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2.4 Neupert effect

Neupert (1968Jump To The Next Citation Point) noted that in the rise phase of the soft X-rays their flux corresponds to the time integral of the centimeter radio flux since the start of the flare. As the centimeter flare emission is emitted by relativistic electrons, it is not surprising that the same correlation was later also found between the soft X-ray flux and the cumulative time integral of the hard X-ray flux, i.e.,
∫ t FSXR (t) ∝ FHXR (t′)dt′ , (1 ) t0
or, equivalently,
d --FSXR (t) ∝ FHXR (t) . (2 ) dt

This empirical relationship is called the “Neupert effect”. An example is presented in Figure 12View Image. Already Neupert (1968Jump To The Next Citation Point) suggested that there may be a direct causal relation between the energetic electrons and the thermal plasma: the soft X-rays may originate chiefly from a plasma heated by the accumulated energy deposited through flare accelerated electrons. It should be remarked here that Equation 1View Equation is only an approximation and that the correspondence with theory works only if cooling (by conduction or radiation) is negligible.

View Image

Figure 12: The derivation of the soft X-ray flux (observation by the GOES satellite) correlates in 80% of the flares with the hard X-ray flux (observation of the RHESSI satellite). This is an example of what is called the Neupert effect (from Dennis and Zarro, 1993Jump To The Next Citation Point).

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