Solar Wind Protons Research Articles

Ion density and temperature profiles along (XGSM) and across (ZGSM) the magnetotail as observed by THEMIS, Geotail, and ARTEMIS

AbstractCharacteristics of the two‐dimensional configuration of the magnetotail current sheet are important for modeling magnetotail motion/evolution and charged particle energization. Because of the magnetotail current sheet's dynamical nature, however, simultaneous plasma and magnetic field measurements at different radial distances are required to reveal this configuration. Simultaneous observations of the magnetotail current sheet from Time History of Events and Macroscale Interactions during Substorms (THEMIS) D (around 10RE downtail), Geotail (around 30RE downtail), and Acceleration, Reconnection, Turbulence and Electrodynamics of the Moons Interaction with the Sun (ARTEMIS) P1 (around 55RE downtail) are used to study distributions of plasma (ion) density and temperature along (Earth‐Sun direction) and across (north‐south direction) the magnetotail. Fourteen events (each including several current sheet crossings at different downtail distances) are studied. We demonstrate that the plasma temperature along and across the magnetotail varies more significantly than plasma density does. The temperature decrease from equatorial plane to current sheet boundaries is a major contributor to the cross‐tail pressure balance. The Alfven velocity VA,B calculated at the current sheet boundaries increases significantly toward the Earth from 700 km/s at lunar orbit ∼55RE to 2200 km/s around ∼10RE downtail. The corresponding energy (mp is the proton mass) is 4 times larger than the plasma temperature T0 in the magnetotail's equatorial plane, whereas the ratio EA/T0 is constant along the magnetotail. The plasma temperature T0 measured around lunar orbit in the magnetotail agrees well with the simultaneously measured energy of solar wind protons (VSW is the solar wind speed).

Interplay of Electron and Proton Instabilities in Expanding Solar Wind

Abstract Protons and electrons observed in the solar wind possess temperature anisotropies for which upper and lower bounds appear to be partially regulated by marginal conditions associated with various kinetic plasma instabilities. Such features are most clearly seen when a collection of measurements is plotted as a two-dimensional histogram in phase space. While the partial outer boundaries of such data distribution may well be explained by various instability threshold conditions, an outstanding issue is that the majority of data points are actually located sufficiently away from the boundaries and reside in near isotropic conditions. This implies that certain processes are operative that counteract the adiabatic effect in the radially expanding solar wind, without which solar wind plasma will inexorably be forced to proceed toward the marginal firehose condition. A number of physical processes have been proposed in the literature to explain such a feature. The present paper suggests yet another mechanism. It considers dynamic electrons and protons in the quasilinear evolution of anisotropy-driven instabilities, which is in contrast to previous studies where either protons or electrons are assumed to be stationary when considering the dynamics of the other particle species. It is shown that the dynamical interplay between the two species during the quasilinear development of parallel electron firehose and proton–cyclotron instabilities leads to a counter-balancing effect, which prevents the uniform progression of the solar wind protons toward the marginal firehose state.

The statistical mechanics of solar wind hydroxylation at the Moon, within lunar magnetic anomalies, and at Phobos

AbstractWe present a new formalism to describe the outgassing of hydrogen initially implanted by the solar wind protons into exposed soils on airless bodies. The formalism applies a statistical mechanics approach similar to that applied recently to molecular adsorption onto activated surfaces. The key element enabling this formalism is the recognition that the interatomic potential between the implanted H and regolith‐residing oxides is not of singular value but possess a distribution of trapped energy values at a given temperature, F(U,T). All subsequent derivations of the outward diffusion and H retention rely on the specific properties of this distribution. We find that solar wind hydrogen can be retained if there are sites in the implantation layer with activation energy values exceeding 0.5 eV. We especially examine the dependence of H retention applying characteristic energy values found previously for irradiated silica and mature lunar samples. We also apply the formalism to two cases that differ from the typical solar wind implantation at the Moon. First, we test for a case of implantation in magnetic anomaly regions where significantly lower‐energy ions of solar wind origin are expected to be incident with the surface. In magnetic anomalies, H retention is found to be reduced due to the reduced ion flux and shallower depth of implantation. Second, we also apply the model to Phobos where the surface temperature range is not as extreme as the Moon. We find the H atom retention in this second case is higher than the lunar case due to the reduced thermal extremes (that reduces outgassing).

