Solar Wind Flow Research Articles

Magnetospheric boundary perturbations on MHD and kinetic scales

AbstractTo study the magnetopause on both MHD and kinetic scales, we have analyzed two Time History of Events and Macroscale Interactions during Substorms/Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun magnetopause crossings under steady slow‐solar wind and minimum magnetic shear conditions. These events approximate a ground state of the magnetospheric boundary with minimum influences from large‐solar wind disturbances and magnetic reconnection. Our observations reveal evidence for the Kelvin‐Helmholtz instability, the quasi‐periodicity of magnetopause surface waves accompanied by highly asymmetrical plasma signatures between the inbound (from magnetosheath to low‐latitude boundary layer (LLBL)) and the outbound (from LLBL to magnetosheath) magnetopause crossings. Stronger plasma and magnetic gradients were observed during the outbound crossings but more gradual and volatile variations at higher frequencies during the inbounds. The scale lengths of the magnetic and plasma gradients were comparable or less than the proton gyroradius. Enhancements of lower hybrid waves occurred at the locations of strong gradients or variations. We interpreted the collocations of the lower hybrid waves and plasma gradients and their variations in terms of (1) lower hybrid instabilities that directly convert solar wind flow energy into lower hybrid waves and other wave modes in the LLBL, or (2) Kelvin‐Helmholtz instability and magnetic reconnection which produce the conditions for the lower hybrid instabilities to grow. The rate of ion diffusion across the magnetopause caused by the lower hybrid instability is marginally sufficient to populate the LLBL. The diffusion coefficient of O+ is ~30 times larger than that of H+. The lower hybrid waves could contribute to the energy dissipation at plasma gradients in magnetopause surface wave structures and limit Kelvin‐Helmholtz instability growth further downstream.

Solar Sector Structure

The interplanetary magnetic field near has a characteristic "sector" structure that reflects its polarity relative to the solar direction. Typically we observe large-scale coherence in these directions, with two or four "away" or "towards" sectors per solar rotation, from any platform in deep space and near the ecliptic plane. In a simple picture, this morphology simply reflects the idea that the sources of the interplanetary field lie mainly in or near the Sun, and that the solar-wind flow enforces a radial component in this field. Although defined strictly via the interplanetary field near one AU, recent evidence confirms that this pattern also appears clearly at the level of the photosphere, with signatures including not only the large-scale structures (e.g., the streamers) but also highly concentrated fields such as those found in sunspots and even solar flares. This association with small-scale fields strengthens at the Hale sector boundary, defining the Hale boundary as the one for which the polarity switch matches that of the leading-to-following polarity alternation in the sunspots of a given hemisphere.

Spectral features of solar plasma flows

Research to the identification of plasma flows in the Solar wind by spectral characteristics of solar plasma flows in the range of magnetohydrodynamics is devoted. To do this, the wavelet skeleton pattern of Solar wind parameters recorded on Earth orbit by patrol spacecraft and then executed their neural network classification differentiated by bandwidths is carry out. This analysis of spectral features of Solar plasma flows in the form of magnetic clouds (MC), corotating interaction regions (CIR), shock waves (Shocks) and highspeed streams from coronal holes (HSS) was made. The proposed data processing and the original correlation-spectral method for processing information about the Solar wind flows for further classification as online monitoring of near space can be used. This approach will allow on early stages in the Solar wind flow detect geoeffective structure to predict global geomagnetic disturbances.

CXO X-ray spectroscopy of comets and abundances of heavy ions in the solar wind

X-rays from comets originate in charge exchange between heavy ions of the solar wind and cometary species. Spectra of nine comets observed by the Chandra X-ray Observatory (CXO) are analyzed using the time-dependent instrument sensitivity and the energy-dependent spectral resolution. X-ray emissions are extracted from the spectra in the range of 150–1100eV using the χ2-fitting. Production of X-rays varies in the observed comets by a factor of 500 from 4.4×1013ergs−1 in Comet 73P to 2.2×1016ergs−1 in Comet Ikeya–Zhang. The measured solar wind flow varies within a factor of 20, being the weakest in Comet 73P and the strongest in 9P/Tempel 1. The retrieved X-ray line intensities vary within a factor of 5×104. These lines above 300eV are attributed to emissions of the H- and He-like ions, and laboratory data on the excitation cross sections for these emissions (Greenwood et al. [2000]. Astrophys. J. 533, L175–L178) are used to convert the observed emissions into abundances of heavy ions in the solar wind. Continuity equations for charge exchange in comets are solved analytically and result in relationships between the X-ray emissions and the ion fluxes. The flux of O7+ scaled to 1AU varies within a factor of 35 with a mean value of 1.6×104cm−2s−1. The retrieved ratios of O8+/O7+, C6+/C5+, Ne10+/Ne9+, C6+/O7+, N6+/O7+, and Ne9+/O7+ demonstrate significant variations, while their mean values for O, C, and N agree with those recommended by Schwadron and Cravens (Schwadron and Cravens [2000]. Astrophys. J. 544, 558–566) for the slow and fast solar wind. (Data on Ne9+ and Ne10+ are lacking in Scwadron and Cravens (Schwadron and Cravens [2000]. Astrophys. J. 544, 558–566).) The results are compared with the ion ratios from Bodewits et al. (Bodewits et al. [2007]. Astron. Astrophys. 469, 1183–1195) that were obtained from the same CXO spectra of comets, and some significant differences are briefly discussed. CXO X-ray spectroscopy of comets is a diagnostic tool to study the composition of the solar wind and its variations.

