L. K. Jian

Ion-Driven Instabilities in the Solar Wind: Wind Observations of 19 March 2005

Abstract Intervals of enhanced magnetic fluctuations have been frequently observed in the solar wind. But it remains an open question as to whether these waves are generated at the Sun and then transported outward by the solar wind or generated locally in the interplanetary medium. Magnetic field and plasma measurements from the Wind spacecraft under slow solar wind conditions on 19 March 2005 demonstrate seven events of enhanced magnetic fluctuations at spacecraft‐frame frequencies somewhat above the proton cyclotron frequency and propagation approximately parallel or antiparallel to the background magnetic field B o. The proton velocity distributions during these events are characterized by two components: a more dense, slower core and a less dense, faster beam. Observed plasma parameters are used in a kinetic linear dispersion equation analysis for electromagnetic fluctuations at k x B o = 0; for two events the most unstable mode is the Alfvén‐cyclotron instability driven by a proton component temperature anisotropy T⊥/T|| > 1 (where the subscripts denote directions relative to B o), and for three events the most unstable mode is the right‐hand polarized magnetosonic instability driven primarily by ion component relative flows. Thus, both types of ion anisotropies and both types of instabilities are likely to be local sources of these enhanced fluctuation events in the solar wind.

Generation of ion cyclotron waves in the corona and solar wind

To examine the generation and nonlinear evolution of ion cyclotron waves in the corona and solar wind, we perform electromagnetic simulations using a wide range of plasma conditions and ion velocity distribution functions. The source of the instability is temperature anisotropy of ions with temperature perpendicular to the magnetic field larger than parallel. For velocity distribution we use Maxwellian, bi-Maxwellian, and Fermi-accelerated functions with perpendicular temperature larger than parallel with the aim to understand the extent to which the details of the distribution function impact the general properties and the nonlinear evolution of the instability. The results show that in a proton-electron plasma, ion cyclotron waves are generated over a wide range of temperature anisotropies and plasma beta. Also, the general properties of the instability and the nonlinear evolution of the waves are not sensitive to the details of the velocity distribution functions. Allowing for the presence of minor ion species we show that these ions by themselves can drive the instability and generate waves with frequencies below the gyrofrequency of the minor ions. In the event that protons also have temperature anisotropy, waves on the proton branch are also generated. Results using bi-Maxwellian or Fermi-accelerated velocity distribution functions show similar properties for the instability and the nonlinear evolution of the waves. However, differences are found when allowing for relative drifts between the protons and minor ions in that when using Fermi-accelerated distribution functions oblique ion cyclotron waves are generated that are not observed in simulations using bi-Maxwellian distribution function.

Ion-driven instabilities in the solar wind: Wind observations of 19 March 2005

Abstract Intervals of enhanced magnetic fluctuations have been frequently observed in the solar wind. But it remains an open question as to whether these waves are generated at the Sun and then transported outward by the solar wind or generated locally in the interplanetary medium. Magnetic field and plasma measurements from the Wind spacecraft under slow solar wind conditions on 19 March 2005 demonstrate seven events of enhanced magnetic fluctuations at spacecraft‐frame frequencies somewhat above the proton cyclotron frequency and propagation approximately parallel or antiparallel to the background magnetic field B o. The proton velocity distributions during these events are characterized by two components: a more dense, slower core and a less dense, faster beam. Observed plasma parameters are used in a kinetic linear dispersion equation analysis for electromagnetic fluctuations at k x B o = 0; for two events the most unstable mode is the Alfvén‐cyclotron instability driven by a proton component temperature anisotropy T⊥/T|| > 1 (where the subscripts denote directions relative to B o), and for three events the most unstable mode is the right‐hand polarized magnetosonic instability driven primarily by ion component relative flows. Thus, both types of ion anisotropies and both types of instabilities are likely to be local sources of these enhanced fluctuation events in the solar wind.

