Magnetic Fluctuation Power Near Proton Temperature Anisotropy Instability Thresholds in the Solar Wind

Abstract

The proton temperature anisotropy in the solar wind is known to be constrained by the theoretical thresholds for pressure anisotropy-driven instabilities. Here we use approximately 1 million independent measurements of gyroscale magnetic fluctuations in the solar wind to show for the first time that these fluctuations are enhanced along the temperature anisotropy thresholds of the mirror, proton oblique firehose, and ion cyclotron instabilities. In addition, the measured magnetic compressibility is enhanced at high plasma beta (β 1) along the mirror instability threshold but small elsewhere, consistent with expectations of the mirror mode. The power in this frequency (the 'dissipation') range is often considered to be driven by the solar wind turbulent cascade, an interpretation which should be qualified in light of the present results. In particular, we show that the short wavelength magnetic fluctuation power is a strong function of collisionality, which relaxes the temperature anisotropy away from the instability conditions and reduces correspondingly the fluctuation power.PACS numbers: 96.50. Ci, 52.35.Ra, 95.30.Qd The physical processes that regulate the expansion of the super-Alfvénic solar wind into space include adiabatic particle motion, plasma instabilities, and binary particle collisions. As the wind expands, plasma density n p and magnetic field |B| decrease radially. The Chew-Goldberger-Low (CGL) relations [1] predict that the plasma ions should become anisotropic in the sense of T > T ⊥ if the particle motion is adiabatic and collisionless; here T is the ion temperature parallel and perpendicular to the background magnetic field. However, Coulomb collisions and pressure-anisotropy instabilities act to pitch-angle scatter the plasma back towards isotropy [2]. At 1 AU, the most probable value of the proton temperature anisotropy is T ⊥ /T ≈ 0.89 (see Figure 1, top panel below). If CGL were valid, this would imply a proton temperature anisotropy of T ⊥ /T ≥ 200 at 5 solar radii.The same pressure-anisotropy instabilities that operate in the solar wind are believed to operate in other low-collisionality astrophysical plasmas, including clusters of galaxies [3] and some accretion disks onto black holes [4,5]. In the latter environment, these instabilities not only modify the thermodynamics of the plasma (as in the solar wind), but they also play a crucial dynamical role, regulating the anisotropic stress that helps transport angular momentum, allowing accretion to proceed. vice, where isotropization and magnetic fluctuations were observed corresponding to the Alfvén ion cyclotron instability [6]. Similar results were obtained in the solar wind at 1 AU [7] (for T ⊥ /T > and solar wind speeds greater than 600 km/s), suggesting that the proton cyclotron instability plays a role. Both mirror mode and ion cyclotron anisotropy instabilities appear to be active in the terrestrial magnetosheath, inferred from observed constraints on the temperature anisotropy [8,9]. More recently, Kasper et al. Growth of ion...

Solar Wind Electrons Alphas and Protons (SWEAP) Investigation: Design of the Solar Wind and Coronal Plasma Instrument Suite for Solar Probe Plus

“…Closer to the Sun β ||p is lower, so the allowable range of R p should be larger, but the heating may also be stronger. Statistical studies such as these, drawn from a large ensemble of measurements, are a power tool to identify the relative roles of heating and instabilities in modifying the VDFs (Bale et al 2009). A statistical determination of the effects of instabilities within 0.25 AU requires a large ensemble (at least several million) of solar wind measurements over all conditions near the Sun; since the growth rates of the instabilities are small compared to the ion gyro-frequency time resolution is not a driver; ion and electron temperature anisotropies measured to 20 % accuracy.…”

“…Since telemetry rates on SPP are highly constrained, we place sufficient storage within the SWEM to run and archive SPAN observations at their maximum data rates throughout each encounter, and then downloaded high resolution data for regions of interest identified after the encounter and an initial examination of summary observations. Finally, to search for wave-particle interactions at the highest cadences, we employ a direct high speed interface between the SWEAP and FIELDS (Bale et al 2015, this issue) instrument suites. Particle counts are relayed to FIELDS through the interface, and a wave-particle correlator calculates correlations and phase delays between particles and electromagnetic fields.…”

Kinetic instabilities in the solar wind driven by temperature anisotropies

“…The magnetosheath temperature anisotropy instability problem was first extended to the solar wind proper and solar corona, by Gary et al (2001a, b). Many subsequent works followed including those by , Kasper et al (2003Kasper et al ( , 2008, Marsch et al (2004Marsch et al ( , 2006, , , Matteini et al (2007Matteini et al ( , 2012, Bale et al (2009), Bourouaine et al (2010, Maruca et al (2011Maruca et al ( , 2012, Osman et al (2011Osman et al ( , 2012Osman et al ( , 2013, Marsch (2012), , Adrian et al (2016), and others that the present review might have missed. These works further investigated the effects of temperature anisotropy instabilities in the solar wind by analyzing data obtained from an armada of spacecraft including Helios, Ulysses, WIND, ACE, Stereo, etc.…”

“…1, the proton data distribution obtained near 1 AU, in ðb ki ; T ?i =T ki Þ parameter space. Figure 1 is adopted from the paper by Michno et al (2014), where perpendicular and parallel temperatures as well as the density and magnetic field intensity in the solar wind are calculated using data from WIND SWE and MFI instruments (through the SPDF CDAWeb service) (Bale et al 2009;Ogilvie et al 1995;Lepping et al 1995). We then constructed the proton temperature ratio and the parallel plasma beta.…”

Braginskii magnetohydrodynamics for arbitrary magnetic topologies: coronal applications

“…Schekochihin & Cowley 2006;Kunz et al 2012;Santos-Lima et al 2014Berlock & Pessah 2015) and solar wind (e.g. Bale et al 2009) plasmas have demonstrated that instabilities due to anisotropic effects can have a large influence on dynamics.…”