M. Dryer

Improvements to the HAF solar wind model for space weather predictions

We have assembled and tested, in real time, a space weather modeling system that starts at the Sun and extends to the Earth through a set of coupled, modular components. We describe recent efforts to improve the Hakamada-Akasofu-Fry (HAF) solar wind model that is presently used in our geomagnetic storm prediction system. We also present some results of these improvement efforts. In a related paper, Akasofu [2001] discusses the results of the first 2 decades using this system as a research tool and for space weather predictions. One key goal of our efforts is to provide quantitative forecasts of geoeffective solar wind conditions at the L1 satellite point and at Earth. Notably, we are addressing a key problem for space weather research: the prediction of the north-south component (Bz) of the interplanetary magnetic field. This parameter is important for the transfer of energy from the solar wind to the terrestrial environment that results in space weather impacts upon society. We describe internal improvements, the incorporation of timely and accurate boundary conditions based upon solar observations, and the prediction of solar wind speed, density, magnetic field, and dynamic pressure. HAF model predictions of shock arrival time at the L1 satellite location are compared with the prediction skill of the two operational shock propagation models: the interplanetary shock propagation model (ISPM) and the shock-time-of-arrival (STOA) model. We also show model simulations of shock propagation compared with interplanetary scintillation observations. Our modeling results provide a new appreciation of the importance of accurately characterizing event drivers and for the influences of the background heliospheric plasma on propagating interplanetary disturbances.

Dynamical models of coronal transients and interplanetary disturbances

Abstract We review the status of the best “off-the-shelf” tool available for the study of dynamical behavior of coronal transients and traveling interplanetary disturbances. This tool involves numerical solution of the initial-boundary value problem of multi-dimensional time-dependent magnetohydrodynamics. While this tool cannot address questions of turbulence and kinetic behavior, we suggest that deeper understanding of large scale phenomena can be obtained by direct comparison of the MHD models with multi-disciplinary synoptic observations of specific events on the sun, and in the corona and interplanetary space. Conclusions reached after a recent critique (based on a limited set of observational and numerical data) of the MHD paradigms application to coronal transients are examined and found to have limited validity. Substantial observational progress was achieved during SMY through ground- and space-based observations of solar and interplanetary events. Many of these observations can confidently be associated with one another for specific events. These associations can be combined into a reasonable scenario of geometrical extent and mass, energy and momentum transfer in the framework of the solar-terrestrial chain of cause and effect. Several of these events during STIP Interval VII in August 1979 are used to provide test cases for an MHD simulation that is described with some details. The bringing-together of diverse observations is necessary in order to outline a program for the testing of dynamical models and their more physically-restricted approximations.

Magnetohydrodynamic models of coronal transients in the meridional plane. II - Simulation of the coronal transient of 1973 August 21

A two-dimensional, planar MHD model of solar atmospheric transient events is used to simulate the flare-associated events of 1973 August 21. This event, observed in H..cap alpha.., He II 304 A, soft X-ray, and coronal white light, provided sufficient information (especially in the latter diagnostic) for an assessment to be made of the models ability to simulate major features of an actual solar event. It was found that a thermodynamic input pulse based on data provided by the NASA Marshall Space Flight Center--Aerospace Corporation X-ray telescope (S-056) on Skylab was sufficient to produce the global geometry, shock and contact surface velocities, excess mass contours, and energy budget which were, for the most part, observed by the High Altitude Observatory white-light coronagraph (S-052) on Skylab in the form of a forerunner and coronal transient.

Propagation of an interplanetary shock along the heliospheric plasma sheet

Propagation of an interplanetary shock along the heliospheric plasma sheet (HPS) is simulated using a high-resolution numerical MHD model in the meridional plane. The ambient solar wind contains two opposite orientations of the interplanetary magnetic field above and below the equatorial plane. These regions are separated by a thin transition layer that represents the heliospheric current sheet contained within the HPS, A pulse is introduced at the inner boundary (0.1 AU) into this steady state to initiate the interplanetary shock. The HPS with its weaker intensity of the magnetic field, larger mass density, and slower flow velocity modifies the global shock structure. A dimple is formed at the forward shock front, a reverse dimple is formed at the reverse shock, and the contact discontinuity is significantly distorted. Weak compression of the HPS occurs beyond the forward shock front due to the postshock increase of the azimuthal magnetic pressure. Although slight collimation of mass flow takes place toward the axis of the HPS, an antisunward protrusion (“pimple”) within the shock fronts dimple did not form in our simulation.

