Plasma Plume protects the Earth against solar storms

The Earth's magnetic field, or magnetosphere, stretches from the planet's core out into space, where it meets the solar wind, a stream of charged particles emitted by the sun. 

For the most part, the magnetosphere acts as a shield to protect the Earth from this high-energy solar activity.

But when this field comes into contact with the sun's magnetic field, a process called "magnetic reconnection," powerful electrical currents from the sun can stream into Earth's atmosphere, whipping up geomagnetic storms and space weather phenomena that can affect high-altitude aircraft, as well as astronauts on the International Space Station.

Now scientists at MIT and NASA have identified a process in the Earth's magnetosphere that reinforces its shielding effect, keeping incoming solar energy at bay.

By combining observations from the ground and in space, the team observed a plume of low-energy plasma particles that essentially hitches a ride along magnetic field lines, streaming from Earth's lower atmosphere up to the point, tens of thousands of kilometers above the surface, where the planet's magnetic field connects with that of the sun.

In this region, which the scientists call the "merging point," the presence of cold, dense plasma slows magnetic reconnection, blunting the sun's effects on Earth.

John Foster

"The Earth's magnetic field protects life on the surface from the full impact of these solar outbursts," says John Foster, associate director of MIT's Haystack Observatory.

"Reconnection strips away some of our magnetic shield and lets energy leak in, giving us large, violent storms."

"These plasmas get pulled into space and slow down the reconnection process, so the impact of the sun on the Earth is less violent."

Foster and his colleagues publish their results in this week's issue of Science.

Philip Erickson

The team includes Philip Erickson, principal research scientist at Haystack Observatory, as well as Brian Walsh and David Sibeck at NASA's Goddard Space Flight Center.

Mapping Earth's magnetic shield
For more than a decade, scientists at Haystack Observatory have studied plasma plume phenomena using a ground-based technique called GPS-TEC, in which scientists analyze radio signals transmitted from GPS satellites to more than 1,000 receivers on the ground.

Large space-weather events, such as geomagnetic storms, can alter the incoming radio waves—a distortion that scientists can use to determine the concentration of plasma particles in the upper atmosphere.

Using this data, they can produce two-dimensional global maps of atmospheric phenomena, such as plasma plumes.

These ground-based observations have helped shed light on key characteristics of these plumes, such as how often they occur, and what makes some plumes stronger than others but as Foster notes; "this two-dimensional mapping technique gives an estimate only of what space weather might look like in the low-altitude regions of the magnetosphere."

To get a more precise, three-dimensional picture of the entire magnetosphere would require observations directly from space.

Toward this end, Foster approached Walsh with data showing a plasma plume emanating from the Earth's surface, and extending up into the lower layers of the magnetosphere, during a moderate solar storm in January 2013.

Walsh checked the date against the orbital trajectories of three spacecraft that have been circling the Earth to study auroras in the atmosphere.

As it turns out, all three spacecraft crossed the point in the magnetosphere at which Foster had detected a plasma plume from the ground.

The team analyzed data from each spacecraft, and found that the same cold, dense plasma plume stretched all the way up to where the solar storm made contact with Earth's magnetic field.

More information: "Simultaneous Ground- and Space-Based Observations of the Plasmaspheric Plume and Reconnection" Science, 2014.

Magnetic shielding of exomoons

A new study on magnetic fields around extrasolar giant planets sheds first light on the magnetic environment of extrasolar moons. 

The work, authored by René Heller of the Department of Physics and Astronomy at McMaster University (Canada) and Jorge I. Zuluaga of the FACom group in the Institute of Physics of the University of Antioquia (Colombia), is the first to explore the complex magnetic environment of exomoons and its impact on the habitability of these peculiar bodies.

Regrettably the results are not completely encouraging. Even the most massive moons that can be expected from a formation point of view will be small compared to Earth.

Thus, the only possibility these moons can be magnetically protected from the stellar and cosmic high-energy radiation is that they are encased by their giant planet's magnetosphere.

Yet, in orbits close to the planet, these moons can be subject to enormous tidal heating, potentially making them uninhabitable.

These results represent just the beginning of an interesting research branch, which introduces a new key factor for the habitability of those "Pandora"-like environments.

Probably the first image that comes to our minds when thinking of an inhabited extrasolar moon shows the beautiful landscapes of Pandora, the hypothetical moon of James Cameron's movie Avatar.

But the environments of extrasolar moons seem to be less favored than the idealized version shown on the big screen.

Even if located around planets in the stellar "habitable zone," where the amount of incoming light allows for the existence liquid water and hence life, exomoons are subject to a number of other perturbing effects making things harsher for life than previously thought.

René Heller

In a paper accepted for publication in the Astrophysical Journal Letters, Prof. Jorge I. Zuluaga and Dr. René Heller have studied the relationship among the magnetic field of extrasolar giant planets in the stellar habitable zone and the so-called "habitable edge" around those planets.

This edge defines the minimum distance to the planet at which a moon would just avoid to undergo a runaway greenhouse effect—an uninhabitable state similar to that on Venus.

Heller and Zuluaga studied moons having a mass and size similar to that of Mars orbiting planets with masses and compositions ranging from that of Neptune to Jupiter.

Larger moons, with masses similar to that of Earth, are unlikely to have formed even around the largest planets, while moons lighter than Mars will not be easily detected in the near future.

Structure of the magnetosphere. Dashed lines represent the orbits of moons which are completely shielded (CS), partially shielded (PS) and unshielded (US)

The core of this new work is in the calculation of the size of the magnetospheres of giant planets that are located in the habitable zones of their host stars.

Planetary magnetospheres are "bubbles" made of fields and plasma created by the shock between the stellar wind and the intrinsic magnetic field of the planet.

These bubbles separate the immediate magnetic environment of the planet from the very different environment of the interplanetary space. Magnetospheres could be really huge.

In the solar system, for example, Jupiter is surrounded by a magnetosphere that ends at distances up to 50 times the size of the planet (3.5 million of kilometers or almost 10 times the distance from the Earth to the Moon) on the dayside and stretches out almost as far as the orbit of Saturn in the night or trailing side, creating an "invisible" plasma tail.

Although the magnetosphere and its elongated tail could cover billions of kilometers, the most important part in terms of permanent moon coverage is restricted to a region the size of the dayside radius, or "standoff radius."

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