Solar energetic particles (SEPs) come as bursts of high-energy particles from the direction of the Sun that last for hours or sometimes days. In this chapter, we introduce properties of SEPs after reviewing some properties of the solar and interplanetary environment in which they exist.
The Structure of the Sun
Within the tachocline, which lies at the bottom of the convective zone, the Sun rotates (from east to west) as a rigid body, but throughout the convective zone, the Sun rotates differently, faster at the equator than at the poles. Azimuthal surfaces of constant rotational speed run radially through the convection zone and form conical shells around the axis of rotation that extend inward only to the tachocline and not to the peak at the center of the Sun.
The Solar Magnetic Field
Coronal holes, often seen near the poles, are regions of open magnetic field lines that extend into the outer heliosphere and are stretched by the plasma of the solar wind. This usually defines the "top" of the corona and usually occurs near 2 RS, where it is ~ 106 cm3.
Coronal Mass Ejections (CMEs)
However, CMEs and the solar wind drag some of the field lines far into the outer heliosphere. However, βP increases with height in the corona and when βP>1, the plasma is no longer trapped in magnetic loops; it can spread out into space and pull magnetic fields outward into the solar wind.
Solar Energetic Particles
- Time Duration
- The Solar Cycle
- Relativistic Kinematics
The abundances of elements and isotopes were powerful indicators of the origin, acceleration and transport of SEPs. It differs from abundances in the photosphere by a factor that depends on the first ionization potential (FIP) of the element as shown in Fig.1.6 and listed in Table 1.1 (Reames 1995,2014).
What Do We “ See ” at the Sun?
Reames, D.V.: The "FIP effect" and the origin of solar particles and the solar wind. arXiv Reames, D.V.: Four Different Pathways to Element Abundance in Solar Particles. Schmelz, J.T., Reames, D.V., von Steiger, R., Basu, S.: Composition of the solar corona, solar wind and solar particles.
The First SEPs
Solar Radio Bursts and Electrons
This differed from the large proton events in which the associated electrons were mainly relativistic. For type III bursts, frequencies drift rapidly, produced by 10–100 keV electrons streaming from the Sun; frequencies in type II bursts drift outward from a source moving at the speed (~1000 km s1) of a shock wave.
The Spatial Distribution
- Lateral Diffusion and the Birdcage Model
- Large Scale Shock Acceleration and CMEs
- The Longitude Distribution
- Scatter-Free Events
- Field-Line Random Walk
Does ambient turbulence in the interplanetary medium cause pitch-angle scattering of the particles emanating from the Sun? In Fig.2.4 this contributes to the longitude distribution of the impulsive events shown in the right panel.
If this process continues indefinitely, it can lead to a power law spectrum where power depends on shock compression ratio. However, there is no stronger collaboration effect; shock acceleration occurs within a modest number of proton gyro radii of the shock, and CME-powered shocks crossing each other spend negligible time at such a small separation.
First Ionization Potential (FIP) and Powers of A/Q
It also became apparent that there were other abundance increases, such as Fe/O, which was about 10 times larger than in the solar wind (e.g. Gloeckler et al. 1975). The next generation of measurements of 3He-rich events (Figure 2.6) led to their association with non-relativistic electron events (Reames et al. 1985) and with type III radio bursts (Reames and Stone 1986).
The Seed Population for Shocks
We will see that in most cases these ESP peaks represent coronal environmental material, although suprathermal ions may also contribute. 2005) found that in two otherwise similar large SEP events, the energy dependence of Fe/C above ~10 MeV amu1 suddenly increased in one event and decreased in the other, as shown in the left panel of Fig. The right panel shows hypothetical spectra of two sources of suprathermal ions where different injection thresholds will yield different abundance ratios (Tylka et al.2005).
For further studies of the dependence of spectral refraction, of the power-law dependence of Q/A and the variation with shock geometry, see Li et al. Recent studies of the A/Q dependence in gradual SEP events (Reames2016) have found that most of these events (69%) have source-plasma temperatures of 1.6 MK, consistent with the acceleration of the ambient coronal plasma shock (see Sect. 5.6).