The magnetic structure of Saturn's magnetosheath

AbstractA planet's magnetosheath extends from downstream of its bow shock up to the magnetopause, where the solar wind flow is deflected around the magnetosphere and the solar wind‐embedded magnetic field lines are draped. This makes the region an important site for plasma turbulence, instabilities, reconnection, and plasma depletion layers. A relatively high Alfvén Mach number solar wind and a polar‐flattened magnetosphere make the magnetosheath of Saturn both physically and geometrically distinct from the Earth's. The polar flattening is predicted to affect the magnetosheath magnetic field structure and thus the solar wind‐magnetosphere interaction. Here we investigate the magnetic field in the magnetosheath with the expectation that polar flattening is manifested in the overall draping pattern. We compare an accumulation of Cassini data between 2004 and 2010 with global magnetohydrodynamic (MHD) simulations and an analytical model representative of a draped field between axisymmetric boundaries. The draping patterns measured are well captured and in broad agreement for given upstream conditions with those of the MHD simulations (which include polar flattening). The deviations from the analytical model, based on no polar flattening, suggest that nonaxisymmetry is invariably a key feature of the magnetosphere's global structure. Our results show a comprehensive overview of the configuration of the magnetic field in a nonaxisymmetric magnetosheath as revealed by Cassini. We anticipate our assessment to provide an insight to this barely studied interface between a high Alfvénic bow shock and a dynamic magnetosphere.

Ion cyclotron waves at Mars: Occurrence and wave properties

AbstractIon cyclotron waves (ICWs) are generated during the interaction between the solar wind and the Martian exosphere in a process called ion pickup. Mars Global Surveyor (MGS) detected waves near the proton gyrofrequency, indicating pickup of the exospheric hydrogen. To analyze these waves, we first improve the zero levels of the MGS magnetic field data taken during the first aerobreaking phase and then perform a statistical study of the ICWs observed from just outside the Martian bow shock to over 14 Mars radii away. These ICW events typically last for 5 to 30 min but can occasionally last for hours. The wave power decreases slowly with distance on both the upstream and downstream sides. From the variation of wave properties with the strength of the background field, we find that there are likely still remaining offsets in at least some the data sets even after applying our calibration technique. Thus, we use the events with a strong background field to examine the wave properties that depend on an accurate determination of the field direction and strength. We find the pickup angle associated with the largest occurrence rate of ICWs to be around 45°, but neither the wave amplitude, nor wave frequency, nor wave duration appear to vary with pickup angle. Finally, we find the waves with background field strength greater than 4 nT occur on both the positive and negative electric field sides of Mars but have a larger occurrence rate on the side of Mars in the positive electric field direction (which is defined as the direction of the cross product of the magnetic field vector and solar wind flow vector).

A vortical dawn flank boundary layer for near‐radial IMF: Wind observations on 24 October 2001

