Solar Wind Frame Of Reference Research Articles

What is the Solar Wind Frame of Reference?

Abstract Various solar wind ion species move with different speeds and theoretical considerations as well as limited observations in a region close to the Sun show that heavy solar wind ions tend to flow faster than protons, at least in less-aged fast solar wind streams. The solar wind flow carries the frozen-in interplanetary magnetic field (IMF) and this situation evokes three related questions: (i) what is the proper solar wind speed, (ii) is this speed equal to the speed of the dominant component, whatever that may be, and (iii) what is the speed of the magnetic field? We show that simple theoretical considerations based on the MHD approximation as well as on the dynamics of charged particles in electric and magnetic fields suggest that the IMF velocity of motion (de Hoffmann–Teller (HT) velocity) would be deliberated as the velocity appropriate for solar wind studies. Our analysis based on the Wind, Helios, ACE, and SOHO observations of differential streaming of solar wind populations shows that their energy is conserved in the HT frame. On the other hand, the noise and temporal resolution of the data do not allow us to decide whether the total momentum is also conserved in this frame.

Determination of Plasma, Pickup Ion, and Suprathermal Particle Spectrum in the Solar Wind Frame of Reference

Abstract Particle spectra directly measured by a plasma or low-energy particle experiment on spacecraft often contain instrumental effects due to a limited field of view or angular resolution. It is because the particle distribution function at low energies is highly anisotropic in the spacecraft frame of reference, in which the measurements are made. In this paper, we present a new mathematical method of transforming the particle spectrum to the solar wind frame of reference, where the particle distribution can be assumed to be nearly isotropic. The transformed particle spectrum allows us to investigate the properties of the solar wind, pickup ions, and suprathermal particles without concern for instrumental effects. We apply the method to the measurements made by the Solar Wind Ion Composition Spectrometer on Ulysses. Our results demonstrate that the transformed spectrum can improve the determination of the solar wind density and temperature from the previously published methods. A brief survey of Ulysses data shows that suprathermal ions in the slow solar wind frequently display a velocity distribution very close to the v −5 power law.

GEOTAIL observation of upstream ULF waves associated with lunar wake

Left-handed, circular polarized ULF waves with frequency of 0.3−1.1 Hz were detected by GEOTAIL at 27 lunar radii upstream of the moon when the spacecraft was magnetically connected with the lunar wake. The wave was detected twice at 16:45–17:00 and 18:55–19:02 on October 25, 1994, when the spacecraft and the moon were on the dawn side of the Earth’s magnetosphere. The ULF wave was propagating in a direction nearly parallel to the background magnetic field. The observed frequency and polarization are explained by reversal of polarization of right-handed, sunward-propagating electron whistler waves with frequencies above 1.4 Hz in the solar wind frame of reference, which were excited through the interaction with electron beams flowing in anti-sunward direction downstream of the lunar wake. The downstream flow of electron beam is explained by filtering effect of the potential drop at the boundary of the lunar wake. Low-energy components of electrons are reflected back by the potential drop, and the rest components, with energies higher than that of the electric potential penetrate through the wake. The velocity distribution of downstream electrons would be modified to have some bump or shoulder in energy range to form a beam, which is likely to excite whistler mode wave through cyclotron resonance. The lowest energy of the resonant electrons was calculated to be 0.96–2.5 (keV) from the lower boundary of the detected frequency. The variation in the lowest frequency suggests that there are some regions of the lunar wake where potential drop is reduced.

A self‐consistent model for the particles and fields upstream of an outgassing comet: 2. A time‐dependent description

In a previous paper [Flammer et al., 1991], we determined the global variation of the magnetic field and solar wind flow parameters in the unshocked region upstream of an outgassing comet using a kinetic treatment for the cometary ions. Two different assumptions were made concerning the cometary ion distribution function: the pickup cometary ions formed either a velocity space gyrotropic ring distribution or a velocity space isotropic shell distribution in the solar wind frame of reference. In the present paper we consider the general case wherein the newly picked up ions are elastically pitch angle scattered from the initial ring distribution to a shell with some characteristic time scale τ. Using theoretically determined parameters which reflect the conditions expected at comet Kopff (a typical short‐period comet and a possible target for the CRAF/Cassini mission) for various heliocentric distances, we determine how the pitch angle scattering rate affects the global morphology.

