Solar Electron Research Articles

Deterioration mechanism of perovskite solar cells by operando observation of spin states

Perovskite solar cells are attractive because of their remarkably improved power conversion efficiency. In view of their application, however, it is important not only to increase the power conversion efficiency, but also to elucidate the deterioration mechanism. Here, we show operando direct observation of spin states in the cells using electron spin resonance, thereby investigating the operation and deterioration mechanisms from a microscopic viewpoint. By simultaneous measurements of solar cell characteristics and electron spin resonance, the spin states in the hole transport material spiro-OMeTAD are demonstrated to change in accordance with the device performance variation under operation. These variations are ascribed to the change of hole transport and to interfacial electric dipole layers. Reverse electron transfer from TiO2 to the hole transport material layer is demonstrated under ultraviolet light irradiation, which decreases hole doping. Conducting such operando microscopic investigation will be useful to obtain further guidelines for improving the device performance and durability.

Using Video Photometry to Measure the 3 MeV Proton Half Brightness Fluence for Tetrakis (Dibenzoylmethide) Europium (III) Triethylammonium

In recent years, luminescent materials have been proposed for use as smart sensors to detect structural damage using triboluminescence, measure incident radiation fluence using radioluminescence, and monitor surface temperature using phosphor thermometry. To sense structural damage using triboluminescence, appropriate materials are embedded in a composite host structure for various applications. When the damage occurs in the host structure, it leads to the fracture of the triboluminescent crystals resulting in light emission. This process provides a real-time warning that structural damage has occurred. The TL emission of the candidate material has to be sufficiently bright, so that the light signal reaching from the point of fracture to the detector, through a fiber optics cable, is strong enough to be detected. The majority of the known triboluminescent materials do not emit light with sufficient intensity to allow detection with compact and inexpensive detectors. Over the past forty years, some materials have been reported with TL of sufficient intensity for the light emission to be easily observed with the naked eye. Out of these triboluminescent materials, only few could satisfy the above criterion. Tetrakis (Dibenzoylmethide) Europium (III) Triethylammonium (EuD4TEA), one of the brightest known TL materials and is a potential candidate for application to this type of hybrid smart sensor in extreme environments. Recent measurements by the authors indicate that the decay time for EuD4TEA is bi-exponential and can be used to measure temperatures from -10 to 80 °C. Ionizing radiation in space poses a significant challenge to future human and robotic exploration missions. Any intelligent sensor material used in space must also be resistant to ionizing radiation. Sources and types of such radiation include galactic and solar electrons and protons, x-rays, ultraviolet light, and magnetically trapped charged particles in radiation belts. While much research has been devoted to characterizing material degradation due to electron exposure, surprisingly little work seems to have been done on proton damage. Research has shown that proton bombardment in the keV to MeV range, like that occurring in space, reduces the intensity of fluorescence. Thus, proton irradiation will likely reduce the ability of a candidate smart material to emit fluorescence.Practical space-based sensors based on luminescence will depend heavily upon research investigating the resistance of these materials to ionizing radiation and the ability to anneal or self-heal from damage caused by such radiation. In 1951, Birks and Black showed experimentally that the fluorescence efficiency of anthracene bombarded by alphas varies with total fluence (N) as (I/I0) = 1/(1 + AN), where I is the luminescence yield, I0 is the initial luminescence yield, and A is a constant. The half brightness fluence (N1/2) is a material figure of merit and is defined as the reciprocal of A. Broser and Kallmann developed a similar relationship to the Birks and Black equation for inorganic phosphors irradiated using alpha particles. From 1990 to the present, we have found that the Birks and Black relation describes the reduction in emission yield for every tested luminescent material except lead phosphate glass due to proton irradiation.These results indicate that radiation produced quenching centers compete with emission centers for absorbed energy. In 2016, the N1/2 for EuD4TEA irradiated with 3 MeV protons was measured to be about 3 x 1010 mm-2. Conversely, ZnS:Mn compounds tested were found to have N1/2 values for 3 MeV protons that are nearly one thousand times larger (~1013 mm-2). According to Tribble, a spacecraft at 1 AU from the Sun will receive a 1 MeV proton fluence of less than 1011 mm-2 from a large solar event. Likewise, 1 MeV proton fluences in the Earth’s radiation belts and the Earth-Moon-Sun Lagrange points will be even less than the 1011 mm-2 value from large solar events. For this reason, EuD4TEA might be a candidate sensor material to monitor large-scale solar events for astronauts in vehicles flying in near earth space. Since their N1/2 values are larger, ZnS:Mn compounds are more suited for use as sensors in higher radiation environments or for very long duration space missions. The purpose of this presentation is to present preliminary results from a 1U CubeSat payload using a EuD4TEA-based sensor to measure ionizing radiation in Earth orbit. This flight experiment is a natural outgrowth of earlier ground-based radiation exposure measurements that will be used to determine if EuD4TEA-based materials can be used as a real-time sensor for applications in space.

