Thomas R. Detman

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.

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.

MHD simulation of an interaction of a shock wave with a magnetic cloud

Interplanetary shock waves, propagating in the heliosphere faster than earlier-emitted coronal ejecta, penetrate them and modify their parameters during this interaction. Using two and one half dimensional MHD simulations, we show how a magnetic cloud (flux rope) propagating with a speed 3 times higher than the ambient solar wind is affected by an even faster traveling shock wave overtaking the cloud. The magnetic field increases inside the cloud during the interaction as it is compressed in the radial direction and becomes very oblate. The cloud is also accelerated and moves faster, as a whole, while both shocks (driven by the cloud and the faster interplanetary shock) merge upstream of the cloud. This interaction may be a rather common phenomenon due to the frequency of coronal mass ejections and occurrence of shock waves during periods of high solar activity.

Solar wind velocity and normalized scintillation index from single-station IPS observations

The recently refurbished Ooty Radio Telescope in southern India was used in a two-month campaign of interplanetary scintillation (IPS) observations in collaboration with the Cambridge IPS array in England during April–May 1992. The unique feature of this campaign was that, for the first time, scintillation enhancements were predicted in real time by observing solar events on 7–8 May, 1992 and then detected at both Ooty and Cambridge. Also, for the first time, high spatial resolution (∼ 100 sources sr−1) solar wind all-sky velocity maps were obtained at Ooty. Good consistency is found between the IPS observations from both observatories andin-situ shocks detected at Earth by IMP-8.Yohkoh soft X-ray images were used to infer the generation of a coronal mass ejection on 7 May, 1992.

Parametric study of loop‐like magnetic cloud propagation

Propagation and evolution of loop-like magnetic clouds in the ambient solar wind flow are studied self-consistently using ideal MHD equations in the 2½ dimensional approximation. Magnetic clouds, as ideal force-free objects (cylinders lying in the ecliptic plane), are ejected near the Sun and followed beyond the Earths orbit. We investigate the influence of various initial parameters, like the injection velocity or different steady states of the solar wind, on their propagation and evolution. Simulation results are compared with an analytical theory of magnetic cloud evolution (expansion) published by Osherovich et al. [1993a, b]; good agreement is found, although no need to use a polytropic index less than 1 (as in the analytical approach) is required.

Propagation of a spheromak: 1. Some comparisons of cylindrical and spherical magnetic clouds

A series of our papers in the Journal of Geophysical Research, 1995-1996, was devoted to simulations of propagation of cylindrical magnetic clouds (flux ropes) having different orientation of their axes to the ecliptic plane and initial parameters. In this paper we supplement our study with the case of detached spherical plasmoids. By varying the velocity, density, temperature, and the magnetic field strength inside clouds, we simulate a number of plasmoid scenarios that can be compared with observations and with existing models and simulations of flux ropes. Initially, the spherical clouds have a poloidal magnetic field configuration within a sphere. During the propagation they evolve into toroids (i.e., closed flux ropes). Radial profiles of magnetic field and plasma quantities in these toroids are similar to cylindrical magnetic clouds. However, they are different in the central (now external) part of the cloud, where the poloidal axis was originally situated, that is, in the toroids hole. Here the magnetic field is greatly enhanced but does not rotate, and the temperature decrease is absent. The deceleration and transit time to 1 AU is comparable between spherical and cylindrical clouds. The shock wave ahead of a spherical cloud is about 2 times closer than for a corresponding cylindrical cloud.

Key Links to Space Weather: Forecasting Solar-Generated Shocks and Proton Acceleration

Forecasting the arrival of solar-generated shocks and accelerated protons anywhere in the heliosphere presents an awesome challenge in the new field of space weather. Currently, observations of solar wind plasmas and interplanetary magnetic fields are made at the sun-Earth libration point, L1, about 0.01 astronomical units (∼245 Earth radii) sunward of our planet. An obvious analogy is the pilot tube that protrudes ahead of a supersonic vehicle. The Advanced Composition Explorer and Solar and Heliospheric Observatory spacecraft, currently performing this function, provide about -1 h advance notice of impending arrival of interplanetary disturbances. The signatures of these disturbances may be manifested as interplanetary shock waves and/or coronal mass ejecta. We describe a first-generation procedure, based on first-principles numerical modeling, that provides the key links required to increase the advance notice (or lead time) to days, or even weeks. This procedure, instituted at the start of the present solar cycle 23, involves three separate models, used in real time, to predict the arrival of solar-event-initiated interplanetary shock waves at the L1 location. We present statistical results, using L1 observations as ground truth for 380 events. We also briefly discuss how one of these models (Hakamada-Akasofu-Fry version 2) may be used with a model that predicts the flux and fluence of energetic particles, for energies up to 100 MeV, that are generated by these propagating interplanetary shock waves.

Magnetic traps in the interplanetary medium associated with magnetic clouds

MHD simulations of the propagation of magnetic clouds in the interplanetary medium show that interplanetary magnetic field (IMF) lines, draping around the cloud, are often bent in a complicated way. The magnetic field along these field lines (even on nonbent sections) is not smoothly decreasing with increasing distance from the Sun but usually exhibits several extreme values (minima and maxima). Depressions in the IMF strength may trap energetic particles with suitable energies and pitch angles. These particles may remain trapped (in the expanding region) until the IMF configuration changes. Possible locations of magnetic traps are shown in this paper.

Real-time Kp predictions from ACE real time solar wind

The Advanced Composition Explorer (ACE) spacecraft provides nearly continuous monitoring of solar wind plasma, magnetic fields, and energetic particles from the Sun-Earth L1 Lagrange point upstream of Earth in the solar wind. The Space Environment Center (SEC) in Boulder receives ACE telemetry from a group of international network of tracking stations. One-minute, and 1-hour averages of solar wind speed, density, temperature, and magnetic field components are posted on SEC’s World Wide Web page within 3 to 5 minutes after they are measured. The ACE Real Time Solar Wind (RTSW) can be used to provide real-time warnings and short term forecasts of geomagnetic storms based on the (traditional) Kp index. Here, we use historical data to evaluate the performance of the first real-time Kp prediction algorithm to become operational.

On the large-scale effects of two interplanetary shocks on the associated particle events

Abstract An evolutionary model, including particle and shock propagation through the interplanetary medium, has been used to reproduce the evolution of the flux and anisotropy in the upstream region of two low-energy particle events observed by ISEE-3. These events, on 24 April 1979 and 18 February 1979, originated at solar (helio)longitudes ≈ 50 apart. By fitting the observed particle fluxes and anisotropies, the conditions for the propagation of the particles through the interplanetary medium and the injection rates at the shock have been determined as a function of time. The results are discussed in terms of the interplanetary magnetic field connection between the observer and the shock front and they are related to the heliolongitude of the parent solar activity.