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Associated Research Papers

SolPhys_Lugaz2009.pdf - Lugaz et al. Solar Terrestrial Simulation in the STEREO Era: The January 24-25, 2007 Eruptions

JGR_109_A02107.pdf – Manchester et al., Modeling a space weather event from the Sun to the Earth: CME generation and interplanetary propagation

 

    Research


    Propagation of CMEs in the Heliosphere

    Once a CME is initiated and ejected into space, it is crucial to understand how the CME evolves as it travels towards Earth. A key factor affecting CME evolution is the way in which it interacts with the solar wind and interplanetary magnetic fields. In order to get a clear picture of how a CME will affect Earth, it is necessary to accurately describe a CME’s structure at 1 AU after it has propagated through the heliosphere.

    At C2H2, astronomers work to understand, model, and predict a CME’s structure at 1 AU after it has interacted with the heliosphere. In order to be accurate, the models must 1) include a realistic mechanism for the initiation and expansion a CME, 2) use a solar wind model that accurately reproduces the major attributes of the real solar wind, and 3) invoke physics that realistically describes the magnetic and fluid interactions of the CME plasma with the solar wind.

    In carrying out models of a propagating and interacting CME, it is necessary to consider the coupled system comprising of the Sun, solar wind, magnetosphere, ionosphere, and thermosphere. These models must include magnetic behavior as well as the fluid interactions of plasma, so the equations of magneto-hydrodynamics (MHD) lend themselves to describe the space environment between the Sun-Earth system. MHD models provide only a low order approximation of the state of the plasma, but accurately describe the bulk transport of plasma and magnetic flux throughout the system.

    In work done by Manchester et al. in 2004, which included C2H2’s Ilia Roussev and which continues at C2H2, a CME was modeled traveling through the heliosphere out to 1 AU. The model started with a system out of equilibrium and then evolved over time. The coronal magnetic fields were modeled for the quiet Sun with open field lines near the poles and closed field lines near the equatorial regions, forming a streamer belt. The CME was modeled using a 3-D magnetic flux rope placed within the streamer belt. The flux rope began out of equilibrium and rapidly expanded to drive a shock as it was expelled from the corona. The CME and its related shocks and magnetic fields were then tracked as it traveled through the heliosphere out to 1 AU.

    Modeling CME evolution through the heliosphere is not possible without an accurate model of the solar wind. In this study, a steady state solar wind at solar minimum with the following essential features is employed: 1) open magnetic field lines forming coronal holes at high latitudes; 2) closed magnetic field lines forming a streamer belt at low latitudes; 3) a bi-modal solar wind with fast wind over the poles and slow wind over the equator, which is simulated by using 1) & 2). Solar rotation is included because the Parker spiral effect is important at large distances, such as 1 AU.

    The steady-state solar wind model used in this simulation is as follows (see figure below). At high latitudes, the magnetic field is carried out with the solar wind, creating open magnetic field lines. Close to the equator, the magnetic field lines are closed, forming helmet streamers. A current sheet forms at the tip of the helmet streamers along the neutral line. As the magnetic field expands away from the Sun frozen into the solar wind, the Sun’s rotation alters the orientation of the magnetic fields as described by the Parker spiral. More and more distant fields become more and more angled until they are at an approximately 45 degree angle compared to the radial direction at 1 AU. Finally, the latitudinal variation in wind speed is determined by temperature and flux expansion. The Sun is hotter at the poles by approximately a factor of 2, but the factor that describes flux expansion is more than two times larger at the equator. A function based on these two values results in latitudinal wind speed variation.

    Care was taken in this model to describe a CME as physically realistically as possible. The CME is modeled using a flux rope that experiences a force imbalance between the magnetic pressure within the flux rope and the plasma pressure of the corona. The flux rope is initially added to the corona and made to be slightly buoyant. The magnetic field in the flux rope is much higher than the surrounding corona, providing the energy for the CME explosion. To spark the explosion, a large amount of plasma is suddenly removed, leading to flux rope expansion.

    When the CME explodes, it quickly accelerates to over 1000 km/s in the low corona, creating a shock front. The CME also expands and begins decelerating as it is expelled from the corona. The CME continues to expand and decelerate this way for a few hours, but eventually the dense plasma of the solar wind begins to slow expansion in the radial direction. The CME continues to expand outwards in the direction perpendicular to its travel. This results in a pancake shaped CME. At high solar latitudes, the fast solar wind, moving at 750 km/s, is actually traveling faster than the CME and sweeps the ends of the CME forward. The CME is now in the shape of a crescent that faces away from the Sun. Like the CME, the solar wind distorts the shock front in a similar way, creating a crescent shape, as well.

    The shape of the CME when it reaches the Earth is important because it will affect the field orientation at the Earth (affecting the way it interacts with the Earth’s magnetic field). The CME’s shape also affects the shock inclination, which is significant to the Earth’s magnetospheric response, as well. The modeled CME at 1 AU is shown in the figure below.

    Understanding the way a CME interacts with the heliosphere will allow astronomers to predict how a CME will affect the Earth when it arrives. In this model, the CME moved at similar speeds to the solar wind. Some CMEs may travel much faster than the fast solar wind and thus will be affected in a different way. Slower CMEs may also result in different interactions. At C2H2, we continue to study CME-heliosphere interactions to further refine the physics and understanding behind these explosive solar events.