O. Norberg

Ion acceleration in the Martian tail: Phobos observations

The measurements carried out on the spacecraft Phobos-2 have revealed that the plasma sheet of the Martian magnetosphere consists mainly of ions of planetary origin, accelerated up to ∼ 1 keV/q. Such an acceleration may result from the action of magnetic shear stresses of the draped field, the ion energy increasing toward the center of the tail where magnetic stresses are stronger. The energy gained by heavy ions does not depend on their mass and are proportional to the ion charge. The mechanism of the ion acceleration is related with the generation of a charge separation electric field, which extracts ions from “ray” structures in the Martian tail.

Transverse ion energization and wave emissions observed by the Freja satellite

Observations by the Freja satellite at altitudes of about 1700 km in the auroral zone sometimes show energization of both light and heavy ions to characteristic energies of up to about 50–100 eV. This ion heating is often associated with lower hybrid waves, and also with wave emissions at lower frequencies. No obvious decrease of electric field wave power below the proton gyrofrequency, reported by other spacecraft at similar altitudes, is observed in the regions of intense ion heating. Furthermore, localized density depletions associated with lower hybrid waves have not been detected in these regions. We present a preliminary model where waves observed during an event in the evening-side auroral zone are used to explain the observed ion energies.

Ionospheric signature of the cusp as seen by incoherent scatter radar

Measurements with the Sondre Stromfjord incoherent scatter radar, co-ordinated with the observations by the Freja satellite, have been performed during three campaigns, April 1993, February 1994, and May–June 1994. Radar signatures of various types of magnetosheath particle injections in the cusp-cleft region are investigated. The measurement days represent very different geomagnetic conditions, from very quiet to a Kp index of 7+. On three occasions both Freja and the radar detected the cusp. A unique cusp signature is found for a relatively stable cusp, distinguishing it from the many other soft precipitation features seen around noon. The signature includes extremely high electron temperatures in a latitudinally well-defined region with a sharp equatorward border, some F region electron density enhancement, ion outflow, and mainly poleward plasma flow. Enhanced ion temperatures are also seen in the vicinity of, but not exactly coincident with, the electron temperature enhancements. Other day side precipitation features observed with an intense soft component are narrow arcs, which usually have an accompanying accelerated electron component of several hundred eV to some keV energy. These are typically seen in, or bordering, convection regions where the plasma flow vorticity implies upward field-aligned currents.

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.

Energetic neutral atom imaging at low altitudes from the Swedish microsatellite Astrid: Images and spectral analysis

Observations of energetic neutral atoms (ENA) in the energy range 26–52 keV are reported from four occasions during geomagnetically disturbed periods. The data were acquired by the ENA imager flown on the Swedish microsatellite Astrid in a 1000 km circular orbit with 83° inclination. The ENA imager separates charged particles from neutrals through an electrostatic deflection system in the energy range between 0.1 and 114 keV. ENA images obtained from vantage points in the polar cap and in the afternoon magnetic local time (MLT) hours looking into the antisunward hemisphere show intense ENA fluxes (∼ 104 (cm2 sr s)−1 over 26–37 keV) coming from the dusk region and low altitudes (∼300 km). The morphology shows no relation to local magnetic field excluding the possibility of charged particle detection. It is concluded that the source of these ENAs are precipitating/mirroring ions from the ring current/trapped radiation interacting with the exobase on auroral L-shells and in the dusk region. The observed ENA fluxes show a relation with Kp and Dst geomagnetic indices. The observed ENA spectrum from a geomagnetic storm on February 8, 1995, is investigated in more detail and compared to the parent ion spectrum obtained by the Defense Meteorological Satellite Project (DMSP) satellite, F12, during the same period on L=6±2 around dusk. The observed ENA spectral slope is used to derive the parent ion spectral temperature. The derived ion temperatures range is 3.0–6.0 keV for H and 4.5–8.5 keV for O. The higher of these ion temperatures comes closest in agreement to the extrapolated DMSP spectrum leading us to favor O over H as the species of the detected ENAs. It is shown that the detected ENAs must have been produced at L ≥6 to reach the detector without atmospheric attenuation and that the main energy dependence of the ENA spectrum, apart from the parent ion spectrum, is governed by the energy dependence of the charge exchange cross section between ions and exospheric oxygen.

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.

ENA imaging from the Swedish micro satellite Astrid during the magnetic storm of 8 February, 1995

Abstract The generation of energetic neutral atoms (ENA) through charge exchange processes between hot (≥ 1 keV) plasma and cold ( D st ≈−80 nT). The equatorial ion distribution is modeled by a 13 parameter model using a three component (H, He and O) Chamberlain exosphere model with parameters from the MSISE90 extended thermospheric model. The equatorial ion distribution, deduced from the ENA-images through forward modeling is located at MLT (magnetic local time) 1900 hours and occupies L-shell 4–6. Very low ENA fluxes were detected at energies >37 keV. It is suggested that the ENAs are produced by precipitating/mirroring ions on auroral/subauroral field lines coming from the near Earth current sheet charge exchanging with exospheric neutrals at near-exobase altitudes (300–400 km).

Proton flow in the Martian magnetosheath

The Automatic Space Plasma Experiment with a Rotating Analyzer (ASPERA) measurements on board the Phobos 2 spacecraft gave, for the first time, a three-dimensional (3-D) picture of the proton flow around Mars. The measurements from the circular orbits of Phobos 2 are well suited to study the bow shock at the terminator region, the nightside magnetosheath, and the tail region. Moreover, measurements from the elliptical orbits offer dayside magnetosheath data. In this work, all circular orbits (11) where there was enough information for 3-D velocity calculations are analysed. The solar wind deflection at the bow shock and the disappearance of the flow near the optical shadow of Mars are found to be typical features on all circular orbits. A dawn-dusk asymmetry is detected in many cases as well. When the results are compared to a gasdynamic model, the locations of the observed boundaries and the general behavior of the flow are found to be quite consistent with the model. The region where proton particle flux decreases significantly, referred to as a magnetopause near the optical shadow of Mars, was typically found near the magnetic field maximum. The magnetopause was thus inside the so-called magnetic tail boundary, which is defined to be at the broad magnetic minimum between the bow shock and the central current sheet. The magnetic tail boundary may be related to the O+ pick-up ions because the mass loading boundary also lies between the shock and the magnetopause. Because the proton flow may behave differently in the dayside than in the nightside magnetosheath, the 3-D velocities are calculated on two elliptical orbits as well. However, in these cases the nature of the flow is not possible to determine as reliably as near the terminator.

The ionospheric signature of the cusp: A case study using Freja and the Sondrestrom radar

Radar measurements in the cusp region, in close conjunction with the Freja satellite, show evidence of the ionospheric signature of the cusp. The intense low-energy fluxes of ions and electrons associated with the cusp, and seen with the satellite, produce a distinct region of enhanced electron density and electron temperature in the ionospheric F-region. The ionization due to the precipitating ions is rather weak, of the same order of magnitude, and maximizing at the same altitude, as the solar EUV ionization. That due to the electrons is much stronger, maximizes at higher altitude and is clearly identifiable in this case. This is partly because of the unusually high altitude of the cold, EUV and convection controlled F-region, creating a distinct height difference as compared to the precipitation region, and partly because ion outflow prevents the cold daytime plasma from convecting into the precipitation region. A narrow region of high ion temperatures most likely indicating strong electric fields is seen within the precipitation region but is not coincident with the most intense precipitation. The equatorward boundary of the precipitation region is seen to move equatorward, while the convection of the plasma within the region is poleward.