The CSHKP model, named after some of the major contributors towards the de- velopment of the model (Carmichael 1964; Sturrock 1966; Hirayama 1974; Kopp & Pneuman 1976), is often referred to as the standard model of solar flares. The model can explain compact flares, as well as two-ribbon flares if the approximately 2D geometry for compact flares is extended into the third dimension.
The basic idea of the CSHKP model begins with a plasmoid or filament. A plasmoid is a compact structure consisting of cool, dense plasma, whereas a filament is similar but refers to the case where the plasmoid is elongated. The plasmoid/filament is suspended inside a twisted magnetic flux tube (referred to as a flux rope) in the corona above a magnetic polarity inversion line (also referred to as a neutral line) of an active region. The formation of a flux rope is illustrated in Fig. 1.6. In this scenario, proposed by Van Ballegooijen & Martens (1989), flux cancellation occurs in a highly sheared magnetic field, resulting in the formation of helical field lines that may support a cool, dense plasma structure within them. The strong shear required to form the flux rope means that when formed it lies almost parallel to the polarity inversion line. The sheared flux rope can be seen as the classic ‘sigmoid’ shape that often appears in SXR observations prior to a flare (Moore et al. 2001). Free magnetic energy is stored by the stretching and twisting of the flux rope, and this process is thought to be due to the perturbation of the flux rope by turbulent motion in the convective zone and/or emergence of new flux (the details of this are still a matter of debate). This stretching, twisting, and shearing of the magnetic field can be described quantitatively by magnetic helicity.
The plasmoid erupts if the system eventually evolves into a non-equilibrium state. Normally the flux rope will be surrounded by a much simpler magnetic field which runs perpendicular to the polarity inversion line, and since magnetic field lines move with perfectly conducting fluids (referred to as Alfv´en’s frozen-in theorem; Alfv´en 1942), the erupting plasmoid drags the surrounding magnetic field upwards with it. As the magnetic field lines are stretched, the oppositely directed field lines below the filament are brought together and form a current sheet. This is illustrated in Fig. 1.7. The orange region of this figure represents the diffusion region, where the magnetic field approaches zero and hence the frozen-in condition no longer applies. The diffusion of plasma in this region allows the reconfiguration of the magnetic field into a lower energy state in a process known as magnetic reconnection.
During the impulsive phase of the flare, the magnetic reconnection process forms a post-flare coronal loop below the current sheet, and converts stored magnetic energy into kinetic energy by accelerating charged particles away from the recon- nection site. Those particles that are accelerated downwards move along the field lines of the newly formed coronal loop. These particles (mainly electrons) may ap-
Figure 1.6: Different stages of the formation of a magnetic flux rope over a polarity inversion line. Image courtesy of Van Ballegooijen & Martens (1989).
Figure 1.7: A simple illustration of 2D magnetic reconnection and associated plasma outflows in a current sheet (indicated by the orange region), where parallel but oppositely directed magnetic field lines (indicated by the blue lines) are brought together. Image courtesy of Zweibel & Yamada (2009).
proach relativistic speeds, meaning they release microwaves via the gyrosynchrotron mechanism as they spiral along the magnetic field lines of the post-flare loop, and when they reach the more cool, dense plasma of the chromosphere they are rapidly decelerated, emitting X-ray radiation via bremsstrahlung. Gamma rays may also be produced during this phase via collisions of accelerated protons and ions.
