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The MAVEN Solar Energetic Particle (SEP) instrument is designed to measure the energetic charged particle input to the Martian atmosphere. SEP consists of two sensors mounted on corners of the spacecraft deck, each utilizing a dual, double-ended solid-state detector telescope architecture to separately measure fluxes of electrons from 20 to 1000 keV and ions from 20–6000 keV, in four orthogonal look directions, each with a field of view of 42 ∘ by 31 ∘ . SEP, along with the rest of the MAVEN instrument suite, allows the effects of high energy solar particle events on Mars’ upper atmospheric structure, temperatures, dynamics and atmospheric escape rates, to be quantified and understood. Given that solar activity was likely substantially higher in the early solar system, understanding the relationship between energetic particle input and atmospheric loss today will enable more confident estimates of total atmospheric loss over Mars’ history.

The night side ionosphere of Mars is known to be highly variable: essentially nonexistent in certain geographic locations, while occasionally nearly as strong as the photoionization‐produced dayside ionosphere in others. The factors controlling its structure include thermospheric densities, temperatures and winds, day‐night plasma transport, plasma temperatures, current systems, solar particle events, crustal magnetic fields, and electron precipitation, none of which are adequately understood at present. Using a kinetic Monte Carlo approach called Mars Monte Carlo Electron Transport (MarMCET), we model the dynamics of precipitating solar wind electrons on the nightside ionosphere of Mars to study the effects of these last two factors on ionospheric density and structure. We calculate ionization rate profiles and, using simple assumptions concerning atmospheric chemistry, also calculate electron density profiles, total electron content, and equivalent ionosphere slab thickness. We present the first model investigation of the coupled effects of crustal magnetic field gradients and precipitating electron pitch angle distributions (PADs). Including such effects, particularly in cases of nonisotropic PADs, is found to be essential in accurately predicting ionization rate and electron density profiles: peak ionization rates can vary by a factor of 20 or more when these effects are included.

Solar flares efficiently accelerate electrons to several tens of MeV and ions to 10 GeV. The acceleration is usually thought to be associated with magnetic reconnection occurring high in the corona, though a shock produced by the Coronal Mass Ejection (CME) associated with a flare can also accelerate particles. Diagnostic information comes from emission at the acceleration site, direct observations of Solar Energetic Particles (SEPs), and emission at radio wavelengths by escaping particles, but mostly from emission from the chromosphere produced when the energetic particles bombard the footpoints magnetically connected to the acceleration region. This paper provides a review of observations that bear upon the acceleration mechanism.

The solar wind electrons observed at 1 AU are characterized by velocity distribution functions (VDF) that deviate from the Maxwellian form in a high energy regime. Such a feature is often modeled by a kappa distribution. In the present paper a self-consistent theory of quiet-time solar wind electrons that contain a power-law tail component, f ∝ v-α is discussed. These electrons are assumed to be in dynamic equilibrium with enhanced electrostatic fluctuations with peak frequency near the plasma frequency (i.e., the Langmuir turbulence). In order to verify the theoretical prediction, the solar wind electrons in the high-energy range known as the super-halo distribution detected by WIND and STEREO spacecraft are compared against the theoretical model where it was found that the theoretical power-law index is intermittent with regard to the observed range of indices, thus indicating that the turbulent equilibrium model of suprathermal solar wind electrons may be valid.

The shape of the Sun subtly reflects its rotation and internal flows. The surface rotation rate, ~2 kilometers per second at the equator, predicts an oblateness (equator-pole radius difference) of 7.8 milli-arc seconds, or ~0.001%. Observations from the Reuven Ramaty High-Energy Solar Spectroscopic Imager satellite show unexpectedly large flattening, relative to the expectation from surface rotation. This excess is dominated by the quadrupole term and gives a total oblateness of 10.77 ± 0.44 milli-arc seconds. The position of the limb correlates with a sensitive extreme ultraviolet proxy, the 284 angstrom limb brightness. We relate the larger radius values to magnetic elements in the enhanced network and use the correlation to correct for it as a systematic error term in the oblateness measurement. The corrected oblateness of the nonmagnetic Sun is 8.01 ± 0.14 milli-arc seconds, which is near the value expected from rotation.