Interplanetary shocks and foreshocks observed by STEREO during 2007–2010

Interplanetary shocks in the heliosphere modify the solar wind through which they pass. In particular, shocks play an important role in particle acceleration. During the extended solar minimum (2007–2010) STEREO observed 65 forward shocks driven by stream interactions (SI), with magnetosonic Mach numbers Mms ≈ 1.1–4.0 and shock normal angles θBN ~ 20–87°. We analyze the waves associated with these shocks and find that the region upstream can be permeated by whistler waves (f ~ 1 Hz) and/or ultra low frequency (ULF) waves (f ~ 10−2–10−1 Hz). While whistlers appear to be generated at the shock, the origin of ULF waves is most probably associated with local kinetic ion instabilities. We find that when the Mach number (Mms) is low and the shock is quasi-perpendicular ( θBN > 45°) whistler waves remain close to the shock. As Mms increases, the shock profile changes and can develop a foot and overshoot associated with ion reflection and gyration. Whistler precursors can be superposed on the foot region, so that some quasi-perpendicular shocks have characteristics of both subcritical and supercritical shocks. When the shock is quasi-parallel ( θBN < 45°) a large foreshock with suprathermal ions and waves can form. Upstream, there are whistler trains at higher frequencies whose characteristics can be slightly modified probably by reflected and/or leaked ions and by almost circularly polarized waves at lower frequencies that may be locally generated by ion instabilities. In contrast with planetary bow shocks, most of the upstream waves studied here are mainly transverse and no steepening occurs. Some quasi-perpendicular shocks (45° <  θBN < 60°) are preceded by ULF waves and ion foreshocks. Fluctuations downstream of quasi-parallel shocks tend to have larger amplitudes than waves in the sheath of quasi-perpendicular shocks. We compare SI-driven shock properties with those of shocks generated by interplanetary coronal mass ejections (ICMEs). During the same years, STEREO observed 20 ICME-driven shocks with Mms ≈ 1.2–4.0 and θBN ~ 38–85°. We find that shocks driven by ICMEs tend to have larger proton foreshocks (dr ~ 0.1 AU) than shocks driven by stream interactions (dr ≤ 0.05 AU). This difference of ion foreshock size should be linked to shock age: ICME-driven shocks form at shorter distances to the Sun and therefore can energize particles for longer times as they propagate to 1 AU, while stream interaction shocks form closer to Earths orbit and have been accelerating ions for a shorter interval of time.

The impact of a slow interplanetary coronal mass ejection on Venus

We present Venus Express observations of the impact of a slow interplanetary coronal mass ejection (ICME), which struck Venus on 23 December 2006, creating unusual quasi steady state upstream conditions for the 2 h close to periapsis: an enhanced (∼ nT) interplanetary magnetic field (IMF), radially aligned with the Sun-Venus line; and a dense (∼ cm−3) solar wind. Contrary to our current understanding and expectations, the ionosphere became partially demagnetized. We also find evidence for shocked sheathlike solar wind protons and electrons in the wake of Venus, and powerful (≈ nT2/Hz) foreshock whistler mode waves radiating from the bow shock at an unexpectedly low frequency (0.6 Hz). Given the abnormally high density of escaping heavy ions at the magnetopause boundary (295 cm−3, one of the highest of the whole mission) and the enhanced density of escaping heavy ions in the wake, we find that even weak ICMEs with no driving shocks can increase atmospheric loss rates at Venus and suggests that the Bx component of the IMF may be a factor in atmospheric escape rates.

MESSENGER survey of in situ low frequency wave storms between 0.3 and 0.7 AU

MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER)magnetometer data was surveyed between 0.3 and 0.7AU from 6 June 2007 to 23 March 2011 for low-frequency wave (LFW) storms, when the magnetometer was sampling at a rate of at least 2 s . A total of 12,197 LFW events were identified, of which 5506 lasted 10min or longer. The events have a high degree of polarization, are circularly polarized, with wave vectors nearly aligned or antialigned with the interplanetary magnetic field (IMF) at frequencies in the vicinity of the proton cyclotron frequency. These events are observed about 6% of the time, preferentially associatedwith radially directed inward or outward IMF. Their occurrence rate andmedian duration do not change much with R, where R is the heliocentric radial distance. For a narrow-frequency window in the solar wind frame, left-handed storms in the spacecraft frame have a power drop off that is roughly proportional to R 3 which is consistent with a source close the Sun, while right-handed storms have a power drop off roughly proportional R 1 which is not consistent with a source close to the Sun. The power in the left-handed LFW storms is on average greater than the right-handed ones by a factor of 3. In the solar wind frame, the wave frequency decreases from 0.13 to 0.04Hz moving from 0.3 to 0.7AU, but the frequency normalized by the local proton cyclotron frequency does not change much with the running median varying from 0.35 to 0.5. The normalized frequency band widths of the wave power spectra increase slightly with R, possibly associated with energy dissipation.

