Solar wind access to the plasma sheet along the flanks of the magnetotail

Low-energy particle trajectories in an idealized magnetotail magnetic field are investigated to determine the accessibility of magnetosheath protons and electrons to the plasma sheet along the flanks of the tail magnetopause. The drift motion of the positively (negatively) charged particles incident on the dawn (dusk) magnetotail flank causes such particles to penetrate deeper into the magnetotail. For certain combinations of particle energy, incident velocity vector and initial penetration point on the tail magnetopause, the incident particles can become trapped in the plasma sheet, after which their net drift motion then provides a current capable of supporting the entire observed magnetotail field. The results further indicate that the bulk of the solar wind plasma just outside the distant tail boundary, which streams preferentially in a direction along the magnetopause away from the Earth at velocities around 400 km s^?1, can be caught up in the tail if the initial penetration point is within about 2R_E, of the quasi-neutral sheet. It is suggested that a large fraction of the magnetotail plasma is composed of former solar wind particles which have penetrated the magnetospheric boundary at the tail flanks.     

Interfacing MHD Single Fluid and Kinetic Exospheric Solar Wind Models and Comparing Their Energetics

An exospheric kinetic solar wind model is interfaced with an observation-driven single-fluid magnetohydrodynamic (MHD) model. Initially, a photospheric magnetogram serves as observational input in the fluid approach to extrapolate the heliospheric magnetic field. Then semi-empirical coronal models are used for estimating the plasma characteristics up to a heliocentric distance of 0.1 AU. From there on, a full MHD model that computes the three-dimensional time-dependent evolution of the solar wind macroscopic variables up to the orbit of Earth is used. After interfacing the density and velocity at the inner MHD boundary, we compare our results with those of a kinetic exospheric solar wind model based on the assumption of Maxwell and Kappa velocity distribution functions for protons and electrons, respectively, as well as with in situ observations at 1 AU. This provides insight into more physically detailed processes, such as coronal heating and solar wind acceleration, which naturally arise from including suprathermal electrons in the model. We are interested in the profile of the solar wind speed and density at 1 AU, in characterizing the slow and fast source regions of the wind, and in comparing MHD with exospheric models in similar conditions. We calculate the energetics of both models from low to high heliocentric distances.     

On the screening of the interstellar electric and magnetic fields by the solar wind

Fast-streaming solar-wind plasma with high conductivity screens the heliosphere from the penetration of the interstellar electric and magnetic fields. The simplest model with the constant solar wind conductivity and radial velocity is considered and the boundary electrodynamic problem is solved for static external fields. The results show that screening of the external fields takes place in the heliosphere according to the exponential law.     

Interplanetary dust particles: Disintegration and orbital motion

Abrupt or gradual disintegration of the interplanetary dust particle causes increase of its distance from the Sun due to the solar radiation pressure. The problem of the orbital evolution of the interplanetary dust particles under such disintegration processes is discussed. The process of gradual disintegration due to the solar wind particles is calculated in detail. Obtained results represent corrections to the changes of orbital elements for the Poynting-Robertson effect and effect of the solar wind.     

Core, Halo and Strahl Electrons in the Solar Wind

Electron velocity distribution functions (VDF) observed in the low speed solar wind flow are generally characterized by ‘core’ and ‘halo’ electrons. In the high speed solar wind, a third population of ‘strahl’ electrons is generally observed. New collisional models based on the solution of the Fokker-Planck equation can be used to determine the importance of the different electron populations as a function of the radial distance. Typical electron velocity distribution functions observed at 1 AU from the Sun are used as boundary conditions for the high speed solar wind and for the low speed solar wind. Taking into account the effects of external forces and Coulomb collisions with a background plasma, suprathermal tails are found to be present in the electron VDF at low altitudes in the corona when they exist at large radial distances. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the relationship of cosmic ray intensity with solar,interplanetary, and geophysical parameters

