Low Latitude Ionospheric Electrodynamics

The low latitude ionosphere is strongly affected by several highly variable electrodynamic processes. Over the last two decades ground-based and satellite measurements and global numerical models have been extensively used to study the longitude-dependent climatology of low latitude electric fields and currents. These electrodynamic processes and their ionospheric effects exhibit large ranges of temporal and spatial variations during both geomagnetic quiet and disturbed conditions. Numerous recent studies have investigated the short term response of equatorial electric fields and currents to lower atmospheric transport processes and solar wind-magnetosphere driving mechanisms. This includes the large electric field and current perturbations associated with arctic sudden stratospheric warming events during geomagnetic quiet times and highly variable storm time prompt penetration and ionospheric disturbance dynamo effects. In this review, we initially describe recent experimental and numerical modeling results of the global climatology and short term variability of quiet time low latitude electrodynamic plasma drifts. Then, we examine the present understanding of equatorial electric field and current perturbation fields during periods of enhanced geomagnetic activity.     

Optimization of transfers under constraints on the thrust direction: I

Transfers with a low thrust are considered under constraints imposed on the thrust vector direction. These constraints can be caused by peculiarities of the attitude control system and the mode of stabilization of a spacecraft, and, in the general case, they are functions of the time and state vector. The constraints specified by equalities and inequalities are investigated. It is shown that the optimal thrust is directed along the projection of the Lawden’s primer vector onto the restricting set.     

A perspective on the importance of reproductive mode in astrobiology

Reproduction is a vital characteristic of life, and sex is the most common reproductive mode in the eukaryotic world. Sex and reproduction are not necessarily linked mechanisms: Sexuality without reproduction exists, while several forms of asexual reproduction are known. The occurrence of sexuality itself is paradoxical, as it is very costly in evolutionary terms. Most of the hypotheses (more than 20) attempting to explain the prevalence of sex fall into two categories: Sex either creates good gene combinations for adaptation to environments or eliminates bad gene combinations counteracting the accumulation of mutations. In spite of this apparent wealth of beneficial effects of sex, asexuality is not rare. Most eukaryotic, asexual lineages are short-lived and can only persist through the presence of sexual roots, but at least two animal groups, bdelloid rotifers and darwinulid ostracods, seem to claim the status of ancient asexuals. Research on (a)sexuality is relevant to astrobiology in a number of ways. First, strong relationships between the origin and persistence of life in extreme environments and reproductive mode are known. Second, the "habitability" of nonterrestrial environments to life greatly depends on reproductive mode. Whereas asexuals can do equally well or better in harsh environments, they fail to adapt fast enough to changing abiotic and biotic environments. Third, it has been shown that plants reproduce mainly asexually in space, and sperm production and motility in some vertebrates are hampered. Both findings indicate that extraterrestrial life under conditions different from Earth might be dominated by asexual reproduction. Finally, for exchange of biological material between planets, the choice of reproductive mode will be important.     

Geophysical Constraints on the Evolution of Mars

The evolution of Mars is discussed using results from the recent Mars Global Surveyor (MGS) and Mars Pathfinder missions together with results from mantle convection and thermal history models and the chemistry of Martian meteorites. The new MGS topography and gravity data and the data on the rotation of Mars from Mars Pathfinder constrain models of the present interior structure and allow estimates of present crust thickness and thickness variations. The data also allow estimates of lithosphere thickness variation and heat flow assuming that the base of the lithosphere is an isotherm. Although the interpretation is not unambiguous, it can be concluded that Mars has a substantial crust. It may be about 50 km thick on average with thickness variations of another ±50 km. Alternatively, the crust may be substantially thicker with smaller thickness variations. The former estimate of crust thickness can be shown to be in agreement with estimates of volcanic production rates from geologic mapping using data from the camera on MGS and previous missions. According to these estimates most of the crust was produced in the Noachian, roughly the first Gyr of evolution. A substantial part of the lava generated during this time apparently poured onto the surface to produce the Tharsis bulge, the largest tectonic unit in the solar system and the major volcanic center of Mars. Models of crust growth that couple crust growth to mantle convection and thermal evolution are consistent with an early 1 Gyr long phase of vigorous volcanic activity. The simplest explanation for the remnant magnetization of crustal units of mostly the southern hemisphere calls for an active dynamo in the Noachian, again consistent with thermal history calculations that predict the core to become stably stratified after some hundred Myr of convective cooling and dynamo action. The isotope record of the Martian meteorites suggest that the core formed early and rapidly within a few tens of Myr. These data also suggest that the silicate rock component of the planet was partially molten during that time. The isotope data suggest that heterogeneity resulted from core formation and early differentiation and persisted to the recent past. This is often taken as evidence against vigorous mantle convection and early plate tectonics on Mars although the latter assumption can most easily explain the early magnetic field. The physics of mantle convection suggests that there may be a few hundred km thick stagnant, near surface layer in the mantle that would have formed rapidly and may have provided the reservoirs required to explain the isotope data. The relation between the planform of mantle convection and the tectonic features on the surface is difficult to entangle. Models call for long wavelength forms of flow and possibly a few strong plumes in the very early evolution. These plumes may have dissolved with time as the core cooled and may have died off by the end of the Noachian.     

