Solar Wind Electron Heat Flux Research Articles

The Regulation of the Solar Wind Electron Heat Flux by Wave–Particle Interactions

The solar wind electrons carry a significant heat flux into the heliosphere. The weakly collisional state of the solar wind implicates collisionless processes as the primary factor that constrains nonthermal features of the velocity distribution function (VDF), including the heat flux. Previous observational work suggests that the electron VDF sometimes becomes unstable to the whistler wave, but reliance on model VDFs (e.g., drifting bi-Maxwellians) has proven insufficient for an exact description of the behavior of the solar wind electrons—in particular, the regulation of the heat flux. The characterization of these processes requires methods to obtain fine details of the VDF and quantification of the impact of kinetic processes on the VDF. We employ measurements of the electron VDF by Solar Orbiter’s Solar Wind Analyser and of the magnetic field by the Radio and Plasma Waves instrument to study an unstable solar wind electron configuration. Through a Hermite–Laguerre expansion of the VDF, we implement a low-pass filter in velocity space to remove velocity space noise and obtain a VDF suitable for analysis. With our method, we directly measure the instability growth rate and the rate of change of the electron heat flux through wave–particle interactions.

Electron heat flux in the near-Sun environment

Aims. We survey the electron heat flux observed by the Parker Solar Probe (PSP) in the near-Sun environment at heliocentric distances of 0.125–0.25 AU. Methods. We utilized measurements from the Solar Wind Electrons Alphas and Protons and FIELDS experiments to compute the solar wind electron heat flux and its components and to place these in context. Results. The PSP observations reveal a number of trends in the electron heat flux signatures near the Sun. The magnitude of the heat flux is anticorrelated with solar wind speed, likely as a result of the lower saturation heat flux in the higher-speed wind. When divided by the saturation heat flux, the resulting normalized net heat flux is anticorrelated with plasma beta on all PSP orbits, which is consistent with the operation of collisionless heat flux regulation mechanisms. The net heat flux also decreases in very high beta regions in the vicinity of the heliospheric current sheet, but in most cases of this type the omnidirectional suprathermal electron flux remains at a comparable level or even increases, seemingly inconsistent with disconnection from the Sun. The measured heat flux values appear inconsistent with regulation primarily by collisional mechanisms near the Sun. Instead, the observed heat flux dependence on plasma beta and the distribution of suprathermal electron parameters are both consistent with theoretical instability thresholds associated with oblique whistler and magnetosonic modes.

The electron heat flux in the polar solar wind: Ulysses observations

Ulysses measurements of the solar wind electron heat flux as a function of heliographic latitude are presented. Contamination from a light leak at the highest latitudes has been removed from the data by interpolating between uncontaminated pixels. After normalization by the radial gradient in the electron heat flux obtained during the inecliptic phase of the Ulysses mission, the electron heat flux is essentially constant from −80° to 80° of heliographic latitude. During brief periods of the fast latitude scan, both uncorrelated and anti‐correlated modulations of the electron heat flux and the solar wind velocity are observed. The implications of the observations with regards to different models of electron heat flux regulation are discussed.

Properties of interplanetary magnetic sector boundaries based on electron heat‐flux flow directions

We have used the solar wind electron heat flux flow directions to determine the interplanetary magnetic field polarities for the ISEE 3 period from 1978 to 1982. This technique assumes that the heat flux electrons flow away from the Sun along magnetic field lines. It provides the field polarities independently of the field directions. The resulting distribution of sector durations and the changes in that distribution with solar activity cycle are presented for four 1‐year periods. The large‐scale sectors expected from extrapolation of the Stanford source surface maps are present along with a population of small‐scale sectors with a peak in the time range of 9 hour to 1 day. About half the small‐scale sectors contain bidirectional electron (BDE) flows, suggesting their origins in coronal mass ejections. We also examine cases of false polarities, in which the directions of the fields imply polarities opposite to those determined from the heat‐flux directions. These constitute only 6 to 8% of all the hourly averages of the data. The majority (78%) of these false polarity regions were not associated with BDEs, and 75% were of only 1 or 2 hour durations. False polarity regions tended to lie nearly orthogonal to the spiral field angles and at relatively high latitudinal angles. While multiple (≥ 3 in 24 hours) current sheet crossings are common, we find no cases consistent with a wavy current sheet.

