Transcript Electromagnetic energy conversion at reconnection fronts

Electromagnetic energy conversion
at reconnection fronts
Angelopoulos et al., Science, Sep. 27, 2013
Supporting Material for Education, Public
Outreach and the Press
Vassilis Angelopoulos
Dept. of Earth, Planetary and Space Sciences, UCLA
For release: 2pm U.S. E.T. on Thursday, 09/26/13
Even steady solar wind can lead to dynamic space weather
Movie courtesy: NASA/SOHO (soho_movie9CME.avi)
Movie courtesy: NASA/SVS (NASA_substorm_a010104_H264_....mp4)
Solar activity can cause space weather at Earth. Even steady, slow solar winds can drive the unsteady energy loading
and sudden energy releases at near-Earth space called substorms. Storms and substorms are driven by similar
processes that are important to understand. Sun’s corona - the glow visible during a solar total eclipse - is a mix of hot,
positively charged atoms and electrons that expands outward, towards interstellar space at a million miles an hour. This
“solar wind” carries with it Sun’s own magnetic field. During solar maximum the Sun’s surface develops sunspots where
the magnetic field is 1000 times more intense than typical. The energy in that magnetic field near the solar surface can
heat and accelerate the solar wind above it explosively, occasionally releasing billions of tons of mass into space. (left,
from NASA’s SOHO mission). When Earth is in the path of one of these high speed, dense solar wind streams the SunEarth interaction is very intense, causing high energy particles near the Earth, inducing currents that damage power
distribution lines and powering intense auroras. Such space storms are the most dramatic space weather phenomena.
However, even during steady solar wind (right, artist’s concept), space weather is still dynamic.
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Solar wind distorts Earth’s magnetic field, creates
magnetosphere
The solar wind blowing past Earth distorts Earth’s magnetic field into a long magnetosphere. As the solar wind buffets
magnetized planets it distorts their magnetic fields into windsock-shaped regions, called magnetospheres (For a movie of that
distortion see: BarMagneC.m1v, or click the image above). Earth’s magnetosphere extends more than a million miles behind the
Earth, along the Sun-Earth line, on the nightside (to the right of the image). Like the bow wave ahead of a fast moving ship, or a
shock wave of a supersonic jet, the bow shock stands ahead of the sunward side of the magnetosphere. Space weather at Earth
is powered by the solar wind through a series of energy transformations common to storms and substorms. The Sun’s rotation
winds up energy in its magnetic field, accelerating the solar wind outward. The Earth’s magnetic field captures a fraction of this
energy and re-releases it internal to the magnetosphere to accelerate and heat particles near Earth. It is this last elusive step,
the impulsive energy conversion internal to the magnetosphere, that is the point of our study.
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The magnetosphere contains the radiation belts, where
most communications and remote sensing satellites orbit
The radiation belts are filled with high-energy electrons, protons and other charged particles. The charged particles penetrate
satellite walls and electronics causing damage through buildup of electrical charge, and also emit radiation (X rays, gamma rays).
During magnetic storms particle fluxes become so intense that sensitive electronics can be disabled and satellites can become
inoperable. Astronauts and future space tourists, immersed in this environment can receive in a short time more than a
lifetime’s dose of radiation; storm-time radiation levels can be lethal. Understanding and predicting Earth’s space weather, is
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important for our technologies, space exploration and the increasing presence of humans into space.
Magnetic reconnection allows energy coupling
nightside
dayside
Image courtesy: N. A. Tsyganenko, SPbU
A cut through a computer model of the magnetosphere under steady, southward solar wind magnetic field. Sun is on
the left (+X). North is to the top (+Z). When the solar wind’s magnetic field (left, red line) is directed nearly opposite to
the Earth’s (left, blue line) and the two field lines meet, they snap and re-connect such that solar wind field lines
become connected to Earth’s poles. Not much energy is released at first, but dayside reconnection sets the stage for
what follows… The fast-moving solar wind sets the magnetosphere into motion. It strips magnetic field lines from the
dayside, transports them over the poles and compresses them in the magnetosphere behind the Earth, at the
nightside. This increases the magnetic field (and the energy) in Earth’s long tail, the magnetotail. Eventually, the field
lines in the magnetotail reconnect again (right), to initiate the transfer of the magnetic energy back to particle energy.
The accelerated particles enter Earth’s radiation belts and also precipitate into the atmosphere causing brilliant auroras.
Where and how this nightside energy conversion from magnetic to particle energy occurs has been, so far, unclear.
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Reconnection fronts power Earth’s radiation belts and cause
brilliant auroras, leading to intense space weather.
