S. M. Han

THE PHOTOSPHERIC CONVECTION SPECTRUM

Spectra of the cellular photospheric flows are determined from observations acquired by the MDI instrument on the SOHO spacecraft. Spherical harmonic spectra are obtained from the full-disk observations. Fourier spectra are obtained from the high-resolution observations. The p-mode oscillation signal and instrumental artifacts are reduced by temporal filtering of the Doppler data. The resulting spectra give power (kinetic energy) per wave number for effective spherical harmonic degrees from 1 to over 3000. Significant power is found at all wavenumbers, including the small wavenumbers representative of giant cells. The time evolution of the spectral coefficients indicates that these small wavenumber components rotate at the solar rotation rate and thus represent a component of the photospheric cellular flows. The spectra show distinct peaks representing granules and supergranules but no distinct features at wavenumbers representative of mesogranules or giant cells. The observed cellular patterns and spectra are well represented by a model that includes two distinct modes – granules and supergranules.

A time‐dependent, three‐dimensional MHD numerical study of interplanetary magnetic draping around plasmoids in the solar wind

A spherical plasmoid is injected into a representative solar wind at 18 solar radii, which is chosen as the lower computational boundary of a 3-dimensional MHD model. The field line topology of the injected plasmoid resembles the streamline topology of a spherical vortex. Evolution of the plasmoid and its surrounding interplanetary medium is described out to approximately 1 AU for three cases with different velocities imparted to the plasmoid. In the first case a plasmoid enters the lower boundary with a velocity of 250 km s{sup {minus}1} equal to the steady state background solar wind velocity at the lower boundary. In the second and third cases the plasmoid enters with peak velocities of twice and 3 times the background velocity. A number of interesting features are found. For instance, the evolving plasmoid retains its basic magnetic topology although the shape becomes distorted. As might be expected, the shape distortion increases with the injection velocity. Development of a bow shock occurs when the plasmoid is injected with a velocity greater than the sum of the local fast magnetosonic speed and the ambient solar wind velocity. The MHD simulation demonstrates magnetic draping around the plasmoid.

Non-planar MHD model for solar flare-generated disturbances in the heliospheric equatorial plane

An analysis, with a representative (canonical) example of solar-flare-generated equatorial disturbances, is presented for the temporal and spatial changes in the solar wind plasma and magnetic field environment between the Sun and one astronomical unit (AU). Our objective is to search for first order global consequences rather than to make a parametric study. The analysis - an extension of earlier planar studies - considers all three plasma velocity and magnetic field components (Vr, Vφ, V0, and Br, B0, Bφ) in any convenient heliospheric plane of symmetry such as the ecliptic plane, the solar equatorial plane, or the heliospheric equatorial plane chosen for its ability (in a tilted coordinate system) to order northern and southern hemispheric magnetic topology and latitudinal solar wind flows. Latitudinal velocity and magnetic field gradients in and near the plane of symmetry are considered to provide higher-order corrections of a specialized nature and, accordingly, are neglected, as is dissipation, except at shock waves.The representative disturbance is examined for the canonical case in which one describes the temporal and spatial changes in a homogeneous solar wind caused by a solar-flare-generated shock wave. The ‘canonical’ solar flare is assumed to produce a shock wave that has a velocity of 1000 km s#X2212;1 at 0.08 AU. We have examined all plasma and field parameters at three radial locations: central meridian and 33° W and 90° W of the flares central meridian. A higher shock velocity (3000 km s#X2212;1) was also used to demonstrate the models ability to simulate a strongly-kinked interplanetary field. Among the global (first-order) results are the following: (i) incorporation of a small meridional magnetic field in the ambient magnetic spiral field has negligible effect on the results; (ii) the magnetic field demonstrates strong kinking within the interplanetary shocked flow, even reversed polarity that - coupled with low temperature and low density - suggests a viable explanation for observed ‘magnetic clouds’ with accompanying double-streaming of electrons at directions ∼ 90° to the heliocentric radius.

Magnetohydrodynamic modelling of interplanetary disturbances between the Sun and Earth

A time-dependent, nonplanar, two-dimensional magnetohydrodynamic computer model is used to simulate a series, separately examined, of solar flare-generated shock waves and their subsequent disturbances in interplanetary space between the Sun and the Earths magnetosphere. The ‘canonical’ or ansatz series of shock waves include initial velocities near the Sun over the range 500 to 3500 km s−1. The ambient solar wind, through which they propagate, is taken to be a steady-state homogeneous plasma (that is, independent of heliolongitude) with a representative set of plasma and magnetic field parameters. Complete sets of solar wind plasma and magnetic field parameters are presented and discussed.Particular attention is addressed to the MHD models ability to address fundamental operational questions vis-à-vis the long-range forecasting of geomagnetic disturbances. These questions are: (i) will a disturbance (such as the present canonical series of solar flare shock waves) produce a magnetospheric and ionospheric disturbance, and, if so, (ii) when will it start, (iii) how severe will it be, and (iv) how long will it last? The models output is used to compute various solar wind indices of current interest as a demonstration of the models potential for providing ‘answers’ to these questions.

