Magnetohydrodynamic (MHD) turbulence in the solar wind is observed to show the spectral behavior of classical Kolmogorov fluid turbulence over an inertial subrange and departures from this at short ...wavelengths, where energy should be dissipated. Here we present the first measurements of the electric field fluctuation spectrum over the inertial and dissipative wave number ranges in a Beta > or approximately = 1 plasma. The k(-5/3) inertial subrange is observed and agrees strikingly with the magnetic fluctuation spectrum; the wave phase speed in this regime is shown to be consistent with the Alfvén speed. At smaller wavelengths krho(i) > or = 1 the electric spectrum is enhanced and is consistent with the expected dispersion relation of short-wavelength kinetic Alfvén waves. Kinetic Alfvén waves damp on the solar wind ions and electrons and may act to isotropize them. This effect may explain the fluidlike nature of the solar wind.
We present a theoretical analysis of electron heat flux inhibition in the solar wind when a significant portion of the heat flux is carried by strahl electrons. We adopt core-strahl velocity ...distribution functions typical for the solar wind at 0.3-4 au to demonstrate that strahl electrons are capable of generating highly oblique whistler waves at wave numbers k e ∼ 1, where e is typical thermal electron gyroradius. The whistler waves are driven by electrons in the anomalous cyclotron resonances (the fan instability) and propagate at typical angles of about 70°-80° to the strahl that is usually anti-sunward. The group velocity of the whistler waves is predominantly parallel to the strahl, thereby facilitating efficient scattering of strahl electrons. We suggest that the highly oblique whistler waves drive pitch-angle scattering of strahl electrons, resulting in halo formation and suppressing the heat flux of strahl electrons below a threshold that is shown to depend on βe. The proposed fan instability is fundamentally different from the whistler heat flux instability driven by the normal cyclotron resonance with halo electrons and being ineffective in suppressing the heat flux of the strahl.
Despite the importance of millisecond duration spatial structures chorus wave nonlinearity or time domain structures (TDS) to plasma dynamics, there have been no direct observations of the generation ...and interaction of these waves and TDS with electrons at the millisecond timescale required for their understanding. Through superposition of 0.195 ms Magnetospheric Multiscale Satellite electron measurements inside 37 superposed, millisecond duration electron holes, the first observations of electron spectra and pitch angle distributions on a submillisecond timescale have been obtained. They show that keV electrons inside the superposed electron hole are accelerated by several hundred volts and that the spectrum of electrons inside the electron hole contain several maxima and minima that are explained by a model of electron energy changes on entering the holes. We report the first observation of trapped electrons inside the TDS, in accordance with the theoretical requirement that such electrons must be present in order to form the phase space holes. Mechanisms of electron acceleration by electron holes (through perpendicular energy gain as the TDS moves into a converging magnetic field) and scattering (due to the perpendicular electric field) are discussed.
The structure of magnetic flux ropes injected into the solar wind during reconnection in the coronal atmosphere is explored with particle-in-cell simulations and compared with in situ measurements of ...magnetic “switchbacks” from the Parker Solar Probe. We suggest that multi-x-line reconnection between open and closed flux in the corona injects flux ropes into the solar wind and that these flux ropes convect outward over long distances before eroding due to reconnection. Simulations that explore the magnetic structure of flux ropes in the solar wind reproduce the following key features of the switchback observations: a rapid rotation of the radial magnetic field into the transverse direction, which is a consequence of reconnection with a strong guide field; and the potential to reverse the radial field component. The potential implication of the injection of large numbers of flux ropes in the coronal atmosphere for understanding the generation of the solar wind is discussed.
The double probe technique for measuring DC and low‐frequency electric fields in space plasmas is described with an emphasis on uncertainties in the measurements, data analysis techniques, and ...methods for improving the electric field measurements in future missions.
Key Points
It is explained how and why the double probe electric field experiment works
Electric field data analysis techniques are described
Methods for improving the electric field measurement are described
The mechanisms for accelerating electrons from thermal to relativistic energies in the terrestrial magnetosphere, on the sun, and in many astrophysical environments have never been verified. We ...present the first direct observation of two processes that, in a chain, cause this acceleration in Earth's outer radiation belt. The two processes are parallel acceleration from electron-volt to kilovolt energies by parallel electric fields in time-domain structures (TDS), after which the parallel electron velocity becomes sufficiently large for Doppler-shifted upper band whistler frequencies to be in resonance with the electron gyration frequency, even though the electron energies are kilovolts and not hundreds of kilovolts. The electrons are then accelerated by the whistler perpendicular electric field to relativistic energies in several resonant interactions. TDS are packets of electric field spikes, each spike having duration of a few hundred microseconds and containing a local parallel electric field. The TDS of interest resulted from nonlinearity of the parallel electric field component in oblique whistlers and consisted of ∼ 0.1 msec pulses superposed on the whistler waveform with each such spike containing a net parallel potential the order of 50 V. Local magnetic field compression from remote activity provided the free energy to drive the two processes. The expected temporal correlations between the compressed magnetic field, the nonlinear whistlers with their parallel electric field spikes, the electron flux and the electron pitch angle distributions were all observed.
