Excitation of alfven turbulence in the solar wind ahead of the Earth bow shock by beams of high-velocity protons

One of the mechanisms of Alfven turbulence generation in the foreshock region is investigated by the example of the Earth bow shock. The effect of the temperature of high-velocity beams on characteristics of generated disturbances is examined. It is shown that the beam temperature has a significant impact on transverse scales of disturbances. The higher the temperature, the greater the limitations on transverse wavelengths. The development of instability in the propagation of reflected, intermediate, and diffusion proton beams in the foreshock region of the Earth bow shock is considered. Perturbation motion dynamics in foreshock region is analyzed.     

Initial Venus Express magnetic field observations of the Venus bow shock location at solar minimum

In this study, magnetic field measurements obtained by the Venus Express spacecraft are used to determine the bow shock position at solar minimum. The best fit of bow shock location from solar zenith angle 20-120° gives a terminator bow shock location of 2.14 R_V (1 R_V=6052 km) which is 1600 km closer to Venus than the 2.40 R_V determined during solar maximum conditions, a clear indication of the solar cycle variation of the Venus bow shock location. The best fit to the subsolar bow shock is 1.32 R_V, with the bow shock completely detached. Finally, a global bow shock model at solar minimum is constructed based on our best-fit empirical bow shock in the sunlit hemisphere and an asymptotic limit of the distant bow shock which is a Mach cone under typical Mach number of 5.5 at solar minimum. We also describe our approach to making the measurements and processing the data in a challenging magnetic cleanliness environment. An initial evaluation of the accuracy of measurements shows that the data are of a quality comparable to magnetic field measurements made onboard magnetically clean spacecraft.     

On the generation of low-frequency waves in the solar wind in the front of the bow shock

The generation of low-frequency waves in the solar wind by the flux of protons accelerated in the magnetosheath is considered. It is shown that pulsations are produced in two partly overlapping frequency ranges. The growth rate of waves is maximal when the angle θ between the direction of the interplanetary magnetic field and the front of the bow shock is not equal $\text{π}\text{2}$. The dependence of the increment of perturbation on the solar wind velocity is analysed. A satisfactory agreement between theory and experimental results on the connection of Pc3–4 properties and parameters of the solar wind is obtained.     

On turbulence in the quasi-perpendicular bow shock

In this paper we propose to review the fundamental aspects of turbulence theories and their relevance to particle distribution functions observed by the cluster satellites in the quasi-perpendicular shock. The paper focusses on the hierarchical model describing the different levels of plasma turbulence; from the linear theory, through the quasi-linear remedy, to strong turbulence theories in the context of the earth's bow shock. We will discuss very briefly the validity of these approximations, and their relevance as far as satellite observations are concerned. In particular, we will discuss the development of non-Gaussian features in the ion distribution functions through the evaluation of higher order moments such as the kurtosis or flatness and the skewness. We have found that the profile of the kurtosis versus skewness tends to collapse to a parabolic line. This in turn allows us to draw analogies with neutral fluid turbulence where such a collapse of the kurtosis-skewness profile has been observed.     

Acceleration of pickup interstellar protons by shock waves in corotating solar wind regions

It follows from numerous measurements of the differential fluxes of energetic charged particles in corotating interaction regions between solar wind streams with different speeds that the spectra of particles accelerated by reverse shocks are harder than those of particles accelerated by forward shocks. The measurements cannot be explained in terms of the theory of diffusive acceleration (first-order Fermi acceleration). We show that the measurements can be easily explained in terms of the theory of drift acceleration of charged particles by shock waves with allowance made for their multiple scattering from the front.     

Interaction of interplanetary shocks with the bow shock

Fast forward interplanetary (IP) shocks have been identified as a source of large geomagnetic disturbances. However, the shocks can evolve in the solar wind, they are modified by interaction with the bow shock and during their propagation through the magnetosheath. A few previous papers refer the inclination and deceleration of the IP shock front in this region. Our contribution continues this effort and presents the study of an IP shock interaction with the bow shock. Since the bow shock is a reversed fast shock, the interaction of the IP shock and bow shock is a problem of interaction of two fast MHD shocks.