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.

The X-ray signature of solar coronal mass ejections

The coronal response to six solar X-ray flares has been investigated. At a time coincident with the projected onset of the white-light coronal mass ejection associated with each flare, there is a small, discrete soft X-ray enhancement. These enhancements (precursors) precede by typically ∼20 m the impulsive phase of the solar flare which is dominant by the time the coronal mass ejection has reached an altitude above 0.5 R⊙. We identify motions of hot X-ray emitting plasma, during the precursors, which may well be a signature of the mass ejection onsets. Further investigations have also revealed a second class of X-ray coronal transient, during the main phase of the flare. These appear to be associated with magnetic reconnection above post-flare loop systems.

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.

Performance of interplanetary shock prediction models: STOA and ISPM

Abstract The shock time of arrival (STOA) model and the interplanetary shock propagation model (ISPM) give predictions of the time of arrival and strength of solar-initiated interplanetary shocks. This paper presents the first operational predictions made of interplanetary shocks that follow solar events. The time interval of this study was February 1997–March 1999 (the rise of Solar Cycle 23). The results are presented in contingency-table form (whether or not a shock was predicted and/or observed) and also as the time differences (errors) between the predicted and observed shock arrivals. These results are compared to the accuracies that would be obtained using a constant, representative value (Rule of Thumb or R-T) for the transit time. The results show the percentage of successful predictions to within an accuracy of 12 h of shocks for the STOA, ISPM and R-T are 53, 58, and 33%, respectively. The corresponding root mean square (rms) errors of the shock arrival times are 15.0, 15.1 and 14.8 h. Note that the rms heavily weights outlying points, so although only 3 of the ISPM-predictions were off by more than 12 h, the ISPM rms is worse than that for STOA and R-T models; both of which had prediction errors >|12| h for eight events. Statistics presented in this paper not only show the capabilities of these models, but also allow for comparison to future shock-arrival forecasting models. These results may be considered as reference metrics in evaluating forecasting skill. The relationship of the interplanetary shocks to geomagnetic activity is also briefly considered.

Simulation of magnetic cloud propagation in the inner heliosphere in two-dimensions: 1. A loop perpendicular to the ecliptic plane

We present results of simulations of a magnetic clouds evolution during its passage from the solar vicinity (18 solar radii) to approximately 1 AU using a two-dimensional MHD code. The cloud is a cylinder perpendicular to the ecliptic plane. The external flow is explicitly considered self-consistently. Our results show that the magnetic cloud retains its basic topology up to 1 AU, although it is distorted due to radially expanding solar wind and magnetic field lines bending. The magnetic cloud expands, faster near the Sun, and faster in the azimuthal direction than in the radial one; its extent is approximately 1.5–2× larger in the azimuthal direction. Magnetic clouds reach approximately the same asymptotic propagation velocity (higher than the background solar wind velocity) despite our assumptions of various initial conditions for their release. Recorded time profiles of the magnetic field magnitude, velocity, and temperature at one point, which would be measured by a hypothetical spacecraft, are qualitatively in agreement with observed profiles. The simulations qualitatively confirm the behavior of magnetic clouds derived from some observations, so they support the interpretations of some magnetic cloud phenomena as magnetically closed regions in the solar wind.

The influence of the large-scale interplanetary shock structure on a low-energy particle event

We have developed a numerical model to study the influence of the large-scale shock topology on the associated low-energy (less than 2 MeV) particle event upstream of the shock. It includes particle injection at the solar corona, two-dimensional MHD simulation of the propagation of the shock, modeling of particle propagation through the interplanetary medium, and particle injection at the shock. The model does not attempt to simulate the physical processes of shock particle acceleration, therefore the injection at the shock is represented by a numerical source function in the equation that describes particle propagation