Macroscopic quasilinear theory of parallel electron firehose instability associated with solar wind electrons

A number of different microinstabilities are known to be responsible for regulating the upper bound of temperature anisotropies in solar wind protons, alpha particles, and electrons. In the present paper, quasilinear kinetic theory is employed to investigate the time variation in electron temperature anisotropies in response to the excitation of parallel electron firehose instability in homogeneous and non-collisional solar wind plasma under the condition of T∥e>T⊥e. By assuming the bi-Maxwellian form of velocity distribution functions, various velocity moments of the particle kinetic equation are taken in order to reduce the theory to macroscopic model in which the wave-particle interaction is incorporated, hence, the macroscopic quasilinear theory. The threshold condition for the parallel electron firehose instability, empirically constructed as a curve in (β∥e,T⊥e/T∥e) phase space, is implicit in the present macroscopic quasilinear calculation. Even though the present calculation excludes the oblique firehose instability, which is known to possess a higher growth rate, the basic methodology may be further extended to include such a mode. Among the findings is that the parallel electron firehose instability dynamically couples the electrons and protons, which implies that this instability may be important for overall solar wind dynamics. The present analysis shows that the macroscopic quasilinear approach may eventually be incorporated in global-kinetic models of the solar wind electrons and ions.

Structure, dynamics, and seasonal variability of the Mars‐solar wind interaction: MAVEN Solar Wind Ion Analyzer in‐flight performance and science results

AbstractWe report on the in‐flight performance of the Solar Wind Ion Analyzer (SWIA) and observations of the Mars‐solar wind interaction made during the Mars Atmosphere and Volatile EvolutioN (MAVEN) prime mission and a portion of its extended mission, covering 0.85 Martian years. We describe the data products returned by SWIA and discuss the proper handling of measurements made with different mechanical attenuator states and telemetry modes, and the effects of penetrating and scattered backgrounds, limited phase space coverage, and multi‐ion populations on SWIA observations. SWIA directly measures solar wind protons and alpha particles upstream from Mars. SWIA also provides proxy measurements of solar wind and neutral densities based on products of charge exchange between the solar wind and the hydrogen corona. Together, upstream and proxy observations provide a complete record of the solar wind experienced by Mars, enabling organization of the structure, dynamics, and ion escape from the magnetosphere. We observe an interaction that varies with season and solar wind conditions. Solar wind dynamic pressure, Mach number, and extreme ultraviolet flux all affect the bow shock location. We confirm the occurrence of order‐of‐magnitude seasonal variations of the hydrogen corona. We find that solar wind Alfvén waves, which provide an additional energy input to Mars, vary over the mission. At most times, only weak mass loading occurs upstream from the bow shock. However, during periods with near‐radial interplanetary magnetic fields, structures consistent with Short Large Amplitude Magnetic Structures and their wakes form upstream, dramatically reconfiguring the Martian bow shock and magnetosphere.

SELF-VLF Electromagnetic Signals and Solar Wind Proton Density Variations that Preceded the M6.2 Central Italy Earthquake on August 24, 2016

The authors of this study wanted to verify a possible relationship between the M6.2 earthquake, which occurred Accumoli (RI), Italy on August 24, 2016 at 01:36:33 UTC and solar and geomagnetic activity. In recent years was found that the global M6 + seismic activity is always preceded by an increase in solar activity and the future of scientific research on earthquake prediction will be destined also to the study of the Sun (heliophysics). The analysis of the characteristics of the solar wind near Earth and the analysis of the Earth's geomagnetic activity confirmed that the strong M6.2 earthquake recorded in Italy on August 24, 2016 was preceded by an increase in solar activity and in Earth's geomagnetic activity.

