The Evaporating Sun

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Number: 55
1st Author: Ryan Milligan
2nd Author:
Published: 1 January 2007
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The standard model of a solar flare requires that a considerable fraction of the energy released is used to accelerate electrons to very high energies. During the impulsive phase these electrons travel along newly reconnected magnetic field lines, from the corona to the underlying chromosphere, where they lose their energy by collisions with the dense plasma. This transfer of energy heats the chromospheric material to tens of millions of degrees causing it to rise up into the corona and expand to fill the post-flare loop, which is then familiarly observed at SXR and EUV wavelengths (see Figure 1). This process is known as chromospheric evaporation and is an important component of flare energetics (see this earlier Yohkoh nugget).

(Note: the term evaporation is somewhat of a misnomer: evaporation implies a change of state of matter. The process could be more appropriately termed chromospheric ablation. It is too late to change the astronomers' terminology now, though.)

Figure 1. A Cartoon of the basic model of chromospheric evaporation. Electrons injected into the loop at the apex make their way to the footpoints where they heat and subsequently "evaporate" the local plasma, forcing it to rise back up the legs of the loop.

Theoretical expectations

The basic idea of chromospheric evaporation was first proposed by Werner Neupert in 1968 (original reference) to explain the delay between the peak of the hard and soft X-ray emission during solar flares. The physical interpretation of this premise states that the non-thermal electrons that make it to the chromosphere either produce thick-target bremsstrahlung emission in the form of hard X-rays or transfer their energy to the ambient plasma via Coloumb collisions, resulting in intense heating and an increase in pressure (only 1 part in 105 of the total energy of the electrons appears as bremsstrahlung photons.) This results in the rapid expansion of the chromospheric plasma into the tenuous corona to fill the overlying flare loops, emitting (blue-shifted) soft X-ray and EUV emission in the process.

In recent decades, many simulations have been developed to model the dynamic response of the lower solar atmosphere to various intensities of injected electron beams. The general consensus is that the more energy that is deposited in the chromosphere by the electrons, the faster the evaporated material should rise. However, direct observations to complement these theories have been hard to come by as there is no one instrument that can simultaneously measure both the injected electon spectrum and the velocity of the upflowing material. We therefore require a combination of instruments to investigate the cause and effect of chromospheric evaporation. RHESSI is perfectly suited to diagnosing the HXR emission and therefore deriving the properties of the driving electron beam, while the Coronal Diagnostic Spectrometer (CDS) onboard SOHO can measure plasma velocities. Thus, coordinated observations with these two instruments can measure the direct response of the chromosphere.


The spectroscopic capabilities of RHESSI allow us to infer the properties of the driving electron beam from the HXR emission, while imaging tells us the location of the deposited energy. The associated upflow velocities can be measured through simultaneous observations from an instrument capable of observing Doppler shifts in high-temperature emission lines, such as CDS (see Figure 2). Thus, for the first time we are able to see if the location of the footpoints in HXR are the same as the regions of high upflow velocities as expected if the chromospheric evaporation model is correct.

Figure 2. Left: An EUV Imaging Telescope (EIT) 195A image (prodominantly Fe XII emission at 1.5 MK) showing a post-flare loop on 19 January 2005. Right: A velocity map taken from the same region using the Fe XIX (~8 MK) emission line as observed by CDS. Strong blueshifts are clearly visible at the HXR footpoints as observed by RHESSI (shown as yellow and blue contours of the >25 keV emission) indicating the hot, upflowing material. For the case of limb flares, however, RHESSI can actually image the upflowing material as it travels up the legs of the post-flare loop. This provides direct evidence of chromospheric evaporation, which increases the density in the loop and thus the stopping distance of nonthermal electrons becomes shorter. This can give rise to the apparent shifts of the emission centroids toward the loop top, as can be seen in Figure 3.

Figure 3. RHESSI 12-15 keV images integrated over 4 seconds and taken at 2 second intervals. The blue line indicates the solar limb. The two major X-ray sources shift from the footpoints up the legs of the loop as a function of time.


Combining RHESSI and SOHO/CDS observations of the same flare allows us to understand the effects associated with the energy released. In particular, RHESSI enables the energy and location of the associated electrons to be determined while CDS shows the location and velocity of the hot plasma. The combination of the two sets of observations provides powerful direct evidence of chromospheric plasma upflows (evaporation) driven by beams of non-thermal electrons. In the future, we hope to greatly improve upon CDS observations using data from the EUV Imaging Spectrometer (EIS) onboard the recently launched Hinode mission (the satellite formerly known as Solar-B).

Biographical note: Ryan Milligan is currently a Postdoctoral Research Associate at NASA's Goddard Space Flight Center. He would also like to thank Wei Liu for the images of the RHESSI limb flare.

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