An investigation of magnetic fields of solar wind transient disturbances at R = 1 AU and determination of their solar sources

Typical features of the magnetic field two types of solar wind transient disturbances with shock waves are investigated and compared: 1. (1) When the shock wave is accompanied by a magnetic cloud (MC). 2. (2) When the shock wave is followed by a region with bidirectional solar wind electron heat flux (BEHF), but with no MC present, however. In this case, a separate study was made of the field features in two typical transient disturbance structures: in the region of impact-compressed solar wind between the shock wave and MC or BEHF, as well as in MC and BEHF. It has been found that the plasma and magnetic field characteristics of the two studied types of transient disturbances are distinguished markedly. The study has provided new results on the influence of the ambient SW upon the transient disturbance magnetic field, and on characteristics of the generation of the B z -component of the magnetic field in each transient disturbance structure. A new test for the existence of interplanetary magnetic field draping around MC and BEHF is proposed and verified. It is concluded that the magnetic field configuration around MC is more adequately consistent with the concept of magnetic line draping than is the case around BEHF. Two methods are proposed to infer the location of solar sources of transient disturbances from their characteristics at R = 1 AU.

Latitudinal variations in the solar wind electron heat flux

Ulysses measurements of the solar wind electron heat flux as a function of heliographic latitude are presented. The latitudinal in the electron heat flux presented have been normalized by the radial gradient in the electron heat flux obtained during the in-ecliptic phase of the Ulysses mission (qe∼ R−3.0). We find no significant variation in electron heat flux with latitude.

Regulation of the solar wind electron heat flux from 1 to 5 AU: Ulysses observations

In this study we use observations from the three‐dimensional electron spectrometer and magnetometer aboard the Ulysses spacecraft to examine the solar wind electron heat flux from 1.2 to 5.4 AU in the ecliptic plane. Throughout Ulysses' transit to 5.4 AU, the electron heat flux decreases more rapidly (∼R−3.0) than simple collisionless expansion along the local magnetic field and is smaller than expected for a thermal gradient heat flux, q∥e(r)=−κ∥▽∥Te(r).The radial gradients and magnitudes expected for a number of electron heat flux regulatory mechanisms are examined and compared to the observations. The best agreement is found for heat flux regulation by the whistler heat flux instability. The upper bound and radial scaling for the electron heat flux predicted for the whistler heat flux instability are consistent with the observations.

Counterstreaming suprathermal electron events upstream of corotating shocks in the solar wind beyond ∼2 Au: Ulysses

Enhanced fluxes of suprathermal electrons are commonly observed upstream of corotating forward and reverse shocks in the solar wind at heliocentric distances beyond ∼2 AU by the Los Alamos plasma experiment on Ulysses. The average duration of these events, which are most intense immediately upstream from the shocks and which fade with increasing distance from them, is ∼2.4 days near 5 AU. These events are caused by the leakage of shock‐heated electrons into the upstream region. The upstream regions of these shocks face back toward the Sun along the interplanetary magnetic field, so these leaked electrons commonly counterstream relative to the normal solar wind electron heat flux. The observations suggest that conservation of magnetic moment and scattering typically limit the sunward propagation of these electrons as beams to field‐aligned distances of ∼15 AU. Although it seems unlikely that these shock‐associated events are an important source of counterstreaming events near 1 AU, remnants of the backstreaming beams may contribute importantly to the diffuse solar wind halo electron population there.

