B. A. Whalen

EXOS D (Akebono) suprathermal mass spectrometer observations of the polar wind

We report observations of the H+, He+, and O+ polar wind ions in the polar cap (>80° invariant latitude, ILAT) above the collision-dominated altitudes (>2000 km), from the suprathermal mass spectrometer (SMS) on EXOS D (Akebono). SMS regularly observes low-energy (a few eV) upward ion flows in the high-altitude polar cap, poleward of the auroral oval. The flows are typically characteristic of the polar wind, in that they are field-aligned and cold (Ti 80° ILAT), the average H+ velocity reached 1 km/s near 2000 km, as did the He+ velocity near 3000 km and the O+ velocity near 6000 km. At Akebono apogee (10,000 km), the averaged H+, He+, and O+ velocities were near 12,7, and 4 km/s, respectively. Both the ion velocity and temperature distributions exhibited a day-to-night asymmetry, with higher average values on the dayside than on the nightside.

EXOS D (Akebono) observations of molecular NO+ and N2 + upflowing ions in the high‐altitude auroral ionosphere

The authors report on observation of molecular ion drift upward in the ionosphere in regions near to the cleft or auroral oval. In particular they observed NO{sup +}, N{sub 2}{sup +}, and even O{sub 2}{sup +} molecular ion flow upward. These fluxes were typically 5 to 15% of the total ion flux. Molecular ion drift is not observed in all passes through the polar region, and seems to correlate with periods of more intense activity. Their observations are compared with, and correlated with other observations at lower altitudes.

Thermal ion observations of depletion and refilling in the plasmaspheric trough

Thermal (0–25 eV) ion observations in the altitude range 5000 km to 10,000 km and at invariant latitudes greater than ∼60°, made by the Suprathermal Ion Mass Spectrometer (SMS) on the EXOS-D satellite, are used to estimate temperatures, densities, composition, and drift velocities of the local major thermal ion population during a 22-day period in February 1990. This preliminary study indicates that equatorward of a high-latitude boundary, higher-density cold ions corotate with Earth and field-aligned drift velocities are low (<1 km/s). These observations are consistent with a plasmaspheric origin for these ions; however, densities measured near the boundary (10-100 cm−3) suggest that this boundary lies in or near the trough region. Poleward of this boundary, convection patterns deviate from corotation, and large upward directed field-aligned flows of ionospheric plasma, consistent with a “polar wind like” source mechanism, are detected. Typically, parallel drift velocities of 12, 7, and 3 km/s for H+, He+, and O+ are observed. Correlations of these plasma parameters, which were obtained approximately once per day, with magnetic activity (Kp) suggest response times of 2–3 days between the onset of Kp changes and the establishment of new, large-scale, steady state conditions. The implications of these results with regard to ionospheric plasma dynamics associated with depletion and replenishment of high-latitude flux tubes are discussed.

Observations in the transverse ion energization region

We present here the first direct observations of thermal (0 ≤ E/Q ≤ 25 Volts) and suprathermal (E/Q > 50 Volts) ion distributions from the Suprathermal Ion Mass Spectrometer (SMS) on the EXOS-D spacecraft in and near the Transverse Ion Energization (TIE) region. It is shown that the TIE region exists on field lines closely related to the auroral zone and at altitudes which vary between 3000 to 6000 km on the day side. This process also occurs on the night side at lower altitudes, but no clear examples when the spacecraft was in the TIE region at these local times have been found to date. The altitude range over which the energization occurs is narrow, less than 100 km. In the region, all ions (major and minor species) are energized to approximately the same energy (temperature) perpendicular to the magnetic field ( ) and ejected into the magnetosphere by the gradient force. Above the region, the energized plasma expands outward along magnetic field lines forming “conic” distributions. However, distribution function observations above the TIE region indicate that significant variations from that expected from conservation of the first invariant occur when the ions have travelled only a few thousand kilometers upwards from the source.

On the sources of energization of molecular ions at ionospheric altitudes

During geomagnetically active times, the suprathermal mass spectrometer on the Akebono satellite frequently observes upflowing molecular ions (NO+, N2+, O2+) in the 2-3 Earth radii geocentric distance regions in the auroral zone. Molecular ions originating at ionospheric altitudes must acquire an energy of the order of 10 eV in order to overcome gravitation and reach altitudes greater than 2 RE. This energy must be acquired in a time short compared with the local dissociative recombination lifetime of the ions; the latter is of the order of minutes in the F region ionosphere (300-500 km altitude). Upflowing molecular ions thus provide a test particle probe into the mechanisms responsible for heavy ion escape from the ionosphere. In this paper we analyze the extensive complement of plasma, field, and wave data obtained on the Akebono satellite in a number of upflowing molecular ion events observed at high altitudes (5000 - 10,000 km). We use these data to investigate the source of energization of the molecular ions at ionospheric altitudes. We show that Joule heating and ion resonance heating do not transfer enough energy or do not transfer it fast enough to account for the observed fluxes of upflowing molecular ions. We found that the observed field-aligned currents were too weak to support large-scale field-aligned current instabilities at ionospheric altitudes. The data suggest but in the absence of high-resolution wave measurements in the 300 to 500 km altitude range cannot ascertain the possibility that a significant fraction of escape energy is transferred to molecular ions in localized regions from intense plasma waves near the lower hybrid frequency. We also compared the energization of molecular ions to that of the geophysically important O+ ions in the 300 to 500 km altitude range, where the energy transfer to O+ is believed to occur via small-scale plasma instabilities, ion resonance, and ion-neutral frictional heating. Direct observation of energy input to the ionosphere from all of these sources in combination with in situ measurements of the density and temperature of neutral and ionized oxygen in the 300 to 500 km range are required to determine the relative importance of these energy sources in providing O+ with sufficient energy to escape the ionosphere.