The thick-target model of Brown (1971) shows how the observed HXR source above the loop top can result from an outflow jet of charged particles produced by magnetic reconnection colliding with the top of the post-flare loop below and generating a shock. This shock causes the further acceleration of charged particles, and sends them spiralling down along the magnetic field lines of the post-flare loop. These charged particles are decelerated via coulomb collisions in the chromosphere until they reach thermal energies. This intense heating of the chromosphere causes an increase in gas pressure, since the energy cannot be dissipated away quickly enough by radiation, which allows the plasma to be able to overcome gravity and flow upwards. Due to the conservation of momentum, this upflow is accompanied by a downflow of plasma to the lower chromosphere. The upflow of hot plasma from the chromosphere, known as chromospheric evaporation, fills the post-flare coronal loop formed by magnetic reconnection (Hirayama 1974). This hot plasma in the post-flare loop is the source of the thermal SXR and EUV emission observed in flares. The decay phase of the flare begins after reconnection has ended, and in this phase the hot plasma cools down. The main features of the CSHKP model are shown in Fig. 1.8.
While the standard flare model has proved to be highly successful in explain- ing many observational features of solar flares, some inconsistencies have been found. The main example relating to this thesis is the presence of quasi-periodic pulsations observed in a large fraction of solar flares, which are not naturally explained by the current standard model. Several modifications to the standard model have been proposed to explain these pulsations, however, and these are discussed further in Section 1.3.
Other examples relate to the electron beam produced by magnetic reconnec- tion that generates the HXR emission at the loop footpoints. Some studies suggest that the necessary beam density would result in an unstable beam (Krucker et al. 2011), and that in some cases the height of the beam source is too low to explain the deposited energy (Mart´ınez Oliveros et al. 2012). An alternative mechanism for the transport of energy from the reconnection site to the loop footpoints via Alfv´en waves could help avoid problems with the electron beam model (Fletcher & Hudson 2008).
Another limitation of the standard flare model is the failure to explain the often-observed second peak in the EUV emission, referred to as the EUV late phase
Figure 1.8: Diagram showing the basic geometry and various features of the standard model of a solar flare. Image courtesy of Shibata et al. (1995).
(Woods et al. 2011). There is evidence to suggest that this is the result of a more complex magnetic topology, where the main flaring structure is linked to another magnetic structure and triggers a second magnetic reconnection (Liu et al. 2013).
An ongoing mystery of solar flares is the origin of the white-light and in- frared emission. Despite the first observed solar flare being viewed in the visual waveband, white-light emission from subsequent flares has been notoriously chal- lenging to measure due to the poor contrast with the solar photosphere. While white-light emission tends to be associated with the more energetic solar flares, it has been observed in weak C-class flares as well (Matthews et al. 2003; Hudson et al. 2006), suggesting that white-light emission is common to all flares even if it cannot always be observed. The fact that the white-light emission tends to correlate well in both space and time with the HXR emission (Krucker et al. 2011) suggests that it is also a product of the non-thermal electrons, although some observations have shown that the white-light emission originates from a lower height, in the up- per photosphere or lower chromosphere, than the HXR emission (Watanabe et al. 2010; Mart´ınez Oliveros et al. 2012). As mentioned above, Mart´ınez Oliveros et al. (2012) found that the source region of both the HXR and white-light emission for the studied flare was too low in the atmosphere to be explained by the thick-target model, since the model did not produce enough electrons with energies high enough to reach the source region to explain the observed white-light flux. Watanabe et al. (2010) also found that the source height of the white-light emission was too low to be explained by the standard thick-target model, and suggest that the ionisation enhanced continuum and radiative back-warming ideas discussed by Hudson (1972) and Metcalf et al. (2003) merit further development as a potential explanation.
There is currently a severe lack of observations of solar flares in the infrared waveband, although future observatories will remedy this (Sim˜oes et al. 2017). De- spite this there has been recent progress in understanding the origin of the infrared emission. Current observations suggest that the infrared emission is impulsive and well-correlated with the HXR and white-light emission (Penn et al. 2016). Radia- tive hydrodynamic simulations of Sim˜oes et al. (2017) suggest that the origin of the infrared continuum emission is mainly in the chromosphere and via the thermal ion free-free emission mechanism (Ohki & Hudson 1975), which is consistent with the observations of Penn et al. (2016). The term ‘free-free’ is used as free electrons are scattered by ions, producing thermal bremsstrahlung, but remain free.