Observations of Electromagnetic Whistler Precursors at Supercritical Interplanetary Shocks L.B
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https://ntrs.nasa.gov/search.jsp?R=20120011758 2019-08-30T21:05:04+00:00Z CORE Metadata, citation and similar papers at core.ac.uk Provided by NASA Technical Reports Server GEOPHYSICAL RESEARCH LETTERS, VOL. ???, XXXX, DOI:10.1029/, Observations of Electromagnetic Whistler Precursors at Supercritical Interplanetary Shocks L.B. Wilson III,1 A. Koval,5,1 A. Szabo,1 A. Breneman,2 C.A. Cattell,2 K. Goetz,2 P.J. Kellogg,2 K. Kersten,2 J.C. Kasper3 B.A. Maruca3 M. Pulupa4 We present observations of electromagnetic precursor [Marsch and Chang, 1983; Wu et al., 1983; Matsukiyo and waves, identified as whistler mode waves, at supercritical Scholer, 2006], and can interact with both ions and electrons interplanetary shocks using the Wind search coil magne- [Wu et al., 1983]. Recent PIC simulations have found that tometer. The precursors propagate obliquely with respect whistler precursors may play an important role in particle acceleration at higher Mach number shocks [e.g. Riquelme to the local magnetic field, shock normal vector, solar wind and Spitkovsky, 2011]. Thus, whistlers are thought to play velocity, and they are not phase standing structures. All an important role in shock wave dynamics through both dis- are right-hand polarized with respect to the magnetic field persion and anomalous resistivity. (spacecraft frame), and all but one are right-hand polar- In this study, we present the first search coil observa- ized with respect to the shock normal vector in the normal tions of whistler precursors in and around the ramp region incidence frame. They have rest frame frequencies fci < f of four supercritical interplanetary (IP) shocks observed by the Wind spacecraft. We also present evidence for wave fce and wave numbers 0.02 . kρce . 5.0. Particle dis- ≪ tributions show signatures of specularly reflected gyrating heating and particle acceleration in one event. This work presents the first 3-D magnetic field waveform observations ions, which may be a source of free energy for the observed of waves with fsc . f and f < fsc < fce at supercritical IP modes. In one event, we simultaneously observe perpendic- lh lh shocks, where fsc is the spacecraft frame frequency and flh ular ion heating and parallel electron acceleration, consis- (= √f cef ci) is the lower hybrid resonance frequency. tent with wave heating/acceleration due to these waves. Al- though the precursors can have δB/Bo as large as 2, fluxgate 2. Observations magnetometer measurements show relatively laminar shock transitions in three of the four events. [Date: 03-27-2012] Waveform captures were obtained from the Wind/WAVES instrument [Bougeret et al., 1995], using the time domain sampler slow (TDSS) receiver, providing three magnetic field components and one electric field component, with 2048 1. Introduction data points sampled at 1875 samples/s. The ambient mag- netic field was obtained from the dual, triaxial fluxgate mag- Collisionless shock waves, predicted over 50 years ago netometers [Lepping et al., 1995] sampled at 11 samples/s. [Petschek, 1958], are a ubiquitous nonlinear structure ob- Ion and electron moments obtained from the∼ Wind/3DP served in nearly every space plasma environment. Shock EESA and PESA particle detectors [Lin et al., 1995] were waves require energy dissipation – transfer of energy from used in conjunction with shock parameters found at [Kasper, one form to another through an irreversible process – where 2007] to calculate Mcr , assuming a polytrope index, γ = 5/3 the type of energy dissipation in collisionless shock waves [Edmiston and Kennel, 1984]. Supplemental ion distribu- depends upon the Mach number, Mf , and has implica- tions were calculated from the two Faraday Cup (FC) ion Coroniti tions for the global structure of the shock wave [ , instruments from the Wind/SWE experiment [Ogilvie et al., 1970]. Theory and observations suggest that low Mach num- 1995]. For more details about the WAVES and 3DP instru- Kennel et al. ber shocks rely upon wave dispersion [ , 1985] ments and analysis, see Wilson III et al. [2010]. and/or anomalous resistivity due to wave-particle interac- The top half of Table 1 lists the relevant shock param- Sagdeev Wilson III et al. tions for energy dissipation [ , 1966; , eters, provided by [Kasper, 2007], for the four events pre- 2007]. Above the first critical Mach number, Mcr , the shock sented herein, which include: the shock normal speed in the requires additional energy dissipation in the form of particle spacecraft frame, Vshn; the angle between the shock normal reflection to limit wave steepening [Edmiston and Kennel, vector and upstream magnetic field, θ ; fast mode Mach 1984]. Bn number, M ; and the shock compression ratio N 2/N 1. The Observations of low Mach number quasi-perpendicular f i i ◦ bottom half shows