Magnetosheath Particles Research Articles

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.

Rocket‐borne measurements of particles and ion convection in dayside aurora

As part of Operation Periquito, two sounding rockets were launched into regions of particle precipitation at invariant latitude 76°N and magnetic local time 13 hours from Cape Parry, Northwest Territories, Canada, in November 1975. On one of the flights, which did not cross any well‐defined auroral arcs, the electron spectra were always soft, the thermal ion drift velocity was variable but generally westward at 500 m/s, and energetic particles (>70 keV) were always well above cosmic ray background, indicating a region of closed magnetic field lines. The other flight crossed two auroral arcs. Electrons with soft (mean energy of 200 eV) power law spectra were observed on the equatorward and poleward sides, whereas intense 1‐keV precipitation was measured within the arcs. The mean drift velocity of thermal ions was seen to change from 1500 m/s westward, parallel to the boundary, on the equatorward side to ∼200 m/s southward on the poleward side of the arcs. Energetic (E > 70 keV) particle measurements showed that the high‐latitude boundary was well poleward of the auroral arcs (Heikkila and Winningham's ‘cleft’ precipitation and ion convection velocity reversals), which indicates that the entire region was on closed magnetic field lines. Comparison of these features with those from midnight aurora shows that the two are qualitatively similar, strongly suggesting that dayside and nightside arcs are different aspects of the same process. No support was seen for the idea that direct penetration of magnetosheath particles is responsible for the dayside aurora.

Penetration of solar wind plasma elements into the magnetosphere

Assuming that the solar wind is unsteady and non-uniform, it is suggested that field aligned plasma elements dent the magnetopause surface. This indentation makes the magnetopause boundary convex and therefore locally unstable with respect to flute instabilities. The intruding element is slowed down and stopped within 1 or 2 Earth radii from the magnetopause. Its excess convection kinetic energy is dissipated in the lower polar cusp ionosphere after ~ 50–500 s, depending on the value of the integrated Pedersen conductivity. Once the plasma element has been engulfed, keeping its identity, the warm plasma content is dissipated by precipitation and by drifting. The magnetosheath particles with large pitch angles are mirrored, and feed the plasma mantle flow. Several consequences of this penetration mechanism are pointed out: Ionospheric heating beneath the polar cusp; Birkeland currents on the eastward and westward edges of the plasma element; Diamagnetic field fluctuations within 1–2 R E from the magnetopause (multiple magnetopause crossings); Oscillation of the magnetopause surface after a new element has penetrated; Exit of energetic particles out of the magnetosphere, and entry of energetic solar wind particles into the magnetosphere along the magnetic field lines of the intruding element; Subtraction of magnetic flux from the dayside magnetosphere and its addition to the geomagnetic tail when the magnetic field of the element has a southward component.

Field aligned distribution of plasma mantle and ionospheric plasmas

The density and bulk velocity distributions of warm magnetosheath particles and cold ionospheric O + and H + ions are calculated along a polar cusp (cleft) magnetic field line. The warm collisionless plasma is injected at 12 earth radii, and interacts with the cold polar wind electrons and ions of ionospheric origin. After magnetic reflections in the exosphere (above 1400km altitude) most of the ions move upwards along tail magnetic field lines and feed the plasma mantle flow. The bulk velocity of these warm protons is approximately 200km s −2 and happens to be proportional to the average thermal speed of the magnetosheath ions at their injection point in the magnetosphere. The density in the plasma mantle is found to be about half that in the entry layer. The electric potential distribution is deduced from the quasi-neutrality condition, A large parallel electric field corresponding to an electrostatic double layer is obtained for the assumed boundary conditions.

The Low Energy Charged Particle (LECP) experiment on the Voyager spacecraft

The Low Energy Charged Particle (LECP) experiment on the Voyager spacecraft is designed to provide comprehensive measurements of energetic particles in the Jovian, Saturnian, Uranian and interplanetary environments. These measurements will be used in establishing the morphology of the magnetospheres of Saturn and Uranus, including bow shock, magnetosheath, magnetotail, trapped radiation, and satellite-energetic particle interactions. The experiment consists of two subsystems, the Low Energy Magnetospheric Particle Analyzer (LEMPA) whose design is optimized for magnetospheric measurements, and the Low Energy Particle Telescope (LEPT) whose design is optimized for measurements in the distant magnetosphere and the interplanetary medium. The LEMPA covers the energy range from ∼10 keV to > 11 MeV for electrons and from ∼15 keV to ≳ 150 MeV for protons and heavier ions. The dynamic range is ∼0.1 to ≳ 1011 cm−2 sec−1 sr−1 overall, and extends to 1013 cm−2 sec−1 sr−1 in a current mode operation for some of the sensors. The LEPT covers the range ∼0.05 ≤ E ≳ 40 MeV/nucleon with good energy and species resolution, including separation of isotopes over a smaller energy range. Multi-dE/dx measurements extend the energy and species coverage to 300–500 MeV/nucleon but with reduced energy and species resolution. The LEPT employs a set of solid state detectors ranging in thickness from 2 to ∼2450 μ, and an arrangement of eight rectangular solid state detectors in an anticoincidence cup. Both subsystems are mounted on a stepping platform which rotates through eight angular sectors with rates ranging from 1 revolution per 48 min to 1 revolution per 48 sec. A ‘dome’ arrangement mounted on LEMPA allows acquisition of angular distribution data in the third dimension at low energies. The data system contains sixty-two 24-bit sealers accepting data from 88 separate channels with near 100% duty cycle, a redundant 256-channel pulse height analyzer (PHA), a priority system for selecting unique LEPT events for PHA analysis, a command and control system, and a fully redundant interface with the spacecraft. Other unique features of the LECP include logarithmic amplifiers, particle identifiers, fast (∼15 ns FWHM) pulse circuitry for some subsystems, inflight electronic and source calibration and several possible data modes.

