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RHESSI measurements relevant to the fundamental processes of energy release and particle acceleration in flares are summarized. RHESSI’s precise measurements of hard X-ray continuum spectra enable model-independent deconvolution to obtain the parent electron spectrum. Taking into account the effects of albedo, these show that the low energy cut-off to the electron power-law spectrum is typically ≲tens of keV, confirming that the accelerated electrons contain a large fraction of the energy released in flares. RHESSI has detected a high coronal hard X-ray source that is filled with accelerated electrons whose energy density is comparable to the magnetic-field energy density. This suggests an efficient conversion of energy, previously stored in the magnetic field, into the bulk acceleration of electrons. A new, collisionless (Hall) magnetic reconnection process has been identified through theory and simulations, and directly observed in space and in the laboratory; it should occur in the solar corona as well, with a reconnection rate fast enough for the energy release in flares. The reconnection process could result in the formation of multiple elongated magnetic islands, that then collapse to bulk-accelerate the electrons, rapidly enough to produce the observed hard X-ray emissions. RHESSI’s pioneering γ -ray line imaging of energetic ions, revealing footpoints straddling a flare loop arcade, has provided strong evidence that ion acceleration is also related to magnetic reconnection. Flare particle acceleration is shown to have a close relationship to impulsive Solar Energetic Particle (SEP) events observed in the interplanetary medium, and also to both fast coronal mass ejections and gradual SEP events. New instrumentation to provide the high sensitivity and wide dynamic range hard X-ray and γ -ray measurements, plus energetic neutral atom (ENA) imaging of SEPs above ∼2 R ⊙ , will enable the next great leap forward in understanding particle acceleration and energy release is large solar eruptions—solar flares and associated fast coronal mass ejections (CMEs).

Terrestrial gamma-ray flashes (TGFs) from Earth's upper atmosphere have been detected with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite. The gamma-ray spectra typically extend up to 10 to 20 megaelectron volts (MeV); a simple bremsstrahlung model suggests that most of the electrons that produce the gamma rays have energies on the order of 20 to 40 MeV. RHESSI detects 10 to 20 TGFs per month, corresponding to ~50 per day globally, perhaps many more if they are beamed. Both the frequency of occurrence and maximum photon energy are more than an order of magnitude higher than previously known for these events.

Feel the Force Magnetic reconnection is a process in which pairs of magnetic field lines merge to convert magnetic energy into particle energy. Kinks formed in the merged field lines produce a slingshot effect that accelerates high-speed plasma jets away from the merger site. The process supplies energy to solar flares and the space storms near Earth that interfere with electric power grids and telecommunications. Space physicists have long debated whether reconnection occurs over great distances, or randomly in localized patches. On 2 February 2002, the Cluster, ACE and Wind spacecraft, widely separated in interplanetary space, all detected similar plasma jets within the same passing current sheet. It was direct evidence of a 2.5-million-kilometre reconnection region, confirming that magnetic reconnection can occur on a very large scale over long periods. On the cover, kinked magnetic field lines accelerate a pair of particle jets. Magnetic reconnection in a current sheet converts magnetic energy into particle energy, a process that is important in many laboratory 1 , space 2 , 3 and astrophysical contexts 4 , 5 , 6 . It is not known at present whether reconnection is fundamentally a process that can occur over an extended region in space or whether it is patchy and unpredictable in nature 7 . Frequent reports of small-scale flux ropes and flow channels associated with reconnection 8 , 9 , 10 , 11 , 12 , 13 in the Earth's magnetosphere raise the possibility that reconnection is intrinsically patchy, with each reconnection X-line (the line along which oppositely directed magnetic field lines reconnect) extending at most a few Earth radii ( R E ), even though the associated current sheets span many tens or hundreds of R E . Here we report three-spacecraft observations of accelerated flow associated with reconnection in a current sheet embedded in the solar wind flow, where the reconnection X-line extended at least 390 R E (or 2.5 × 10 6  km). Observations of this and 27 similar events imply that reconnection is fundamentally a large-scale process. Patchy reconnection observed in the Earth's magnetosphere is therefore likely to be a geophysical effect associated with fluctuating boundary conditions, rather than a fundamental property of reconnection. Our observations also reveal, surprisingly, that reconnection can operate in a quasi-steady-state manner even when undriven by the external flow.

