Solar Wind Heat Flux Research Articles

Oceanic response to tropical cyclone in the northern South China Sea observed by underwater gliders during 2018 and 2020

In this study, we mainly used in-situ observations from underwater gliders to analyze the ocean response in the northern South China Sea affected by Son-tinh (2018), Mandal et al. (2018) Mangkhut (2018)and Noul (2020). The results showed that these TCs caused 0.6 °C, 1.1 °C and 1.7 °C maximum sea surface temperature cooling respectively, which were weaker than general conditions because of long distance, weak intensity and fast movement speed. Net solar radiation, precipitation, 10-m wind and sea surface heat flux also made contribution in changes of SST. The mixed layer depth (MLD) became shallower after Son-Tinh and Noul passed through, while during Mangkhut it did not change significantly. After TCs passed through, the stratification around MLD became more obvious, with a banded distribution and stronger high-value areas of buoyancy frequency. Within 1 week after the shortest distance, the temperature and salinity responses in the upper ocean were stronger than those at the sea surface, and the gradients of temperature and salinity and their anomalies were more evident in the subsurface layer. The results of this study show that underwater glider observations are important for understanding oceanic responses to tropical cyclones and are useful for studying tropical cyclone activities.

Whistler Waves in the Young Solar Wind: Statistics of Amplitude and Propagation Direction from Parker Solar Probe Encounters 1–11

In the interplanetary space solar wind plasma, whistler waves are observed in a wide range of heliocentric distances (from ∼20 solar radii (RS) to Jupiter’s orbit). They are known to interact with solar wind suprathermal electrons (strahl and halo) and to regulate the solar wind heat flux through scattering the strahl electrons. We present the results of applying the technique to determine the whistler wave propagation directions to the spectral data continuously collected by the FIELDS instruments on board Parker Solar Probe (PSP). The technique was validated based on the results obtained from burst mode magnetic and electric field waveform data collected during Encounter 1. We estimated the effective length of the PSP electric field antennas for a variety of solar wind conditions in the whistler wave frequency range and utilized these estimates for determining the whistler wave properties during PSP Encounters 1–11. Our findings show that (1) the enhancement of the whistler wave occurrence rate and wave amplitudes observed between 25 and 35 RS is predominantly due to the sunward-propagating whistler wave population associated with the switchback-related magnetic dips; (2) the antisunward or counterpropagating cases are observed at 30–40 RS; (3) between 40 and 50 RS, sunward and antisunward whistlers are observed with comparable occurrence rates; and (4) almost no sunward or counterpropagating whistlers were observed at heliocentric distances above 50 RS.

Whistler waves generated inside magnetic dips in the young solar wind: Observations of the search-coil magnetometer on board Parker Solar Probe

Context. Whistler waves are electromagnetic waves produced by electron-driven instabilities, which in turn can reshape the electron distributions via wave–particle interactions. In the solar wind they are one of the main candidates for explaining the scattering of the strahl electron population into the halo at increasing radial distances from the Sun and for subsequently regulating the solar wind heat flux. However, it is unclear what type of instability dominates to drive whistler waves in the solar wind. Aims. Our goal is to study whistler wave parameters in the young solar wind sampled by Parker Solar Probe (PSP). The wave normal angle (WNA) in particular is a key parameter to discriminate between the generation mechanisms of these waves. Methods. We analyzed the cross-spectral matrices of magnetic field fluctuations measured by the search-coil magnetometer (SCM) and processed by the Digital Fields Board (DFB) from the FIELDS suite during PSP’s first perihelion. Results. Among the 2701 wave packets detected in the cross-spectra, namely individual bins in time and frequency, most were quasi-parallel to the background magnetic field; however, a significant part (3%) of the observed waves had oblique (> 45°) WNA. The validation analysis conducted with the time series waveforms reveal that this percentage is a lower limit. Moreover, we find that about 64% of the whistler waves detected in the spectra are associated with at least one magnetic dip. Conclusions. We conclude that magnetic dips provide favorable conditions for the generation of whistler waves. We hypothesize that the whistlers detected in magnetic dips are locally generated by the thermal anisotropy as quasi-parallel and can gain obliqueness during their propagation. We finally discuss the implications of our results for the scattering of the strahl in the solar wind.

Parametric analysis of heat flux inhibition in the solar wind: a macroscopic quasilinear approach

Abstract Magnitudes of electron temperature anisotropy and solar wind heat flux are defined with different physical mechanisms e.g. microinstabilities, interparticle collisions, and adiabatic expansion. In the dilute space plasma limit, the present study assumes the interplay between anisotropic core-halo electron components, their relative drift, and relative density of the halo electrons to determine the dynamics of backward and forward-propagating whistler heat flux instabilities along the ambient magnetic field. To investigate the feedback effects of these micro-instabilities in reshaping solar wind distributions and the total heat flux regulation, we formulate quasilinear kinetic equations on the basis of taking the macroscopic velocity moments. For the same input parameters of linear analysis, numerical solutions of the quasilinear equations indicate the time-scale variations, electrons and protons population, wave intensities, and constraints on the heat flux. In future perspective of the global-kinetic solar wind model, the present formalism may be an important step with the inclusion of radial and nonthermal effects.

