Superhalo Electrons Research Articles

Quiet-time Solar Wind Suprathermal Electrons of Different Solar Origins

Abstract The energy spectrum of solar wind strahl, halo, and superhalo electrons likely carries crucial information on their possible origin and acceleration at the Sun. Here we statistically investigate the energy spectrum of solar wind strahl/halo electrons at ∼0.1–1.5 keV and superhalo electrons at ∼20–200 keV measured by Wind/3D Plasma and Energetic Particle during quiet times from 1998 to 2014, according to the types of their Potential Field Source Surface–mapped coronal source regions (CSRs). We adopt the classification scheme developed by Zhao et al. to categorize the CSRs into four types: active region (AR), quiet Sun (QS), coronal hole (CH), and helmet-streamer associated region (HS). We find that for the quiet-time strahl, the AR and HS (QS and CH) correspond to a smaller (larger) kappa index κ strahl with the most frequent value of 7–8.5 (8.5–10) and a larger (smaller) n strahl with the most frequent value of 0.013–0.026 cm−3 (0.006–0.0013 cm−3). For the quiet-time halo, κ halo behaves similarly to κ strahl, but n halo appears similar among the four CSR types. For the superhalo, the AR (QS) corresponds to a larger (smaller) power-law index β with the most frequent value of 2.2–2.4 (1.8–2.0), while the HS and CH have a β not different from either the AR or QS; n sup appears similar, with the most frequent value of 3 × 10−8–3 × 10−7 cm−3, among the four CSR types. These results suggest that the strahl (superhalo) from the hotter CSRs tends to be more (less) efficiently accelerated.

The Strongest Acceleration of >40 keV Electrons by ICME-driven Shocks at 1 au

Abstract We present two case studies of the in-situ electron acceleration during the 2000 February 11 shock and the 2004 July 22 shock, with the strongest electron flux enhancement at 40 keV across the shock, among all the quasi-perpendicular and quasi-parallel ICME-driven shocks observed by the WIND 3DP instrument from 1995 through 2014 at 1 au. We find that for this quasi-perpendicular (quasi-parallel) shock on 2000 February 11 (2004 July 22), the shocked electron differential fluxes at ∼0.4–50 keV in the downstream generally fit well to a double-power-law spectrum, J ∼ E −β , with an index of β ∼ 3.15 (4.0) at energies below a break at ∼3 keV (∼1 keV) and β ∼ 2.65 (2.6) at energies above. For both shock events, the downstream electron spectral indices appear to be similar for all pitch angles, which are significantly larger than the index prediction by diffusive shock acceleration. In addition, the downstream electron pitch-angle distributions show the anisotropic beams in the anti-sunward-traveling direction, while the ratio of the downstream over ambient fluxes appears to peak near 90° pitch angles, at all energies of ∼0.4–50 keV. These results suggest that in both shocks, shock drift acceleration likely plays an important role in accelerating electrons in situ at 1 au. Such ICME-driven shocks could contribute to the formation of solar wind halo electrons at energies ≲2 keV, as well as the production of solar wind superhalo electrons at energies ≳2 keV in interplanetary space.

Discrete energetic (∼50–200 keV) electron events in the high-altitude cusp/polar cap/lobe

We identified 28 discrete electron events (DEEs) with enhanced fluxes at ~50–200 keV in the high-altitude cusp/polar cap/lobe, using the electron measurements by the BeiDa Image Electron Spectrometer (BD-IES) instrument onboard an inclined (55°) geosynchronous orbit (IGSO) satellite from October 2015 to January 2016. We find that among the 28 DEEs, 22 occur in the nightside and mostly in the northern cusp/polar cap/lobe, while 6 occur in the dayside and all in the southern cusp; 24 events correspond to an average interplanetary magnetic field (IMF) component B z >0, 3 correspond to an average IMF B z <0, and 1 has no OMNI IMF data. In these DEEs, the observed average omnidirectional electron differential flux generally fits well to a power-law spectrum, J ~ E − γ , with the spectral index γ ranging from 2.6 to 4.6, while the average electron flux varies over three orders of magnitude from event to event. The spectral index of these cusp DEEs are (strongly) larger than the spectral index of solar wind superhalo electrons (radiation belt electrons) observed by the WIND 3D Plasma & Energetic Particle instrument (the BD-IES). At ~110 keV, the electron flux of DEEs in the cusp/polar cap/lobe shows a weak positive correlation with the solar wind superhalo electron flux but no obvious correlation with the radiation belt electron flux. These results suggest that these DEEs probably originate from transient processes acting on the solar wind superhalo electrons, e.g., the mid/high-latitude reconnection.

