Whistler waves are intermittently present in the solar wind, while their origin and effects are not entirely understood. We present a statistical analysis of magnetic field fluctuations in the ...whistler frequency range (above 16 Hz) based on about 801,500 magnetic field spectra measured over 3 yr aboard the Artemis spacecraft in the pristine solar wind. About 13,700 spectra (30 hr in total) with intense magnetic field fluctuations satisfy the interpretation in terms of quasi-parallel whistler waves. We provide estimates of the whistler wave occurrence probability, amplitudes, frequencies, and bandwidths. The occurrence probability of whistler waves is shown to strongly depend on the electron temperature anisotropy. The whistler wave amplitudes are in the range from about 0.01 to 0.1 nT and typically below 0.02 of the background magnetic field. The frequencies of the whistler waves are shown to be below an upper bound that is dependent on βe. The correlations established between the whistler wave properties and local macroscopic plasma parameters suggest that the observed whistler waves can be generated in local plasmas by the whistler heat flux instability. The whistler wave amplitudes are typically small, which questions the hypothesis that quasi-parallel whistler waves are capable to regulate the electron heat flux in the solar wind. We show that the observed whistler waves have sufficiently wide bandwidths and small amplitudes, so that effects of the whistler waves on electrons can be addressed in the frame of the quasi-linear theory.
We present an analysis of simultaneous particle and field measurements from the ARTEMIS spacecraft, which demonstrates that quasi-parallel whistler waves in the solar wind can be generated locally by ...a bulk flow of halo electrons (whistler heat flux instability). ARTEMIS observes quasi-parallel whistler waves in the frequency range ∼0.05−0.2fce simultaneously with electron velocity distribution functions that are a combination of counter-streaming core and halo populations. A linear stability analysis shows that the plasma is stable when there are no whistler waves, and it is unstable in the presence of whistler waves. In the latter case, the stability analysis shows that the whistler wave growth time is from a few to 10 seconds at frequencies and wavenumbers that match the observations. The observations clearly demonstrate that the temperature anisotropy of halo electrons crucially affects the heat flux instability onset: a slight anisotropy T /T > 1 may quench the instability, while a slight anisotropy T /T < 1 may significantly increase the growth rate. These results demonstrate that heat flux inhibition is strongly dependent on the microscopic plasma properties.
A survey of electrons with energies between ∼1 eV and 30 keV was conducted using measurements from the THEMIS spacecraft over radial distances between 3 and 12 Earth radii. Two distinct populations ...are observed, one with a peak energy near 10 eV and one at approximately 1 keV. These populations are present 88% of the time in the magnetosphere. The warm population (∼10 eV) is generally more dense (∼1 cm−3) and extends across the dayside. These warm electron characteristics are similar to the ion warm plasma cloak. The hot distribution (∼1 keV) peaks in number density (∼0.2 cm−3) near midnight and into the morning sector. Since the populations are transported through different evolutionary paths, there are spatial regions within the magnetosphere where the density ratio of the populations (warm to hot, nw/nh) is much larger or smaller than unity. At L shells greater than ∼6, near midnight and into the dawn sector, the density of the hot population is close to a factor of 10 larger than the warm populations. In other regions, the warm electron population is typically slightly more dense (∼2×) than the hot population.
Key Points
Multiple distinct electron populations are present in the Earth's magnetosphere
The ratio of the density of electron populations depends on geomagnetic activity
On the nightside of the Earth, there are often regions where the density of hot electrons is larger than that of warm electrons
Abstract
We present particle-in-cell simulations of a combined whistler heat flux and temperature anisotropy instability that is potentially operating in the solar wind. The simulations are performed ...in a uniform plasma and initialized with core and halo electron populations typical of the solar wind beyond about 0.3 au. We demonstrate that the instability produces whistler-mode waves propagating both along (anti-sunward) and opposite (sunward) to the electron heat flux. The saturated amplitudes of both sunward and anti-sunward whistler waves are strongly correlated with their initial linear growth rates,
B
w
/
B
0
∼
(
γ
/
ω
ce
)
ν
, where for typical electron betas we have 0.6 ≲
ν
≲ 0.9. We show that because of the relatively large spectral width of the whistler waves, the instability saturates through the formation of quasi-linear plateaus around the resonant velocities. The revealed correlations of whistler wave amplitudes and spectral widths with electron beta and temperature anisotropy are consistent with solar wind observations. We show that anti-sunward whistler waves result in an electron heat flux decrease, while sunward whistler waves actually lead to an electron heat flux increase. The net effect is the electron heat flux suppression, whose efficiency is larger for larger electron betas and temperature anisotropies. The electron heat flux suppression can be up to 10%–60% provided that the saturated whistler wave amplitudes exceed about 1% of the background magnetic field. The experimental applications of the presented results are discussed.
We present a statistical analysis of large-amplitude bipolar electrostatic structures measured by Magnetospheric Multiscale spacecraft in the Earth's bow shock. The analysis is based on 371 ...large-amplitude bipolar structures collected in nine supercritical quasi-perpendicular Earth's bow shock crossings. We find that 361 of the bipolar structures have negative electrostatic potentials, and only 10 structures (< 3%) have positive potentials. The bipolar structures with negative potentials are interpreted in terms of ion phase space holes produced by ion streaming instabilities, particularly the two-stream instability between incoming and reflected ions. We obtain an upper estimate for the amplitudes of the ion phase space holes that is in agreement with the measurements. The bipolar structures with positive potentials could be electron phase space holes produced by electron two-stream instabilities. We argue that the negligible number of electron phase space holes among large-amplitude bipolar structures is due to the electron hole transverse instability, the criterion for which is highly restrictive at ωpe/ωce ≫ 1, a parameter range typical of collisionless shocks in the heliosphere and various astrophysical environments. Our analysis indicates that the original mechanism of electron surfing acceleration involving electron phase space holes is not likely to be efficient in realistic collisionless shocks.
