Small `scope catches a big bang

On 4 November 2003 a solar flare occurred that was so large that the X-ray sensors on the NOAA GOES satellites — the defacto standard for quantifying flares since 1974 — saturated for about 12 minutes. Because of this sensor saturation, solar scientists had to try and extrapolate what the peak magnitude and timing might have been, coming up with a preliminary estimate for the peak of soft X-ray flux at 0.1 – 0.8 nm, of X28 (2.8 mW/m2 at the Earth) peaking at 19:50 UT.

Meanwhile, at the Narrabri Visitor's Centre, a pair of "RadioJove" kit receivers were assiduously taking 20-MHz flux measurements, which were being recorded to disk on an obsolete PC. This inexpensive radio telescope started as a hobby for local staff, and was described in the article "Narrabri Simple 20-MHz Interferometer" in the October 2002 ATNF News. It turned out that "Simple" made very high fidelity recordings of this exceptional solar flare and we have been able to use these data to better characterise the flare's X-ray peak.

Figure 1: X-ray profile of the largest solar flare on record. The data from the two "Simple" receivers allows us to measure beyond the GOES-12 saturation level and constrain the flare peak to the indicated bounds.

How is it that this "backyard-class" telescope was able to provide the intensity of solar X-rays some ten billion times higher in frequency than it's observing band? The answer involves the Earth's atmosphere acting like a giant X-ray detector: Solar X-rays cause atmospheric molecules to become ionised. As the X-ray intensity increases during large flares, the ionosphere becomes opaque to low-frequency radio waves. The resulting decrease in detected power on the ground is called a sudden ionospheric disturbance, or ionospheric fadeout.

In order to determine the solar X-ray emission from our fadeout data we had to perform several steps:

♦ Compare the fluxes during the flare to "quiet day" fluxes from the same sidereal times, in order to quantify the ionospheric attenuation of the 20-MHz Galactic background. An instrument that does this is called a relative ionospheric opacity meter, or "riometer".

♦ Correct for the solar radiation cross-section of the zenithal ionosphere, in order to ensure our data represented the intrinsic solar X-ray emission, rather than just its local ionospheric effects.

♦ Use the unsaturated GOES-12 satellite measurements to determine the parameters of fit for a published model that relates low-frequency ionospheric absorption to the solar X-ray intensity.

We have been able to demonstrate that the whole process works by using data from other large "X-class" flares. These flares reveal that, even when we simulate saturation of the GOES-12 sensor, we can still determine the flare peak-magnitude to better than 10% and the peak time to better than three minutes.

Analysis of the 4 November 2003 data was further complicated by the presence of some radio interference at the flare peak. This meant we had to use the additional step of fitting a polynomial to the interference-free data in order to infer the properties at the peak. To reflect the uncertainty associated with this process our error estimate was increased by another 10%.

The end result of this number crunching is a new set of constraints on the magnitude and timing of the largest solar flare on record. Our best estimate gives a peak magnitude of X40 peaking at 19:46 UT. The full range of fits consistent with our data spans X34 – X48, peaking at 19:44-48 UT. This suggests the flare was substantially larger, and peaked earlier, than the X28 preliminary estimate. However our results are comparable to the X28 – X40 (peaking at 19:44-50 UT) and X40 – X50 (at 19:45-46 UT) estimates that have since been published in the literature.

Our observations, described in the paper X-Ray Magnitude of the November 4 2003 Solar Flare Inferred from the Ionospheric Attenuation of the Galactic Radio Background have been accepted for publication in the Journal of Geophysical Research and Space Physics.

Just as remarkable as making a scientifically valuable observation with such an inexpensive instrument, was the serendipitous case of "being in the right place at the right time". An hour earlier and the sun would have been well below the horizon from Narrabri, an hour later and the local ionosphere would have been so highly exposed to the solar X-rays that our technique could not be applied!

Many people have assisted with this work, including Tim Kennedy, Daron Brooke, Thomas Ashcraft, Rodney Viereck from the NOAA, Rose Roche, Michael Dahlem, Rob Chapman, Charles Belling and Alex Davey. Sincere thanks also go to our anonymous referees.

David Brodrick, Steven Tingay and Mark Wieringa
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