Prolonged, Weak C1.2 Flares: A Source of Protons and Electrons

Two proton events (>100 MeV; GOES, ACS SPI) associated with weak flares (C1.2, GOES class) in the soft X-ray range that occurred on November 26, 2011, and September 29, 2013, are analyzed. These events were accompanied by significant fluxes of relativistic electrons (>1 MeV; SOHO EPHIN) in the interplanetary space. The time profiles of these protons and electron fluxes were similar during the growth phase, which suggested a common mechanism of their acceleration and propagation. The gradual increase in the flux intensities, in our opinion, shows a prolonged and gradual injection of particles into the interplanetary space, which may be the result of prolonged acceleration or capture. Since prolonged electron capture is unlikely, it is most probable that a prolonged acceleration took place. The prolonged proton acceleration is realized at the post-eruptive phase of solar flares or shock-wave coronal mass ejection. According to the existing theoretical concepts, the acceleration of solar electrons to ~10 MeV by a shock wave is hardly possible. In this case, relativistic electrons and protons were apparently accelerated in the post-eruptive phase of solar flares, where the stochastic acceleration by magnetohydrodynamic turbulence is considered the most probable mechanism.

Leptonic new force and cosmic-ray boosted dark matter for the XENON1T excess

The recently reported excess in XENON1T is explained by new leptonic forces, which are free from gauge anomalies. We focus on two scenarios with and without dark matter. In Scenario #1, the gauge boson of gauged lepton number U(1)Le−Lj, j=μ or τ provides non-standard interaction between solar neutrino and electron that enhances the number of electron recoil events in the XENON1T detector. In Scenarino #2, the new gauge boson exclusively couples to electron and dark matter, then cosmic-ray electrons can transfer their momenta to dark matter in halo. The boosted dark matter generates the electron recoil signals of O(1) keV. The dark matter, aided by the new gauge interaction, efficiently heats up a neutron star in our Galaxy more than ∼1500 K as a neutron star captures the halo dark matter. Therefore, we propose to utilize the future infrared telescope to test our scenario.

Sodium deca-tungstate /polyacrylic acid self-assembled flexible wearable photochromic composite fabric for solar UV detector

A flexible and lightweight solar ultraviolet (UV) detector compatible with cotton fabric is prepared by simple hydrogen bonding self-assembly of polyacrylic acid (PAA)and sodium deca-tungstate (SDT). Based on photochromism, the solar UV detector with good detection capabilities and the detection effect are obtained. The as-designed solar UV-induced electron transfer process imparts the detector quick response and visual color changes. The detector was tested on the spot in direct solar light and simulated solar light, which proved its outstanding performance as a flexible, wearable and visualized UV detector. Furthermore, the influence of solar doses and on-off cycle stability were studied. Therefore, the detection device fabricated can be used to indicate the intensity of UV lights by sensitive color changes, which provides a feasible idea for the preparation of novel UV detection devices.