Signatures of shock drivers in the solar wind and their dependence on the solar source location

Solar wind and energetic ion observations following 40 interplanetary shocks with well‐established solar source locations have been examined in order to determine whether signatures characteristic of the coronal material forming the shock driver are present. The signatures considered include magnetic‐field‐aligned bidirectional ion flows observed by the ISEE 3 and IMP 8 spacecraft; bidirectional solar wind electron heat fluxes; solar wind plasma proton and electron temperature depressions; low‐β plasma; enhanced, low‐variance magnetic fields; and energetic ion depressions. Several shock driver signatures are commonly observed following shocks originating from within ∼50° of central meridian and are generally absent for other events. We conclude that shock drivers generally extend up to ∼100° in longitude, centered on the solar source longitude. A particular shock driver signature is typically found following ∼80% of shocks from near‐central meridian events. However, only ∼20% of shocks from near‐central meridian events are associated with the bidirectional 35‐1000 keV ion events discussed by Marsden et al. (1987). Since shocks from central meridian events are not usually associated with all the shock driver signatures examined, the absence of a driver cannot be confirmed from consideration of one of these signatures alone. We also find evidence that a few bidirectional energetic ion and solar wind electron heat flux events following shocks (in particular from far eastern sources) may occur on open field lines outside of shock drivers.

Interplanetary magnetic field connection to the Sun during electron heat flux dropouts in the solar wind

Gosling (1975) and MacQueen (1980) pointed out that coronal mass ejections (CMEs) add significant magnetic flux which is connected to the Sun, to the interplanetary medium. Since the interplanetary magnetic field is observed to stay at a roughly constant level, some compensating removal of flux is necessary. McComas et al. (1989) suggested that magnetic reconnection in current sheets near the Sun removes flux by producing U‐shaped magnetic structures in the interplanetary medium which are disconnected from the Sun at both ends. Since solar wind halo electrons normally carry heat flux outward from the Sun's hot corona to the cool interplanetary medium, McComas et al. (1989) interpreted observations of dropouts of the solar wind electron heat flux as evidence for such disconnections. Here we use 2‐ to 8.5‐keV solar electrons observed by ISEE 3 as tracers of the magnetic topology for these heat flux dropout (HFD) events. These electrons are superior to the ∼0.1‐ to 1‐keV halo electrons as test particles since they have longer mean free paths, they are less affected by electric fields, and their transit times from the Sun to 1 AU are only ∼1–2 hours compared to 3–10 hours for the halo electrons. Furthermore, ≥2‐keV electrons are often accelerated in impulsive events, which are sometimes associated with solar flares, and they also generate type III solar radio bursts, which can be tracked from the Sun to 1 AU by the ISEE 3 radio experiment. In at least eight of the 25 HFDs we find strong streaming of the ≥2‐keV electrons outward from the Sun. In one HFD an impulsive solar electron event was observed with an associated type III radio burst which could be tracked from the Sun to ∼1 AU. We conclude that in many HFDs the interplanetary field is still connected to the Sun, and that some energy‐dependent process may produce HFDs without significantly perturbing electrons of higher energies. For two of the 25 HFDs, however, the ≥2‐keV electron observations exhibit all the characteristics of real magnetic disconnection events, including a depletion in the total ≥2‐keV fluxes.

Electron heat flux dropouts in the solar wind: Evidence for interplanetary magnetic field reconnection?

Electron heat flux dropout events have been observed in the solar wind using the ISEE 3 plasma electron data set. These events manifest themselves as dropouts of the solar wind halo electrons which are normally found streaming outward along the local magnetic field. These dropouts leave nearly isotropic distributions of solar wind halo electrons, and consequently, the electron heat flux in these events is reduced to near the observational noise level. We have examined ISEE 3 data from shortly after launch (August 16, 1978) through the end of 1978 and identified 25 such events ranging in duration from 20 min to over 11 hours. Comparison with the ISEE 3 magnetometer data indicates that these intervals nearly always occur in conjunction with large rotations of the interplanetary magnetic field, particularly those rotations associated with spacecraft passages through the heliospheric current sheet. Statistical analyses of the plasma and magnetic field data for the 25 dropout intervals indicate that heat flux dropouts generally occur in association with high plasma densities, low plasma velocities, low ion and electron temperatures, and low magnetic field magnitudes. A second set of 25 intervals chosen specifically to lie at large field rotations, but at times at which no heat flux dropouts were observed, do not show these characteristic plasma variations. This suggests that the dropout intervals comprise a unique set of events. Since the hot halo electrons normally found streaming outward from the Sun along the interplanetary magnetic field (the solar wind electron heat flux) are a result of direct magnetic connection to the hot solar corona, heat flux dropout intervals may indicate that the spacecraft is sampling plasma regimes which are magnetically disconnected from the Sun and instead are connected to the outer heliosphere at both ends. The close association of electron heat flux dropout events with sector boundaries, where the field topology is best suited for magnetic reconnection, provides additional support for this interpretation.

