As with other types of plasma emission, radio noise storms can be identified based on their appearance in a dynamic spectrum, which is an image showing how the radio emission spectrum evolves, with frequency on the vertical axis, and time on the horizontal axis. Noise storms are generally observed to have two components; long–duration, wide–band emission, overlaid by short–lived (<1 s), narrow–band (<10 MHz) spikes in emission, called Type I bursts. An example high–resolution spectrum is shown in Figure 1.17, from Iwaiet al. (2013). Type I burst peak flux distributions have previously been shown to exhibit a power–law structure, with
Figure 1.17: A type I radio noise storm dynamic spectrum. Top: Left– and right– circularly polarised spectra for a four hour period, exhibiting a storm particularly in the latter two hours of the observation. Bottom: Zoomed–in subsets of the above spectra, denoted there by the red box in the RCP image. Here the fine structure of the storm is revealed. Overplotted diamonds denote where the automated detection
an index of 2–3 (Mercier & Trottet, 1997; Rameshet al., 2013). Iwai et al.(2013) however made use of the high spectral resolution of the Assembly of Metric-band Aperture TElescope and Real-time Analysis System (AMATERAS; Iwai et al., 2012b) and an automated burst–finding procedure to show this spectral index may actually be between 4 and 5. In either case, such a power–law form suggests, similar to flare distributions, the presence of an avalanche process producing the accelerated particles responsible for noise storms.
Spectral observations of radio noise storms often reveal highly polarised emis- sion with extremely high brightness temperatures, leading to their common in- terpretation as plasma emission (Kerdraon & Mercier, 1983; Sundaram & Subra- manian, 2004). As storms are often long–lived and comprised of numerous small bursts, they have been presented as evidence for ambient or quiet–sun acceler- ation outside of solar flares (Raulin & Klein, 1994). Indeed, the possibility of storms being produced in nano– and pico–flares have had important implications for coronal heating (Mercier & Trottet, 1997; Ramesh et al., 2013).
1.2.2
Spatial Structure
Type I storms have been observed to originate from, or above, active regions since their very early observations (Payne-Scott & Little, 1951; Wild & Zirin, 1956). As shown in Figure 1.18, these emissions are interpreted as a column of material, emitting at the plasma frequency, and so at higher frequencies closer to the solar surface (McLean, 1981). The sources themselves have sizes of several arcminutes for the continuum storm, and arcminutes for individual bursts, with source size generally decreasing with emission frequency (Malik & Mercier, 1996).
Figure 1.18: Interpreted spatial structure of Type I noise storms. Shown are the solar limb as a black curve, with marked sunspots. Above this active region is the volume producing the type I storm. Higher frequency plasma emission originates from lower heights in the corona, where density is higher (McLean, 1981).
Interestingly, noise storms are commonly seen in the high corona, and associate with areas where active regions interact, rather than above them individually (Brueckner, 1983; Lang & Willson, 1989; Willsonet al., 1990).
Noise storms, like flare emission, are commonly used as tools to gain informa- tion on the nature of reconnection and particle acceleration in the corona. For example, joint observations of storms and consistent coronal upflows have been put forward as evidence for gradual interchange reconnection in the corona (Del Zannaet al., 2011, ; see Figure 1.19). Observations of storms have also been asso- ciated with transequatorial reconnection between active regions (Willson, 2005) and ‘slipping’ reconnection during a solar flare (Dud´ıket al., 2014). However, an
Figure 1.19: Noise storm association with SXR active regions. Shown are three full–disk X–ray images produced by the Hinode X–Ray Telescope, for three different times during the passage of an active region across the disk. Overplotted contours show 80% of peak flux levels of 408 (blue), 327 (green), 237 (orange), and 151 MHz (red), observed by the Nan¸cay Radioheliograph (Del Zanna et al., 2011).
alternative acceleration mechanism has been put forward, which notes emerging flux could be super–Alfv´enic, and so produce shocks causing acceleration and eventually plasma emission making up a noise storm (Spicer et al., 1982).
