Chasing White-Light Flares

Abstract

In this memoir I describe my life in research, mostly in the area of solar physics. The recurring theme is "white-light flares," and several sections of this paper deal with this and related phenomena; I wind up describing how I see the state of the art in this stillinteresting and crucially important (as it has been since 1859) area of flare research. I also describe my participation in two long-lived satellite programs dedicated to solar observations (Yohkoh and RHESSI) and elaborate on their discoveries. These have both helped with white-light flares both directly and also with closely related X-ray and γ -ray emissions), with the result that this article leans heavily in that direction.

Current‐driven flare and CME models

“…There is one flare-related context where current neutralization is required: a return current is needed to neutralize the direct current associated with precipitating electrons. The implied current ≈ 10 17 A greatly exceeds the maximum current in a coronal flux loop [Holman, 1985], requiring a return current such that the net current is consistent with a redirected coronal current (≈ 10 11 A). Early models for the return current [Brown and Melrose, 1977;Brown and Bingham, 1984;Spicer and Sudan, 1984; van den Oord, 1990] assumed that it is 10.1002/2017JA024035 cospatial with the direct current.…”

“…In these models, the precipitating electron beam was assumed to be turned on in the corona, and that the return current was attributed to ambient electrons accelerated by either an electrostatic or inductive electric field [van den Oord, 1990] in response to the postulated beam of electrons. Such a model cannot apply if the precipitating electrons are accelerated by a parallel electric field: the same parallel electric field cannot accelerate the precipitating electrons downward and the electrons in the return current upward on the same field line [Holman, 1985;Emslie and Henoux, 1995], cf. also Fletcher and Hudson [2008].…”

“…This analogy with the acceleration of auroral electrons suggests that the acceleration of electrons occurs due to a parallel electric field in the upward current region [e.g., Ergun et al, 2002]. In the application to a flare, the model requires a large number of neighboring up-and down-current paths [Holman, 1985;Melrose and Wheatland, 2014]. Specifically, with the numbers adopted here, 10 6 up and down currents of 10 11 A, each corresponding to 10 30 electrons per second, are required to give a net 10 36 s −1 precipitating electrons on the up-current paths.…”

“…The high density of the precipitating electrons [Krucker et al, 2011] is consistent with a model in which the source of the accelerated electrons is upflow from the chromosphere. The acceleration is due to the EMF becoming localized along field lines on the upward current paths; the total Φ = 10 10 V separates into potential changes of 10 4 V along 10 6 upward current paths [Holman, 1985;Melrose and Wheatland, 2013], and the electrons are accelerated by the resulting E ∥ just above the chromosphere [Haerendel, 2012;Melrose, 2012b;Melrose and Wheatland, 2013] on each upward current path, analogous to the acceleration of auroral electrons just above the ionosphere. The effective rate of dissipation is then identified in terms of the power going into electrons through the acceleration by E ∥ .…”

“…Which of the flare and CME is the cause and which effect has long been (and remains) controversial? The acceleration of a CME can be attributed to a current-current force, which can be derived from the Lorentz force in an approach based on magnetohydrodynamics (MHD), whereas the EMF and bulk energization of electrons, due to a parallel electric field E ∥ [Holman, 1985], require a nonfluid-based approach.…”