Identification and Extraction of Solar Radio Spikes Based on Deep Learning

Solar radio spikes are short-duration, narrow-band burst signals, which are a fine structure of solar radio bursts. The processing and analysis of their observed data are of great significance in the study of electron acceleration in the process of solar flares and electron acceleration during the explosion and diagnosis of corona parameters. Deep learning interprets data by mimicking the mechanism of the human brain. Faster Region-based Convolutional Neural Network (Faster R-CNN) is a branch of deep learning based on region nomination, and its classification results have considerable advantages in accuracy. In this paper, Faster R-CNN will be used to identify and extract solar radio spikes. In order to improve the detection ability of small events, a multi-scale detection frame and a multi-layer feature fusion training method are used. The analysis results show that the Average Precision (AP) value of the improved network is close to 91%, which is nearly 10% higher than the original network. So the improved Faster R-CNN method can also be used for the identification and extraction of small-scale fine structures in other fields.

Probing solar flare accelerated electron distributions with prospective X-ray polarimetry missions

Solar flare electron acceleration is an extremely efficient process, but the method of acceleration is not well constrained. Two of the essential diagnostics, electron anisotropy (velocity angle to the guiding magnetic field) and the high energy cutoff (highest energy electrons produced by the acceleration conditions: mechanism, spatial extent, and time), are important quantities that can help to constrain electron acceleration at the Sun but both are poorly determined. Here, by using electron and X-ray transport simulations that account for both collisional and non-collisional transport processes, such as turbulent scattering and X-ray albedo, we show that X-ray polarization can be used to constrain the anisotropy of the accelerated electron distribution and the most energetic accelerated electrons together. Moreover, we show that prospective missions, for example CubeSat missions without imaging information, can be used alongside such simulations to determine these parameters. We conclude that a fuller understanding of flare acceleration processes will come from missions capable of both X-ray flux and polarization spectral measurements together. Although imaging polarimetry is highly desired, we demonstrate that spectro-polarimeters without imaging can also provide strong constraints on electron anisotropy and the high energy cutoff.

Recurrent Solar Energetic Particle Flux Enhancements Observed near Earth and Mars

Abstract The period 2016 August 1 to November 15 was characterized by the presence of corotating interaction regions (CIRs) and a few weak coronal mass ejections (CMEs) in the heliosphere. In this study we show recurrent energetic electron and proton enhancements observed near Earth (1 au) and Mars (1.43–1.38 au) during this period. The observations near Earth use data from instruments on board the Advanced Composition Explorer, the Solar and Heliospheric Observatory, and Solar Dynamics Observatory and those near Mars are from the Solar Energetic Particle, Solar Wind Ion Analyzer, and Magnetometer instruments on board the Mars Atmosphere and Volatile EvolutioN (MAVEN). During this period, the energetic electron fluxes observed near Earth and Mars showed prominent periodic enhancements over four solar rotations, with major periodicities of ∼27 days and ∼13 days. Periodic radar blackouts/weakenings of radar signals at Mars were observed by the Mars Advanced Radar for Subsurface and Ionosphere Sounding/Mars Express, and are associated with these solar energetic electron enhancements. During this period, a weak CME and a high-speed stream (HSS)-related interplanetary shock could have interacted with the CIR and enhance energetic proton fluxes near 1.43–1.38 au, causing ∼27 days periodicity in proton fluxes to be significantly diminished at 1.43–1.38 au. These events also have an unexpected impact on the Martian topside ionosphere, such as topside ionospheric depletion and compression observed by the Langmuir Probe and Waves and Neutral Gas and Ion Mass Spectrometer on board MAVEN. These observations are unique not only because of the recurring nature of electron enhancements seen at two vantage points, but also because they reveal the unexpected impact of the weak CME and interplanetary shock on the Martian ionosphere, which provides new insights into the impact of CME–HSS interactions on the Martian plasma environment.