Abstract
The origin of switchbacks in the solar wind is discussed in two classes of theory that differ in the location of the source being either near the transition region near the Sun or in the ...solar wind itself. The two classes of theory differ in their predictions of the switchback rate (the number of switchbacks observed per hour) as a function of distance from the Sun. To distinguish between these theories, one-hour averages of Parker Solar Probe data were averaged over five orbits to find the following: (1) The hourly averaged switchback rate was independent of distance from the Sun. (2) The average switchback rate increased with solar wind speed. (3) The switchback size perpendicular to the flow increased as
R
, the distance from the Sun, while the radial size increased as
R
2
, resulting in an increasing switchback aspect ratio with distance from the Sun. (4) The hourly averaged and maximum switchback rotation angles did not depend on the solar wind speed or distance from the Sun. These results are consistent with switchback formation in the transition region because their increase of tangential size with radius compensates for the radial falloff of their equatorial density to produce switchback rates that are independent of radial distance. This constant switchback rate is inconsistent with an in situ source. The switchback size and aspect ratio, but not their hourly average or maximum rotation angle, increased with radial distance to 100 solar radii. Additionally, quiet intervals between switchback patches occurred at the lowest solar wind speeds.
Chorus waves are among the most important natural electromagnetic emissions in the magnetosphere as regards to their potential effects on electron dynamics. They can efficiently accelerate or ...precipitate electrons trapped in the outer radiation belt, producing either fast increases of relativistic particle fluxes or auroras at high latitudes. Accurately modeling their effects, however, requires detailed models of their wave power and obliquity distribution as a function of geomagnetic activity in a particularly wide spatial domain, rarely available based solely on the statistics obtained from only one satellite mission. Here we seize the opportunity of synthesizing data from the Van Allen Probes and Cluster spacecraft to provide a new comprehensive chorus wave model in the outer radiation belt. The respective spatial coverages of these two missions are shown to be especially complementary and further allow a good cross calibration in the overlap domain. We used 4 years (2012–2016) of Van Allen Probes VLF data in the chorus frequency range up to 12 kHz at latitudes lower than 20°, combined with 10 years of Cluster VLF measurements up to 4 kHz in order to provide a full coverage of geomagnetic latitudes up to 45° in the chorus frequency range 0.1fce–0.8fce. The resulting synthetic statistical model of chorus wave amplitude, obliquity, and frequency is presented in the form of analytical functions of latitude and Kp in three different magnetic local time sectors and for two ranges of L shells outside the plasmasphere. Such a synthetic and reliable chorus model is crucially important for accurately modeling global acceleration and loss of electrons over the long run in the outer radiation belt, allowing a comprehensive description of electron flux variations over a very wide energy range.
Key Points
The lower and upper band chorus model based on Van Allen Probes and Cluster VLF measurements is developed
The modeled parameters are chorus amplitude and wave normal angle distribution, wave frequency on L, MLT, MLat, Kp
The diffusion rates are estimated and compared with previous model results
Time domain structures (TDS) (electrostatic or electromagnetic electron holes, solitary waves, double layers, etc.) are ≥1 ms pulses having significant parallel (to the background magnetic field) ...electric fields. They are abundant through space and occur in packets of hundreds in the outer Van Allen radiation belts where they produce magnetic‐field‐aligned electron pitch angle distributions at energies up to a hundred keV. TDS can provide the seed electrons that are later accelerated to relativistic energies by whistlers and they also produce field‐aligned electrons that may be responsible for some types of auroras. These field‐aligned electron distributions result from at least three processes. The first process is parallel acceleration by Landau trapping in the TDS parallel electric field. The second process is Fermi acceleration due to reflection of electrons by the TDS. The third process is an effective and rapid pitch angle scattering resulting from electron interactions with the perpendicular and parallel electric and magnetic fields of many TDS. TDS are created by current‐driven and beam‐related instabilities and by whistler‐related processes such as parametric decay of whistlers and nonlinear evolution from oblique whistlers. New results on the temporal relationship of TDS and particle injections, types of field‐aligned electron pitch angle distributions produced by TDS, the mechanisms for generation of field‐aligned distributions by TDS, the maximum energies of field‐aligned electrons created by TDS in the absence of whistler mode waves, TDS generation by oblique whistlers and three‐wave‐parametric decay, and the correlation between TDS and auroral particle precipitation, are presented.
Key Points
Time domain structures produce field‐aligned electron pitch angle distributions
Field‐aligned electrons are seeds for relativistic electron acceleration
TDS cause electron precipitation that produces some types of auroras
Whistler mode chorus waves are present throughout the Earth's outer radiation belt as well as at larger distances from our planet. While the generation mechanisms of parallel lower band chorus waves ...and oblique upper band chorus waves have been identified and checked in various instances, the statistically significant presence in recent satellite observations of very oblique lower band chorus waves near the resonance cone angle remains to be explained. Here we discuss two possible generation mechanisms for such waves. The first one is based on Landau resonance with sporadic very low energy (<4 keV) electron beams either injected from the plasma sheet or produced in situ. The second one relies on cyclotron resonance with low‐energy electron streams, such that their velocity distribution possesses both a significant temperature anisotropy above 3–4 keV and a plateau or heavy tail in parallel velocities at lower energies encompassing simultaneous Landau resonance with the same waves. The corresponding frequency and wave normal angle distributions of the generated very oblique lower band chorus waves, as well as their frequency sweep rate, are evaluated analytically and compared with satellite observations, showing a reasonable agreement.
Key Points
Very oblique chorus waves could be generated by low‐energy beams or streams
Landau and cyclotron resonance generation mechanisms are examined analytically
Analytical frequency sweep rates correspond with Van Allen Probes measurements
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