The eastward deflection of fast coronal mass ejecta in interplanetary space

Previous work has shown that a bidirectional solar wind electron heat flux is one of the more prominent signatures of a coronal mass ejection event in the solar wind at 1 AU. Using ISEE 3 solar wind electron measurements obtained during 1978 and 1979, we have used this signature to identify the fast coronal mass ejecta driving 19 interplanetary shocks. In 17 of the 19 shock events an eastward deflection of the ejection plasma (apparent arrival from west of the sun) was observed. The average eastward deflection for all of the events was ∼3°, corresponding to a typical transverse velocity of ∼25 km/s. Usually an oppositely directed (i.e., westward) flow deflection of comparable magnitude was observed within the compressed ambient plasma ahead of the ejecta. The sense of these deflections—first westward within the compressed ambient plasma and then eastward within the ejecta—is the same as is observed near the leading edges of quasi‐stationary, corotating high speed streams. However, in contrast to the case of corotating streams, the west‐east flow reversal does not coincide with a local maximum in static pressure. No consistent flow deflection pattern perpendicular to the ecliptic has been observed for these events, and no similar deflections have been observed for ejecta (bidirectional electron heat flux events) traveling at the same speed as or slower than the ambient plasma ahead. We suggest that the preferred eastward deflection of these fast coronal mass ejecta is a consequence of solar rotation and the spiral geometry of the ambient solar wind.

Bidirectional solar wind electron heat flux events

Normally the ≳ 80‐eV electrons which carry the solar wind electron heat flux are collimated along the interplanetary magnetic field (IMF) in the direction pointing outward away from the sun. Occasionally, however, collimated fluxes of ≳ 80‐eV electrons are observed traveling both parallel and antiparallel to the IMF. Here we present the results of a survey of such bidirectional electron heat flux events as observed with the plasma and magnetic field experiments aboard ISEE 3 at times when the spacecraft was not magnetically connected to the earth's bow shock. The onset of a bidirectional electron heat flux at ISEE 3 usually signals spacecraft entry into a distinct solar wind plasma and field entity, most often characterized by anomalously low proton and electron temperatures, a strong, smoothly varying magnetic field, a low plasma beta, and a high total pressure. Significant field rotations often occur at the beginning and/or end of bidirectional heat flux events, and, at times, the large field rotations characteristic of “magnetic clouds” are present. Approximately half of all bidirectional heat flux events are associated with and follow interplanetary shocks, while the other events have no obvious shock associations. When shock associated, the delay from shock passage typically is ∼13 hours, corresponding to a radial separation of ∼0.16 AU. Independent of any shock association, bidirectional heat flux events typically are ∼0.13 AU thick in the radial direction, although considerable variability is evident from one event to another. Near solar activity maximum, bidirectional heat flux events occurred at a rate of ∼3 per month, and the solar wind electron heat flux was bidirectional ∼5% of the time. Bidirectional heat flux events often contain strong out‐of‐the‐ecliptic field components and thus can be effective in producing geomagnetic disturbances. This is particularly true for shock‐associated events where the intrinsically strong fields in the leading portions of the events are amplified by compression in transit from the sun and where strong out‐of‐the‐ecliptic field components resulting from compression and draping of the ambient field are often present within the shocked plasma immediately ahead. Consistent with previous work we interpret the bidirectional heat flux as evidence for a closed field topology in interplanetary space. Further, we suggest that these events are one of the more prominent signatures of coronal mass ejection events in the solar wind at 1 AU.

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 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.