Thermal electron densities and temperatures in the dayside cusp region

Measurements of the density and temperature of electrons in the energy range 0–2 eV henceforth referred to as thermal electrons, observations of particle fluxes in the energy range 50 eV–15 keV and measurements of the d.c. electric field on board the INJUN V satellite have been investigated in the dayside cusp region and over the polar cap. Limiting the observations to the 14–15 MLT interval we find the following features at altitudes between 2000 and 2500 km. On the dayside there is a region typically three to nine degrees wide in which the thermal charged particles are influenced by magnetosheath particle fluxes. This region may be subdivided into two parts. Region A: An equatorial edge, which is characterized by an increase in poleward direction in temperature and often a decrease in density of thermal electrons, a plasma trough. Within this region, which is of the order of two degrees in width, there is usually observed an electric field reversal and the highest intensity of particle fluxes in the energy range 50 eV–15 keV. Region B, which extends poleward of Region A: In this region the density of thermal electrons is enhanced, while their temperature is close to that in the surrounding region and in particular lower than in Region A. The width of Region B is highly variable. The average width observed in the 35 cases selected for this study is six degrees. The equatorial borders of Regions A and B move equatorward with increasing magnetic activity measured by Kp. The increase of the density of thermal electrons in Region B could be due to the redistribution of ionization resulting from the heating of the F-region by magnetosheath particles precipitated in Region A and successive poleward convection. There could also be a contribution from charged particles with energies below the particle detector threshold of 50 eV on INJUN V produced above the F-Layer maximum in Region B.

Auroral Zones in a Quadrupole Magnetosphere

The nondipolar component of the geomagnetic field is likely to dominate during at least some geomagnetic reversals. The magnetosphere and auroral zones for a nondipolar field are more complex than for a dipole field. To begin to explore the nature of this complexity, we have studied the auroral zones appropriate to a pure quadrupole field. The quadrupole coefficients for the historical geomagnetic field were used in the study. The result is that such auroral zones are much more extensive than at present. They extend in association with two great circles from the tropics to midlatitudes in both north and south hemispheres. As a consequence, bombardment of the upper atmosphere by magnetosheath (polar cusp) and quasitrapped energetic magnetospheric (plasma sheet) particle is also much more extensive.

Some properties of the current sheet in the geomagnetic tail

The topic of this report is that of the influence of noise, and of the finite length and width of the tail on the behaviour of the current sheet. The presence of a weak magnetic field linking through the current sheet leads to plasma containment and counterstreaming, with the consequence that both the plasma temperature and density are increased in the vicinity of the current sheet. The effect of these changes on the relationship between steady bulk parameters is discussed. The finite length of the tail significantly modifies the equilibrium situation in the near Earth tail, for streams mirroring at the Earthwards end of field lines lead to a reduction of merging. The finite width of the tail restricts the region of reduced merging rate to a triangular shaped area extending from the dusk magnetopause into the tail. The finite tail width is also important in the more distant tail, where magnetosheath particles which penetrate the magnetopause ends of the current sheet may become major current carriers, especially if B z , is small and northwards. Finally, it is shown that the above factors, together with a non-adiabatic current sheet, are important to our understanding of the temporal behaviour of the tail.

A model of the distributed magnetospheric currents

A model of the quiet time ring current and the tail currents has been developed for quiet magnetic conditions and perpendicular incidence of the solar wind on the main (dipolar) field of the earth. Previous work of several authors has shown that these distributed currents flow throughout the magnetosphere and that they are produced by the drift of charged particles in the nonuniform magnetic field B and by plasma diamagnetism. The currents are modeled from a few thousand kilometers above the earth's surface to distances of over 200 RE back in the tail of the magnetosphere. This procedure allows the computation of the magnetic field BD, associated with these distributed currents to beyond lunar orbit. The currents are chosen so that BD correctly models observed field features in the inner magnetosphere and the magnetotail. Although the tail currents can be formed continuously by the entry of magnetosheath particles into the equatorial flanks of the tail even during quiet times, the ring currents must be supplied at irregular (substorm or storm) intervals when dynamic processes are operative. These currents make significant contributions to the earth's surface field variations (e.g., in Sq and Dst). The formation of the tail currents implies that the magnetosphere is open (in the equatorial regions of the tail) to low-energy charged particles. This model work also suggests that when magnetospheric particles and fields are considered self-consistently, it is not possible to produce a magnetically closed magnetosphere. The static magnetic field associated with this distributed current model exhibits a neutral line beyond the orbit of the moon. The inclusion of the field associated with these distributed currents in a model of the total magnetic field should yield better agreement between observed magnetospheric particle and field phenomena and model calculations.

Isis 1 observations of the high-latitude ionosphere during a geomagnetic storm

The Isis 1 satellite has made measurements of several ionospheric and related parameters, and the results of the various measurements have been compared in detail for two north transpolar passes during the geomagnetic storm of February 3, 1969. Simultaneous measurements were made of local electron and ion densities and temperatures, electron density between the satellite and the peak of the F layer, radio noise, and particle fluxes over a wide energy range extending down to 10 eV. Several features of the ionosphere (in particular, enhancements of radio noise, scale height, and plasma temperatures) appear to be due to soft-particle (100 eV to 1 keV) precipitation, which is related to magnetospheric structure as delineated by the observation of more energetic particles. The magnetosheath particles precipitating on the dayside of the polar cap are particularly effective.