RHESSI and IRIS

From RHESSI Wiki

Introduction

The IRIS satellite is the next SMEX on the blocks, scheduled to join RHESSI in low Earth orbit in December 2012. Its stated goal is to study "the flow of energy and plasma through this foundation of the corona and heliosphere," where "this foundation" is the interface region between the [corona] and the somewhat fuzzy surface of the Sun. Conceptually, this interface is broken up into a [photosphere], a [chromosphere], and a [transition region] imagined all to be spherical entities somewhat reminiscent of [Aristotle]'s celestial spheres.

Now we know better: these are not spheres at all, but highly structured and dynamic regions that have evaded explanation to date, partly because of weak theoretical insight, but mainly because the right observations just were not there. Their large-scale geometry is close to that of concentric spheres because solar gravity is so strong, though. The complex structure and the physics that produces it is where IRIS fits in. This new observatory will be especially suited for studying RHESSI's main specialty, flares. It will do the first comprehensive UV imaging spectroscopy of flares, among other things (note that [SUMER] was incapable of studying flares directly).

UV and hard X-rays

The UV and EUV emissions from a solar flare, ie the products of excitation of the "interface layer" that IRIS will study, often have an excellent correlation with hard X-rays and the powerful energy release of the impulsive phase of a flare. This was first made clear long ago by the UVSP instrument on NASA's pioneering Solar Maximum Mission, 1980-1989, although discovered in earlier missions (probably by Neupert) and also seen via the "Sudden Frequncy Deviation" type of ionospheric effect, as developed by Donnelly. Figure 1 illustrates this impulsive-phase connection via the very impulsive flare SOL2010-06-12.

Figure 1: Time histories of He II 304 A, observed by the EVE instrument on SDO, with RHESSI hard X-rays. This is the very impulsive flare SOL2010-06-12.

The spectrum of neutral helium is complicated, but the singly-ionized state (He II to astronomers) is simple: it is hydrogen with twice the nuclear charge and four times the nuclear mass, rougly speaking. Thus it has a Lyman-alpha analog, which is at 304 A. Normally this reflects a temperature high enough to ionize helium once, but not twice. Such temperatures also produce a flood of other UV and EUV lines, and these are the ones that show the impulsive phase that IRIS can study.

What happens in the chromosphere?

References

IRIS (goddess)