REVISED MODEL OF THE STEADY-STATE SOLAR WIND HALO ELECTRON VELOCITY DISTRIBUTION FUNCTION

ABSTRACT A recent study discussed the steady-state model for solar wind electrons during quiet time conditions. The electrons emanating from the Sun are treated in a composite three-population model—the low-energy Maxwellian core with an energy range of tens of eV, the intermediate ∼102–103 eV energy-range (“halo”) electrons, and the high ∼103–105 eV energy-range (“super-halo”) electrons. In the model, the intermediate energy halo electrons are assumed to be in resonance with transverse EM fluctuations in the whistler frequency range (∼102 Hz), while the high-energy super-halo electrons are presumed to be in steady-state wave–particle resonance with higher-frequency electrostatic fluctuations in the Langmuir frequency range (∼105 Hz). A comparison with STEREO and WIND spacecraft data was also made. However, ignoring the influence of Langmuir fluctuations on the halo population turns out to be an unjustifiable assumption. The present paper rectifies the previous approach by including both Langmuir and whistler fluctuations in the construction of the steady-state velocity distribution function for the halo population, and demonstrates that the role of whistler-range fluctuation is minimal unless the fluctuation intensity is arbitrarily raised. This implies that the Langmuir-range fluctuations, known as the quasi thermal noise, are important for both halo and super-halo electron velocity distribution.

Simulation of Quiet-Sun Hard X-Rays Related to Solar Wind Superhalo Electrons

In this paper, we propose that the accelerated electrons in the quiet Sun could collide with the solar atmosphere to emit Hard X-rays (HXRs) via non-thermal bremsstrahlung, while some of these electrons would move upwards and escape into the interplanetary medium, to form a superhalo electron population measured in the solar wind. After considering the electron energy loss due to Coulomb collisions and the ambipolar electrostatic potential, we find that the sources of the superhalo could only occur high in the corona (at a heliocentric altitude $\gtrsim 1.9$ R$_\odot$ (the mean radius of the Sun)), to remain a power-law shape of electron spectrum as observed by STEREO at 1AU near solar minimum (Wang et al., 2012). The modeled quiet-Sun HXRs related to the superhalo electrons fit well to a power-law spectrum, $f \sim \varepsilon^{-\gamma}$, with an index $\gamma$ $\approx$ 2.0 - 2.3 (3.3 - 3.7) at 10 - 100 keV, for the warm/cold thick-target (thin-target) emissions produced by the downward-traveling (upward-traveling) accelerated electrons. These simulated quiet-Sun spectra are significantly harder than the observed spectra of most solar HXR flares. Assuming that the quiet-Sun sources cover 5% of the solar surface, the modeled thin-target HXRs are more than six orders of magnitude weaker than the RHESSI upper limit for quiet-Sun HXRs (Hannah et al., 2010). Using the thick-target model for the downward-traveling electrons, the RHESSI upper limit restricts the number of downward-traveling electrons to at most $\approx$3 times the number of escaping electrons. This ratio is fundamentally different from what is observed during solar flares associated with escaping electrons where the fraction of downward-traveling electrons dominates by a factor of 100 to 1000 over the escaping population.