The flux rate of cosmic rays incident on the Earth’s upper atmosphere is modulated by the solar wind and the Earth’s magnetic field. The amount of solar wind is not constant due to changes in solar activity in each solar cycle, and hence the level of cosmic ray modulation varies with solar activity. In this context, we have investigated the variability and the relationship of cosmic ray intensity with solar, interplanetary, and geophysical parameters from January 1982 through December 2008. Simultaneous observations have been made to quantify the exact relationship between the cosmic ray intensity and those parameters during the solar maxima and minima, respectively. It is found that the stronger the interplanetary magnetic field, solar wind plasma velocity, and solar wind plasma temperature, the weaker the cosmic ray intensity. Hence, the lowest cosmic ray intensity has good correlations with simultaneous solar parameters, while the highest cosmic ray intensity does not. Our results show that higher solar activity is responsible for a higher geomagnetic effect and vice versa.     

Interstellar matter and the location of the shock front

The initially supersonic flow of the solar wind passes through a magnetic shock front where its velocity is supposed to be reduced to subsonic values. The location of this shock front is primarily determined by the energy density of the external interstellar magnetic field and the momentum density of the solar wind plasma. Interstellar hydrogen penetrating into the heliosphere undergoes charge exchange processes with the solar wind protons and ionization processes by the solar EUV radiation. This results in an extraction of momentum from the solar wind plasma. Changes of the geometry and the location of the shock front due to this interaction are studied in detail and it is shown that the distance of the magnetic shock front from the Sun decreases from 200 to 80 AU for an increase of the interstellar hydrogen density from 0.1 to 1.0 cm⁻³. The geometry of the shock front is essentially spherical with a pronounced embayment in the direction opposite to the approach of interstellar matter which depends very much on the temperature of the interstellar gas. Due to the energy loss by the interaction with neutral matter the solar wind plasma reduces its velocity with increasing distance from the Sun. This modifies Parker's solution of a constant solar wind velocity.     

Consequences of an isotropic static plasma sheet in models of the geomagnetic tail

The equation of momentum balance and magnetic flux conservation are given for a static tail model with an isotropic plasma sheet. The possibility of magnetic field leakage into the solar wind and across the neutral sheet is allowed. Numerical integrations for a wide variety of adjustable model parameters are presented that give the dependence on distance from Earth of all tail parameters (field strength inside and outside of the plasma sheet, plasma pressure, plasma sheet area, tail radius, and normal field component to the neutral sheet). The model gives good agreement with the observed distance dependence of the tail field strength, and accounts for the scatter in the data in terms of a mixture of the fields inside and outside the plasma sheet in the data averages. However, compared with the present interpretations of the observations the model gives a too large plasma pressure at large distances and a too small normal component to the neutral sheet. The discrepancies imply that plasma flow and/or pressure anisotropy are required for an adequate model.     

Deep Space 1 at comet 19P/Borrelly: Magnetic field and plasma observations

On September 22, 2001 the Deep Space 1 spacecraft performed a flyby at comet 19P/Borrelly at a solar distance of 1.36 AU leading the Earth by 74^° in longitude. The spacecraft-comet distance at closest approach was 2171 km. The bow shock had a magnetic compression ratio of 2.5 at a distance of 147 100 km from the nucleus. Deep Space 1 first entered the sheath region essentially from the north polar region. Fluctuations from the cometary ion pickup were present throughout the sheath region and even well upstream of the shock, as expected. The magnetic field pileup region had a peak field strength of 83 nT and was shown to be consistent with a pressure equal to the solar wind ram pressure. The peak field location was offset from the time of closest approach. It is uncertain whether this is a spatial or temporal variation. Draping of magnetic fields around the nucleus was sought, but evidence for this was not apparent in the data. A possible explanation is that the interplanetary solar wind was composed of turbulent short-scale fields, and thus the fields were not symmetric about the point of closest approach. During the flyby phase there were in general few intervals of ACE data where there were large scale Parker spiral fields. With the addition of plasma data, the shock properties are investigated. The characteristics of magnetic draping, pileup and fluctuations are explored. These comet 19P/Borrelly results are contrasted with other cometary flyby results.     