AbstractWe present an example of a boundary layer tailward of the dawn terminator which is entirely populated by rolled‐up flow vortices. Observations were made by Wind on 24 October 2001 as the spacecraft moved across the region at X ∼−13 RE. Interplanetary conditions were steady with a near‐radial interplanetary magnetic field (IMF). Approximately 15 vortices were observed over the 1.5 h duration of Wind's crossing, each lasting ∼5 min. The rolling up is inferred from the presence of a hot tenuous plasma being accelerated to speeds higher than in the adjoining magnetosheath, a circumstance which has been shown to be a reliable signature of this in single‐spacecraft observations. A blob of cold dense plasma was entrained in each vortex, at whose leading edge abrupt polarity changes of field and velocity components at current sheets were regularly observed. In the frame of the average boundary layer velocity, the dense blobs were moving predominantly sunward and their scale size along X was ∼7.4 RE. Inquiring into the generation mechanism of the vortices, we analyze the stability of the boundary layer to sheared flows using compressible magnetohydrodynamic Kelvin‐Helmholtz theory with continuous profiles for the physical quantities. We input parameters from (i) the exact theory of magnetosheath flow under aligned solar wind field and flow vectors near the terminator and (ii) the Wind data. It is shown that the configuration is indeed Kelvin‐Helmholtz (KH) unstable. This is the first reported example of KH‐unstable waves at the magnetopause under a radial IMF.

Bowl‐shaped deformation found in Earth's magnetosphere

The decade‐long Cassini mission has yielded invaluable information about Saturn's magnetosphere. One feature of Saturn's magnetosphere that has puzzled scientists is a bowl‐shaped deformation of Saturn's equatorial current sheet—the disk‐like sheet of charged particles across which the planet's magnetic field abruptly reverses direction. So far, there are few data available to explain this deformation, which is seen only during periods when the planet's magnetic axis is not perpendicular to the solar wind flow.

Reconstruction of geomagnetic activity and near-Earth interplanetary conditions over the past 167 yr – Part 4: Near-Earth solar wind speed, IMF, and open solar flux

Abstract. In the concluding paper of this tetralogy, we here use the different geomagnetic activity indices to reconstruct the near-Earth interplanetary magnetic field (IMF) and solar wind flow speed, as well as the open solar flux (OSF) from 1845 to the present day. The differences in how the various indices vary with near-Earth interplanetary parameters, which are here exploited to separate the effects of the IMF and solar wind speed, are shown to be statistically significant at the 93% level or above. Reconstructions are made using four combinations of different indices, compiled using different data and different algorithms, and the results are almost identical for all parameters. The correction to the aa index required is discussed by comparison with the Ap index from a more extensive network of mid-latitude stations. Data from the Helsinki magnetometer station is used to extend the aa index back to 1845 and the results confirmed by comparison with the nearby St Petersburg observatory. The optimum variations, using all available long-term geomagnetic indices, of the near-Earth IMF and solar wind speed, and of the open solar flux, are presented; all with ±2σ uncertainties computed using the Monte Carlo technique outlined in the earlier papers. The open solar flux variation derived is shown to be very similar indeed to that obtained using the method of Lockwood et al. (1999).

Grid-free 2D plasma simulations of the complex interaction between the solar wind and small, near-Earth asteroids

We present results from a new grid-free 2D plasma simulation code applied to a small, unmagnetized body immersed in the streaming solar wind plasma. The body was purposely modeled as an irregular shape in order to examine photoemission and solar wind plasma flow in high detail on the dayside, nightside, terminator and surface-depressed ‘pocket’ regions. Our objective is to examine the overall morphology of the various plasma interaction regions that form around a small body like a small near-Earth asteroid (NEA). We find that the object obstructs the solar wind flow and creates a trailing wake region downstream, which involves the interplay between surface charging and ambipolar plasma expansion. Photoemission is modeled as a steady outflow of electrons from illuminated portions of the surface, and under direct illumination the surface forms a non-monotonic or “double-sheath” electric potential upstream of the body, which is important for understanding trajectories and equilibria of lofted dust grains in the presence of a complex asteroid geometry. The largest electric fields are found at the terminators, where ambipolar plasma expansion in the body-sized nightside wake merges seamlessly with the thin photoelectric sheath on the dayside. The pocket regions are found to be especially complex, with nearby sunlit regions of positive potential electrically connected to unlit negative potentials and forming adjacent natural electric dipoles. For objects near the surface, we find electrical dissipation times (through collection of local environmental solar wind currents) that vary over at least 5 orders of magnitude: from 39μs inside the near-surface photoelectron cloud under direct sunlight to ≫1s inside the particle-depleted nightside wake and shadowed pocket regions.

