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Ward Manchester IV, Janet Kozyra, Sue Lepri, University of Michigan and Benoit Lavraud, Universite de Toulouse

 

The Earth is connected to the Sun through the sunlight that heats our planet, but also through plasmas (electrically charged gas) and magnetic fields that blow out from the solar corona both in steady streams as in the solar wind and in explosions called coronal mass ejections (CMEs). CMEs are clouds of plasma containing solar magnetic fields sometimes still attached to the Sun, hence the name magnetic clouds. The direction of the magnetic field within a cloud determines its potential for producing severe space weather at Earth – also called its geo-effectiveness. Southward interplanetary magnetic fields produce the most severe storms because in this orientation, the Earth’s magnetic field becomes attached to the Sun’s magnetic field allowing dangerous particles and energy to enter geospace from the solar wind. (Severe space storms can damage satellites in orbit around the Earth, induce ground currents that damage or disrupt power grids, create radiation hazards for astronauts and airline passengers, and redistribute ionospheric material in ways that disrupt navigation and communications systems). But even a harmless cloud with magnetic fields that point northward in the same direction as the Earth’s magnetic field can cause a space storm. Here is where the complications arise.

Interplanetary magnetic clouds can be modified as they propagate through the interplanetary space on their way to Earth. They can be deflected from their paths towards or away from Earth by high-speed flows in the background solar wind. They can also collide and merge with previous clouds released in earlier explosions to form a larger disturbance. They can snow-plow the background solar wind plasmas and magnetic fields at their leading edge to form a stormdriving sheath around a cloud that would have otherwise been harmless. And finally, in rare instances, a piece of a massive solar filament can impact the Earth. This is important because high-density plasma can amplify the effects of a magnetic storm.

Often involved in CME eruptions are solar filaments, which are ribbons of dense plasma supported in the Sun’s outer atmosphere (corona) by strong horizontal magnetic fields. Filament material is 100 times denser and 100 times cooler than the surrounding solar atmosphere. When supporting field erupts, the solar filaments are caught up in the explosive release of plasma and magnetic fields. Despite observations indicating that more than 70% of active region eruptions involve solar filaments, they are rarely identified in the disturbances that reach Earth. The reasons are a long-standing mystery in space science.

On 21 January 2005, one of the fastest CMEs of recent times hit the Earth. Observations indicate that its velocity may have reached 3000 km/s between 3 and 50 solar radii before decelerating down to 1000 km/s upstream of Earth. Its travel time to Earth was just 34 hours. In comparison, the typical solar wind travels at 400-700 km/s and takes 3-4 days to reach Earth. The disturbance had two extremely unusual features.

First, it contained a large amount of solar filament material – only one of a handful of such cases ever reported. Second, the filament material was displaced from its expected position. It was at the leading edge of the magnetic cloud, while filament material should be following behind.

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Illustration of a decelerating magnetic cloud with solar filament material pushing its way


through to the front.

 

Using a large-scale simulation of a fast magnetic cloud, we found that the unusual location of the filament material was a consequence of two processes. As the CME decelerated on its way to Earth, the momentum of the massive solar filament caused it to push through the twisted magnetic fields of the magnetic cloud. Diverging flows in front of the filament moved magnetic flux to the sides of the magnetic cloud. These two processes combined to move the filament material to the very front of the CME and produced a pattern of unbalanced magnetic flux at the nose of the cloud.

Within 1 hour after impact of the magnetic cloud and under northward interplanetary magnetic fields, a cold dense plasma sheet formed within the magnetosphere from the filament material. This dense plasma sheet continued to move through the magnetosphere for more than 6 hours as the filament passed by the Earth. Densities were high enough to inflate the magnetotail despite the northward IMF conditions and low levels of magnetic activity. The interaction with the solar filament was linked to an array of unusual features throughout the magnetosphere, ionosphere, and atmosphere. These results raise questions about whether rare collisions with solar filaments may, under the right conditions, be a factor in producing even more extreme space weather events. There is evidence that high-density plasma was involved in some of the most severe space storms ever recorded.

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