The temperature structure of the solar corona holds many important clues as to how the solar atmosphere is heated. Recent observations with EIS/Hinode and AIA/SDO have shown that well constrained temperature measurements can be made over a wide range of solar conditions. In this talk I will present results from a systematic study of the differential emission measure distribution in 15 active region cores. We focus on measurements in the “inter-moss” region, that is, the region between the loop footpoints, where the observations are easier to interpret. To reduce the uncertainties at the highest temperatures we present a new method for isolating the Fe XVIII emission in the AIA/SDO 94 channel. The resulting differential emission measures show that the temperature distribution in an active region core is often strongly peaked near 4MK, suggesting that these loops are close to equilibrium. We will compare these results to the analysis of evolving million degree loops, which show a similar, sharply peaked temperature distribution.

Abstract: The solar corona is at a glance a simpel physical domain. There is no complications due to radiation, the magnetic field is dominating the dynamics everywhere and we know that the corona is heated by a process that is tightly connected to the magnetic field. In spite of the relatively simple physics involved, we still do not even know which heating mechanism or mechanisms are responsible for maintaining the coronal plasma at several million degrees Kelvin. The responsible culprit is the dynamics of the magnetic field and the driver that injects energy into the magnetic field which is then released as heat in the corona. Numerical modeling has within the last few years become a realistic tool to help resolve this question, but modelling is made difficult by the fact that in order to answer any questions about the corona, it is necessary to include deeper layers of the atmosphere to account for the forcing of the magnetic field. Here we still need more observational data to get the forcing correct and verifying the simulations. I will give an overview of the modelling effort to date, the complications models face and the problems in verifying the modelling results.

The formation and subsequent ejection of cool transition region plasma into the corona will be discussed, as observed in our three-dimensional magnetohydrodynamic (3D MHD) model of the solar corona. To investigate the dynamics of the ejection, a comparison between the 3D MHD model and a 1D loop model will be presented. In addition, observations from Hinode/EIS and SDO/AIA will be compared to the output of the numerical models. In the 3D case, the pressure gradient acting upon the plasma is the main driver of the ejection. The pressure gradient is caused by a heating event that takes place just above the chromosphere following Ohmic dissipation of currents that have been produced through braiding of magnetic field lines by photospheric plasma motions. The parameters extracted from the 3D model serve as input parameters for the 1D loop model. A heating pulse is injected at different heights along the loop with varying amplitude and width to mimic the situation in 3D prior to the ejection. As a consequence, the heating rate is strongly increased in a localized area and leads to enhanced evaporation of material that causes the material to rise. In contrast to earlier studies, where similar heating events lead to both transition region redshifts and coronal blueshifts, our parameter study shows consistent blueshifts along the loop and almost no redshifts. In particular, we do not see the transition region plasma being pushed down significantly following the heating event. We will discuss these findings in terms of the mass cycle between the chromosphere and corona.

A study of the outflow velocity field in the quiet corona and coronal hole has been carried out using the EIS spectrometer on board the satellite Hinode. This concentrates on the temperature range from the upper transition region to the base of the true corona. The first part, the subject of a preliminary publication, treats the fine-scale structure of this field. The statistics of this structure strongly supports a model of coronal heating and wind acceleration due to stochastic magnetic reconnection low down in the transition region. To extend this to a larger scale, involving hole / non-hole flows, requires an absolute calibration of the velocity zero. This poses important difficulties for the instrument EIS, particularly for the low luminosity regions of the quiet corona and coronal holes. With this calibration, we are in a position to integrate the fine scale structure into an overall velocity structure. This leads to a clearer understanding of the relation between small-scale transition region reconnection and the onset region of the solar wind.

Magnetic reconnection at the interface between coronal holes and loops, so-called interchange reconnection, can release the hotter and denser plasma from the magnetically confined regions into the heliosphere contributing to the formation ofthe highly variable slow solar wind.

While it has been shown that it must take place, e.g., to explain the quasi-rigid rotation of holes in presence of the underlying photospheric differential rotation, we know very little of how interchange reconnection actually occurs. In order to understand its dynamics we have performed high-resolution reduced MHD simulations in Cartesian geometry (with a straightened out coronal loop next to an open region) of the interface region between open and closed corona. This boundary is not stationary, becomes fractal, and field-lines change connectivity continuously, becoming alternatively open and closed. We will discuss quantitatively the implications for coronal heating and sources of the solar wind.

Furthermore, we are able for the first time to include (self-consistently) fluctuations in the coronal magnetic field, naturally generated by photospheric convection. This is of crucial importance, in fact even small fluctuations in the super-radially expanding magnetic field at the boundary between coronal holes and streamers allow field-lines random walk, diffusing the plasma injected from coronal loops into the heliosphere away from the heliospheric current sheet, as observed in situ. In this way all the boundary between open and closed region is a source of slow wind, unlike current models that limit its source to few special boundary sections, a small fraction of the whole boundary.