Role of ionospheric effects and plasma sheet dynamics in the formation of auroral arcs

At the ionospheric level, the substorm onset (expansion phase) is marked by the initial brightening and subsequent breakup of a pre-existing auroral arc. According to the field line resonance (FLR) wave model, the substorm-related auroral arc is caused by the field-aligned current carried by FLRs. The FLRs are standing shear Alfvén wave structures that are excited along the dipole/quasi-dipole lines of the geomagnetic field. The FLRs (that can cause auroral arc) thread from the Earthward edge of the plasma sheet and link the auroral arc to the plasma sheet region of 6–15 R _E. The region is associated with magnetic fluctuations that result from the nonlinear wave-wave interactions of the cross-field current-instability. The instability (excited at the substorm onset) disrupts the cross-tail current which is built up during the growth phase of the substorms and results in magnetic fluctuations. The diversion of the current to polar regions can lead to auroral arc intensification. The current FLR model is based on the amplitude equations that describe the nonlinear space-time evolution of FLRs in the presence of ponderomotive forces exerted by large amplitude FLRs (excited during substorms). The present work will modify the FLR wave model to include the effects arising from magnetic fluctuations that result from current disruption near the plasma sheet (6–15 R _E). The nonlinear evolution of FLRs is coupled with the dynamics of plasma sheet through a momentum exchange term (resulting from magnetic fluctuations due to current disruption) in the generalized Ohm's law. The resulting amplitude equations including the effects arising from magnetic fluctuations can be used to study the structure of the auroral arcs formed during substorms. We have also studied the role of feedback mechanism (in a dipole geometry of the geomagnetic field) in the formation of the discrete auroral arc observed on the nightside magnetosphere. The present nonlinear dispersive model (NDM) is extended to include effects arising from the low energy electrons originating from the plasma sheet boundary layer. These electrons increase the ionospheric conductivity in a localized patch and enhance the field-aligned current through a feedback mechanism. The feedback effects were studied numerically in a dipole geometry using the the NDM. The numerical studies yield the magnitude of the field-aligned current that is large enough to form a discrete auroral arc. Our studies provide theoretical support to the observational work of Newell et al. that the feedback instability plays a major role in the formation of the discrete auroral arcs observed on the nightside magnetosphere.     

Multidecadal Signal of Solar Variability in the Upper Troposphere During the 20th Century

Studies based on data from the past 25–45 years show that irradiance changes related to the 11-yr solar cycle affect the circulation of the upper troposphere in the subtropics and midlatitudes. The signal has been interpreted as a northward displacement of the subtropical jet and the Ferrel cell with increasing solar irradiance. In model studies on the 11-yr solar signal this could be related to a weakening and at the same time broadening of the Hadley circulation initiated by stratospheric ozone anomalies. Other studies, focusing on the direct thermal effect at the Earth’s surface on multidecadal scales, suggest a strengthening of the Hadley circulation induced by an increased equator-to-pole temperature gradient. In this paper we analyse the solar signal in the upper troposphere since 1922, using statistical reconstructions based on historical upper-air data. This allows us to address the multidecadal variability of solar irradiance, which was supposedly large in the first part of the 20th century. Using a simple regression model we find a consistent signal on the 11-yr time scale which fits well with studies based on later data. We also find a significant multidecadal signal that is similar to the 11-yr signal, but somewhat stronger. We interpret this signal as a poleward shift of the subtropical jet and the Ferrel cell. Comparing the magnitude of the two signals could provide important information on the feedback mechanisms involved in the solar climate relationship with respect to the Hadley and Ferrel circulations. However, in view of the uncertainty in the solar irradiance reconstructions, such interpretations are not currently possible.     