Characteristics of solar wind suprathermal halo electrons

A suprathermal halo population of electrons is ubiquitous in space plasmas, as evidence of their departure from thermal equilibrium even in the absence of anisotropies. The origin, properties, and implications of this population, however, are poorly known. We provide a comprehensive description of solar wind halo electrons in the ecliptic, contrasting their evolutions with heliospheric distance in the slow and fast wind streams. At relatively low distances less than 1 AU, the halo parameters show an anticorrelation with the solar wind speed, but this contrast decreases with increasing distance and may switch to a positive correlation beyond 1 AU. A less monotonic evolution is characteristic of the high-speed winds, in which halo electrons and their properties (e.g., number densities, temperature, plasma beta) exhibit a progressive enhancement already distinguishable at about 0.5 AU. At this point, magnetic focusing of electron strahls becomes weaker and may be counterbalanced by the interactions of electrons with wave fluctuations. This evolution of halo electrons between 0.5 AU and 3.0 AU in the fast winds complements previous results well, indicating a substantial reduction of the strahl and suggesting that significant fractions of strahl electrons and energy may be redistributed to the halo population. On the other hand, properties of halo electrons at low distances in the outer corona suggest a subcoronal origin and a direct implication in the overheating of coronal plasma via velocity filtration.

Small Electron Events Observed by Parker Solar Probe/IS⊙IS during Encounter 2

Abstract The current understanding of the characteristics of solar and inner heliospheric electron events is inferred almost entirely from observations made by spacecraft located at 1 astronomical unit (au). Previous observations within 1 au of the Sun, by the Helios spacecraft at ∼0.3–1 au, indicate the presence of electron events that are not detected at 1 au or may have merged during transport from the Sun. Parker Solar Probe’s close proximity to the Sun at perihelion provides an opportunity to make the closest measurements yet of energetic electron events. We present an overview of measurements of electrons with energies between ∼17 keV and ∼1 MeV made by the Parker Solar Probe Integrated Science Investigation of the Sun (IS⊙IS) instrument suite during Encounter 2 (2019 March 31–April 10 with perihelion of ∼0.17 au), including several small electron events. We examine these events in the context of the electromagnetic and solar wind environment measured by the FIELDS and SWEAP instruments on Parker Solar Probe. We find most of these electron enhancements to be associated with type III radio emissions that reach the local plasma frequency and one enhancement that appears to be primarily associated with abrupt changes in the local magnetic field. Together, these associations suggest that these are indeed the first measurements of energetic electron events within 0.2 au.

Solar wind charge exchange in cometary atmospheres

Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet. The ESA/Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P) provides a unique opportunity to study charge-changing processes in situ. To understand the role of these reactions in the evolution of the solar wind plasma, and interpret the complex in-situ measurements made by Rosetta, numerical or analytical models are necessary. We use an extended analytical formalism describing solar wind charge-changing processes at comets along solar wind streamlines. The model is driven by solar wind ion measurements from the Rosetta Plasma Consortium-Ion Composition Analyzer (RPC-ICA) and neutral density observations from the Rosetta Spectrometer for Ion and Neutral Analysis-Comet Pressure Sensor (ROSINA-COPS), as well as charge-changing cross sections of hydrogen and helium particles in a water gas. A mission-wide overview of charge-changing efficiencies at comet 67P is presented. Electron capture cross sections dominate and favor the production of He and H energetic neutral atoms, with fluxes expected to rival those of H+ and He2+ ions. Neutral outgassing rates are retrieved from local RPC-ICA flux measurements, and match ROSINA's estimates very well. From the model, we find that solar wind charge exchange is unable to fully explain the magnitude of the sharp drop of solar wind ion fluxes observed by Rosetta for heliocentric distances below 2.5 AU. This is likely because the model does not take into account the relative ion dynamics and, to a lesser extent, ignore the formation of bow shock-like structures upstream of the nucleus. This work also shows that the ionization by solar EUV radiation and energetic electrons dominates the source of cometary ions, although solar wind contributions may be significant during isolated events.

