Y. C. Whang

Solar wind in the distant heliosphere

This paper presents a quantitative MHD model to study the effects of pickup protons on the solar wind. The model includes the kinetic theory solution of interstellar neutral hydrogen, the ionization process for the production of pickup protons, the spiral interplanetary magnetic field, and the full MHD equations. In order to study the effects of pickup protons, we first obtain the kinetic theory solutions of interstellar neutral hydrogen. This solution is then used to calculate the flow of interstellar hydrogen and the production of pickup protons. We study a one-dimensional, steady state model of the pickup proton solar wind in the upwind direction. The solar wind proton parameters predicted by this solution agree reasonably well with observational data from Voyager 2. Throughout the heliosphere the temperature of pickup protons is higher than the temperature of the solar wind protons by two orders of magnitude. Outside the ionization radius, the solar wind temperature and the fast speed begin to increase and the bulk speed and the Mach number of the wind begin to decrease. The component of the equation of motion along the streamline direction can be written in the form of a Bernoulli equation; it can clearly explain the deceleration of the solar wind. In the distant heliosphere the sum of the kinetic energy and the thermal energy decreases with the heliocentric distance due to the charge exchange process; at the same time, the thermal energy of the solar wind plasma increases due to heating by the pickup process. The combined action causes a significant radial decrease in the kinetic energy of the wind.

Pickup protons and pressure-balanced structures from 39 to 43 AU: Voyager 2 observations during 1993 and 1994

The pressure of interstellar pickup protons in the distant heliosphere can be determined by analyzing pressure-balanced structures, observed on a scale of a few hundredths of an AU. This paper extends the earlier work of L. F. Burlaga et al. (Journal of Geophysical Research, 99, 21,511, 1994) by analyzing pressure-balanced structures observed by Voyager 2 from 39.3 to 40.6 AU in 1993 and from 42.6 to 43.2 AU during 1994. The pickup proton temperature is high in the region of the distant heliosphere that we considered: (5.4 ± 0.1) × 106 K at 39–41 AU and (6.0 ± 0.4) × 106 K at ≈43 AU. The density of the pickup protons is (1.6 ± 0.3) × 10−4 cm−3 at 39–41 AU and (1.2 ± 0.2) × 10−4 cm−3 at ≈43 AU. The ratio of the pickup proton density to the solar wind proton density (Ni/N) is small, only 0.03 ± 0.01 during both 1993 and 1994. Nevertheless, the pickup proton pressures are relatively high because of their high temperatures. The pickup proton pressure was (11 ± 2) × 10−14 erg cm−3 at 39–41 AU and (9 ± 1) × 10−14 erg cm−3 at 43 AU. There is a possible decrease in Pi with increasing distance from the Sun. The pickup proton pressure is an order of magnitude greater than the solar wind proton pressure: Pi/Pswp = 10 ± 2 at 39–41 AU and 8 ± 2 at 43 AU. Our results support the hypothesis of Burlaga et al. that the pickup proton pressure is more important than the solar wind thermal pressure in the dynamics of the distant heliosphere. The ratio of the pickup ion pressure to the magnetic pressure is Pi/(B2/8π) = 1.7 ± 0.3 at 39–41 AU and 1.7 ± 0.72 at ≈43 AU. These results are compared with a model.

Global structure of the out‐of‐ecliptic solar wind

[1] We use the observed photospheric field maps and the wind speed observed from Ulysses to study the out-of-ecliptic solar wind. The model calculates the wind speed from the rate of magnetic flux tube expansion factors using a conversion function that is determined by least squares fit of all currently available data from Ulysses. Using the best fit conversion function, we investigate the global solar wind covering a 36-year period from 1968 through 2003. The results complement and expand upon earlier studies conducted with interplanetary scintillation and other in situ spacecraft observations. The rotationally averaged wind speed is a function of two parameters: the heliolatitude and the phase of the solar cycle. The out-of-ecliptic solar wind has a recurrent stable structure, and the average wind speed varies like a sine square of latitude profile spanning more than 5 years during the declining phase and solar minimum in each solar cycle. Ulysses has observed this stable structure in its first polar orbit in 1992-1997. Near solar maximum the structure of the out-of-ecliptic solar wind is in a transient state lasting 2-3 years when the stable structure breaks down during the disappearance and reappearance of the polar coronal holes.

Magnetohydrodynamic waves and instabilities in the heat-conducting solar wind plasma

Abstract The solar wind plasma is a heat-conducting, thermally anisotropic plasma. This paper studies the various wave modes and instabilities in such a plasma, based on a closed system of governing equations obtained by Whang (1971, J. geophys. Res. 76 , 7503). The inclusion of heat fluxes leads to the presence of two new wave modes, in addition to the Alfven, slow magnetosonic and fast magnetosonic modes. The presence of the zeroth-order heat fluxes also results m an asymmetry in the wave speed and a new MHD instability. The new unstable wave propagates in the direction of the heat fluxes. It is demonstrated that the Alfven wave and the associated fire-hose instability are not affected by the presence of heat fluxes, while the exclusion of heat fluxes may suppress the development of the mirror instability. The results are also applied to the heat-conducting solar wind.

Merging of slow‐mode compressive waves in low‐β plasma

A unsteady one-dimensional model can be used to study the interaction of two colliding streams for the formation of MHD shocks. If both β and θ are small, the interaction evolves to a shock system consisting of slow shocks only. At increasing β and θ, the interaction may evolve to a system consisting of both slow and fast shocks and eventually to a system consisting of fast shocks only. We study the merging rates of neighboring characteristic curves for compressive waves associated with the interaction region. We first study a linearized wave system produced by an initial small perturbation. The results show that at small β and θ, the merging rate of neighboring characteristic curves for the slow-mode compressive waves is far greater than that for the fast-mode compressive waves. Then we use a nonlinear example to demonstrate that at small β and θ, the slow-mode characteristic curves merge rapidly to form a pair of slow shocks, while there is no merging of fast-mode characteristic curves. The results explain why β and θ have an important effect on the nature of shocks.

Statistical properties of interplanetary fluctuations behind the bow shock

Abstract Snells transmission model can determine all modes of refracted waves if the incident angle is between the two critical angles. However, the model cannot be used to study transmission of waves at all incident angles. Assuming that interplanetary fluctuations impinge on the bow shock from all directions, an alternative model is introduced in this paper to simulate their interaction. The results show that the state of fluctuations undergoes a significant jump across Earths bow shock. Both the amplitude of fluctuations and the degree of fluctuation anisotropy increase across the Earths bow shock. The calculated probability distribution functions of fluctuations immediately behind the bow shock resemble those reconstructed from observed magnetosheath data. Simulations are carried out to study the spatial variation for the state of fluctuations behind the bow shock under typical interplanetary conditions. We have found that the dawn-dusk (or South-North) asymmetry of the state of fluctuations is present in the magnetosheath when the background magnetic field is in the ecliptic (or meridian). A numerical experiment for MHD waves with incident angles within the limits of the two critical incident angles is also carried out to compare the result calculated from this proposed model with those from the well-developed Snells transmission model.