Suprathermal Density Research Articles

Suprathermal electron moments in the ionosphere

The ionospheric electron population is divided into two groups. The ambient electrons are thermalized. Their energy is usually smaller than one electron volt. Their densities and temperatures are the usual ones measured by incoherent scatter radars, or modeled by international codes such as International Reference Ionosphere (IRI). There is however a second population called the suprathermal electrons. This one is either due to photoionization or to electron impact between the thermosphere and the precipitation in the high latitude zone. In the frame of space weather, it may be the source of scintillations, plasma bulks and other physical phenomena. The suprathermal electron population can only indirectly be measured through the plasmaline and had never been modeled. Its modeling requires the computation of the electron stationary flux by solving the Boltzmann transport equation. This flux is multiplied by various powers of the velocity v and integrated to obtain the different order moments. By integrating f over v0dv, one deduces the suprathermal electron density. An integration of v1fdv allows the computation of their mean velocity. Higher moments give access to their temperature and finally to their heat flux. In this work, we demonstrate for the first time the full and rigorous calculation of the ionospheric electron moments up to three. As two case studies, we focus on high latitude in the auroral oval and low magnetic latitude over Algiers for different solar and geophysical conditions. We compare the suprathermal densities and temperatures to the thermal electron parameters. Our results highlight that – as expected – the suprathermal density is small compared to the thermal one. Although it is close to 3 × 103 m−3 at 180 km during the day, it drops drastically at night, to hardly reach 3 m−3. Contrarily to the density, the velocity is about 10 times more important during the nighttime when precipitation occurs than during the daytime under the electromagnetic solar flux. At 400 km, it varies during the day between 700,000 m s−1 (active solar conditions) and 900,000 m s−1 (quiet Sun). At night, the velocity varies between 3 × 106 m s−1 (low mean energy precipitation) and 3 × 107 m s−1 (high mean energy precipitation) at 400 km. The suprathermal temperature increases as the solar activity decreases or as the mean energy of the electron precipitation increases. It may reach values close to 3 × 108 K. The heat flux may be fully oriented downward or experiences a reversal with some flux going up depending on the forcing.

Steady Electron Runaway Model SERM: Astrophysical Alternative for the Maxwellian Assumption

Abstract A Steady Electron Runaway Model (SERM) is formulated describing plasmas in the astrophysical “condition” having finite (rather than infinitesimal) Knudsen number, , suggesting an omnipresent leptokurtic, nonthermal, and heat-conducting electron velocity distribution function (eVDF) as the replacement for the Maxwellian ansatz typically made. The shape parameters of SERM’s eVDFs are functionals of the local dimensionless electric field, , shown to be nearly interchangeable with the pressure Knudsen number, . The eVDF is determined by the total density and pressure, heat flux, and with the Maxwellian as a special case when . The nonthermal part of the eVDF is caused by local and global runaway physics and its density fraction is monotonically dependent on . SERM explains the distinguishable conduction band of suprathermal electrons to be the result of the inhomogeneities of astroplasmas that require to enforce quasi-neutrality. SERM shows that the direction of the heat flow should be that of . Almost all reported space age correlations among the shape parameters of the solar wind eVDF are reproduced by this modeling, including scaling of: (i) nonthermal spectral break energy, and (ii) partition of suprathermal density and partial pressure, with solar wind speed. SERM, together with eVDF observations, indirectly bracket , producing a steady-state eVDF, consistent with in situ (i) heat flows, (ii) strahl pitch angle features in high-speed winds, (iii) , and (iv) non-negative probability at all velocities. Because finite is the identified prerequisite for SERM modeling, nonthermal eVDF’s are expected nearly everywhere in astrophysics where .

Why is solar cycle 24 an inefficient producer of high-energy particle events?

