We still do not know which modes of non-thermal energy power the chromosphere, transition region, corona, and solar wind. We know that waves, electrical currents, and magnetic reconnection all may release substantial energy, and that non-thermal particles, resistive dissipation, and wave damping occur. Yet, it remains unclear how much each of these contributes, how that depends on local conditions, and how the conversion of non-thermal to thermal energy happens in detail.
Waves: The role of virtually pure acoustic power in the heating of the chromosphere and its potential role in the corona remains a puzzle: some report too little power (Fossum & Carlsson 2005, 2006; Carlsson et al 2007), while others argue there should be enough (Cuntz et al 2007; Wedemeyer-Böhm et al 2007).
Various types of (mostly transverse) MHD waves are likely more efficient at travelling into the corona and solar wind (Cranmer & Ballegooijen 2005, Cranmer et al. 2007) than mostly compressible waves. We recently combined observations in the Ca II H line from Hinode with numerical simulations to show that the swaying motion observed with Hinode is caused by Alfvenic waves with power large enough to be of importance for the acceleration of the solar wind (Figure 1, from De Pontieu et al 2007b).
Much of the non-thermal line widths seen with spectrographs such as SOHO’s SUMER may well be caused by waves such as these (McIntosh et al. 2008), which are prime candidates for powering the solar wind. In the closed corona, they may be less important energetically, but there they serve valuable diagnostic purposes in coronal seismology (Tomczyk et al. 2007).
High-cadence observations combined with modeling will allow us to quantify reflection, transmission, and absorption of the waves. The radiation magnetohydrodynamic models will be guided by IRIS’s observations and coordinated vector-magnetic field observations with SDO. With the SAM code-development we will develop the second generation chromospheric models; these will provide sufficient realism in field-plasma coupling, radiative transfer, energy deposition, and wave properties for a direct comparison.
Currents: Establishing the energy associated with resistive dissipation of electrical currents within the chromosphere is challenging. We expect these currents to be intense in compact fibrils or sheets. Any dissipation within the chromosphere is expected to be associated with rapid temperature increases. IRIS will enable the spectroscopic tracing of changes in the thermal structure even in very thin strands. Such strands are seen with Hinode’s SOT in Ca II H for only some tens of seconds (De Pontieu et al. 2007a,b), possibly because changes in temperature cause them to fade from the passband. The broad thermal coverage of IRIS spectra and images from 5,000 to over a hundred thousand Kelvin will allow us to follow such an evolution.
The evolution of the thermal structure within the chromosphere will provide information on how much electromagnetic energy is transformed locally into heat and how much energy propagates downward from the corona as thermal energy or energetic particles (Rappazzo et al. 2007; Stirling et al. 1993) in events ranging from microflares to X-class flares. We expect both to occur, of course, but do not know their relative roles under a variety of conditions.