Multifractal analysis of high resolution solar wind proton density measurements

The solar wind is a highly turbulent medium, with a high level of field fluctuations throughout a broad range of scales. These include an inertial range where a turbulent cascade is assumed to be active. The solar wind cascade shows intermittency, which however may depend on the wind conditions. Recent observations have shown that ion-scale magnetic turbulence is almost self-similar, rather than intermittent. A similar result was observed for the high resolution measurements of proton density provided by the spacecraft Spektr-R. Intermittency may be interpreted as the result of the multifractal properties of the turbulent cascade. In this perspective, this paper is devoted to the description of the multifractal properties of the high resolution density measurements. In particular, we have used the standard coarse-graining technique to evaluate the generalized dimensions Dq, and from these the multifractal spectrum f(α), in two ranges of scale. A fit with the p-model for intermittency provided a quantitative measure of multifractality. Such indicator was then compared with alternative measures: the width of the multifractal spectrum, the peak of the kurtosis, and its scaling exponent. The results indicate that the small-scale fluctuations are multifractal, and suggest that different measures of intermittency are required to fully understand the small scale cascade.

The interplanetary and magnetospheric causes of extreme dB/dt at equatorial locations

AbstractThe 1 min resolution solar wind and geomagnetic data obtained from seven equatorial/low‐latitude stations during four extreme geomagnetic activities are used to investigate the extreme dB/dt perturbations. Simulations of the magnetospheric‐ionospheric environment were also performed for varying amplitudes of the solar proton density. Simulations were carried out using the Space Weather Modeling Framework/BATS‐R‐US + RCM model. Both the observations and simulations demonstrated that the appearance time of the extreme dB/dt perturbations at equatorial stations during disturbed conditions is instantaneous and equitable to those experienced at auroral regions yielding time lags of the order of a few seconds. We find that the rapid dB/dt enhancements are caused by the electric field of magnetospheric current origin, which is being enhanced by solar wind density and ram pressure variations and boosted by the equatorial electrojet. Our results indicate that the solar wind proton density variations could be used as a predictor of extreme dB/dt enhancement at equatorial latitudes.

LONG-TERM TRENDS IN THE SOLAR WIND PROTON MEASUREMENTS

ABSTRACT We examine the long-term time evolution (1965–2015) of the relationships between solar wind proton temperature (T p) and speed (V p) and between the proton density (n p) and speed using OMNI solar wind observations taken near Earth. We find a long-term decrease in the proton temperature–speed (T p–V p) slope that lasted from 1972 to 2010, but has been trending upward since 2010. Since the solar wind proton density–speed (n p–V p) relationship is not linear like the T p–V p relationship, we perform power-law fits for n p–V p. The exponent (steepness in the n p–V p relationship) is correlated with the solar cycle. This exponent has a stronger correlation with current sheet tilt angle than with sunspot number because the sunspot number maxima vary considerably from cycle to cycle and the tilt angle maxima do not. To understand this finding, we examined the average n p for different speed ranges, and found that for the slow wind n p is highly correlated with the sunspot number, with a lag of approximately four years. The fast wind n p variation was less, but in phase with the cycle. This phase difference may contribute to the n p–V p exponent correlation with the solar cycle. These long-term trends are important since empirical formulas based on fits to T p and V p data are commonly used to identify interplanetary coronal mass ejections, but these formulas do not include any time dependence. Changes in the solar wind density over a solar cycle will create corresponding changes in the near-Earth space environment and the overall extent of the heliosphere.

