Gurbax S. Lakhina

Some Basic Concepts of Wave-Particle Interactions in Collisionless Plasmas

The physical concepts of wave-particle interactions in a collisionless plasma are developed from first principles. Using the Lorentz force, starting with the concepts of particle gyromotion, particle mirroring and the loss cone, normal and anomalous cyclotron resonant interactions, pitch angle scattering, and cross-field diffusion are developed. To aid the reader, graphic illustrations are provided.

Extremely intense ELF magnetosonic waves: A survey of polar observations

A Polar magnetosonic wave (MSW) study was conducted using 1 year of 1996–1997 data (during solar minimum). Waves at and inside the plasmasphere were detected at all local times with a slight preference for occurrence in the midnight-postmidnight sector. Wave occurrence (and intensities) peaked within~±5° of the magnetic equator, with half maxima at ~±10°. However, MSWs were also detected as far from the equator as +20° and 60° MLAT but with lower intensities. An extreme MSW intensity event of amplitude Bw = ~± 1 nT and Ew = ~± 25 mV/m was detected. This event occurred near local midnight, at the plasmapause, at the magnetic equator, during an intense substorm event, e.g., a perfect occurrence. These results support the idea of generation by protons injected from the plasma sheet into the midnight sector magnetosphere by substorm electric fields. MSWs were also detected near noon (1259 MLT) during relative geomagnetic quiet (low AE). A possible generation mechanism is a recovering/expanding plasmasphere engulfing preexisting energetic ions, in turn leading to ion instability. The wave magnetic field components are aligned along the ambient magnetic field direction, with the wave electric components orthogonal, indicating linear wave polarization. The MSW amplitudes decreased at locations further from the magnetic equator, while transverse whistler mode wave amplitudes (hiss) increased. We argue that intense MSWs are always present somewhere in the magnetosphere during strong substorm/convection events. We thus suggest that modelers use dynamic particle tracing codes and the maximum (rather than average) wave amplitudes to simulate wave-particle interactions.

Auroral zone dayside precipitation during magnetic storm initial phases

Abstract Significant charged-particle precipitation occurs in the dayside auroral zone during and after interplanetary shock impingements on the Earths magnetosphere. The precipitation intensities and spatial and temporal evolution are discussed. Although the post-shock energy flux (10– 20 erg cm −2 s −1 ) is lower than that of substorms, the total energy deposition rate may be considerably greater (∼ an order of magnitude) than nightside energy rates due to the greater area of the dayside portion of the auroral oval (defined as extending from 03 MLT through noon to 21 MLT). This dayside precipitation represents direct solar wind energy input into the magnetosphere/ionosphere system. The exact mechanisms for particle energization and precipitation into the ionosphere are not known at this time. Different mechanisms are probably occurring during different portions of the storm initial phase. Immediately after shock compression of the magnetosphere, possible precipitation-related mechanisms are: (1) betatron compression of preexisting outer zone magnetospheric particles. The anisotropic plasma is unstable to loss-cone instabilities, leading to plasma wave growth, resonant particle pitch-angle scattering and electron and proton losses into the upper ionosphere. (2) The compression of the magnetosphere can also lead to enhanced field-aligned currents and the formation of dayside double-layers. Finally (3) in the latter stages of the storm initial phase, there is evidence for a long-lasting viscous-like interaction occurring on the flanks of the magnetopause. Ground-based observations identifying the types of dayside auroral forms would be extremely useful in identifying the specific solar wind energy transfer mechanisms.

