This article is based on a paper presented at the 1992 World Space Congress and submitted for publication to the COSPAR journal Advances in Space Research.
The magnetospheres of the Earth and the Sun, the latter also being known as the heliosphere, differ in physical dimension by a factor of 105 but share some characteristics of all magnetized astrophysical systems interacting with external plasma winds in their local environments. In that the laws of plasma physics, magnetohydrodynamics, and gas flow dynamics are universal, some parameters in the earth's magnetosphere can in principle be scaled to the heliosphere and perhaps to larger systems such as the galactic magnetosphere and extragalactic radio sources. Since the earth's magnetosphere has been explored extensively by in-situ spacecraft measurements of magnetic fields, plasmas, photonic emissions (e.g.: optical, radio, x-ray), and energetic particles, it would clearly be advantageous to find magnetospheric processes and parameters which can be scaled to other astrophysical systems for which we have only remote observations. In this article we extend previous work /1/ and discuss processes affecting electron acceleration and transport, the electrons being considered "test particles" for the dominant magnetic structures and dynamics of magnetospheric systems such as those of the Earth and the Sun.
Many years of measurements by earth-orbiting spacecraft experiments have established that energetic electrons provide useful probes of magnetospheric field configurations and internal dynamics. The outer terrestrial magnetosphere is strongly affected by the direction of the interplanetary magnetic field (IMF), which changes polarity towards and away from the Sun during the 27-day synodic rotation period. The transient connectivity of the interplanetary magnetic field (IMF) lines into the polar cap region, shown in Fig. 1 (a), was established by correlative observations of bidirectional anisotropies for 50-500 eV electrons between the distant tail lobe at ISEE-3 and the low altitude polar cap as measured by the DMSP spacecraft /2/. Suprathermal (> 200 eV) electrons appear over the north geomagnetic pole when the IMF direction is outward from the Sun and can thereby connect into the inward dipolar field, and the shift of this "polar rain" to the south pole for IMF directions toward the Sun provides a measure of the IMF polarity even in the absence of IMF measurements.
Correlated measurements by IMP-8, ISEE-3, and geosynchronous orbit experiments have been utilized /3/ to determine that energetic electrons trapped at MeV energies within the magnetosphere have little temporal correlation to the known thirteen-month cycle of intensity variation for interplanetary electrons propagating along the spiral interplanetary magnetic field from the dominant jovian source in the inner heliosphere (R < 101 AU). Since solar wind or solar flare acceleration processes cannot account for the energy or intensity of such electrons during periods of little or no flare activity, acceleration to MeV energies and transport within the magnetosphere has been attributed to magnetospheric recirculation /4/ which is represented in the four panels of Fig. 1 (b). This circulatory process couples conventional inward diffusion and acceleration with pitch angle scattering in an active wave-particle interaction region of the inner magnetosphere (e.g., the well-known "slot" region of equatorially trapped electron depletion near L = 2) and energy-conserving outward transport across field lines at low altitudes towards the outer magnetosphere, where scattering again isotropizes the angular distributions. The recirculated electrons are observed /5/ at geosynchronous orbit as pitch angle distributions with "butterfly" shapes arising from electrons mirroring at low altitudes and outside the atmospheric loss cone.
The acceleration of energetic electrons and ions up to MeV energies in the earth's magnetotail is evidently produced by reconnection electric fields in the neutral sheet region. The large scale electric field across the tail from solar wind plasma convection through the magnetosphere is less than 102 kV and field-aligned currents arising from magnetospheric coupling to the ionosphere only account for auroral electron acceleration to keV energies. Recent numerical code simulations /6/ indicate that the rate of magnetic reconnection, and hence the rate of charged particle acceleration, is increased significantly by magnetic turbulence in the nominal X-point region. Acceleration to maximum energies of a few MeV follows from trapping of particles in small magnetic bubbles convecting away from the X-point and containing strong local electric fields. The maximum acceleration energy for electrons is proportional to the square of the Alfven velocity VA, the total dimension L of the reconnection region, and the plasma frequency wp. Thus regions of high magnetic field, low density, and large dimensions give the highest maximum energies of acceleration.