We compare profiles of magnetic field and plasma parameters observed by several spacecraft in the solar wind and magnetosheath with the profiles of the same parameters resulting from the MHD numerical model. The MHD model suggests that the interaction of an IP shock with the bow shock results in an inward bow shock displacement that is followed by its outward motion. Such motion will result in an indentation propagating along the bow shock surface. This scenario is confirmed by multipoint observations. Moreover, the model confirms also previous suggestions on the IP shock deceleration in the magnetosheath.     

Hydromagnetic wave interaction with the magnetopause and the bow shock

The results of analyses of hydromagnetic reflection and refraction at a shear layer and at a shock are applied to situations representative of the magnetopause and the Earth's bow shock.The almost complete absence of magnetic fluctuations in the outer fringes of the magnetopause is explicable in terms of the magnetopause behaving like a near perfect reflector to the turbulent hydromagnetic waves in the magnetosheath. Therefore it seems unlikely that the turbulent sound wave refraction mechanism is very effective in producing a viscous-like interaction between the solar wind and the magnetosphere. For conditions typical of the solar wind and the magnetotail during quiet times the stability analysis given here indicates that the tail of the magnetopause is unlikely to undergo Kelvin-Helmholtz instability. However, during times when the solar wind blows harder than it usually does the distant tail of the magnetopause will exhibit not only instability but also greatly enhanced hydromagnetic wave transmission into the tail from the magnetosheath. It is noted that both longitudinal and transverse waves can be amplified on passage through a strong shock. Thus the amplification of the turbulent spectrum of hydromagnetic waves in the solar wind on passage through the Earth's bow shock may account for the (at least) order of magnitude increase of the noise spectrum in the magnetosheath over that in the unshocked solar wind.     

Amplification of hydromagnetic waves through the earth's bow shock

This paper sets out to determine if the turbulence observed in the magnetosheath can be generated by amplification, through the Earth's bowshock, of the hydromagnetic fluctuation carried by the solar wind. This problem is closely related to the study of the transmission and generation of hydromagnetic waves through an oblique fast shock wave. An incident wave (Alfvén, magnetosonic or entropy wave) striking the front of the shock, is refracted and gives rise to five other hydromagnetic waves. In this paper, a system of equationsis given to determine the transmission and generation coefficients of these diverging waves. Given a model of the Earth's bow shock, the ratio of refracted to incident energy fluxes is computed. It appears that there is always amplification, at least for one refracted wave, but its value varies greatly with the direction of the incident wave. Therefore, further information about the angular distribution of the hydromagnetic perturbations carried by the solar wind are required to give an order of magnitude of the energy of the turbulence created.     

Angular distributions of solar protons and electrons

High angular-resolution measurements of directional fluxes of solar particles in space have been obtained with detectors aboard OGO-5 during the cosmic ray event of 18 November 1968. This is the only case on record for which sharply-defined directional observations of protons and electrons covering a wide rigidity range (0.3 MV to 1.5 GV) are available.The satellite experiment provided data for determining pitch-angle distributions with respect to the direction of the local interplanetary magnetic field lines during the lengthy highly anisotropic phase of the event. It was found that the unidirectional differential intensities j(θ) of 3- to 25-MeV protons varied in accordance with the relationship j(θ) = b₀ + b₁cosθ + b₂cos²θ, where b₀ and b₁ ? 0, and b₂, is positive, zero or negative. Soon after onset, 79–266-keV electrons arriving from the direction of the Sun displayed an anisotropic component with the intensity varying as cos θ. Later, a double-peaked distribution appeared at the lower energies, whereas the flux at the upper end of the range covered by the experiment became isotropic. These results have been interpreted in the light of the temporal flux profiles and the state of the interplanetary medium.The observation of the unusually large and long-lasting anisotropies lead to several conclusions including: (1) If injection of the solar particles was instantaneous, the diffusion coefficient was either constant or increasing with distance from the Sun. (2) If the solar source emitted particles over an extended period, and there is evidence to that effect, there was weak scattering in the region between the Sun and the Earth and a strong scattering region beyond the Earth's orbit. (3) Solar electrons were stored near the Sun. (4) The observed angular distribution of 200-MV protons in the magnetosheath was in good agreement with that deduced in an earlier analysis of polar orbiting satellite observations and trajectory calculations.     

Propagation of solar protons and the interplanetary magnetic field

Solar proton events are classified into three types, F,F¹ and S, based on the intensity time variation and related solar phenomena. The propagation mechanism for each is considered. The structure of the magnetic fields near the Sun and in interplanetary space are deduced, and the structure of the flare nimbus is discussed. The influence of the interplanetary magnetic fields on the propagation of solar protons is examined in detail.     