Dynamical evidence for nonlinear Langmuir wave processes in type III solar radio bursts

AbstractThe nonlinear processes and evolution of Langmuir waves in the source regions of type III solar radio bursts are explored in detail. Langmuir waves recorded by the Time Domain Sampler of the STEREO/WAVES instrument can be roughly classified into six groups based on the waveform, power spectra, and field strength perpendicular to the local magnetic field. It is argued that these groups correspond to either different stages of the evolution of Langmuir waves generated by electron beams or differ due to the direction of the magnetic field relative to the solar wind velocity. Approximately half of the observed Langmuir waves have strong perpendicular fields, meaning that understanding how these fields are produced is crucial for understanding type III sources. Most events recorded are either localized waveforms consistent with Langmuir eigenmodes or have two or more spectral peaks consistent with electrostatic (ES) decay of Langmuir/z mode waves. The remaining events appear to correspond to either earlier or later stages of Langmuir wave evolution or are decay events for which the Doppler shift is insufficient to distinguish the beam‐driven and product Langmuir waves. This is supported by the fact that most events exceed the threshold for ES decay even though their spectra show no evidence for decay and some of the events are observed when the solar wind flow is approximately perpendicular to the magnetic field, minimizing Doppler shifting. Low‐frequency fields produced by intense Langmuir waves are quantitatively consistent with density perturbations produced by the ponderomotive force, ion‐acoustic waves produced by ES decay, or sheath rectification. Above the observed nonlinear threshold, quantitative analysis suggests that the observed low‐frequency signals are consistent with perturbations produced by ponderomotive effects and ion‐acoustic waves produced by ES decay, but effects of sheath rectification may also contribute.

Tensor anisotropy of cosmic rays

Long-termobservations of the muon intensity of galactic cosmic rays at the Nagoya (35°10′ N, 136°58′ E) and Yakutsk (62°01′ N, 129°43′ E) stations have revealed amplitude-phase annual and semiannual oscillations of the semidiurnal variation. These oscillations are attributable to the properties of the cosmic-ray anisotropy tensor that result from shielding by the interplanetary magnetic field and solar-wind shear flow. The mentioned tensor is also shown to have a north-south asymmetry.

DISCOVERY OF A NEW CLASS OF CORONAL STRUCTURES IN WHITE LIGHT ECLIPSE IMAGES

White light images of the solar corona, taken during total solar eclipses, capture the complex dynamic relationship between the coronal plasma and the magnetic field. This relationship can be recorded on timescales of seconds to minutes, within a few solar radii above the solar surface. Rays, large-scale loops, and streamers, which are the brightest structures in these images, have shaped current models of the coronal magnetic field and solar wind flow. We show in this work how the application of novel image processing techniques to unique high-resolution white light eclipse images reveals the presence of a new class of structures, reminiscent of smoke rings, faint nested expanding loops, expanding bubbles, and twisted helical structures. These features are interpreted as snapshots of the dynamical evolution of instabilities developing at prominence-corona interfaces and propagating outward with the solar wind.

On the “bowl‐shaped” deformation of planetary equatorial current sheets

Arridge et al. (2008) reported evidence for the formation of a “bowl‐shaped” equatorial current disk in Saturn's magnetosphere during epochs with large planetary dipole tilt angle from perpendicularity with the Sun‐planet axis. Using a large multiyear set of Geotail, Polar, and Time History of Events and Macroscale Interactions spacecraft data and a simple model of the magnetic field reversal surface, we found that a similar kind of the tilt‐related magnetic field deformation is present in the Earth's magnetosphere. Based on the properties of a tilted vacuum magnetic field configuration, we conclude that the bowl‐shaped distortion is a universal feature of magnetospheres with a tilted planetary dipole, resulting from the joint effect of the north‐south asymmetry due to the tilt and of the day‐night asymmetry imposed by the solar wind flow.

Harmonic waves and sheath rectification in type III solar radio bursts

AbstractIn type III solar radio bursts and planetary foreshocks, Langmuir waves are produced by electron beams and converted partially to radio waves by linear and nonlinear processes. Lower amplitude second harmonic electric fields are observed simultaneously during the most intense Langmuir wave events in type III source regions. The electric fields at the harmonic frequencies can arise from various mechanisms, such as radio wave emission by either coalescence or antenna mechanisms, nonlinear currents, harmonics of Langmuir waves, electron trapping in Langmuir wave potentials, and Langmuir wave rectification at the sheath surrounding the spacecraft, or they can result from instrumental harmonics. In this paper the relative powers and electric field vectors of Langmuir waves and the harmonic fields are compared for multiple events. The structure of the harmonic field is shown to be determined by the Langmuir waveform, but the harmonic field direction is typically closely aligned with the solar wind flow. The magnitude, structure, and orientation of the harmonic fields is used to determine which processes are responsible. It is shown that the dominant process generating the observed harmonic fields is Langmuir wave rectification at the sheath surrounding the spacecraft.