Statistical analysis of suprathermal electron drivers at 67P/Churyumov–Gerasimenko

We use observations from the Ion and Electron Sensor (IES) onboard the Rosetta spacecraft to study the relationship between the cometary suprathermal electrons and the drivers that affect their density and temperature. We fit the IES electron observations with the summation of 2 kappa distributions, which we characterize as a dense and warm population (∼10 cm−3 and ∼16 eV) and a rarefied and hot population (∼0.01 cm−3 and ∼43 eV). The parameters of our fitting technique determine the populations’ density, temperature, and invariant kappa index. We focus our analysis on the warm population to determine its origin by comparing the density and temperature to the neutral density and magnetic field strength. We find that the warm electron population is actually two separate sub-populations: electron distributions with temperatures above 8.6 eV and electron distributions with temperatures below 8.6 eV. The two sub-populations have different relationships between their density and temperature. Moreover, the two sub-populations are affected by different drivers. The hotter sub-population temperature is strongly correlated with neutral density, while the cooler sub-population is unaffected by neutral density and is only weakly correlated to magnetic field strength. We suggest that the population with temperatures above 8.6 eV is being heated by lower hybrid waves driven by counterstreaming solar wind protons and newly-formed, cometary ions created in localized, dense neutral streams. To the best of our knowledge, this represents the first observations of cometary electrons heated through wave-particle interactions.

PROTON HEATING BY PICK-UP ION DRIVEN CYCLOTRON WAVES IN THE OUTER HELIOSPHERE: HYBRID EXPANDING BOX SIMULATIONS

ABSTRACT Using a one-dimensional hybrid expanding box model, we investigate properties of the solar wind in the outer heliosphere. We assume a proton–electron plasma with a strictly transverse ambient magnetic field and, aside from the expansion, we take into account the influence of a continuous injection of cold pick-up protons through the charge-exchange process between the solar wind protons and hydrogen of interstellar origin. The injected cold pick-up protons form a ring distribution function, which rapidly becomes unstable, and generate Alfvén cyclotron waves. The Alfvén cyclotron waves scatter pick-up protons to a spherical shell distribution function that thickens over that time owing to the expansion-driven cooling. The Alfvén cyclotron waves heat solar wind protons in the perpendicular direction (with respect to the ambient magnetic field) through cyclotron resonance. At later times, the Alfvén cyclotron waves become parametrically unstable and the generated ion-acoustic waves heat protons in the parallel direction through Landau resonance. The resulting heating of the solar wind protons is efficient on the expansion timescale.

Modification of Velocity Power Spectra by Thermal Plasma Instrumentation

The upcoming Solar Probe Plus mission (Launch 2018) will launch with the newest and fastest space plasma instrumentation to date. The Solar Wind Electrons, Alphas, and Protons (SWEAP) instrument suite, which measures thermal plasma, will make measurements faster than the local gyro-frequency and proton plasma frequency. By developing an end-to-end computer model of a SWEAP instrument, this work explores the specific instrumental effects of thermal space plasma measurement, particularly in the reproduction of velocity power spectra, or Power Spectral Densities (PSDs). This model reproduces the slowest measurement cadence of the Solar Probe Cup (SPC), a Faraday cup (FC) style instrument on that will measure thermal plasma density, velocity, and temperature on SPP. By using the calibrated model to model measurement of fully determined and synthetic turbulent time series data, a consistent underestimation of the velocity power spectral indices has been quantified, with possible implications for previous missions flying similar instrumentation.

Transport of solar wind plasma onto the lunar nightside surface

AbstractWe present first measurements of energetic neutral atoms that originate from solar wind plasma having interacted with the lunar nightside surface. We observe two distinct energetic neutral atom (ENA) distributions parallel to the terminator, the spectral shape, and the intensity of both of which indicate that the particles originate from the bulk solar wind flow. The first distribution modifies the dayside ENA flux to reach ∼6° into the nightside and is well explained by the kinetic temperature of the solar wind protons. The second distribution, which was not predicted, reaches from the terminator to up to 30° beyond the terminator, with a maximum at ∼102° in solar zenith angle. As most likely wake transport processes for this second distribution we identify acceleration by the ambipolar electric field and by the negatively charged lunar nightside surface. In addition, our data provide the first observation indicative of a global solar zenith angle dependence of positive dayside surface potentials.