A theoretical model of the bow shock and the magnetosphere of Jupiter

Using the data obtained from the Pioneer 10 and 11 observations, a theoretical model is proposed for the bow shock and the magentosphere of Jupiter. This indicates that the distance of the magnetopause from Jupiter on the sunlit side is (50–55) × r_J (r_J: Jupiter radius, = 7 × 10⁹ cm) and that the ratio of the stand-off distance to this distance is about equal to or slightly larger than unity. Hence the Mach number of the solar wind seems to be less than 1.5 at Jupiter's orbit. This result necessarily leads to a blunt body model of the Jovian magnetosphere, the tail region of which is not as extended as observed in the Earth's case.     

Bow shock protons in the lunar environment

Protons from the Earth's bow shock are observed by the Suprathermal Ion detector Experiment (SIDE) in two regions of the lunar orbit. The dawn region begins at the dawn side bow shock crossing and ends ç 5 days later and the dusk region begins at ç 2 days prior to entering the dusk side magnetosheath and ends at the inbound bow shock crossing. Dusk and dawn refer to a terrestrial coordinate system. The dominant contribution to the ion spectra observed by the SIDE in these regions is from particles with energies between ç 750 eV q^–1 and 3500 eV q^–1. 3500 eVq^–1 is the upper limit of the energy range of the detector. Analysis of simultaneous data from the Explorer 35 magnetometer and the SIDE indicates that the observability of bow shock protons at the lunar distance is dependent on the configuration of the interplanetary magnetic field.Paper presented at the Conference on Interactions of the Interplanetary Plasma with the Modern and Ancient Moon, sponsored by the Lunar Science Institute, Houston, Texas and held at the Lake Geneva Campus of George Williams College, Wisconsin, between September 30 and October 4, 1974.     

Interplanetary propagation of relativistic solar protons

Particle fluxes and pitch angle distributions of relativistic solar protons at Earth's orbit have been determined by Monte Carlo calculations. The analysis covers two hours after the release of the particles from the Sun and total of 8 × 10⁶ particle trajectories were simulated. The pitch angle scattering was assumed to be isotropic and the scattering mean free path was varied from 0.1 to 4 AU.The intensity-time profiles after a delta-like injection from the Sun show that the interplanetary propagation is clearly non-diffusive at scattering mean-free paths above 0.5 AU. All pitch angle distributions have a steady minimum at 90 °, and they become similar about 20 min after the arrival of first particles.As an application, the solar injection profile and the interplanetary scattering mean-free path of particles that gave rise to the GLE on 7 May, 1978 were determined. In contrast to the values of 3–5 AU published by other authors, the average scattering mean-free path was found to be about 1 AU.     

Propagation of flare protons in the solar atmosphere

The velocity dispersion for a large number of solar proton events is analyzed in the energy regime of 10–60 MeV. It is found for all events that the time from the flare to particle maximum t _m is well represented by a sum of two components. The first component which is energy independent describes the propagation in the solar atmosphere, the second component describes the propagation in the interplanetary medium giving a velocity dispersion v × t _m = const. The additional study of time intensity profiles, onset times, and multispaceprobe observations reveals that the propagation in the solar atmosphere consists of three processes: (1) A rapid transport process in the initial ( 1 h) phase after the event fills up a fast propagation region (FPR), which may extend up to 60° from the flare site and which is tentatively identified with a large unipolar magnetic cell as seen on H synoptic charts, (2) a large-scale drift process which is energy independent with drift velocities v _D in the range 1° v _D 4°h^-1, and simultaneously (3) a diffusion process which yields the general broadening of the intensity time profiles for eastern hemisphere events, which is, however, of less importance than previously assumed.     

Mean free paths and diffusion coefficients for energetic protons at small heliodistances calculated using Helios 1 and 2 data

Pitch angle scattering of energetic particles (100 MeV) in the interplanetary medium are studied using Helios 1 and 2 magnetometer and plasma data during 1976 near the minimum of solar activity. An IMF configuration was used in the computer experiments which allowed the pitch angle diffusion coefficient, D and hence the parallel mean free path, to be determined. The radial mean free path was found to vary as _r r ^-0.9 between 0.4 and 1 AU, but between 0.3 and 0.4 AU it decreases significantly. To reconcile our value of _r at 1 AU, lying between 0.01 and 0.02 AU, with the average prompt solar proton event profile, an increasing value of _r at lower radial distances would be required.     

