19 research outputs found
A latitude-dependent wind model for Mira's cometary head
We present a 3D numerical simulation of the recently discovered cometary
structure produced as Mira travels through the galactic ISM. In our simulation,
we consider that Mira ejects a steady, latitude-dependent wind, which interacts
with a homogeneous, streaming environment. The axisymmetry of the problem is
broken by the lack of alignment between the direction of the relative motion of
the environment and the polar axis of the latitude-dependent wind. With this
model, we are able to produce a cometary head with a ``double bow shock'' which
agrees well with the structure of the head of Mira's comet. We therefore
conclude that a time-dependence in the ejected wind is not required for
reproducing the observed double bow shock.Comment: 4 pages, 4 figures, accepted for publication in ApJ
Influence of He++ and Shock Geometry on Interplanetary Shocks in the Solar Wind: 2D Hybrid Simulations
After protons, alpha particles (He) are the most important ion species in the solar wind, constituting typically about 5\% of the total ion number density. Due to their different charge-to-mass ratio protons and He particles are accelerated differently when they cross the electrostatic potential in a collisionless shock. This behavior can produce changes in the velocity distribution function (VDF) for both species generating anisotropy in the temperature which is considered to be the energy source for various phenomena such as ion cyclotron and mirror mode waves. How these changes in temperature anisotropy and shock structure depend on the percentage of He particles and the geometry of the shock is not completely understood. In this paper we have performed various 2D local hybrid simulations (particle ions, massless fluid electrons) with similar characteristics (e.g., Mach number) to interplanetary shocks for both quasi-parallel and quasi-perpendicular geometries self-consistently including different percentages of He particles. We have found changes in the shock transition behavior as well as in the temperature anisotropy as functions of both the shock geometry and He particle abundance: The change of the initial leads to variations of the efficiency with which particles can escape to the upstream region facilitating or not the formation of compressive structures in the magnetic field that will produce increments in perpendicular temperature. The regions where both temperature anisotropy and compressive fluctuations appear tend to be more extended and reach higher values as the He content in the simulations increases.Data set in h5 format corresponding to each panel of the figures of the publicatio
Variability of the Magnetic Field Power Spectrum in the Solar Wind at Electron Scales
At electron scales, the power spectrum of solar-wind magnetic fluctuations can be highly variable and the dissipation mechanisms of the magnetic energy into the various particle species is under debate. In this paper, we investigate data from the Cluster mission's STAFF Search Coil magnetometer when the level of turbulence is sufficiently high that the morphology of the power spectrum at electron scales can be investigated. The Cluster spacecraft sample a disturbed interval of plasma where two streams of solar wind interact. Meanwhile, several discontinuities (coherent structures) are seen in the large-scale magnetic field, while at small scales several intermittent bursts of wave activity (whistler waves) are present. Several different morphologies of the power spectrum can be identified: (1) two power laws separated by a break, (2) an exponential cutoff near the Taylor shifted electron scales, and (3) strong spectral knees at the Taylor shifted electron scales. These different morphologies are investigated by using wavelet coherence, showing that, in this interval, a clear break and strong spectral knees are features that are associated with sporadic quasi parallel propagating whistler waves, even for short times. On the other hand, when no signatures of whistler waves at similar to 0.1-0.2f(ce) are present, a clear break is difficult to find and the spectrum is often more characteristic of a power law with an exponential cutoff.Peer reviewe
ULF Wave Transmission Across Collisionless Shocks : 2.5D Local Hybrid Simulations
We study the interaction of upstream ultralow frequency (ULF) waves with collisionless shocks by analyzing the outputs of 11 2D local hybrid simulation runs. Our simulated shocks have Alfvenic Mach numbers between 4.29 and 7.42 and their theta BN angles are 15 degrees, 30 degrees, 45 degrees, and 50 degrees. The ULF wave foreshocks develop upstream of all of them. The wavelength and the amplitude of the upstream waves exhibit a complex dependence on the shock's MA and theta BN. The wavelength positively correlates with both parameters, with the dependence on theta BN being much stronger. The amplitude of the ULF waves is proportional to the product of the reflected beam velocity and density, which also depend on MA and theta BN. The interaction of the ULF waves with the shock causes large-scale (several tens of upstream ion inertial lengths) shock rippling. The properties of the shock ripples are related to the ULF wave properties, namely their wavelength and amplitude. In turn, the ripples have a large impact on the ULF wave transmission across the shock because they change local shock properties (theta BN, strength), so that different sections of the same ULF wavefront encounter shock with different characteristics. Downstream fluctuations do not resemble the upstream waves in terms the wavefront extension, orientation or their wavelength. However, some features are conserved in the Fourier spectra of downstream compressive waves that present a bump or flattening at wavelengths approximately corresponding to those of the upstream ULF waves. In the transverse downstream spectra, these features are weaker.Peer reviewe
Investigating the anatomy of magnetosheath jets - MMS observations
Karl-Heinz Glassmeier,
Hans-Ulrich Auster and Wolfgang Baumjohann for the use of FGM
data provided under the lead of the Technical University of Braunschweig
and with financial support through the German Ministry for
