Terrestrial Ring Current Research Articles

Latitudinal dependence of nonlinear interaction between electromagnetic ion cyclotron wave and terrestrial ring current ions

Electromagnetic ion cyclotron (EMIC) waves can lead to the rapid decay (on a timescale of hours) of the terrestrial ring current. Such decay process is usually investigated in the framework of quasi-linear theory. Here, both theoretical analysis and test-particle simulation are performed to understand the nonlinear interaction between ring current ions and EMIC waves. In particular, the dependence of the nonlinear wave-particle interaction processes on the ion initial latitude is investigated in detail. These nonlinear processes are classified into the phase trapping and phase bunching, and the phase bunching is further divided into the channel and cluster effects. Compared to the prediction of the quasi-linear theory, the ring current decay rate can be reduced by the phase trapping, increased by the channel effect phase bunching, but non-deterministically influenced by the cluster effect phase bunching. The ion initial latitude changes the occurrence of the phase trapping, modulates the transport direction and strength of the cluster effect phase bunching, and only slightly affects the channel effect phase bunching. The current results suggest that the latitudinal dependence of these nonlinear processes should be considered in the evaluation of the ring current decay induced by EMIC waves.

Ring Current Dynamics

This chapter reviews the current understanding of ring current dynamics. The terrestrial ring current is an electric current flowing toroidally around the Earth, centered at the equatorial plane and at altitudes of ∼10,000 to 60,000 km. Enhancements in this current are responsible for global decreases in the Earth’s surface magnetic field, which have been used to define geomagnetic storms. Intense geospace magnetic storms have severe effects on technological systems, such as disturbances or even permanent damage of telecommunication and navigation satellites, telecommunication cables, and power grids. The main carriers of the ring current are positive ions, with energies from ∼1 keV to a few hundred keV, which are trapped by the geomagnetic field and undergo an azimuthal drift. The ring current is formed by the injection of ions originating in the solar wind and the terrestrial ionosphere into the inner magnetosphere. The injection process involves electric fields, associated with enhanced magnetospheric convection and/or magnetospheric substorms. The quiescent ring current is carried mainly by protons of predominantly solar wind origin, while active processes in geospace tend to increase the abundance (both absolute and relative) of O+ ions, which are of ionospheric origin. During intense geospace magnetic storms, the O+ abundance increases dramatically. This increase has been observed to occur concurrently with the rapid intensification of the ring current in the storm main phase and to result in O+ dominance around storm maximum. This compositional change can affect several dynamic processes, such as species-and energy-dependent charge-exchange and wave-particle scattering loss.

Ring current and the magnetosphere‐ionosphere coupling during the superstorm of 20 November 2003

We investigated the impact on the terrestrial ring current of a coronal mass ejection (CME) and the associated magnetic cloud that severely disturbed the Earth's magnetosphere on 20 November 2003. This CME decreased the Dst index to −472 nT, which makes it the second largest storm, based on the minimum Dst index values, observed between 1957 and 2004. Data from the DMSP, NOAA, and LANL satellites showed the unique characteristics of this storm; a polar cap potential that increased to at least 200 kV, a polar cap boundary that moved as low as about 60° MLAT, a plasma sheet density that increased to 5 cm−3 at L = 6.6 when the Dst index was near its minimum, and the inner edge of the plasma sheet ion population that penetrated into a region for which L ≤ 1.5. In order to study the dynamics of the ring current and the associated magnetosphere‐ionosphere coupling, we performed a ring current simulation that computed the evolution of the phase space density of the ring current ions and the closure of the electric current between the magnetosphere and the ionosphere. Major results were as follows: (1) The ring current, in terms of the Dst index and the inner edge of the plasma sheet, can result from the enhancement of the convection electric field, given the polar cap potentials used in the model; (2) The solar wind particles probably penetrated quickly into the geosynchronous altitude on the nightside with a lag of about 80 min, resulting in further enhancement of the ring current; (3) Dense geocoronal neutral hydrogen or a large coefficient of pitch angle diffusion (>10−4 s−1) is probably needed to account for the rapid motion of the inner edge of the plasma sheet (or the ring current) population to a higher L value; (4) Both the simulated and observed field‐aligned current (FAC) distributions show multiple current sheets, rather than the normally expected two current sheets. Fluctuations in the polar cap potential and the plasma sheet density are believed to cause the multiple sheets of field‐aligned currents; (5) The equatorward edge of the Region 2 type field‐aligned currents was observed to expand as low as 40° MLAT, which is consistent with the simulation; and (6) The convection pattern can be much more complicated than an average one due to a strong Region 2 FAC. A noticeable feature was the reversal of the zonal ionospheric plasma flow that emerged on the dawnside. In particular, a westward flow was observed in the equatorial region of the eastward plasma flow at dawn. Its speed had a local maximum of about 5° equatorward of the flow reversal. The flow reversal is thought to have resulted from the relatively strong shielding electric field.