Simultaneous satellite and ground-based observations of solar protons

During September 1963 two solar proton events occurred. The particles were detected directly by the satellite Alouette and indirectly, as ionospheric absorption, by a network of riometers distributed in latitude across the northern and southern polar caps of the Earth. Since one of the riometers was located at the South Pole, where it was continuously sunlit during the events, the absorption measurements there were unaffected by the usual day-to-night variation. Hence the absorption has been unambiguously compared with the proton flux measured by the satellite, and an empirical relation between the two has been derived.The riometer records have also been used to indicate continuously the variation of absorption with latitude across the polar caps. These latitude profiles were compared with the latitudinal variation of the proton flux as determined by the satellite, and there is a close correspondence between the profiles obtained for the same time by the two techniques. In particular the change in the cut-off energies following the sudden commencement of a magnetic storm is clearly evident. The “noon-recovery” of the absorption in the polar cap is shown to be compatible with a change in the geomagnetic cut-off at locations near the edge of the polar cap.     

Multi-spacecraft study of foreshock cavitons upstream of the quasi-parallel bow shock

In this work we perform the first multi-spacecraft analysis of two foreshock cavitons observed by the Cluster spacecraft. We also study the characteristics of their surrounding regions. Foreshock cavitons are a relatively new type of phenomena in the Earth's foreshock. They appear in regions deep inside the foreshock and are therefore always immersed in a sea of ULF waves and suprathermal particles. In the observational data the cavitons appear as simultaneous depressions of interplanetary magnetic field and plasma density. The two cavitons presented here have highly structured interiors and exhibit surface irregularities. They propagate sunwards in the reference frame of the solar wind plasma. Since their velocities are smaller than the solar wind velocity, the cavitons are convected towards the Earth by the solar wind flow. Their sizes are comparable to the size of the Earth. We show that the cavitons are different from other foreshock phenomena, such as cavities. The latter are thought to form by thermal expansion due to the excess of thermal pressure caused by intense flux of suprathermal ions in their interiors. Thermal pressure inside the cavitons is the same as in their surroundings, so they cannot form in this way. The proposed mechanism for the caviton formation includes nonlinear interactions between different types of ULF waves deep inside the foreshock.     

Observations of electron plasma waves upstream of the Jovian bow shock

The Ulysses spacecraft encountered the planet Jupiter in February 1992, on its journey towards high heliospheric latitude. During the approach to the planet, as well as on the outbound pass, while receding from the Jovian bow shock, the Plasma Frequency Receiver that is part of the Unified Radio and Plasma Wave experiment (URAP) recorded bursts of plasma waves in the frequency range of a few kHz. These emissions, first observed by the PWS experiment onboard the Voyager spacecraft, have been identified as upstream electron plasma waves. In this paper, we present the first analysis of the characteristics of these emissions, which are very similar to those found in the Earth's electron foreshock, upstream of the Earth's bow shock. These bursty emissions, with a peak frequency very close to the local electron plasma frequency F_pe, have a typical electric field amplitude in the range 0.01–0.1 mV m⁻¹, with some bursts above 1 mV m⁻¹. The frequency bandwidth over which significant power can be found above the instrument background noise ranges from below 0.2 F_pc to about 2 F_pc. On the basis of our present knowledge of similar emissions observed at Earth, we suggest that the broadband emissions are triggered by suprathermal (a few tens of eV) electrons, streaming back from Jupiter's bow shock.     

The effect of the earth's bow shock and magnetosheath on the interaction of a discontinuity in the solar wind with the magnetosphere

A theoretical model is proposed for the interaction of a plane discontinuity in the solar wind with the magnetosphere. The presence of the bow shock and magnetosheath are taken into account, the calculation being based on the Spreiter et al. (1966) gas-dynamic model for a solar wind Mach Number M = 5. The model proposed predicts the manner in which the shape of the interplanetary discontinuity is distorted in its passage through the magnetosheath; it is found that the point of first impact with the magnetopause makes an angle of 56° with the Sun-Earth line for relatively quiet solar wind conditions.