The aim of the study is to investigate the reason for the low productivity of high-energy SEPs in the present solar cycle. We employ scaling laws derived from diffusive shock acceleration theory and simulation studies including proton-generated upstream Alfv\'en waves to find out how the changes observed in the long-term average properties of the erupting and ambient coronal and/or solar wind plasma would affect the ability of shocks to accelerate particles to the highest energies. Provided that self-generated turbulence dominates particle transport around coronal shocks, it is found that the most crucial factors controlling the diffusive shock acceleration process are the number density of seed particles and the plasma density of the ambient medium. Assuming that suprathermal populations provide a fraction of the particles injected to shock acceleration in the corona, we show that the lack of most energetic particle events as well as the lack of low charge-to-mass ratio ion species in the present cycle can be understood as a result of the reduction of average coronal plasma and suprathermal densities in the present cycle over the previous one.

A Model for Spectral and Compositional Variability at High Energies in Large, Gradual Solar Particle Events

Shocks driven by fast coronal mass ejections (CMEs) are generally believed to be the dominant accelerators in large, gradual solar energetic particle (SEP) events. A key challenge for this notion has been the highly variable spectral and compositional characteristics of these events above a few tens of MeV per nucleon. We have recently proposed that this variability results from the interplay of two factors: evolution in the shock-normal angle as the shock moves outward from the Sun; and a compound seed population, typically comprising at least suprathermals from the corona (or solar wind) and suprathermals from flares. We present here a simple analytical implementation of these ideas. Our calculations semiquantitatively reproduce key features of the observed variability, including spectral morphologies and energy dependence in Fe/O, 3He/4He, and mean ionic charges, in ways that are consistent with correlations in the data. The model makes a prediction for the average high-energy Fe/O enhancement that is borne out by 30 years of observations; the model also provides a quantitative explanation for the Breneman & Stone fractionation effect, a fundamental but previously unexplained aspect of SEP phenomenology. Our calculations must be bolstered by future efforts incorporating realistic CME-shock simulations and a rigorous treatment of particle transport. Suprathermal densities in the corona, as well as details of the injection process at shocks of arbitrary obliquity, require further investigation. Nevertheless, these first results suggest a comprehensive framework for understanding the complexity of high-energy variability in terms of shock physics for most, if not all, large SEP events.

Suprathermal phonons in superfluid4He

We show that the decay probability for high-energy (\ensuremath{\geqslant}10 K) phonons in a dense cloud of low-energy (\ensuremath{\sim}1 K) phonons tends to zero when the phonons are in a beam in superfluid helium. This is in distinct contrast to their behavior in an isotropic distribution where their lifetime is several orders of magnitude shorter. The low decay probability arises because the high-energy phonons interact by the four phonon process and this requires a large angle between the two incoming phonons, however in a narrow-angle beam such large angle processes are impossible. The creation probability of high-energy phonons is hardly affected by being in a beam and as these phonons essentially do not decay, we predict that their density increases and becomes suprathermal, i.e., it is much greater than that in an isotropic distribution. The possible processes that do limit the decay lifetime of high-energy phonons in anisotropic phonon distributions are discussed, and we suggest how this suprathermal density could be detected.

The creation of supra-thermal densities of high-energy phonons in He II

We report the new phenomenon that high-energy phonons can be created from low-energy phonons. This arises because the dynamics of phonons in propagating pulses are quite different to those in isotropic phonon distributions. A pulse of low-energy phonons rapidly thermalises by three-phonon processes. On a much longer time scale four-phonon processes occur within this phonon cloud which create high-energy (10 K) phonons that cannot spontaneously decay. These phonons have a lower velocity and so are lost from the back of phonon cloud; their deficit is restored continuously by four-phonon processes. These now isolated high-energy phonons are very stable and propagate ballistically behind the low-energy phonons, so giving the two pulses which are detected in experiments. For long pulses the high-energy phonons may also decay within the cloud, however the available low-energy phonons for scattering are confined to a narrow-angle cone, so the decay probability is very low because the four phonon process requires large angle scattering. A supra-thermal density of these high-energy phonons is predicted.