A new satellite mission has observed electron acceleration by electric field waves moving along the magnetic boundary between the Earth and the solar wind. |
Connecting the dots. This artist's drawing shows the four MMS spacecraft flying through the magnetopause, where the magnetic field of the solar wind (yellow-orange) confronts the Earth's magnetic field (blue). At this boundary, magnetic field reconnection converts the field energy into particle energy. MMS has observed electric-field waves that likely play a role in this conversion
Charged particles around the Earth, Sun, and
other astrophysical bodies appear to be accelerated to high energies in
regions where magnetic fields break up and reconnect, but the exact
mechanism is unclear. A recently launched multi-satellite mission has
now flown through such a region and directly observed electron
acceleration by fast-moving electric-field waves, suggesting a possible
role for these waves in the production of high-energy particles. The new
data may be an important step in unraveling the mysteries behind solar
flares and other energetic cosmic events.
To
learn about the strong interactions between particles and magnetic
fields that occur near many planets and stars, researchers study the
magnetopause, where the solar wind meets the Earth’s magnetic field. The
solar wind is a collection of mostly protons and electrons streaming
out from the Sun, carrying with it the interplanetary magnetic field
(IMF), which spirals outward from the Sun. Within the magnetopause, the
IMF and the geomagnetic field often point in nearly opposite directions
in a region called the X-line. The field misalignment forces the field
lines to break and reconnect. This reconnection, which also occurs
around the Sun and in other plasma regions, converts magnetic energy
into kinetic energy for charged particles. Studying reconnection is
important for understanding the generation of high-energy particles
around the Earth (which endanger satellites and high-altitude airplane
passengers) and also for explaining high-energy events like solar
flares.
Acceleration of electrons
in a reconnection region has been much harder to measure than the
acceleration of ions. Forrest Mozer of the University of California,
Berkeley, and his colleagues now report on a direct observation of
electron acceleration occurring in the magnetopause, using the
Magnetospheric Multiscale (MMS) mission. Launched in the spring of 2015,
MMS consists of four satellites flying in a tetrahedral formation. Each
probe records electric and magnetic fields as well as the numbers of
electrons and ions in various energy ranges. Compared with other
multi-satellite missions, the MMS probes have a smaller separation (as
little as 10 km), which affords them much higher spatial resolution for
measuring localized acceleration mechanisms in reconnection regions.
On
October 5, 2015, the MMS flotilla was passing the X-line in the dayside
magnetopause. Two of the probes recorded a set of sharp spikes in the
electric field pointing parallel to the local magnetic field. The spikes
were part of a traveling wave called a time domain structure (TDS).
TDSs have been detected many times by satellite missions in other
regions [1].
They have not generally been considered as acceleration mechanisms
because the electric field in a spike has both positive (push) and
negative (pull) peaks, giving a net electric potential of only 10 volts
or less. “They were thought to be the result of some other processes
rather than significant mechanisms on their own,” Mozer says.
However,
a TDS can accelerate particles because it acts like a fast-moving
barrier that “bumps into slow-moving electrons,” Mozer explains. By
comparing the spike arrival times at the two MMS spacecraft, he and his
colleagues were able to directly measure the velocity of a TDS, whereas
previous TDS observations could only infer the velocity. They found that
the wave was moving away from the X-line at 4000 km/s. The MMS
instruments confirmed that the wave led to particle acceleration by
observing a 50% jump in the number of modestly-high-energy electrons
after the wave’s passing. The TDS boosted electrons to around 200 eV, 40
times their initial energy.
This is not the first detection of TDS-induced acceleration [2],
but it is the first direct observation of electron acceleration by TDS
within a reconnection region. Mozer admits that the energy gain is not
enough to explain 100-keV electrons that have been observed in other
reconnection zones. But he believes that faster moving TDSs may be
observed in the future. “I think we are seeing just the tip of the
iceberg,” Mozer says.
The MMS
observations are new and significant because they are “able to
characterize TDSs and directly investigate the associated particle
acceleration simultaneously,” says Daniel Graham of the Swedish
Institute of Space Physics. James Drake of the University of Maryland
says the acceleration “was more than you might expect for such a small
electric field.” Still, the total energy gain is not very large, so
other mechanisms may play a dominant role in accelerating electrons.
Even so, Drake believes these new observations are important for
providing benchmarks for computer simulations of reconnection regions.
This research is published in Physical Review Letters.
References
- F. S. Mozer, O. V. Agapitov, A. Artemyev, J. F. Drake, V. Krasnoselskikh, S. Lejosne, and I. Vasko, “Time Domain Structures: What and Where They Are, What They Do, and How They Are Made,” Geophys. Res. Lett. 42, 3627 (2015).
- F. S. Mozer, O. Agapitov, V. Krasnoselskikh, S. Lejosne, G. D. Reeves, and I. Roth, “Direct Observation of Radiation-Belt Electron Acceleration from Electron-Volt Energies to Megavolts by Nonlinear Whistlers,” Phys. Rev. Lett. 113, 035001 (2014).
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