Only 24% of the events have active region temperatures of 2.5–3.2 MK and thus include dominant enhancements due to impulsive suprathermal seed ions. Thus, the properties of the SEP are controlled by those of the CME and the shock.
Wave Generation and the Streaming Limit
The controversy raised by Gosling's (1993) paper led to an invited discussion of three alternative perspectives in Eoswhere Hudson (1995) argued that the term. While the broadening of the term "flare" has some philosophical merit, it is important for SEP studies to distinguish a point-source flare, or now, a localized jet, from the source of acceleration in a broad, solar-extending CME. . pushed shock wave, especially when they involve different physical mechanisms.
SEP – CME Correlation
Of course, the "peak intensity" is in fact a strong function of longitude, as expected from Fig. 2.2 (see also Fig. 5.16), as is the speed of the shock driven by the CME; these factors contribute to the spread of the measurement, which, as we will see, can be reduced by using the measurements of several spacecraft in a single SEP event (see Fig. has greatly improved the CME-SEP relationship by modeling the full 3D geometry of shock waves using the three coronagraph images from SOHO/LASCO and STEREO A and B. Much of the remaining scattering must be due to differences in particle transport conditions that scatter the particles in space and give time variations and delays in reaching peak intensities .
SEPs Actually Cause Flares, Not the Reverse
Reames, D.V., Cliver, E.W., Kahler, S.W.: Abundance enhancements in impulsive solar energetic particle events with associated coronal mass ejections. Reames, D.V., Cliver, E.W., Kahler, S.W.: Variations in abundance enhancements in impulsive solar energetic particle events and related CMEs and flares.
SEP Onset Times
This may be the width of the source shock surface above closed loops that was once erroneously referred to as the "rapid propagation region". Surely there are a few GLEs where only the SPR timing would allow some sort of (unspecified) acceleration at the time of the associated flare.
Realistic Shock-SEP Timing and Correlations
The mean abundances of the elements in gradual events, relative to those in Fig. Reames, D.V.: The "FIP Effect" and the Origin of Solar Energetic Particles and of the Solar Wind.
Injection Pro ﬁ les
High-Energy Spectra and Spectral Knees
In any case, none of the 16 GLEs showed evidence of high-energy spectral enhancement that could suggest the existence of a new source that could dominate higher energies. Tylka and Dietrich (2009) have used the stiffness of the geomagnetic discontinuity at neutron monitoring stations to develop integral stiffness spectra, using data from the worldwide neutron monitoring network for 53 GLEs.
Intensity Dropouts and Compact Sources
Subsequent observations (Chollet and Giacalone 2011) showed that the boundaries between flux tubes with and without SEPs were extremely sharp. Differences in the scattering in some magnetic flux tubes can have a particularly strong influence on the intensities and angular distributions of non-relativistic electrons.
These profound sudden changes in particle intensity mostly occur when differently coupled flux tubes are sampled within a passing CME, as measured by multiple spacecraft in the 2006 December 14 SEP event (von Rosenvinge et al. 2009). In the following chapters (Sections 4.6 and 5.6), we will see that the pattern of dependence of the stepwise abundance increase on A/Q ions leads to the determination of the value of Q and the associated temperature of the source plasma T.
Type II emission from the shock nose and flanks can be distinguished because the nose is farther from the Sun, at lower, and therefore lower frequency; the frequencies can be correlated with the coronagraph image. Electron acceleration at the quasi-parallel nose of a shock probably occurs because real shocks are not flat but very complex structures, varying in space and time.
Why Not Flares?
Modern instruments allow measurement of the reconnection magnetic flux, and a recent database contains reconnection fluxes for 3137 solar-flare band events (Kazachenko et al. 2017). The observed properties of SEPs in space are not compatible with such hot flare plasma and closed fields do not simply open to release them.
SEPs as Probes
Kahler, S.W., Reames, D.V.: Panagsukisok kadagiti magnetiko a topolohia dagiti magnetiko nga ulep babaen dagiti solar nga enerhia a partikula. Rouillard, A., Sheeley Jr., N.R., Tylka, A., Vourlidas, A., Ng, C.K., Rakowski, C., Cohen, C.M.S., Mewaldt, R.A., Mason, G.M., al., D. : Dagiti longitudinal a tagikua ti maysa a nasikap a pasamak ti partikulo ti init a naimbestigaan babaen ti panangusar iti moderno a solar imaging.