Accelerated cometary ions observed downstream of the comet Halley bow shock

Fluxes of energetic ions with energies exceeding 100 keV were observed upstream of the bow shock of comet Halley by the Tünde instrument which was on board the VEGA 1 spacecraft. Downstream of the shock, ion fluxes in the energy range 100 to 800 keV were observed. Cometary ions, such as O+, newly picked up by the solar wind have energies of about 15 keV in the solar wind frame of reference; hence the measured ion fluxes indicate that acceleration processes must have been operating near comet Halley. The measured ion fluxes were transformed into distribution functions in the solar wind frame using a variety of assumptions concerning the energy dependence of the distribution function and the identity of the ion species. The derived distribution function upstream of the shock falls off steeply with energy between 100 and 150 keV, with an effective temperature of about 7 keV or spectral index about −15. The distribution function increases with decreasing cometocentric distance, on average, reaching a maximum at the bow shock. Downstream of the shock, the spectra can be represented in two ways: (1) one population with a single spectral index based on a power law energy dependence or (2) two exponential function populations with different effective temperatures. In the latter case a soft component exists with an effective temperature of about 30 keV for energies less than about 250 keV and a harder component exists for higher energies with an effective temperature of about 100 keV. Downstream of the shock, the ion fluxes decrease with decreasing cometocentric distance. The measured distribution functions are compared with those obtained by similar instruments on Giotto and the International Cometary Explorer as well as with the predictions of several theoretical models that employ different acceleration mechanisms.

A self‐consistent model for the particles and fields upstream of an outgassing comet: 1. Gyrotropic and isotropic ion distributions

The magnetic field and solar wind flow parameters in the vicinity of a weakly outgassing comet are determined using a self‐consistent model which treats the cometary ions kinetically. Two different assumptions are made concerning the cometary ion distribution function; the pickup cometary ions form either a velocity space gyrotropic ring distribution or a velocity space Isotropic shell distribution in the solar wind frame of reference. The individual currents due to the solar wind and cometary ions, which determine the magnetic field, are calculated in each case. Using theoretically determined parameters which reflect the conditions expected at comet Kopff (a typical short‐period comet and one possible target for the future CRAF/Cassini mission) for various heliocentric distances, we are able to determine how the global field and flow characteristics are influenced by the nature of the cometary ion distribution function.

Quasilinear diffusion rates of cometary ions

The freshly created ions in the upstream region of the cometary bow shock usually form a ring-beam velocity distribution in the solar wind frame of reference. These newborn ions subsequently excite a variety of instabilities which in turn cause the ions to diffuse in pitch angle space to form a quasispherical shell velocity distribution. Such a process can be described by quasilinear kinetic theory. In the present paper, the detailed properties of the initial diffusion rates associated with the cometary newborn ions are studied for various physical parameters including the so-called injection angle α (that is, the average initial pitch angle associated with the ions) ranging from 0° (pure beam distribution) to 90° (pure ring distribution). It is shown that initial diffusion rates are strongly dependent upon the angle α, such that for 0°≤α≤60° (quasiparallel regime) the pitch angle diffusion rate remains relatively small, while for 60°≤α≤90° (quasiperpendicular regime) the diffusion rate increases substantially.

Geomagnetic Sudden impulses and storm sudden commencements: A note on terminology

The geomagnetic field occasionally exhibits abrupt, worldwide variations that have a morphology similar to that shown in Figure 1. Known as sudden impulses (SIs) or storm sudden commencements (SSCs), these signatures have been successfully explained as a compression of the magnetosphere caused by the passage of a shock or tangential discontinuity in the solar wind [e.g., Nishida, 1978]. Shocks propagate through the solar wind (outward from the Sun for fast forward shocks and inward for reverse shocks, in the solar wind frame of reference) and tangential discontinuities are simply carried with the solar wind. The purpose of this note is to examine the definitions of and distinctions between SSCs and SIs; to modernize present definitions of SIs and SSCs, in which SSCs are a subset of SIs depending on subsequent observed values of Dst or alternate geomagnetic indices; and to recommend quantitative definitions of the two terms for open discussion.

Preshock region acceleration of implanted cometary H+ and O+

One of the most interesting results of the cometary flyby missions was the realization that magnetohydrodynamic turbulence generated by the pickup process of freshly ionized cometary particles results in fast pitch angle scattering of the newly implanted ions. This process rapidly modifies the newly implanted ion pitch angle distribution in the solar wind frame of reference, from the original pickup ring to a pickup shell distribution. It also appears that the implanted ions have an isotropic pitch angle distribution in the decelerating solar wind frame of reference. This paper presents a self‐consistent description of the multispecies plasma flow (composed of solar wind protons and cometary H+ and O+) in the unshocked upstream region. The solar wind is treated as a fluid, while the evolution of the implanted ion distributions is described by generalized transport equations. The ambient solar wind population is depleted by charge exchange, while implanted protons and oxygen ions are produced by photoionization and charge transfer and lost by charge exchange. The transport equations take into account the adiabatic velocity change due to the deceleration of the flow, velocity diffusion, charge exchange loss, and the pickup of newly created ions. The model equations were numerically solved for the comet Halley flyby conditions, and the results are compared with spacecraft observations. Our calculations imply that second‐order Fermi acceleration can explain the implanted spectra observed in the unshocked solar wind. The comparison of the measured and calculated distribution functions also indicates that spatial diffusion (and consequently first‐order Fermi acceleration) of implanted ions is likely to play an important role in forming the energetic particle environment in the vicinity of the shock.

