Energetic electrons in the cusp and in the high latitude plasma sheet: Evidence for source regions

HEOS-2 has observed energetic electrons (> 40 keV) in the high latitude magnetosphere appearing as one or more peaks outside and often well separated from the trapping boundary. Most of the observations are between 70° and 80° invariant latitudes both in the day and nightside. The peaks are located in the dayside adjacent to the polar cusp and coincide in the nightside with the edge of the plasma sheet. The electron peak intensity on the nightside shows a clear correlation with AE. The electron peak intensities on the dayside exceed those on the nightside and are generally higher in the pre-noon than in the afternoon sector. Observations on the dayside in the distant cusp region and in the adjacent magnetosheath show high and fluctuating intensities of energetic electronswith an energy spectrum much harder than in the outermost trapping region.

This observational evidence suggests different source regions for these energetic electrons: one in the distant geomagnetic tail and another one around the dayside cusp indentation.     

HEOS-2 observations of the boundary layer from the magnetopause to the ionosphere

HEOS-2 low energy electron data (10 eV–3.7 keV) from the LPS Frascati plasma experiment have been used to identify three different magnetospheric electron populations. Magnetosheathlike electron energy spectra (35–50 eV) are characteristic of the plasma mantle, entry layer and cusps from the magnetopause down to 2–3 R_E Plasma sheet electrons (energy > 1 keV) are found at all local times, with strong intensities in the early morning quadrant and weaker intensities in the afternoon quadrant. The plasma sheet shows a well defined inner edge at all local times and latitudes, the inner edge coinciding probably with the plasmapause. The plasma sheet does not reach the magnetopause, but it is separated from it by a boundary layer electron population that is very distinct from the other two electron populations, most electrons having energies 100–300 eV.We map these three electron populations from the magnetopause down to the high latitude near earth regions, by making use of the HEOS-2 low latitude inbound passes and the high latitude outbound passes (in Solar Magnetic (SM) coordinates). The boundary layer extends along the magnetopause up to 5–7 R_E above the equator; at higher latitudes it follows the magnetic lines of force and it is found closer and closer to the earth, so that it has the same invariant latitudes of the system 1 currents observed by Iijima and Potemra (1976) in their region 1. The plasma sheet can be mapped into their region 2 and the cusp-entry layer-plasma mantle can be mapped into their cusp currents region. The boundary layer is observed for any Interplanetary Magnetic Field (IMF) direction. We speculate that magnetosheath particles penetrate into the magnetosphere everywhere along the magnetopause. The electron energization, however, is observed only in the boundary layer, on both dawn and dusk side and could be due to the polarization electric field at magnetopause generated by the magnetosheath plasma bulk motion in the region where such motion is roughly perpendicular to the magnetospheric magnetic field. The electron energization is absent in the regions (entry layer and plasma mantle) where the sheath plasma motion is roughly parallel or antiparallel to the magnetospheric magnetic field.     

Low energy electrons and protons in the magnetosphere

Observations made by HEOS-2 of low energy electrons and protons in the high latitude magnetosphere are presented. Plasma in the magnetosphere is observed in the cusp (which extend down to low altitudes) and over large areas adjacent to the high latitude magnetopause both on the dayside and on the nightside (the entry layer and the plasma mantle respectively).A comparative study of the plasma properties in the various parts of the magnetosphere is performed. An ion bulk motion directed tailward along the geomagnetic field lines is observed both in the entry layer and in the plasma mantle; in the cusp, on the contrary, the bulk motion is practically absent. Moreover the electron thermal anisotropy is parallel to the magnetic field in the magnetosheath, and perpendicular to it in the plasma mantle. One possible explanation (suggested by Rosenbauer et al., 1975) of the origin of these populations is that plasma, penetrated from the magnetosheath in the entry layer, flows tailward along the field lines, is then reflected in the cusp region and convected in the plasma mantle.     

Plasma properties in the dayside cusp region

A statistical study of the cusp plasma has been performed using mainly electron data from the LPS, Rome, plasma experiment flown onboard HEOS-2. We have located the cusp by means of 35–50 eV electrons, from 1.5 to 2.5R_E (south pole) and from 3R_E up to 11R_E (north pole) at 60–70° SM latitude within ±60° of SM longitude from the noon meridan plane. The average cusp thickness is 4.2° of invariant latitude. The location of the cusp in invariant latitude around the noon meridian plane depends on the IMF component B_zGSM according to the linear best fit: Λ = 78.7° + 0.48B_zGSM(γ). Away from the noon meridian plane the invariant latitude of the cusp decreases from 79–84° to 70–74° (at ±50° SM Longitude). At the equatorward edge of the north pole cusp, at all radial distances and at all SM longitudes, we have found a population of electrons with a harder energy spectrum than in the cusp itself. These electrons show a peak at 170–280 eV in our data. They are not the cusp (35–50 eV) electrons and are easily distinguishable from the 1 keV magnetospheric electrons. In the south pole auroral oval they are found at any SM longitude mainly poleward of the 1 keV electrons. The cusp electrons (35–50 eV) and protons have anisotropies that vary with radial distance and SM latitude, both flowing earthward more or less along the magnetic field.     

