H. Koskinen

Pseudobreakup and substorm growth phase in the ionosphere and magnetosphere

We present observations made in space and on the ground during the growth phase and the onset of a substorm on August 31, 1986. About 20 min after the e parameter at the magnetopause had exceeded 1011 W, magnetic field dipolarization with an increase of energetic particle fluxes was observed by the AMPTE Charge Composition Explorer (CCE) spacecraft at the geocentric distance of 8.7 RE close to magnetic midnight. The event exhibited local signatures of a substorm onset at AMPTE CCE and a weak wedgelike current system in the midnight sector ionosphere. However, it did not lead to a full-scale substorm expansion, as determined by several ground-based instruments, nor did it produce large particle injections at geostationary orbit. Only after another 20 min of continued growth phase the entire magnetosphere-ionosphere system could apparently allow the onset of a regular substorm expansion. The initial activation is interpreted in the present paper as a “pseudobreakup.” We examine the physical conditions in the near-Earth plasma sheet using spacecraft observations and analyze the development in the ionosphere using ground-based magnetometers and electric field observations from the STARE radar. We find that the main observable differences between pseudobreakups and ordinary breakups are the strength and consequences. Furthermore, it is shown that ionospheric activity at the time of a pseudobreakup is not necessarily as localized in longitude as generally believed.

The loss of ions from Venus through the plasma wake

Venus, unlike Earth, is an extremely dry planet although both began with similar masses, distances from the Sun, and presumably water inventories. The high deuterium-to-hydrogen ratio in the venusian atmosphere relative to Earth’s also indicates that the atmosphere has undergone significantly different evolution over the age of the Solar System. Present-day thermal escape is low for all atmospheric species. However, hydrogen can escape by means of collisions with hot atoms from ionospheric photochemistry, and although the bulk of O and O2 are gravitationally bound, heavy ions have been observed to escape through interaction with the solar wind. Nevertheless, their relative rates of escape, spatial distribution, and composition could not be determined from these previous measurements. Here we report Venus Express measurements showing that the dominant escaping ions are O+, He+ and H+. The escaping ions leave Venus through the plasma sheet (a central portion of the plasma wake) and in a boundary layer of the induced magnetosphere. The escape rate ratios are Q(H+)/Q(O+) = 1.9; Q(He+)/Q(O+) = 0.07. The first of these implies that the escape of H+ and O+, together with the estimated escape of neutral hydrogen and oxygen, currently takes place near the stoichometric ratio corresponding to water.

Particle scattering and current sheet stability in the geomagnetic tail during the substorm growth phase

The degree of pitch angle scattering and chaotization of various particle populations in the geomagnetic tail during the substorm growth phase is studied by utilizing the Tsyganenko 1989 magnetic field model. A temporally evolving magnetic field model for the growth phase is constructed by enhancing the near-Earth currents and thinning the current sheet from the values given by the static Tsyganenko model. Changing the field geometry toward an increasingly taillike configuration leads to pitch angle scattering of particles whose Larmor radii become comparable to the field line radius of curvature. Several different cases representing substorms with varying levels of magnetic disturbance have been studied. In each case, the field development during the growth phase leads to considerable scattering of the thermal electrons relatively close to the Earth. The current sheet regions where the electron motion is chaotic are magnetically mapped to the ionosphere and compared with low-altitude measurements of electron precipitation. The chaotization of the thermal electron population occurs within a few minutes of the substorm onset, and the ionospheric mappings of the chaotic regions in the equatorial plane compare well with the region of brightening auroras. Even though the temporal evolution of the complex plasma system cannot be self-consistently described by the temporal evolution of the empirical field model, these models can provide the most accurate estimates of the field parameters for tail stability calculations.

Correlation dimension and affinity of AE data and bicolored noise

This paper is concerned with the general question of the dynamics of the magnetosphere. In general, to solve the dynamics of the magnetosphere one has to solve magnetohydrodynamic equations with some appropriate set of boundary conditions. This results in a very complex solution, which gives indications of being chaotic. The question of the chaotic nature of the magnetospheric dynamics has been addressed by various authors by looking at the correlation dimension of the auroral electrojet index. There has been disagreement on the outcome of such experiments, so the authors report on a detailed analysis of the auroral electrojet index time series. They find a correlation dimension of 3.4. For comparison they have generated a bicolored noise signal, and show that it shares many of the characteristics of the auroral electrojet data. They also find that the auroral electrojet time series is self-affine in nature.

Supermagnetosonic Jets behind a Collisionless Quasiparallel Shock

The downstream region of a collisionless quasiparallel shock is structured containing bulk flows with high kinetic energy density from a previously unidentified source. We present Cluster multispacecraft measurements of this type of supermagnetosonic jet as well as of a weak secondary shock front within the sheath, that allow us to propose the following generation mechanism for the jets: The local curvature variations inherent to quasiparallel shocks can create fast, deflected jets accompanied by density variations in the downstream region. If the speed of the jet is super(magneto)sonic in the reference frame of the obstacle, a second shock front forms in the sheath closer to the obstacle. Our results can be applied to collisionless quasiparallel shocks in many plasma environments.

