Astrid M. Veronig

Physics of the Neupert effect: Estimates of the effects of source energy, mass transport, and geometry using Rhessi and Goes data

The empirical Neupert effect (ENE) is the observed temporal correlation of the hard X-ray (HXR) flux FHXR(t) with the time derivative of the soft X-ray (SXR) flux SXR(t) in many flares. This is widely taken to mean that the energetic electrons responsible for FHXR(t) by thick-target collisional bremsstrahlung are the main source of heating and mass supply (via chromospheric evaporation) of the SXR-emitting hot coronal plasma. If this interpretation were correct, one would expect better correlation between the beam power supply Pbeam(t), inferred from the HXR spectrum, and the actual power Pin(t) required to explain the SXR flux and spectrum, allowing for variations in both emission measure (EM) and temperature T, for radiative and conductive cooling losses, and for complexities of geometry like multiple loops. We call this the theoretical Neupert effect (TNE). To test if it is true that Pbeam(t) and Pin(t) inferred from data are better correlated than FHXR(t) and SXR(t), we use an approximate approach for a simple single-loop geometry and rough estimates of the particle and energy transport and apply the model to RHESSI and GOES data on four flares. We find that if the beam low cutoff energy E1 is taken as constant, the correlation of Pbeam(t), Pin(t) is no better than that of FHXR(t), SXR(t). While our modeling contains many approximations to cooling and other physics, ignored entirely from ENE data considerations, there seems to be no reason why their order-of-magnitude inclusion should make the TNE worse rather than better, although this should be checked by more accurate simulations. These results suggest that one or more of the following must be true: (1) fast electrons are not the main source of SXR plasma supply and heating, (2) the beam low cutoff energy varies with time, or (3) the TNE is strongly affected by source geometry. These options are discussed in relation to possible future directions for TNE research.

A CORONAL THICK-TARGET INTERPRETATION OF TWO HARD X-RAY LOOP EVENTS

We report a new class of solar flare hard X-ray (HXR) sources in which the emission is mainly in a coronal loop so dense as to be collisionally thick at electron energies up to 50 keV. In most of the events previously reported, most of the emission is at the dense loop footpoints, although sometimes with a faint high-altitude component. HXR RHESSI data on loop dimensions and nonthermal electron parameters and GOES soft X-ray data on hot loop plasma parameters are used to model coronal thick-target physics for two discovery events (2002 April 14 [23:56 UT] and 2002 April 15 [23:05 UT]). We show that loop column densities N are consistent with (1) a nonthermal coronal thick-target interpretation of the HXR image and spectrum; (2) chromospheric evaporation by thermal conduction from the hot loop rather than by electron beam heating; and (3) the hot loop temperature being due to a balance of thick-target collisional heating and (mainly) conductive cooling.

The Confined X-class Flares of Solar Active Region 2192

The unusually large active region (AR) NOAA 2192, observed in 2014 October, was outstanding in its productivity of major two-ribbon flares without coronal mass ejections. On a large scale, a predominantly north?south oriented magnetic system of arcade fields served as a strong top and lateral confinement for a series of large two-ribbon flares originating from the core of the AR. The large initial separation of the flare ribbons, together with an almost absent growth in ribbon separation, suggests a confined reconnection site high up in the corona. Based on a detailed analysis of the confined X1.6 flare on October 22, we show how exceptional the flaring of this AR was. We provide evidence for repeated energy release, indicating that the same magnetic field structures were repeatedly involved in magnetic reconnection. We find that a large number of electrons was accelerated to non-thermal energies, revealing a steep power-law spectrum, but that only a small fraction was accelerated to high energies. The total non-thermal energy in electrons derived (on the order of 1025 J) is considerably higher than that in eruptive flares of class X1, and corresponds to about 10% of the excess magnetic energy present in the active-region corona.