On the Role of Solar Wind Expansion as a Source of Whistler Waves: Scattering of Suprathermal Electrons and Heat Flux Regulation in the Inner Heliosphere

Abstract The role of solar wind expansion in generating whistler waves is investigated using the EB-iPic3D code, which models solar wind expansion self-consistently within a fully kinetic semi-implicit approach. The simulation is initialized with an electron velocity distribution function modeled after observations of the Parker Solar Probe during its first perihelion at 0.166 au, consisting of a dense core and an antisunward strahl. This distribution function is initially stable with respect to kinetic instabilities. Expansion drives the solar wind into successive regimes where whistler heat flux instabilities are triggered. These instabilities produce sunward whistler waves initially characterized by predominantly oblique propagation with respect to the interplanetary magnetic field. The excited waves interact with the electrons via resonant scattering processes. As a consequence, the strahl pitch angle distribution broadens and its drift velocity reduces. The strahl electrons are scattered in the direction perpendicular to the magnetic field, and an electron halo is formed. At a later stage, resonant electron firehose instability is triggered and further affects the electron temperature anisotropy as the solar wind expands. Wave–particle interaction processes are accompanied by a substantial reduction of the solar wind heat flux. The simulated whistler waves are in qualitative agreement with observations in terms of wave frequencies, amplitudes, and propagation angles. Our work proposes an explanation for the observations of oblique and parallel whistler waves in the solar wind. We conclude that solar wind expansion has to be factored in when trying to explain kinetic processes at different heliocentric distances.

Constraining Suprathermal Electron Evolution in a Parker Spiral Field With Cassini Observations

AbstractSuprathermal electrons in the solar wind consist of the “halo,” present at all pitch angles, and the “strahl” which is a field‐aligned, beam‐like population. Examining the heliospheric evolution of strahl beams is key to understanding the in‐transit processing of solar wind suprathermal electrons, in particular, to identify electron scattering mechanisms and to establish the origin of the halo population. Not only does this have significant implications with regard to the kinetic processes occurring within the solar wind but also its thermodynamic evolution, as the suprathermal electrons carry the majority of the solar wind heat flux. In this investigation, an established model for suprathermal electron evolution in a Parker spiral interplanetary magnetic field is adapted from its original use. The model is constrained using solar wind strahl observed by the Cassini mission on its interplanetary journey to Saturn. The effects of large scale IMF geometry due to different solar wind velocities and application of different electron scattering factors are examined. It is found that slow solar wind speeds provide the closest match to the strahl width observations, both in terms of radial distance and electron energy trends, and that predominantly slower solar wind speeds were therefore likely observed by the Cassini mission en‐route to Saturn. It is necessary to include a strahl scattering factor which increases with electron energy in order to match observations, indicating that the strahl scattering mechanism must have an inherent energy dependence.

Clarifying the solar wind heat flux instabilities

In the solar wind electron velocity distributions reveal two counter-moving populations which may induce electromagnetic (EM) beaming instabilities known as heat flux instabilities. Depending on plasma parameters two distinct branches of whistler and firehose instabilities can be excited. These instabilities are invoked in many scenarios, but their interplay is still poorly understood. An exact numerical analysis is performed to resolve the linear Vlasov-Maxwell dispersion and characterize these two instabilities, e.g., growth rates, wave frequencies and thresholds, enabling to identify their dominance for conditions typically experienced in space plasmas. Of particular interest are the effects of suprathermal Kappa-distributed electrons which are ubiquitous in these environments. The dominance of whistler or firehose instability is highly conditioned by the beam-core relative velocity, core plasma beta and the abundance of suprathermal electrons. Derived in terms of relative drift velocity the instability thresholds show an inverse correlation with the core plasma beta for the whistler modes, and a direct correlation with the core plasma beta for the firehose instability. Suprathermal electrons reduce the effective (beaming) anisotropy inhibiting the firehose modes while the whistler instability is stimulated.