Dawn‐dusk asymmetry of geosynchronous magnetopause crossings

Geosynchronous magnetopause crossing (GMC) data were collected from literature sources from 1967 to 1993 (189 GMCs) and from the experimental data on magnetic measurements on GOES (129 GMCs) and plasma measurements on LANL (197 GMCs) geosynchronous satellites in 1994 to 2001. The dawn‐dusk asymmetry of the magnetopause at geosynchronous orbit was examined by two independent methods using the collected data set of 515 GMCs. First, the large amount of accumulated GMCs permitted the revealing of a substantial dawn‐dusk asymmetry in the local time (LT) distribution of the GMC occurrence probability, with a statistically significant maximum in the range from 1000 LT to 1100 LT. Second, an analysis of the dawn‐dusk asymmetry dependence on the upstream solar wind conditions was performed using a scatter plot of the solar wind total pressure versus local time for various IMF Bz. There was no asymmetry revealed for large positive Bz. Under strong negative Bz we found a prominent magnetopause dawn‐dusk asymmetry. The asymmetry is characterized by a shifting of the GMCs with the minimal required solar wind total pressure toward the dawn and by a significantly lower (about 3 times) solar wind pressures required for the GMCs in the dawn sector relative to the dusk sector. We found that the asymmetry cannot be attributed to the IMF orientation along the Parker spiral, which is not revealed for strongly disturbed solar wind conditions accompanying the GMCs. An application of the dawn‐dusk asymmetry effect for the Chao et al. [2002] model provided a substantial increase in the model predictive capability interim of the geosynchronous magnetopause crossings. The standard deviation decreased by 20% from 0.55 RE for the initial version to 0.45 RE for the asymmetrical version of the model, with the magnetopause axis rotated by an angle of about 15° toward the dawn. The physical processes responsible for the magnetopause dawn‐dusk asymmetry are discussed. We indicate the two most probable magnetospheric phenomena, which would contribute to the substantial dawn‐dusk asymmetry of the magnetopause under disturbed solar wind and geomagnetic conditions. The first one is that magnetopause erosion would operate more intensively in the prenoon sector. The second phenomenon is an asymmetrical terrestrial ring current that would develop during geomagnetic storms.

Numerical Simulation of the Ring Current: Review

Numerical simulation of the terrestrial ring current is reviewed. After mentioning ‘modules’ which are needed to be taken into consideration in a ring current simulation, we discuss growth and decay of the ring current. At least four different paradigms have been proposed to account for the ring current development in the past forty years, i.e., the convection paradigm, the substorm paradigm, the diffusion paradigm, and the ionosphere paradigm. As for the proton ring current, a simulation under the convection paradigm gives reasonable results which are in fair agreement with observations with respect to the Dst variation as well as the radial and longitudinal energy density variation of protons when the convection electric field depending on solar wind parameters is given. The proton energy density is observed to be enhanced (weakened) on the nightside, and be weakened (enhanced) near noon during a storm main phase (recovery phase). This characteristic is probably understood to mean that a large-scale and long-standing electric field dominates other electric fields during the storm main phase, e.g., a locally induced electric field (the substorm paradigm) and a highly fluctuated electric field (the diffusion paradigm). The declining of the ring current is shown to be triggered by the decrease in the convection electric field at the beginning of a storm recovery phase, but the decrease in the convection electric field hardly contributes the decay of the ring current. The charge exchange or other loss processes is needed for the substantial decay of it. An ultimate decay rate (several hours) is achieved when the strong diffusion takes place, or when the plasma sheet density drastically decreases while the charge exchange is estimated to provide rather slow decay (a half of day). Diagnosis tools for investigating the ring current, which are expected to bring us a new insight, are proposed in the latter section.