Lightning‐generated whistler waves are electromagnetic plasma waves in the very low frequency (VLF) band, which play an important role in the dynamics of radiation belt particles. In this paper, we ...statistically analyze simultaneous waveform data from the Van Allen Probes (Radiation Belt Storm Probes, RBSP) and global lightning data from the World Wide Lightning Location Network (WWLLN). Data were obtained between July to September 2013 and between March and April 2014. For each day during these periods, we predicted the most probable 10 min for which each of the two RBSP satellites would be magnetically conjugate to lightning producing regions. The prediction method uses integrated WWLLN stroke data for that day obtained during the three previous years. Using these predicted times for magnetic conjugacy to lightning activity regions, we recorded high time resolution, burst mode waveform data. Here we show that whistlers are observed by the satellites in more than 80% of downloaded waveform data. About 22.9% of the whistlers observed by RBSP are one‐to‐one coincident with source lightning strokes detected by WWLLN. About 40.1% more of whistlers are found to be one‐to‐one coincident with lightning if source regions are extended out 2000 km from the satellites footpoints. Lightning strokes with far‐field radiated VLF energy larger than about 100 J are able to generate a detectable whistler wave in the inner magnetosphere. One‐to‐one coincidences between whistlers observed by RBSP and lightning strokes detected by WWLLN are clearly shown in the L shell range of L = 1–3. Nose whistlers observed in July 2014 show that it may be possible to extend this coincidence to the region of L≥4.
Key Points
Whistlers are observed in 80% of data at predicted satellite locations
About 22.9% of the RBSP whistlers are related with a possible WWLLN source lightning
About 40.1% more of coincident whistlers are found if source regions are extended
There have been many significant advances in understanding magnetic field reconnection as a result of improved space measurements and two-dimensional computer simulations. While reviews of recent ...work have tended to focus on symmetric reconnection on ion and larger spatial scales, the present review will focus on asymmetric reconnection and on electron scale physics involving the reconnection site, parallel electric fields, and electron acceleration.
The “zebra stripes” are peaks and valleys commonly present in the spectrograms of energetic particles trapped in the Earth's inner belt and slot region. Several theories have been proposed over the ...years to explain their generation, structure, and evolution. Yet, the plausibility of various theories has not been tested due to a historical lack of ground truth, including in situ electric field measurements. In this work, we leverage the new visibility offered by the database of Van Allen Probes electric drift measurements to reveal the conditions associated with the generation of zebra stripe patterns. Energetic electron fluxes by the Radiation Belt Storm Probes Ion Composition Experiment between 1 January 2013 and 31 December 2015 are systematically analyzed to determine 370 start times associated with the generation of zebra stripes. Statistical analyses of these events reveal that the zebra stripes are usually created during substorm onset, a time at which prompt penetration electric fields are present in the plasmasphere. All the pieces of experimental evidence collected are consistent with a scenario in which the prompt penetration electric field associated with substorm onset leads to a sudden perturbation of the trapped particle drift motion. Subsequent drift echoes constitute the zebra stripes. This study exemplifies how the analysis of trapped particle dynamics in the inner belt and slot region provides complementary information on the dynamics of plasmaspheric electric fields. It is the first time that the signature of prompt penetration electric fields is detected in near‐equatorial electric field measurements below L = 3.
Key Points
An algorithm is designed to analyze the zebra stripes present in the spectrograms of energetic electrons trapped below L = 3
A superposed epoch analysis of 370 events is performed to determine the experimental conditions associated with zebra stripe generation
Experimental evidence suggests that prompt penetration electric fields associated with substorm onsets routinely generate zebra stripes
Accurate knowledge of the full, three-dimensional electric field vector is of fundamental importance in understanding electrodynamics of a vast variety of space plasmas. However, heliophysics ...research still lacks access to the reliable parallel electric field measurements required to close many significant science questions. This uncertainty represents a significant barrier to progress in the field. The only way to close this major observational gap is a profound change in electric field instrument design. A new electric field instrument called Grotifer is now being designed to address the need for highly accurate three-dimensional electric field measurements while enabling lower cost missions and constellation missions in deep space. Grotifer (Giant rotifer) is a reference to the rotifer, also known as the “wheel animalcule.” Similarly, Grotifer consists of mounting detectors on two rotating plates, orthogonal to each other, on a non-rotating central body. The two rotating plates provide continuous high-accuracy three-dimensional measurements of both electric fields and magnetic fields. The Grotifer design leverages more than 50 years of expertise in delivering highly accurate spin plane electric field measurements, while overcoming inaccuracies generated by spin axis electric field measurements. Our current efforts focus on designing Grotifer as a SmallSat (27U CubeSat). That said, Grotifer could also become part of the payload on a much larger platform. In the future, one could imagine fleets of Grotifers studying electrodynamics at many points, facilitating differentiation between spatial and temporal dynamics. Plasma detectors could also be added to the rotating plates to cover the full phase space better than is done on spinning spacecraft, leading to more complete correlation studies of the fields and plasmas.
We present surprising observations by the NASA Van Allen Probes spacecraft of whistler waves with substantial electric field power at harmonics of the whistler wave fundamental frequency. The wave ...power at harmonics is due to a nonlinearly steepened whistler electrostatic field that becomes possible in the two-temperature electron plasma due to the whistler wave coupling to the electron-acoustic mode. The simulation and analytical estimates show that the steepening takes a few tens of milliseconds. The hydrodynamic energy cascade to higher frequencies facilitates efficient energy transfer from cyclotron resonant electrons, driving the whistler waves, to lower energy electrons.
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