Solar energetic particle observations
3.1 Introduction
This chapter focusses on SEP observations, including the broad range of electromagnetic fre- quencies and wavelengths that are used as diagnostic tools to observe and measure solar ac- tivity. The frequency ranges are complementary to each other and thus provide an in-depth picture of what processes are taking place from the photosphere out to the corona. This chap- ter starts of by discussing solar radio emission, infra-red (IR) emission, white-light emission, Hα emission, ultra-violet (UV) emission, solar X-rays, and solar gamma rays. Although the emissions are discussed in a particular order (from lower energies to higher energies), it is of- ten times impossible to refrain from mentioning the next emission process since most of these mechanisms are associated with each other. Terrestrial SEP observations, in the form of ground level events (GLEs), are also discussed. A few solar spacecraft missions are discussed, in- cluding the most recently launched Parker Solar Probe (PSP) and Solar Orbiter (SolO), as well as other important historic and planned future missions. Lastly, the important role of space weather (SWx) and SWx predictions are explained.
26 3.2. SOLAR RADIO EMISSION Solar radio emission is subdivided into coherent and incoherent emission mechanisms. Co- herent emission occurs by kinetic instabilities from unstable particle distributions, where as incoherent emission results from continuum processes such as free-free emission in microwave and millimetre wavelengths [Aschwanden, 2005]. There are four main mechanisms that are able to produce solar radio emission and include i) coherent plasma emission, ii) linear mode conversion emission, iii) cyclotron maser emission, and finally iv) gyrosynchrotron emission [see e.g.Melrose, 1980;Bastian et al., 1998;Cairns et al., 2000]. In the first case, during coherent plasma emission, Langmuir waves (plasma oscillations) are driven by non-thermal electron beams close to the fundamental plasma frequency, fp, which is then converted to transverse electromagnetic waves (by means of wave-wave interactions) with frequencies close tofp, and its harmonic,2fp, or both [Li et al., 2011]. Third harmonics have also been observed in the past [Zlotnik et al., 1998]. The fundamental plasma frequency is defined as
fp = 1 2π
nee2 me0
1/2
, (3.1)
wherene is the electron number density,e is the electron charge, me the electron mass, and finally0 the permittivity of free space [Cairns, 2011]. Coherent plasma emission is the focus of the next two sections since it is responsible for type II and type III radio bursts. For the second case, linear mode conversion emission takes place when wave modes in inhomoge- neous plasma are coupled to each other and one mode may be converted to another mode. The second harmonic of the fundamental plasma frequency,2fp, can be caused by the non-linear interaction between two (primary and reflected) Langmuir waves. In the third case, cyclotron maser (microwave amplified stimulated emission radiation) emission is also able to produce solar radio emission by energetic electrons moving in a magnetic field [Aschwanden, 2005; Li et al., 2011;Humphreys, 2015]. This radio emission is close to the electron cyclotron frequency, fce= (1/2π)(eB/me), and its second harmonic,2fce. Lastly, incoherent gyro-synchrotron emis- sion is produced by the spiralling motion of energetic electrons around a magnetic field line.
This emission occurs at the harmonics offce/γwhereγis the Lorentz factor [Bastian et al., 1998].
3.2.1 Coronal type II radio bursts
Type II solar radio bursts are non-thermal radio emission originating from the fast mode mag- netohydrodynamics (MHD) shocks where the shocks accelerate the non-thermal electrons which produce the radio emission at the fundamental plasma frequency [Gopalswamy, 2006]. These shocks are usually associated with CMEs. Figure 3.1a shows an example of the intensity of ra- dio emission in the frequency-time plane of a type II radio burst observed on4October2002at the Learmonth Observatory. Note that in this case the frequency axis is orientated from higher frequencies to lower frequencies. Type II radio bursts are characterised by their slow drift rate of less than1 MHzs−1 [Li et al., 2011] and they usually occur between∼ 20 - 400MHz. The slope of the emission is associated with the speed of the shock and the density scale is associ- ated with the height in the medium. Since the radio emission occurs at the plasma frequency (and its harmonics), two sets of emission clouds are observed in figure 3.1a. Each of these clouds are also divided into the upper and lower bands thought to be caused by the density
(a) (b)
Figure 3.1: (a) The dynamic spectrum of the split-band type II radio burst starting on 4 Oc- tober2002at22h44UT (universal time) observed at the Learmonth Observatory [Lobzin et al., 2008]. (b) A schematic of the dynamic spectrum showing the different type II radio bursts often observed. Schematic adapted from [Gopalswamy, 2006].
structures of the shock [Vrˇsnak et al., 2001;Gopalswamy, 2006]. The high-frequency part of type II burst corresponds to radio emission that is close to the Sun where the density of the corona is higher. In contrast, the low-frequency part of these bursts corresponds to radio emission originating far away from the Sun where the ambient density is much lower.