Evidence against an independent solar wind density driver of the terrestrial ring current

The interplanetary electromagnetic fields are generally considered the drivers of the storm‐time terrestrial ring current. Recently, the solar wind density has been advocated as an additional driver. Solar wind density partially determines the density in the plasma sheet, which, in turn, is the likely source for the ring current. Therefore, the solar wind density may drive the ring current by enhancing the ring current source plasma. Some studies, using a few years of data, have found a strong statistical signal for a solar wind density driver with a few hours lag prior to the maximum ring current intensity, as measured by a minimum in Dst. However, we show, using a much larger database of storms, that no consistent role for density alone is evident on time scales of a few hours. The previously reported statistical signal seems to be isolated to a particular interval from November 1994 through September 1995.

The terrestrial ring current: Origin, formation, and decay

The terrestrial ring current is an electric current flowing toroidally around the Earth, centered at the equatorial plane and at altitudes of ∼10,000–60,000 km. Changes in this current are responsible for global decreases in the Earth's surface magnetic field, which are known as geomagnetic storms. Intense geomagnetic storms have severe effects on technological systems, such as disturbances or even permanent damage to telecommunication and navigation satellites, telecommunication cables, and power grids. The main carriers of the storm ring current are positive ions, with energies from ∼1 keV to a few hundred keV, which are trapped by the geomagnetic field and undergo an azimuthal drift. The ring current is formed by the injection of ions originating in the solar wind and the terrestrial ionosphere. The injection process involves electric fields, associated with enhanced magnetospheric convection and/or magnetospheric substorms. The quiescent ring current is carried mainly by protons of predominantly solar wind origin, while geospace activity tends to increase the abundance (both absolute and relative) of O+ ions, which are of ionospheric origin. During intense magnetic storms, the O+ abundance increases dramatically, resulting in a rapid intensification of the ring current and an O+ dominance around storm maximum. This compositional change affects, among other processes, the decay of the ring current through the species‐ and energy‐dependent charge exchange and wave‐particle scattering loss. Energetic neutral atoms, products of charge exchange, enable global imaging of the ring current and are the most promising diagnostic tool of ring current evolution. This review will cover the origin of ring current particles, their transport and acceleration, the effects of compositional variations in the ring current, the effects of substorms on ring current growth, and the dynamics of ring current decay with an emphasis on the process of charge exchange and the potential for wave scattering loss.

Plasma near Io: Estimates of some physical parameters

Models of Io's interaction with the Jovian plasma are examined to assess whether conditions hazardous to a spacecraft flying by Io at about 1000 km above its surface should be anticipated. Only two models are regarded as consistent with most of the presently available data. The ‘ionospheric’ model allows field‐aligned currents to close through Io and its ionosphere. The ‘magnetospheric’ model allows the currents to close on an Ionian magnetopause and across a tail neutral sheet. Although only the latter model provides an explanation for the pitch angle distribution of energetic electrons measured by the LECP detector on Voyager 1, the implications of both models for near‐Io plasma are explored. The plasma density is found to be roughly the same near Io and elsewhere in the torus. In Io's wake the plasma is hotter than elsewhere in the torus but in the extreme case the ion temperature is merely of order the temperature in the terrestrial ring current. No special problems are anticipated for a spacecraft subject to these plasma conditions.

EUV observations of the equatorial aurora

The extreme ultraviolet telescope on the Apollo‐Soyuz mission observed the equatorial aurora from an altitude of 220 km on four separate occasions in July 1975 in quiet geomagnetic conditions (Ap=6). In all cases signals well above ambient background in the 50 to 150‐Å, 114 to 150‐Å, 170 to 600‐Å, and 500 to 780‐Å bands were recorded as the spacecraft moved across the equator and the instrument viewed the atmosphere below it. The observed emissions are confined to a band roughly 10° to 20° in width, the peak emission occurring in the range ∼ −15° to +2° magnetic latitude. No enhancement in the 1350 to 1550‐Å channel was noted. The observed signals are interpreted as recombination radiation of energetic helium and, possibly, oxygen ions originating in the terrestrial ring current.

Radiation Measurements to 658,300 Km. with Pioneer IV

The flight of deep-space probe Pioneer IV is described, and radiation observations obtained are reported and discussed in comparison with Pioneer II and the first Soviet cosmic rocket results. In particular, the discussion includes: paucity of energetic plasma in interplanetary space March 3-6, 1959, lack of influence of the moon, re-determination of interplanetary cosmicray intensity, effective extent of the geomagnetic field, and the terrestrial ring current of Dolginov and Pushkov. (T.R.H.)