Loading‐unloading processes in the nightside ionosphere

The loading-unloading processes during magnetospheric substorms are examined using the ’IL index’ from the IMAGE magnetometer chain together with solar wind and IMF data from the WIND spacecraft. The IL index is a local, midnight sector, AL index. Energy input throughout the substorm is evaluated by integrating the epsilon parameter from the start of the growth phase until the end of the substorm. Energy dissipated in the ionosphere is estimated by integrating the IL index from the substorm onset to the end of the substorm. We show that the best correlation with the energy dissipated in the ionosphere is given by the energy input to the system after the substorm onset. Hence, we conclude that the energy loaded during the growth phase is necessary for the magnetospheric reconfiguration before the substorm onset, but that the size of the substorm as measured by the IL index is mostly governed by the direct energy input during the expansion phase.

A test particle simulation of the motion of oxygen ions and solar wind protons near Mars

Outflowing oxygen ions (O+) have been observed to be a persistent feature in the Martian tail. However, not much is known of the spatial distribution of oxygen ions, of the correlation between oxygen ions and solar wind protons (H+), and, especially, of the acceleration of oxygen ions in the tail. We present a test particle simulation study of the motion of O+ and H+ ions and compare the results to Automatic Space Plasma Experiment with a Rotating Analyzer (ASPERA) Phobos 2 particle measurements. We have studied the spatial distribution, velocity, density, and flux of oxygen ions in the nightside. In the simulation the oxygen ions were ionized from the Martian hot oxygen corona and the H+ ions were solar wind protons. We used an empirical flow model to derive the magnetic and electric field everywhere around the planet. The work is the first test particle simulation for Mars where a fully three-dimensional magnetotail configuration has been used. The model reproduces many observed plasma features. The solar wind protons flow fluid-like near the terminator despite their finite Larmor radii. The oxygen ions produce a plasmasheet-like layer near the cross-tail current sheet and empty magnetic lobes, and have north-to-south asymmetry in the magnetosheath and in the tail much as observed. The energy of oxygen ions in the tail is close to, but slightly less than, observed. The particle density and the particle flux of oxygen ions also agree quite well with the observations, suggesting that the total O+ outflow rate is ∼ 2 × 1025 s−1. Overall, the study suggest that most of the observed O+ outflow features can be understood by assuming that the ions are accelerated by the convective electric field associated with the flow of the solar wind protons.

The D-CIXS X-ray mapping spectrometer on SMART-1

The D-CIXS Compact X-ray Spectrometer will provide high quality spectroscopic mapping of the Moon, the primary science target of the ESA SMART-1 mission. D-CIXS consists of a high throughput spectrometer, which will perform spatially localised X-ray fluorescence spectroscopy. It will also carry a solar monitor, to provide the direct calibration needed to produce a global map of absolute lunar elemental abundances, the first time this has been done. Thus it will achieve ground breaking science within a resource envelope far smaller than previously thought possible for this type of instrument, by exploiting two new technologies, swept charge devices and micro-structure collimators. The new technology does not require cold running, with its associated overheads to the spacecraft. At the same time it will demonstrate a radically novel approach to building a type of instrument essential for the BepiColombo mission and potential future planetary science targets.

Energetic neutral atom imaging by the Astrid microsatellite

The microsatellite Astrid carried the first instrument (PIPPI, Prelude in Planetary Particle Imaging) specifically designed to perform energetic neutral atom (ENA) imaging. It made measurements from a low altitude (1000 km) polar orbit in the energy range ∼13–140 keV. The ENA images, obtained from near-pole vantage points, adequately reflect general morphological features of the ring current such as a global dawn — dusk asymmetry. The detected ENA peak fluxes (500 – 2000 cm−2s−1sr−1keV−1 for 26 – 37 keV) and structure of the ENA images correlate well with magnetospheric activity throughout the entire data set. The Astrid results demonstrate a considerable potential for ENA imaging from low altitude polar orbits. High ENA fluxes, large angular size of the generation region and simultaneous sampling over all local times are major advantages of such imaging.

Freja observations of heating and precipitation of positive ions

The experiments on board Freja are designed to measure auroral particle energization processes with very high temporal and spatial resolution. One main scientific objective is to study ion heating transverse to the magnetic field lines in the auroral region. The Freja orbit with an inclination of 63° allows us to make detailed measurements in the nightside auroral oval during all disturbance levels. We concentrate here on two different observations of transverse ion energization at an altitude of about 1700 km in the northern hemisphere auroral region. The three-dimensional ion mass spectrograph has shown that both heavy and light ions are heated to energies most often in the range from a few eV to some hundred eV. Transversely heated ions are, however, also seen up to the present high energy limit of the hot plasma instrument, 4.5 keV. Ion conics are produced in regions with anisotropic electron fluxes as well as in regions of intense keV proton precipitation. Waves above the lower hybrid frequency are observed in the events presented in this report. These waves may play an important role in the ion heating process. The Freja data indicate that the waves are generated in different ways in these events. Thus, this preliminary investigation confirms that several scenarios are needed to explain the heating of ionospheric plasma and shows some of the possibilities for future studies.