the ratios of the Mf to Mcr as well as (shock normal angle, θBn, 45 ) low-β shocks show a rel- to the three whistler critical Mach numbers [Krasnoselskikh atively laminar transition,≥ with a compressive electromag- et al., 2002]: Mw corresponds to the maximum Mach num- netic (EM) precursor wave supplying the dispersive dissi- ber at which a linear whistler can phase stand with respect pation [Kennel et al., 1985]. The precursor wave has been to the shock; Mgr is the maximum Mach number at which identified as a right-hand polarized wave with fci < f fce, ≪ the wave can carry energy into the upstream; and Mnw is where fcs is the cyclotron frequency of species s and i(e) the maximum Mach number for which a stationary solution represent ions(electrons), consistent with a whistler mode can be found above which the wave breaks. All four events wave. These ’whistlers’ have been observed in nearly ev- have M /Mcr > 1 (i.e., are supercritical) and two of the ery space plasma environment ([e.g. Wilson III et al., 2009, f events have M /Mnw > 1. and references therein]), can couple to multiple wave modes f 3. Analysis Copyright 2012 by the American Geophysical Union. Figure 1 plots the Wind/MFI data at 11 samples/s for 0094-8276/12/$5.00 the four supercritical IP shocks examined.∼ The only event 1 X - 2 L.B. WILSON III ET. AL.: LOWER HYBRID WAVES – PARTICLES with resolvable magnetic fluctuations is shown in Figure 1A. 2009] of whistlers. At these frequencies, whistlers lie on the The other three shock profiles show relatively laminar tran- same branch of the dispersion relation as both EM lower sitions, and magnetic foots are noticeable in Figure 1B and hybrid [Marsch and Chang, 1983] and magnetosonic wave 1D. All four IP shocks exhibit magnetic overshoots, consis- modes [Wu et al., 1983]. tent with previous observations of supercritical shocks [Far- We examined the particle distribution functions (Figures ris et al., 1993]. Note that Figure 1A was examined by 3i and 3e) from the Wind/3DP instrument for evidence of Wilson III et al. [2009], but only with fluxgate magnetome- free energy or signatures of ion reflection. Only three of the ters. The important observation here is that the magnetic events (see Figures 1A, 1B, and 1D) had the burst parti- fields are under-sampled and that higher frequency fluctu- cle data (full distribution every 3 seconds) needed to exam- ations (discussed and shown below) with amplitudes com- ine the distributions in and around the shock ramps. We parable to the shock ramp cannot be observed in fluxgate were able to produce the electron distributions using 11 ∼ magnetometer data at this sample rate. High time resolu- sample/s MFI data, reducing the aliasing across the shock, tion measurements ( 40 samples/s) are needed to resolve similar to method introduced by Schwartz et al. [2011]. This the ramp as one can≥ see the spiky nature of these plots is was not possible for the PESA distributions. However, when indicative of undersampling. we compared the distributions produced using multiple in- Figure 2 shows the four TDSS samples that span the stantaneous magnetic field vectors to those produced using ramps of the four supercritical IP shocks. Each panel shows only one (typical method), the results were consistent with the three normal incidence frame (NIF) components [defined each other for the Figure 1B event. This is primarily due by Sundkvist et al., 2012] of the search coil magnetic field. to the field direction being predominantly in the same di- The ratio of the maximum peak-to-peak wave amplitude to rection throughout the duration of the distribution. Thus, we believe that the PESA High distributions produced using the ambient magnetic field at the start of each event, δB/Bo, is: (A) 0.9; (B) 2.1; (C) 2.0; and (D) 0.5. Thus, at only one magnetic field vector (Figure 3i) accurately repre- least for∼B and C,∼ the wave amplitudes∼ significantly∼ modify sent the ion distributions across the shock in Figure 1B. the ramp structure shown in Figure 1. All three events with burst particle data showed evidence The waveforms were analyzed using Minimum Variance for gyrating ions (example in Figure 3i) at/near the pre- Analysis (MVA) [Khrabrov and Sonnerup, 1998] to deter- dicted gyrospeed for specular reflection [Thomsen et al., mine the wave vector, k; polarization with respect to the 1983a]. The particle distributions in Figure 3 have color- ambient magnetic field; and wave normal angles with re- coded outlines that correspond to the color-coded regions spect to the local magnetic field (θ ); shock normal vector in Figure 1B. The IP shock associated with Figure 1B also kB showed evidence for strong perpendicular (with respect to (θkn); and local solar wind velocity (θkV ). Wavelet analysis revealed that these waves exhibit fluctuations at both high the magnetic field) ion heating (second panel of Figure 3i), which was supported by PESA Low and SWE observations.