While the long duration of storms indicates a lack of clear correlation with solar flares or CMEs, there still exists a relationship with solar activity. A study has shown that noise storms tend to precede CMEs (Ramesh & Sundaram, 2001), and more recently, a decline in noise storm emission has been noted during the release of a CME (Iwai et al., 2013). A number of studies have also shown that storm brightness temperature decreases in association with solar flares (Aurass
et al., 1990, 1993; Boehme & Krueger, 1982). This behaviour is explored and interpreted in a study of a recent coronal event in Chapter 6.
expected based on the collisional thick target model (CTTM; see Section 2.3.1), that the commonly–observed ‘soft–hard–soft’ evolution of the electron spectral index should have a measurable effect on nonthermal HXR footpoints – producing a variation in height as the beam hardens and softens. This effect has not been observed to date, likely due to the fact that in most flares, emissions at lower energies – which are most sensitive to this variation – and earlier in the flare are dominated by thermal SXRs from heated chromospheric and coronal plasma. This issue is addressed by performing a detailed study of an ‘early impulsive flare’, when thermal SXRs are absent. This study reveals the expected evolution of the HXR footpoints, and has been published in O’Flannagain et al.(2013).
Secondly, we address the issue of HXR source sizes outlined above. As men- tioned, it has been revealed by detailed RHESSI studies of HXR footpoints that source vertical extents appear 3–6 times larger in observations than those pre- dicted by theory for standard chromospheric targets. Modelling work has been performed to address this issue by taking into account various coronal density structures and other physical processes than collisions, such as pitch–angle scat- tering and magnetic mirroring, with no major predicted change in source size as a result. This problem is addressed with a model of locally ionised chromo- spheric plasma produced by the electron beam, which should serve to vertically
extend HXR sources. This successfully produces an increase in vertical extent to levels matching observations within commonly observed energy ranges of ∼30 to 70 keV. A paper outlining this work has been published in O’Flannagain et al.
(2015).
Finally, a potential observation of the source of accelerated particles in active regions is outlined. To date, a handful of observations of magnetic reconnection exist, usually of apparently 2D reconnecting EUV structures at the solar limb. However, as reconnection in the corona is a 3D process, it is important to also look for examples of null–point reconnection in solar observations (see Section 2.2.1.1 for details). To assist in this, we outline an observation of a collaps- ing transequatorial EUV loop structure detected by AIA, with an accompaning dynamic Type I noise storm imaged by the Nan¸cay Radioheliograph. The be- haviour of the storm emission during the passage of the active region across the disk, and particularly during its collapse, is interpreted as a process of gradual and then enhanced reconnection along a separator produced by the quadrupolar magnetic field. This work is currently in preparation as a paper for publication (O’Flannagain et al., 2015).
fields in coronal plasma, the chapter begins by introducing plasma physics and magnetohydrodynamics. These topics are then built upon to describe the various theories of magnetic reconnection, which is believed to be the primary energy re- lease mechanism in solar flares and CMEs. We then introduce the standard solar flare model as it is currently understood. Particular attention is given to the role of reconnection in the nonthermal acceleration of particles and their propagation through the corona and chromosphere. Finally, we outline the emission mecha- nisms for the two primary types of emission analysed in this thesis: collisional thick target X–rays, and plasma emission.
2.1
Plasma Physics
A plasma is often defined as an ionised gas, but in fact need not be fully ionised, and can be more accurately described by the three following criteria. First, the plasma oscillation period, τp must be much smaller than the collision timescale,
τc: τc τp >> 1,whereτp = 2π ωp , ωp = ne2 me0 (2.1) A plasma oscillation is caused when a perturbed charged particle experiences a restoring force due to charge separation. Second, for the length scale being considered, L, the Debye length,λD must have a value such that:
L >> λD. (2.2)
The Debye length is the distance at which an electron, given an amount of kinetic energy equal to the average thermal energy of the plasma, will travel before returning due to the electrostatic force. This length therefore defines a scale above which collective effects such as shielding of magnetic charge take place, which are a crucial component of plasma behaviour. Finally, a Debye sphere, or a sphere with a radius equal to the Debye length, must be sufficiently populated:
Λ>>1. (2.3) Here, Λ is the plasma parameter, equal to three times the number of particles contained in a Debye sphere, or Λ = 4πλ3
Dn, where n is the plasma electron
2.1.1
Maxwell’s Equations
Maxwell’s equations form the foundation of classical electrodynamics by describ- ing the interaction between magnetic and electric fields,B and E, and how they evolve in time and space. In a vacuum, they are given as:
∇ ·E= ρc 0 (2.4) ∇ ·B= 0 (2.5) ∇ ×E=−∂B ∂t (2.6) ∇ ×B=µ0J+µ00 ∂E ∂t =µ0J+ 1 c2 ∂E ∂t (2.7)
whereρcis the charge density andJis the current density. It is important to note
that, asµ00 is by definition equal to 1/c2, then the second term in Equation 2.7, which includes the displacement current, can be neglected when typical plasma velocities are significantly below the speed of light, which we hereafter assume to be valid.