H. E. J. Koskinen

Evolution of the ion environment of comet 67P/Churyumov-Gerasimenko - Observations between 3.6 and 2.0 AU

Context. The Rosetta spacecraft is escorting comet 67P/Churyumov-Gerasimenko from a heliocentric distance of >3.6 AU, where the comet activity was low, until perihelion at 1.24 AU. Initially, the solar wind permeates the thin comet atmosphere formed from sublimation. Aims. Using the Rosetta Plasma Consortium Ion Composition Analyzer (RPC-ICA), we study the gradual evolution of the comet ion environment, from the first detectable traces of water ions to the stage where cometary water ions accelerated to about 1 keV energy are abundant. We compare ion fluxes of solar wind and cometary origin. Methods. RPC-ICA is an ion mass spectrometer measuring ions of solar wind and cometary origins in the 10 eV–40 keV energy range. Results. We show how the flux of accelerated water ions with energies above 120 eV increases between 3.6 and 2.0 AU. The 24 h average increases by 4 orders of magnitude, mainly because high-flux periods become more common. The water ion energy spectra also become broader with time. This may indicate a larger and more uniform source region. At 2.0 AU the accelerated water ion flux is frequently of the same order as the solar wind proton flux. Water ions of 120 eV–few keV energy may thus constitute a significant part of the ions sputtering the nucleus surface. The ion density and mass in the comet vicinity is dominated by ions of cometary origin. The solar wind is deflected and the energy spectra broadened compared to an undisturbed solar wind. Conclusions. The flux of accelerated water ions moving from the upstream direction back toward the nucleus is a strongly nonlinear function of the heliocentric distance.

Cold ions at the Martian bow shock : Phobos observations

The measurements carried out by the plasma spectrometer ASPERA, on-board the Phobos 2 spacecraft show that the Martian bow shock is characterized by a sudden increase of ionization of the neutral corona. It acts as a source of new ions that can strongly modify the process of ion heating behind the shock front. The loss of momentum of solar wind protons due to their interaction with exospheric ions may lead to an increase in the effective scale of the obstacle.

Mapping between the ionospheric and the tail electric fields in a time-dependent Earth's magnetosphere

In this paper we use the concept of the Faraday loop to connect the ionosphere and the tail along the geomagnetic field lines to study the coupling between ionospheric and magnetospheric electric fields. The formulation using the Faraday loop shows that the coupling consists of three contributions: One is the familiar mapping of the ionospheric field to the tail along equipotential magnetic field lines. The other two are parallel potential differences between the tail and the ionosphere distributed across the magnetic field lines and magnetic flux transport across the Faraday loop due to time evolution of the magnetic field, which gives rise to an inductive electric field perpendicular to the magnetic field. Application of this method requires a model for the ionospheric electric field, a model distribution of the parallel potential differences, and a time-evolving magnetic field model. In the present study the ionospheric electric field pattern is described using a statistical model. The parallel potential differences are modeled by Gaussian distributions in latitude as narrow longitudinally elongated ridges. The magnetic field model describes the time evolution of the geomagnetic field during the substorm growth phase. We show that the effects of parallel potential differences and the inductive fields are of the same order as the mapped ionospheric field, and hence they must be taken into account when the large-scale coupling is studied.

Dispersive magnetosheath‐like ion injections in the evening sector on January 11, 1997

While the plasma cloud of the January 1997 CME event was passing the Earth, the Interball Auroral Probe (Interball-2) crossed the duskside auroral oval toward a contracted polar cap. The PROMICS-3 plasma instrument observed several consecutive dispersion ramps of magnetosheath-like protons over a wide range of magnetic local times (16–21 MLT). The last dispersion ramps were observed at later local times than previously reported, likely caused by the extreme conditions during a period when the magnetosphere was immersed in the dense plasma cloud. The Polar/VIS images of the oval show a very contracted polar cap. During the first clear dispersion event energetic oxygen ions were detected. They were also observed by the the particle detectors of Polar which was in close conjugation with Interball-2 right at this moment. We suggest that the dispersion ramps are signatures of ion injections formed by impulsive entry of magnetosheath ions through the dusk-side flank into the magnetosphere during a period of strongly northward IMF and contracted polar cap.

Dissipation to the joule heating: Isolated and stormtime substorms

Abstract We discuss the energy input from the solar wind and its dissipation in the nightside ionosphere, as a form of Joule heating, during both isolated and storm-time substorms. The energy supplied to the magnetosphere by the solar wind is estimated by computing integrals of Akasofus epsilon parameter determined from WIND satellite measurements. The northern hemisphere Joule dissipation is estimated using the local electrojet index, IL, derived from the IMAGE magnetometer chain observations in the Scandinavian sector. The integrals of the epsilon parameters and the electrojet index are computed from the beginning of enhanced energy input (southward turning of the IMF) to the end of the recovery phase. The main difference between isolated and storm-time substorms is that while the total ionospheric dissipation of storm-time substorms is considerably larger, its relative role is smaller by a factor of two. As there are cases where the ratio between the input and output energies is much larger than in typical cases, we have computed the epsilon parameter also without the IMF X -component. Statistically this does not make large difference but in single events the input energy computed without the X -component can be only half of what the usual way of computing the epsilon parameter yields. We also note that in events where the input seems to remain small as compared to the output there may well have been considerable input prior to the selected start of integration, indicating that the magnetosphere may have stored energy longer than expected.

Observations of plasma entry into the magnetosphere at late magnetic local times

Abstract After the plasma cloud from a coronal mass ejection had passed the Earth on January 11, 1997, the Interball-2 satellite observed several consecutive dispersion ramps of magnetosheath protons above the evening sector auroral region. These protons were encountered at high magnetic latitudes over a wide local time sector, closer to midnight than has been previously reported. At the time of observations the IMF was strongly northward and the polar cap was highly contracted. Simultaneous observations from Interball-2, Polar, and SuperDARN indicate that magnetosheath plasma was observed in the region of sunward convection, and had most likely entered the magnetosphere either through the evening sector flank magnetopause or in the tail. The characteristics of the dispersion ramps suggest that they were caused by several temporally limited injections through a spatially wide area.

Time‐dependent modeling of particles and electromagnetic fields during the substorm growth phase: Anisotropy of energetic electrons

We use a bounce averaged drift model with realistic electromagnetic fields together with magnetic field and electron data obtained by CRRES to study energetic electron distributions during the growth phase of an isolated substorm on December 12, 1990. The magnetic field model includes the actual time evolution of the geomagnetic field as measured by CRRES. The inductive electric field caused by the time evolution of the magnetic field configuration is included in the drift model to consider fully electromagnetic fields. The drift motion is computed for all pitch angles and for the entire energy range covered by the medium-energy spectrometer on CRRES. By using the Liouville theorem we are able to map electron distributions from orbit to orbit to model their time evolution in the model fields. To test the model predictions, we examine the substorm growth phase on December 12, 1990: A quiet period of about 20 hours preceded the growth phase that led to the expansion phase of a 500-nT substorm. The outer belt energetic electron distributions showed a clear development of magnetic field-aligned pitch angle anisotropy. This period was covered by two CRRES orbits, 339 and 340. During orbit 339, CRRES measured a quiet time distribution of energetic electrons. During orbit 340 the substorm onset was seen as a rapid dipolarization of the magnetic field and by a dispersionless electron injection. The quiet time fluxes were used as initial conditions for the model for fluxes during the growth phase. We conclude that pitch angle dependent energization of the drifting electrons caused by the inductive electric field plays an essential role in development of the outer belt electron distributions during the substorm growth phase.