Selecting Impulsive Events
A more recent version of the bimodal abundance survey is the two-dimensional histogram shown in Fig. Time periods near coordinates (1, 1) in fig. 4.1 occurs during large gradual SEP events for which the normalization was chosen.
Sample Impulsive Events
Fe/O is stored for a 19-year period, and this time we have the luxury of requiring 20% accuracy to prevent excessive dispersion of the distributions. The peak near (6, 3) in the figure represents impulsive events, but the Ne/O value was not actually used for the selection of candidate periods to define impulsive SEP events.
Of course, it is still true that gradual events occupy many more 8-hour periods, and lower intensity impulsive events are less likely to reach 20%.
Abundances for Z 26
Average element enhancements from 4He to Fe were summarized by Reames as shown in Figure 4.6.
Abundances for 34 Z 82
Power-Law Enhancements in A/Q: Source-Plasma
It is also possible to determine the most suitable temperature and a power law fit for individual impulsive SEP events. Thus, impulsive SEP events outside solar active regions are rare and very small (but see also Sect.4.8).
Associations: CMEs, Flares, and Jets
Particle-in-cell simulations show Fermi acceleration of ions reflected back and forth from the tips of the collapsing islands of reconnection (Drake et al. 2009). Magnetic flux rise is still considered an important triggering mechanism for solar flares (Paraschiv et al. 2020).
Can We Have It Both Ways?
Active-region jets tend to have temperatures of ~3 MK (see Fig. 14 of Raouaﬁet al.2016) like those we infer from impulsive SEP events (Fig. 4.10). Solar jets near active regions have temperatures around 3 MK while those of coronal holes are closer to 1.5 MK (see Fig. 14 of Raouaﬁ et al. 2016).
Nuclear Reactions: Gamma-Ray Lines and Neutrons
Nuclear reactions in the corona also produce 2H, 3H, positrons, π-mesons and isotopes of Li, Be and B as deduced from the γ-ray line spectroscopy. Neutrons are also produced in nuclear reactions in solar flares and 50–300 MeV neutrons have been observed directly in space (Chupp et al. 1982; Chupp 1984).
Mason, G.M., Klecker, B.: A possible mechanism for heavy ion enrichment in 3He-rich solar powered particle events. Gradual Solar Energetic Particle Events (SEPs) are "large proton events" and are usually much more "gradual" in their decay than in their onset.
- Diffusive Transport
- Wave Growth
- Particle Transport
- Initial Abundance Ratios
- The Streaming Limit
- Electron Transport
The growth rate of the σ polarization mode of Alfvén waves (see Ng et al. 2003; Stix 1992; Melrose 1980) produced by protons is clearest and simplest in the wave frame, where it is given by. 5.6) represents focusing of the particles in the diverging magnetic field, while the fourth term represents pitch-angle scattering with the diffusion coefficient Dμμ.
Low-energy electrons typically propagate without scattering with a highly anisotropic angular distribution mainly due to solar wind absorption of frequencies of 0.1–1 Hz that would resonate with these electrons. However, small impulsive and gradual events tend to remain undispersed and the angular distributions are rapidly isotropized in more intense gradual events and especially in GLEs (see Reames et al. 2001 ).
Models and Shock Acceleration
The time-dependent self-consistent model of wave-amplified particle transport (Ng et al.2003) was applied to shock acceleration by Ng and Reames (2008) resulting in the modeling of the time evolution of the proton spectrum in the shock shown in Fig. The right panel shows the time evolution of the spatial distribution of 12.3 MeV protons upstream of the shock.
Shock Acceleration In Situ
The shock and background spectra are shown on the right with the spectral slopes indicated (Reames2012#AAS). Particle intensity peaks at shock transit time in almost all events in the Reames (2012) study.
Averaging SEP Abundances
This is the issue of cosmic ray mediated shocks discussed by Terasawa et al. 2006) for two additional interplanetary shocks.