The magnetic and plasma configurations of the outer heliosphere beyond about 50 AU from the Sun have yet to explored by in situ measurements (e.g., by the Pioneer 10/11 and Voyager 1/2 spacecraft) and have been considered in recent reviews /7/. As sketched in Fig. 2 the Parker spiral magnetic field emanating from the Sun requires about sixteen solar rotations to reach an expected inner shock, called the solar wind termination shock, which marks transition of 400 km/sec supersonic solar wind to subsonic flow at about 100 AU. Within this shock the region of solar wind affected by solar rotation corresponds in the magnetosphere to the corotating plasma and fields within the plasmapause boundary.
The interstellar gas flows around the heliopause outside the termination shock at a low speed of 20 km/s and sweeps the solar wind gas and fields back into a turbulent wake perhaps extending downstream many thousands of AU. The twenty-two year cycle of solar magnetic polarity reversals periodically switches the north and south lobe polarities of solar wind field lines extending back into the heliotail where plasma flows eventually merge into the interstellar gas. Downstream turbulence and the lobe field reversals would lead to magnetic reconnection at many points within the heliotail.
The outer boundary layers (e.g., heliosheath) of the upstream heliosphere are presently ill- defined in the absence of direct measurements of magnetic fields and plasmas in the Very Local Interstellar Medium (VLISM). Dependent on flow speed and density the interstellar gas interaction with the upstream heliosphere may be subsonic or supersonic with a detached external shock similar to the earth's bow shock in the latter case. The magnitude of the interstellar magnetic field may be of order six microgauss but the direction in the VLISM is unknown. Alignment with the galactic spiral arms in the Local Interstellar Medium (LISM) could lead to a large north or south component relative to the solar equatorial plane. This field may strongly affect the outer boundary layers and could channel interstellar cosmic rays into the heliosphere at energies up to 102 GeV in analogy to solar wind electron entry into earth's magnetosphere. Cosmic ray ions from the VLISM, or from acceleration at the heliospheric boundaries, may also affect the boundary layer thickness and location.
The lack of rigid plasma corotation in the heliosphere leads to the Parker spiral configuration of longitudinal IMF components becoming dominant beyond 1 AU from the Sun. Whereas dipolar fields with relatively negligible longitudinal components lead to particle trapping and longitudinal drift in planetary magnetospheres, the spiral field produces gradient-curvature drift of electrons and ions in opposite directions between the solar equatorial plane and the polar heliosphere inside the termination shock. For the A < 0 (polar magnetic field directed inward towards north pole and outward from south pole) solar minimum heliosphere shown in Fig. 2 the electrons drift equatorward from the poles within the solar "plasmasphere" and poleward from the equator at the termination shock due to VxB electric fields at the shock. Electrons undergo adiabatic deceleration during transport within the radially expanding solar wind and increase in energy to maxima of a few hundred MeV /8/ during propagation along the termination shock.
The degree of connection between the polar heliospheric magnetic fields and the VLISM field may govern the efficiency of heliospheric "recirculation," since a good connection would enhance leakage out of an "open" heliosphere while a poor connection would enforce recirculation along the termination shock back into the equatorial or polar heliosphere. Conversely, interstellar electrons would gain easier entry into the polar heliosphere when good magnetic connection existed. Unusually high intensities of MeV-GeV electrons observed at 1 AU /9/ and 40 AU /10/ during the 1987 (A < 0) solar minimum might then be related to poor or good VLISM connection, depending on whether the observed electrons were of jovian or galactic origin, respectively. Since a galactic origin is currently preferred at GeV energies beyond the limits of termination shock acceleration for jovian electrons /9,10/, there may be an indication here of a good VLISM connection in 1987. Alternatively, it may be necessary to consider effects of further electron acceleration in the heliotail, where the cross-tail potential scaled from the earth case is of order 101 MV and where turbulent reconnection could accelerate electrons to 103 MeV.
The flows of heliospheric electrons sketched in Fig. 3 for A > 0 (outward fields at north pole) solar minima resemble the circulatory pattern of trapped electrons in the earth's magnetosphere. Gradient- curvature drifts drive heliospheric electrons inward along the equatorial neutral sheet and inward diffusion compresses magnetospheric electrons towards the dipole equator. Diffusion and drifts in the heliospheric case, and wave-particle scattering in the magnetospheric one, drive electrons poleward from the neutral sheet during inward propagation. In analogy to low altitude scattering across dipole field lines in magnetospheric recirculation the strong Alfven turbulence in the solar wind transition region below 20-30 solar radii may drive enhanced diffusion between the equatorial plane and the solar poles. Strong magnetic fields in flare loops near the sun's surface will produce magnetic mirroring, bidirectional pitch angle distributions, and charged particle trapping as in the terrestrial magnetosphere.