Properties of energetic electrons of magnetospheric origin in the magnetosheath and in the solar wind—correlation with geomagnetic activity

Bursts of energetic electrons (from >40keV up to 2MeV) as distinct from the magnetopause electron layer observed by Domingo et al. (1977) have been observed in the magnetosheath and in the solar wind by HEOS-2 at high-latitudes. Although these electrons are occasionally found close to the bow shock and simultaneously with low frequency (magnetosonic) upstream waves our observations strongly indicate that these electrons are of exterior cusp origin. Indeed, the flux intensity is highest in the exterior cusp region and decreases as the spacecraft moves away from it both tailward or upward. The energy spectrum becomes harder with increasing radial distance from the exterior cusp. The measured anisotropy indicates that the particles are propagating away from the exterior cusp. The magnetic field points to the exterior cusp region when these electrons are observed, being, for solar wind observations, centred at longitude 0° or 180° rather than along the spiral and in the magnetosheath, being usually different from the 90° or 270° orientation typical of that region. We exclude, therefore, that acceleration in the bow shock is the source of these particles because $\text{B}$ is not tangent to the shock when bursts are observed. We have also found a one to one correlation between geomagnetic storms' recovery phases and intense, continuous observations of >40 keV electrons in the magnetosheath, while, on the other hand, during geomagnetically quiet (Dst) periods bursts are observed only if AE is much larger than average.     

The three-dimensional shape of the magnetopause

Nearly 1000 magnetopause crossings from HEOS-2, HEOS-1, OGO-5 and 5 IMP space-craft covering most of the northern and part of the southern dayside and near-Earth tail magnetopause (X >?15 R_E) have been used to perform a detailed study of the three-dimensional shape and location of the magnetopause. The long-term influence of the solar wind conditions on the average magnetopause geometry has been reduced by normalising the radial distances of the observed magnetopause crossings to an average dynamical solar wind pressure. Best-fit ellipsoids have been obtained to represent the average magnetopause surface in geocentric solar ecliptic (GSE) and (as a function of tilt angle) in solar magnetic (SM) coordinates. Average geocentric distances to the magnetopause for the 1972–1973 solar wind conditions (density 9.4 cm^?3, velocity 450 km s^?1) are 8.8 R_E in the sunward direction, 14.7 R_E in the dusk direction, 13.4 R_E in the dawn direction and 13.7 R_E in the direction normal to the ecliptic plane. The magnetopause surface is tilted by 6.6° ± 2° in a direction consistent with that expected from the aberration effect of the radial solar wind. Our data suggest that the solar wind plasma density and the interplanetary magnetic field (IMF) orientation affect the distance to the polar magnetopause, larger distances corresponding to higher plasma density and southward fields. Our best-fit magnetopause surface shows larger geocentric distances than predicted by the model of Choe et al. [Planet Space Sci. 21, 485 (1973).] normalised to the same solar wind pressure.     

A Martian PFS average spectrum: Comparison with ISO SWS

The evaluation of the planetary Fourier spectrometer performance at Mars is presented by comparing an average spectrum with the ISO spectrum published by Lellouch et al. [2000. Planet. Space Sci. 48, 1393.]. First, the average conditions of Mars atmosphere are compared, then the mixing ratios of the major gases are evaluated. Major and minor bands of CO₂ are compared, from the point of view of features characteristics and bands depth. The spectral resolution is also compared using several solar lines. The result indicates that PFS radiance is valid to better than 1% in the wavenumber range 1800-4200 cm⁻¹ for the average spectrum considered (1680 measurements). The PFS monochromatic transfer function generates an overshooting on the left-hand side of strong narrow lines (solar or atmospheric). The spectral resolution of PFS is of the order of 1.3 cm⁻¹ or better. A large number of narrow features to be identified are discovered.     