On the Shape of SEP Electron Spectra: The Role of Interplanetary Transport

Abstract We address the effect of particle scattering on the energy spectra of solar energetic electron events using (i) an observational and (ii) a modeling approach. (i) We statistically study observations of the STEREO spacecraft, using directional electron measurements made with the Solar Electron and Proton Telescope in the range of 45–425 keV. We compare the energy spectra of the anti-Sunward propagating beam with that of the backward-scattered population and find that, on average, the backward-scattered population shows a harder spectrum with the effect being stronger at higher energies. (ii) We use a numerical solar energetic particle (SEP) transport model to simulate the effect of particle scattering (both in terms of pitch angle and perpendicular to the mean field) on the spectrum. We find that pitch-angle scattering can lead to spectral changes at higher energies (E > 100 keV) and further away from the Sun (r > 1 au), which are also often observed. At lower energies, and closer to the Sun, the effect of pitch-angle scattering is much more reduced, so that the simulated energy spectra still resemble the injected power-law functions. When examining pitch-angle-dependent spectra, we find, in agreement with the observational results, that the spectra of the backward-propagating electrons are harder than that of the forward (from the Sun) propagating population. We conclude that Solar Orbiter and Parker Solar Probe will be able to observe the unmodulated omnidirectional SEP electron spectrum close to the Sun at higher energies, giving a direct indication of the accelerated spectrum.

The Sun, neutrinos and Super-Kamiokande.

In the standard model of elementary particle physics neutrinos are massless, and therefore the actuality of finite neutrino mass indicates a theory beyond the standard model. The Sun produces abundant neutrinos due to nuclear fusion reactions. A pioneering experiment in the early '70s detected neutrinos from the Sun, but found that the observed flux was smaller than expected, which was then called the missing solar neutrino problem. Tremendous efforts were made both experimentally and theoretically to solve this problem. In 2001, almost 30 years after the first indication, data from Super-Kamiokande in Japan and SNO in Canada together provided evidence that neutrino oscillation effectively converts the solar (electron) neutrinos to non-electron type neutrinos. Neutrino oscillation can occur only for those neutrinos with finite neutrino mass.

Counterstreaming Strahls and Heat Flux Dropouts as Possible Signatures of Local Particle Acceleration in the Solar Wind

Abstract Suprathermal electrons with energies of ∼70 eV and above are observed at 1 au as dispersionless halo electrons and magnetic field-aligned beams of strahls. For a long time, it has been thought that both populations originate only from the solar corona, and that the only active process impacting their properties in the solar wind is scattering. This view has consequently impacted the interpretation of typical patterns of pitch-angle distributions (PADs) of suprathermal electrons. Meanwhile, recent observational studies supported by numerical simulations have shown that there is an unaccounted population of electrons accelerated to suprathermal energies at reconnecting current sheets (RCSs) and 3D dynamical plasmoids (or 2D magnetic islands (MIs)) directly in the heliosphere. We present multispacecraft observations of counterstreaming strahls and heat flux dropouts in PADs within a region filled with plasmoids and RCSs unaffected by interplanetary shocks, comparing observed PAD features with those predicted by particle-in-cell simulations. We show typical PAD patterns determined by local acceleration of thermal-core electrons up to hundreds of electron volts. Resulting PAD views depend on properties and topology of particular RCSs, MIs, and plasma/magnetic field parameters. Our study suggests that solar wind-borne suprathermal electrons coexist with those of solar origin. Therefore, some of heat flux dropout and bidirectional strahl events can be explained by local dynamical processes involving magnetic reconnection. Possible implications of the results for the interpretation of the actively debated decrease in the strahl/halo relative density with heliocentric distance and puzzling features of suprathermal electrons observed at crossings of the heliospheric current sheet and cometary comas are also discussed.