WHISPER, A RESONANCE SOUNDER AND WAVE ANALYSER: PERFORMANCES AND PERSPECTIVES FOR THE CLUSTER MISSION

The WHISPER sounder on the Cluster spacecraft is primarily designed to provide an absolute measurement of the total plasma density within the range 0.2–80 cm^-3. This is achieved by means of a resonance sounding technique which has already proved successful in the regions to be explored. The wave analysis function of the instrument is provided by FFT calculation. Compared with the swept frequency wave analysis of previous sounders, this technique has several new capabilities. In particular, when used for natural wave measurements (which cover here the 2–80 kHz range), it offers a flexible trade-off between time and frequency resolutions. In the basic nominal operational mode, the density is measured every 28 s, the frequency and time resolution for the wave measurements are about 600 Hz and 2.2 s, respectively. Better resolutions can be obtained, especially when the spacecraft telemetry is in burst mode. Special attention has been paid to the coordination of WHISPER operations with the wave instruments, as well as with the low-energy particle counters. When operated from the multi-spacecraft Cluster, the WHISPER instrument is expected to contribute in particular to the study of plasma waves in the electron foreshock and solar wind, to investigations about small-scale structures via density and high-frequency emission signatures, and to the analysis of the non-thermal continuum in the magnetosphere.     

Origin of the diffuse galactic gamma-ray emission at low and medium gamma-ray energies

Recently the galactic plane has been observed in the low and medium energy gamma-ray range in the directions towards the center and anticenter. Spectral measurements are now available at those energies, where the contribution from π°-decay gamma rays can be neglected. The high MeV-fluxes observed in both parts of the Galaxy are an indication of either a strong electron induced component or a high contribution from unresolved sources. Several interstellar cosmic-ray electron spectra have been used to calculate the contribution from electron bremsstrahlung and inverse Compton collisions with optical, infrared and 2.7 K black-body photons. From these calculations restrictions on the interstellar electron spectrum are derived.     

Bursty Bulk Flow Driven Turbulence in the Earth’s Plasma Sheet

During the last several years significant progress has been made in understanding MHD turbulence in the Earth’s plasma sheet. Due to the statistically transitory properties of fluctuations, finite size and boundary effects, however, issues of fundamental importance remain unresolved. Here we concentrate on such intrinsic features of plasma sheet turbulence as its origin and dynamical nature. In particular, we investigate bursty bulk flow driven multi-scale transfer of energy towards the dissipation scale, and provide evidence for the presence of non-linear interactions. We show that, in contrast with previous results, Alfvénic fluctuations together with 2D eddy interactions may appear as important constituents of turbulence in the plasma sheet.     

The Magnetic Field Pile-up and Density Depletion in the Martian Magnetosheath: A Comparison with the Plasma Depletion Layer Upstream of the Earth's Magnetopause

Using magnetometer and electron observations from the Mars Global Surveyor (MGS) and the Wind spacecraft we show that the region of magnetic field pile-up and density decrease located between the Martian ionosphere and bow shock exhibit strong similarities with the plasma depletion layer (PDL) observed upstream of the Earth's magnetopause in the absence of magnetic reconnection when the magnetopause is a solid obstacle in the solar wind. A PDL is formed upstream of the terrestrial magnetopause when the magnetic field piles up against the obstacle and particles in the pile-up region are squeezed away from the high magnetic pressure region along the field lines as the flux tubes convect toward the magnetopause. We here discuss the possibility that at least part of the region of magnetic field pile-up and density depletion upstream of Mars may be formed by the same physical processes which generate the PDL upstream of the Earth's magnetopause. More complete ion, electron, and neutral measurements are needed to conclusively determine the relative importance of the plasma depletion process versus exospheric processes.