L. B. Wilson

Low-frequency whistler waves and shocklets observed at quasi-perpendicular interplanetary shocks

[1] We present observations of low-frequency waves (0.25 Hz < f < 10 Hz) at five quasi-perpendicular interplanetary (IP) shocks observed by the Wind spacecraft. Four of the five IP shocks had oblique precursor whistler waves propagating at angles with respect to the magnetic field of 20–50 and large propagation angles with respect to the shock normal; thus they do not appear to be phase standing. One event, the strongest in our study and likely supercritical, had low-frequency waves consistent with steepened magnetosonic waves called shocklets. The shocklets are seen in association with diffuse ion distributions. Both the shocklets and precursor whistlers are often seen simultaneously with anisotropic electron distributions unstable to the whistler heat flux instability. The IP shock with upstream shocklets showed much stronger electron heating across the shock ramp than the four events without upstream shocklets. These results may offer new insights into collisionless shock dissipation and wave-particle interactions in the solar wind.

The properties of large amplitude whistler mode waves in the magnetosphere: Propagation and relationship with geomagnetic activity

using waveform capture data from the Wind spacecraft. Weobserved 247 whistler mode waves with at least one electricfield component (105/247 had≥80 mV/m peak!to!peakamplitudes) and 66 whistler mode waves with at least onesearch coil magnetic field component (38/66 had≥0.8 nTpeak!to!peak amplitudes). Wave vectors determined fromevents with three magnetic field components indicate that30/46 propagate within 20° of the ambient magnetic field,though some are more oblique (up to ∼50°). No relationshipwas observed between wave normal angle and GSM lati-tude. 162/247 of the large amplitude whistler mode waveswere observed during magnetically active periods (AE >200 nT). 217 out of 247 total whistler mode waves exam-ined were observed inside the radiation belts. We presenta waveform capture with the largest whistler wave magneticfield amplitude (^8nTpeak!to!peak) ever reported in theradiation belts. The estimated Poynting flux magnitude asso-ciated with this wave is ^300 mW/m

Observation of relativistic electron microbursts in conjunction with intense radiation belt whistler-mode waves

We present multi-satellite observations indicating a strong correlation between large amplitude radiation belt whistler-mode waves and relativistic electron precipitation. On separate occasions during the Wind petal orbits and STEREO phasing orbits, Wind and STEREO recorded intense whistler-mode waves in the outer nightside equatorial radiation belt with peak-to-peak amplitudes exceeding 300 mV/m. During these intervals of intense wave activity, SAMPEX recorded relativistic electron microbursts in near magnetic conjunction with Wind and STEREO. The microburst precipitation exhibits a bursty temporal structure similar to that of the observed large amplitude wave packets, suggesting a connection between the two phenomena. Simulation studies corroborate this idea, showing that nonlinear wave--particle interactions may result in rapid energization and scattering on timescales comparable to those of the impulsive relativistic electron precipitation.

THEMIS observations of the magnetopause electron diffusion region: Large amplitude waves and heated electrons

Received 14 April 2013; revised 9 May 2013; accepted 14 May 2013; published 18 June 2013. [1 ]W e present the first observations of large amplitude waves in a well-defined electron diffusion region based on the criteria described byScudderet al.[2012]atthe subsolarmagnetopause using data from one Time History of Events and Macroscale Interactions during Substorms (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 s 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 which enable us to probe electron dynamics in the diffusion region were analyzed in detail. The energetic electrons (~30keV) within the electron diffusion region have anisotropic distributions with Te! /Tek > 1t hat 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. Citation: Tang, X., C. Cattell, J. Dombeck, L. Dai, L. B. Wilson III, A. Breneman, and A. Hupach (2013), THEMIS observations of the magnetopause electron diffusion region: Large amplitude waves and heated electrons, Geophys. Res. Lett., 40 ,2 884‐2890, doi:10.1002/ grl.50565.

Observations of electromagnetic whistler precursors at supercritical interplanetary shocks

] We present observations of electromagnetic precursorwaves, identified as whistler mode waves, at supercriticalinterplanetary shocks using the Wind search coil magneto-meter. The precursors propagate obliquely with respect tothe local magnetic field, shock normal vector, solar windvelocity, and they are not phase standing structures. All areright-hand polarized with respect to the magnetic field(spacecraft frame), and all but one are right-hand polarizedwith respect to the shock normal vector in the normal inci-dence frame. They have rest frame frequencies f

Interplanetary and interstellar dust observed by the Wind/WAVES electric field instrument

Observations of hypervelocity dust particles impacting the Wind spacecraft are reported here for the first time using data from the Wind/WAVES electric field instrument. A unique combination of rotating spacecraft, amplitude-triggered high-cadence waveform collection, and electric field antenna configuration allow the first direct determination of dust impact direction by any spacecraft using electric field data. Dust flux and impact direction data indicate that the observed dust is approximately micron-sized with both interplanetary and interstellar populations. Nanometer radius dust is not detected by Wind during times when nanometer dust is observed on the STEREO spacecraft and both spacecraft are in close proximity. Determined impact directions suggest that interplanetary dust detected by Wind/WAVES at 1 AU is dominated by particles on bound trajectories crossing Earths orbit, rather than dust with hyperbolic orbits.

