1. Scientific Objective: The aim is to simultaneously determine the 3-D geometric structure, temperature, density and flows of active region loops. This will be possible for a few months near the end of 2007 when the two STEREO spacecraft are at an optimum separation for loop stereoscopy. Later, when the spacecraft are further apart, we expect there will be more confusion in identifying the same loops from the two different perspectives. For an unambiguous reconstruction of loops, extrapolation of vector magnetic fields is very helpful. Hinode/SOT provides high resolution vector fields and will be essential. We also put a lot of weight on
simultaneous ground-based observations. These provide the magnetic vector near the base of the corona, and display cool loops to complement the hotter loops seen by other instruments. The spectrograph Hinode/EIS observes lines from Fe VIII-XXIV, thus providing good temperature resolution for the main structure and flows in the loop. In addition line intensity ratios of Fe XII, Fe XIII, Fe XIV provide density diagnostics of the coronal plasma. The SOHO/SUMER wavelength has many strong lines coming from the chromosphere and transition region, and can be used to obtain details of the structure and flows at the base of the corona. As a secondary objective, we will be hoping to capture microflares triggering Doppler shift oscillation. The Doppler shift oscillations are thought to be signatures of magnetoacoustic standing waves in coronal loops. As such, their oscillation period is determined by the wave time along the loop. This depends on loop length, loop magnetic field, plasma density and temperature. The temperature and density can be obtained to better than 20% by spectroscopy. Previously we have used circular loops models to estimate the loop length and derive the magnetic field. The loop length is the most critical parameter in the equation for loop magnetic field. With STEREO we will be able to obtain a much better picture of the loop, and so for the first time be able to derive realistic magnetic field strengths in loops exhibiting Doppler shift oscillation.
Target: Active region observed at several positions, 6 hours per position, as it crosses the disk.
2. Scientific Objectives: In the interface between the photosphere and the corona, the magnetic field plays a crucial role in guiding waves, in the build-up of magnetic stress and its dissipation through reconnection. One likely chromospheric heating mechanism is thought to be dissipation of hydro-magnetic shocks, especially in weak field and force-free regions of the chromosphere (e.g. De Pontieu & Erdélyi 2005, Nature, 430, 536). Most of the corona, transition region and chromosphere is force-free and currents are along field lines (parallel currents). Another possibility is direct magnetic field dissipation into plasma heat and flow, that is, by reconnection. This occurs in sheets of perpendicular current, where the resistivity is high enough to trigger dissipation. Distinguishing when, where and what the signatures of shocks and reconnection are, requires detailed high cadence, high resolution observations of all layers of the solar atmosphere. Until recently it was thought that brightenings with periods of 3-5 min indicated shock dissipation, but studies of explosive events suggest that reconnection may also be triggered by p-modes and
then reconnection signatures would show the same 3-5 min periods expected
from shock dissipation (Ning et al. 2004, A&A, 419, 114; Chen and Priest 2006, A&A, 238, 313). The goal of this proposal is to determine the heating associated with specific magnetic field structure or magnetic field evolution. Flux
cancellation, emergence and merging can be seen in the photosphere and the
structure in the transition region and corona extrapolated. This identifies regions known as Quasi-Separatrix Layers (QSLs) across which the magnetic connectivity changes (Démoulin et al. 1996, A&A, 308, 643) and current sheets form. Our plan is to use high cadence magnetic field, intensity and velocity SOT observations at different photospheric and chromospheric heights to determine wave fluxes and sites of energy dissipation such as Ca II bright points. These will be combined with SUMER and EIS observations of the chromosphere, transition region and lower corona to detect flows, and XRT images to find bright points in the corona. Both quiet and active region observations should be made. In active regions, the magnetic fields are stronger and more organized than in the quiet Sun so can be observed more easily, but there is likely to be more confusion in the chromosphere due to overlapping events.
Operational Considerations: These observations do not require a special target. Their success lies in observing with many instruments simultaneously the same part of the Sun. Planning and target choice can be done days in advance which makes it a good program for multi-observatory observations. This HOP-JOP is also planned as a back-up for HOP-JOPs targeting specific features on the Sun such as
active regions, filaments etc in the SOHO-Hinode-Ground campaigns. A typical
observation is 6 hours.