GOES Hard X-rays?
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Nugget | |
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Number: | 290 |
1st Author: | Hugh Hudson, |
2nd Author: | Janet Machol and Rodney Viereck |
Published: | 16 January 2017 |
Next Nugget: | TBD |
Previous Nugget: | Constant Density |
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Contents |
Introduction
Solar flares invariably emit soft X-rays, due to their injection of hot plasma into the corona. These soft X-rays have become the defining property of a flare, and the GOES X-ray class (A, B, C, M, X) its most popular metric. The database really began with the launch of SOLRAD-1 in 1960; this earliest astronomical observatory in space initially found the relationship between short-wave fadeouts and solar flares. Sensors similar to SOLRAD's X-ray photometersfound their way onto the GOES series of weather satellites. The very first GOES satellite, GOES-1, carried a "Space Environment Sensor" package that extended the SOLRAD soft X-ray observations begun with SOLRAD. Other early soft X-ray photometers appeared on other satellites of that early era, notably OSO-1, launched in 1962, and Ariel-1, launched the same year.
Some confusion might arise from the cultural gap between RHESSI scientists, who tend to think of spectra in terms of energy units (keV), and X-ray spectroscopists, who may be happier in wavelength space (nm or Å). In particular the GOES/XRS channel A (nominally 0.5-4 Å) may sound like a hard X-ray sensor, since the lower-limit wavelength seems to correspond to 25 keV or so. This Nugget clarifies that conversion and also points out a tricky background source for this channel in particular.
The first entry in the GOES database of solar flares would now be termed SOL1975-09-01T15:19 (C 0), although that sounds a bit strange. The classification C1.0 would now be the weakest C-class flare, with the C representing a base-10 logarithm of the peak energy flux, and 1.0 the mantissa of the logarithm converted back into a proper number. The meaning of the letter designations may be lost in history, and although flares weaker than A-class and more powerful than X-class are well-known now, no generally accepted extension of the letter-logarithms has taken hold.
The GOES/XRS solar X-ray photometers
Now called the XRS (X-ray Spectrometer) instrument, up until the present these sensors consist of pairs of gas-filled ionization chambers with spectral response limited to about 1-8 Å and 0.5-4 Å by choice of fill gas (argon or xenon) plus the thickness of the entrance window. Although broad-band and hardly spectroscopic in nature, these two bands allow for a basic measurement of the temperature of the flare plasma - typically in the narrow range 10-30 MegaK.
Because the plots in Figure 1 are on logarithmic scales, one wants to be careful about the actual contributions, which must be assessed directly. To see the distribution of the response of each channel, we should take a model spectrum; for soft X-rays we get excellent modeling from atomic-physics codes such as CHIANTI. Figure 2 shows the same response curve as in the right-hand panel of Figure 1, convolved with CHIANTI isothermal fits at 5 MegaK and 20 MegaK, characteristic of flares early and late in their time development. Active regions and the quiet corona typically have much lower temperatures.
These plots have some really interesting aspects. For the low-temperature one, the linear version of the plot shows that the two GOES bands have peak responses at 1.24 and 2.48 keV (a nice coincidence), and bandwidths of about 20% around these photon energies. For the hot spectrum, though, the XRS A band (0.5-4.0 Å) does become a broad band: its full width embraces the Fe feature, which the convolved model spectra show at 6-7 keV.
This sensitivity of GOES to the K-line feature opens an interesting question, which has not been addressed in the literature (we think) but might be. A solar flare typically excites a broad range of temperatures in the footpoints of its coronal magnetic loops - how hot does this chromospheric material actually get? The absence of a clear impulsive-phase signature in the GOES/XRS A channel (nominally 0.5-8 Å in breadth, actually covering 2-9 keV pretty well for a hot thermal spectrum (Figure 2, lower panel) suggests that Fe XXV does not form in the chromosphere despite the intense energy release there. This remains an interesting research question.
Non-solar signatures
The advent of solar minimum, and the improved instrumentation in the current GOES spacecraft series (GOES-13, -14, and -15) have allowed us to get a clear picture of a non-solar effect prevalent in the data, especially in the XRS A channel (0.5-8 Å). Figure 3 illustrates this effect via recent GOES-15 data.
This non-solar contribution to the high-energy channel can be confusing, because the phenomena causing it have some flare-like properties: often fast-rise/slow-decay time profiles, for example. But the spectrum is entirely wrong, as the right-hand panel of the figure shows. Flare soft X-rays have a distinctive pattern in a hard-vs-soft correlation plot, namely a clockwise circulation with time that reflects the early temperature peak. The ratio also follows a well-defined mean line, which we interpret with CHIANTI to be the line defined by thermal sources at 5-20 MegaK temperatures at varying emission measures.
What is the origin of these non-solar counts? GOES/XRS shields against high-energy particles, but they may leak through and/or produce secondary effects such as bremsstrahlung or neutrons. We can see a clear diurnal phase, suggesting that the interference has a local-time organization, as the geotail would.
Conclusion
The "new" GOES data (GOES-13, -14 and -15) have improved sampling that lets us see the weaker solar fluxes better. We are now getting a good look at this parameter space because solar minimum is almost upon us. These low-flux data are important because of new observations; not just RHESSI but also from programs like SphinX, MESSENGER, and MinXSS. We also expect much better soft X-ray observations from the new series of GOES satellites, GOES-N,O,P.
In studying these data we note here that we must be careful interpreting the GOES/XRS data at low flux levels because of background interference from the Earth's magnetic environment - but we don't know in detail what the mechanisms really are.
References
[1] "he Solar Flare 3.8-10 keV X-Ray Spectrum"
RHESSI Nugget Date | 16 January 2017 + |
RHESSI Nugget First Author | Hugh Hudson, + |
RHESSI Nugget Index | 290 + |
RHESSI Nugget Second Author | Janet Machol and Rodney Viereck + |