90 research outputs found
Atypical Particle Heating at a Supercritical Interplanetary Shock
We present the first observations at an interplanetary shock of large amplitude (> 100 mV/m pk-pk) solitary waves and large amplitude (approx.30 mV/m pk-pk) waves exhibiting characteristics consistent with electron Bernstein waves. The Bernstein-like waves show enhanced power at integer and half-integer harmonics of the cyclotron frequency with a broadened power spectrum at higher frequencies, consistent with the electron cyclotron drift instability. The Bernstein-like waves are obliquely polarized with respect to the magnetic field but parallel to the shock normal direction. Strong particle heating is observed in both the electrons and ions. The observed heating and waveforms are likely due to instabilities driven by the free energy provided by reflected ions at this supercritical interplanetary shock. These results offer new insights into collisionless shock dissipation and wave-particle interactions in the solar wind
Characteristics of Electron Distributions Observed During Large Amplitude Whistler Wave Events in the Magnetosphere
We present a statistical study of the characteristics of electron distributions associated with large amplitude whistler waves inside the terrestrial magnetosphere using waveform capture data as an addition of the study by Kellogg et al., [2010b]. We identified three types of electron distributions observed simultaneously with the whistler waves including beam-like, beam/flattop, and anisotropic distributions. The whistlers exhibited different characteristics dependent upon the observed electron distributions. The majority of the waveforms observed in our study have f/fce or = 8 nT pk-pk) whistler wave measured in the radiation belts. The majority of the largest amplitude whistlers occur during magnetically active periods (AE > 200 nT)
Kinetic Theory and Fast Wind Observations of the Electron Strahl
We develop a model for the strahl population in the solar wind -- a narrow,
low-density and high-energy electron beam centered on the magnetic field
direction. Our model is based on the solution of the electron drift-kinetic
equation at heliospheric distances where the plasma density, temperature, and
the magnetic field strength decline as power-laws of the distance along a
magnetic flux tube. Our solution for the strahl depends on a number of
parameters that, in the absence of the analytic solution for the full electron
velocity distribution function (eVDF), cannot be derived from the theory. We
however demonstrate that these parameters can be efficiently found from
matching our solution with observations of the eVDF made by the Wind
satellite's SWE strahl detector. The model is successful at predicting the
angular width (FWHM) of the strahl for the Wind data at 1 AU, in particular by
predicting how this width scales with particle energy and background density.
We find the strahl distribution is largely determined by the local temperature
Knudsen number , which parametrizes solar wind
collisionality. We compute averaged strahl distributions for typical Knudsen
numbers observed in the solar wind, and fit our model to these data. The model
can be matched quite closely to the eVDFs at 1 AU, however, it then
overestimates the strahl amplitude at larger heliocentric distances. This
indicates that our model may be improved through the inclusion of additional
physics, possibly through the introduction of "anomalous diffusion" of the
strahl electrons
THEMIS Observations of the Magnetopause Electron Diffusion Region: Large Amplitude Waves and Heated Electrons
We present the first observations of large amplitude waves in a well-defined
electron diffusion region at the sub-solar magnetopause using data from one
THEMIS satellite. These waves identified as whistler mode waves, electrostatic
solitary waves, lower hybrid waves and electrostatic electron cyclotron waves,
are observed in the same 12-sec waveform capture and in association with
signatures of active magnetic reconnection. The large amplitude waves in the
electron diffusion region are coincident with abrupt increases in electron
parallel temperature suggesting strong wave heating. The whistler mode waves
which are at the electron scale and enable us to probe electron dynamics in the
diffusion region were analyzed in detail. The energetic electrons (~30 keV)
within the electron diffusion region have anisotropic distributions with
T_{e\perp}/T_{e\parallel}>1 that may provide the free energy for the whistler
mode waves. The energetic anisotropic electrons may be produced during the
reconnection process. The whistler mode waves propagate away from the center of
the 'X-line' along magnetic field lines, suggesting that the electron diffusion
region is a possible source region of the whistler mode waves
