Neupertianity
| Nugget | |
|---|---|
| Number: | 502 |
| 1st Author: | Hugh HUDSON |
| 2nd Author: | |
| Published: | August 25, 2025 |
| Next Nugget: | TBD |
| Previous Nugget: | Synchrotron Radiation and the Foundations for a Cosmic Bridge |
Introduction
The terminology of the "Neupert Effect" has entered the flare literature, both solar and stellar. Werner Neupert (Ref. [1]) had recognized something quite important in his 1967 paper, via the comparison of flare soft X-ray emission with radio signatures of sudden energy release. Figure 1 shows two examples from this early paper, comparing the time integral of microwave Synchrotron radiation with the growth of soft X-ray emission measure.
The soft X-ray datum here was a pioneering spectroscopic product of Neupert's [OSO-III] instrument; it is the "Fe feature" detectable above about 10 MK, and representing Fe ions ionized down to at least their "He-like" state, ie an iron nucleus with only two electrons. Such data have not really been obtained recently; OSO-III ended its mission in the 1960s.
Significance
The Neupert Effect is often taken as evidence for the "thick-target model", frequently identified with John Brown. It is more general than that, however, since the general action of solar activity is heating. The heating of solar plasma at any height in the atmosphere tends to drive it upwards against its previous state of hydrostatic equilibrium; once in the corona, the cooling timescale becomes long and the result is "Neupertian", if the heated material gathers in closed magnetic fields. Thus any theory of solar activity predicts the Neupert Effect in some form. The alternative is upward flow on open field lines and rapid effective cooling due to expansion. Could one have closed fields totally in the corona, such that the heated plasma does not connect with the gas reservoir of the lower solar atmosphere? This is very unlikely, because it would suggest the outlandish configuration shown in Figure 2.
The configuration in Figure 2 has probably never been observed in the solar corona, but it is not completely impossible. Plasmas forming such a trefoil knot pattern have actually been observed in laboratory fluid flows (Ref. [2]). There is also substantial theoretical work (starting with Kelvin, it turns out) on various applications of these and more complex topological states. As always, a stellar atmosphere has a serious artifact in the form of the interface layer between the very different photospheric and coronal plasma volumes.
Conclusions
The Neupert Effect is pervasive in solar flares because of mass ejection into closed coronal field lines. The weird configuration in Figure 2 would allow truly isolated coronal plasmas to develop, perhaps in different ways. The tension force from the Neupertian green field captures the truly closed red field. Of course any energy release in the red field might well drive magnetic reconnection that would spoil the cartoon!
References
[1] "Comparison of Solar X-Ray Line Emission with Microwave Emission during Flares"