Expected IPS variations due to a disturbance described by a 3-D MHD model

Abstract The variations of interplanetary scintillation due to a disturbance described by a threedimensional, time-dependent, magnetohydrodynamical (MHD) model of the interplanetary medium are calculated. The resulting simulated IPS maps are compared with observations of real disturbances and it is found that there is some qualitative agreement. We are able to conclude that the MHD model with a more realistic choice of input conditions would probably provide a useful description of many interplanetary disturbances.

Three-dimensional, time-dependent, MHD model of a solar flare-generated interplanetary shock wave

Three-dimensional model of the propagation of an interplanetary shock wave into a representative ambient three-dimensional heliospheric solar wind is demonstrated. The numerical MHD simulation is initialized by assuming a peak shock velocity of 1000 km sec-1 at the center of a right circular cone of 18° included angle at 18 solar radii. Examination of the shocked plasma and IMF parameters reveals several fundamental results that differ from our earlier 2D and 2-1/2D simulations under similar input conditions that were confined to the ecliptic plane. The differences include: (i) diminution of the solar wind peak velocity occurs in the 3D example because of flow divergence introduced in the heliolatitudinal direction; (ii) concentration of the peak density at each radius in an annular region with the minimum density at the central axis due, again, to flow divergence; (iii) similar behavior of the IMF magnitude that starts (near the sun) with peaks at high latitudes and to the west of the simulated flare’s central meridian; and (iv) twisted, helical-like, IMF rotation due to a large amplitude, non-linear Alfven wave in the shocked plasma. The amplitude is highest along the central axis and decays to a minimum at the sides of the MHD shock where the fast-mode jump conditions prevail.

Three-dimensional, time-dependent MHD simulations of interplanetary plasmoids

Abstract Plasmoids in the interplanetary medium have been hypothesized for nearly 2 decades by many observers. These suggestions can be classified into two categories: (1) solar-ejected diamagnetic plasmoids that retain their closed, albeit expanding, topology to and beyond Earth; and (2) plasmoids that are formed in the corona near the Sun or in the interplanetary medium as a consequence of reconnection of opposite-directed IMF lines. We present a 3-D MHD, time-dependent simulation of a plasmoid in the first category under the assumption that, before entering the computational domain, the plasmoid already exists at or near the Sun. Also, we present a simulation of a plasmoid in the second category where a dipolar solar IMF with an initially flat heliospheric current sheet and a representative solar wind is disturbed by a simulated shock wave. Of particular interest is the draping of the IMF about the plasmoid in both examples. Although the shock in the second example propagates across the current sheet with negligible large-scale distortion, the strong transverse pressure gradients behind it apparently cause reconnection and formation of a plasmoid. The code, which has a grid too coarse for examination of kinetic reconnection studies, has both inherent and explicit numerical diffusion that allows reconnection. A fast forward MHD shock precedes the plasmoid as it expands into heliospheric space. We will describe the plasma and magnetic properties of the expanding plasmoid as it moves toward an observer at 1 AU.

Interplanetary disturbances produced by a simulated solar flare and equatorially-fluctuating heliospheric current sheet

A recently developed nonplanar, time-dependent magnetohydrodynamic (MHD) model (Wuet al., 1983) was used to study the interplanetary disturbances produced by a compound event in the heliosphere. That is, a steady-state interplanetary medium is first disturbed by a simulated equatorially-fluctuating current sheet. After a few days (100 hr), the disturbed interplanetary medium is again perturbed by a solar-flare-generated shock wave. Attention is directed toward the differences that are caused by the presence of the equatorially-fluctuating (warped) current sheet.

Interplanetary shock collisions: Forward with reverse shocks

When one interplanetary shock overtakes another, the structure that results depends upon the nature of the interacting shocks. We examine, numerically, the results of collisions of forward with reverse shocks, in two dimensions, and show that the results depend primarily upon shock strength. We also note that such interactions could explain why many energy outbursts on the Sun that would be expected to cause geomagnetic effects at Earth, do not.

Recent Developments on the Numerical Simulation of Astrogeophysical Flows

In this paper, we present the recent developments of numerical simulation models for astrogeophysical flows, in particular, the propagation of solar disturbances in the heliospheric space.