Spontaneous emission of electromagnetic and electrostatic fluctuations in magnetized plasmas: Quasi-parallel modes

The present paper is devoted to the theoretical and numerical analysis of the spontaneously emitted electromagnetic fluctuations characterized by quasi-parallel wave vectors relative to the ambient magnetic field. The formulation is based upon the Klimontovich plasma kinetic theory. The comparative study is carried out between the spontaneously emitted field fluctuation spectrum constructed on the basis of a single Maxellian velocity distribution function (VDF) and the spectrum that arises from multi-component electron VDFs similar to those found in the solar wind. Typical solar wind electron VDF is composed of a Gaussian core and kappa distributions of halo and super-halo components. Of these, the halo and super-halo populations represent tenuous but energetic components. It is found that the energetic electrons make important contributions to the total emission spectrum. It is also found that the halo electrons are largely responsible for the emission spectrum in the whistler frequency range, whereas the more energetic super-halo electrons emit quasi-longitudinal fluctuations in the Langmuir frequency range, thus validating the recent quasi-steady state model of the solar wind electrons put forth by the present authors [Kim et al., Astrophys. J. 806, 32 (2015); Yoon et al., Astrophys. J. 812, 169 (2015)].

A Brief Review of Interplanetary Investigations in China from 2014 to 2016

Great progress has been made in the research of solar corona and interplanetary physics by the Chinese scientists during the past two years (2014-2016). Nearly 100 papers were published in this area. In this report, we will give a brief review to these progresses. The investigations include:solar corona, solar wind and turbulence, superhalo electron and energetic particle in the inner heliosphere, solar flares and radio bursts, Coronal Mass Ejections (CMEs) and their interplanetary counterparts, Magnetohydrodynamic (MHD) numerical modeling, CME/shock arrival time prediction, magnetic reconnection, solar variability and its impact on climate. These achievements help us to better understand the evolution of solar activities, solar eruptions, their propagations in the heliosphere, and potential geoeffectiveness. They were achieved by the Chinese solar and space scientists independently or via international collaborations.

THE ANGULAR DISTRIBUTION OF SOLAR WIND SUPERHALO ELECTRONS AT QUIET TIMES

The angular distribution of superhalo electrons provides important information on the generation of superhalo electrons in the solar wind. Here we present a comprehensive study of the angular distribution of ~20–200 keV superhalo electrons measured at 1 AU by the WIND 3DP instrument during quiet times from 1995 January through 2005 December. For a given energy, we define the anisotropy of superhalo electrons (with respect to the interplanetary magnetic field) as where Jout (Jin) is the observed differential flux of electrons traveling outward from (inward toward) the Sun. We found that for ~96% of the selected quiet-time samples, superhalo electrons have isotropic angular distributions, while for ~3% (~1%) of quiet-time samples, superhalo electrons have outward-anisotropic (inward-anisotropic) angular distributions. All three groups of superhalo electrons show no correlation with the local solar wind plasma, interplanetary magnetic field, turbulence, and energetic electrons accelerated/reflected at the terrestrial bow shock. For the generation of quiet-time superhalo electrons, one possibility is that they are accelerated by nonthermal processes related to the solar wind source, followed by strong scattering/reflection in the interplanetary medium. Another possibility is that these superhalo electrons are formed due to the acceleration of solar wind halo/strahl electrons, e.g., by turbulence, throughout the interplanetary medium.

Solar Wind Electron Energization by Plasma Turbulence

The solar wind electrons are made of the low-energy Maxwellian core, intermediate-energy halo, field-aligned strahl, and the highly-energetic super-halo electrons. The present paper discusses a model in which the halo electrons interact with the whistler fluctuation via cyclotron wave-particle resonance, and the super-halo electrons interact through Landau resonance with the Langmuir fluctuation, thus maintaining a local steady state.