Image courtesy: E. V. Masongsong, UCLA
Triggered by reconnection, the energy release in Earth’s magnetosphere happens at reconnection fronts. Past studies had
shown that magnetic reconnection initiates the avalanche of energy in substorms. In each substorm cycle, the solar wind
motional energy is stored in the Earth’s stretched and compressed magnetic field lines on the nightside over tens of minutes to
hours. The field lines then snap and reconnect impulsively (in minutes), somewhere near midnight (opposite the Sun), roughly
midway between Earth and the moon’s orbit. But where does the stored magnetic energy get converted to particle kinetic
energy, once nightside reconnection takes off? The present paper shows that this conversion happens inside thin fronts
(boundaries) of intense magnetic flux that are launched on both sides of the reconnection region (see Figure). The converted
energy causes particle flows (red arrows), heating (yellow volume), and excites waves. On the earthward side, these energize the
radiation belts and cause particle precipitation into the atmosphere that drive brilliant auroras. On the tailward side, giant
plasmoids are launched out to interplanetary space. Just as weather fronts come with wind shear, heat waves or rain and drive
atmospheric weather, reconnection fronts cause space particle flow shears, heating and precipitation, driving space weather. 6
An alignment of ARTEMIS in the tail
and six other satellites enabled this study.
Image courtesy: E. V. Masongsong, UCLA
The recently commissioned, dual lunar orbiter mission ARTEMIS provides a unique vantage-point, a lunar perspective of Earth’s
space weather phenomena, enabling measurements of the total magnetic flux transported and energy converted in the
magnetotail during each substorm. While measuring the global energy responsible for such space weather phenomena,
ARTEMIS, together with THEMIS and ancillary Earth-orbiting spacecraft, can also measure the local flux transport and energy
conversion on either side of the reconnection site to determine if these match expectations from global transport measurements.
During such conjunctions (twice yearly) between ARTEMIS (P1, P2), THEMIS (P3-P5), Geotail (GT), and GOES (G15, G13) it is now
possible to observe the reconnection fronts and measure their contribution to magnetic flux transport per unit Y-distance (the
electric field), and power conversion rate (like in an ordinary resistor, this power density is current density times the electric field
– both measured quantities). This study was based on such a conjunction.
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Local measurements of power conversion agree with
global estimate of these quantities by ARTEMIS
0.5GWb  3x1015J
55GW/RE2
x3x10RE2
x1800s
P3:50GW/RE2
----------3x1015J
P2: 5GW/R 2
E
The total energy conversion at ARTEMIS matched the integral of the power conversion rate past THEMIS and ARTEMIS,
providing the first self-consistent accounting of global energy conversion by local processes in the magnetosphere. Top panel,
red line: global magnetic flux input in the magnetotail determined from upstream solar wind monitors; black line with hash:
ARTEMIS’s raw measure of the total flux contained in the magnetotail; black smooth line: same, but excluding local transients
marked "plasmoids” and “earthward flow bursts”. Tail reconnection is operating when the black smooth line deviates
significantly from the red line, indicating magnetotail flux reduction (green and blue hatched areas). The energy converted from
magnetic to particle energy or waves was estimated from this magnetic flux reduction to be ~3x10 15J. Bottom panel: cumulative
local power conversion rate at P3 (THEMIS) and at P2 (ARTEMIS). When further integrated by YZ area (3x10RE2) and duration
(1042UT-1112UT, 1800s) it also amounts to energy conversion ~3x1015J, in agreement with the global estimate (mostly at
reconnection fronts, the vertical lines). Power conversion rate was similar to the total power generating capacity of all power
plants on Earth. The total energy converted was approximately equal to the energy released by a 7.1 magnitude earthquake.
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Global power conversion occurs at gradients sharp enough
to perturb electron microscopic motion
Unlike atmospheric weather, where kinetic gas processes, such as shocks, have little effect on global dynamics, global space
weather is very much controlled by particle dynamics at a microscopic level. Charged particles gyrate around the magnetic
field and move with it. If sharp gradients or small-scale waves perturb the particle gyration the particles start to move
independently of the magnetic field. Electric fields that arise under such conditions can have a component along the magnetic
field, and are then able to accelerate particles to high energies along the magnetic field. This can result in direct
electromagnetic energy conversion from fields to particle motion. In the past, such conditions were thought possible only near
the reconnection site. However, in the event shown, far from the reconnection site, power conversion occurred at spatial
gradients of the order of the electron motion: 20-150km (time is transformed to space using the measured flow, 400-600km/s).
