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Magnetopause & Boundary Layer

Equatorial Magnetosphere

The IASTP Context

MISSION OBJECTIVES

The objectives of the EQUATOR-S mission are to provide high-resolution plasma and magnetic field measurements in the equatorial magnetosphere and across the low-latitude dayside magnetopause and boundary layer. These regions play key roles for our understanding of the global perspective of solar-terrestrial relations as well as the detailed plasmaphysical processes. EQUATOR-S thus complements the Inter-Agency Solar-Terrestrial Physics (IASTP) program. Another important objective is also the validation of an advanced instrument for the measurement of electric fields that is based on electron-beam sensing.

Magnetopause & Boundary Layer

The magnetopause separates two plasmas of widely different nature: the dense, relatively cool and fast streaming solar wind on one side, the dilute, hot and slowly convecting magnetospheric plasma that is coupled to the ionosphere and on the other side. Such plasma boundaries are abundant in the universe. The Earth's magnetopause is one of the few examples that we can study in much detail.

Much is already known from earlier missions such as ISEE and AMPTE. To lowest order the boundary is impermeable (except perhaps in the cusp regions). However, mass and momentum are being exchanged across the boundary by processes violating the "frozen-in" condition which prevails on large scales in cosmical plasmas. These processes are one of the prime study goals of the IASTP. EQUATOR-S, because of its special orbit and its high time resolution measurements can elucidate the roles of the ion and electron inertial scales (a few hundred km and a few km, respectively) in the still mysterious reconnection process.

Equatorial Magnetosphere

Another important object of research with EQUATOR-S will be the near-Earth edge of the geomagnetic tail with its hot plasma constituent, the plasma sheet. This plasma has a very dynamic behavior. Prior to the onset of a so-called substorm, the magnetic field stretches and the plasma sheet becomes thin. At substorm onset, the magnetic field quickly assumes a more dipolar configuration, plasma is jetting towards the Earth, and, further inward, one expects that it is rapidly convecting in longitude, away from the midnight sector. Besides striking local energization processes, the generation of current systems that connect to the ionosphere and thereby transfer momentum and energy to the atmosphere are most important. These currents are the origin of structured auroral displays and of a host of ion and electron beams or other exotic distributions (e.g., ion and electron "conics"). These beams originate largely in not yet well understood energy conversion processes at lower altitudes in and above the topside ionosphere, but are also detected at equatorial latitudes. They create a strong spatial variation in the plasma population (including ionic composition).

Judging from the thinness of auroral arcs and their spacing there should be much fine structure in the near-Earth plasma sheet, in particular in the electric field and pressure distributions. Because of its unprecedented time-resolution, EQUATOR-S is well equipped to resolve this structure and thus will make substantial contributions to the clarification of the generator processes driving the auroral currents. In conjunction with the GEOTAIL and POLAR missions the morphological and causal interrelations of aurora, currents, beams, and plasma sheet can be studied. In particular, the question of substorm initiation and of the significance of the so-called injection boundary near the geostationary orbit can be addressed.

The IASTP Context

The prime goal of the Inter-Agency Solar-Terrestrial Physics program is to improve our understanding of the flow of energy, mass and momentum into and out of the magnetosphere by measuring simultaneously the solar wind properties in front of the magnetosphere, the dynamics of the geomagnetic tail, the polar regions at high and low altitudes, and the equatorial regions of magnetopause, near-earth plasma sheet and ring-current. The equatorial region plays a central role, in that not only the inflow of mass, momentum and energy into inner magnetosphere and ionosphere is to a large extent initiated in this region, but also outflow and loss manifest themselves there. The most important dayside processes in this context are reconnection at the subsolar magnetopause, the formation of boundary layers with the accompanying mass and momentum transfers on the dayside. On the nightside, the injection of hot plasma from the tail, the generation of currents continuing to and depositing momentum and energy in the ionosphere, generating aurora and extracting mass from the ionosphere, and the storage of mass and energy in the ring-current are key topics.

Simultaneous measurements in the solar wind (WIND, SOHO, ACE, IMP8) and in the magnetospheric boundary regions (GEOTAIL, INTERBALL, EQUATOR-S ) will allow a more quantitative understanding of the relation between input parameters and transfer efficiencies. Simultaneous measurements at equatorial and polar latitudes (POLAR) at distances of about 10 Re will provide clues about the origin and low-latitude connection of entry layer and mantle on the dayside, and between plasma injections from the tail and large-scale morphology and dynamics of auroral sheet currents, Alfvén waves, particle beams and plasma flows on the nightside. In the latter context the energy flow from the deeper tail as well as the sequence of events constituting a magnetospheric substorm can only be assessed with simultaneous measurements in the near-earth plasma sheet ( EQUATOR-S ) and further outward (GEOTAIL).

One of the unsolved problems pertains to the trigger of a substorm, what it is, and whether it is located near the earth (7 Re) or at a neutral line at 25 Re in the central plasma sheet. Another important open question concerns the high-altitude closure of the auroral currents.


Last update of this page 1999-04-06 by Helmut Steinle . Please send comments to hcs@mpe.mpg.de !

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