Proton impact charge transfer on hydantoin – Prebiotic implications

Formation and destruction of prebiotic compounds in astrophysical environments is a major issue in reactions concerning the origin of life. Detection of hydantoin in laboratory irradiation of interstellar ice analogues has confirmed evidence of this prebiotic compound and its stability to UV radiation or collisions may be crucial. Considering the different astrophysical environments, we have investigated theoretically proton-induced collisions with hydantoin in a wide energy range, from eV in the interstellar medium, up to keV for processes involving solar wind or supernovae shock-waves protons. Results are compared to previous investigations and qualitative trends on damage under spatial radiations are suggested.

On the origin of the ionosphere at the Moon using results from Chandrayaan‐1 S band radio occultation experiment and a photochemical model

AbstractThe origin of the Moon's ionosphere has been explored using Chandrayaan‐1 radio occultation (RO) measurements and a photochemical model. The electron density near the Moon's surface, obtained on 31 July 2009 (∼300 cm−3), is compared with results from a model which includes production and recombination of 16 ions, solar wind proton charge exchange, and the electron impact ionization. The model calculations suggest that in the absence of transport, inert ions, namely Ar+, Ne+, and He+, dominate lunar ionosphere (density ∼5 × 104 cm−3). Interaction with solar wind, however, leads to their complete removal (∼2–3 cm−3). Assuming the Moon's exosphere to have CO2, H2O, O, OH, H2, CH4, and CO molecules in addition to the inert gases, the model calculations suggest that the lunar ionosphere is dominated by molecular ions, namely H2O+, CO , and H3O+, with near‐surface density ∼250 cm−3. We surmise that lunar ionosphere can be molecular in nature.

The Low-Energy Neutral Imager (LENI).

To achieve breakthroughs in the areas of heliospheric and magnetospheric energetic neutral atom (ENA) imaging, a new class of instruments is required. We present a high angular resolution ENA imager concept aimed at the suprathermal plasma populations with energies between 0.5 and 20 keV. This instrument is intended for understanding the spatial and temporal structure of the heliospheric boundary recently revealed by Interstellar Boundary Explorer instrumentation and the Cassini Ion and Neutral Camera. The instrument is also well suited to characterize magnetospheric ENA emissions from low‐altitude ENA emissions produced by precipitation of magnetospheric ions into the terrestrial upper atmosphere, or from the magnetosheath where solar wind protons are neutralized by charge exchange, or from portions of the ring current region. We present a new technique utilizing ultrathin carbon foils, 2‐D collimation, and a novel electron optical design to produce high angular resolution (≤2°) and high‐sensitivity (≥10−3 cm2 sr/pixel) ENA imaging in the 0.5–20 keV energy range.

On the evolution of the κ distribution of protons in the inner heliosheath

AbstractThe evolution of the solar wind proton distribution function along the plasma flow downstream of the heliospheric termination shock is studied. Starting from a kinetic phase space transport equation valid in the bulk frame of the plasma flow that takes into account convective changes, cooling, velocity diffusion, and charge exchange‐induced injection and loss, the associated moment equation for the evolution of the pressure of the total proton population consisting of thermal solar wind and suprathermal pickup protons is derived. Assuming that the local joint proton distribution can always be represented by a so‐called κ distribution, an ordinary differential equation for the variation of the parameter κ along a given streamline is obtained. This way, the proton velocity distribution can be computed for the whole inner heliosheath. It is demonstrated that the accelerating effect of velocity diffusion is to be expected to overcompensate the loss effect on the proton distribution at higher velocities due to charge exchange with cold interstellar hydrogen atoms. While this corroborates a value of κ < 1.65 in the inner (upwind) heliosheath, at the same time it reveals that the assumption of a constant κ, which was commonly made in earlier studies, should be abandoned.