Effects of Nonextensive Ions (Heavier and Lighter) on Ion Acoustic Solitary Waves in a Magnetized Five Component Cometary Plasma with Kappa Described Electrons

We have investigated the propagation characteristics of Ion-Acoustic Solitary Waves (IASWs) in a magnetized, cometary plasma consisting of hydrogen ions, positively and negatively charged oxygen ions, kappa described hot solar electrons, and slightly colder cometary electrons. The effects of q-nonextensive distributions, on both lighter and heavier ions have been studied by deriving the Zakharov–Kuznetsov (ZK) equation. The basic features of IASWs such as amplitude, width, and phase speed have been extensively studied by a numerical analysis of the ZK equation. It is found that superthermality of the electrons and nonextensivity of ions significantly modify the characteristics of the solitary waves. The amplitudes of the solitary waves seem to be well correlated to the presence of water molecules in a cometary plasma and the associated photoionization processes.

Statistical Results for Solar Energetic Electron Spectra Observed over 12 yr with STEREO/SEPT

Abstract This work presents a statistical analysis of near-relativistic solar energetic electron event spectra near 1 au. We use measurements of the Solar Electron and Proton Telescope (SEPT) on board STEREO in the energy range of 45–425 keV and utilize the SEPT electron event list containing all electron events observed by STEREO A and STEREO B from 2007 through 2018. We select 781 events with significant signal-to-noise ratios for our analysis and fit the spectra with single or broken-power-law functions of energy. We find 437 events showing broken power laws and 344 events only showing a single power law in the energy range of SEPT. For those events with broken power laws, we find a mean break energy of about 120 keV. We analyze the dependence of the spectral index on the rise times and peak intensities of the events as well as on the presence of relativistic electrons. The results show a relation between the power law spectral index and the rise times of the events with softer spectra belonging to rather impulsive events. Long rise-time events are associated with hard spectra as well as with the presence of higher-energy (>0.7 MeV) electrons. This group of events cannot be explained by a pure flare scenario but suggests an additional acceleration mechanism, involving a prolonged acceleration and/or injection of the particles. A dependence of the spectral index on the longitudinal separation from the parent solar source region was not found. A statistical analysis of the spectral indices during impulsively rising events where the rise times are below 20 minutes is also shown.

Solar Electrons and Protons in the Events of September 4–10, 2017 and Related Phenomena

The solar proton events on September 4–10, 2017 motivated us to reconsider the hypothesis of the presence of two phases of acceleration of charged particles in solar flares in which nonrelativistic electrons are accelerated in the first phase, while relativistic electrons and protons are accelerated during the second phase. According to the data of SOHO/EPHIN (relativistic electrons) and ACS SPI (hard X-rays and protons with energy of >100 MeV), the populations of electrons and protons accelerated at the first and second phases of a flare could be separated in these events near the Earth. The data of observations are indicative of the realization of a stochastic mechanism of acceleration in flares according to which protons and electrons gain energy in many elementary acts, whose duration is much shorter than that of the flare itself. To reconcile the stochastic acceleration process with the existence of two phases in solar flares, it is necessary taking into account the gyrosynchrotron radiation losses of electrons that can be neglected at the first phase. The energy of accelerated protons at the first phase is too low for their detection in the Sun. However, in the second phase, it can reach levels sufficient for detection of nuclear and pion decay gamma lines. In this case, the role of coronal mass ejection consists in (1) involvement of an increasingly larger number of loops in the flare process at altitudes ranging from the chromosphere to the corona; (2) return of the accelerated particles into the flare region; (3) additional acceleration of particles at the shock front; (4) creation of conditions for escaping of particles into the interplanetary space in a wide spatial angle.