The atmosphere of Mars, lacking a global magnetic field, is exposed to the precipitation of solar energetic particles (SEPs), resulting in impact ionization and the production of secondary electrons, some of which may escape the atmosphere. In this study, we examine upward traveling fluxes of superthermal electrons between ∼100 and 650 eV, measured by the Mars Global Surveyor Magnetometer/Electron Reflectometer at 400 km altitude during nine of the largest and clearest SEP events of the last solar maximum from November 2000 until the “Halloween” storms of late 2003. We subtract the contribution from backscattered low‐energy precipitating electrons and find that, for the highest and most rarely observed SEP fluxes, we detect a statistically significant flux of SEP‐produced superthermal electrons escaping the Martian atmosphere. The measured fluxes are found to be in broad agreement with a calculation of expected upward electron fluxes resulting from ionization of neutrals by energetic proton impact. Peak SEP ionization rates on the nightside from the Halloween storms are found to be comparable to (although lower than) typical dayside photoionization rates and at least 3 orders of magnitude higher than average nightside electron impact ionization rates. Further advances in our knowledge of SEP effects on the Martian ionosphere await data from the Radiation Assessment Detector (RAD) instrument on the Mars Science Laboratory rover in 2012 and the MAVEN orbiter in 2014. Key Points Solar energetic particle events cause atmospheric ionization Largest SEP events produce detectable superthermal electrons First extraterrestrial detection of SEP‐produced ionospheric electrons

Neutrons produced on the Sun during the M2 flare on 31 December 2007 were observed at 0.48 AU by the MESSENGER Neutron Spectrometer. These observations are the first detection of solar neutrons inside 1 AU. This flare contained multiple acceleration episodes as seen in type III radio bursts. After these bursts ended, both the energetic particle and neutron fluxes decayed smoothly to background with an e‐folding decay time of 2.84 h, spanning a 9 h time period. This time is considerably longer than the mean lifetime of a neutron, which indicates that either the observed neutrons were generated in the spacecraft by solar energetic particle protons, or they originated on the Sun. If most of the neutrons came from the Sun, as our simulations of neutron production on the spacecraft show, they must have been continuously produced. A likely explanation of their long duration is that energetic ions were accelerated over an extended time period onto closed magnetic arcades above the corona and then slowly pitch angle–scattered by coronal turbulence into their chromospheric loss cones. Because of their relatively low energy loss in the Neutron Spectrometer (0.5–7.5 MeV), most of these neutrons beta decay to energetic protons and electrons close to the Sun, thereby forming an extended seed population available for further acceleration by subsequent shocks driven by coronal mass ejections in interplanetary space.