The heating of ions downstream of the x‐line during magnetic reconnection is explored using full‐particle simulations, test particle simulations, and analytic analysis. Large‐scale particle simulations reveal that the ion temperature increases sharply across the boundary layer that separates the upstream plasma from the Alfvénic outflow. This boundary layer, however, does not take the form of a classical switch‐off shock as discussed in the Petschek reconnection model, so the particle heating cannot be calculated from the magnetohydrodynamic, slow‐shock prediction. Test particle trajectories in the fields from the simulations reveal that ions crossing the narrow boundary into the exhaust instead behave like pickup particles: they gain both a directed outflow and an effective thermal speed given by the flow speed v0 of the exhaust. The detailed dynamics of these particles are explored by taking 1‐D cuts of the simulation data across the exhaust, transforming to the deHoffman‐Teller frame, and calculating explicitly the increment in the temperature, miv02/3, with mi, the ion mass. We compare the model predictions with the temperature increment in solar wind exhausts measured by the ACE and Wind spacecraft, confirming that the temperature increment is proportional to the ion mass. The Wind data from 22 high‐shear exhaust encounters confirm the scaling of the proton temperature increment with the square of the exhaust velocity. However, the temperature increments are consistently lower than the model prediction. Implications for understanding the production of high‐energy ions in flares and the broader universe are discussed.

THEMIS was launched on February 17, 2007 to determine the trigger and large-scale evolution of substorms. During the first seven months of the mission the five satellites coasted near their injection orbit to avoid differential precession in anticipation of orbit placement, which started in September 2007 and led to a commencement of the baseline mission in December 2007. During the coast phase the probes were put into a string-of-pearls configuration at 100 s of km to 2 R E along-track separations, which provided a unique view of the magnetosphere and enabled an unprecedented dataset in anticipation of the first tail season. In this paper we describe the first THEMIS substorm observations, captured during instrument commissioning on March 23, 2007. THEMIS measured the rapid expansion of the plasma sheet at a speed that is commensurate with the simultaneous expansion of the auroras on the ground. These are the first unequivocal observations of the rapid westward expansion process in space and on the ground. Aided by the remote sensing technique at energetic particle boundaries and combined with ancillary measurements and MHD simulations, they allow determination and mapping of space currents. These measurements show the power of the THEMIS instrumentation in the tail and the radiation belts. We also present THEMIS Flux Transfer Events (FTE) observations at the magnetopause, which demonstrate the importance of multi-point observations there and the quality of the THEMIS instrumentation in that region of space.

Magnetic reconnection is the process by which magnetic field lines of opposite polarity reconfigure to a lower-energy state, with the release of magnetic energy to the surroundings. Reconnection at the Earth's dayside magnetopause and in the magnetotail allows the solar wind into the magnetosphere 1 , 2 . It begins in a small ‘diffusion region’, where a kink in the newly reconnected lines produces jets of plasma away from the region. Although plasma jets from reconnection have previously been reported 3 , 4 , 5 , 6 , 7 , the physical processes that underlie jet formation have remained poorly understood because of the scarcity of in situ observations of the minuscule diffusion region. Theoretically, both resistive and collisionless processes can initiate reconnection 8 , 9 , 10 , 11 , 12 , 13 , 14 , but which process dominates in the magnetosphere is still debated. Here we report the serendipitous encounter of the Wind spacecraft with an active reconnection diffusion region, in which are detected key processes predicted by models 8 , 9 , 10 , 11 , 12 , 13 of collisionless reconnection. The data therefore demonstrate that collisionless reconnection occurs in the magnetotail.