Low‐frequency waves within isolated magnetic clouds and complex structures: STEREO observations

Complex Structures (CSs) formed by the interaction of magnetic cloud (MC)-like structures with other transients (e.g., another MC, a stream interaction region, or a fast stream of solar wind) were frequently observed in the interplanetary space by STEREO spacecraft during the solar minimum 23 and the rising phase of the solar cycle 24. Here we report the presence of low-frequency waves (LFWs) inside some isolated MCs (IMCs) and inside the CSs observed by STEREO during such period (2007–2011). It is important to study in detail the properties of waves in space plasmas since particle distribution functions can be modified by wave-particle interactions. We compare wave characteristics within IMCs with those waves observed inside CSs. Both left-handed (LH) and right-handed (RH), near-circularly polarized, transverse and almost parallel-propagating LFWs (around the proton cyclotron frequency) were sporadically observed inside both IMCs and CSs. In contrast, compressive mirror-mode waves (MMs) were observed only within CSs. We studied local plasma conditions inside the IMCs and CSs to gain insight about wave origin: most of the MMs within CSs were observed in regions with enhanced plasma beta (β>1); the majority of the LH waves were found in low beta plasmas (β<1), and the RH waves were predominantly observed at moderate betas (0.4<β≤2). These observations are in agreement with linear kinetic theory predictions for the growth of the mirror, the LH ion cyclotron, and the RH ion firehose instability, respectively. It is possible that the waves were generated locally inside the IMCs and CSs via temperature anisotropies. The plasma beta enhancements that were frequently observed inside the CSs may be the result of compressions and heating taking place inside the interacting structures.

Generation and propagation of ion cyclotron waves in nonuniform magnetic field: Application to the corona and solar wind

With the objective to understand the generation, propagation, and nonlinear evolution of ion cyclotron waves (ICWs) in the corona and solar wind, we use electromagnetic hybrid (kinetic ions and fluid electrons) simulations with a nonuniform magnetic field. ICWs are generated by the temperature anisotropy of O5+ ions as minority species in a proton-electron plasma with uniform density. A number of magnetic field models are used including radial and spiral with field strength decreasing linearly or with the square of the radial distance. O5+ ions with perpendicular temperature larger than parallel are initially placed in the high-magnetic field regions. These ions are found to expand outward along the magnetic field. Associated with this expansion, ion cyclotron waves propagating along the magnetic field are also seen to expand outward. These waves are generated at frequencies below the local gyrofrequency of O5+ ions propagating parallel and antiparallel to the magnetic field. Through analysis of the simulation results we demonstrate that wave generation and absorption take place at all radial distances. Comparing the simulation results to observations of ICWs in the solar wind shows some of the observed wave characteristics may be explained by the mechanism discussed in this paper.

Ninety degrees pitch angle enhancements of suprathermal electrons associated with interplanetary shocks

We report the results of the first systematic analysis of 90° pitch angle (PA) enhancements or the ring distributions of suprathermal (E ∼70 eV–2 keV) electrons at interplanetary (IP) shocks. We analyze 2 h time intervals around 232 IP shocks observed by the two STEREO spacecraft between 2007 and 2011. The ring distributions were detected downstream of 114 events (49%). In 52 (22.4%) cases they were detected at the shock ramp. We also found 90° enhancements upstream of 11 (4.7%) events. Statistical analysis of basic shock properties did not reveal substantial differences between the shocks that are associated with the enhancements and those that are not. The data from the STEREO/WAVES instruments revealed that the 90° PA enhancements tend to be associated with magnetic and electric field fluctuations. Although at this point we do not have a satisfactory explanation for the mechanism that produces these distributions, our findings suggest that wave-particle interactions play a role, while pure focusing and mirroring effects due to adiabatic motion of electrons across the shock fronts cannot fully account for the observations.

In situ analysis of heliospheric current sheet propagation

The heliospheric current sheet (HCS) is an important structure not only for understanding the physics of interplanetary space but also for space weather prediction. We investigate the differences of the HCS arrival time between three spacecraft separated in heliolongitude, heliolatitude and radial distance from the Sun (STEREO A, STEREO B, and ACE) to understand the key factors controlling the HCS propagation. By assuming that the source of the solar wind does not evolve except for the effects of solar rotation, we first test the first-order approach method (ignoring latitudinal differences), using STEREO observation during the year 2007, when the Sun was quiet and the two STEREO spacecraft were separated in heliolongitude by less than 44°. The first-order approach method matches well with observations for many events except for those events when the HCS has a small inclination angle to the ecliptic plane. The latitudinal effect is suggested to account for such discrepancies. The predictions are not improved much by considering the HCS inclination angle obtained from the potential field source surface (PFSS) model. However, the predictions match well with the observations when the HCS inclination angle at 1 AU is obtained from the time differences of HCS arrival times between the STEREO B and ACE spacecraft. An improved model of calculating the inclination of the heliospheric current sheet other than PFSS is needed.