C/He, for intervals during the gradual SEP events, in both panels, with Tas symbols in the lower panel and power of A/Qas symbols in the upper. The region of abundances showing active-region temperatures T 2 MK is immediately distinguishable, clustering in the upper left of the lower panel of Fig.5.15.
Spatial Distributions and the Reservoir
Reservoirs, Loops, and Long-Duration γ Rays
Non-thermal Variations: Impulsive vs. Gradual SEPs
More significantly, the spread in the distribution of gradual events is much smaller than that of impulsive events in Fig. The distribution in the impulsive events must originate from non-thermal abundance variations in the local plasma of jets where the magnetic reconnection occurs.
The Abundance of He and the FIP Effect
However, if we really expect to reduce the spread of the distributions as seen in the gradual events, we need to average over several small jets that produce impulsive SEP events rather than just one; no events will reduce the spread by a factor of√n. We need only a small increase in the number of jets producing impulsive SEP events that are too small to resolve as separate events, yet sufficient to contribute to the seed population in the pool of impulsive suprathermal ions above a solar active region that can be sampled later and averaged by a shock wave.
Ng, C.K., Reames, D.V., Tylka, A.J.: The effect of proton-enhanced waves on the compositional evolution of solar energetic particles in gradual events. Sandroos, A., Vainio, R.: Simulation results for the spectral variability of heavy ions in large step solar-powered particle events.
Most GLEs have spectral stiffness indices between 5 and 7. The parameters of these double power-law fits to GLE stiffness spectra are tabulated by Raukunen et al. 2018), who, based on these results, also discusses the interdependence of ﬁt parameters and ﬂuence models. They find enhanced acceleration of GV protons in high Mach number shock regions and highlight the importance of velocity differences between the upscattering and downscattering centers.
The Streaming Limit
The black dashed lines in Fig. 6.4 are power laws below the streaming limit that decay as ~0.4 power of intensity. The rate of increase of proton intensity can also be a factor in the establishment of flux limit equilibrium, as shown in Figure 6.6.
Most events have slower evolution and do not exceed the limit. 2009) pointed out that trapping can also allow intensities to exceed the current limit. Shortly after arriving at a given radius, intensities rise to the current limit at that radius.
Radiation Hazards and an SEP Storm Shelter
However, the probability of the occurrence of a large gradual SEP event during a short passage of the spacecraft's perihelion may be small. On planets or moons, it may be possible to build effective shelters from local materials.
A Mission to Mars
GCR radiation is not reduced by shielding; it is actually increased by the production of secondary nuclear reaction products, including highly penetrating neutrals (Carnell et al.2016). The problem actually comes when there is a continuous human presence outside the Earth's magnetosphere; then it's not a matter of if, but when.
The Upper Atmosphere of Earth
For the timing of a manned mission to Mars, one can go during solar maximum when SEP events are more likely but GCR intensity is reduced, or during solar minimum when SEPs are reduced but GCRs are at a maximum (see Fig. 1.8 ). It is assumed that the risk of SEP can be somewhat reduced by a safe shelter with protection of 20-40 g cm2, combined with an adequate warning system.
SEPs and Exoplanets
Tylka, A.J., Boberg, P.R., McGuire, R.E., Ng, C.K., Reames, D.V.: Temporal evolution in the spectra of gradual solar particle events. ed.). Verkhoglyadova, O.P., Li, G., Ao, X., Zank, G.P.: Radial dependence of peak proton and iron ion fluxes in solar particle events: application of the PATH code.
At relativistic energies, dE/dx reaches a broad minimum at ∼2.5 GeV amu1 and then rises slightly from density effects not included here. Unfortunately, these low-priority hitchhikers may even be turned off in transit to the mission destination to save resources, further reducing their finite value.
Δ E Versus E Telescopes
- An Example: LEMT
- Isotope Resolution: SIS
- Angular Distributions
- Onboard Processing
Figure 7.5 shows the resolution of Ne isotopes by the Solar Isotope Spectrometer (SIS) on the Advanced Composition Explorer (ACE) in two different SEP events. Onboard microprocessors can now handle up to ~10,000 s1 particles, correct the geometry, and identify particle types and energies by looking up 6464 elemental log-log tables (one for 3He-4He, one for 6Z26) that would overlap the regions of Fig. 7.3, for example.