WHISTLER MODE WAVES AND THE ELECTRON HEAT FLUX IN THE SOLAR WIND:CLUSTEROBSERVATIONS

The nature of the magnetic field fluctuations in the solar wind between the ion and electron scales is still under debate. Using the Cluster/STAFF instrument, we make a survey of the power spectral density and of the polarization of these fluctuations at frequencies $f\in[1,400]$ Hz, during five years (2001-2005), when Cluster was in the free solar wind. In $\sim 10\%$ of the selected data, we observe narrow-band, right-handed, circularly polarized fluctuations, with wave vectors quasi-parallel to the mean magnetic field, superimposed on the spectrum of the permanent background turbulence. We interpret these coherent fluctuations as whistler mode waves. The life time of these waves varies between a few seconds and several hours. Here we present, for the first time, an analysis of long-lived whistler waves, i.e. lasting more than five minutes. We find several necessary (but not sufficient) conditions for the observation of whistler waves, mainly a low level of the background turbulence, a slow wind, a relatively large electron heat flux and a low electron collision frequency. When the electron parallel beta factor $\beta_{e\parallel}$ is larger than 3, the whistler waves are seen along the heat flux threshold of the whistler heat flux instability. The presence of such whistler waves confirms that the whistler heat flux instability contributes to the regulation of the solar wind heat flux, at least for $\beta_{e\parallel} \ge$ 3, in the slow wind, at 1 AU.

Solar cycle variations in the electron heat flux: Ulysses observations

Solar wind observations by the Ulysses spacecraft now include nearly ten years of continuous ion and electron measurements. In this study, we report detailed measurements of the electron heat flux in the solar wind. In particular, we examine the heat flux measurements for long‐term correlations with wave activity and solar wind speed. We find that the average heat flux, when scaled by R2.9 to account for variations due to distance from the Sun, is constant and independent of heliographic latitude or solar cycle. We find that during both solar maximum and solar minimum, there is no significant correlation between the magnitude of the electron heat flux and the solar wind speed. Comparison of the electron heat flux data with wave activity indicates that the whistler heat flux instability does not play an important role in limiting the solar wind heat flux.

Heat flux dropouts in the solar wind and Coulomb scattering effects

Measurements of solar wind electrons at ISEE 3 located 0.01 AU upstream from the Earth (McComas et al., 1989) indicate periods of time when the flux of antisunward suprathermal electrons decreases suddenly, leaving the velocity distribution nearly isotropic and causing the solar wind heat flux to drop. These heat flux dropouts (HFDs) are usually found in regions of increased plasma density and decreased electron temperature, and they are associated with sector boundaries. It has been suggested that HFDs may be due either to disconnection from the Sun of the magnetic flux tube in which they are found, or to enhanced Coulomb scattering of halo electrons in transit from the Sun to the Earth. Using the vector electron spectrometer on ISEE 1, we have found eight intervals of greatly reduced heat flux which appear to be associated with HFDs at ISEE 3. Five of the eight events were delayed by an appropriate convection time and had approximately the same duration as the corresponding ISEE 3 event. The onset times for the other three intervals of reduced heat flux could not be determined in the ISEE 1 data. Other events from the ISEE 3 set were not observed because ISEE 1 either was not in the solar wind or was inside the foreshock, which obscured the HFD identification. Velocity distributions during HFDs at ISEE 1 show that the depletion of halo electrons traveling away from the Sun is most pronounced in the 100‐eV range, while there is essentially no depletion in the 1‐keV range, and that in four cases the magnitude of the halo depletion and its upper velocity limit both depend on the density increase in the HFD. These results are shown to be in agreement with the υ−3 dependence of the Coulomb collision frequency. Thus we conclude that Coulomb scattering effects play a substantial role in at least some heat flux dropout events.

Bidirectional solar wind electron heat flux and hemispherically symmetric polar rain

The nearly continuous precipitation of low‐energy electrons into the earth's polar caps is known as polar rain. Normally polar rain is much more intense in one polar cap than in the other. Recently Makita and Meng have shown that polar rain is occasionally (approximately once a month for a period of ∼10 hours at a time) observed at high intensity levels simultaneously in both polar caps. Such events are known as hemispherically symmetric polar rain. We have examined ISEE 3 solar wind electron data obtained concurrent with reported symmetric polar rain events and have found that a bidirectional solar wind electron heat flux is present whenever such polar cap events occur. We interpret this result as a demonstration that the solar wind heat flux is the prime source of polar rain. A bidirectional solar wind heat flux is evidence that the local interplanetary magnetic field is part of either a magnetic bottle rooted at both ends in the sun or a closed magnetic loop entirely disconnected from the sun. Thus, we infer that, in contrast to the normal situation when only one of the earth's polar caps is magnetically connected to the sun, during hemispherically symmetric polar rain events either both of the earth's polar caps are magnetically connected to the sun, or else both are connected to a magnetic loop which is entirely disconnected from the sun. The relative timing between bidirectional solar wind heat flux and symmetrical polar rain events can be utilized to determine certain magnetospheric quantities such as the cross‐tail convection speed.