Orbital Evolution of Dust Particles in the Sublimation Zone near the Sun

We have performed the calculations of the orbital evolution of dust particles from volcanic glass (p-obsidian), basalt, astrosilicate, olivine, and pyroxene in the sublimation zone near the Sun. The sublimation (evaporation) rate is determined by the temperature of dust particles depending on their radius, material, and distance to the Sun. All practically important parameters that characterize the interaction of spherical dust particles with the radiation are calculated using the Mie theory. The influence of radiation and solar wind pressure, as well as the Poynting–Robertson drag force effects on the dust dynamics, are also taken into account. According to the observations (Shestakova and Demchenko, 2016), the boundary of the dust-free zone is 7.0–7.6 solar radii for standard particles of the zodiacal cloud and 9.1–9.2 solar radii for cometary particles. The closest agreement is obtained for basalt particles and certain kinds of olivine, pyroxene, and volcanic glass.     

The Solar Ionization Rate of the Interplanetary Hydrogen as a Function of a Heliomagnetic Latitude: A New Model for the Interplanetary Lyman Alpha Studies

In this work we have modelled the solar wind proton flux which varies as a function of distance to the heliomagnetic equator and its effects on the interplanetary Lyman α radiation. The results imply that a groove observed in Lyman α intensity patterns toward the upwind direction Bertaux et al. disappears when the tilt angle of the heliomagnetic equator is larger than 20^°.The observations by Bertaux et al. were measured during the solar wind minimum when the tilt angle of the streamer belt is low. During the solar wind maximum when the tilt angle of the streamer belt is large the Lyman α groove should disappear according to our results. This revised version was published online in July 2006 with corrections to the Cover Date.     

Azimuthal structure of the solar wind velocity andK-corona brightness in the heliospheric current sheet

The solar wind velocity near Earth shows systematic structure in and around the heliospheric current sheet. The solar wind velocity measurements at IMF sector boundary crossings at 1 AU during 1972–1977 have been used to infer the azimuthal structure of the solar wind velocity in the current sheet. We found that the solar wind velocity in the in-ecliptic portion of the current sheet varies from longitude to longitude, where it originates from the corona. Also, the yearly average value of solar wind velocity in the HCS is found to vary with the phase of the solar cycle; with a maximum value around 1974. TheK-corona brightness on the source surface corresponding to the IMF sector boundary crossings during the period of study also shows a similar but opposite pattern of variation when the data are averaged over a long period. However, this relation is not observed when we considered them individually. So, we conclude that there exists a longitudinal variation of solar wind velocity in the heliospheric current sheet.     

Tracking a major interplanetary disturbance

The severe geomagnetic storm which occurred during 27–29 August 1978 was remarkable because it arrived unexpectedly and was not related to a solar flare or long-lived coronal hole. Observations on 900 celestial radio sources show that the storm was associated with a large-scale region causing enhanced interplanetary scintillation which enveloped the Earth at the same time. The disturbance was first detected on 26 August, when the outer boundary had reached a distance of about 0.8 a.u. from the Sun and it was tracked until 30 August. The enhancement was followed by a fast solar wind stream and its shape suggests that it was a compression zone caused by the birth of the stream.     

Meridional (north-south) motions of the solar wind

The steady state hydrodynamic equations which describe the solar wind flow are linearized and used to study the spatial behavior of zonal pressure perturbations. Such perturbations produce meridional (north-south) motions in the solar wind. To simplify the spatial dependence of the zero-order variables and to take advantage of the supersonic regime, the analysis is restricted to heliocentric distances greater than 0.1 AU (astronomical unit). A simplified problem involving a north-south magnetic field asymmetry is also treated. The emphasis of the paper is to determine what pressure perturbations are required at the inner boundary (0.1 AU) to produce at earth north-south deviations from radial flow of 1° to 3°.     

Expansion of the solar wind from a two-hole corona

A one-fluid model is employed to study the global expansion of the solar wind from a two-hole corona, under the assumptions that the holes are confined to polar caps within 30° of heliographic colatitude, the flow is steady and axisymmetric, and the geometry of streamlines is prescribed. The boundary conditions are adjusted in such a way that the calculated solar wind properties at 1 AU are in a reasonable agreement with observational results. A series of numerical solutions are obtained, the series produces a maximum terminal speed of 829 km s^?1 at the pole. The calculated solar wind speeds are strongly latitude dependent and are positively correlated with local divergence factor of a stream tube. The solutions imply that most plasma properties are highly inhomogeneous at the polar caps. The flow velocity, the temperature, the proton number flux and the conduction heat flux all increase towards the hole center.     