There are several different varieties of type II radio bursts characterised according to their re- spective wavelength domains. Figure 3.1b shows a basic schematic of the six different type II bursts in the frequency-time plane. Since frequency is proportional to the reciprocal of wave- length, the wavelengths start at the kilometre (km) range at the bottom and the metric (m) range at the top of the y-axis, with the decametre-hectometric (DH) range in the middle. Burst Ais confined to the metric range, burstBstarts in the metric range and continues into the DH range, burstCis confined to the DH range, burstDstarts in the DH range and continues into the km domain, burstEruns across all wavelengths from metric to kilometric, and finally burst Fis confined to the kilometric domain. Note that the harmonic structures and band-splitting (shown in figure 3.1a) are not shown in the schematic of 3.1b, but could be observed in radio emission data. BurstAis the only burst that can be observed from Earth-based radio telescopes since its wavelength is short enough to penetrate the Earth’s ionosphere. All the other burst types can only be observed by spacecraft outside the Earth’s ionosphere. Coronal densities similar to those of the terrestrial ionospheric densities are located more or less at∼3RJ(the approximate location of the SWSS) above the photosphere and has a frequency of∼20MHz.
Therefore, metric (m) coronal type II bursts are those above the ionospheric cut-off frequency (20MHz), and those below the cut-off frequency are interplanetary type II bursts [Gopalswamy, 2004]. Hence, in order to take all type II bursts into consideration, data from ground based telescopes as well as spacecraft radio telescopes should be incorporated.
Dodson and Hedeman[1971] made the first connection between metric type II bursts and SEPs.
Furthermore,Kahler et al.[1978] explain that a mass ejection event is necessary to prompt a pro- ton event. These findings suggests the physical link between CME-driven shocks and particle acceleration. Also,Cliver et al.[2004] found that there is a 100% association between DH type
28 3.2. SOLAR RADIO EMISSION
Figure 3.2: A solar metric type III burst observed by the Unified Radio and Plasma Wave Ex- periment (URAP) on-board Ulysses on6July1994between13h30and16h00UT. The figure is adapted fromAschwanden[2005].
II bursts and SEP events. Even type II bursts below2MHz are also associated with SEPs [Cane and Stone, 1984]. Taking all these findings into consideration, it can be concluded that the same shock accelerates the ions and the electrons that in turn, produce the type II burst [Gopalswamy, 2004].
3.2.2 Coronal type III radio bursts
Type III radio emission bursts are much more frequent than type II bursts and therefore are the most studied source of radio emission in the solar system. As mentioned in the previous section, the slope of the radio emission in the frequency-time plane alludes to the speed of the shock. Hence, type III bursts are characterised by a fast frequency drift of∼ 100MHzs−1 in the metric range due to the speed of the electron beams reaching up to30%the speed of light [Suzuki and Dulk, 1985]. Figure 3.2 shows the dynamic spectra of a metric type III radio burst observed on6July1994by the URAP instrument on-board Ulysses. The black, darkened re- gion in the figure shows that as the electron beam travels upward in the corona (away from the Sun), into the interplanetary medium with decreasing plasma density, the type III burst drifts
to lower frequencies over time, revealing its negative frequency drift rate where∂f /∂t <0[As- chwanden, 2005]. Type III bursts are excellent probes of the electron acceleration mechanisms during solar flares, shed light on the magnetic field configuration along which the electrons travel, and reveal the plasma density along the electrons’ trajectory [Li et al., 2011]. The tim- ing of these bursts are especially important when investigating the injection function. Type III bursts are subdivided into normal bursts and reverse-slope (RS) bursts. The former are pro- duced when electrons propagate upward along open magnetic field line into the heliosphere (away from the Sun), and the latter are produced when electrons propagate downward, back towards the Sun. These bidirectional type III bursts are observed in both metric and decimet- ric ranges. Several type III burst simulation models have been developed to improve space weather (SWx) predictability, see for exampleLi et al.[2006],Li[2007], andLobzin et al.[2009].