The acceleration of pickup ions

The well-established association of pickup ions with anomalous cosmic rays shows that acceleration of pickup ions to energies above 1 GeV occurs. At present, diffusive shock acceleration of the pickup ions at the termination shock of the solar wind seems to be the best candidate for acceleration to the high energies of anomalous cosmic rays, accounting well for many of their observed properties. However, it is shown that acceleration of pickup ions from their initial energies by this process appears to be difficult at very strong, nearly perpendicular shocks such as the termination shock. This injection problem remains without a clear solution. A number of alternatives have been proposed for the initial acceleration of pickup ions to the point where diffusive acceleration at the termination shock can take over, but none of these processes has yet emerged as a clear favorite.     

The Observational Motivation for Computational Advances in Solar Flare Physics

A solar flare is a violent and transient release of energy in the corona of the Sun, associated with the reconfiguration of the coronal magnetic field. The major mystery of solar flare physics is the precise nature of the conversion of stored magnetic energy into the copious accelerated particles that are observed indirectly by the radiation that they produce, and also directly with in situ detectors. This presents a major challenge for theory and modeling. Recent years have brought significant observational advances in the study of solar flares, addressing the storage and release of magnetic energy, and the acceleration and propagation of fast electrons and ions. This paper concentrates on two topics relevant to the early phase of a flare, magnetic reconnection and charged particle acceleration and transport. Some recent pertinent observations are reviewed and pointers given for the directions that, this reviewer suggests, computational models should now seek to take.     

Influence of the Interplanetary Magnetic Field on the Solar Wind Flow about Planetary Obstacles

The main effects caused by the interplanetary magnetic field (IMF) are analyzed in cases of supersonic solar wind flow around magnetized planets (like Earth) and nonmagnetized (like Venus) planets. The IMF has a relatively weak strength in the solar wind but it is enhanced considerably in the so-called plasma depletion layer or magnetic barrier in the vicinity of the streamlined obstacle (magnetopause of a magnetized planet, or ionopause of a nonmagnetized planet). For magnetized planets, the magnetic barrier is a source of free magnetic energy for magnetic reconnection in cases of large magnetic shear at the magnetopause. For nonmagnetized planets, mass loading of the ionospheric particles is very important. The new created ions are accelerated by the electric field related to the IMF, and thus they gain energy from the solar wind plasma. These ions form the boundary layer within the magnetic barrier. This mass loading process affects considerably the profiles of the magnetic field and plasma parameters in the flow region.     

X-Rays From Mars

X-rays from Mars were first detected in July 2001 with the satellite Chandra. The main source of this radiation was fluorescent scattering of solar X-rays in its upper atmosphere. In addition, the presence of an extended X-ray halo was indicated, probably resulting from charge exchange interactions between highly charged heavy ions in the solar wind and neutrals in the Martian exosphere. The statistical significance of the X-ray halo, however, was very low. In November 2003, Mars was observed again in X-rays, this time with the satellite XMM-Newton. This observation, characterized by a considerably higher sensitivity, confirmed the presence of the X-ray halo and proved that charge exchange is indeed the origin of the emission. This was the first definite detection of charge exchange induced X-ray emission from the exosphere of another planet. Previously, this kind of emission had been detected from comets (which are largely exospheres) and from the terrestrial exosphere. Because charge exchange interactions between atmospheric constituents and solar wind ions are considered as an important nonthermal escape mechanism, probably responsible for a significant loss of the Martian atmosphere, X-ray observations may lead to a better understanding of the present state of the Martian atmosphere and its evolution. X-ray images of the Martian exosphere in specific emission lines exhibited a highly anisotropic morphology, varying with individual ions and ionization states. With its capability to trace the X-ray emission out to at least 8 Mars radii, XMM-Newton can explore exospheric regions far beyond those that have been observationally explored to date. Thus, X-ray observations provide a novel method for studying processes in the Martian exosphere on a global scale.     

Observations of the Martian Subsolar ENA Jet Oscillations

The Neutral Particle Detector (NPD) of the ASPERA-3 experiment (Analyser of Space Plasmas and Energetic Atoms) on board the Mars Express (MEX) spacecraft observed an intense flux of H ENAs (energetic neutral atoms) with average energy of about 1.5 keV emitted anisotropically from the subsolar region of Mars. The NPD detected the ENA jet near the bow shock at radial distances of about 1 R _M from the Martian surface as the spacecraft moved outbound, while the NPD continuously pointed towards the subsolar region. The jet intensity shows oscillative behavior. These intensity variations occur on two clearly distinguishable time scales. The majority of the identified events have an average oscillation period of about 50 sec. The second group consists of events with long-scale variations with a time scale of approximately 300 sec. The fast oscillations of the first group exhibit a periodic structure and are detected in every orbit, while the slow variations of the second group are identified in ∼40% of orbits. The intensity of the fast oscillations have a peak-to-valley ratio about 20 to 30% of the peak intensity. One of the possible mechanisms to explain fast oscillations is the formation of the low frequency ion waves at the subsolar region of Mars. Slow variations may be explained by either temporal variations in the ENA generation source or by a specific structure of the ENA generation source, in which hair-like ENA subjets can be present.     