Time-of-Flight Versus E
Buffering is also possible, so that rates can be sampled over longer or shorter periods, multiple samples vs. The resolution using this technique can be greatly improved by adding an additional time plane, using electrostatic mirrors to reflect electrons, and using microchannel plates with position-sensing anodes.
Plots of GOES particle and X-ray data are also shown correlated with CME height-time plots, as shown in the example in Fig.7.8. Note the steep height-time plots (i.e., fast CMEs) in the middle panel near the start of the two large gradual SEP events in the top panel.
Other data in the catalog include movies of CME evolution and EUVflares along with radio data from Wind/WAVES. While these instruments must deal with geomagnetic field limitations, as neutron monitors do, they can directly measure spectra and abundances and represent a major improvement in the accuracy of measurements at high energies.
Problems and Errors
Mason, G.M., Gold, R.E., Krimigis, S.M., Mazur, J.E., et al.: Ultra-low-energy isotope spectrometer (ULEIS) for the ACE spacecraft. Here, we study the energetic particles themselves as samples of the solar corona, which is their origin, distinguishing the corona from the photosphere and the SEP from the solar wind.
Element Abundances in the Sun
The Solar Wind
A more complete scheme for distinguishing the different regions of the solar wind has been developed with Table 8.1 Photospheric, SEP, CIR and SSW reference abundances. eV]. However, the variations of H and He in the SEP do not appear to be related to those in the solar wind.
Corotating Interaction Regions: Accelerated Solar Wind
The 4He, C and O spectra show similar shapes for all elements in the events in the lower panel of Figure 8.3 and for the normalized Fe and 4He spectra for most of the events in the upper panel. These energetic CIR ions give us another measure of the FIP effect in the solar corona, probably similar to the combined solar wind.
Comparing FIP Patterns of SEPs and the Solar Wind
Theory suggests that SEPs may be more likely to be accelerated by closed field lines while the solar wind should come from open field lines near the base of the corona (Reames2018a; Laming2015; Laming et al.2019). Spectral line measurements of the corona also show suppressed values of S/O (Schmelz et al. 2012) apparently in closed field lines.
FIP Theory: The Sources of SEPs and the Solar Wind
Thus, at energies above a few MeV amu1, SEP cannot therefore simply be reaccelerated solar wind; they are an independent sample of coronal material (Reames2018a). This irreconcilable difference in FIP patterns was first noted by Mewaldt et al. 2003) also noted that SEPs were not simply reaccelerated solar wind.
A Full-Sun Map of FIP
The most significant difference between all the observed open and closed field patterns is in C/O as shown in Table 8.2. Actually, the closed field measurements are significantly lower than the recent photospheric values, contrary to any expectations.
A Possible SW-SEP Model
In the upper panel, the CME-driven shock (gray) accelerates the SEP from weakly closed loops and from suprathermal ions and plasma remnants from jets when present (Reames2018b). Therefore, a single CME-driven shock shown in the top panel of Fig. 8.8 can reaccelerate material from impulsive SEPs and jet bursts over the active region and can also accelerate cooler plasma from closed loops on its flanks.
FIP-Dependent Variations in He
Fisk, L.A., Lee, M.A.: Energetic particle shock acceleration in corotating interaction regions in the solar wind. However, in Chapter 8 we saw that the SEP abundances are not related to those of the solar wind.
Impulsive SEP Events
Event numbers 3 and 4, which indicate the time of event initiation, refer to the list of Reames et al. 2014); the coordinates of the solar source are also given. Event numbers 34 and 35, which indicate the start time of the event, refer to the list of Reames et al. 2014); the coordinates of the solar source are also given.
Gradual SEP Events
Waves Coupling Proton Velocity with A/Q
Compound Seed Particles
CME Associations of Impulsive and Gradual Events
Four Subtypes of SEP Events
Rigidity-Dependence: Acceleration or Transport?
Correlations Between Spectra and Abundances