Solar wind observations on the lunar surface with the Apollo-12 ALSEP

The Apollo-12 ALSEP solar wind spectrometer obtained data from the lunar surface starting November 20, 1969. To a first approximation, the general features of the positive ion flux depend only on the instrument's orientation and location in space relative to the Sun-Earth system. However, there are some detectable effects of the interaction of the solar wind with the local magnetic field and surface, including the deceleration of incident positive ions and the enhancement of fluctuations in the plasma. The expected asymmetry of sunset and sunrise times due to the motion of the Moon about the Sun is not observed. On one occasion, the solar wind was incident on the ALSEP site as early as 36 hr (18°) before sunrise.     

Investigation of the force balance in the Titan ionosphere: Cassini T5 flyby model/data comparisons

Cassini’s Titan flyby on 16 April, 2005 (T5) is the only encounter when the two main ionizing sources of the moon’s atmosphere, solar radiation and corotating plasma, align almost anti-parallel. In this paper a single-fluid multi-species 3D MHD model of the magnetospheric plasma interaction for T5 conditions is analyzed. Model results are compared to observations to investigate the ionospheric dynamics at Titan as well as to understand the deviations from a typical solar wind interaction, such as Venus’ interaction with the solar wind. Model results suggest that for the T5 interaction configuration, corotating plasma is the dominant driver determining the global interaction features at high altitudes. In the lower ionosphere below ～1500 km altitude – where the control of the ionospheric composition transfers from dynamic to chemical processes – magnetic and thermal pressure gradients oppose each other locally, complicating the ionospheric dynamics. Model results also imply that the nightside ionosphere – produced only by the impact ionization in the model – does not provide enough thermal pressure to balance the incident plasma dynamic pressure. As a result, the induced magnetic barrier penetrates into the ionosphere by plasma convection down to ～1000 km altitude and by magnetic diffusion below this altitude. Moreover, strong horizontal drag forces due to ion-neutral collisions and comparable drag forces estimated from possible neutral winds in the lower ionosphere below ～1400 km altitude oppose over local regions, implying that the Titan interaction must be treated as a 3D problem. Ion and electron densities calculated from the model generally agree with the Cassini Ion Neutral Mass Spectrometer and Langmuir probe measurements; however, there are significant differences between the calculated and measured magnetic fields. We discuss possible explanations for the discrepancy in the magnetic field predictions.     

Magnetically-Driven Planetary Radio Emissions and Application to Extrasolar Planets

At least six intense nonthermal planetary radio emissions are known in our solar system: the auroral radio emissions from the Earth, Jupiter, Saturn, Uranus and Neptune, and the radio bursts from the Io-Jupiter flux tube. The former are thought to be driven by the solar wind flow pressure or energy flux on the magnetospheric cross-section, while the latter is a consequence of the Io-Jupiter electrodynamic interaction. Although in the solar wind, the flow ram pressure largely dominates the magnetic one, we suggest that the incident magnetic energy flux is the driving factor for all these six radio emissions, and that it can be estimated in the same way in all cases. Consequences for the possible radio emission from extrasolar planets are examined. ‘Hot Jupiters’, if they are magnetized, might possess a radio emission several orders of magnitude stronger than the Jovian one, detectable with large ground-based low-frequency arrays. On the other hand, `giants' analogous to the Io-Jupiter interaction in the form of a pair star/hot-Jupiter are unlikely to produce intense radio emissions, unless the star is very strongly magnetized. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stability of a collisionless contact discontinuity

The stability of a contact discontinuity in a collisionless plasma is examined. It is shown that the discontinuity can become unstable when the pressure component normal to the magnetic field is not continuous across the discontinuity. Even when the system is unstable, the growth rates are very small and unimportant in the context of wave propagation, except when the propagation is almost normal to the field. The instability could, however, lead to the slow dissipation of contact discontinuities in the solar wind and in the day side of the cusp region of the solar wind-magnetosphere boundary.