Minor ion composition in the polar ionosphere

Ion composition measurements from the EXOSD Suprathermal Ion Mass Spectrometer (SMS) are presented. Ions other than H+, notably O+, He+, O++ N+ and N++, are found to constitute a significant (>0.1) and at times dominant (>0.5) component of the thermal ion population in the high-altitude polar ionosphere. Their relative abundance and occurrence are highly variable. Ion flux ratios in the range of 0.1–0.5 are typical for 0+/H+, 0.1–0.3 for He+/H+, 0.1–0.3 for 0++/O+, and 0.05–0.1 for N++/N+. Our observations show that (1) ions other than H+, notably He+, O+, O++ and N+, often constitute a significant component (>0.1) of the thermal ion population in the high-altitude polar ionosphere; (2) doubly charged oxygen and nitrogen (O++ and N++) are sometimes present with fluxes up to 0.1 of the singly-charged (O+ and N+) ion fluxes; and (3) the He+ and O++ fluxes are sometimes comparable to the H+ and O+ fluxes.

The E-region rocket/radar instability study (ERRRIS): scientific objectives and campaign overview

Abstract The E -region Rocket/Radar Instability Study (Project ERRRIS) investigated in detail the plasma instabilities in the low altitude ( E -region) auroral ionosphere and the sources of free energy that drive these waves. Three independent sets of experiments were launched on NASA sounding rockets from Esrange, Sweden, in 1988 and 1989, attaining apogees of 124, 129 and 176km. The lower apogee rockets were flown into the unstable auroral electrojet and encountered intense two-stream waves driven by d.c. electric fields that ranged from 35 to 115 mV/m. The higher apogee rocket returned fields and particle data from an active auroral arc, yet observed a remarkably quiescent electrojet region as the weak d.c. electric fields (~ 10–15 mV/m) there were below the threshold required to excite two-stream waves. The rocket instrumentation included electric field instruments (d.c. and wave), plasma density fluctuation ( δn / n ) receivers, d.c. fluxgate magnetometers, energetic particle detectors (ions and electrons), ion drift meters, and swept Langmuir probes to determine absolute plasma density and temperature. The wave experiments included spatially separated sensors to provide wave vector and phase velocity information. All three rockets were flown in conjunction with radar backscatter measurements taken by the 50MHz CUPRI system, which was the primary tool used to determine the launch conditions. Two of the rockets were flown in conjunction with plasma drift, density, and temperature measurements taken by the EISCAT incoherent scattar radar. The STARE radar also made measurements during this campaign. This paper describes the scientific objectives of these rocket/radar experiments, provides a summary of the geophysical conditions during each launch, and gives an overview of the principal rocket and radar observations.

Observations of ion-neutral collisional effects in the auroral E region

Thermal ion energy distribution functions and local electric and magnetic fields were directly measured for the first time in the ionospheric E region on board rockets which were launched as part of the E Region Rocket/Radar Instability Study (ERRRIS) from Esrange, Sweden, in 1988 and 1989 to investigate plasma instabilities in the auroral ionosphere. Measured ion distribution functions were fitted to shifted Maxwellian distributions, and their resulting ion drift velocities were compared with E {times} B/B{sup 2} velocities from the double-probe electric field observations. The results show that the ion drift direction rotates with respect to the local electric field direction and that the ratio of the magnitudes of the ion velocity to the E {times} B/B{sup 2} velocity decreases with decreasing altitudes. Using these observations, the quiet time ion-neutral collision frequencies and neutral wind velocities were estimated and found to be consistent with theoretical estimates. However, significant discrepancies between observations and theory are found in the disturbed E region near auroral particle precipitation regions. These data indicate that the auroral atmosphere is significantly perturbed due to Joule as well as particle heating effects.

Soft ion precipitation at very high latitudes during northward interplanetary magnetic field

During extended periods of northward interplanetary magnetic field (IMF), soft ion precipitation was frequently observed at very high latitudes (>80° ILAT) on EXOS D (Akebono). The precipitating protons typically had temperatures of a few hundreds of eV, and were accompanied by precipitating electrons with temperatures of several tens of eV. The densities of the precipitating protons were of the order of 10−1 cm−3, and were usually lower than those of the electrons. During these periods, the ion convection at the highest latitudes was chaotic or weak, but often had an average antisunward component. During periods of strongly northward IMF, ion precipitation was observed at all magnetic local times (MLT). A flux minimum, sometimes below the level of detection of the Akebono suprathermal mass spectrometer instrument, was often found just poleward of the dayside auroral oval. This region is believed to coincide with the open magnetic field lines in the tail lobe. The region of ion precipitation was observed at auroral latitudes on the dawnside when the IMF By changed polarity from negative to positive; it was located at higher latitudes (>80° ILAT) on the duskside when the IMF By was positive. These observations suggest a connection between the ion precipitation and the auroral oval.

Low energy upflowing ion events observed by EXOS‐D: Initial results

Initial observations of low-energy upflowing ion events from the Suprathermal Ion Mass Spectrometer (SMS) on EXOS-D are presented. The SMS instrument on EXOS-D measures the mass composition, energy and angular distributions of thermal and suprathermal ions in the 0.1–4000 eV/q energy-per-charge and 0.8–70 amu/q mass-per-charge ranges. Two events are examined. The first features predominantly conical ion pitch-angle distributions and the second has mainly field-aligned distributions. Both events displayed small scale variations in upward flux, composition, and angular characteristics, and coincided with small-scale magnetic field fluctuations. In both events, the peak He+ and H+ fluxes were comparable; the O+ flux was a factor of 5–10 larger. The pitch-angle distribution often switched rapidly between field-aligned and conical.