ASYMPTOTIC THEORY OF SOLAR WIND ELECTRONS

This paper presents a theory for the asymptotically steady-state solar wind electron velocity distribution function (VDF) in a local equilibrium with plasma wave turbulence. By treating the local solar wind electron VDF as a superposition of three populations—the low-energy Maxwellian core electrons with an energy range of tens of eV, the intermediate –103 eV energy-range halo electrons, and the high –105 eV energy-range superhalo electrons—the present paper puts forth a model in which the halo electrons are in dynamical steady state with the pervasive whistler fluctuations, while the superhalo electrons maintain dynamical steady-state equilibrium with the Langmuir fluctuations, known as the quasi-thermal noise. Customary models of the solar wind electrons include only the Maxwellian core and the halo (plus highly field-aligned strahl). While the present paper does not consider the strahl population in the discussion, the highly energetic superhalo component, which is observed to be present in all solar conditions, is explicitly taken into account as part of the total solar wind electron model. Comparisons with STEREO and WIND spacecraft observations are also made.

SOLAR WIND ∼20–200 keV SUPERHALO ELECTRONS AT QUIET TIMES

High-energy superhalo electrons are present in the interplanetary medium (IPM) even in the absence of any significant solar activity, carrying important information on electron acceleration in the solar wind. We present a statistical survey of ~20–200 keV superhalo electrons measured at 1 AU by the WIND 3D Plasma & Energetic Particle instrument during quiet-time periods from 1995 January through 2013 December. The selected 242 quiet-time samples mostly occur during the rising, maximum and decay phases of solar cycles. The observed omnidirectional differential flux of these quiet-time superhalo electrons generally fits to a power-law spectrum , with β ranging from ~1.6 to ~3.7 and the integrated density nsup ranging from 10−8 to 10−5 cm−3. In solar cycle 23 (24), the distribution of β has a broad maximum between 2.4 and 2.8 (2.0 and 2.4). Both β and the logarithm of nsup show no obvious correlation with sunspot number, solar flares, solar wind core population, etc. These superhalo electrons may form a quiet-time energetic electron background/reservoir in the IPM. We propose that they may originate from nonthermal processes related to the acceleration of the solar wind such as nanoflares, or could be formed in the IPM due to further acceleration and/or long-distance propagation effects.

Numerical simulation of superhalo electrons generated by magnetic reconnection in the solar wind source region**Supported by the National Natural Science Foundation of China.

Superhalo electrons appear to be continuously present in the interplanetary medium, even during very quiet times, with a power-law spectrum at energies above ∼2keV. Here we numerically investigate the generation of superhalo electrons by magnetic reconnection in the solar wind source region, using magnetohydrodynamics and test particle simulations for both single X-line reconnection and multiple X-line reconnection. We find that the direct current electric field, produced in the magnetic reconnection region, can accelerate electrons from an initial thermal energy of T ∼ 105 K up to hundreds of keV. After acceleration, some of the accelerated electrons, together with the nascent solar wind flow driven by the reconnection, propagate upwards along the newly-opened magnetic field lines into interplanetary space, while the rest move downwards into the lower atmosphere. Similar to the observed superhalo electrons at 1AU, the flux of upward-traveling accelerated electrons versus energy displays a power-law distribution at ∼ 2–100 keV, f(E) ∼ E−δ, with a δ of ∼ 1.5 – 2.4. For single (multiple) X-line reconnection, the spectrum becomes harder (softer) as the anomalous resistivity parameter α (uniform resistivity η) increases. These modeling results suggest that the acceleration in the solar wind source region may contribute to superhalo electrons.