Leaving the heliosphere: Voyager 2 reports back On 30 August 2007 Voyager 2 began to cross the termination shock, a boundary produced by the inter-action of the Sun with the rest of the Galaxy, where the supersonic solar wind abruptly slows as it presses outward against the surrounding interstellar matter. Five Letters in this issue present the data that the probe sent back. The Voyager 2 crossings occurred about 1.5 billion kilometres closer to the Sun than those of Voyager 1, illustrating the asymmetry of the heliosphere. The results from the plasma experiment, low-energy particle, cosmic ray, magnetic field and plasma-wave detectors reveal a complex and dynamic shock, reforming itself in hours rather than days. The cover graphic of Voayer 2 on the brink of entering interstellar space is by Henry Kline of JPL. It may be decades before another probe crosses the termination shock but remote observations can now bridge the gap — as shown by Wang et al . who report measurements of energetic neutral atoms in the heliosheath from the STEREO A and B spacecraft that complement the Voyager in situ observations made at the same time. In News & Views, J R Jokipii puts the Voyager findings into context. For more on the on Voyager odyssey, see page 24, and the Author page, and go to the movie on http://www.nature.com/nature/videoarchive/voyager . The recent Voyager 2 measurements across the termination shock found that the shocked solar wind plasma contains only ∼20 per cent of the energy released by the termination shock, whereas energetic particles above ∼28 keV contain only ∼10 per cent. This paper reports the detection and mapping of energetic neutral atoms produced by charge exchange of suprathermal ions with interstellar neutrals. These termination shock-energized pickup ions contain the missing ∼70 per cent of the energy dissipated in the termination shock, and they dominate the pressure in the heliosheath. The solar wind blows an immense magnetic bubble, the heliosphere, in the local interstellar medium (mostly neutral gas) flowing by the Sun 1 . Recent measurements by Voyager 2 across the termination shock, where the solar wind is slowed to subsonic speeds before entering the heliosheath, found that the shocked solar wind plasma 2 contains only ∼20 per cent of the energy released by the termination shock, whereas energetic particles 3 above ∼28 keV contain only ∼10 per cent; ∼70 per cent of the energy is unaccounted for, leading to speculation 2 , 3 that the unmeasured pickup ions or energetic particles below 28 keV contain the missing energy. Here we report the detection and mapping of heliosheath energetic (∼4–20 keV) neutral atoms produced by charge exchange of suprathermal ions with interstellar neutral atoms. The energetic neutral atoms come from a source ∼60° wide in longitude straddling the direction of the local interstellar medium. Their energy spectra resemble those of solar wind pickup ions, but with a knee at ∼11 keV instead of ∼4 keV, indicating that their parent ions are pickup ions energized by the termination shock. These termination-shock-energized pickup ions contain the missing ∼70 per cent of the energy dissipated in the termination shock, and they dominate the pressure in the heliosheath.

A compilation of various spacecraft measurements made over the last 30 years suggests that the pitch angle distribution (PAD) width of a solar electron burst may have a complex energy signature. To date, no study has considered the PAD width over a broad energy range during a single solar electron burst. Here we use Wind 3DP data to examine the energy dependence of the PAD width between 100 eV to 100 keV during the solar electron burst of 22 March 2002. We find that the PAD width during this event varied nonmonotonically with energy, with a minimum in the width‐energy profiles often being observed between 10 and 20 keV. In addition, we find time periods during and after the burst when the PAD width‐energy profile had a distinct maximum or protuberance below 10 keV. The times at which the protuberance appeared were coincident with Type III radio events. This suggests that the protuberance in the width‐energy profiles may be a manifestation of electron scattering by the electron/electron instability, as previously predicted by simulations. Key Points We use Wind 3DP data to examine the energy dependence of the PAD on 22 March 2002 We find a distinct maximum in the width‐energy profile below 10 keV The maximum may be a due to scattering by the e/e instability

Issue Title: Topical Issue: The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) - Mission Description and Early Results Although designed primarily as a hard X-ray imager and spectrometer, the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) is also capable of measuring the polarization of hard X-rays (20-100 keV) from solar flares. This capability arises from the inclusion of a small unobstructed Be scattering element that is strategically located within the cryostat that houses the array of nine germanium detectors. The Ge detectors are segmented, with both a front and rear active volume. Low-energy photons (below about 100 keV) can reach a rear segment of a Ge detector only indirectly, by scattering. Low-energy photons from the Sun have a direct path to the Be and have a high probability of Compton scattering into a rear segment of a Ge detector. The azimuthal distribution of these scattered photons carries with it a signature of the linear polarization of the incident flux. Sensitivity estimates, based on Monte Carlo simulations and in-flight background measurements, indicate that a 20-100 keV polarization sensitivity of less than a few percent can be achieved for X-class flares.[PUBLICATION ABSTRACT]