Photometric properties of Titan''s surface from Cassini VIMS: Relevance to titan''s hemispherical albedo dichotomy and surface stability

The Visual and Infrared Mapping Spectrometer (VIMS) instrument on the Cassini Saturn Orbiter returned spectral imaging data as the spacecraft undertook six close encounters with Titan beginning 7 July, 2004. Three of these flybys each produced overlapping coverage of two distinct regions of Titan's surface. Twenty-four points were selected on approximately opposite hemispheres to serve as photometric controls. Six points were selected in each of four reflectance classes. On one hemisphere each control point was observed at three distinct phase angles. From the derived phase coefficients, preliminary normal reflectances were derived for each reflectance class. The normal reflectance of Titan's surface units at 2.0178 μm ranged from 0.079 to 0.185 for the most absorbing to the most reflective units assuming no contribution from absorbing haze. When a modest haze contribution of τ=0.1 is considered these numbers increase to 0.089–0.215. We find that the lowest three reflectance classes have comparable normal reflectance on either hemisphere. However, for the highest brightness class the normal reflectance is higher on the hemisphere encompassing longitude 14–65° compared to the same high brightness class for the hemisphere encompassing 122–156° longitude. We conclude that an albedo dichotomy observed in continental sized units on Titan is due not only to one unit having more areal coverage of reflective material than the other but the material on the brighter unit is intrinsically more reflective than the most reflective material on the other unit. This suggests that surface renewal processes are more widespread on Titan's more reflective units than on its less reflective units.

We note that one of our photometric control points has increased in reflectance by 12% relative to the surrounding terrain from July of 2004 to April and May of 2005. Possible causes of this effect include atmospheric processes such as ground fog or orographic clouds; the suggestion of active volcanism cannot be ruled out.

Several interesting circular features which resembled impact craters were identified on Titan's surface at the time of the initial Titan flyby in July of 2004. We traced photometric profiles through two of these candidate craters and attempted to fit these profiles to the photometric properties expected from model depressions. We find that the best-fit attempt to model these features as craters requires that they be unrealistically deep, approximately 70 km deep. We conclude that despite their appearance, these circular features are not craters, however, the possibility that they are palimpsests cannot be ruled out.

We used two methods to test for the presence of vast expanses of liquids on Titan's surface that had been suggested to resemble oceans. Specular reflection of sunlight would be indicative of widespread liquids on the surface; we found no evidence of this. A large liquid body should also show uniformity in photometric profile; we found the profiles to be highly variable. The lack of specular reflection and the high photometric variability in the profiles across candidate oceans is inconsistent with the presence of vast expanses of flat-lying liquids on Titan's surface. While liquid accumulation may be present as small, sub-pixel-sized bodies, or in areas of the surface which still remain to be observed by VIMS, the presence of large ocean-sized accumulations of liquids can be ruled out.

The Cassini orbital tour offers the opportunity for VIMS to image the same parts of Titan's surface repeatedly at many different illumination and observation geometries. This creates the possibility of understanding the properties of Titan's atmosphere and haze by iteratively adapting models to create a best fit to the surface reflectance properties.     

Albedo and photometric study of Mars with the Planetary Fourier Spectrometer on-board the Mars Express mission

The Short Wavelength Channel of the Planetary Fourier Spectrometer (PFS) covers the 8333-1750 cm⁻¹ (1.2-5.7 μm) spectral range, that is well suited to study the reflectance properties of the martian soil. These properties vary with time due to the dust dynamics in the martian environment. Wind can blow off dust exposing soil and fresh rocks and can support grain mobility inducing local dust settling. We have analyzed PFS data from January 2004 to April 2005. A detailed photometric study of the radiance acquired from the planet has been performed in order to compare correctly measurements obtained at different viewing geometries and to produce a mosaic image of the planet. The results show good agreement with data from the Thermal Emission Spectrometer (on-board NASA Mars Global Surveyor orbiter), although some variations are observed. Some albedo changes could be due to small to medium scale dust storms. A very accurate estimation of the limb-darkening parameter has been computed from the analyzed data. The obtained values are compared with a surface roughness and a thermal inertia map in order to assess the relation between the limb-darkening parameter and the physical properties of surface.     

Calibration of the Planetary Fourier Spectrometer short wavelength channel

The Planetary Fourier Spectrometer (PFS) experiment on board the Mars Express mission has two channels covering the 1.2-5.5 μm short wavelength channel (SWC) and the 5.5-45 μm (LWC). The SWC measures part of the thermal emission spectrum and the solar reflected spectrum of Mars between 1700 and 8200 cm⁻¹ with a spectral resolution of 1.3 cm⁻¹, in absence of apodisation. We present here the calibration of this channel and its performance. The instrument calibration has been performed on ground, before launch, in space during near earth verification (NEV) measurements, and at Mars. Special attention has been given to the problem of microvibrations on board the spacecraft.In order to obtain correct results, the source-instrument-detector interaction for the thermal part is studied very accurately. The instrument shows a nonlinear behaviour with source intensity. The SNR increases with amplification, hence high gain factors are usually used. The detector is, in space, cooled by a passive radiator, and works around 210-215 K. The calibration source (an internal lamp) shows variations during a pericentre pass and therefore impose a complex procedure for the SW channel calibration. Mechanical microvibrations strongly affect part of the spectrum. We discuss the validity of the present calibration, and indicate possible future developments. Samples of the calibrated data are given to show the performance of the experiment and its scientific potentialities.