Characteristics of energetic particle events associated with interplanetary shocks

We present observations of the 35‐ to 1600‐keV proton intensity‐time profiles and the three‐dimensional 35‐ to 56‐keV anisotropy distributions recorded during two interplanetary shock events on ISEE 3, and discuss these in the light of current particle acceleration models. The large April 5, 1979, event associated with a quasi‐parallel shock shows an extended foreshock region with a strong increase of the upstream proton flux, a downstream plateaulike profile, upstream flow from the shock, and downstream isotropy in the solar wind frame of reference. The small March 9, 1979, event has a structured intensity‐time profile, a narrow shock spike, and anisotropic angular distributions both upstream and downstream, the anisotropies immediately behind the shock exhibiting an intensity peak at pitch angles around 90°. The April event, representative for a class of large energetic storm particle events, shows many observational features which are in agreement with predictions made by diffusive shock acceleration models. The March event, representative for a class of events with irregular profiles and mainly associated with quasi‐perpendicular shocks, exhibits features which are characteristic of shock drift acceleration. We conclude that both acceleration models are operative in association with interplanetary shocks.

Observations of three‐dimensional anisotropies of 35‐ to 1000‐keV protons associated with interplanetary shocks

We present results of a detailed analysis of three‐dimensional anisotropies of protons in the energy range 35–1000 keV observed in association with interplanetary shocks on ISEE 3. We compare observations of high time resolution anisotropies made close to the shock in seven energy channels with theoretical predictions for Fermi acceleration and shock drift acceleration, and we find good evidence for both types of acceleration. We find a small number (six) of events exhibiting the signature of Fermi acceleration, and a somewhat larger number (20) exhibiting the signature of shock drift acceleration, the “Fermi” events being associated with strong, fast quasi‐parallel shocks and the “shock drift” events being associated with weaker, slower quasi‐perpendicular events. In the solar wind frame of reference the Fermi events have moderate upstream anisotropies, with flow away from the shock persisting for periods of one to two hours, the anisotropy decreasing with increasing energy, whereas downstream these events are isotropic. These events exhibit slow quasi‐exponential intensity increases of 1–2 orders of magnitude, peaking at the shock, and slowly decaying after the shock, often rising to a secondary peak some hours later. The shock drift events have large upstream first‐order anisotropies close to the shock, with flow away from the shock, and moderate downstream first‐order anisotropies, with flow toward the shock. The most notable feature of the shock drift events is a large negative second harmonic immediately downstream of the shock, signifying protons gyrating around the magnetic field at pitch angles of around 90°. These events have shock spike intensity increases lasting for a few minutes or tens of minutes. At all energies the largest intensity increases are observed with the Fermi events. Since the Fermi events are associated with the largest fluxes of solar flare protons, this may be due to a combination of solar particle background and protons accelerated in the vicinity of the shock.

Acceleration of low‐energy protons and alpha particles at interplanetary shock waves

We have investigated low‐energy protons and alpha particles in the energy range 30 keV/charge to 150 keV/charge associated with three different interplanetary shock waves in the immediate preshock and postshock region. The data were obtained with the Max‐Planck‐Institut/University of Maryland sensor system on ISEE 3. In particular, we present spatial distributions in the preshock and postshock medium as measured in the spacecraft frame and after transformation into the solar wind and the shock frame of reference, respectively, the dependence of the phase space density at different energies on distance from the shock, and the form of the distribution function of both species immediately at the shock. In the preshock region particles are flowing in the solar wind frame of reference away from the shock and in the postshock medium the distribution is more or less isotropic in this frame of reference. This is similar to findings at the earth's bow shock during times when diffuse particles are present. The distribution function in the postshock region can be represented by a power law in energy with the same spectral exponent for both protons and alpha particles. In the preshock medium the phase space densities falls off exponentially (after subtraction of a background value) with distance from the shock and the e‐folding distance scale of the proton phase space densities increases approximately proportional to the square root of energy. In addition, there is a much slower intensity increase further upstream over a time period of several hours. Although the spectra of diffuse bow shock associated particles are different from the spectra of the interplanetary shock‐associated particles in the immediate vicinity of the shock, it is concluded that the first‐order Fermi acceleration process can consistently explain the data. It is shown that independent of the acceleration mechanisms invoked, the mean free path of the low energy ions in the preshock medium is considerably smaller than the mean free path determined by the turbulence of the background interplanetary medium. This suggests that these particles generate a wave field of their own making, which in turn scatters the particles.