Observations of energetic electrons from solar flares often indicate fast progagation over large heliocentric angles at coronal altitudes /11,12/. Complexity and time dependence of the coronal fields at other times, including solar minima, may give rise to rapid poleward transport of electrons propagating along the IMF towards the coronal region. In the corresponding magnetospheric case VLF waves are thought /4/ to drive cross-field diffusion at low magnetospheric altitudes in the case of magnetospheric recirculation. MHD turbulence and Alfven waves, the latter being an energy source for solar wind acceleration, in the low altitude corona would be the heliospheric equivalents of VLF and other plasma waves in the terrestrial magnetosphere. Cross-field diffusion and drifts of electrons in the IMF may be less significant in the inner heliosphere inside 1 AU where more radial IMF field lines allow fast interplanetary propagation along the field into the low altitude region of strong scattering.
Measurements of jovian electrons between 0.5 and 1 AU /13,14/ show unexpected anomalies, including occasional increases of intensity towards the Sun and intensity changes correlated to solar magnetic reversals, which should be investigated further for potential effects due to low- altitude scattering. The competing effects of drifts, diffusion, and coronal propagation on jovian electron flows between the solar equator and the polar heliosphere can be directly measured by comparison of future Ulysses spacecraft measurements /15/ with corresponding ones at 1 AU in the Ecliptic. Extension of the Ulysses solar polar mission, current scheduled for flights over the south and north solar poles in 1994 and 1995, respectively, would be helpful for investigation of solar cycle effects and would correspond to the magnetospheric equivalent of the DMSP/ISEE-3 electron measurements. The proposed Solar Probe mission to several solar radii above the Sun's surface would increase our knowledge of field, plasma, and particle processes in this unexplored region which is most comparable within the present discussion to the terrestrial ionosphere.
In some cases the experimental techniques and data analysis methodologies developed for magnetospheric studies may find direct application to investigations of the heliosphere. The developing techniques of global magnetospheric imaging /16/ for ultraviolet, visible, radio, x-ray, and energetic neutral atom (ENA) sources in the planetary magnetospheres could be applied to measurements of solar UV backscattering from the VLISM, heliospheric radio emissions from electron synchrotron processes, reflected jovian radio waves at the outer heliospheric boundary regions and in the heliotail, and ENA particles produced at the termination shock or in the heliotail. With regard to the latter, the single-ionized "anomalous component" of 10-30 MeV ions, thought to be VLISM atoms stripped of one electron by solar UV radiation and accelerated at the termination shock, have already (or are) being investigated by cosmic ray experiments on magnetospheric spacecraft (e.g.: Cosmos, STS/Spacelab, CRRES, SAMPEX) which have used the geomagnetic field as a momentum/charge filter to isolate low charge state ions from the otherwise dominant populations of fully ionized solar and galactic cosmic ray ions. Theoretical and numerical models for magnetospheric boundary layers and regions (e.g., the magnetosheath and magnetopause) may be applicable with appropriate scaling and other adjustments to comparable heliospheric boundaries and regions. Direct knowledge of hot plasma and energetic particle interactions with neutral gas, dust, rings, and satellites in the outer planet magnetospheres, as obtained by the Pioneer and Voyager flyby missions, may be applicable to analogous interactions with comparable heliospheric components including the heliospheric "rings" formed by the Kuiper disk comets at 30-100 AU and the neutral gas and cometary bodies further out in the VLISM. It is possible, for example, that heliospheric electron and ion acceleration in the outer heliosphere might contribute significantly to VLISM gas ionization and to radiation processing of cometary surfaces.
The fleet of earth-based and heliospheric spacecraft probing the equatorial and polar heliosphere will eventually send back a wealth of data requiring correlative data analysis approaches similar to those long used for coordinated observation campaigns by multiple satellites in the earth space environment. The International Solar Terrestrial Physics (ISTP) program and the Coordinated Data Analysis Workshop (CDAW 1-9) series, both centered at NASA Goddard Space Flight Center, are programmatic models for what may eventually be required to fully exploit multiple spacecraft measurements and simulations of global heliospheric processes. Current efforts in this direction include the International Heliospheric Study (IHS), NASA's Coordinated Heliospheric Observations (COHO) program /17/, and the COHO and OMNI on-line data bases /18,19/ for heliospheric field, plasma, and energ>