Transformation of solar wind energy into the energy of magnetospheric processes

When the solar wind flows round the magnetosphere, its flow structure and interplanetary magnetic field lines are affected. This indicates the appearance of an electric current system in near-Earth space. the magnetized solar wind plasma moving at the solar wind velocity in the coordinate system of near-earth bow shock induces an electric field in this system. When crossing the bow shock front at the nose point, the tangential magnetic field component increases nearly four times, and the magnetic field energy density—15 times.

Energization of planetary pickup ions in the solar system

We study the solar wind‐induced ion escape from planetary atmospheres at different radial heliospheric distances in the solar system. We derive histograms of the gyroaveraged E×B velocities, energies, and Larmor radii of planetary ions in the solar wind at Mercury, Venus, Earth, and Mars. The statistical analysis is based on the interplanetary Pioneer Venus Orbiter and OMNI solar wind data sets. In addition to the energization in the undisturbed solar wind we also model how planetary heavy ions get energized in the solar wind interaction of an unmagnetized planet at different distances to the Sun. We found that due to the Parker spiral, pickup ions are expected to be found on average at lower energies and at velocities more perpendicular to the solar wind flow, the closer to the Sun a planet or a comet is. According to a global hybrid simulation, planetary heavy ion energization is influenced qualitatively in a similar way in the presence of an induced magnetosphere than in the upstream solar wind under different Parker spiral angles due to fact that the structure of an induced magnetosphere depends strongly on the interplanetary magnetic field and solar wind conditions. Finally, the energization and dynamics of the pickup ions vary considerably with the solar activity. The variation is stronger the farther away from the Sun an object is. The Larmor radii of the pickup ions are largest during a solar minimum while the pickup ion energies are highest during the declining phase of a solar cycle.

Modeling solar wind mass‐loading in the vicinity of the Sun using 3‐D MHD simulations

Collisionless shocks due to mass‐loading were first discussed to describe the solar wind flow around a cometary atmosphere, showing its choking effects on the flow. Recent observations have led to an increased interest in mass‐loading occurring in the solar corona due to both sungrazing comets and collisional debris production by sunward migrating interplanetary dust particles. The 1‐D simulations with a hydrodynamic model have illustrated the impact on the solar wind from abrupt mass‐loading in the coronal region. Full 3‐D magnetohydrodynamic (MHD) simulations using a solar corona model based on the Block‐Adaptive‐Tree‐Solarwind‐Roe‐Upwind‐Scheme code provide a more realistic coronal environment for modeling specific events applicable to modeling the mass‐loaded coronal wind. A specific application is introduced modeling the mass‐loading effects from a sungrazing comet.

IMPLICATIONS OF THE RECENT LOW SOLAR MINIMUM FOR THE SOLAR WIND DURING THE MAUNDER MINIMUM

The behavior of the Sun and near-Earth space during grand solar minima is not understood; however, the recent long and low minimum of the decadal-scale solar cycle gives some important clues, with implications for understanding the solar dynamo and predicting space weather conditions. The speed of the near-Earth solar wind and the strength of the interplanetary magnetic field (IMF) embedded within it can be reliably reconstructed for before the advent of spacecraft monitoring using observations of geomagnetic activity that extend back to the mid-19th century. We show that during the solar cycle minima around 1879 and 1901 the average solar wind speed was exceptionally low, implying the Earth remained within the streamer belt of slow solar wind flow for extended periods. This is consistent with a broader streamer belt, which was also a feature of the recent low minimum (2009), and yields a prediction that the low near-Earth IMF during the Maunder minimum (1640-1700), as derived from models and deduced from cosmogenic isotopes, was accompanied by a persistent and relatively constant solar wind of speed roughly half the average for the modern era.

Reconstruction of geomagnetic activity and near-Earth interplanetary conditions over the past 167 yr – Part 2: A new reconstruction of the interplanetary magnetic field

Abstract. We present a new reconstruction of the interplanetary magnetic field (IMF, B) for 1846–2012 with a full analysis of errors, based on the homogeneously constructed IDV(1d) composite of geomagnetic activity presented in Part 1 (Lockwood et al., 2013a). Analysis of the dependence of the commonly used geomagnetic indices on solar wind parameters is presented which helps explain why annual means of interdiurnal range data, such as the new composite, depend only on the IMF with only a very weak influence of the solar wind flow speed. The best results are obtained using a polynomial (rather than a linear) fit of the form B = χ · (IDV(1d) − β)α with best-fit coefficients χ = 3.469, β = 1.393 nT, and α = 0.420. The results are contrasted with the reconstruction of the IMF since 1835 by Svalgaard and Cliver (2010).