Plasmaspheric hiss properties: Observations from Polar

In the region between L = 2 to 7 at all Magnetic Local Time (MLTs) plasmaspheric hiss was detected 32% of the time. In the limited region of L = 3 to 6 and 15 to 21 MLT (dusk sector), the wave percentage detection was the highest (51%). The latter plasmaspheric hiss is most likely due to energetic ~10–100 keV electrons drifting into the dusk plasmaspheric bulge region. On average, plasmaspheric hiss intensities are an order of magnitude larger on the dayside than on the nightside. Plasmaspheric hiss intensities are considerably more intense and coherent during high-solar wind ram pressure intervals. A hypothesis for this is generation of dayside chorus by adiabatic compression of preexisting 10–100 keV outer magnetospheric electrons in minimum B pockets plus chorus propagation into the plasmasphere. In large solar wind pressure events, it is hypothesized that plasmaspheric hiss can also be generated inside the plasmasphere. These new generation mechanism possibilities are in addition to the well-established mechanism of plasmaspheric hiss generation during substorms and storms. Plasmaspheric hiss under ordinary conditions is of low coherency, with small pockets of several cycles of coherent waves. During high-solar wind ram pressure intervals (positive SYM-H intervals), plasmaspheric hiss and large L hiss can have higher intensities and be coherent. Plasmaspheric hiss in these cases is typically found to be propagating obliquely to the ambient magnetic field with θkB0 ~30° to 40°. Hiss detected at large L has large amplitudes (~0.2 nT) and propagates obliquely to the ambient magnetic field (θkB0 ~70°) with 2:1 ellipticity ratios. A series of schematics for plasmaspheric hiss generation is presented.

Evolution of Nonlinear Alfvén Waves in Streaming Inhomogeneous Plasmas

A nonlinear evolution equation for Alfven waves, propagating in streaming plasmas with nonuniform densities and inhomogeneous magnetic fields, is obtained by using the reductive perturbation technique. The governing equation is a modified derivative nonlinear Schrodinger (MDNLS) equation. The numerical solution of this equation shows that inhomogeneities exhibit their presence as an effective dissipation. The spatiotemporal evolution of long-wavelength Alfvenic fluctuations shows that the wave steepens as it propagates. High-frequency radiation is also observed in our simulations. Unlike coherent Alfven waves in homogeneous plasmas, which can become noncoherent/chaotic only in the presence of a driver, MDNLS evolves into noncoherent/turbulent state without any driver simply because of inhomogeneities. This clearly indicates that the integrability property of the derivative nonlinear Schrodinger equation, which allows coherent solitary solutions, is destroyed by inhomogeneities.

Broadband plasma waves observed in the polar cap boundary layer: Polar

Polar observations indicate the presence of intense broadband plasma waves nearly all of the time (96% occurrence frequency in this study) near the apogee of the Polar trajectory (∼6–8 RE). The region of wave activity bounds the dayside (0500 to 1800 LT) polar cap magnetic fields, and we thus call these waves polar cap boundary layer (PCBL) waves. The waves are spiky signals spanning a broad frequency range from ∼101 to 2 × 104 Hz. The waves have a rough power law spectral shape. The wave magnetic component has on average a ƒ−2.7 frequency dependence and appears to have an upper frequency cutoff of ∼(6–7) × 103 Hz, which is the electron cyclotron frequency. The electric component has on average a ƒ−2.2 frequency dependence and extends up to ∼2 × 104 Hz. The frequency dependences of the waves and the amplitude ratios of B′/E′ indicate a possible mixture of obliquely propagating electromagnetic whistler mode waves plus electrostatic waves. There are no clear intensity peaks in either the magnetic or electric spectra which can identify the plasma instability responsible for the generation of the PCBL waves. The wave character (spiky nature, frequency dependence and admixture of electromagnetic and electrostatic components) and intensity are quite similar to those of the low-latitude boundary layer (LLBL) waves detected at and inside the low-latitude dayside magnetopause. Because of the location of the PCBL waves just inside the polar cap magnetic field lines, it is natural to assume that these waves are occurring on the same magnetic field lines as the LLBL waves, but at lower altitudes. Because of the similar wave intensities at both locations and the occurrence at all local times, we rule out an ionospheric source. We also find a magnetosheath origin improbable. The most likely scenario is that the waves are locally generated by field-aligned currents or current gradients. We find a strong relationship between the presence of ionospheric and magnetosheath ions and the waves near the noon sector. These waves may thus be responsible for ion heating observed near the cusp region. Antisunward convection of these freshly accelerated oxygen ions over the polar cap during intense wave events (occurring during southward Bz events) might lead to enhanced plasma sheet O+ population. For magnetic storm intervals this mechanism would lead to a natural delay between the main phase onset and the appearance of oxygen ions in the ring-current.