Shocklets, SLAMS, and field‐aligned ion beams in the terrestrial foreshock

We present Wind spacecraft observations of ion distributions showing field-aligned beams (FABs) and large-amplitude magnetic fluctuations composed of a series of shocklets and short large-amplitude magnetic structures (SLAMS). We show that the SLAMS are acting like a local quasi-perpendicular shock reflecting ions to produce the FABs. Previous FAB observations reported the source as the quasi-perpendicular bow shock. The SLAMS exhibit a foot-like magnetic enhancement with a leading magnetosonic whistler train, consistent with previous observations. The FABs are found to have T_b ~ 80-850 eV, V_b/V_sw ~ 1-2, T_{b,perp}/T{b,para} ~ 1-10, and n_b/n_i ~ 0.2-14%. Strong ion and electron heating are observed within the series of shocklets and SLAMS increasing by factors \geq 5 and \geq 3, respectively. Both the core and halo electron components show strong perpendicular heating inside the feature.

Electromagnetic waves and electron anisotropies downstream of supercritical interplanetary shocks

We present waveform observations of electromagnetic lower hybrid and whistler waves with f_ci 1.01. Thus, the whistler mode waves appear to be driven by a heat flux instability and cause perpendicular heating of the halo electrons. The lower hybrid waves show a much weaker correlation between \partialB and normalized heat flux magnitude and are often observed near magnetic field gradients. A third type of event shows fluctuations consistent with a mixture of both lower hybrid and whistler mode waves. These results suggest that whistler waves may indeed be regulating the electron heat flux and the halo temperature anisotropy, which is important for theories and simulations of electron distribution evolution from the sun to the earth.

A statistical analysis of properties of small transients in the solar wind 2007–2009: STEREO and Wind observations

We present a comprehensive statistical analysis of small solar wind transients (STs) in 2007–2009. Extending work on STs by Kilpua et al. (2009) to a 3 year period, we arrive at the following identification criteria: (i) a duration < 12 h, (ii) a low proton temperature and/or a low proton beta, and (iii) enhanced field strength relative to the 3 year average. In addition, it must have at least one of the following: (a) decreased magnetic field variability, (b) large, coherent rotation of the field vector, (c) low Alfven Mach number, and (d) Te/Tp higher than the 3 year average. These criteria include magnetic flux ropes. We searched for STs using Wind and STEREO data. We exclude Alfvenic fluctuations. Case studies illustrate features of these configurations. In total, we find 126 examples, ∼81% of which lie in the slow solar wind (≤ 450 km s−1). Many start or end with sharp field and flow gradients/discontinuities. Year 2009 had the largest number of STs. The average ST duration is ∼4.3 h, 75%<6 h. Comparing with interplanetary coronal mass ejections (ICMEs) in the same solar minimum, we find the major difference to be that Tp in STs is not significantly less than the expected Tp. Thus, whereas a low Tp is generally considered a very reliable signature of ICMEs, it is not a robust signature of STs. Finally, since plasma β∼1, force-free modeling of STs having a magnetic flux rope geometry may be inappropriate.

Ion distributions in the Earth's foreshock: Hybrid-Vlasov simulation and THEMIS observations

We present the ion distribution functions in the ion foreshock upstream of the terrestrial bow shock obtained with Vlasiator, a new hybrid-Vlasov simulation geared toward large-scale simulations of the Earths magnetosphere (http://vlasiator.fmi.fi). They are compared with the distribution functions measured by the multispacecraft Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. The known types of ion distributions in the foreshock are well reproduced by the hybrid-Vlasov model. We show that Vlasiator reproduces the decrease of the backstreaming beam speed with increasing distance from the foreshock edge, as well as the beam speed increase and density decrease with increasing radial distance from the bow shock, which have been reported before and are visible in the THEMIS data presented here. We also discuss the process by which wave-particle interactions cause intermediate foreshock distributions to lose their gyrotropy. This paper demonstrates the strength of the hybrid-Vlasov approach which lies in producing uniformly sampled ion distribution functions with good resolution in velocity space, at every spatial grid point of the simulation and at any instant. The limitations of the hybrid-Vlasov approach are also discussed.