J. Šimůnek

Artificial electron and ion beam effects: Active Plasma Experiment

The Active Plasma Experiment uses intensive electron beam emission for the study of dynamic processes in the magnetosphere and upper ionosphere. The beam energy and current are as high as 8 keV and 100 mA and the pitch angle of the emission varies in the range from 50° to 80°. The basic cycle of electron injection is formed by current pulses of different duration, intensity, and frequency. The spacecraft potential is balanced by a low-energy xenon plasma generator during the electron beam injection. The spacecraft potential is measured by the floating probe, and the response of the environment is studied by the charged particle spectrometer working in the energy range from 0.05 to 25 keV. During the neutral or ionized Xe release without the electron gun operation the spacecraft potential remains nearly unchanged and the observed energy spectra of charged particles do not exhibit the presence of any acceleration process. The spacecraft potential during electron beam emission does not exceed 50 V if the Xe plasma or the neutral gas was released together with the electrons. The electron gun firing creates a disturbance which produces a broad spectrum of energetic electrons extending up to 1.5 keV. The acceleration process can be explained by the introduction of the electric field with intensity of about 100 V/m. This intensity is in agreement with the observed E × B drift velocity. The spatial extent of the disturbance is established to be tens of meters.

Two point observation of magnetopause motion: The INTERBALL project

Abstract On August 3, 1995, two satellites of the INTERBALL mission were launched into a highly elliptical dawnside orbit with an apogee near 200 000 km and a perigee of 800 km. The distance separating the satellites varies from one to several thousand km. Both satellites carry sets of Faraday cups which cover a spatial angle approaching nearly 4π. The cups provide high resolution data (up to 16 Hz for the main satellite and 10 Hz for the subsatellite) which we supplement with magnetic field measurements and observations of the electron/ion energy and angular distributions. The configuration of both satellites allows us to determine small-scale variations of the magnetopause shape and position and to estimate the velocity of the magnetopause motion. A preliminary analysis of the data suggests that the nature of magnetopause motion depends significantly on the geomagnetic latitude. Wavy motion of the magnetopause boundaries seems to be preferred at the low-latitude boundary layer near the equatorial plane whereas a radial expansion/contraction seems to be most common at the high latitude plasma mantle.

Configuration of the outer cusp after an IMF rotation

Abstract We present the results of a two-point study of the topology and dynamics of high-altitude cusp regions under changing interplanetary magnetic field (IMF) direction. The study uses INTERBALL-1 and MAGION-4 measurements on April 2, 1996. An analysis of the MAGION-4 ion energy spectra indicates the presence of magneto sheath-like plasma well below the expected magnetopause position and a large region occupied by low-temperature dense plasma with the very low bulk velocity near the magnetopause. The examinations of two-point observations of the cusp-magnetosheath transition have confirmed our knowledge of the structure of a plasma entry region during stable IMF. Our study reveals a complicated topology of the magnetic field during cusp reformation from an initial to a final quasisteady state. We have found that such a reformation can take 10 – 15 minutes.

Magion-4 High-Altitude Cusp Study

The polar cusps have traditionally been described as narrow funnel-shaped regions of magnetospheric magnetic field lines directly connected to magnetosheath ones, allowing the magnetosheath plasma to precipitate into the ionosphere. However, recent middle- to highaltitude observations (i.e., the Interball, Hawkeye, Polar, Image, and Cluster spacecraft) reported the cusps to encompass a broad area near local noon. The present paper focuses on a statistical study of the high-altitude cusp and surrounding magnetosheath regions as well as on some peculiarities of the cusp-magnetosheath transition. For a comparison of high- and lowaltitude cusp determination, we present a mapping of two-year Magion-4 (a part of the Interball project) observations of cusp-like plasma along model magnetic field lines (according to the Tsyganenko 96 model) down to the Earth’s surface. The footprint positions show a substantial latitudinal dependence on the dipole tilt angle. The dependence can be fitted by a line with a slope of 0.14° MLAT per 1° of tilt. In contrary to previously reported IMF or solar wind influences on the cusp shape or location, some differences exist: (1) a possible IMF BB X dependence of the cusp location, (2) a split cusp for B Y ≠ 0, and (3) a smaller cusp during periods of higher solar wind dynamic pressure. The conclusions following from the statistical analysis are confirmed by case studies which reveal the physical mechanisms leading to the observed phenomena. Results have shown that (1) reconnection near the cusp does not necessarily lead to observable precipitation, (2) the cusp precipitation in one hemisphere can be supplied from the conjugate hemisphere, and (3) the cusp geometry at a certain time depends on the IMF history.

Two-Point Interball Observations of the LLBL

The low-latitude boundary layer (LLBL) is encountered as an interface between two plasma regions — the magnetosheath and plasma sheet and thus contains a mixture of both plasma populations. Several mechanisms have been discussed as candidates for a formation of the LLBL. These mechanisms can be divided into magnetic reconnection between the magnetospheric and magnetosheath magnetic fields, impulsive penetration of magnetosheath plasma, and viscous/diffusive mixing of plasma populations at the magnetopause. The observed fluctuations of plasma parameters inside the LLBL are attributed either to transient nature of the phenomena forming the layer or to sweeping of deformations of the magnetopause or inner edge of the LLBL along the spacecraft.

Artificial ion beam effects on spacecraft potential

Abstract The Active Plasma Experiment (APEX) uses intensive electron beam emission for the study of dynamic processes in the magnetosphere and upper ionosphere. In order to balance the spacecraft potential while the electron beam is operating, a low-energy Xenon plasma generator emits an ion beam. This paper reports on the ability of the Xenon plasma generator to influence the vehicle potential which is estimated from the potential difference between a floating probe and the vehicle itself. The potential difference is measured under quiet conditions and with the plasma generator operating. We discuss the influence of the parameters of the ambient plasma, spacecraft position (altitudes up to 3000 km) and Xe injector working modes on the spacecraft charging/discharging process.