A Vortical Dawn Flank Boundary Layer for Near-Radial IMF: Wind Observations on 24 October 2001
We present an example of a boundary layer tailward of the dawn terminator which is entirely populated by rolled-up flow vortices. Observations were made by Wind on 24 October 2001 as the spacecraft moved across the region at the X plane approximately equal to 13 Earth radii. Interplanetary conditions were steady with a near-radial interplanetary magnetic field (IMF). Approximately 15 vortices were observed over the 1.5 hours duration of Wind's crossing, each lasting approximately 5 min. The rolling up is inferred from the presence of a hot tenuous plasma being accelerated to speeds higher than in the adjoining magnetosheath, a circumstance which has been shown to be a reliable signature of this in single-spacecraft observations. A blob of cold dense plasma was entrained in each vortex, at whose leading edge abrupt polarity changes of field and velocity components at current sheets were regularly observed. In the frame of the average boundary layer velocity, the dense blobs were moving predominantly sunward and their scale size along the X plane was approximately 7.4 Earth radii. Inquiring into the generation mechanism of the vortices, we analyze the stability of the boundary layer to sheared flows using compressible magnetohydrodynamic Kelvin-Helmholtz theory with continuous profiles for the physical quantities. We input parameters from (i) the exact theory of magnetosheath flow under aligned solar wind field and flow vectors near the terminator and (ii) the Wind data. It is shown that the configuration is indeed Kelvin-Helmholtz (KH) unstable. This is the first reported example of KH-unstable waves at the magnetopause under a radial IMF
Interpretation of flat energy spectra upstream of fast interplanetary shocks
Interplanetary shocks are large-scale heliospheric structures often caused by
eruptive phenomena at the Sun, and represent one of the main sources of
energetic particles. Several interplanetary shock crossings by spacecraft at
AU have revealed enhanced energetic-ion fluxes that extend far upstream of
the shock. Surprisingly, in some shock events, ion fluxes with energies between
keV and about MeV acquire similar values (which we refer to as
``overlapped'' fluxes), corresponding to flat energy spectra in that range. In
contrast, closer to the shock, the fluxes are observed to depend on energy. In
this work, we analyze three interplanetary shock-related energetic particle
events observed by the Advanced Composition Explorer spacecraft where flat ion
energy spectra were observed upstream of the shock. We interpret these
observations via a velocity filter mechanism for particles in a given energy
range. This reveals that low energy particles tend to be confined to the shock
front and cannot easily propagate upstream, while high energy particles can.
The velocity filter mechanism has been corroborated from observations of
particle flux anisotropy by the Solid-State Telescope of Wind/3DP
Multi-point Assessment of the Kinematics of Shocks (MAKOS): A Heliophysics Mission Concept Study
Collisionless shocks are fundamental processes that are ubiquitous in space
plasma physics throughout the Heliosphere and most astrophysical environments.
Earth's bow shock and interplanetary shocks at 1 AU offer the most readily
accessible opportunities to advance our understanding of the nature of
collisionless shocks via fully-instrumented, in situ observations. One major
outstanding question pertains to the energy budget of collisionless shocks,
particularly how exactly collisionless shocks convert incident kinetic bulk
flow energy into thermalization (heating), suprathermal particle acceleration,
and a variety of plasma waves, including nonlinear structures. Furthermore, it
remains unknown how those energy conversion processes change for different
shock orientations (e.g., quasi-parallel vs. quasi-perpendicular) and driving
conditions (upstream Alfv\'enic and fast Mach numbers, plasma beta, etc.).
Required to address these questions are multipoint observations enabling direct
measurement of the necessary plasmas, energetic particles, and electric and
magnetic fields and waves, all simultaneously from upstream, downstream, and at
the shock transition layer with observatory separations at ion to
magnetohydrodynamic (MHD) scales. Such a configuration of spacecraft with
specifically-designed instruments has never been available, and this white
paper describes a conceptual mission design -- MAKOS -- to address these
outstanding questions and advance our knowledge of the nature of collisionless
shocks.Comment: White paper submitted to the Decadal Survey for Solar and Space
Physics (Heliophysics) 2024-2033; 9 pages, 3 figures, 5 table
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