Economy and Technology and the German Center for Aviation and
Space (DLR) under contract 50 OC 0302. The work of Heli Hietala
is supported by NASA grant NNX17AI45G and contract NAS5-
02099
Transient Foreshock Structures Upstream of Mars: Implications of the Small Martian Bow Shock
We characterize the nature of magnetic structures in the foreshock region of
Mars associated with discontinuities in the solar wind. The structures form at
the upstream edge of moving foreshocks caused by slow rotations in the
interplanetary magnetic field (IMF). The solar wind plasma density and the IMF
strength noticeably decrease inside the structures' core, and a compressional
shock layer is present at their sunward side, making them consistent with
foreshock bubbles (FBs). Ion populations responsible for these structures
include backstreaming ions that only appear within the moving foreshock, and
accelerated reflected ions from the quasi-perpendicular bow shock. Both ion
populations accumulate near the upstream edge of the moving foreshock which
facilitates FB formation. Reflected ions with hybrid trajectories that straddle
between the quasi-perpendicular and quasi-parallel bow shocks during slow IMF
rotations contribute to formation of foreshock transients.Comment: Submitted to Geophysical Research Letter
Magnetosheath jet properties and evolution as determined by a global hybrid-Vlasov simulation
We use a global hybrid-Vlasov simulation for the magnetosphere, Vlasiator, to investigate magnetosheath high-speed jets. Unlike many other hybrid-kinetic simulations, Vlasiator includes an unscaled geomagnetic dipole, indicating that the simulation spatial and temporal dimensions can be given in SI units without scaling. Thus, for the first time, this allows investigating the magnetosheath jet properties and comparing them directly with the observed jets within the Earth's magnetosheath. In the run shown in this paper, the interplanetary magnetic field (IMF) cone angle is 30 degrees, and a foreshock develops upstream of the quasi-parallel magnetosheath. We visually detect a structure with high dynamic pressure propagating from the bow shock through the magnetosheath. The structure is confirmed as a jet using three different criteria, which have been adopted in previous observational studies. We compare these criteria against the simulation results. We find that the magnetosheath jet is an elongated structure extending earthward from the bow shock by similar to 2.6 R-E, while its size perpendicular to the direction of propagation is similar to 0.5 R-E. We also investigate the jet evolution and find that the jet originates due to the interaction of the bow shock with a high-dynamic-pressure structure that reproduces observational features associated with a short, large-amplitude magnetic structure (SLAMS). The simulation shows that magnetosheath jets can develop also under steady IMF, as inferred by observational studies. To our knowledge, this paper therefore shows the first global kinetic simulation of a magnetosheath jet, which is in accordance with three observational jet criteria and is caused by a SLAMS advecting towards the bow shock
Geo‐effectiveness and radial dependence of magnetic cloud erosion by magnetic reconnection
Magnetic flux erosion by magnetic reconnection occurs at the front of at least some magnetic clouds (MCs). We first investigate how erosion influences the geo‐effectiveness of MCs in a general sense and using a south‐north magnetic polarity MC observed on 18–20 October 1995. Although the magnetic shear at its front may not be known during propagation, measurements at 1 AU show signatures of local reconnection. Using a standard MC model, an empirical model of the geomagnetic response ( Dst ), and an observational estimate of the magnetic flux erosion, we find that the strength of the observed ensuing storm was ~30% lower than if no erosion had occurred. We then discuss the interplay between adiabatic compression and magnetic erosion at the front of MCs. We conclude that the most geo‐effective configuration for a south‐north polarity MC is to be preceded by a solar wind with southward IMF. This stems not only from the formation of a geo‐effective sheath ahead of it but also from the adiabatic compression and reduced (or lack thereof) magnetic erosion which constructively conspire for the structure to be more geo‐effective. Finally, assuming simple semiempirical and theoretical Alfvén speed profiles expected from expansion to 1 AU, we provide first‐order estimates of the erosion process radial evolution. We find that the expected reconnection rates during propagation allow for significant erosion, on the order of those reported. Calculations also suggest that most of the erosion should occur in the inner heliosphere, and up to ~50% may yet occur beyond Mercury's orbit.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106123/1/jgra50756.pd
First Observations of Irregular Surface of Interplanetary Shocks at Ion Scales by Cluster
We present the first observational evidence of the irregular surface of
interplanetary (IP) shocks by using multi-spacecraft observations of the
Cluster mission. In total we discuss observations of four IP shocks that
exhibit moderate Alfv\'enic Mach numbers (M6.5). Three of them are
high- shocks with upstream = 2.2--3.7. During the times when
these shocks were observed, the Cluster spacecraft formed constellations with
inter-spacecraft separations ranging from less than one upstream ion inertial
length (d) up to 100~d. Expressed in kilometers, the distances ranged
between 38~km and 10~km. We show that magnetic field profiles and the
local shock normals of observed shocks are very similar when the spacecraft are
of the order of one d apart, but are strikingly different when the
distances increase to ten or more d. We interpret these differences to be
due to the irregular surface of IP shocks and discuss possible causes for such
irregularity. We strengthen our interpretation by comparing observed shock
profiles with profiles of simulated shocks. The latter had similar
characteristics (M, , upstream ion ) as observed shocks
and the profiles were obtained at separations across the simulation domain
equivalent to the Cluster inter-spacecraft distances