beyond the scope of this short scientific justification. However, concentrating upon the importance of observing simultaneously this dynamic behaviour from several different vantage points, we see that there are some fundamental issues that could be addressed. These include:
(i) for waves:
It should be possible to observe the “same” propagating wave phenomena at the base of active region loops. Part of the problem with observing these oscillations on the disk from along the Sun-Earth line is that the angle of the emerging loop to the solar surface is unknown. Of course, it can be inferred indirectly from magnetic field extrapolations to a certain degree. Naturally this problem of a single viewing angle has an effect on the accurate calculation of the wave speed. Measuring this propagation speed from different viewpoints, will allow us to obtain a better-constrained value. Also, given that often the change in intensity related to these oscillations can only be <10 percent above the average intensity of the loop, the proper application of an appropriate background subtraction is very important. The different view-points should help to improve/verify the best way of doing this.
(ii) for dynamic brightenings:
We can ask: can we observe the “same brightening” from the three vantage points? Are there any discernable differences? If so, what are they? Does the opportunity of tracking the possible evolution of the 3D geometry of a “large” brightening help us understand their existence in the first place? Can we get obtain a rough estimate of the possible preferred height of these brightening above the limb (or are they ALL very low down in the atmosphere)?
Of course, the above is certainly not an exhaustive list of what could be examined by this proposed unique dataset.
In order to achieve these observations, a short, constrained (2 to 3 hour, approx. 20-30s cadence in a single filter) observing run using SECCHI/EUVI would be undertaken. This will take SECCHI out of its usual synoptic observing mode and possibly suspend all other STEREO observations during this observing period. A short test case at higher cadence (10s) for 10 minutes was undertaken on STEREO-A only in February 2007. Apart from that dataset, no other coordinated high cadence observing programme (and hence data) as suggested here exists.
Thus, when these high cadence SECCHI observations are combined with observations from SOHO, TRACE and Hinode located along the Sun-Earth line, we have the capability to obtain near simultaneous observations of dynamic changes in the solar atmosphere from three lines of sight. EUV imaging in the same filter as SECCHI but from TRACE and SOHO/EIT will be requested along with complementary Hinode/XRT full disk observations. EUV spectroscopy will be undertaken by SOHO/CDS and Hinode/EIS to both increase the wavelength coverage as well as provide plasma diagnostics for the selected targets. Magnetograms from both SOHO/MDI and Hinode/SOT will provide details of the underlying photospheric magnetic field structures.
The obtained time-series observations will be investigated by employing, say, wavelet analysis techniques and/or Bayesian analysed methods across the datasets.
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