Before reading this, It's a good idea to go over jimm's last poster , on SXT and BCS differential emission measures for solar flares, presented at the June, 97 SPD meeting. This is a continuation of that work, and will be presented as another poster at the Fall, 97 AGU meeting in San Fransisco.
Given the Soft X-ray DEM , we would like to see how the solar flare plasma relates to the hard X-ray burst as a function of temperature. It is well-known that soft X-ray time derivative is similar to the hard X-ray (or microwave) light curve. This is thought of as evidence for "chromosphseric evaporation"; i.e., the idea that the soft X-ray emission comes from a plasma that is heated by the nonthermal electrons which are responsible for the hard X-rays. The nonthermal electrons lose most of their energy via Coulomb collisions in the chromosphere. The temperature rises and a high pressure region forms which drives material both upward and downward ("chromospheric evaporation and condensation"). The coronal temperature and density increase as the evaporated material rises, resulting in soft X-rays. The hard X-ray light curves are proportional to the time profile of the input electrons. The soft X-rays, which come from the plasma heated by the nonthermal electrons, are proportional to the accumulated energy of the electrons up to a given time. The soft X-ray emission is thus proportional to the time integration of the profile of the input electrons, and the time derivative of the soft X-ray profile will look like the hard X-ray time profile. This is known as "derivativity" or the "Neupert Effect".
In a study of large flares observed by HXRBS, B. Dennis and D. Zarro (1993, Solar Physics, 146, 177) found that 80% of the flares showed good correlations between hard X-ray peaks and peaks in the time derivatives of the soft X-rays. They noted that gradual hard X-ray flares were less likely to show correlations. Here we will use a less restrictive definition of the Neupert effect: Instead of requiring that the peaks in the soft X-ray time derivative be correlated with the hard X-ray peaks, we simply require that the time derivative of the soft X-ray energy be positive during the hard X-ray burst.
The question here is: How does the Neupert effect manifest itself at different temperatures?
The answer is: It's different for different flares; not too surprising. Figure 1 shows typical behavior for some flares, in particular those flares that have a distinct high temperature hump in the DEM . The top plot is of the energy in the SXR plasma for the flare of 13-Jan-1992. The white dashed line shows the total energy, the red line shows the energy in high temperature plasma (T > 18 MK), the blue line shows the energy in low temperature plasma (3 MK < T < 18 MK). Note that for the energy calculation, we've assumed that the volume and filling factor are the same for all temperatures. ("Filling Factor": how much of a given volume actually has hot plasma in it.)
You can see the effect of how the DEM varies with time in the SXT and BCS data. The top plot of Figure 2 shows the data from the Al.1 (white) and Be119 (blue) filters of SXT, and the FeXXV channel of BCS (red). The bottom plot shows the time derivatives of the data, along with the HXT data. For this case, the curves have been arbitrarily scaled. The FeXXV channel is only sensitive to high T plasma, and its derivative closely follows the hard X-ray time profile. (You can find plots of the temperature responses in the SPD 97 poster .) The Be119 filter is sensitive to low and high T plasma; its derivative has a peak with the HXR burst, and remains positive long after the HXR burst. The Al.1 filter is sensitive to high and low T plasma, but is much more sensitive to low T plasma than the Be119 filter. Its derivative has a peak with the HXR burst, but also has a peak long after the HXR burst. Al.1 brightness increases (i.e., has a positive derivative) until 17:37 UT. The late increase in Al.1 counts isn't due to extra heating, however; the total energy decreases after 17:32 UT. Instead, it's due to cooling of the high T plasma, and the corresponding increase in low T plasma. The plasma moves into the temperature range where the Al.1 filter is more sensitive, and the Al.1 brightness increases, even though the total energy in the plasma is decreasing.
Now that we have a reasonably neat explanation for how the Neupert effect works in some cases, let's look at a flare where it doesn't work so well at all. Figure 3 has the same format as Figure 1, for a flare that occurred on 19-Feb-1992. Note that the DEM for this flare never showed a high-T hump. The top plot is of the energy in the SXR plasma. The white dashed line shows the total energy, the red line shows the energy in high temperature plasma (T > 18 MK), the blue line shows the energy in low temperature plasma (3 MK < T < 18 MK). The bottom plot shows the time derivatives for each component, compared with the > 33 keV hard X-ray time profile as seen by the HXT.
Figure 4. HXT data, all four channels, for the 19-feb-1992 flare.
So what can we conclude from this work? First, we can say that for many flares, in particular those (18 out of 79 analyzed so far) which have distinct high-T components in the DEM, the Neupert effect applies to high-T plasma, and not necessarily to the Low-T plasma. Typically there is some heating that exists after the HXR burst is gone, thus all of the soft X-ray plasma is not due to "chromospheric evaporation". And some flares do not seem to fit well with the Neupert effect at all. Still a work in progress.
Comments to: jimm@ssl.berkeley.edu
16-Oct-1997, jmm