Electromagnetic lower hybrid instability in the solar wind

Low frequency electromagnetic lower hybrid waves (so-called hybrid whistlers) propagating nearly transverse to the magnetic field can be driven unstable by a resonant interaction with ‘halo’ electron distributions carrying solar wind heat flux. The electromagnetic lower hybrid instability is excited when the ‘halo’ electron drift exceeds the parallel phase velocity of the wave. The growth rate attains a maxima at a certain value of the wavenumber. The maximum growth rate decrease by an increase in β⊥e (the ratio of electron pressure to magnetic field pressure) and ‘halo’ electron temperature anisotropy. At 0.3 AU the growth time of the electromagnetic lower hybrid instability is of the order of 25 ms or shorter, whereas the most unstable wavelengths associated with the instability fall typically in a range of 27 to 90 km. The instability would give rise to a local heating of solar wind ions and electrons in the perpendicular and parallel directions relative to the magnetic field, B0. The observations of low frequency whistlers having high values ofB/E ratios (B andE being the magnitude of the wave magnetic and electric field, respectively) and propagating at large oblique angles to B0 behind interplanetary shocks, can be satisfactorily explained in terms of electromagnetic lower hybrid instability. The instability is also relevant to the generation mechanism of correlated whistler and electron plasma oscillation bursts detected on ISEE-3.

Electromagnetic lower hybrid instability driven by solar wind heat flux

Under a fully electromagnetic treatment, the threshold for excitation of the lower hybrid instability driven by solar wind electron heat flux is found to be much higher than that predicted by electrostatic approximation. For average solar wind conditions at 1 AU, the fully electromagnetic lower hybrid instability is excited when the core electron drift speed is about 8VA, whereVA is the Alfven speed. The region between the Sun and 1 AU is expected to be more favourable than 1 AU for this instability.

Regulation of solar wind heat flux by ordinary mode instability

The ordinary mode can frequently become unstable in the solar wind at 1 AU provided the ratio of halo to core electrons density does not exceed the value 0.05. The growth rates corresponding to the average conditions are typically ∼10ΩP (ΩP being the proton cyclotron frequency). Because of low threshold for onset of instability forβ⊥C ≳ 1 (whereβ⊥C is the transverse beta for the core electrons), the mode is expected to play an important role in regulating the solar wind heat flux at 1 AU.

Solar wind heat flux regulation by the whistler instability

This paper studies the role of the whistler instability in the regulation of the solar wind heat flux near 1 AU. A comparison of linear and second-order theory with experimental results provides strong evidence that the whistler may at times contribute to the limitation of this heat flux.

Evidence for the regulation of solar wind heat flux at 1 AU

Observational evidence favoring the local regulation of solar-wind heat flux at 1 AU is reviewed, and four months of IMP 6 plasma and magnetic-field data are merged and analyzed in order to investigate what might be regulating the heat flux. A statistical analysis of the data shows that the solar-wind Alfven speed is probably regulating the heat flux locally at 1 AU and that the Alfven speed, the velocity difference between the peak of low-energy electrons and the bulk plasma velocity, and the solar-wind velocity component projected along the local spiral angle are statistically well correlated for Alfven speeds not exceeding about 70 km/s. A time-series analysis of the data indicates that only the Alfven speed and the velocity difference between the peak of low-energy electrons and the bulk plasma velocity are well correlated both qualitatively and quantitatively on a microscopic time scale. It is strongly suggested that, at times, the solar-wind heat flux is locally regulated by the magnitude of the Alfven speed at 1 AU. Uncertainties in the results are discussed.

Electron parameter correlations in high-speed streams and heat flux instabilities

Statistical electron parameter correlations associated with high-speed streams are determined with the aim of identifying one or more locally active solar wind heat flux instabilities. Evidence that points toward local regulation of the heat flux at 1 AU is presented, and the results of a search for special signatures expected from the action of the Alfven, magnetosonic, and whistler flux instabilities are discussed. It is shown that under certain conditions, the whistler mode can be active in regulating the heat flux at 1 AU.

Solar wind electrons

Average characteristics of solar wind electron velocity distributions as well as the range and nature of their variations are presented. The measured distributions are generally symmetric about the heat flux direction and are adequately parameterized by the superposition of a nearly bi-Maxwellian function which characterizes the low-energy electrons and a bi-Maxwellian function which characterizes a distinct, ubiquitous component of higher-energy electrons. An alternate self-consistent description of the higher-energy component is presented in terms of an unbound population of hot electrons with energy greater than some breakpoint energy of ≃60 V. The largest-scale parameter variations appear to come most often in association with high-speed streams. The salient electron parameter variations associated with these structures are presented and discussed. The mechanism by which interplanetary electrons conduct heat is convection of the hot component relative to the bulk speed. Arguments are presented which favor the local regulation of the solar wind heat flux at 1 AU.