An extreme coronal mass ejection and consequences for the magnetosphere and Earth

A “perfect” interplanetary coronal mass ejection could create a magnetic storm with intensity up to the saturation limit (Dst ~ −2500 nT), a value greater than the Carrington storm. Many of the other space weather effects will not be limited by saturation effects, however. The interplanetary shock would arrive at Earth within ~12 h with a magnetosonic Mach number ~45. The shock impingement onto the magnetosphere will create a sudden impulse of ~234 nT, the magnetic pulse duration in the magnetosphere will be ~22 s with a dB/dt of ~30 nT s−1, and the magnetospheric electric field associated with the dB/dt ~1.9 V m−1, creating a new relativistic electron radiation belt. The magnetopause location of 4 RE from the Earths surface will allow expose of orbiting satellites to extreme levels of flare and ICME shock-accelerated particle radiation. The results of our calculations are compared with current observational records. Comments are made concerning further data analysis and numerical modeling needed for the field of space weather.

Parametric analysis of positive amplitude electron acoustic solitary waves in a magnetized plasma and its application to boundary layers

Received 27 August 2007; revised 5 November 2007; accepted 19 December 2007; published 20 June 2008. [1] The existence domain of a fully nonlinear positive amplitude electron acoustic solitary wave has been studied in a four-component plasma composed of warm magnetized electrons, warm electron beam, and energetic multi-ion species with ions hotter than the electrons (Ti > Te). A Sagdeev pseudopotential technique has been used to obtain the nonlinear evolution equation for the wave propagating obliquely with the ambient magnetic field. It is observed that the ion temperatures and concentrations play a crucial role in determining the characteristics and the existence domain of the electron acoustic solitary wave. With a large cold ion population and/or a large cold to hot ion temperature ratio, the plasma tends to behave like a single ion-dominated one. The corresponding Sagdeev pseudopotential shows an extremely narrow and deep profile producing small-amplitude, narrow width, spiky solitary waves. Such solutions are found to be applicable in the bow shock, magnetosheath, and cusp regions. Comparison with CLUSTER observations agrees well with the analytical model. It has been shown that in the magnetosheath, cooler He 2+ ions are necessary to produce a positive polarity solution while a hotter species may produce a compressive (negative polarity) solution.

A pair of forward and reverse slow‐mode shocks detected by Ulysses at ∼5 AU

We report the first finding of a pair of forward and reverse slow-mode shocks in the distant heliosphere using plasma and magnetic field data from the Ulysses spacecraft located at 5.3 AU and 9° S heliolatitude. The slow-mode shocks are found to occur in a compressed magnetic field (low plasma ) region within a co-rotating interaction region (CIR). We find Mach numbers to be 0.3-0.5 with respect to forward/reverse slow shocks. Across each shock, the solar wind velocities jump by at least 40 km/s. The increases in plasma density and ion temperature accompany a decrease in the magnetic field. The shocks are also found to have velocities of 60 km/s and 115 km/s and thicknesses between 7.5 - 12.6 x 10 4 km (much larger than the ion inertial length, ∼ 10 3 km). Low frequency plasma waves are detected by the Ulysses URAP instrument at the slow-mode shock transition regions. However, the waves are not of sufficient amplitude to provide enough anomalous resistivity through wave-particle interactions for shock dissipation. Low energy (∼30 - 90 keV) electron enhancements directed along the local magnetic field are also found associated with the slow shocks, indicating the ability of the shocks to accelerate interplanetary particles. This finding imply that more slow shocks might be found in the CIR magnetic compressed regions (where plasma is squeezed out) at large heliospheric distances.