QUIET-TIME INTERPLANETARY ∼2-20 keV SUPERHALO ELECTRONS AT SOLAR MINIMUM

We present a statistical survey of ~2-20 keV superhalo electrons in the solar wind measured by the SupraThermal Electron instrument on board the two STEREO spacecraft during quiet-time periods from 2007 March through 2009 March at solar minimum. The observed superhalo electrons have a nearly isotropic angular distribution and a power-law spectrum, fv –γ, with γ ranging from 5 to 8.7, with nearly half between 6.5 and 7.5, and an average index of 6.69 ± 0.90. The observed power-law spectrum varies significantly on a spatial scale of 0.1 AU and a temporal scale of several days. The integrated density of quiet-time superhalo electrons at 2-20 keV ranges from ~10–8 cm–3 to 10–6 cm–3, about 10–9-10–6 of the solar wind density, and, as well as the power-law spectrum, shows no correlation with solar wind proton density, velocity, or temperature. The density of superhalo electrons appears to show a solar-cycle variation at solar minimum, while the power-law spectral index γ has no solar-cycle variation. These quiet-time superhalo electrons are present even in the absence of any solar activity—e.g., active regions, flares or microflares, type III radio bursts, etc.—suggesting that they may be accelerated by processes such as resonant wave-particle interactions in the interplanetary medium, or possibly by nonthermal processes related to the acceleration of the solar wind such as nanoflares, or by acceleration at the CIR forward shocks.

Langmuir Turbulence and Suprathermal Electrons

Charged particle acceleration takes place ubiquitously in the Universe including the near-Earth heliospheric environment. Typical in situ spacecraft measurements made in the solar wind show that the charged particle velocity distribution contains energetic components with quasi scale-free power-law velocity dependence, f∼v −α, for high velocity range. In this Review a theory of quiet-time solar-wind electrons that contain a suprathermal component is discussed, in which these electrons are taken to be in dynamical equilibrium with Langmuir turbulence. This Review includes an overview of the Langmuir turbulence theory, as well as a discussion on asymptotic equilibrium solution of Langmuir turbulence/suprathermal electron system. Theoretical predictions of high-energy electron velocity power-law distribution index is then compared against the recent observations of the superhalo electron velocity distribution made by instruments onboard WIND and STEREO spacecraft. It is shown that the theoretical prediction of velocity power-law index is intermediate to the observed range.

Landau damping in relativistic plasmas with power-law distributions and applications to solar wind electrons

The relativistic plasma dispersion relation is derived for Langmuir waves in a spatially homogeneous unmagnetized plasma in which the electrons have an isotropic power-law distribution in momentum space. The theory is applied to the study of Langmuir waves in the quiescent solar wind near the orbit of the Earth assuming that the electron distribution function can be approximated as a power-law from thermal energies ∼10eV to relativistic energies ≳100keV. Numerical solutions of the dispersion relation show that in the regime of weak Landau damping the phase speeds of the waves match the velocities of the high-energy particles, known in the solar wind literature as the superhalo, which lie in the range 0.09&amp;lt;v∕c&amp;lt;0.8. While this result raises the question of whether wave-particle interactions with a spectrum of Langmuir waves plays a role in the formation of the superhalo, the observed energy density of the waves near the orbit of the Earth at 1AU is so small that active acceleration of superhalo electrons by Langmuir waves, if it occurs at all, has ceased by the time the solar wind reaches the Earth.

The STEREO IMPACT Suprathermal Electron (STE) Instrument

The Suprathermal Electron (STE) instrument, part of the IMPACT investigation on both spacecraft of NASA’s STEREO mission, is designed to measure electrons from ∼2 to ∼100 keV. This is the primary energy range for impulsive electron/3He-rich energetic particle events that are the most frequently occurring transient particle emissions from the Sun, for the electrons that generate solar type III radio emission, for the shock accelerated electrons that produce type II radio emission, and for the superhalo electrons (whose origin is unknown) that are present in the interplanetary medium even during the quietest times. These electrons are ideal for tracing heliospheric magnetic field lines back to their source regions on the Sun and for determining field line lengths, thus probing the structure of interplanetary coronal mass ejections (ICMEs) and of the ambient inner heliosphere. STE utilizes arrays of small, passively cooled thin window silicon semiconductor detectors, coupled to state-of-the-art pulse-reset front-end electronics, to detect electrons down to ∼2 keV with about 2 orders of magnitude increase in sensitivity over previous sensors at energies below ∼20 keV. STE provides energy resolution of ΔE/E∼10–25% and the angular resolution of ∼20° over two oppositely directed ∼80°×80° fields of view centered on the nominal Parker spiral field direction.