The Middle Atmospheric Ozone Response to the 11-Year Solar Cycle

Because of its chemical and radiative properties, atmospheric ozone constitutes a key element of the Earth’s climate system. Absorption of sunlight by ozone in the ultraviolet wavelength range is responsible for stratospheric heating, and determines the temperature structure of the middle atmosphere. Changes in middle atmospheric ozone concentrations result in an altered radiative input to the troposphere and to the Earth’s surface, with implications on the energy balance and the chemical composition of the lower atmosphere. Although a wide range of ground- and satellite-based measurements of its integrated content and of its vertical distribution have been performed since several decades, a number of uncertainties still remain as to the response of middle atmospheric ozone to changes in solar irradiance over decadal time scales. This paper presents an overview of achieved findings, including a discussion of commonly applied data analysis methods and of their implication for the obtained results. We suggest that because it does not imply least-squares fitting of prescribed periodic or proxy data functions into the considered times series, time-domain analysis provides a more reliable method than multiple regression analysis for extracting decadal-scale signals from observational ozone datasets. Applied to decadal ground-based observations, time-domain analysis indicates an average middle atmospheric ozone increase of the order of 2% from solar minimum to solar maximum, which is in reasonable agreement with model results.     

The Solar Wind Interaction With the Martian Ionosphere/Atmosphere

The interaction of the solar wind with the Martian exosphere and ionosphere leads to significant loss of atmosphere from the planet. Spacecraft data confirm that this is the case. However, the issue is how much is actually lost. Given that spacecraft coverage is sparse, simulation is one of the few ways for these estimates to be made. In this paper the evolution of our attempts to place bounds on this loss rate will be addressed. Using a hybrid particle code the loss rate with respect to solar EUV flux is addressed as well as a variety of numerical and chemical issues. The progress made has been of an evolutionary nature, with one approach tried and tested followed by another as the simulations are improved and better estimates are produced. The results to be reported suggest that the ion loss rates are high enough to explain the loss of water from Mars during earlier solar epochs.     

The Response of the Middle Atmosphere to Solar Cycle Forcing in the Hamburg Model of the Neutral and Ionized Atmosphere

This paper studies the response of the middle atmosphere to the 11-year solar cycle. The study is based on numerical simulations with the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA), a chemistry climate model that resolves the atmosphere from the Earth’s surface up to about 250 km. Results presented here are obtained in two multi-year time-slice runs for solar maximum and minimum conditions, respectively. The magnitude of the simulated annual and zonal mean stratospheric response in temperature and ozone corresponds well to observations. The dynamical model response is studied for northern hemisphere winter. Here, the zonal mean wind change differs substantially from observations. The statistical significance of the model’s dynamical response is, however, poor for most regions of the atmosphere. Finally, we discuss several issues that render the evaluation of model results with available analyses of observational data of the stratosphere and mesosphere difficult. This includes the possibility that the atmospheric response to solar variability may depend strongly on longitude.     

Satellite Measurements of Middle Atmospheric Impacts by Solar Proton Events in Solar Cycle 23

Solar cycle 23 was extremely active with seven of the largest twelve solar proton events (SPEs) in the past forty years recorded. These events caused significant polar middle atmospheric changes that were observed by a number of satellites. The highly energetic protons produced ionizations, excitations, dissociations, and dissociative ionizations of the background constituents in the polar cap regions (>60 degrees geomagnetic latitude), which led to the production of HO_x (H, OH, HO₂) and NO_y (N, NO, NO₂, NO₃, N₂O₅, HNO₃, HO₂NO₂, BrONO₂, ClONO₂). The HO_x increases led to short-lived ozone decreases in the polar mesosphere and upper stratosphere due to the short lifetimes of the HO_x constituents. Polar middle mesospheric ozone decreases greater than 50 % were observed and computed to last for hours to days due to the enhanced HO_x. The NO_y increases led to long-lived polar stratospheric ozone changes because of the long lifetime of the NO_y family in this region. Upper stratospheric ozone decreases of >10 % were computed to last for several months past the solar events in the winter polar regions because of the enhanced NO_y.     

Acceleration of Solar-Energetic Particles by Shocks

Our current understanding of the acceleration of solar-energetic particles is reviewed. The emphasis in this paper is on analytic theory and numerical modeling of the physics of diffusive shock acceleration. This mechanism naturally produces an energy spectrum that is a power law over a given energy interval that is below a characteristic energy where the spectrum has a break, or a rollover. This power law is a common feature in the observations of all types of solar-energetic particles, and not necessarily just those associated with shock waves (e.g. events associated with impulsive solar flares which are often described in terms of resonant stochastic acceleration). Moreover, the spectral index is observed to have remarkably little variability from one event to the next (about 50%). Any successful acceleration mechanism must be able to produce this feature naturally and have a resulting power-law index that does not depend on physical parameters that are expected to vary considerably. Currently, only diffusive shock acceleration does this.