Comment on “Comment on the abundances of rotational and tangential discontinuities in the solar wind” by M. Neugebauer

[1] Neugebauer [2006] has very nicely reviewed the current status of work done on identifying the abundance of rotational discontinuities (RDs) and tangential discontinuities (TDs) occurring in interplanetary space. This has been a topic of great interest and heated debate since the 1970s [Smith, 1973; Belcher and Solodyna, 1975; Burlaga et al., 1977; Lepping and Behannon, 1980] (see also discussion by Neugebauer et al. [1984]). Neugebauer [2006] has also reexamined jump conditions across discontinuities and has ended up with inconclusive answers. [2] We wish to make some suggestions that may help clarify the apparently conflicting results of the RD/TD occurrence ratio existing in the literature. We will argue that in many cases discontinuities are ‘‘contaminated’’ by overlying plasma and induced magnetic fields (see also discussion by Sonnerup and Scheible [1998], Horbury et al. [2001], and Knetter et al. [2004] concerning contamination by electromagnetic plasma waves), leading to errors (in interpretation) of results using the Sonnerup and Cahill [1967] minimum variance method (MVA). We also will argue that the establishment of pure (or nearly pure) solar wind convection of discontinuities does not necessarily lead to the conclusion that they are TDs. [3] Tsurutani et al. [1994] have argued that interplanetary discontinuities are (often) the phase steepened edges of nonlinear Alfvén waves as Neugebauer [2006] notes. Another feature detected in interplanetary space are decreases in the interplanetary magnetic field magnitude. These have been given the name magnetic holes (MHs), magnetic decreases (MDs) and other descriptive names in the literature [Turner et al., 1977; Winterhalter et al., 1994, 2000; Tsurutani and Ho, 1999]. These magnetic field magnitude (pressure) decreases are supplanted by enhanced, anisotropic plasma [Fränz et al., 2000; Neugebauer et al., 2001]. The total pressure is constant across these structures, to first order [Winterhalter et al., 1994]. It has recently been shown that these MHs/MDs are (often) collocated with the discontinuities/phase steepened edges of Alfvén waves [Tsurutani et al., 2002a, 2002b]. Dasgupta et al. [2003] and Tsurutani et al. [2002b, 2005a] have argued that the ponderomotive force associated with the steepened Alfvén wave edges (the discontinuities) accelerate solar wind ions (and electrons) perpendicular to the ambient magnetic field and thus create the MHs/MDs by plasma diamagnetic effects. In this scenario, the plasma blobs and their resultant magnetic decreases are external features to the discontinuities and not parts of the discontinuities/Alfvén waves themselves. From this viewpoint, MHs/MDs can thus be thought of as byproducts of the Alfvén wave dissipation process. [4] We view MH/MD plasma and induced field decreases which are collocated with the discontinuities as possible contaminants to the intrinsic discontinuity structures. In some cases the MHs/MDs can appear to be bounded by a pair of TD-like structures as well [Tsurutani and Ho, 1999]. Minimum variance analysis results of this very complex region of multiple discontinuities will be difficult, if not impossible to interpret. [5] Tsurutani et al. [2005b] have examined several (7) events from the Knetter [2005] Cluster discontinuity data set where the discontinuities were collocated with MHs/MDs. All of the discontinuities were associated with Alfvén waves. The same discontinuities were identified at ACE, 0.01 AU upstream of Cluster, by their similar field rotational characteristics. The time delay from detection at ACE to that at Cluster was measured. It was found that the discontinuities/MHs/MDs propagated at almost the solar wind convection speed (determined by plasma measurements), within measurement uncertainties. Tsurutani et al. [2005b] speculated that the low wave propagation speed relative to the ambient solar wind was due to a ‘‘slowing’’ of the wave phase speed through the high-density plasma (the MHs/MDs), oblique wave propagation, or a combination of both factors. [6] As to why most directional discontinuities (DDs) in the solar wind have small values of BN [Knetter et al., 2004; JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, A03101, doi:10.1029/2006JA011973, 2007