Case Study of Solar Wind Suprathermal Electron Acceleration at the Earth’s Bow Shock

Abstract We present a case study of the in situ acceleration of solar wind suprathermal electrons at the two quasi-perpendicular-bow-shock crossings on 2015 November 4, combining the Wind 3D Plasma and Energetic Particle measurements of ambient solar wind suprathermal electrons and Magnetospheric Multiscale mission measurements of shocked suprathermal electrons. In both cases, the omnidirectional differential fluxes of shocked suprathermal electrons in the downstream exhibit a double-power-law energy spectrum with a spectral index of ∼3 at energies below a downward break ε brk near 40 keV and index of ∼6 at energies above, different from the unshocked suprathermal electrons observed in the ambient solar wind. At energies below (above) ε brk, the observed electron flux ratio between the downstream and ambient solar wind, J D /J A , peaks near 90° PA (becomes roughly isotropic). Electrons at ε brk have an average electron gyrodiameter (across bow shock) comparable to the shock thickness. These suggest that the bow-shock acceleration of suprathermal electrons is likely dominated by the shock drift acceleration mechanism. For electrons at energies below (above) ε brk, their estimated drift time appears to be roughly energy independent (decrease with energy), leading to the formation of a double-power-law spectrum substantially steepening at a break that’s determined by the shock thickness.

Propagation of Scatter-free Solar Energetic Electrons in a Meandering Interplanetary Magnetic Field

Abstract We investigate energetic electron transport in a meandering interplanetary magnetic field under the scatter-free regime. The meandering magnetic field is adopted from the Giacalone model in which a single parameter V rms is used to characterize how the interplanetary magnetic field deviates from the Parker field. The trajectories of energetic electrons are followed in this meandering field using test particle simulations. Ten thousand electrons are injected in the ecliptic plane and the path length distributions are obtained at distances 0.2, 0.5, 1.0, 2.0, and 3.0 au from the Sun for five different V rms, 0.3, 0.6, 1.0, 1.5, 2.0, and 2.5 km s−1. By generating 10,000 different realizations of the meandering field line, we also obtain the path length distribution of the field lines. Our simulations show that the path length distributions of the electrons and that of the field lines are different and the difference increases with V rms. When the V rms approaches zero, the field lines approach the Parker field, and the differences between particle path lengths and field path lengths are small but nonzero due to the gradient and curvature drifts. The path lengths for 1 MeV electrons do not differ much from those for 100 MeV electrons. Our results of the distribution of electron path length can be compared to the observations of Zhao et al. to set constraints on V rms. We also calculate both the longitudinal and latitudinal displacements from the source when electrons arrive at 1 au. This provides some basis for understanding simultaneous observations of impulsive events made at multiple spacecraft.

Plasma properties of suprathermal electrons near comet 67P/Churyumov-Gerasimenko with Rosetta

Context. The Rosetta spacecraft escorted comet 67P/Churyumov-Gerasimenko from 2014 to September 2016. The mission provided in situ observations of the cometary plasma during different phases of the cometary activity, which enabled us to better understand its evolution as a function of heliocentric distance. Aims. In this study, different electron populations, called warm and hot, observed by the Ion and Electron Sensor (IES) of the Rosetta Plasma Consortium (RPC) are investigated near the comet during the escorting phase of the Rosetta mission. Methods. The estimates for the suprathermal electron densities and temperatures were extracted using IES electron data by fitting a double-kappa function to the measured velocity distributions. The fitting results were validated using observations from other RPC instruments. We give upgraded estimates for the warm and hot population densities compared to values previously shown in literature. Results. The fitted density and temperature estimates for both electron populations seen by IES are expressed as a function of heliocentric distance to study their evolution with the cometary activity. In addition, we studied the dependence between the electron properties and cometocentric distance. Conclusions. We observed that when the neutral outgassing rate of the nucleus is high (i.e., near perihelion) the suprathermal electrons are well characterized by a double-kappa distribution. In addition, warm and hot populations show a significant dependence with the heliocentric distance. The populations become clearly denser near perihelion while their temperatures are observed to remain almost constant. Moreover, the warm electron population density is shown to be strongly dependent on the radial distance from the comet. Finally, based on our results we reject the hypothesis that hot electron population seen by IES consists of solely suprathermal (halo) solar wind electrons, while we suggest that the hot electron population mainly consists of solar wind thermal electrons that have undergone acceleration near the comet.