We present Mars Global Surveyor measurements of bipolar out‐of‐plane magnetic fields at current sheets in Mars' magnetosphere. These signatures match predictions from simulations and terrestrial observations of collisionless magnetic reconnection, and could similarly indicate differential ion and electron motion and the resulting Hall current systems near magnetic X lines. Thus, these observations may represent passages through or very near reconnection diffusion regions at Mars. Out of 28 events found at 400 km altitude with well‐defined current sheet orientations, 26 have magnetic fields consistent with the expected polarities of Hall fields near diffusion regions. For these events, we find an average ratio of Hall field to main field of 0.51 ± 0.13, and an average ratio of normal to main field (reconnection rate) of 0.16 ± 0.09, consistent with terrestrial observations of reconnection. These events do not consistently correlate with the location of crustal fields or with IMF reversals, indicating that magnetic field draping alone (perhaps enhanced by high solar wind dynamic pressure) may generate current sheets capable of reconnection. For some events, we observe field‐aligned electrons that may carry parallel currents that close the Hall current loop. Electron distributions around current sheets often indicate magnetic connection to the collisional exosphere. For crossings sunward of the X line, we usually observe an electron flux minimum at the current sheet, consistent with the resulting closed magnetic structure. For crossings antisunward of the X line, we do not observe flux minima, consistent with field lines open downstream. Collisionless reconnection, if common at Mars, could represent a significant atmospheric loss process.

Mars currently has no global magnetic field of internal origin but must have had one in the past when the crust acquired intense magnetization, presumably by cooling in the presence of an Earth-like magnetic field (thermoremanent magnetization). A new map of the magnetic field of Mars, compiled by using measurements acquired at an ≈400-km mapping altitude by the Mars Global Surveyor spacecraft, is presented here. The increased spatial resolution and sensitivity of this map provide new insight into the origin and evolution of the Mars crust. Variations in the crustal magnetic field appear in association with major faults, some previously identified in imagery and topography (Cerberus Rupes and Valles Marineris). Two parallel great faults are identified in Terra Meridiani by offset magnetic field contours. They appear similar to transform faults that occur in oceanic crust on Earth, and support the notion that the Mars crust formed during an early era of plate tectonics.

Vector magnetic field observations of the martian crust were acquired by the Mars Global Surveyor (MGS) magnetic field experiment/electron reflectometer (MAG/ER) during the aerobraking and science phasing orbits, at altitudes between ∼100 and 200 kilometers. Magnetic field sources of multiple scales, strength, and geometry were observed. There is a correlation between the location of the sources and the ancient cratered terrain of the martian high-lands. The absence of crustal magnetism near large impact basins such as Hellas and Argyre implies cessation of internal dynamo action during the early Naochian epoch (∼4 billion years ago). Sources with equivalent magnetic moments as large as 1.3 × 10$^{17}$ ampere-meter$^2$ in the Terra Sirenum region contribute to the development of an asymmetrical, time-variable obstacle to solar wind flow around Mars.

The Solar Wind Ion Analyzer (SWIA) on the MAVEN mission will measure the solar wind ion flows around Mars, both in the upstream solar wind and in the magneto-sheath and tail regions inside the bow shock. The solar wind flux provides one of the key energy inputs that can drive atmospheric escape from the Martian system, as well as in part controlling the structure of the magnetosphere through which non-thermal ion escape must take place. SWIA measurements contribute to the top level MAVEN goals of characterizing the upper atmosphere and the processes that operate there, and parameterizing the escape of atmospheric gases to extrapolate the total loss to space throughout Mars’ history. To accomplish these goals, SWIA utilizes a toroidal energy analyzer with electrostatic deflectors to provide a broad 360 ∘ ×90 ∘ field of view on a 3-axis spacecraft, with a mechanical attenuator to enable a very high dynamic range. SWIA provides high cadence measurements of ion velocity distributions with high energy resolution (14.5 %) and angular resolution (3.75 ∘ ×4.5 ∘ in the sunward direction, 22.5 ∘ ×22.5 ∘ elsewhere), and a broad energy range of 5 eV to 25 keV. Onboard computation of bulk moments and energy spectra enable measurements of the basic properties of the solar wind at 0.25 Hz.