Although very thin, multiple reconnection fronts have been observed to move across a large volume of the magnetosphere,
and survive over long time-scales (10s of minutes). The reconnection fronts’ longevity, number, propagation length
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(>200,000km, from reconnection site to radiation belts), and intensity all contribute to their global consequences.
New perspectives on space weather can lead to
better understanding of astrophysical objects too
Image courtesy of NASA/GALEX
Mira, a mass-shedding red giant, moving thru the interstellar medium, creates a
13-light year long tail with a bow shock standing against the upstream flow. Its
tail-length to stand-off distance ratio is comparable to Earth’s.
Coordinated NASA Heliophysics active missions (2013) are our
best hope for understanding fundamental space processes
Space weather prediction is where atmospheric weather prediction was a century ago. Imagine trying to understand (let
alone predict) atmospheric weather with a couple of weather stations in each continent, or with a few stations in the US but
no other stations elsewhere in the world… it would be impossible. Just like a century ago, when dozens of weather stations
relaying global data via telegraph brought rapid advancements in our understanding of atmospheric weather, space
weather’s understanding to the point of predictability relies on studies with multiple, highly-capable, well-coordinated
observatories, or constellations of small satellites. The next step in that future is coming next year with the launch of NASA’s
four-satellite mission MMS to study reconnection phenomena with unprecedented temporal and spatial resolution. THEMIS
will be using its remaining fuel to further optimize its coordination with MMS and other NASA and international missions
during the upcoming solar maximum. By simultaneously measuring: electron dynamics with MMS (4); ion dynamics with
THEMIS (3); radiation belts responses with Van Allen probes (2); and global dynamics with NASA’s ARTEMIS (2), and other
international missions the emerging “System-Observatory” represents the best hope the field has ever had for resolving the
vexing problems of space weather. Because processes of reconnection, turbulence and electromagnetic energy conversion
occur everywhere in the Universe, including our Sun, other stars, accretion disks and magnetars, studies here at Earth, at our
own astrophysical back yard, can help us understand more distant objects, well beyond reach for our in-situ instruments.10
More on ARTEMIS and its lunar capture
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Critical to the determination of the total energy was
ARTEMIS observations of the global energy input.
The Acceleration, Reconnection, Turbulence and Electrodynamics of
the Moon’s Interaction with the Sun (ARTEMIS) mission is a novel
NASA approach for efficient exploration of new, compelling science
goals by repurposing capable, versatile assets already in space. A spinoff of NASA’s THEMIS mission, ARTEMIS utilized the remaining on-board
fuel on two THEMIS Earth-orbiting satellites to reposition them into
lunar orbits. Proposed in 2010 to study the lunar space environment,
Earth’s interaction with the solar wind and magnetotail particle
acceleration, ARTEMIS was inserted into lunar orbit in the summer of
2011 and has been acquiring high-quality data from there since.
ARTEMIS was the first mission to conduct routine operations at the
Earth-Moon Lagrange points, the region where Earth’s and Moon’s
gravity match each other. During its 9 months there it proved the
navigation and operations solutions that will help future lunar
exploration from this unique location, while obtaining data critical for
the study of plasmoids, the solar wind and the lunar wake. Early next
year it is slated to conduct coordinated exploration of the lunar
environment with NASA’s recently launched LADEE mission. ARTEMIS
provides a unique and critical node for NASA’s Heliophysics System
Observatory: in addition to its two-point measurements that can
resolve spatial from temporal effects, it provides the total energy
content of the magnetosphere during storms and substorms, energy
that is principally responsible for space weather activity.
http://www.nasa.gov/ARTEMIS ; http://artemis.ssl.berkeley.edu;
http://link.springer.com/journal/11214/165/1/page/1
THEMIS and ARTEMIS are part of NASA’s Explorer program, managed by the Goddard Space Flight Center. The University of California
Berkeley’s Space Sciences Laboratory is responsible for mission operations, and built several of the on-board and ground-based instruments.
ATK (formerly Swales Aerospace), Beltsville, MD, built the THEMIS probes. ARTEMIS orbit design, navigation and execution was a collaborative 12
effort between JPL, GSFC and UCB. The University of California at Los Angeles built the ground magnetometers and is the PI institution.
The sequence of maneuvers that turned THEMIS P1 and P2
into ARTEMIS P1 and P2 took >1year, but was very efficient.
See movie at: http://www.youtube.com/watch?v=V2vj8nJ83i4&list=PLiuUQ9asub3RLY8-H4kQU5t58UX1nYJPR&index=6
Also at: NASA_ARTEMIS_SVS_lunar_orbit_insertion.mp4
Movie credits: NASA/SVS
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