The causes of the hardest electron precipitation events seen with SAMPEX

AbstractWe studied the geomagnetic, plasmaspheric, and solar wind context of relativistic electron precipitation (REP) events seen with the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), Proton Electron Telescope (PET) to derive an exponential folding energy E0 for each event. Events with E0< 400 keV peak near midnight, and with increasing E0, the peak magnetic local time (MLT) moves earlier but never peaks as early as the MLT distribution of electromagnetic ion cyclotron (EMIC) waves in the outer belt, and a distinct component near midnight remains. Events with E0>750 keV near dusk (1400 < MLT < 2000) show correlations with solar wind dynamic pressure and proton density, AE index, negative Dst index, and an extended plasmasphere, all supporting an EMIC wave interpretation. Events with 500 keV <E0< 600 keV near midnight (MLT 2200–0200) do not show these correlations. Comparing these two samples to all events with E0>500 keV (“hard REP”), we estimate that roughly 45% of the whole population has the distributions of geomagnetic and solar wind parameters associated with EMIC waves, while 55% does not. We hypothesize that the latter events may be caused by current sheet scattering (CSS), which can be mistaken for EMIC wave scattering in that both simultaneously precipitate MeV electrons and keV protons. Since a large number of MeV electrons are lost in the near‐midnight hard REP events, and in the large number of E0< 400 keV events that show no dusk‐like peak at all, we conclude that CSS should be studied further as a possibly important loss channel for MeV electrons.

Solar wind‐driven variations of electron plasma sheet densities and temperatures beyond geostationary orbit during storm times

AbstractThe empirical models of the plasma sheet electron temperature and density on the nightside at distances between 6 and 11 RE are constructed based on Time History of Events and Macroscale Interactions During Substorms (THEMIS) particle measurements. The data set comprises ∼400 h of observations in the plasma sheet during geomagnetic storm periods. The equatorial distribution of the electron density reveals a strong earthward gradient and a moderate variation with magnetic local time symmetric with respect to the midnight meridian. The electron density dependence on the external driving is parameterized by the solar wind proton density averaged over 4 h and the southward component of interplanetary magnetic field (IMF BS) averaged over 6 h. The interval of the IMF integration is much longer than a typical substorm growth phase, and it rather corresponds to the geomagnetic storm main phase duration. The solar wind proton density is the main controlling parameter, but the IMF BS becomes of almost the same importance in the near‐Earth region. The root‐mean‐square deviation between the observed and predicted plasma sheet density values is 0.23 cm−3, and the correlation coefficient is 0.82. The equatorial distribution of the electron temperature has a maximum in the postmidnight to morning MLT sector, and it is highly asymmetric with respect to the local midnight. The electron temperature model is parameterized by solar wind velocity (averaged over 4 h), IMF BS (averaged over 45 min), and IMF BN (northward component of IMF, averaged over 2 h). The solar wind velocity is a major controlling parameter, and IMF BS and BN are comparable in importance. In contrast to the density model, the electron temperature shows higher correlation with the IMF BS averaged over ∼45 min (substorm growth phase time scale). The effect of BN manifests mostly in the outer part of the modeled region (r > 8RE). The influence of the IMF BS is maximal in the midnight to postmidnight MLT sector. The correlation coefficient between the observed and predicted plasma sheet electron temperature values is 0.76, and the root‐mean‐square deviation is 2.6 keV. Both models reveal better performance in the dawn MLT sector.

Modified ion-Weibel instability as a possible source of wave activity at Comet 67P/Churyumov-Gerasimenko

Abstract. We analytically discuss wave excitation in a homogeneous three component plasma consisting of solar wind protons, electrons and a beam of cometary water ions applied to the plasma environment of comet 67P/Churyumov-Gerasimenko. The resulting dispersion relations are studied in a solar wind rest frame, where a cometary current is solely generated by the water ion beam, and a cometary rest frame representing the rest frame of the Rosetta spacecraft. A modified ion-Weibel instability is excited by the cometary current and predominantly grows perpendicular to this current. The corresponding water ion mode is connected to a frequency of about 40 mHz in agreement with wave measurements of Rosetta's magnetometer in the cometary rest frame. Furthermore, the superposition of the strongest growing waves result in a fan-like phase structure close to the comet.